C Loisel

Z Maache-Rezzoug

C Esneault

J L Doublier

EFFECT OF HYDROTHERMAL TREATMENT ON THE PHYSICAL AND RHEOLOGICAL PROPERTIES OF MAIZE STARCHES

Keywords: Hydrothermal treatment, Maize starch, Pregelatinization, Rheological properties hal-00413504

Standard maize (SMS) and waxy maize starch (WMS) were hydrothermally treated at their residual moisture level (~12 %) by Instantaneous Controlled Pressure Drop process in order to obtain pre-gelatinised starches in a single step. The effect of two parameters of this process, namely the steam pressure level and processing time, on the structural and rheological properties of the native maize starches were described. The occurrence of partial gelatinization for DIC treated starches was clearly attested by the increase of the median volume diameter in cold water, the decrease of the gelatinization enthalpy and a loss of birefringence under polarized light, this was more prominent for the highest pressure and longest time: 2.7 to 3 bar for 200 to 300 seconds.

Sensitivity of starches to the process was also dependent on their origin, SMS being more affected than WMS.

Introduction

Starch is essential in many formulated foodstuffs. It constitutes an important source of energy and contributes to the structure and the texture of food, as a thickening or gelling agent. These properties occur under heating (pasteurization or sterilization) in an excess of water.

Pregelatinized starches are widely used for many foods as a major ingredient to provide thickening texture at temperatures below the gelatinization temperature. They are obtained from native or modified starch, by drum [START_REF] Vallous | Performance of a double drum dryer for producing pregelatinized maize starches[END_REF], roll, or spray drying after cooking and also by extrusion cooking [START_REF] Mercier | Comparative modifications of starch and starchy products by extrusion cooking and drum-drying[END_REF]. Annealing [START_REF] Tester | Annealing of starch[END_REF] and heat-moisture treatment (HMT) [START_REF] Gunaratne | Effect of heat-moisture treatment on the structure and physicochemical properties of tuber and root starches[END_REF] are also two common physical means by which the treated starch can acquire modified properties without rupturing the granule. These processes require a hydration step followed by drying. When applying a thermal treatment, the initial dispersion is changed into a starch paste due to the swelling of granules and leaching of amylose in the intergranular phase. This overall phenomenon is referred to as the pasting process and is a result of the gelatinization which corresponds to the loss of crystalline starch granules at ~ 60-70°C. These overall changes include loss of birefringence, loss of X-ray diffraction pattern, absorption of water and swelling, change of shape and size of starch granules and leaching of amylose from the granules [START_REF] Williams | Starch gelatinization : a morphological study of triticeae and other starches[END_REF], [START_REF] Eliasson | Cereals in breadmaking. A molecular colloidal approach[END_REF], [START_REF] Hoover | Composition, molecular structure, and physico-chemical properties of tuber and root starches: a review[END_REF].

Many methods of characterizing structural changes of starch upon pasting have been developed. A large number of techniques, such as differential scanning calorimetry (DSC) [START_REF] Donovan | Differential scanning calorimetry of heat-moisture treated wheat and potato starches[END_REF][START_REF] Lim | Thermal transition characteristics of heat treated corn and potato starches[END_REF][START_REF] Ziegler | Spherulitic crystallization of starch : influence of botanical origin and extent of thermal treatment[END_REF][START_REF] Kiseleva | Influence of growth temperature on the structure and thermodynamic parameters of barley starches[END_REF] and X-ray diffraction [START_REF] Kiseleva | Influence of growth temperature on the structure and thermodynamic parameters of barley starches[END_REF][START_REF] Lim | Thermal transition characteristics of heat treated corn and potato starches[END_REF] have been used to study the gelatinization behavior of starch granule altered by various physical treatments. During recent years differential scanning calorimetry (DSC) has evolved as the preferred method [START_REF] Eliasson | Cereals in breadmaking. A molecular colloidal approach[END_REF][START_REF] Takaya | Thermal studies on the gelatinization and retrogradation of heat-moisture treated starch[END_REF], to hal-00413504, version 1 -4 Sep 2009 measure different aspects of the gelatinization process. DSC is of great value in studying both the loss of crystalline order during gelatinization, which occurs when the starch is heated in the presence of water [START_REF] Cooke | Loss of crystalline and molecular order during starch gelatinization and origin of the enthalpic transition[END_REF], and the re-ordering of such systems during ageing [START_REF] Chung | Physical aging of glassy normal and waxy rice starches: effects of aging temperature on glass transition and enthalpy relaxation[END_REF].

Hydrothermal treatment of starch also leads to a modification of its rheological properties. These properties can be characterized after rehydration of the starches under mild temperature (40°C, [START_REF] Anastasiades | Rheological and physical characterization of pregelatinized maize starches[END_REF] or using Brabender Viscogragh to assess repeatable pasting conditions. The shape of the pasting profile gives then useful indications on the ability of the granules to swell freely before their physical breakdown (peak and plateau values).

The purpose of the present work was to characterize hydrothermally-treated starches by a recent hydrothermal process: the Instantaneous Controlled Pressure Drop (DIC) over a useful range of operating conditions. The DIC process is based on the thermomechanical processing induced by subjecting the product to a rapid transition from high steam pressure to a vacuum. It has been used initially in the field of the drying texturation in order to obtain a more expanded structure than one obtained by classical methods such as hot air. This process was applied to various products such as pasta products (Maache- [START_REF] Maache-Rezzoug | Effect of instantaneous controlled pressure drop treatment on organoleptic and cooking quality of pasta products[END_REF], and a thickening polysaccharide used in the petroleum industry, the scleroglucan [START_REF] Rezzoug | Effect of instantaneous controlled pressure drop proccess on hydration capacity of scleroglucan. Optimisation of operating conditions by response surface methodology[END_REF].

The originality of the DIC treatment compared to other hydrothermal treatments is that the starches are treated at residual moisture content; no hydration step is then required before the DIC treatment.

The effect of process input variables (steam pressure level and processing time) on the structural and functional properties of two native starches, standard maize starch (SMS) and waxy maize (WMS) was investigated.

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Materials and methods

Raw materials

Standard maize starch (SMS) and waxy maize starch (WMS, Waxilys 200) were supplied by Roquette Frères (Lestrem, France). The amylose content was different for the two types of starch: 27-28 % and lower than 1% for SMS and WMS, respectively.

Methods

Moisture content

The starch moisture content was determined by air oven at 105°C during 24 h, according to the A.F.N.O.R. standard method.

DIC hydrothermal treatment

The schematic diagram of the equipment used has been described in a previous study [START_REF] Rezzoug | Effect of instantaneous controlled pressure drop proccess on hydration capacity of scleroglucan. Optimisation of operating conditions by response surface methodology[END_REF]. This equipment is composed of three main elements: a processing vessel, a vacuum container and a valve that connects these two parts. In the DIC treatment, the sample is placed in a vessel which is maintained under vacuum (~ 50 mbar). This initial vacuum allows a better diffusion of the heating fluid through the product and consequently heat transfer is improved. The feed of the treatment vessel was typically 150 g of native starch at residual moisture content (~ 12 % w/w).

The thermal treatment is performed by injection of saturated steam, at a fixed steam pressure for a determined processing time, followed by a rapid pressure drop to vacuum (about 50 mbar). The opening of the valve leads to a rapid transition from processing pressure to a vacuum because the volume of the vacuum container (1600 L) is 130 times larger than the one of the treatment vessel (12 L). The rapid transition of pressure inside the processing vessel induces a rapid cooling of starch from 107-140 °C (depending on the steam pressure level) to about 30°C in less than 1 hal-00413504, version 1 -4 Sep 2009 second. The equilibrium pressure after dropping pressure depends on the operating conditions: the higher the steam pressure level, the higher the equilibrium pressure. At the end of the treatment, the starch sample does not need a drying step because initial and final moisture are similar.

Pasting procedure using Viscograph Brabender

The processed samples are powdery products that have to be rehydrated for analytical purpose. This was performed using the Brabender Viscograph to obtain a starch paste under repeatable conditions. The starch concentrations were chosen in order to lie within the sensitivity range of the Viscograph: i.e. 6% for SMS and 3% (w/w) for WMS. Starch was slurried in demineralized water at room temperature, and then submitted to gradual heating (1.5°C/min) from 30°C to 96°C.; this temperature was maintained for 10 minutes and was followed by a cooling step (1.5°C/min) down to 70°C before sampling. The relevant values obtained from the pasting profile were: peak and plateau (at 96°C) in Brabender units (100 BU for 25 cmg). The moisture content was determined directly after the pasting procedure, to check the starch concentration before rheological measurements.

Rheological measurements

Flow behavior and viscoelastic properties of starch pastes were measured using a controlled stress rheometer (TA Instrument AR1000) with the cone/plate geometry (6 cm / 2°). An aliquot of the starch dispersion pasted in the Viscograph Brabender was poured onto the plate of the rheometer preheated at 60°C, then covered by a layer of paraffin oil to avoid evaporation. For flow measurements, two up-down shear scans from 0 to 660 s -1 (4 min each) were linearly applied, followed by a logarithmic stepwise decrease from 660 to 0.01 s -1 , after equilibrium for each shear rate.

The oscillatory tests were carried out at 4% strain (linear viscoelastic range) on a new aliquot, with the following sequence: a mechanical spectrum at 60°C, then a quenching to 25°C, followed hal-00413504, version 1 -4 Sep 2009 by a time dependent measurement of the viscoelastic behavior during 15 hours at 6.3 rad/s, and a mechanical spectrum at 25°C. The frequency range investigated was from 0.5 to 100 rad/s.

Differential scanning calorimetry

The DSC analysis was performed directly on samples treated by DIC with the Setaram Micro-DSCIII apparatus at a 70% moisture content using the following procedure: gradual heating from 20 to 95°C (1°C/min). From the DSC curves, the peak temperature (T p ) and gelatinization enthalpy (∆H) were calculated. The degree of gelatinization of hydrothermally treated starches was calculated by the following equation [START_REF] Marshall | Determining the degree of gelatinization in parboiled rice: comparison of subjective and an objective method[END_REF]:

( ) 100 1 (%) x H H DG raw t ∆ ∆ - = (1)
where DG is the degree of gelatinization of hydrothermally treated starch, ∆H t the gelatinization enthalpy of DIC treated starch and ∆H raw the gelatinization enthalpy of the native starch.

Granule size and distribution

Particle size determination was carried out at room temperature using a Malvern Master Sizer (Malvern Instruments, Ltd) laser scattering analyser with a 300 mm Fourier cell (range 0.05 µm to 879 µm). The starch dispersion was first diluted (1/10) with demineralized water at 20°C

immediately at the end of the Viscograph procedure, then dispersed in the sample dispersion unit (1 ml/100 ml water) and fed into the measuring cell. Volume distribution was obtained using the Mie scattering theory which requires the refractive index of the media to be specified: we used 1.529 and 1.33, respectively, for starch and liquid phase and 0.1 for the starch granule absorption [START_REF] Loisel | Structure des empois d'amidon en relation avec leur comportement rhéologique[END_REF]. From each distribution, the median volume diameter D(v, 0.5) was chosen to allow comparison with the literature [START_REF] Ziegler | Dynamic measurement of starch granule swelling during gelatinization[END_REF] and the size dispersion of starch granules was evaluated using the dispersion index referred to as the span, by the following equation:

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Span:

) 5 . 0 , ( ) 1 . 0 , ( ) 9 . 0 , ( v D v D v D - (2)
The same experiment was performed on the starch powder directly after the DIC process to yield the median volume diameter of starch granules.

Microscopy

The samples were examined visually using a phase contrast microscope equipped with a CCD camera. Small amounts of sample were suspended in distilled water to 1/20 and observed using a magnification of 400 under direct transmitted or polarized light.

Experimental design

A central composite rotatable design with two independent variables and five levels was used. For the two variables, the design yielded 12 experiments with four (2 2 ) factorial points, four extra points (star points) to form central composite design and four center points for the replications.

The experiments were run in random order to minimize the effects of unexpected variability in the observed responses due to extraneous factors. The experimental range and the central point were based on the results of preliminary trials. Table 1 lists the independent variables and the coded factors levels. The response surfaces were obtained by using the analysis design procedure of Statgraphics Plus for Windows, (4.1 version).

Results and discussion

Effect of the DIC treatment on the physical properties of starch

The effect of the DIC process on the overall physical properties of the two starches is summarized in Tables 2, 3 and 4. Table 2 presents the gelatinization characteristics for the two starches: peak temperature, gelatinization enthalpy and degree of starch gelatinization (%) induced when applying the DIC process.

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For the two types of starches, the gelatinization temperature as defined by microcalorimetry as the maximum of the endothermic peak did not vary to a large extent when applying the DIC process, unless at the highest pressures. For instance the peak temperature, Tp, jumped from 67.1 (untreated) to 76.6°C for SMS, and from 69.7 to 74.5°C for WMS, when the 3 bar/225 s process was applied. For milder treatment (2.7 bar/300 s) the change was less dramatic: 70.1 against 67.1°C (untreated) for SMS, and 72.5 against 69.7°C (untreated) for WMS.

Meanwhile the gelatinization enthalpy decreased progressively for the two types of starches; this decrease was more pronounced beyond 2 bars, particularly for SMS. The gelatinization enthalpy decreased from 11.6 J/g (untreated) down to 6.4 J/g (2.7 bar/300 s) and 2.8 J/g (3 bar/ 225 s) for SMS, and from 16.8 J/g (untreated) down to 12.7 J/g (2.7 bar/300 s) and 8.2 J/g (3 bar/ 225 s) for WMS.

The decrease of gelatinization enthalpy has been related to loss of crystalline order within the starch granule [START_REF] Cooke | Loss of crystalline and molecular order during starch gelatinization and origin of the enthalpic transition[END_REF]. This loss of cristallinity, as estimated from DG, was up to 76% for SMS against 51% for WMS. This shows that SMS is more sensitive to the DIC treatment than WMS. A progressive loss of cristallinity was experienced as the conditions of the treatment were more intensive.

In spite of lower moisture content of DIC treated starches (~12%) compared to moisture content of HMT treated starches (20 ~30%), quite similar results have been reported in the case of HMT treatment by [START_REF] Takaya | Thermal studies on the gelatinization and retrogradation of heat-moisture treated starch[END_REF] for standard maize starch (at 120 and 130°C under saturated humidity for 20 min) and by [START_REF] Lim | Thermal transition characteristics of heat treated corn and potato starches[END_REF] (120°C, 1 hour, 25% moisture content). These authors observed an increase of the gelatinization peak temperature as well as a decrease of the gelatinization enthalpy. For instance T p increased from 66.9 (untreated) to 74.2°C (120°C, 25% moisture content) in the work by [START_REF] Lim | Thermal transition characteristics of heat treated corn and potato starches[END_REF], whereas the gelatinization enthalpy decreased from 18.7 to 14.4 J/g. [START_REF] Hoover | The effect of heat-moisture treatment on the structure and physicochemical properties of normal maize, waxy maize, dull waxy maize and amylomaize V starches[END_REF] also found a shift towards higher temperatures for maize starches: 66 °C (untreated) against 71°C, and 73 °C (untreated) against 74°C for SMS hal-00413504, version 1 -4 Sep 2009

and WMS, respectively. However, no change was observed in the endothermic value; this difference could be ascribed to the milder process used by these authors: 100°C during 16h for 30% hydrated starch.

It has been postulated that the gelatinization enthalpy is related to the overall cristallinity of amylopectin [START_REF] Tester | Swelling and gelatinization of cereal starches. I. Effect of amylopectin, amylose and lipids[END_REF]. For [START_REF] Cooke | Loss of crystalline and molecular order during starch gelatinization and origin of the enthalpic transition[END_REF] the ∆H value represents the number of double helices that unravel and melt during gelatinization. The difference of T p and ∆H between WMS and SMS may be attributed to starch composition, especially the amylose to amylopectin ratio. It is well known that amylopectin is primarily involved in starch granule cristallinity [START_REF] French | Organization of starch granules[END_REF]. This may explain why SMS, which contains amylose, is more sensitive to the DIC treatment.

Table 3 gives For the two types of results, before and after pasting with the Viscograph, the changes are again dramatic for the most severe treatments. As regards to the calorimetric results, the effect of the treatment differs between SMS and WMS: the later being less prone to swelling.

Figure 1 shows microscopic views of SMS starch granules under polarized light before pasting.

The untreated starch (a) exhibited the usual Maltese cross related to its crystalline order. The same observation could be made for the 2 bar/225s treated starch (b). For the view (c) (2.7 bar/300 s), we observed intact starch granules but also starch granules without their Maltese cross. The presence of the later shows partial gelatinization of starch as already mentioned from the calorimetric results. The coexistence of the two types of starch granules may also be related to the heterogeneity in DIC treatment. A recent study pointed out the existence of a gradient of temperature and moisture content between the surface and the bottom of the powder layer [START_REF] Zarguili | Study of the effect of hydrothermal process conditions on the properties of maize starch[END_REF]). Thus the sampling may have an influence on the microscopic observations. The disappearance of the Maltese cross is more pronounced and the proportion of intact granules is smaller for the view (d) (3 bar/225 s). This confirms the loss of cristallinity for the strongest treatments. It is worth noting that even for the most intensive treatment integrity of the starch granule was not altered whereas the size of the starch granules could vary to a large extent (Table 3). For WMS (pictures not shown) there was no dramatic change: the Maltese cross was still observed, whatever the intensity of the treatment. These microscopic observations confirm the higher resistance of WMS to the DIC treatment.

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Figure 2 shows the Brabender viscograms for untreated (a), 2 bar/331 s (b) and 2.7 bar/300s (c) treated SMS. For the two treatments, no peak could be seen and the plateau values clearly were lower.

The peak and plateau values from the viscograms are presented in Table 3. For SMS, the viscosity peak as well as the plateau value dramatically decreased for pressures higher than 2 bars. SMS treated at 2.7 bar during 300 seconds reached the Brabender peak viscosity of 140 BU while for the untreated SMS, the viscosity peak was of 844 BU. For 3 bar/225 s, no viscosity could be measured by this way. This may be explained by disruption of starch granules, as already mentioned from the results of Table 3, the dispersion being extremely fluid. On the opposite way, WMS was much less affected by the hydrothermal treatment.

Typical flow curves of SMS dispersions at 60°C are shown in Figure 3 for untreated and DIC treated samples at 6% concentration (w/w). These flow curves are typical of a non-newtonian shear-thinning fluid with a yield stress as described by [START_REF] Evans | Rheology of gelatinized starch suspensions[END_REF][START_REF] Evans | Rheology of gelatinized starch suspensions[END_REF][START_REF] Doublier | Rheological studies on starch -Flow behaviour of wheat starch pastes[END_REF]. The Herschel-Bulkley equation fitted the data satisfactorily (R 2 = 0.987), according to the equation (3). As seen in Table 4 the apparent viscosity at a shear rate of 1s -1 and the yield stress values for SMS first decreased for the mildest DIC treatment; this was followed by a slight increase and then a decrease beyond 2 bar/119 s. For the 3 bar/225 s treated SMS, the behavior was also shearthinning, but no yield stress was detected, owing to the fluidity of the paste (0.09 Pas at 1 s -1 ). For WMS, the Herschel-Buckley equation was obeyed (R 2 = 0.983) but no strong effect of the treatment was noticed.

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Figure 4 illustrates the viscoelastic behavior at 60°C of 6% native and 2.7 bar/300 s treated SMS dispersion through the variations of the storage modulus (G') and the loss modulus (G'') as a function of frequency. The native starch exhibited the behavior of a weak gel with G'> G" and G' almost independent of frequency. The SMS treated sample displayed lower and more frequencydependent moduli, but the behavior remained solid-like. As can be seen from Table 4, the storage modulus dramatically dropped for the 2.7 bar/300 s treated starch and completely disappeared for the 3 bar/225 s experiment. In this case the starch paste was extremely fluid. With respect to the waxy maize starch, no change of the moduli was observed after the DIC treatment.

The rheological behavior of starch suspensions is known to be the result of a combination of two major factors: the continuous phase and the volume fraction of the dispersed phase. In the range of concentrations used in this work, the volume fraction appears to be close to unity: the suspension can then be described as a packing of swollen starch granules, the overall behavior being governed by the dispersed phase [START_REF] Doublier | A rheological investigation of cereal starch pastes and gels. Effect of pasting procedures[END_REF]. This explains why the elastic response in the viscoelastic behavior of the suspension is predominant [START_REF] Tecante | Steady flow and viscoelastic behavior of crosslinked waxy corn starch-κ-carrageenen pastes and gels[END_REF]. In such a case, the deformability of the swollen starch granules also plays a role in the rheological properties: the decrease of apparent viscosity and storage modulus with the intensity of the treatment may be related to a higher deformability. the continuous phase. This latter phenomenon would be related to lower ability to form a gel of the amylose in the continuous phase.

To conclude, the Viscograph procedure which is a mean to characterize the starch paste under repeatable conditions produces a dramatic decrease of apparent viscosity and storage modulus of SMS starch previously treated under the most severe DIC conditions. This can be related to the partial gelatinization of SMS obtained through the DIC treatment, which makes it more sensitive to the thermal treatment in the Viscograph and leads to partial disruption of starch granules.

Statistical analysis

Among the response parameters cited in Tables 2, 3 and 4 for SMS, we selected three representative ones namely D v , ∆H and η a for statistical considerations.

The relationship between the processing parameters and the responses can be explained by examining the response surfaces generated by the second order polynomial equation obtained from the experimental analysis (Fig. 6 for SMS). A second degree polynomial equation was assumed to approximate the considered response:

R p =β 0 + β 1 x 1 + β 2 x 2 + β 11 x 1 2 + β 22 x 2 2 + β 12 x 1 x 2 (3)
Where R p is the considered response, β 0 , β 1 , β 2 ,β 11 ,β 11 and β 12 the regression coefficients and x 1 ,

x 2 , the variables related respectively to pressure level and processing time. The regression coefficients are showed in Table 5. Adequacy tests of the models, tested by the coefficients of determination R 2 , revealed that they were quite well adequate with a probability of rejecting the lack of fit greater than 0.05.

The results of statistical analysis are given in Table 6 indicating the p-value and the F-ratio for each response parameter. P-value less than 0.05 indicates that the source (linear, quadratic or cross product term) is significantly different from zero at the 95% confidence level, based on Fisher test.

From this Table, it can be clearly seen that the steam pressure level is the most significant parameter.

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From Figure 6a, it can be seen that the linear and quadratic effects of the steam pressure level are very pronounced for D v which increases from 13 to 43 µm when the pressure level increased from 1.3 to 2.7 bar; in the meantime, the variation of D v for increasing processing time from 150 sec 300 sec is only from 13 to 23 µm. The increase of the pressure level induces an increase of the median volume diameter especially for high values of processing time. This is confirmed by a significant effect of the two-factor interaction illustrated by a distortion of the response surface.

A similar trend is observed for ∆H for which the steam pressure level has a strong effect. Figure 6b shows that the gelatinization enthalpy is directly proportional to steam pressure level, this being the only significant parameter. The higher the steam pressure, the lower the ∆H value. ∆H decreases from 11.2 to 6.2 J/g when the steam pressure level varies from 1.3 to 2.7 bar (Figure 6b). The processing time is not a critical parameter, since an increase from 150 to 300 seconds yields a decrease of enthalpy of only 10 to 9.2 J/g.

The apparent viscosity (Figure 6c) depends not only on the steam pressure level (linear and quadratic significant effects) but also on the processing time. This response decreases with both steam pressure level (beyond 1.6 bar) and processing time.

The analysis of other parameters (not shown) displays a predominance of steam pressure level effect compared to that of processing time. hal-00413504, version 1 -4 Sep 2009

Conclusion

Starch can be modified by Instantaneous Controlled Pressure Drop (Détente Instantanée Contrôlée: DIC) treatment, starch granules being exposed to high temperature obtained by steam under pressure during short time period (<10 min). DIC treatment of starch can be defined as a physical modification that involves treatment at very restricted moisture content (~12%). The DIC process revealed itself to be an interesting technique to obtain partially pre-gelatinized starches without subsequent drying of the product. The methods employed to estimate the physical changes i.e. microcalorimetry, particle size and microscopic analysis, attested the progressive loss of cristallinity while starch granules were not ruptured. These changes became dramatic for the strongest processing conditions: 2.7 to 3 bars for 200 to 300 seconds. The DIC treatment yielded an increased fluidity and a loss of the elastic response of the pastes, as a result of partial pregelatinization of starch granules brought about by the process. We also noticed a marked difference in the sensitivity of the two types of starches to the process, SMS being more affected than WMS. For WMS, prolonged processing times or higher steam pressure are probably required to obtain a same gelatinization degree as SMS. hal-00413504, version 1 -4 Sep 2009

Figures captions

  the median volume diameter before and after pasting in the Viscograph and the Brabender values for the two types of starches. D(v,0.5) obtained before pasting increased with the severity of the treatment. The increase was progressive at first, and then more pronounced for the highest pressures: from 14.5 µm to about 67 µm for SMS and from 15.7 µm to 36 µm in the case of WMS. The value for untreated starch corresponds to the usual starch granule size: since starch granules are known not to swell in cold water, the increase of volume diameter may be ascribed to the loss of cristallinity already discussed: the less organized the starch granule, the more hydrated it is. It is worth noting that the largest median diameters of SMS, 52.0 µm (2.7 bar/300 s) and 66.8 µm (3 bar/225 s) are associated to higher span values: 2.2 instead of usual lower values, 1.2 to 1.5. This heterogeneity of size is due to the presence of larger particles for that samples, probably composed of aggregates of starch granules. D(v,0.5) of pasted samples exhibit the reverse tendency: the largest size of the granules is obtained for untreated samples, 60.9 µm and 39.0 µm for SMS and WMS respectively. Then the median diameter decreases from 60.9µm to about 40 µm (2 bar/225 s) for SMS and from 39.0 µm to 32.8 µm (2.7 bar/150 s) for WMS. The median diameter of 64.2µm (2 bar/225 s) for SMS being related hal-00413504, version 1 -4 Sep 2009 to higher heterogeneity as already mentioned. The decrease of the median diameter of pasted starch granules with increasing DIC conditions, may result from partial disruption of starch granules: the starches which are partially gelatinized by the DIC treatment are more sensitive to the thermal treatment in the Viscograph.

  τ o is the yield stress (Pa), K the consistency index (Pa.s n ) and n the flow behavior index (dimensionless).

Figure 5

 5 Figure5shows changes with time of the storage modulus measured at 25 °C for untreated SMS

Figure 1 .

 1 Figure 1. Microscopic observations of SMS: untreated (a); DIC treated at 2bar/225s (b), at 2.7bar/300s (c) and at 3 bar/225 (d). Scale bar: 50 µm.

Figure 2 .

 2 Figure 2. Pasting profiles obtained with the Brabender Viscograph of SMS: untreated (a); DIC treated at 2bar/331s (b) and at 2.7 bar/300s (c).

Figure 3 .

 3 Figure 3. Flow curves of 6 % SMS dispersion (measurement at 60 °C). ( ): untreated starch; (•): DIC treated at 1 bar/ 225 s; (■): DIC treated at 2.7 bar/300 s. The continuous line represents the regression curve according to Herschel-Buckley equation.

Figure 4 .

 4 Figure 4. Mechanical spectra of 6 % SMS dispersion (measurement at 60 °C; shear strain 4 %). G' (solid), G" (hollow); ( ): untreated starch; (■): DIC treated at 2.7 bar/300 s.

Figure 5 .

 5 Figure 5. G' variations at 25 °C as a function of time of 6 % SMS dispersions. (shear strain 4 %; angular frequency: 6.3 rad/s). ( ): untreated starch; (▲): DIC treated at 2.7 bar/ 150 s; (■): DIC treated at 2.7 bar/300 s.

Figure 6 .

 6 Figure 6. Response surfaces plots for SMS as a function of the hydrothermal processing parameters. hal-00413504, version 1 -4 Sep 2009

  Figure 1

  Figure 3

  Figure 5

Table 1 :

 1 Coded levels for independent variables used in developing experimental data , N is the number of experiments of orthogonal design, i.e. of the factorial design. In this case α = 1.414.

					Coded level
			-α	-1	0	1	 +α
		Pressure level (bar)	1.0	1.3	2	2.7	3.0
		Processing time (s)	119	150	225	300	331
	hal-00413504, version 1 -4 Sep 2009	α(Axial distance) N

4

Table 2 :

 2 DSC characteristics of SMS and WMS after DIC process DG, starch gelatinization of DIC treated starch; (2) Mean absolute error (five repetitions); -too low to be measured. hal-00413504, version 1 -4 Sep 2009

	Processing	Standard Maize Starch	Waxy Maize Starch
	Parameters
	Steam pressure level	T p	∆H	DG (1) (%)	T p	∆H	DG (1) (%)
	(bar)/time (s)	(°C)	(J/g)		(°C)	(J/g)
	Untreated	67.1	11.6	-	69.7	16.8	0
	1/225	67.1	11.5	1.2	69.8	15.9	5.3
	1.3/150	67.3	11.2	3.5	70.0	16.0	4.8
	1.3/300	67.5	11.0	5.1	70.5	15.4	8.3
	2/119	68.5	9.5	18.1	70.4	15.2	9.2
	2/225 (2)	69.1 (0.2)	9.6 (0.3)	17.2 (2.4)	71.2 (0.47) 14.8 (0.4)	11.9 (2.4)
	2/331	69.7	7.8	32.8	72.1	14.9	11.1
	2.7/150	68.1	8.6	25.5	70.9	13.8	18.0
	2.7/300	70.1	6.4	45.0	72.5	12.7	24.1
	3/225	76.6	2.8	76.3	74.5	8.2	51.3
	(1)

Table 5 :

 5 Regression coefficients of the polynomial function (the values of the variables are specified in their original units) and the coefficients of determination

		Coefficients	Apparent	Enthalpy	D (v,0,5)
	hal-00413504, version 1 -4 Sep 2009	β 0 β 1 β 2 β 11 β 12 β 22 R 2	viscosity -7.36 26.03 0.00 -6.07 -0.03 0.00 91.74 %	5.52 6.03 0.02 -1.82 0.01 0.00 88.15 %	113.58 -98.69 -0.21 21.44 0.14 0.00 91.45 %

Table 6 :

 6 Analysis of variance showing the effect of treatment variables as a linear term, quadratic term and interactions (cross product) on the response parameters for SMS

		Apparent viscosity	Enthalpy	D (v,0,5)
	Source	F-ratio	P-value	F-ratio	P-value	F-ratio	P-value
	P	53.04	0.0002*	43.36	0.0003*	42.25	0.0003*
	t	27.00	0.0013*	2.68	0.1456	4.86	0.0632
	PP	32.94	0.0007*	5.07	0.0591	20.94	0.0026*
	Pt	5.34	0.0542	0.99	0.3533	6.39	0.0393*
	tt	0.20	0.6670	0.10	0.7660	0.01	0.9192

* indicates significant at p < 0.05. P : steam pressure level; t : processing time


I Zarguili

Z Maache-Rezzoug

C Loisel

J.-L Doublier

INFLUENCE OF D.I.C HYDROTHERMAL PROCESS CONDITIONS ON THE GELATINIZATION PROPERTIES OF STANDARD MAIZE STARCH

Keywords: DIC hydrothermal treatment, maize starch, pregelatinized starch, DSC

Standard maize starch was hydrothermally treated at residual moisture content (~12 %) by Instantaneous Controlled Pressure Drop for various pressure levels and processing times. In order to examine the homogeneity of the starch treatment, three thicknesses (1.5, 1 and 0.5 cm) were tested at 3 bar of steam pressure. For the three thicknesses, the temperature and the moisture content of starch were measured during the treatment at the bottom and the surface.

The results showed an absence of gradient of temperature and moisture content only for a thickness of 0.5 cm. Gelatinization transition temperatures (T o , T p ) increased after hydrothermal modifications. The change in the gelatinization temperature range was observed after DIC treatment when pressure level or processing time increased. For a high pressure (≥ 4 bar) associated with low processing time (2 minutes) a total gelatinization of starch was observed. It also occurs when the processing time is important (~15 min) and pressure lower than 4 bar.

INTRODUCTION

Starches are used in food industries for their various properties and have many industrial applications as a thickener, colloidal stabilizer, gelling agent and bulking agent. In a native state, they do not have all the required properties for food industries in terms of resistance to thermomechanical treatment and storage stability. Many methods are used to modify starch such as, drum dryer [START_REF] Vallous | Performance of a double drum dryer for producing pregelatinized maize starches[END_REF], extrusion cooking [START_REF] Mercier | Comparative modifications starch and starchy products by extrusion cooking and drum-drying[END_REF][START_REF] Barron | Structural modifications of low hydrated pea starch subjected to high thermomechanical processing[END_REF][START_REF] Brümmer | Effect of extrusion cooking on molecular parameters of corn starch[END_REF], annealing [START_REF] Jacobs | Evaluation of the impact of annealing on gelatinisation at intermediate water content of wheat and potato starches: A differential scanning calorimetry and small angle X-ray scattering study[END_REF][START_REF] Tester | Annealing of maize starch[END_REF], and heat moisture treatment (HMT) [START_REF] Kulp | Heat-moisture treatment of starches. I-Physicochemical properties[END_REF][START_REF] Hoover | Effect of heat-moisture treatment on the structure and physicochemical properties of normal maize, waxy maize, dull waxy maize and amylomaize V starches[END_REF][START_REF] Collado | Heat-moisture treatment effects on sweet potato starches differing in amylose content[END_REF][START_REF] Lim | Thermal transition characteristics of heatmoisture treated corn and potato starches[END_REF][START_REF] Gunarante | Effect of heat-moisture treatment on the structure and physical properties of tuber and root starches[END_REF]. HMT, discovered by [START_REF] Sair | Water sorption by starches, water sorption corn starch and commercial modifications of starches[END_REF] considered as a method for physical modification of starch. It refers to starch exposure to higher temperatures (≈120°C), commonly above gelatinization temperature, at very restricted moisture content (18-30%) during 16 h and in some cases shorter than 16 h [START_REF] Collado | Heat-moisture treatment effects on sweet potato starches differing in amylose content[END_REF][START_REF] Lim | Thermal transition characteristics of heatmoisture treated corn and potato starches[END_REF]. For HMT, pressure is often required to assure sufficient heating, but uniform heat distribution and penetration into the starch is not easy to accomplish [START_REF] Maruta | Reduced-pressurized heat-moisture treatment. A new method for heat-moisture treatment of starch[END_REF].

The general effects of HMT are loss of birefringence, increased gelatinization temperatures, changes in gelatinization range (narrowing or broadening of the DSC endotherm), X-ray diffraction patterns, swelling volume and solubility, with consequent changes in functionality [START_REF] Sair | Water sorption by starches, water sorption corn starch and commercial modifications of starches[END_REF][START_REF] Kulp | Heat-moisture treatment of starches. I-Physicochemical properties[END_REF][START_REF] Donovan | Differential scanning calorimetry of heatmoisture treated wheat and potato starches[END_REF][START_REF] Hoover | Effect of heat-moisture treatment on the structure and physicochemical properties of normal maize, waxy maize, dull waxy maize and amylomaize V starches[END_REF][START_REF] Collado | Heat-moisture treatment effects on sweet potato starches differing in amylose content[END_REF][START_REF] Takaya | Thermal studies on the gelatinisation and retrogradation of heat-moisture treated starch[END_REF][START_REF] Gunarante | Effect of heat-moisture treatment on the structure and physical properties of tuber and root starches[END_REF][START_REF] Adebowale | Microstructure, physicochemical properties and retrogradation behaviour of Mucuna bean (Mucuna pruriens) starch on heat-moisture Treatments[END_REF][START_REF] Adebowale | Microstructure, physicochemical properties and retrogradation behaviour of Mucuna bean (Mucuna pruriens) starch on heat-moisture Treatments[END_REF]. Hoover & Vasanthan (1994) and [START_REF] Hoover | Effect of heat-moisture treatment on the structure and physicochemical properties of normal maize, waxy maize, dull waxy maize and amylomaize V starches[END_REF] reported that the extent of starch chain associations within amorphous regions and the degree of crystallinity are altered during heat-moisture treatment of wheat, oat, normal maize, waxy maize, high amylose maize, potato and lentil starches. The magnitude of these changes was found to be dependent upon the moisture content during heat treatment and upon the starch source.

The purpose of the present work is to characterize the hydrothermally-treated starch by a recent hydrothermal process: the Instantaneous Controlled Pressure Drop (DIC) developed for some years in our laboratory [START_REF] Rezzoug | Effect of instantaneous controlled decompression process on hydration capacity of scleroglucan. Optimisation of operating conditions by response surface methodology[END_REF]. This technique is based on a hydro-thermo-mechanical effect induced by the abrupt transition from a high steam pressure level to a vacuum. We applied this process for starch heat treatment in order to obtain its gelatinization and to modify its functional properties. In this paper, a preliminary study was performed to optimise the thick layer of starch which leads to a homogeneous treatment. After this optimisation, the effect of the processing parameters (pressure and processing time) on the thermal properties of starch was evaluated.

2: MATERIALS AND METHODS

: RAW MATERIALS

Standard maize starch (SMS) was supplied by Roquette Frères (Lestrem, France). The moisture content was 12.5% wet basis.

: DIC APPARATUS

The general DIC apparatus presented in figure 1 is constituted of four major parts:

1. A processing vessel (2), in which products are treated at high temperature/high steam pressure (up to respectively 160°C and 6 bar).

2. A pneumatic valve (3) (complete motion in less than 1/10 th of a second), in order to realise an abrupt pressure drop. This valve is opened rapidly connecting the processing vessel to the vacuum tank.

3. A vacuum tank (4), its volume is 130 fold greater than the processing vessel.

4. Manual and automatic control panel and acquisition system.

: EXPERIMENTAL PROTOCOL

The Standard maize starch (12.5% wb) disposed in circular containers (diameter: 20 cm; height: 5 cm) was treated in a processing vessel in which a vacuum of 50 mbar was established through a short communication with the vacuum tank (figure 2b). The initial vacuum is carried out to facilitate the diffusion of steam into product. Consequently, the time necessary for the temperature of the starch to reach that of the steam is reduced. Saturated steam was then introduced into the vessel at fixed pressure level (figure 2c) and maintained for a predetermined time (figure 2d). This step is followed by a sudden decompression towards vacuum (figure 2e). After the vacuum phase, an atmospheric air is injected to return to atmospheric pressure (figure 2f) and to recover the product for various measurements.

: MOISTURE CONTENT

The starch moisture content was determined by air oven at 105°C during 24 h, according to the A.F.N.OR standard method.

2.5: DIFFERENTIAL SCANNING CALORIMETRY

Thermal characteristics of treated starch were studied by using a DSC Q10 (TA-instrument, France). Starch samples (10 mg) were placed in a stainless pan; distilled water was added to get a ratio of 1:9 (w/w) starch:water, mix, and then the sample pan was sealed. The DSC analyser was calibrated using indium and the thermogram was recorded with water as reference. Sample pan was heated at a rate of 10°C/min from 25 to 120°C. Thermal transition of starch samples were defined as T o (onset temperature), T P (peak temperature) and ∆H was referred to as the gelatinization enthalpy. The gelatinization temperature range (R) was calculated as 2 (T p -T o ) as described by [START_REF] Krueger | A differential scanning calorimetry study on the effect of annealing on gelatinization behavior of corn starch[END_REF]. The degree of gelatinization of treated starch was also calculated by the following equation [START_REF] Marshall | Determining the degree of gelatinization in parboiled rice: comparison of subjective and an objective method[END_REF]

. ) 1 ( (%) × ∆ ∆ - = raw t H H SG (Equation 1)
Where SG is the gelatinization degree, ∆H t the transition enthalpy of DIC treated starch and ∆H raw the transition enthalpy of native starch.

: MICROSCOPIC OBSERVATIONS

The samples were examined visually using a phase contrast microscope equipped with a CCD camera. Small amounts of sample were suspended in distilled water to 1/20 and observed using a magnification of 400 under polarised light.

3: RESULTS AND DISCUSION

3.1: TEMPERATURE AND MOISTURE CONTENT PROFILES IN STARCH DURING THE DIC TREATMENT

When the steam is in contact with the layer of starch it is ultimately absorbed as liquid water.

Consequently, the latent heat of condensation is absorbed and the temperature of the product increases until reaching the surrounding steam temperature or equilibrium temperature. In this study, the temperature was measured on-line, during DIC treatment, by two thermocouples placed at the surface and the bottom of starch. Figure 3 (a)-(c) presents the evolution of temperature with processing time at pressure of 3 bar for three thicknesses of 0.5 (a), 1 (b) and 1.5 cm (c). The results show the existence of a temperature gradient between the surface and the bottom of the three thick layers, which increases with the thickness. For a thickness of 0.5 cm, the temperature gradient is eliminated in 40 seconds (figure 3-a), while it needs, under similar conditions, 170 and 300 seconds for the 1 cm and 1.5 cm thick layers (figures 3-b and 3-c), respectively. The equilibrium temperature at 3 bar of steam pressure is 136°C.

According to figure 3, the time to reach the equilibrium temperature at this pressure is shorter when the thickness of products layer is low. The equilibrium temperature is reached after 60, A slight decrease of the moisture content at the bottom is observed for 1.5 cm thickness. This reduction may be caused by water evaporation resulting from the heating of the cupel. This evaporation enables the moisture content to increase on the upper surface of starch. After 600 seconds, the moisture content recovers its initial value caused by water diffusion from the upper surface to the bottom. In order to avoid heterogeneity of treatment, we retained a thickness of 0.5 cm.

3.2: EFFECT OF PROCESSING TIME

3.2.1: Gelatinization properties

The results of thermal properties, measured by DSC, of treated starch at various processing times and for 3 bar steam pressure and 0.5 cm thickness are shown in table 1 where the raw starch properties are included for comparison. From this table it appears that the DIC process influences onset temperature (T o ), peak temperature (T p ) and the enthalpy of gelatinization (∆H t ). T o and T p increased with the processing time and shifted to higher values. For the sample treated during 15 minutes, these temperatures shifted respectively from 63.3°C

(native) to 80.0°C and from 69.9°C (native) to 84.2°C. Similar results were observed by Hoover & Manual (1996) for standard maize starch heat treated at 100°C for 16 h at moisture content of 30%. They also found a shift towards higher temperature for standard maize starches: 66 (untreated) against 71°C. The increase in T o and T p has been attributed by [START_REF] Hoover | Effect of heat-moisture treatment on the structure and physicochemical properties of cereal, tuber, and legume starches[END_REF] to structural changes within the starch granules, which involves mainly amylose-amylose and amylose-lipid interactions. The broadening of temperature range (R) reaches a maximum value (3.6 °C) for the sample treated at 2 minutes. R value shifted from 13.4 °C to 17.0°C for native and starch treated at 2 minutes, respectively. R decreased for higher processing times and beyond 5 minutes, the temperature range narrowed below the value of the native starch. Hoover & Manual (1996) observed an increase in gelatinization temperature range for standard maize starch (6°C). The authors suggested that amylose-lipid interactions during heat-moisture treatment may lead to the formation of crystallites of different stabilities. The gelatinization enthalpy of native maize starch was 11.6 J/g, whereas ∆H of treated DIC starch decreased strongly to 0.57 J/g for 15 minutes processing time. From table 1, it can be seen that SG depends strongly on the conditions of DIC heat treatment. A partial and rapid gelatinization was observed during the first minute of treatment, while it became increasingly slow with time.

: Microscopic observations

The observation of native starch granules in the optic microscope with polarized light, shows the existence of the cross polarization (figure 4-a) for sample treated during 30 seconds (figure 4-b). The polarization cross is still clear, although a gelatinization degree of 59 % (table 1). For samples treated during longer times, the polarization cross disappears and after 15 minutes (figure 4-d), it completely disappears, which indicate that the starch is almost completely gelatinized.

3.3: EFFECT OF DIC PRESSURE LEVEL

For the studied pressures, the temperatures profiles were on-line measured on the surface and the bottom of starch. The results presented in figure 5 show that the time to reach the equilibrium temperature is low when the pressure is high. It is respectively of 80, 60, 45 and time for studying the effect of pressure level on starch properties. This time allows reaching the equilibrium temperature for all samples.

3.3.1: Gelatinization properties

The influence of pressure level during DIC treatment on the gelatinization transition temperatures (T o , T p , R) and gelatinization enthalpy (∆H) are presented in table 2 andfigure 6. The DSC analysis showed that increasing steam pressure increased substantially T o and T p and decreased ∆H of all treated samples. The heat treatment caused a broadening of 3.6°C and 4.7°C of gelatinization temperature range for the starch treated at 3 and 4 bar, respectively, in comparison with native starch. However, no significant change in R for starch treated at 2 bar was found. Similar results were obtained by [START_REF] Lim | Thermal transition characteristics of heatmoisture treated corn and potato starches[END_REF] for heat moisture treated maize and potato starches (120°C/30 % and 130°C/25 %). The gelatinization range of treated maize starch increased by 3.9°C and 4°C for treatments at 130°C/25% and 120°C/30%, respectively.

From table 2 and figure 6, it can be seen that increasing steam pressure causes the reduction in ∆Η t until disappearance for sample treated at 5 bar. [START_REF] Kudo | Proceedings of Australia/Japan Symposium on Food Science and Technology[END_REF] observed that the value of endothermic enthalpy of the gelatinization decreases with the severity of the heat-moisture treatment. The reduction of enthalpy when the steam pressure increases is related to heating effect associated to this increasing. When the saturated steam pressure increases, the temperature inside the processing vessel increases. The equilibrium temperature of the saturated steam at 2 bar is 122°C, whereas it is of 153°C at 5 bar. [START_REF] Lim | Thermal transition characteristics of heatmoisture treated corn and potato starches[END_REF] observed a reduction in enthalpy during HMT. The authors argued that this reduction may be attributed to the transformation of the intercrystalline amorphous parts, and thus the crystalline regions could melt more easily, i.e. with lower energy. Concerning the gelatinization degree, a substantial decreasing was observed when processing pressure increases. SG(%) is of 23.4% for starch treated at 2 bar whereas it was estimated at 100% for starch treated at 5 bar, indicating a total loss of crystallinity.

3.3.2: Microscopic observations

Microscopic observations under polarized light of treated starch at different pressures (figure 7), showed obvious polarized cross for starch granules treated at 2 bar (figure 7

Figure 3

 3 Figure 3(d)-(f) presents the variation of the moisture content of starch during the

Figure 1 .

 1 Figures captions

Figure 2 .

 2 Figure 2. Typical pressure-time profile of a DIC processing cycle. (a) sample at atmospheric pressure; (b) establishment of a vacuum; (c) steam injection to reach the selected pressure; (d) treatment time; (e) pressure drop; (f) atmospheric pressure for the recovery of sample.

Figure 3 .

 3 Figure 3. Evolution of temperature and moisture content versus processing time for DIC treated starch at pressure of 3 bar for three thick layers (a and d = 0.5 cm ; b and e = 1 cm ; c and f = 1.5 cm). The symbols ■ and ▲, represent respectively the temperature and the moisture content on the surface and on the bottom of product.

Figure 4 .

 4 Figure 4. Microscopic observations of native starch (a) and DIC treated starch at 3 bar during 0.5 min (b), 10 min (c) and 15 min (d).

Figure 5 .

 5 Figure 5. Evolution of temperature on the surface and bottom (thickness = 0.5 cm), versus processing time for different pressures. (a): 2 bar ; (b): 3 bar; (c): 4 bar and (d):5 bar.

Figure 6 .

 6 Figure 6. DSC thermograms for native and DIC treated starch at different pressures.

Figure 7 .

 7 Figure 7. Microscopic observations of DIC treated starch during 2 minutes at different pressures: (a): 2 bar ; (b): 3 bar; (c): 4 bar and (d):5 bar.

  Figure 1.


Z Maache-Rezzoug

K Allaf

STUDY OF THE EFFECT OF HYDROTHERMAL PROCESS CONDITIONS ON PASTA QUALITY

Keywords: Hydrothermal treatment, Pasta, Hydration properties, RVA

Effect of hydrothermal treatment on the pasting, hydration properties and colour quality of commercial fresh pasta were studied. The Instantaneous Controlled Pressure Drop (DIC) treatment is defined as a hydrothermal treatment that involves a physical modifications at high temperature (<144°C) and restricted moisture contents (<30%) during a very short time (15 -40 sec) followed by a rapid pressure drop to a vacuum (50 mbar). Two process variables (steam pressure level and processing time) were investigated using response surface methodology. Steam pressure level had the greatest effect on hydration and pasting parameters. Increasing of pressure level resulted in elevation of mass ratio between cooked and uncooked pasta (W i /W 0 ) 100°C and reduction of optimum cooking time (OCT) for all treated pasta. The OCT of untreated pasta was 7 min (W/W o =2.4), while for the same ratio it was less than 1 minute for pasta treated at pressures ranging between 2.5 and 3.5 bar. The viscosity at 10 minutes (V 10 ) increased linearly when pressure level increased. V 10 was 146 cP for the untreated pasta whereas it reached 2659 cP for the pasta treated at 3.9 bar (144°C) during 25 second. The cold paste viscosity (CPV) and setback viscosity (SB) decreased when the pressure varied from 1 to 3.9 bar.

1: Introduction

Durum wheat semolina is considered to be the best material for making high quality pasta products. The intrinsic quality attributed to the pasta are influenced primarily by the properties of the protein and starch fraction and by many others factors such as the origin of the semolina [START_REF] Wood | A comparison of Australian and imported spaghetti[END_REF], and the pasta production process (mixing, extrusion and drying conditions) [START_REF] Kobrehel | Influence de la température de séchage des pâtes alimentaires sur l'activité et la composition des peroxydases en relation avec la couleur des produits[END_REF][START_REF] Debbouz | Effect of process variables on spaghetti quality[END_REF]. Starch and proteins are responsible for most desirable characteristics of cereals products. The many textures and culinary quality of pasta are related to the interactions of these two biopolymers in presence of water [START_REF] Resmini | Ultrastructure studies of pasta : a review[END_REF][START_REF] Güler | Effects of industrial pasta drying temperatures on starch properties and pasta quality[END_REF]. The influence of protein on the properties of pasta has received a large research attention [START_REF] Dexter | Spaghetti stickiness : some factors influenncing stickiness and relationship to other cooking quality characteristics[END_REF][START_REF] Fortini | Durum, Chemistry and Technology[END_REF][START_REF] Favier | Suivi de la cinétique thermique d'insolubilisation des protéines de la pâte alimentaire par chromatographie d'exclusion-diffusion haute performance[END_REF], however, less informations are available on the influence of starch on pasta quality although this macromolecule is the major component of durum wheat semolina (approx.

70%).

Few studies exist on the application of hydrothermal treatments to the pasta for the production of instant products, or for improvement of the cooking quality. [START_REF] Abecassis | Amélioration de la qualité des pâtes alimentaires par traitement thermique des produits secs[END_REF] noted, on pasta previously dried at 37 °C, an improvement of cooking quality and colour after hydrothermal treatment at 90 °C for short time.

The application of high temperature (HT) in the drying process [START_REF] Abecassis | Influence des températures de séchage sur l'aspect et la qualité culinaire des pâtes alimentaires[END_REF][START_REF] Anese | Effects of drying processing on the Maillard reaction in pasta[END_REF] is the most noticeable innovation in pasta industry in recent years.

The temperature usually reaches 70°C-80°C and in some cases more than 100°C [START_REF] Güler | Effects of industrial pasta drying temperatures on starch properties and pasta quality[END_REF]. Under these conditions the objective of drying is not only lowering the moisture content from 30% to 12.5% w.b., but also causing transformations of the physicochemical characteristics [START_REF] Zweifel | Thermal modifications of starch during high-temperature drying of pasta[END_REF]. The main advantages of HT drying are the control of micro-organisms, the shortening of drying time and the improvement of the cooking quality [START_REF] Dexter | High temperature drying : Effect on spaghetti properties[END_REF]De Stefanis and Sgruletta, 1990). The disadvantage is often a red colour development as a consequence of an increase in Maillard reactions [START_REF] Anese | Effects of drying processing on the Maillard reaction in pasta[END_REF][START_REF] Zardetto | Effect of different heat treatment on the furosine content in fresh filled pasta[END_REF].

The pasta quality has been shown to be highly influenced by starch gelatinisation and protein network formation [START_REF] Resmini | Ultrastructure studies of pasta : a review[END_REF][START_REF] Fardet | Textural images analysis of pasta protein networks to determine influence of technology processes[END_REF][START_REF] Riva | Starch retrogradation in cooked pasta and rice[END_REF]. During cooking, gelatinisation and formation of the protein network occur simultaneously which confers the viscoelastic characteristics to the cooked product [START_REF] Feillet | The biochemical basis of pasta cooking quality. Its consequences for durum wheat breeders[END_REF][START_REF] Riva | Starch gelatinization in pasta cooking: Differential flux calorimetry investigations[END_REF].

For rapid evaluation of cooking and processing properties of cereal products, the Rapid Visco-Analyser (RVA) is commonly used to measure the viscosity of starch-based materials over a heating-holding-cooling cycle to assess functional properties [START_REF] Lai | Effects of hydrothermal treatment on the physicochemical properties of pregelatized rice flour[END_REF][START_REF] Yunt | Physicochemical properties of australian wheat flours for white salted noodles[END_REF]. The viscographs of pasta, from the measurement of the changes in viscosity over time-temperature profile, can reflect the molecular changes of starch and functional properties of final product [START_REF] Güler | Effects of industrial pasta drying temperatures on starch properties and pasta quality[END_REF][START_REF] Whalen | Measurement of extrusion effects by viscosity profile using the Rapid ViscoAnalyser[END_REF][START_REF] Panozzo | The Rapid Viscoanalyser as a method of testing for noodle quality in a wheat breeding program[END_REF].

Pasta colour is the first important quality characteristic that greatly influences consumer acceptance. Some authors showed that the colour depends mainly on two colour indexes: yellow index (b) and browning index (100-L) [START_REF] Abecassis | Influence des températures de séchage sur l'aspect et la qualité culinaire des pâtes alimentaires[END_REF]. These indexes are highly related to the genetic and agronomic characters, to the milling and to the pastification conditions [START_REF] De Stefanis | Effects of high-temperature drying on technological properties of pasta[END_REF]. The yellow colour is related to the presence of carotenoïd pigments and affected by lipoxygenases activity. [START_REF] Kobrehel | Influence de la température de séchage des pâtes alimentaires sur l'activité et la composition des peroxydases en relation avec la couleur des produits[END_REF] showed that the browning of pasta was correlated to the peroxidasic activity of the semolina and that HT drying at 90 °C decreases this activity and induces an improvement of pasta colour.

The purpose of the present work is to study the effect of a recent hydrothermal process: the Instantaneous Controlled Pressure Drop (DIC) as a pre-treatment on freshly extruded pasta before drying. The DIC process is based on the thermomechanical effect induced by an abrupt transition from high steam pressure to a vacuum [START_REF] Rezzoug | Effect of instantaneous controlled pressure drop proccess on hydration capacity of scleroglucan. Optimisation of operating conditions by response surface methodology[END_REF].

The most important parameters of this process are the steam pressure which determines the treatment temperature, the processing time and the vacuum pressure. The vacuum level is limited by the power of the vacuum pump. For this reason, we have fixed the pressure at its lowest value (50 mbar). Moreover it's the difference between the high pressure level and the vacuum which determines the amount of steam generated by flash vaporization and thus the intensity of the mechanical constraints responsible for the micro-alveolation phenomenon.

The effect of the two selected process variables (steam pressure level and processing time) was studied according to the characteristics of pasta by using hydration test, Rapid Visco Analyser (RVA) measurements, colour and scanning electron microscopic observations.

2: Materials and methods

2.1: Raw material Commercial fresh pasta (1.5 mm thickness) from a single commercial source (Panzani) was used in all experiments. The delay between the pasta processing and experiments was about 4 days during which the samples were conditioned in sealed bags at 4 °C. The "tagliatelle" pasta shape, was composed from semolina of durum wheat, water and fresh eggs at a rate of 140g /kg of semolina.

Methods

2.2.1: DIC equipment

Figure 1 is a schematic representation of the used equipment. The DIC equipment is composed of three main elements:

-The processing vessel (2) where the samples were placed and treated.

-The vacuum system which consists mainly of a vacuum tank (4) with a volume 130 fold greater than the processing vessel and a vacuum pump (5).

-A pneumatic valve (3) that separates the processing vessel from the vacuum tank.

2.2.2: Procedure of DIC hydrothermal treatment

The fresh pasta was treated in the processing vessel in which a vacuum of 50 mbar was established through a short communication with the vacuum tank (Fig 2b). The initial vacuum is carried out to facilitate the diffusion of steam into pasta. Consequently, the time necessary for the temperature of the pasta to reach that of the steam is reduced. Saturated steam was then introduced into the vessel at fixed pressure level (Fig. 2c) and maintained for a predetermined time (Fig. 2d). This step is followed by a sudden decompression towards vacuum (Fig. 2e). The rapid transition of pressure inside the processing vessel induces a rapid cooling of pasta which passes in less than 1 second from 100-144 °C (according to the steam pressure condition) to about 30°C. The equilibrium after pressure dropping depends on operating conditions: the higher the steam pressure level, the higher the equilibrium pressure. The steam generated by flash vaporization after the decompression to a vacuum produces a micro-texturation which is closely linked to a complex process of micro-alveolation. This process depends on the difference in pressure (or temperature) between the two thermodynamic equilibrium states before and after decompression. After the vacuum phase, atmospheric air is injected to return to atmospheric pressure (Fig. 2f) and to recover the product.

The feed of the treatment vessel was typically 300 g. After treatment the pasta was airdried (50 °C) in oven (Memert 700) to 12 % w.b..

: Moisture content

Moisture content of milled pasta was determined from the loss of weight during drying in a forced-air oven at 105°C (AACC, 1995). The moisture content of the pasta before DIC treatment was 28.7 % w.b. and passed to about 29.8 % after the treatment for all samples.

: Hydration test

The test consists in hydrating 2.5 g of DIC treated pasta after drying, with 1 litre of distilled water at 100°C or 20 °C. These temperatures were maintained constant during the test. During 20 minutes, the pasta was weighed at regular intervals (Maache- [START_REF] Maache-Rezzoug | Influence of instantaneous controlled decompression treatment on texturation & cooking time of the pasta[END_REF]. The mass ratio between hydrated and unhydrated pasta (W i /W 0 ) was calculated. W i and W 0 are the mass of hydrated (T= 20 °C) or cooked (T= 100 °C) pasta at various times and unhydrated pasta respectively.

The optimal cooking time (OCT) corresponds to the occurrence of complete gelatinisation of the starch and the disappearance of the white central core of the pasta.

After observation and tasting of pasta, OCT for untreated sample was estimated at 7 minutes. From figure 3-a, it can be seen that 7 minutes of cooking time corresponds to a mass ratio W/W o =2.4. This value was taken as a reference to estimate OCT for treated pasta, and was thus defined as the time taken by the treated pasta to reach the optimal absorption (ratio=2).

: Pasting properties

Pasting properties of ground pasta sample were determined using a Rapid Visco Analyser (RVA 3C, Newport Scientific, Sydney, Australia). Four grams of ground pasta [250-500 µm] at 14 % moisture basis was mixed with distilled water (25 ml) in disposable aluminium sample cup. The mix was then homogenized and introduced in the instrument. A programmed heating and cooling cycle was used, the suspension was held at 25 °C for 6 min, heated to 95 °C in 7 min at the rate of 10 °C/min. The temperature was held at 95 °C for 6 min, cooled to 25 °C in 7 min and then held at 25 °C for 6 min. The number of replication tests used in each case was two and the viscosity was expressed in cP.

From the RVA profile, the viscosity at 10 minutes (V 10 ) measured at 66 °C was measured. The final or cold paste viscosity (CPV) i.e., viscosity at the end of the cycle at 25 °C and setback viscosity (SB) i.e., CPV-HPV (hot paste viscosity i.e., minimum viscosity during stirring at 95°C) were calculated. 1 lists the independent variables and the coded factor levels.

2 1 12 2 2 22 2 1 11 2 2 1 1 0 X X β X β X β X β X β β η + + + + + =
[1] According to the statistical method, a second order polynomial function (Eq.1) was assumed to approximate the considered response. The coefficients of the polynomial equation are represented by β 0 (constant term), β 1 and β 2 (first order terms), β 11 and β 22 (second order terms) and β 12 (interaction involving the variables X 1 and X 2 ). The steam pressure level is denoted by X 1 and the processing time by X 2 .

The contour plots were obtained by using the design analysis procedure of Statgraphics Plus for Windows, (4.1 version). They were generated for each pasta quality by using the predicted values obtained by Eq. ( 1).

The relationships between the responses (absorption characteristics, RVA and colour values) and the processing parameters were judged by finding the correlation coefficients (R 2 ); the significance of the p-values was decided at a probability level of 0.01, 0.05 and 0.1 3: RESULTS AND DISCUSSION 3.1. Water absorption Figure 3 shows the water absorption kinetic curves at 100 and 20°C for the raw and DIC treated pasta. It appears that the DIC treatment modified the behavior of pasta during the absorption process.

Absorption kinetics at 100°C

The absorption kinetics (Fig. 3-a) showed that pasta hydrates rapidly up to 2 min of hydration time. The mass ratio W/W o =2.4 increased and reached the saturation from 12 min and the maximum absorption is obtained for the test carried out at 3.5 bar/15 sec followed by the samples treated at 2.5 bar/25 sec. The statistical analysis indicates that the processing pressure had a significant effect on the absorption parameters (Table 3).

The higher the processing pressure the higher the hydration rate of pasta. The low pressures (≤1.5 bar) do not exert any influence on the water absorption; the values of W/W o are almost identical to that of raw pasta. In contrast, the treatment at p>3.5 bar and t> 15 sec seems to be excessive processing conditions and provided a very sensitive sample to overcooking. For these conditions, a marked deliquescence of pasta and a large amount of solid loss in water for important cooking times were observed. The reduction in W/W o is probably due to this solid loss and suggested that for high conditions of DIC treatment (p>3.5/T>138 °C), the protein network which retained its integrity during cooking is disrupted.

The optimal cooking time (OCT) for treated pasta, evaluated by sensorial analysis, was reduced as a consequence of the improvement of water absorption. The presence of mico-alveolation in the structure favors the internal diffusion of water. The cooking time of untreated pasta was 7 min corresponding to a ratio of 2.4, while for the same ratio OCT was less than 1 min for pasta treated at pressure between 2.5/15 sec and 3.5 bar/15 sec (Table 2). Compared to 7 min, the cooking time of untreated sample, the pasta treated at 3.5 bar/15 sec has a ratio of 3.5 that corresponds to overcooking stage. In these conditions, the important water absorption induced an increase of the cooked volume and makes the pasta softer, thus reducing firmness. According to Del Nobile and [START_REF] Del Nobile | Modeling of water sorption kinetics in spaghetti during overcooking[END_REF], the organoleptic changes of pasta during overcooking are strongly related to the rate at which water molecules penetrate the pasta matrix once the optimal cooking conditions are reached.

Kinetics absorption water of 20°C

To understand the effect of DIC treatment on starch modifications it is convenient to analyze the water absorption at ambient temperature (20°C). In fact, the cooking at 100°C can mask the effect of DIC treatment whereas absorption at 20°C allows separation of the two phenomena knowing that the native starch granules have the characteristic to be insoluble in cold water. From kinetics obtained at 20 °C (Fig. 3-b), it can be noted that the mass ratio (W/W o ) 20°C depends not only on the steam pressure level but also on the processing time, contrary to the absorption kinetics at 100°C. The heating which accompanies the cooking at 100 °C should certainly mask the effect of the processing time. At fixed processing time, the higher the pressure level, the higher the absorption capacity and at fixed pressure level, the higher the processing time, the higher the adsorption capacity.

From figure 3-b, it is clear than at 20 °C the absorption spectrum is wider that the one obtained at 100 °C (Fig. 3-a). The raw and the pasta treated at low pressures (≤1.5 bar) exhibit a low absorption capacity. This is due to a limited moisture content of fresh pasta (28.7 %) for causing a complete gelatinisation of starch at low processing conditions (p≤1.5 bar / T<120°C). Absorption, even moderate, of the untreated pasta proves the existence of some damage of the starch granules caused during the extrusion phase of pasta processing.

For higher pressures (≥2.5 bar) W/W o measured at 5 minute of hydration time is almost identical to the ratio obtained at 100°C (Table 2). The absorption at 20°C may confirm the complete gelatinisation of starch granules.

RVA parameters

The statistical analysis (Table 3) indicates that the pressure level and processing time had a significant linear effect on V 10 while for the two other RVA parameters; only the steam pressure level was significant. The viscosity measured at 10 min (V 10 ) increases strongly with steam pressure and to a lesser extent with processing time (Table 2). The maximal value of V 10 (2659 cP) was obtained for the pasta treated at 3.9 bar and 25 sec while it was only 146 cP for the raw pasta. The high viscosity measured at 10 minute (at 66°C) showed a typical hydration property of pregelatinized starch products. [START_REF] Lai | Effects of hydrothermal treatment on the physicochemical properties of pregelatized rice flour[END_REF] reported a similar observation for hydrotreated rice flour. The gelatinisation is accompanied by a disruption of the molecular order as a result of loss of the starch granule structure and disruption of crystallinity (Cooke and Gidley, 1992). These molecular changes imply cold soluble properties by creating high viscosity at low temperature.

A recent study [START_REF] Loisel | Effect of instantaneous controlled pressure drop on the physical properties of maize starch[END_REF] on native maize starch treated at residual moisture content (~12 % w.b.) by DIC showed that the two process parameters (steam pressure and processing time) have influence on the degree of gelatinisation of starch. The occurrence of partial gelatinisation was clearly attested by the decrease of enthalpy measured by differential scanning calorimetry (DSC) and a loss of birefringence under polarized light, for 2.7 bar and 5 minutes of processing time. For the same native maize starch a total gelatinisation was observed for 3 bar and processing time higher than 15 minutes, [START_REF] Zarguili | Study of the effect of hydrothermal process conditions on the properties of maize starch[END_REF].

DIC treated pasta displayed a lower final viscosity (CPV) than the one of untreated sample. The CPV value for treated pasta varied between 11459 cP (1 bar/25 sec) and 10560 cP (3.9 bar/25 sec) while it was 11581 cP for the raw pasta. [START_REF] Yue | Effect of drying temperature on phisicochemical properties of starch isolated from pasta[END_REF] suggested that the lower final viscosities of HT-dried pasta samples could be partially explained by greater amylose-lipid complex formation that may have restricted recrystallization of amylose during cooling. [START_REF] Güler | Effects of industrial pasta drying temperatures on starch properties and pasta quality[END_REF] confirmed these observations by X-ray diffraction.

As the mixture pasta-water is cooled, re-association between starch molecules, especially amylose, occurs to a greater or lesser degree [START_REF] Zweifel | Thermal modifications of starch during high-temperature drying of pasta[END_REF]. This phase of the pasting curve is commonly referred to as the setback region (SB), and involves retrogradation, or re-ordering of the starch molecules. The pressure level had a strong effect on SB which diminished from 8655 to 7512 cP, when this parameter increased from 1 to 3.9 bar, suggesting that increasing of steam pressure leads to a decreasing of starch retrogradation capacity.

Colour parameters

For the three colour parameters, it can be seen that the processing pressure and the interaction between the two process parameters are significant (Table 3). The analysis of the interactions shows that the steam pressure has a strong effect only at low level of processing time. At 15 seconds, the brightness (L * ) increased from 79.6 to 82.1 when the steam pressure level passed from 1.5 to 3.5 bar, respectively (Table 2). Steam pressure of 3.5 bar associated with a low processing time (15 seconds) seems to maximize the brightness of treated pasta.

The improvement of the brightness observed for pressures ranging between 1.5 and 3.5 bar (corresponding to 110-138 °C) suggests a probable inhibition of peroxidase activity.

These results are in accordance with those of De Stefanis and Sgrulletta (1990) obtained for pasta dried at 90 °C and short time. [START_REF] Kobrehel | Influence de la température de séchage des pâtes alimentaires sur l'activité et la composition des peroxydases en relation avec la couleur des produits[END_REF] suggest a rapid subjecting of pasta to high temperature, just after pasta manufacturing in order to inhibit the enzymatic activities and improve the pasta colour. For the DIC treatment, the rapid release of steam pressure towards vacuum induces a rapid cooling and thus stops the thermal degradation reactions.

When the pressure increased from 1.5 to 3.5 bar, a * varied from 3.19 to 1.71 for a processing time of 15 seconds while it diminished only from 2.7 to 2.3 for a processing time of 35 seconds and a same variation of pressure.

The b * values varied between 30.2 and 25 when the steam pressure increased from 1.5 to 3.5 bar for 15 seconds of processing time. The variation was very weak when the processing time is fixed at 35 seconds. The decreasing of yellowness is probably due to the micro-alveolation of internal structure observed by SEM (section 3.5) of the treated pasta causing a small increase in apparent volume.

Statistical analysis

From table 3, the linear, quadratic and interaction effects can be estimated from the Fisher test (F-ratio). This value tests the statistical significance of each effect by comparing the mean square against an estimate of the standard error. It can be seen that the processing pressure has a strong linear effect on the hydration characteristics and RVA parameters and to a lesser extent on pasta colour. Steam pressure level affected all response parameters with p-value ≤ 0.05 except L * and b * which become significant for p-value ≤ 0.1. The processing time affects only W/W o at 20°C and V 10 , p-value was respectively less than 0.01 and 0.05. For the three hydration characteristics, quadratic effects of the steam pressure level were identified as significant with p-value ≤ 0.05.

The regression equations used to generate the contour plots are shown in table 4. High R 2 values and model significance were obtained for all quality factors tested, except for the two first colour parameters (L * and a * ) for which R 2 was 0.68 in the two cases. The selected contour plots are presented in figure 4, including three representative parameters of cooking quality (OCT), pasting (V 10 ) and colour properties (b * value).

From this figure, it can be seen that the optimal cooking time decreased as the pressure level increased. To obtain a quick cooking pasta of 1 minute, DIC treatment at 2.5 bar steam pressure during 10 seconds is required, according to the developed regression model (R 2 = 0.98). The contour plots for V 10 show that the viscosity of hydrothermally treated pasta increased with pressure level and the magnitude of this increase is more marked for the highest values. According to the developed regression model (R 2 = 0.89) the pasta yellowness (b * value) showed an improvement trend at lower process conditions. The b * value varied from 33 to 29 when the pressure dropped from 1.2 to 2.5 bar. DIC treatment at pressures around 2.5 bar allows to maintain the same pasta yellowness as that of untreated pasta (29.1).

Scanning electron microscopy

Microscopic examinations of cross sections of treated and untreated pasta after drying showed the impact of the DIC treatment on the internal structure and the state of the starch (Figure 5). A more porous structure was observed for treated pasta (photos B, C and D) with starch granules more or less intact according to conditions of the treatment.

In fact, the sample treated at 4 bar during 10 seconds (photo D) has an alveolated texture with pores of important size without visible starch granules. Such a structure would facilitate a rapid water penetration during cooking. In contrast, a photo of untreated pasta (photo A) revealed a very compact structure.

4: Conclusion

The results of this study have shown that cooking quality, pasting properties and colour characteristics are directly influenced by the conditions of DIC treatment. The steam pressure level had the most influence on pasta quality, since it had a significant effect on all response variables investigated. Under studied pressure conditions, the fresh pasta was undergoing physical modifications resulting in the improvement of water absorption, and thus cooking time and colour quality. Increasing of pressure level induces a high apparent viscosity. The microscopy observations showed that the DIC treatment affects the internal structure and the state of starch granules. The rapid water evaporation from the pasta samples creates porous spaces in an internal structure that favours water diffusion and promotes a rapid water uptake during cooking and consequently, a shortening of cooking time. , N is the number of experiments of orthogonal design, i.e. of the factorial design. In this case α = 1.414. 11 and 39 seconds are the approximation of 10.86 and 39.14 seconds that correspond to the values of -α and +α for processing time. -81 + 0.741 X 1 -1.30 X 1 X 2 0.68 a * 2.28 -0.774 X 1 + 0.540 X 1 X 2 0.68 b * 27.3 -1.87 X 1 + 2.45 X 1 X 2 0.89 X 1 : steam pressure level ; X 2 : processing time ; 1 : measured at 5 min of absorption time ; 2 : OCT calculated at W/W 0 = 2.4

Figure Captions

  2.2.6 : Pasta colour The colorimetric analyses were carried out on ground DIC treated samples [250-500 µm]. CIELAB parameters L * , a * and b * were measured using a Hunter Lab D65-10 Reflectance Colorimeter. The L * value expressed the brightness of the sample with 100 as perfect white and 0 as black. A higher L * indicates a brighter sample. Values of a * and b * indicated respectively the red-green and yellow-blue chromaticity. Positive a * and b * values expressed an increasing of redness and yellowness. The results are the average of three replications. 2.2.7 : Scanning electron microscopy Scanning electron micrographs of the control sample and treated pasta were taken with a Jeol 5410 LV SEM. The samples were first splutter-coated with a thin gold film using a Cressington metallizer. 2.2.8 : Experimental design A central composite rotatable design with two independent variables and five levels were used. For the two variables, the design yielded 12 experiments with four (2 2 ) factorial points, four extra points (star points) to form central composite design and four central points for the replications. The experiments were run in random order to minimize the effects of unexpected variability in the observed responses due to extraneous factors. The experimental range and the central point were based on the results of preliminary trials. Table

Figure 1 .

 1 Figure 1. Schematic of apparatus for pasta treatment by DIC. 1. Boiler, 2. Autoclave (stainless steel). 3.Pneumatic valve, 4. Vacuum container, 5. Vacuum pump, 6. Extract container.

Figure 2 .

 2 Figure 2. Typical pressure-time profile of a DIC processing cycle. (a) sample at atmospheric pressure; (b) establishment of a vacuum; (c) steam injection to reach the selected pressure; (d) treatment time; (e) pressure drop; (f) atmospheric pressure for the recovery of sample.

Figure 3 .

 3 Figure 3. Cooking (T=100°C) and absorption (T=20°C) kinetics of tagliatelle pasta treated by DIC at different processing conditions and untreated pasta.

Figure 4 .

 4 Figure 4. Contour plot of optimum cooking time (OCT) of pasta, viscosity at 10 minutes (V 10 ) and yellowness (b* value) as a function of steam pressure level and processing time.

Figure 5 .

 5 Figure 5. Scanning electron micrographs of untreated pasta (A) and DIC treated pasta at 2 bar/10 sec (B), 2 bar/20 sec (C) and 4 bar and 10 sec (D).

  Fig.1

Fig. 2

 2 Fig. 2

  Fig.3

Table 1 :

 1 Coded levels for independent variables used in developing experimental

	data
				Coded level
		-α	-1	0	1	+α
	Steam pressure level (bar)	1.1	1.50	2.50	3.5	3.9
	Processing time (s)	11	15	25	35	39
	α(Axial distance) N 4

Table 2 :

 2 Experimental data and the observed responses values with different combinations of steam pressure level and processing time used in the randomized central composite rotatable design.

	Processing	Absorption characteristics		RVA parameters		Pasta colour
	Parameters
	Steam pressure	W/W 0	1 at	OCT 2	W/W 0	1 at	V 10	CPV	SB	L *	a *	b *
	level (bar)/time (s)	100 °C	(min.)	20°C	(cP)	(cP)	(cP)
	3.5/15	3.28	0.98	2.72	2339	10670	7643	82.1	1.71	25.0
	2.5/11	3.20	1.0	2.09	960	10829	8109	80.0	2.69	29.1
	3.9/25	2.68	3.0	3.0	2659	10560	7512	80.3	2.20	28.4
	2.5/39	3.13	0.90	2.38	1502	10876	8029	80.2	2.47	28.2
	1.5/35	2.57	3.8	1.69	398	10878	8285	80.3	2.68	28.6
	1.5/15	2.40	5.0	1.42	241	11145	8388	79.6	3.19	30.2
	3.5/35	2.86	1.7	2.95	2338	10608	7569	80.2	2.28	28.3
	1.1/25	2.32	6.50	1.48	199	11459	8655	79.9	3.06	29.8
	2.5/25 3	3.18	0.93	2.43	1238	10743	7996	80.9	2.28	27.3
	Untreated	2.20	7.0	1.43	146	11581	8604	80.3	3.15	29.1
	Mean absolute	0.20	0.15	0.05	52	130	91	0.50	0.20	1.10
	error
	1 : measured at 5 min of absorption time ; 2 : OCT (optimal cooking time) was calculated at W/W 0 = 2.4 ; 3 : mean of four repetitions.

Table 3 :

 3 Analysis of variance showing the effect of treatment variables as a linear term, quadratic and interaction (cross product) on the response parameters.

			Hydration characteristics		RVA parameters		Pasta colour
	Independent	W/W 0	1 at	OCT 2	W/W 0	1 at	V 10	CPV	SB	L *	a *	b *
	variables	100 °C	(min.)	20°C	(cP)	(cP)	(cP)
	Linear
	X 1	11.6 **	149 *	521 *	451 *	24.0 *	140 *	3.49 ***	28.0 *	5.25 ***
	X 2	ns		ns	19.4 *	6.80 **	ns	ns	ns	ns	ns
	Quadratic
	X 1	2	24.6 *	206 *	13.1 **	3.68 *** 3.42 ***	ns	ns	ns	ns
	X 2	2	ns		ns	13.7 *	ns	ns	ns	ns	ns	ns
	Interaction
	X 1 X 2	ns		8.94 **	ns		ns	ns	ns	5.37 **	6.81 **	4.50 ***
	X 1 : steam pressure level ; X 2 : processing time ;

1 : measured at 5 min of absorption time ; 2 : OCT calculated at W/W 0 = 2.4 ; * : P-value ≤ 0.01, ** : P-value ≤ 0.05; *** : P-value ≤ 0.1; ns : not significant

Table 4 :

 4 Regression equations describing the response of each quality parameter as a function of the significant effects retained in the models.

	Response parameters	Equation	R 2
	Hydration characteristics
	W/W 0	1 at 100 °C	3.16 + 0.419 X 1 -0.0872 X 1	2	0.87
	OCT 2 (min.)	1.11 -2.77 X 1 + 3.65 X 1	2 + 0.960 X 1 X 2	0.98
	W/W 0	1 at 20°C	2.43 + 1.18 X 1 +0.227 X 2 -0.208 X 1	2 -0.214 X 2	2	0.99
	RVA parameters
	V 10 (cP)	1238 + 1879 X 1 + 231 X 2 -189 X 1	2	0.99
	CPV (cP)	10744 -504 X 1 + 213 X 1	2		0.83
	SB (cP)	7997 -769 X 1				0.96
	Pasta colour
	L *