Émilien Delorme

Benoît Paysant

Philippe Trossat

Yves Gaüzère

Sylvie Pochet

Éric Beuvier

Christine Achilleos
email: christine.achilleos@inrae.fr

Noît Be

Paysant

Yves Ga Üz Èr E

E Sylvi

Pochet

A strategy to assess the sieving time of curd sampled

Keywords: Cheese, Grains of curd, Sampling, Sieving, Syneresis, Whey

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Milk coagulation leads to the formation of a coagulum, a transient gel network trapping the aqueous phase of milk. Most of this aqueous phase is released at cutting, whereas the other part remains trapped in the resulting grains of curd. During cheesemaking, this trapped aqueous phase is expelled from the curd grains as a result of syneresis. Both aqueous phases are whey, but previous studies have shown differences in composition between them, particularly in lactose content [START_REF] Czulak | Lactose, lactic acid and mineral equilibria in Cheddar cheese manufacture[END_REF]Huffman 1985), and they can be distinguished as extraand intra-granular aqueous phases [START_REF] Mietton | Mineraux et transformations fromagères[END_REF]. Thus, an attention should be paid to the separation of curd and whey sampled from the vat for accurate compositional analysis of curd. Sieving is the method described in the literature for physical separation of curd from whey [START_REF] Fox | Effect of curd washing on composition, lactose metabolism, pH, and the growth of non-starter lactic acid bacteria in full-fat Cheddar cheese[END_REF][START_REF] Fagan | The syneresis of rennet-coagulated curd[END_REF][START_REF] Aldalur | Impact of processing conditions on microstructure, texture and chemical properties of model cheese from sheep milk[END_REF][START_REF] Panthi R R | Measurement of syneretic properties of rennet-induced curds and impact of factors such as concentration of milk: A review[END_REF]. The type of the sieve is detailed, but no authors have studied the sieving time to preserve the curd and its aqueous phase. However, the sieving time affects the moisture content of the curd sample and therefore its solute concentrations. This is of particular interest when manufacturing steps are likely to enhance differences in composition between both aqueous phases such as curd washing. Thereby, the curd with its aqueous phase must be separated from the extra-granular aqueous phase in order to characterise the curd solute composition at the time of sampling. Therefore, our objective was to determine a sieving time enabling the elimination of a maximum of the extra-granular aqueous phase while retaining a maximum of the intra-granular aqueous phase.

Considering that the free extra-granular aqueous phase flows faster than the intra-granular aqueous phase, which is retained in the curd, two approaches were investigated in order to assess the transition point between extragranular aqueous phase and intra-granular aqueous phase during sieving. These were the dynamics of flow rates and the lactose content of the flowing whey.

These investigations were carried out during the manufacture of semihard cheeses in the pilot plant of ISBA Franche-Comté (ENILBio, Poligny, FR) from a standardised local cow's milk (3.7% protein; 3.7% fat) pasteurised at 76 °C for 60 s. Milk (300 L) was directly inoculated at 31 °C with 23 g of freeze-dried CHN-11 starter (Chr. Hansen, DK) and left to mature for 60 min. Then, recombinant chymosin (200 IMCU/mL) (CHY-MAX ® Plus, Chr. Hansen, DK) was added to the vat (30 mL/100 L), and coagulation was developed for 25 min. The coagulum was cut into particles (0.6 cm), and the curdwhey mixture was stirred for 10 min at 31 °C. Curd washing was performed by the addition of 117 L of water at 32 °C for 5 min. The curd-whey mixture was heated to 35 °C for 5 min, followed by a final stirring (35 °C, 20 min) until the whey was drained.

Curd-whey samples were collected from the stirred vat with a pitcher and immediately poured over a cheese mould cloth (Kadova ® Saint-Paulin 1.8 kg), which was used as a sieve, and held over a bucket to retrieve the flowing whey (Figure 1).

To study the flow rate of whey during sieving, the curdwhey mixture was sampled (1 L) 1 min before curd washing and poured into the sieve. Four samples were analysed from four different vats. The weight of the whey flowing from the sieved curd was measured every 10 s for 300 s from the moment the sample started to be poured into the sieve (0 s). Calculation of the whey flow rate began when all the curd was in the sieve at 10 s. The first flow rate value, representing the amount of whey that flows between 10 and 20 s of sieving, is the point at 20 s. Dry matter content of the sieved curd was determined according to IDF ( 2004) standard at 0 s, 30 s, 60 s and 300 s of sieving (n = 3).

To study the evolution of the lactose content in the flowing whey, the curd-whey mixture was sampled 30 s after the end of curd washing and poured into the sieve. At this moment, the difference in lactose content between the diluted extra-granular aqueous phase and the less diluted intra-granular aqueous phase is maximised and the identification of the two phases becomes easier. To perform this experiment under similar conditions to the study of the flow rate, the sampling volume was adjusted (2 L) in order to sieve the same curd weight. Two samples were analysed from two different vats. Ten millilitres of the flowing whey were collected under the sieve between 0 s and 260 s for lactose content determination. Lactose content was analysed in whey samples by high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) using a Thermo Scientific Dionex ICS-3000 system. An internal standard was added to the sample, which was then freed from proteins and fats and diluted before injection. Lactose was separated from the other sugars using a CarboPac PA-1 column fitted with a CarboPac PA-1 guard column (250 and 50 mm × 2 mm, respectively) and a sodium hydroxide/sodium acetate gradient. Quantification was performed by referring to a calibration curve and conforming to an internal standard procedure. Samples were prepared and analysed in duplicate.

Flow rate and lactose content dynamics were modelled using linear regression (XLSTAT software version 2019.4.2; Paris, FR). Changes in the dynamics over time were estimated by the intersection of the models.

The flow rates showed a two-stage decrease: at first, a rapid decrease between 20 and 60 s, and then a gradual decrease until 300 s (Figure 2). This is in agreement with the evolution of the curd dry matter during sieving. It showed a rapid increase during the first 60 s (from 16% to 20%), suggesting the loss of a great amount of aqueous phase, probably mainly extra-granular. And then, this increase slowed down until 300 s, suggesting a more progressive draining (Figure 3). These results showed the importance of determining a suitable sieving time because changes in dry matter content of the curd due to an approximate sieving time are likely to impact its solute content. The first 60 s may indicate the loss of whey barely retained by the curd, consisting mainly of extra-granular aqueous phase and a small amount of intra-granular aqueous phase. From about 100 s, we considered that most of the extra- granular aqueous phase was drained and that the flowing whey would be mainly composed of intra-granular aqueous phase, slowly exuded from the curd. The rapid decrease (R) and slow decrease (S) speeds were modelled for each of the 4 sieved curd samples (Figure 2). The model intersection time could be considered as the moment when most of the extra-granular aqueous phase has drained, while most of the intra-granular aqueous phase remains in the curd. The average intersection time was estimated at 60 s, with 2% variation (Table 1).

The lactose content of the whey drained from the sieve showed a two-stage increase: at first a rapid rise from 2.4% to 3.4% during the first 60 s, and then a slower increase from 3.6% to 4.0% between 90 and 260 s (Figure 4). The difference in lactose content between the beginning and the end of sieving confirms the presence of two different aqueous phases: diluted extra-granular aqueous phase and less diluted intra-granular aqueous phase. The increase in lactose content in the first 60 s reflects an increase in the proportion of intra-granular aqueous phase in the stream as the extragranular aqueous phase drained out. Then, the stabilisation of the lactose content from 90 s indicated that the outflow of whey was composed of intra-granular aqueous phase. The intersection of the two linear models of lactose content, both rapid increase and slow increase, indicates a transition around 67 s (Figure 4).

This study showed that it was possible to discriminate between extra-and intra-granular aqueous phases and to remove a maximum of extra-granular aqueous phase while retaining a maximum of intra-granular aqueous phase. Measurements of flow rate and lactose content of the sieved whey allowed the assessment of the transition point between the extra-and intra-granular aqueous phases. The quick evolution the dry matter content during sieving confirmed the impact of the sieving time on the moisture content of the curd and therefore the need to determine a relevant sieving time for the analysis of the curd solute composition. Under the conditions of our experiment, this point could be set between 60 and 67 s. The whey flowing out the sieve after this time can be considered as intra-granular aqueous phase.

The sieving time will be related not only to the type of sieve but also to the manufacturing process applied like the composition of milk, the type of coagulation, the temperature, and the pH, which impacts curd properties and syneresis [START_REF] Panthi R R | Measurement of syneretic properties of rennet-induced curds and impact of factors such as concentration of milk: A review[END_REF].

Figure 1

 1 Figure 1 Device used to sieve the curd-whey mixture.

Figure 2 2 ©

 22 Figure 2 Flow rates (○) of the aqueous phases during the sieving of curd samples (n = 4) and linear regressions modelling the two-stage dynamics.

Figure 3

 3 Figure3Evolution of the dry matter of the curd after 0 s, 30 s, 60 s and 300 s of sieving (n = 3).

Figure 4

 4 Figure4Lactose content (○) evolution in the aqueous phases during the sieving of a curd/whey mixture and the modelling of these dynamics (intersection: 67.4 s).

Table 1

 1 Coefficient of determination, significance, and intersection points on the time axis of the linear models for the two stages (R = rapid decrease, S = slow decrease) during curd sample sieving.

	Sample	Stage	R 2	P-value a	Intersection (s)
	1	R	0.91	0.010	61.4
		S	0.77	<0.001
	2	R	0.86	0.023	60.7
		S	0.77	<0.001
	3	R	0.92	0.011	60.7
		S	0.84	<0.001
	4	R	0.89	0.017	58.6
		S	0.85	<0.001
				Mean (s)	60.4
				Variation (%) b	2.01

a Significant when P < 0.05. b Variation (%) = σ μ Â 100.

© 2021 Society of Dairy Technology

© 2021 Society of Dairy Technology Vol 0

ACKNOWLEDGEMENTS

This work was financially supported by the French Dairy Interbranch Organization (CNIEL, Paris, FR). The authors thank Fabrice Buchin for cheese manufacturing (ISBA Franche-Comté, ENILBio, Poligny, FR) and Fabrice Petitpas and Pascal Barbet for technical help (INRAE, URTAL, Poligny, FR).