Bo Liu
email: boliu@nudt.edu.cn

Baokang Zhao
email: bkzhao@nudt.edu.cn

Chunqing Wu
email: chunqingwu@nudt.edu.cn

An Attribute Based Private Data Sharing Scheme for People-Centric Sensing Networks

Keywords: people-centric sensing networks, relay nodes, privacy, security

In recent years, people-centric sensing networks have attracted much research effort. To date, there are still some significant security and privacy challenges in people-centric sensing networks. In this paper, we focus on the private data sharing and protection in people-centric sensing networks. First, we formalize the network model with relay nodes which improves the data forwarding efficiency of networks. Second, we propose a novel Attribute based Private data sharing protocol in People-centric sensing networks (APP). Relying on the technology of ciphertext policy attribute based encryption, our APP protocol can protect the privacy and integrity with efficient approaches of authentication, encryption, transmission and decryption. Also, we propose an associative data indexing scheme to improve the private data sharing performance. Finally, we discuss the performance evaluation of APP protocol in detail and find that it can achieve much better efficiency.

Introduction

In recent years, people-centric sensing networks, such as BikeNet [START_REF] Eisenman | The BikeNet Mobile Sensing System for Cyclist Experience Mapping[END_REF], CitySense [START_REF] Murty | CitySense: A Vision for an Urban-Scale Wireless Networking Testbed[END_REF], have been subject to extensive research efforts. Unlike traditional sensor networks where humans are passive data consumers that interact with physically embedded static sensors, people-centric sensing networks allow people to collect, store, process, or share information with friends by carrying mobile sensing devices [START_REF] Campbell | People-centric urban sensing[END_REF]. Nowadays, with the widely used popular consumer electronics like PDAs and mobile phones, the people-centric sensor networking issues have attracted a lot of research efforts (such as [START_REF] Oliver | Healthgear: A real-time wearable system for monitoring and analyzing physiological signals[END_REF], [START_REF] Chen | Continuous camera-based monitoring for assistive environments[END_REF]).

In a people-centric sensing network, humans, rather than physical devices, are the focal point of the processed sensor-based information. With the ubiquitous devices, people can gather, analyze, and share targeted information about their daily life patterns and activities like [START_REF] Milenkovi | Wireless sensor networks for personal health mon-itoring: Issues and an implementation[END_REF].

While it brings forth an amazing domain of new applica-tions (such as the MetroSense Project [START_REF] Eisenman | MetroSense Project: People-Centric Sensing at Scale[END_REF], the UCLA Urban Sensing Project [START_REF]CENS Urban Sensing project[END_REF], the CitySense Project [START_REF] Murty | CitySense: A Vision for an Urban-Scale Wireless Network-ing Testbed[END_REF] and so on), there are still some significant security and privacy challeng-es in people-centric sensing networks [START_REF] Giannetsos | People-Centric Sensing in Assistive Healthcare: Privacy Challenges and Directions. Security And Communication Networks[END_REF] [START_REF] Athanasios | Security Threats in Wireless Sensor Networks: Implementation of Attacks & Defense Mechanisms[END_REF].

Secutrity and Privacy challenges

Since sensor devices carried by people are highly mobile, the topology of the network is not fixed at all. The research about people-centric sensing inspires some new architectures and applications, such as [START_REF] Burke | Participatory sensing[END_REF], [START_REF] Eagle | Reality mining: sensing complex social systems[END_REF]. The private data is gathered and transmitted frequently among the sensor nodes and strong cryptographic techniques should be employed [14] [15]. The existing security approaches with fixed topologies cannot be employed in people-centric network at all [16] [17] [18] [START_REF] Yin | Secure routing for large-scale wireless sensor networks[END_REF].

 Privacy:

The people-centric sensing applications entail unrestricted dissemination of consumers' sensor data. Users have to control who can access information about their private data. Critical access controlling and cryptography technologies should be employed.

 Integrity:

Generally, a people-centric sensing system provides anonymity to those nodes that are tasked. It's difficult to guarantee the integrity of information. If a user falsifies data, it's hard to trace the misbehaving. It's always a major challenge to find an approach that balances privacy with data integrity.

In people-centric sensing networks, we can provide the security protections by encrypting data with different keys when users want to share the private information with others.

 Symmetric key encryption:

In symmetric key encryption, consumers have to discuss a uniform key before they share private data. In an people-centric sensing network, the sensor devices are not approached to each other all the time. Key distribution and management is really very hard when somebody wants to send messages.

 Conventional public key encryption:

In conventional public key encryption like RSA, an encryption key and a decryption key are used. One should know the public key before data sharing. The storage of public keys and communication cost both are considerable critical when sensor nodes have to share data with many other nodes. Also, this will lead to similar key management problems as symmetric key encryption.

 Identity based encryption:

Identity based encryption (IBE) is a form of asymmetric cryptography [START_REF] Boneh | Identity-based encryption from the Weil pairing[END_REF] [START_REF] Mont | A flexible role-based secure messaging service: Exploiting IBE technology for privacy in health care[END_REF]. Unlike RSA, IBE has simplified key generation and management approaches. Generally, the public keys are generated from identity strings and only the CA or PKG (private key generator) can create the private keys to decrypt the data. When the sensor node wants to share data with others, it doesn't have to store many public keys [START_REF] Tan | IBE-lite: a lightweight identity-based cryptography for body sensor networks[END_REF]. However, when somebody wants to share data with many friends, he has to encrypt the same data for many copies with different public keys.

 Ciphertext policy attribute based encryption (CP-ABE):

CP-ABE [START_REF] Bethencourt | Ciphertext-policy attribute-based encryption[END_REF] is type of public-key encryption. In CP-ABE scenario, users' private keys are associated with sets of attributes. A user can encrypt a data with a specific access policy, defining types of receivers who will be able to decrypt the ciphertext. Users post sets of attributes to obtain their corresponding secret keys from the third party, private key generator. The decryption of a ciphertext is possible only if the set of attributes of the secret key satisfies the access policy associated to the ciphertext.

In this paper, we focus on designing a high security and private protection protocol based on CP-ABE to solve the privacy and integrity challenges in people-centric sensing networks.

Our Contributions

Based on the above observations, in this paper, we propose a novel Attribute based Privacy aware data sharing protocol in People centric sensing networks (APP). Based on CP-ABE, the proposed APP protocol provides outstanding security and privacy protections in people-centric sensing networks. Specifically, the contributions of this paper are threefold. First, we heuristically propose the people-centric sensing network model with relay nodes. Since data is forwarded in an opportunistic way in people-centric networks, relay nodes can improve the data forwarding efficiency of networks.

Second, we propose the APP protocol, an attribute based privacy aware data sharing protocol in people centric sensing network. With relay nodes, APP protocol can achieve high transmission efficiency. In addition, APP can also resist most existing security threads in people-centric sensing networks, such as data analysis attack, tracing attack and so on.

Third, we give the security analysis and the performance evaluation with commercial ARM experimental platform. The experimental results show that our APP protocol performs reasonable performance.

The rest of the paper is as follows. We give the brief introduction to CP-ABE in Section II. In section III, we formalize the people-centric sensing network and thread models and identify our design goal. Then, we present the APP protocol in Section IV. Security analysis is given in Section V, followed by the experimental results and performance evaluations in Section VI. Conclusion can be found in Section VII.

Ciphertext Policy Attribute Based Encryption

Ciphertext Policy Attribute Based Encryption (CP-ABE) was proposed by Bethencourt et al. in 2007 [START_REF] Bethencourt | Ciphertext-policy attribute-based encryption[END_REF]. In CP-ABE, a set of attributes defines a user and access policy defines which ciphertext an authorized user is able to decrypt. Private Key Generator (PKG), a trusted third party handles the issuance of private keys.

CP-ABE uses a Bilinear Map system to achieve its security goals. In this section, we will give the brief introduction to bilinear maps and show the algorithm of CP-ABE scheme.

Bilinear Maps

In this part, we present a few facts associated to groups with bilinear maps. Given two multiplicative cyclic groups 0 G , 1 G and a generator g for 1 G , a Bi- linear map e can be defined as

0 0 1 : e G G G   [24]
. The bilinear map e satisfies two properties:

 Non-degeneracy: ( , ) 1 e g g  .

 Bilinearity: ( , ) ( , ) e g g e g g  .

2.2

The CP-ABE scheme

The CP-ABE scheme consists of five functions [23] [25].

 Setup The Setup function will choose a random bilinear group 0 G of prime order p and generator g . The function will return a master key ( , )

g   and a public key 0 PK G  , g , h g   , 1/ f g   , ( , ) e g g  with two random exponents , Z p    .  Encrpyt( PK , M , )
This function encrypts a message M with the public key PK and the access policy  .  KeyGen( MK , S ) This function generate a user private key with the master key MK and an attribute set S .  Decrypt( CT , SK , x ) This function is used to extract the secret message associ- ated with the file CT with a private key SK and a node x which comes from ac- cess policy tree .

 Delegate( SK , ' S ) This function enables a user to delegate some or all privileges to another user with his own private key SK .

Models and Design goal

In this section, we formalize the people-centric sensing network model with relay nodes. Then, we give the threat model and our design goal.  Certificate Authority (CA): Every sensing node must be registered in CA if it's first time to access to the people-centric sensing network. CA publishes the public key which is used to encrypt private data. CA can produce the private key with a set of attribute posted by the requester. In this paper, we assume that all information exchange on the internet between CA and consumers uses conventional protocols such as SSL.  Data Service Center (DSC): DSC is isolated with the CA and there are no users' private keys stored in DSC. All data stored in DSC is encrypted. We assume that the DSC is honest. It will not modify or delete users' data and it will never understand the context of private data.  Sender: Sender is a sensing node that wants to share private data with others. All private data should be encrypted with a self-defined access policy before trans-ferred. If the destination node is nearby, the private data will be sent to it directly. Otherwise, the data will be sent to the DSC when the sender node can connect to the DSC server. If can't, the data will be stored in local and transferred to neighbor nodes when it can connect to.  Relay: Relay is a sensing node that is not the destination node but can forward the encrypted data to other nodes. Similar with a sender node, relay will send data to DSC when it can connect to the server. Otherwise, it will gather the encrypted data and transfer it to other nodes when it can connect to.  Receiver: Receiver is the destination of the sender node. In order to gain the private data from sender, receiver posts a query request to the DSC server and gains data from DSC. Receiver tries to decrypt data with its own private key. If receiver gathers data from other nodes is exactly the wishing data, it won't send it to DSC again.

The People-centric Network Model With Relay Nodes

Assumptions

In our people-centric sensing network model, we assume that only CA can generate private keys used for decrypting data. DSC is honest and won't delete and modify users' data. Sensing nodes cannot create the secret keys needed to decrypt the message. We assume that PK is the public key and SK is the private key. Users can obtain the master public key PK and their own private keys preliminary. The communication between CA and users employs security protocols like SSL.

Threat Model

In our threat model, CA and DSC are trustable and honest. However, the sensing nodes are honest but may be curious.

In specific, we consider the adversary can perform the following attacks to subvert privacy and security.

 Eavesdropping Attack: After eavesdropping a packet, the adversary tries to recover the private data and identify the source node.  Obfuscation Attack: The adversary may swap data from different packets to confuse the receiver nodes.  Tracing Attack: The adversary eavesdrops the transmission of a single packet and tries to trace the source and destination locations.  Matching Attack: The adversary may try to generate many keys and encrypt all possible values using different keys to determine whether there is a match for the packet that he eavesdrops.

Design Goal

Our design goal in this paper is to develop an attribute based privacy aware data sharing protocol in people centric sensing network. Specifically, we focus on the following two desirable objectives.

 Resisting privacy-related attacks in people-centric sensing networks. In peoplecentric sensing networks, the collected and delivered data is usually associated to users' private information. Therefore, the users' privacy must be protected in order for people-centric sensing networks wide acceptance to the public.  Achieving effectively private data sharing performance. When users want to share private data with friends, we can encrypt data with different keys for each node. This will cost much computational resource and transmission overhead. Therefore, we must improve the private data sharing performance.

4

Proposed APP Protocol

Security and privacy challenges have been discussed in Section I. In this section, we present our attribute based privacy aware data sharing protocol in people centric sensing network (APP). APP protocol is based on CP-ABE which has been introduced in Section II. The security assumptions are shown in Section III.B. APP protocol consists of the following four phases: First is the initialization phase where the consumer first joins in the network. Second is the data collection phase, which outlines how sensing nodes encrypt the private data. Following is the data delivery phase that describes how a sensing node transfers data to DSC or to a neighbor node. Finally, the data retrieve phase occurs when a receiver node needs to obtain data from DSC. In APP protocol, users should be authenticated with each other first before their communication.

We first describe the authentication approach employed in APP protocol and then delve into the details of our protocol.

Algorithm 1 Authentication between nodes 1. Each node (Alice and Bob) derives a random number N and string A u which is part of its own attribute set S.

Alice generates string m 1 = (N | A u-Bob

) and Bob generates string m 2 = (N | A u- Alice ). 3. Alice calculates c 1 = Encrypt(m 1 ,SK Alice ) and sends it to Bob. 4. Bob calculates Decrypt(c 1 , A u-Alice ) to gain N and A u-Bob . If A u-Bob is incorrect, the authentication will break down. If not, go to step 5. 5. Bob calculates c 2 = Encrypt(m 2 ,SK Bob ) and sends it to Alice. 6. Alice calculates Decrypt(c 2 , A u-Bob ) to gain N and A u-Alice . If A u-Alice or N is incorrect, the authentication will break down. If not, authentication accomplished.

Authentication

The authentication algorithm employs when the sensing nodes (Alice and Bob) need to communicate with each other. When a sensing node accesses to the DSC server, authentication also should be employed.

4.2

Description of the APP Protocol 1) System Initialization: Based on the system requirements, the following steps should be performed to bootstrap the whole system.

 CA first chooses a random bilinear group 0 G of prime order p and generator g ,

PK G  , g , h g   , 1/ f g   , ( , ) e g g  .
 Each user

= (N | A u ), m 2 = (N | data). 4. Calculate c 1 = Encrypt(PK,m 1 , τ). 5. Calculate c 2 = Encrypt(PK,m 2 , τ).
The tuple (c 1 , c 2 ) is then stored in the sender node memory. Note that the access policy τ allows the receiver and himself to decrypt the data. Such policy avoids data duplicate delivering in a loop between nodes.

3) Data delivery:

In APP scheme, sensing nodes will deliver its data to DSC first when it wants to share data with others. If the node cannot reach to the DSC server, it will deliver its data to neighbor nodes.

4) Data retrieve:

Algorithm 3 Data retrieve by receiver nodes 1. Calculate Decrypt(c 1 ,SK), get N 1 and A u . 2. Calculate Decrypt(c 2 ,SK), get N 2 and data.

3. if N 1 equals N 2 then 4.
receiver accepts A u and data 5. else 6.

Drop data.

end if

The sensing node will request data from the DSC periodically when connecting to the DSC server. If the sensing node cannot connect to DSC server, it receives data from relay or sender nodes.

When the data has been gathered in the receiver sensing nodes, the algorithm 3 will be conducted to retrieve data. Since all private data is encrypted, the DSC cannot return specific encrypted data packets associated with users. The DSC server can return c 1 s for the receiver node first. The sensing node tries to decrypt c 1 s first and decides whether to accept the private data from users with attribute A u . Since the length of c 1 is much shorter than that of c 2 , this approach can improve efficiency and reduce the communication time. Since the DSC server understands nothing about the context of the tuples, it's unable to index any of tuples. This feature protects the privacy of the sharing data.

Query Improvement

In APP protocol, the receiver node has to query all the encrypted tuples in DSC in order to gain the shared private data. Assuming there are n tuples in DSC, the receiver node will cost O(n) time to decrypt the c 1 s to determine which tuple should be accepted. We define this produce as Query.

When there are amounts of tuples in DSC, the APP protocol will achieve poor performance. The poor performance is because the DSC server is unable to index any of the tuples. Since the DSC server learns nothing from the encrypted tuples. For instance, consider that the sender node may want to share amounts of private data with receiver nodes. The sender node will encrypt data with the same access policy. We can achieve reasonable performance if the index is constructed.

In order to improve the query performance, we propose the Associative Data Index and query scheme (ADI). The ADI scheme consists of the following two produces.

1) Associative data indexing

 Suppose that the sender node encrypts amounts of private data with access policy tree τ and gains the tuples

{(c 1 , c 2 ) 1 , (c 1 , c 2 ) 2 , •••, (c 1 , c 2 ) k }.
 The sender node calculates the message .

2) Querying

 The receiver node query c 1 s from the DSC server.  If the receiver node decrypts data and gains the index mes-

sage 1 2 ( | | | | ) k N p p p 
, it will drop all the other tuples which are not associated with the index data.

With ADI scheme, we can reasonably improve the query performance. Since DSC learns nothing about the tuples, ADI improves the query performance and also protects the privacy of users.

Security Analysis

In this part, we give the security analysis of our proposed APP protocols. In our security protocol, we assume that CA and DSC are honest. We assume that the private key distribution is security when SSL or other security protocols employed.

 Resilience to Eavesdropping Attack

In the proposed APP protocol, the sender node has encrypted private data into a tuple (c 1 , c 2 ). The adversary eavesdrops the tuple (c 1 , c 2 ) during the secret message delivering to the DSC server or to neighbor nodes. If the adversary is able to recovery the original data after gathering amounts of tuples, he will be success in his attack. The adversary will learn nothing from the ciphertext without the destination node's private key. Therefore, the proposed APP protocol can resist the eavesdropping attack.

 Resilience to Obfuscation Attack

Considering that, a curious relay node can gain encrypted tuples from different communication links. It may swap the c 2 s from different tuples to confuse the receiver node.

Notice that in an encrypted tuple, it embeds the same random number N in both c 1 and c 2 . The receiver node accepts the data in c 2 only if both random numbers match. Therefore, the proposed APP protocol can resist the obfuscation attack.

 Resilience to Tracing Attack

First, the destination node information is encrypted in each tuple with access policy τ. The ciphertext can be decrypted only if the private key SK satisfies τ. The adversary can learn nothing about the destination and can't generate the private keys. Second, every sensing node in people-centric sensing network plays the sender, relay and receiver role at the same time. As it shown in Fig. 2, the sensing node D plays the relay role to node A and C, the sender role to node B and the receiver role to DSC. The adversary can't distinguish where the data really comes from and where is going to.

By summarizing the above, the proposed APP protocol can resist tracing attack.

 Resilience to Matching Attack

The adversary may try to generate many public keys and access policies, and encrypt all possible values using different public keys and policies to determine whether there is a match for the tuple (c 1 , c 2 ). Since the sharing private data is various (such as messages and photographs), and every encrypted tuple uses a random number, the tuples will be quite different even using the same public key, same access policy and by the same sender.

Relying on CP-ABE, our proposed APP protocol can protect the privacy and integrity with efficiently authentication, encryption, transmission and decryption approaches.

Performance Evaluation

In this section, we study the performance of the proposed APP protocol. The performance evaluation metrics are key generation time, encryption time, data transmission overhead and decryption time. The data transmission overhead is defined as the increased data size after encrypted private data with different access policies. In addition, following the earlier design goal, we analyze the private data sharing performance.

In APP protocol, we employ the CP-ABE toolkit [START_REF]Ciphertext-policy attribute-based encryption[END_REF] on commercial ARM experimental platforms (S3C6410, 667MHz, ARM 11 Series (1176) [START_REF]Samsung S3C[END_REF]).

Key Generation Time

All private keys are produced by CA. We use personal computer as CA server (1GB, Intel i5@2.5GHz) and test the private key generation time with different scales of attributes. The result is shown in Fig. 3.

As it shown in Fig. 3, it will cost more time to generate a private key with a larger attribute scale. For example, it costs 0.3401 second to generate the private key with only one attribute while 3.4234 second with sixty-four attributes. The attribute scale should be balanced between security demands and key generation time.

Encryption Time

Encrypting data with different access policies will lead to different time overhead. In CP-ABE scheme, access policy is a 'AND' and 'OR' tree structure [START_REF] Bethencourt | Ciphertext-policy attribute-based encryption[END_REF]. Leaf nodes are the certain access requirements. We measured the encryption efficiency of CP-ABE with different leaf scales and fixed data size (1KB). As is shown in Fig. 4, critical access policy will lead to much encryption overhead.

Data Transmission Overhead

In APP, data transmission overhead mainly means the encrypted data size. We conduct experiment with different access policies and different data sizes and find that the increased encrypted size only associated to access policies. The experiment result is shown in Fig. 5. For example, the increased data size is 9532 bytes when we want to share private data with friends with a certain 32 leaf nodes access policy. The increased data size has less relationship with original data size. Therefore, the access policies should be balanced between the increased data size, the encryption time and security demands.

Decryption Time

The decryption phase greatly affects the APP protocol performance. Since the receiver has to query all the encrypted tuples in DSC to gain the shared private data. We measure decryption time for one receiver node by amount of times and find that it costs about 0.20 second per leaf node to decrypt the private data.

6.5

Query performance Fig. 6. The query performance with ADI scheme

Assuming that there are amounts of tuples in DSC, and about 5% of the tuples are desired to share with the receiver node R. The average query performance is shown in Fig. 6. For example, without ADI scheme, the receiver node has to query about 12000 tuples in total. It's only about 5700 tuples will be queried with ADI. The ADI scheme reasonably improves our APP protocol performance.

Conclusion

In this paper, we have presented a novel Attribute based Privacy aware data sharing protocol in People centric sensing networks (APP). Relying on the CP-ABE scheme, our APP protocol can protect the privacy and integrity with efficiently authentication, encryption, transmission and decryption approaches. With Associative Data Index and query scheme (ADI), our proposed APP protocol achieves effectively performance for private data sharing. We discussed the performance evaluation of APP protocol in detail and found that it can achieve much better efficiency with well-designed index schemes.

Fig. 1 .

 1 Fig. 1. The people-centric network model with relay nodes

3 (

 3 pointing to (c 1 , c 2 ) i .  The sender node calculates 2

Fig. 2 .

 2 Fig. 2. The nodes play as mixed roles in people-centric sensing network

Fig. 3 .

 3 Fig. 3. Key generation time with different attribute scales

Fig. 4 .

 4 Fig. 4. Encryption time with different access leaf scales and fixed data size (1KB)

Fig. 5 .

 5 Fig. 5. The increased data size with different leaf nodes scales

Data collection: Algorithm 2 Encrypting data by sender nodes

  1. Sender node generates a random number N and string A u which is part of its own attribute set S. 2. Sender node defines access policy τ. 3. Calculate m 1

	1 { , , } 2 u U u u i    announces the attribute set i S . The user i u regis-
	ters in CA with his attribute set.
	 CA generates the private key

i SK with i S and MK . Then, CA pushes the private and public keys to the registered user back.

2)

The corresponding author: Dr. Baokang Zhao, with School of Computer Science, National University of Defense Technology, email: bkzhao@nudt.edu.cn.

Acknowledgment The work described in this paper is partially supported by the grants of the National Basic Research Program of China (973 project) under Grant No.2009CB320503, 2012CB315906; the project of National Science Foundation of China under grant No. 61070199, 61103189, 61103194, 61103182, 61202488, 61272482; the National High Technology Research and Development Program of China (863 Program) No. 2011AA01A103, 2012AA01A506, 2013AA013505, the Research Fund for the Doctoral Program of Higher Education of China under Grant No. 20114307110006, 20124307120032, the program for Changjiang Scholars and Innovative Research Team in University (No.IRT1012), Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province("network technology"); and Hunan Province Natural Science Foundation of China (11JJ7003).


Carine Watrelot

Le Bourvellec

Catherine M G C Renard
email: catherine.renard@inra.fr

Aude A Watrelot
email: watrelotaude@yahoo.fr

Carine Le Bourvellec
email: carine.le-bourvellec@inra.fr

Sqpov S Ecurit

Interactions between polyphenols and polysaccharides: Mechanisms and consequences in food processing and digestion

Keywords:

Interactions between polyphenols and polysaccharides:

Mechanisms and consequences in food processing and digestion

Introduction

In intact plant tissues, cell-walls, polyphenols and polyphenoloxidase (PPO) are present in distinct compartments. When cells are ruptured, e.g. by grinding and pressing, these three elements come in contact. Polyphenols react with cell-wall polysaccharides and can be oxidised by PPO. Winemakers, those who treat external timber with creosote, or laboratory workers using any form of chromatography all know this. The literature however when we started working on this topic in 1999 was sparse and disperse, in contrast to the abundance of data for protein/ polyphenol interactions, concentrated on tannin and astringency perception. Three mechanisms can be jointly responsible for formation of polyphenol e cell-walls complexes. The first mechanism is non-covalent and consists in the adsorption of native and oxidised polyphenols to the cell-wall matrix, which will be detailed below. Two distinct mechanisms might lead to formation of covalent bonds, by reaction with cell wall polymers of polyphenols activated either (mechanism 2) by oxidation i.e. as quinones, (resulting primarily from action of polyphenoloxidase) or (mechanism 3) as carbocations, resulting from proanthocyanidin cleavage under acidic conditions, (Beart, Lilley, & Haslam, 1985). The developments in polyphenol analysis and purification allowed to set up simple systems that can be used to quantify these interactions, to modify the conditions and investigate structure/affinity relationships for polyphenols and for polysaccharides (Le Bourvellec, & Renard, 2005;[START_REF] Bourvellec | Non-covalent interactions between procyanidins and apple cell-wall material. Part III : Study on model polysaccharides[END_REF]Le Bourvellec, Guyot & Renard, 2004a;[START_REF] Bourvellec | Pink discoloration of canned pears : Role of procyanidin chemical depolymerization and procyanidin/cell-wall interactions[END_REF][START_REF] Renard | Interactions between apple cell-walls and native apple polyphenols: Quantification and some consequences[END_REF]. These methods and similar approaches have been used also to quantify interactions of proanthocyanidins from grape to grape cell-walls, especially from the grape skin [START_REF] Fournand | Accumulation and extractability of grape skin tannins and anthocyanins at different advanced physiological stages[END_REF][START_REF] Bautista-Ortin | Reactivity of pure and commercial grape skin tannins with cell-wall material[END_REF][START_REF] Bindon | Ripening-induced changes in grape skin proanthocyanidins modifies their interaction with cell-walls[END_REF], Bindon, Basic & Kennedy, 2012[START_REF] Bindon | Factor affecting skin tannin extractability in ripening grapes[END_REF]. More recently physical methods such as isothermal titration calorimetry were used to gain insight in the mechanisms (Le [START_REF] Bourvellec | Impact of processing on the non-covalent interactions between procyanidins and cell-walls[END_REF][START_REF] Watrelot | Interactions between pectic compounds and procyanidins are influenced by methylation degree and chain length[END_REF], 2014). In the last few years, new topics were (1) the impact of thermal treatments (Le [START_REF] Bourvellec | Phenolic and fiber composition of applesauce is close to that of apple flesh[END_REF]2013), with formation of covalent adducts between polyphenols and cell-walls, and (2) consequences, both technological and for polyphenol bioavailability.

Polyphenols are for the most part present in cell vacuoles. The most studied models for polyphenol e polysaccharide interactions are apples and grapes, both of which are rich in condensed tannins, chemically identified as proanthocyanidins. Apple and pear fruits have both high concentrations of polyphenols (up to 7 g/kg fresh weight in the parenchyma of ripe fruit) and a relatively simple composition, notably in cider apples and perry pears. Procyanidins (flavan-3-ol oligomers and polymers) composed essentially of (À)-epicatechin are the main class in the fruit flesh of both apple and pear; their degree of polymerization vary between the cultivars and can be very high (>100). The other classes are phenolic acids, mostly chlorogenic acid, and (in apple only) dihydrochalcones (phloretine glycosides), and monomeric flavan-3-ols, again mainly (À)-epicatechin. All of this makes these two fruit eminently suitable for isolation of well-defined fractions. In grape skin, polyphenols are characterized by the presence of proanthocyanidins of high degree of polymerization (typically mean degree of polymerization of 20e40), including galloylated subunits i.e. presence of both procyanidins and prodelphinidins [START_REF] Man E | Optimization of simultaneous flavanol, phenolic acid, and anthocyanin extraction from grapes using an experimental design: Application to the characterization of champagne grape varieties[END_REF], followed by anthocyanins in red varieties, and phenolic acids.

Plant cell-walls are a complex, porous polysaccharidic material. In fruits and vegetables, they can be described by the type I model of [START_REF] Carpita | Structural models of primary cell walls in flowering plants : Consistency of molecular structure with the physical properties of the walls during growth[END_REF] as composed of three interpenetrating but not interconnected networks: a cellulose/xyloglucan framework (>500 g/kg dry weight) is embedded in a pectin matrix (250e400 g/kg dw), locked into shape by cross-linked glycoproteins (extensin, about 10 g/kg dw). The cell-wall compositions of apple or grapes are well studied, and correspond well to this model [START_REF] Renard | Studies on apple protopectin iv: Apple xyloglucans and influence of pectin extraction treatment their solubility[END_REF], 1990;;[START_REF] Vidal | Polysaccharides from grape berry cell-walls. Part I: Tissue distribution and structural characterization of the pectic polysaccharides[END_REF][START_REF] Doco | Polysaccharides from grape berry cell-walls. Part II. Structural characterization of the xyloglucan polysaccharides[END_REF][START_REF] Vicens | Changes in polysaccharide and protein composition of cell-walls in grape berry skin (cv. Shiraz) during ripening and over-ripening[END_REF].

Consequences of polyphenol e polysaccharide interactions are far reaching in food processing. For example, they contribute to the selective extraction of polyphenols from apple to apple juice (Le Bourvellec, Le Qu er e, & Renard, 2007;[START_REF] Renard | Modulating apple juices phenolic composition by manipulating the pressing conditions[END_REF], and even more important from grape to must, as has been clearly shown by Bindon, Kennedy or Gomez-Plaza's works (e.g. [START_REF] Bautista-Ortin | Reactivity of pure and commercial grape skin tannins with cell-wall material[END_REF]Bindon, Smith, & Kennedy, 2010b, 2010a;[START_REF] Revelette | High-Performance liquid chromatography determination of red wine tannin stickiness[END_REF][START_REF] Ruiz-Garcia | Selective extraction of polysaccharide affects the adsorption of proanthocyanidin by grape cell-walls[END_REF]. They also result in the major part of the so-called "unextractable polyphenols" (Perez-Jimenez, Diaz-Rubio, & Sura-Calixto, 2013) or formation of pomaces, where cell-walls and (oxidised) polyphenols form a single material, with antioxidant capacity but also difficult re-extraction of the polyphenols and colours which may be detrimental for their valorisation as dietary fibers. They also modify the extractability of the cell-wall polymers (Le [START_REF] Bourvellec | Interactions between polyphenols and cell-walls: Modification of polysaccharide extractability[END_REF], decrease their enzyme susceptibility and affect their fermentescibility [START_REF] Aura | Characterization of microbial metabolism of Syrah grape products in an in vitro colon model using targeted and non-targeted analytical approaches[END_REF][START_REF] Bazzocco | The factors affecting the conversion of apple polyphenols to phenolic acids and fruit matrix to short-chain fatty acids by human faecal microbiota in vitro[END_REF]. This has also nutritional impacts: polyphenol -cell-wall interactions limit bioavailability of polyphenols, but they may contribute to the formation of bioactive phenolic metabolites in the gut.

Main results on mechanisms, affinities, and consequences of polyphenol (primarily procyanidins) e cell-wall interactions will be presented.

Initial observations

In wine

Most of the major solutes present in the grape berry at harvest contribute to wine composition in proportion to their amount in the fruit. However [START_REF] Hazak | The phenolic components of grape berries in relation to wine composition[END_REF] found that only a fraction of the tannin present in berries was extracted during winemaking. Studies on the extraction of skin proanthocyanidins in model hydroalcoholic solution have shown that the extraction is incomplete, while only 23% of available skin proanthocyanidins recovered. The structural features of extracted and non-extracted proanthocyanidins in terms of composition and mean degree of polymerization (mDP) were quite different. Extracted proanthocyanidins had a lower mDP, while nonextracted proanthocyanidins had both higher mDP and subunit galloylation percentage [START_REF] Fournand | Accumulation and extractability of grape skin tannins and anthocyanins at different advanced physiological stages[END_REF]. Some of the nonextracted tannins were tightly bound to the insoluble matrix of the grape berry. Winemaking process also influences proanthocyanidins extraction from skin and seed grapes. An increase of soluble solids and a decrease of the rate of proanthocyanidins extraction were observed with a reduction in grape berry crushing during fermentation [START_REF] Cerpa-Calderon | Berry integrity and extraction of skin and seed proanthocyanidins during red wine fermentation[END_REF]. It was suggested that the extraction of phenolic compounds was similar with a diffusion-controlled process influenced by temperature, cell permeability, ethanol concentration. During maceration, the application of pectin-degrading enzymes enhances the degradation of pectic fractions from grape cell-walls and leads to an increase of proanthocyanidins extraction [START_REF] Ducasse | Effect of macerating enzyme treatment on the polyphenol and polysaccharide composition of red wines[END_REF]. As maceration duration increases, concentration of (À)-epicatechin-3-O-gallate subunits (present in higher proportion in seed proanthocyanidins) increased and (À)-epigallocatechin subunit (from skin) decreased, as well as proanthocyanidins molecular size [START_REF] Yacco | Red wine tannin structureactivity relationships during fermentation and maceration[END_REF]. This suggested that seed proanthocyanidin extraction increased with the time of maceration but might also be explained by binding of the proanthocyanidins to yeasts or grape biomass during fining on lees [START_REF] Rodrigues | Effect of winery yeast lees on touriga nacional red wine color and tannin evolution[END_REF]. These findings have significant implications for wine production and have the potential to explain the discrepancies often observed between total proanthocyanidin concentration in grape tissues and the quantity of proanthocyanidins in wine [START_REF] Fournand | Accumulation and extractability of grape skin tannins and anthocyanins at different advanced physiological stages[END_REF]. In fact, the quantities found in wine are frequently lower than expected and show large differences depending on variety (Busse-Valverde, Bautista-Ortin, [START_REF] Busse-Valverde | Influence of skin maceration time on the proanthocyanidin content of red wines[END_REF]2010).

In apple juice

A similar phenomenon has also been observed to occur during apple juice processing and cider production. Studies, carried out at the Unit e de Recherches Cidricoles, have highlighted a discrepancy, both quantitative and qualitative, between procyanidin concentrations in the apples and juices [START_REF] Guyot | Direct thiolysis on crude apple materials for high-performance liquid chromatography characterization and quantification of polyphenols in cider apple tissues and juices[END_REF], 2003) (Fig. 1). Although in apples the main polyphenol class is that of procyanidins, in apple juice phenolic acids become much more important. This was especially obvious for apples of Guillevic (GU) variety, containing primarily procyanidins of high degree of polymerization (DPn of 63) [START_REF] Renard | Modulating apple juices phenolic composition by manipulating the pressing conditions[END_REF] (Fig. 1). Apple cells are disrupted during crushing and pressing steps to produce apple juice. Apple procyanidins can be selectively absorbed by cell-wall material and/or be oxidised which results in lower procyanidins concentration in apple juice, with a lower degree of polymerization [START_REF] Guyot | Variability of the polyphenolic composition of cider apple (Malus domestica) fruits and juices[END_REF]. As it will be explained below, non-covalent interactions between cell-wall material and proanthocyanidins increased with highly polymerized proanthocyanidins. Monomeric catechins concentration slightly decreased from apple juice to cider, which is mainly attributed to the oxidation reactions during fermentation [START_REF] Ye | Evolution of polyphenols and organic acids during the fermentation of apple cider[END_REF].

Quantification and kinetics

In order to address the discrepancies observed and to investigate the phenomena leading to retention of polyphenols, and particularly proanthocyanidins, model experiments are carried out using purified proanthocyanidins and purified cell-walls material from grape or apple. They are put in contact, sometimes varying the concentrations and relative ratios, or the level of maturity of the fruit from which the cell-walls are extracted, and after incubation the free proanthocyanidins are separated by filtration and analysed quantitatively and qualitatively (depolymerization for subunit composition and degree of polymerization and gel permeation chromatography for size distribution determination). Thermodynamic parameters of interaction between pectic fractions and proanthocyanidins with various degree of polymerization have also been studied.

A fast absorption of procyanidins to the apple cell-walls is observed, with most absorption occurring in the first 5 min, and maximum absorption levels being reached in 20 min, whatever the degree of polymerization (Le Bourvellec et al., 2004a). This means that it occurs whenever tissue degradation happens. Similar levels are observed for pHs from 2 to 7 (Le Bourvellec et al., 2004a). Varying procyanidin concentrations are used to obtain binding isotherms, such as the ones pictured in Fig. 2, with procyanidins varying from 3 to 70 monomeric (À)-epicatechin units (Le Bourvellec, & Renard, 2005). Similar results are obtained with grape proanthocyanidins. At low polyphenol concentrations, few proanthocyanidins remain free in solution, particularly for the higher degrees of polymerization. As the proanthocyanidin concentrations increase, the proportion of free proanthocyanidins increases, until a plateau is reached, particularly for the larger proanthocyanidins. The plateau levels are surprisingly high, as the weight of bound procyanidins reaches 80% of the initial cell-wall weight. The data can be fitted by Langmuir isotherm formulation, allowing calculation of affinity constants and saturation levels.

Mechanisms

Non-covalent interactions

As regards the nature of these associations, the question of the mechanism(s) of the association between procyanidins and cellwalls was approached by using physicochemical parameters or compounds that have the capacity to inhibit or to increase ionic interactions, hydrophobic interactions or hydrogen bonding (Le Bourvellec et al., 2004a;[START_REF] Renard | Interactions between apple cell-walls and native apple polyphenols: Quantification and some consequences[END_REF]. Complex formation between procyanidins and cell-walls is not affected by pH in the range 2.2e7. Electrostatic or ionic interactions thus do not appear to play any part in the association between procyanidins and cell- 1998) also showed that the hydrophobic character of proanthocyanidins increases with the degree of polymerization as well as the galloylation. Using isothermal titration calorimetry, pectic fractions with different degree of methylation were titrated by apple procyanidins with two degrees of polymerization. High affinities have been observed between highly methylated (DM 70) homogalacturonans (galacturonic acid chain from pectin) and highly polymerized procyanidins (DP 30) through hydrophobic interactions [START_REF] Watrelot | Interactions between pectic compounds and procyanidins are influenced by methylation degree and chain length[END_REF]. It was suggested that hydrophobic interactions occur between methyl groups of pectic fraction and dihydropyran heterocycles (C-ring) of procyanidins. Hydrogen bonds, mainly driven by enthalpy, have also been shown between some pectic hairy regions and procyanidins, but might be more induced by conformational changes of compounds in solution [START_REF] Watrelot | Neutral sugar side chains of pectins limit interactions with procyanidins[END_REF].

Proanthocyanidins are known to self-associate in solution, which creates a limitation of association with cell-wall and/or protein, as it was suggested by the variation of association of pectins and proteins to immobilized procyanidins [START_REF] Watrelot | Immobilization of flavan-3-ols onto sensor chips to study their interactions with proteins and pectins by SPR[END_REF].

In some respects proanthocyanidin -polysaccharide complexation is similar to proanthocyanidin-assisted aggregation of proteins, that is they are mediated by hydrogen bonds and hydrophobic interactions; however there is a capital difference between the two phenomena. Proanthocyanidins interact with proteins in a series of steps. Initially there is the binding of proanthocyanidin to protein by hydrogen bonds and hydrophobic interaction; these initial, soluble protein-proanthocyanidin complexes then form aggregates by self-association and finally precipitate via colloid formation [START_REF] Jobstl | Molecular model for astringency produced by polyphenol/protein interactions[END_REF][START_REF] Mcrae | Thermodynamics of grape and wine tannin interaction with polyproline : Implication for red wine astringency[END_REF]. Protein compactness is enhanced within the aggregates. On the contrary, interactions of proanthocyanidins with cell-walls appear to be a rapid, direct binding event. Polysaccharides do not collapse nor form closely packed aggregates in the presence of proanthocyanidins, but create either loose oligomers or microgels that are even less compact than single polysaccharide coils [START_REF] Carn | Structural properties of colloidal complexes between condensed tannins and polysaccharide hyaluronan[END_REF]. The difference is easily observable already by moderate centrifugation, where proanthocyanidin-protein micelles are found in the pellet, while proanthocyanidin-polysaccharide complexes remain stable even at ultracentrifugation of 8 Â 10 5 g. The explanation of this difference is in the physical properties of proteins and polysaccharides. So, proanthocyanidins -polysaccharides associations are governed by the same mechanisms as proanthocyanidinprotein interactions but with different kinetics and colloidal consequences.

Covalent interactions

Unequivocal evidence for formation of covalent bonds between polyphenols and polysaccharides is difficult to find. The ideal proof would be formation in vitro of covalent complexes followed by degradation of the polysaccharide to yield a well defined compound, as is successfully possible for the presence of ferulic acids bound to cereal or sugar-beet cell-walls [START_REF] Ralet | Isolation and purification of feruloylated oligosaccharides from cell-walls of sugar-beet ulp[END_REF][START_REF] Saulnier | Ferulic acid and diferulic acids as components of sugar-beet pectins and maize bran heteroxylans[END_REF]. However this has not been possible so far, so that existence of these bonds can only be inferred from the existence of irreversible complexes of polyphenols and polysaccharides. A good example of formation of irreversible complexes between polyphenols and polysaccharides is the formation of pomace during juice extraction. In pomaces high levels of insoluble phenols are present and their separation from the polysaccharides becomes very difficult; polyphenolpolysaccharide complexes are thus frequently observed. Covalent interactions could be mediated by procyanidins oxidation reactions, for example during enzymatic browning. In presence of molecular oxygen when cells are disrupted, polyphenoloxydases (PPO) are able to catalyse monophenols hydroxylation in orthodiphenols through a reversible process, then oxidize these orthodiphenols in ortho-quinones (Rouet-Mayer, Ralambosoa, & Philippon, 1990). The highly reactive ortho-quinones can, depending on the relative redox values of the polyphenol e quinone couples, transfer their oxidation to other polyphenols. Proanthocyanidins are not substrates of PPO [START_REF] Goodenough | Oidation of cider procyanidins by apple oxidase systems[END_REF]; Le Bourvellec, Le Qu er e, Sanoner, Drilleau, & Guyot, 2004b), but they can be converted into corresponding ortho-quinones by coupled oxidation/reduction involving caffeoylquinic acid ortho-quinones. Ortho-quinones are highly electrophilic, which allows nucleophilic addition of polyphenols or proteins and leads to dimer formation and covalent bonds between polyphenols and other compounds [START_REF] Trombley | Capillary electrophoresis methods for the determination of covalent polyphenoleprotein complexes[END_REF]. A systematic investigation was carried out in Le [START_REF] Bourvellec | Interactions between polyphenols and cell-walls: Modification of polysaccharide extractability[END_REF], where purified apple cell-walls, devoid of polyphenols, were put in contact with procyanidins in the absence or presence of caffeoylquinic acid quinones (as drivers of oxidation). Clearly presence of the caffeoylquinic acid quinones enhances binding of the procyanidins to the apple cells walls, as observed gravimetrically. The bound procyanidins become partially resistant to solvolysis, while pectin extractability decreases.

Retention of procyanidins occurs also after plant tissue disruption induced by thermal treatments. This was evidenced in analysis of apple purees and pear wedges, where high concentrations of procyanidins are detected in the solid products (apple puree particles, pear wedges) (Renard, 2005a;[START_REF] Bourvellec | Phenolic and fiber composition of applesauce is close to that of apple flesh[END_REF], 2013). In pears submitted to prolonged thermal treatment a further phenomenon was observed with apparition of a pink discolouration. It can also appear during the processing of other fruits such as quinces [START_REF] Hamauzu | Reddish coloration of Chinese quince (Pseudocydonia sinensis) procyanidins during heat treatment and effect on antioxidant and antiinfluenza viral activities[END_REF] apples, bananas, gooseberries, guavas, and peaches [START_REF] Adams | Discoloration in raw and processed fruits and vegetables[END_REF]. The appearance of pink discolouration depends on various factors, such as pH, temperature, treatment time, type of can used, and storage. However, the main processing factors leading to increased discoloration are excessive heating and delayed cooling of the cans [START_REF] Adams | Discoloration in raw and processed fruits and vegetables[END_REF]. This pink discolouration could be replicated in model solutions or in cell-wall -procyanidin adducts, and was not re-extractable with solvent or with enzymes, leading us to suspect formation of covalent adducts (Fig. 3). This phenomenon is poorly understood. A possible mechanism is chemical depolymerisation of procyanidins, also referred to as condensed tannins, under hot acidic conditions during processing, and their conversion into anthocyanidins [START_REF] Bate-Smith | Leuco-anthocyanins. 1. Detection and identification of anthocyanidins formed leuco-anthocyanins in plant tissues[END_REF]. In hot acidic conditions, the interflavan bond, which is relatively fragile, is cleaved. In this process, terminal units are released as free flavan-3ols, whereas intermediate C-4 carbocations are formed from the extension units. These carbocations are very reactive and can either give anthocyanidins by autoxidation (Bate-Smith, 1954) or react with nucleophilic compounds such as cell-wall polymers to form covalent bonds as in the case of protein [START_REF] Beart | Polyphenol interactions.2. Covelant binding of procyanidins to proteins during acid-catalysed decomposition, observation on some polymeric proathocyanidins[END_REF]. Mass spectrometry identification of thioacidolysis products allowed to identify formation of cyanidin and (À)-epicatechin-cyanidin dimer, probably through acid cleavage of the interflavanic bond followed by oxidation, as described by [START_REF] Beart | Polyphenol interactions.2. Covelant binding of procyanidins to proteins during acid-catalysed decomposition, observation on some polymeric proathocyanidins[END_REF].

Structure activity relationships

Since plant cell-wall contains multiple components, the interactions between proanthocyanidins and cell-walls are likely to be dependent on both the chemical characteristics of proanthocyanidins such as molecular structure and weight, and the physical properties and chemical composition of the cell-walls [START_REF] Hanlin | Review : Condensed tannin and grape cell-wall interactions and their impact on tannin extractability into wine[END_REF][START_REF] Bouvellec | Interactions between polyphenols and Macromolecules: Quantification methods and mechanisms[END_REF]. Associations between proanthocyanidins and cell-walls could be related to a tanning effect as in the case of proanthocyanidins -protein interactions [START_REF] Mcmanus | Polyphenol interactions. Part I. Introduction; some observations on the reversible complexation of polyphenols with proteins and polysaccharides[END_REF]. This tanning effect is a function of both the presence of functional groups able to form hydrogen bonds and hydrophobic interactions, and of the molecular weights, i.e. the size of the polyphenol [START_REF] Mcmanus | Polyphenol interactions. Part I. Introduction; some observations on the reversible complexation of polyphenols with proteins and polysaccharides[END_REF]. Cell-walls contain hydroxyl groups as well as aromatic and glycosidic oxygen atoms that have the ability to form hydrogen bonds and hydrophobic interactions. Proanthocyanidins also contain hydroxyl groups as well as aromatic rings.

Polysaccharides

Cell-wall structure and composition have been shown to influence the association with proanthocyanidins. In particular for grape cell-wall, proanthocyanidins adsorption was consistently lower for skin cell-wall than for flesh (Bindon, Smith, & Kennedy, 2010a, 2010b[START_REF] Bindon | Ripening-induced changes in grape skin proanthocyanidins modifies their interaction with cell-walls[END_REF]. Key structural differences that reduce proanthocyanidins adsorption in skin cell-walls by comparison to flesh cell-walls are: endogenously higher concentrations of insoluble proanthocyanidins, presence of lignin, lower cell-wall bound protein. These differences may confer reduced flexibility and porosity of skin cell-walls relative to flesh cell-walls (Bindon et al., 2010b[START_REF] Bindon | Tissue specific and developmental modificatios of grape cell-walls influence the adsorption of proanthocyanidins[END_REF]. Ripening of grape skin cell-walls also enhances adsorption of proanthocyanidins especially those of high molecular mass [START_REF] Bindon | Tissue specific and developmental modificatios of grape cell-walls influence the adsorption of proanthocyanidins[END_REF][START_REF] Bindon | Factor affecting skin tannin extractability in ripening grapes[END_REF][START_REF] Castro-Lopez | Role of cell-wall deconstructinfg enzymes in the proanthocyanidins-cell-wall adsorption-desorption phenomena[END_REF]. The onset of veraison is characterized by skin cell-walls that showed a low level of proanthocyanidins adsorption, especially of those of high molecular mass. As ripening progresses skin cell-wall isolates show an increase of the retention of proanthocyanidins [START_REF] Bindon | Tissue specific and developmental modificatios of grape cell-walls influence the adsorption of proanthocyanidins[END_REF]. Slight chemical changes that occur in the cell-wall material composition are accompanied by physical and spatial changes in cell-walls during maturation, especially an increase of porosity. This increased cell-wall porosity would facilitate penetration of large proanthocyanidins molecules into a more open cell-wall network (Le [START_REF] Bourvellec | Impact of processing on the non-covalent interactions between procyanidins and cell-walls[END_REF] and explain the highest adsorption of tannins of higher molecular weight in cell-walls from ripe grape skins. In a same manner, harsh drying of apple cell-walls at 100 C was shown to reduce their surface area due to a decrease in porosity and an alteration of the 3-dimensional structure of the cell-wall, decreasing the affinity constant of procyanidins to the apple cell-walls (Le Bourvellec, & Renard, 2005). Modification of the composition of cell-walls by the selective removal of galacturonan-rich pectin fraction of apple cell-walls by boiling (Le [START_REF] Bourvellec | Impact of processing on the non-covalent interactions between procyanidins and cell-walls[END_REF] or enzymatic treatment [START_REF] Renard | Interactions between apple cell-walls and native apple polyphenols: Quantification and some consequences[END_REF] or of grape cell-walls by chelating agent [START_REF] Ruiz-Garcia | Selective extraction of polysaccharide affects the adsorption of proanthocyanidin by grape cell-walls[END_REF] decreases the adsorption of proanthocyanidins by cellwalls. Nevertheless, a high binding capacity of cell-wall hemicellulosic fractions for proanthocyanidins is retained, although lower than observed when pectins are present (Le [START_REF] Bourvellec | Impact of processing on the non-covalent interactions between procyanidins and cell-walls[END_REF][START_REF] Ruiz-Garcia | Selective extraction of polysaccharide affects the adsorption of proanthocyanidin by grape cell-walls[END_REF]. Using purified and commercial proanthocyanidins and cell-wall material from different origin, [START_REF] Bautista-Ortin | Reactivity of pure and commercial grape skin tannins with cell-wall material[END_REF] have shown that cell-walls with higher pectin content and a more flexible conformation have a higher binding capacity. Binding isotherms carried out on model polysaccharides also indicate that the affinity of procyanidins is greatest for pectins followed by xyloglucan, and lowest for cellulose (Le Bourvellec et al., 2005). This higher affinity with pectins may be due to the formation of a three-dimensional gel-like network, forming hydrophobic pockets, able to encapsulate proanthocyanidins. A high affinity of procyanidins for highly methylated galacturonans, the main component of pectins, is confirmed by the modulation of extraction of polysaccharide after procyanidins binding and by isothermal titration calorimetry (Le [START_REF] Bourvellec | Interactions between polyphenols and cell-walls: Modification of polysaccharide extractability[END_REF][START_REF] Bourvellec | Impact of processing on the non-covalent interactions between procyanidins and cell-walls[END_REF]. Moreover, increasing degree of methylation of homogalacturonan and type I rhamnogalacturonans was noted to enhance procyanidins binding [START_REF] Watrelot | Interactions between pectic compounds and procyanidins are influenced by methylation degree and chain length[END_REF][START_REF] Watrelot | Neutral sugar side chains of pectins limit interactions with procyanidins[END_REF], in comparison to non-methylated homogalacturonans and other pectic hairy regions (type II rhamnogalacturonans). Neutral sugars of pectic hairy regions can adopt specific conformations in solution, leading to variation of association with procyanidins. Citrus pectins containing more type I rhamnogalacturonans have higher affinity with immobilized procyanidin dimer than apple pectins [START_REF] Watrelot | Immobilization of flavan-3-ols onto sensor chips to study their interactions with proteins and pectins by SPR[END_REF].

Proanthocyanidins

The extent and strength of proanthocyanidins cell-wall associations will also depend both on concentrations and on the characteristics of the proanthocyanidins, including their molecular mass, degree of galloylation and/or the stereochemistry of their molecules. In general, proanthocyanidins with higher degree of polymerization and degree of galloylation are selectively adsorbed by cell-wall [START_REF] Renard | Interactions between apple cell-walls and native apple polyphenols: Quantification and some consequences[END_REF]Le Bourvellec et al., 2004a). Studies on interactions between proanthocyanidins and cell-wall material from flesh and skin of grape have shown that fining with cell-walls material is size-based (Bindon et al., 2010a). Gel permeation chromatography has shown that the role of proanthocyanidins galloylation can be explained by the variation in size of galloylated material, as opposed to specific selectivity of galloylated proanthocyanidins. As observed using isothermal titration calorimetry, the higher the degree of polymerization, the higher the association and aggregates formation with pectic fractions was [START_REF] Watrelot | Interactions between pectic compounds and procyanidins are influenced by methylation degree and chain length[END_REF] (Fig. 4). An exception to the selective binding of large-size proanthocyanidins has been found for very high molecular mass proanthocyanidins (>15,000 g/mol) from skin of red grapes, which appear to be excluded from bindings when cell-wall porosity is low (Bindon et al., 2010a[START_REF] Bindon | Tissue specific and developmental modificatios of grape cell-walls influence the adsorption of proanthocyanidins[END_REF][START_REF] Bindon | Factor affecting skin tannin extractability in ripening grapes[END_REF][START_REF] Bindon | Ripening-induced changes in grape skin proanthocyanidins modifies their interaction with cell-walls[END_REF]. Proanthocyanidin activity determined by the thermodynamics of interaction between red wine proanthocyanidins and a hydrophobic surface was more correlated to molecular mass than to the degree of polymerization [START_REF] Yacco | Red wine tannin structureactivity relationships during fermentation and maceration[END_REF]. The constitutive subunits of proanthocyanidins can induce variations of conformation and also of polysaccharide interactions. Proanthocyanidins comprising (þ)-catechin as extension units had higher affinity to cell-wall polysaccharides than proanthocyanidins mainly with (À)-epicatechin [START_REF] Mcmanus | Polyphenol interactions. Part I. Introduction; some observations on the reversible complexation of polyphenols with proteins and polysaccharides[END_REF]. Interactions with cell-walls are modulated by flavan-3-ol subunit composition, the stereochemistry of which might lead to changes in proanthocyanidins conformation (Le Bourvellec et al., 2004a), a more open and flexible conformation facilitating formation of hydrogen bonds and hydrophobic interactions. The extent of proanthocyanidins binding on cell-wall also depends on the concentration. An increase of the polyphenol concentration leads to more proanthocyanidin bound per cell-wall mass unit [START_REF] Renard | Interactions between apple cell-walls and native apple polyphenols: Quantification and some consequences[END_REF]Le Bourvellec, & Renard, 2005).

Consequences

Technological consequences

The quantification of adsorption of procyanidins on cell-wall materials was used to develop a model, on the basis of the Langmuir isotherm formulation and a factorial experimental design, to predict the extraction of procyanidins from apple to juice in absence of oxygen [START_REF] Bourvellec | Impact of non-covalent interactions between condensed tannin and apple cell-walls: Elaboration of a quantitative model and its application to transfer from fruit to juice[END_REF]. The parameters that influence the adsorption are the concentration and molecular weight of the procyanidins, the ionic strength of the solution, the temperature at pressing, and the apple cell-wall concentration. From a technological point of view, the two factors ionic strength and temperature are relevant for transfer of polyphenols from apple must. There are limited variations in the ionic strength which could be calculated for apples from their acidity and known ion composition. However, they occur in the part of the ionic strength range where effects are most pronounced. For the cider-maker, this parameter is fixed by the apple, but could be modulated using different apple cultivars. On the other hand, the temperature of pressing, which can vary from 5 to 20 C, will had an influence on the transfer of apple polyphenols to must, and can be modulated by the users. This effect of temperature was validated in apple pressing, for temperatures from 4 C to 25 C: while, in absence of oxidation, extraction is almost quantitative for hydroxycinnamic acids at all temperatures, there is an impact on transfer for all flavan-3-ols, particularly procyanidins [START_REF] Renard | Modulating apple juices phenolic composition by manipulating the pressing conditions[END_REF], with about 30% higher extraction at 25 C than at 4 C (Table 1).

The ability of cell-walls to form complexes with proanthocyanidins makes them an interesting object of study for their possible use as new fining agents as an alternative to animal proteins in winemaking. Such studies were initiated by [START_REF] Bindon | Comparison of the affinity and selectivity of insoluble fibres and commercial proteins for wine proanthocyanidins[END_REF] and [START_REF] Guerrero | Application of insoluble fibers in the fining of wine phenolics[END_REF], who studied the fining action of insoluble fibers prepared from apple flesh and different grape sources and compared them with commercial proteins, reporting that grape fibers may be considered as alternative fining agents for red wine. Proteins and fibers from purified cell-wall of fresh grape and apple fruits and pomaces present a high affinity for proanthocyanidins structures and especially high molecular mass [START_REF] Bautista-Ortin | Remarkable proanthocyanidins adsorption properties of Monastrell pomace cell-wall material highlight its potential use as an alternative fining agent in red wine production[END_REF]Bindon & Smith, 2013;[START_REF] Guerrero | Application of insoluble fibers in the fining of wine phenolics[END_REF], while selectivity for proanthocyanidins by subunit composition is variable between treatments, but differences are minor (Bindon & Smith, 2013). Proanthocyanidins removal by cell-wall materials requires a higher dose to achieve a fining effect comparable with proteins (Bindon & Smith, 2013). Moreover, from all sources of fibers tested, the purified cell-wall of fresh grape and apple fruits show a greater dose effect on adsorption of proanthocyanidins than the corresponding extracts from pomace (Bindon & Smith, 2013). The fact that most fibers selectively remove proanthocyanidins of higher molecular mass is important as large-size proanthocyanidins are known to be a stronger driver of astringency. However, grape fibers not only reduce proanthocyanidins but also reduce anthocyanins, total phenolics and wine colour density [START_REF] Guerrero | Application of insoluble fibers in the fining of wine phenolics[END_REF]. For further development of these alternative fining agents some technological aspects such as the amount of lees formed during their use and their role in wine composition and flavour [START_REF] Bautista-Ortin | Remarkable proanthocyanidins adsorption properties of Monastrell pomace cell-wall material highlight its potential use as an alternative fining agent in red wine production[END_REF]Bindon & Smith, 2013;[START_REF] Guerrero | Application of insoluble fibers in the fining of wine phenolics[END_REF] need to be assessed. Similar observations have been reported in cider processing: in a study on the sensory characteristics of cider, [START_REF] Symoneaux | Prediction of sensory characteristics of cider according to their biochemical composition: Use of a central composite design and external validation by cider professionals[END_REF] used water-extracted apple pomace to decrease procyanidin content and degree of polymerization in cider, obtaining a product with lower astringency. Moreover, the use of enzymes during winemaking process leads to modifications of grape polysaccharides. Those enzymes lead to a decrease of methylated pectins and neutral sugars and to an increase of galacturonic acid and type II rhamnogalacturonans [START_REF] Doco | Effect of flash release and pectinolytic enzyme treatments on wine polysaccharide composition[END_REF]. A higher extraction of proanthocyanidins has been observed during the winemaking process due to cell-wall degradation during maceration stage.

Interactions with solubilized cell wall polymers may on the other hand favor retention of proanthocyanidins as soluble complexes in the liquid phase. Indeed Riou, Vernhet, Doco, & Moutounet, 2002 observed that the different polysaccharides, while they do not modify initial aggregation of tannins, influence further particle size evolution. Arabinogalactan proteins and mannoproteins notably may decrease tannins aggregation (Poncet-Legrand, Doco, Williams, & Vernhet, 2007;[START_REF] Riou | Aggregation of grape seed tannins in model wine -effect of wine polysaccharides[END_REF].

Nutritional consequences

The binding of proanthocyanidins to cell-wall components can have significant effects on the release of phenolic compounds from the food matrix for potential absorption in the human gastrointestinal tract (i.e. bioaccessibility). It is becoming increasingly clear that the biological effects of proanthocyanidins should not be attributed to the native compounds present in foods, but rather to their metabolite products in the colon, which are diverse phenolic acids [START_REF] Williamson | Colonic metabolites of berry polyphenols: The missing link to biological activity[END_REF][START_REF] Del Rio | Polyphenols and health: What compounds are involved?[END_REF][START_REF] Del Rio | Dietary (Poly)phenolics in human Health: Structures, bioavailability, and evidence of protective effects against chronic diseases[END_REF]. These colonic metabolites are produced during fermentation by the colonic microflora of native substances and are strongly absorbed. Proanthocyanidins associated with cell-wall are poorly bioavailable in the upper intestine [START_REF] Donovan | Procyanidins are not bioavailable in rats fed a single meal containing a grapeseed extract or the procyanidin dimer B-3[END_REF] and reach the colon, where they become fermentable substrates for bacterial microflora, which convert them into active metabolites [START_REF] Bazzocco | The factors affecting the conversion of apple polyphenols to phenolic acids and fruit matrix to short-chain fatty acids by human faecal microbiota in vitro[END_REF]. Evidence of the effect of food matrix and noncovalent interactions between procyanidin and cell-wall on the metabolism of procyanidins by human microflora has been recently found [START_REF] Aura | Characterization of microbial metabolism of Syrah grape products in an in vitro colon model using targeted and non-targeted analytical approaches[END_REF][START_REF] Bazzocco | The factors affecting the conversion of apple polyphenols to phenolic acids and fruit matrix to short-chain fatty acids by human faecal microbiota in vitro[END_REF]. Fermentation studies were performed by colon microbiota in in vitro digestion model using freeze-dried apples, procyanidins extracts from these apples and freeze-dried cider made from the same apples. The conversion rate of procyanidins to known microbial metabolites is much lower with isolated procyanidins than with whole fruit, the larger polymers being also less metabolized than the smaller ones. The larger molecules are able to inhibit both the microbial conversion of procyanidins to phenolic acids and of carbohydrate to short-chain fatty acids, but this inhibition is reduced in the presence of the food matrix [START_REF] Aura | Characterization of microbial metabolism of Syrah grape products in an in vitro colon model using targeted and non-targeted analytical approaches[END_REF][START_REF] Bazzocco | The factors affecting the conversion of apple polyphenols to phenolic acids and fruit matrix to short-chain fatty acids by human faecal microbiota in vitro[END_REF]. The inhibitory effect of procyanidins is dose-dependent [START_REF] Aura | Characterization of microbial metabolism of Syrah grape products in an in vitro colon model using targeted and non-targeted analytical approaches[END_REF][START_REF] Bazzocco | The factors affecting the conversion of apple polyphenols to phenolic acids and fruit matrix to short-chain fatty acids by human faecal microbiota in vitro[END_REF]. It seems that cell-wall acts as a nutrient for colonic microorganisms responsible for the conversion of procyanidins into active metabolites.

Conclusion and future trends

Interactions between polyphenols, and particularly proanthocyanidins, and polysaccharides have now been convincingly demonstrated. They are fast and spontaneous, which means that they occur systematically in fruit and vegetable processing. The binding is due to a combination of hydrogen bonds and hydrophobic interactions, and is favoured by increased ionic strength and decreased temperature. Higher affinities are found for proanthocyanidins of high degree of polymerization. Within the cell-walls, higher affinities for pectins are confirmed in solution with various structural pectin parts using isothermal titration calorimetry. Another parameter is the physical state of the cell-walls, with a major impact of porosity and flexibility.

These interactions and their variations can be incurred during food processing, for example as a function of maturation degree of fruit. They can also be used and manipulated: enzyme application and maceration of grapes modify the final phenolic concentration of wines, cell-walls or pomaces have been proposed as finding agents, and modification of pressing temperature modulates cider astringency.

Growing evidence indicates that the biological effects of proanthocyanidins should be attributed to their colonic metabolites rather than to the native proanthocyanidins present in foods. Proanthocyanidins associated with plant cell-walls are poorly bioavailable in the upper gut and reach the colon where they become fermentable substrates for the commensal microbiota, which converts them into readily absorbable metabolites.

Further research needs which can be identified at this point are:

-Improved understanding and quantification of the impact of the proanthocyanidin e cell-wall interactions in food processing, notably in juice extraction, as a function of the state of cell-walls and processing options; -Understanding the nutritional consequences of the proanthocyanidin e cell-wall complexes in the colon: does the formation of complexes modify the fermentation patterns (of either partner), which are the enzymes and the bacteria involved; -More fundamental understanding of the type of interactions and the behaviour of the proanthocyanidins in presence of both polysaccharides and proteins; -Demonstrating convincingly the existence of covalent bonds between proanthocyanidins and cell-walls by isolation and structural elucidation of molecules (or fragments), and by designing experimental models able to replicate these adducts.

Comment citer ce document :

Fig. 1 .

 1 Fig. 1. Polyphenol composition of apples, their juices obtained in oxidation-limited conditions and the ciders. Apple varieties: DCL: 'Douce Co€ et Lign e'; DM 'Douce Moens'; GU: 'Guillevic'; KE: 'Kerrmerien' and 'PJ 'Petit Jaune'. Polyphenol classes (from left to right): in green monomeric catechins; in orange procyanidins; in white phenolic acid; in dark blue dihydrochalcones, and in yellow flavonols. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

  Comment citer ce document : Renard, C. (Auteur de correspondance), Watrelot, A., Le Bourvellec, C. (2017). Interactions between polyphenols and polysaccharides: Mechanisms and consequences in food processing and digestion. Trends in Food Science and Technology, 60, 43-51. DOI : 10.1016/j.tifs.2016.10.022 Manuscrit d'auteur / Author manuscript Manuscrit d'auteur / Author manuscript Manuscrit d'auteur / Author manuscript Version définitive du manuscrit publiée dans / Final version of the manuscript published in : Trends in Food Science & Technology (2016), 9 p., DOI: 10.1016/j.tifs.2016.10.022 Journal homepage : http://www.journals.elsevier.com/trends-in-food-science-and-technology

Fig. 3 .

 3 Fig.3. Pink discoloration in William pear after prolonged heat treatment in acidic conditions: colour effects. A: raw pear; B: pear were processed at 95 C and pH 2.7 for 418 min (Le[START_REF] Bourvellec | Pink discoloration of canned pears : Role of procyanidin chemical depolymerization and procyanidin/cell-wall interactions[END_REF]. Powders are cell-walls isolated from the processed pears by the phenol-buffer method described by[START_REF] Renard | Variability in cell wall preparations : Quantification and comparison of common methods[END_REF]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4 .

 4 Fig.4. Picture of 30 mM 70% methylated homogalacturonans (in galacturonic acid equivalent) with 30 mM low polymerized procyanidins (DP9) (left) or highly polymerized procyanidins (DP30) (right) (in (À)-epicatechin equivalent).

  Renard, C. (Auteur de correspondance), Watrelot, A., Le Bourvellec, C. (2017). Interactions between polyphenols and polysaccharides: Mechanisms and consequences in food processing and digestion. Trends in Food Science and Technology, 60, 43-51. DOI : 10.1016/j.tifs.2016.10.022 Manuscrit d'auteur / Author manuscript Manuscrit d'auteur / Author manuscript Manuscrit d'auteur / Author manuscript Version définitive du manuscrit publiée dans / Final version of the manuscript published in : Trends in Food Science & Technology (2016), 9 p., DOI: 10.1016/j.tifs.2016.10.022 Journal homepage : http://www.journals.elsevier.com/trends-in-food-science-and-technology

Trends in Food Science & Technology (2016), 9 p., DOI: 10.1016/j.tifs.2016.10.022 Journal homepage : http://www.journals.elsevier.com/trends-in-food-science-and-technology

  Comment citer ce document : Renard, C. (Auteur de correspondance), Watrelot, A.,[START_REF] Renard | Interactions between polyphenols and polysaccharides: Mechanisms and consequences in food processing and digestion[END_REF]. Interactions between polyphenols and polysaccharides: Mechanisms and consequences in food processing and digestion. Trends in Food Science and Technology, 60, 43-51. DOI : 10.1016/j.tifs.2016.10.022Version définitive du manuscrit publiée dans / Final version of the manuscript published in :

	Manuscrit d'auteur / Author manuscript
	Manuscrit d'auteur / Author manuscript
	Manuscrit d'auteur / Author manuscript

Table 1

 1 Procyanidins and chlorogenic acid concentrations (mg/L) in apple juices prepared under anaerobic conditions as a function of temperature. CQA: chlorogenic acid; Procya: procyanidins. Data from Renard et al., 2011. Comment citer ce document : Renard, C. (Auteur de correspondance), Watrelot, A., Le Bourvellec, C. (2017). Interactions between polyphenols and polysaccharides: Mechanisms and consequences in food processing and digestion. Trends in Food Science and Technology, 60, 43-51. DOI : 10.1016/j.tifs.2016.10.022 Version définitive du manuscrit publiée dans / Final version of the manuscript published in :

		Douce Moens	Guilllevic		Kermerrien
		CQA	Procya	CQA	Procya	CQA	Procya
	4 C	1099	853	80	121	862	1349
	11 C	1158	986	82	107	1000	1520
	18 C	1213	1067	85	132	950	1821
	25 C	1311	1396	96	146	990	1842

Trends in Food Science & Technology (2016), 9 p., DOI: 10.1016/j.tifs.2016.10.022 Journal homepage : http://www.journals.elsevier.com/trends-in-food-science-and-technology

C.M.G.C. Renard et al. / Trends in Food Science & Technology xxx (2016) 1e9

Acknowledgements

The work leading to this review was supported by funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement n FP7-222654 (DREAM), and from R egion Bretagne and Pays de la Loire under grant agreement CIPOLINA. The authors thank Drs. J.F. Drilleau, S. Guyot, A. Baron, J.M. Le Qu er e, D. Fournand, A. Imberty, S. Quideau, D. Deffieux, K. Bindon, A. Mierczynska-Vasilev, J. Kennedy and P. Mosoni for fruitful discussions through the years.