Pectin-protein interactions

The polysaccharide-protein dispersions have two or more immiscible phases distributed in an emulsion [80]. Parts of these dispersions are mixed to form coacervates and used to accomplish different functional properties [29]. The type of interaction that occurs between proteins and polysaccharides is a veiy important aspect as it affects the rheological properties of the interfaces [80]. 

The interactions between molecules of polysaccharides and proteins, can be given by covalent bonds, known as conjugated, they form veiy stable structures. There also are physical forces, non-covalent interactions such as electrostatic, hydrophobic, steric, hydrogen bonding, and Van der Waals forces. These interactions are involved in the formation and stabilization of dispersions [80]. 

Electrostatic forces are the main ways of interaction between the protein molecule and the ionic polysaccharide. Parameters such as pH, ionic strength, concentration and proportion of the molecules, fillers, and structure and size of the components of the 

---

Phase 3 (5 to 7.5 g/l) At these concentrations, a drop in PG activity is observed. This phenomenom is reprocible and can be attributed to pectin-protein interactions. These interactions only occur between specific pectin-protein couples after a first depolymerization action of PG Depolymerized pectins associate with proteins to yield aggregates which can easily sediment. Their removal during medium centrifugation would explain the lowering in PG activity in the reaction medium over this polygalacturonic acid concentration range. 

In summary, the QCM-D technique has successfully demonstrated the adsorption of pectin on the BSA surface as well as determined the viscoelastic properties of the pectin layer. As pectin concentrations increase, the adsorbed mass of pectin estimated from the Voigt model show higher values than those estimated from the Sau-erbrey equation because the former takes into account the hydrated layer. But the similar increase of thickness of pectin suggests that the pectin chains form a multilayer structure. In agreement with our previous rheology results, the main elastic character of the pectin layer in terms of Q-tool software tells us the network structure of the pectin layer on the BSA surface. In summary, QCM-D cannot only help to better understand the polysaccharide/protein interactions at the interface, but also to gain information of the nanoscale structure of polysaccharide multilayers on protein surface. 

FIGURE 2.12 Cryo-SEM micrograph of fresh cheese elaborated with pectin. Pectin network interacting with the protein shell surrounding fat globules (arrows). 

Figure 3.5 Possible interactions in pectin-protein complexation.

The ability of pectins to interact with other biomolecules confers great versatility as components of various composites for the design of matrix carriers of bioactive biomaterials. This is due to the diversity of functional groups present along their backbone and their polyanionic properties. These features allow one to control to some extent the t es of possible interactions and composites to create new materials based on pectin and other biomolecules, such as proteins and lipids. The various composite structures offer new properties, which enables a variety of applications in the fields of food and pharmaceuticals, especially for the new trends in controlled release of drugs and bioactive molecules. 

Cellulose microfibrils make up the basic framework of the primary wall of young plant cells (3), where they form a complex network with other polysaccharides. The linking polysaccharides include hemicellulose, which is a mixture of predominantly neutral heterogly-cans (xylans, xyloglucans, arabinogalactans, etc.). Hemicellulose associates with the cellulose fibrils via noncovalent interactions. These complexes are connected by neutral and acidic pectins, which typically contain galac-turonic acid. Finally, a collagen-related protein, extensin, is also involved in the formation of primary walls. 

Thermodynamically unfavourable interactions between two biopolymers may produce a significant increase in the surface shear viscosity (rf) of the adsorbed protein layer. This change in surface rheological behaviour is a consequence of the greater surface concentration of adsorbed protein. For instance, with p-casein + pectin at pH = 5.5 and ionic strength = 0.01 M (Ay = 2.6 x 10 m3 mol kg-2), the surface shear viscosity at the oil-water interface was found to increase by 20-30%, i.e., rp = 750 75 and 590 60 mN s m-1 in the presence and absence of polysaccharide. These values of rp refer to data taken some 24 hours following initial protein layer formation (Dickinson et al., 1998 Semenova et al., 1999a). 

Surface shear rheology at the oil-water interface is a sensitive probe of protein-polysaccharide interactions. In particular, there is considerable experimental evidence for a general increase in surface shear viscosity of protein adsorbed layers as a result of interfacial complexation with polysaccharides (Dickinson et al., 1998 Dickinson and Euston, 1991 Dickinson and Galazka, 1992 Semenova et al., 1999a Jourdain et al., 2009). One such example is the case of asi-casein + pectin at pH = 5.5 and ionic strength = 0.01 M (Ay = - 334 x 10 cm /mol) the interfacial viscosity after 24 hours was found to be five times larger in the presence of pectin (i.e., values of 820 80 and 160 20 mN m 1 with and without pectin, respectively) (Semenova et al., 1999a). 

Figure 7.18 Protein-polysaccharide interactions in emulsions subjected to high pressure treatment (HPT). Influence of pH on average effective particle diameter d43 determined by static light scattering (Malvern Mastersizer) in emulsions (20 vol% soybean oil, 0.5 wt% p-lactoglobulin) prepared with untreated protein (open symbols) and high-pressure-treated (800 MPa for 30 min filled symbols) protein in the absence (O, ) and presence (A, ) of 0.5 wt% pectin. Reproduced from Dickinson and James (2000) with permission.

Neirynck, N., van der Meeren, P., Lukaszewicz-Lausecker, M., Cocquyt, J., Verbeken, D., Dewettinck, K. (2007). Influence of pH and biopolymer ratio on whey protein-pectin interactions in aqueous solutions and in O/W emulsions. Colloids and Surfaces A Physicochemical and Engineering Aspects, 298, 99-107. 

Protein-polysaccharide complexation affects the surface viscoelastic properties of the protein interfacial layer. Surface shear rheology is especially sensitive to the strength of the interfacial protein-polysaccharide interactions. Experimental data on BSA+ dextran sulfate (Dickinson and Galazka, 1992), asi-casein + high-methoxy pectin (Dickinson et al., 1998), p-lactoglobulin + low-methoxy pectin (Ganzevles et al., 2006), and p-lactoglobulin + acacia gum (Schmitt et al., 2005) have all demon-... 

At the second critical pH (pH,, ), which is usually below the protein isoelectric point, strong electrostatic interaction between positively charged protein molecules and anionic polysaccharide chains will cause soluble protein/polysaccharide complexes to aggregate into insoluble protein/polysaccharide complexes. For negatively charged weak acid-based (e.g., carboxylic acid) polysaccharides like pectin, with the decrease of pH below the pKa of the polysaccharide, protein (e.g., bovine serum albumin (BSA))/polysaccharide (e.g., pectin) insoluble complexes may dissociate into soluble complexes, or even non-interacted protein molecules and polysaccharide chains, due to the low charges of polysaccharide chains as well as the repulsion between the positively charged proteins (Dickinson 1998). 

---