Interactions between globular proteins and procyanidins of different degrees of polymerization

Abstract 

Interactions of proteins with phenolic compounds occur in food products containing vegetable sources, such as cocoa, cereals, or yogurts containing fruit. Such interactions can modify protein digestion and protein industrial properties. Noncovalent interactions between globular proteins (proteins important in industry) and procyanidins (phenolic compounds present in large quantity in fruits) were studied. The affinity constants between procyanidins of various average degrees of polymerization () and lysozyme or α-lactalbumin were measured by isothermal titration calorimetry. The effects of these interactions on protein solubility and foam properties were examined using α-lactalbumin and BSA. Weak interactions were found with epicatechin and procyanidin dimers. Procyanidins of n=5.5 and n=7.4 showed medium (1.5×10⁵ M⁻¹) and high (8.69×10⁹ M⁻¹) affinities, respectively, for α-lactalbumin at pH 5.5, with n the average number of subunits per oligomer. A positive cooperativity of binding at low procyanidin:protein molar ratios was observed. The affinities of α-lactalbumin and lysozyme for procyanidins increased when the pH was close to the isoelectric pH. Solubility of lysozyme was strongly decreased by procyanidins of n=5.5, whereas α-lactalbumin and BSA were less affected. Protein solubility in the presence of procyanidins was not affected by increased ionic strength but increased slightly with temperature. Procyanidins of n=5.5 and n=7.4 stabilized the average bubble diameter of foam formed with α-lactalbumin but had no effect on foam made from BSA. These results indicate that procyanidins of medium can lead to an undesirable decrease of protein solubility, but may play a positive role in foam stability.

Key words: protein, polyphenol, proanthocyanidin, isothermal titration calorimetry