Non-covalent and covalent associations of polyphenols with food macromolecules are two of the most fundamental factors affecting the quality of polyphenol-rich food products. This review therefore describes the biochemical bases of associations between polyphenols and macromolecules, that is, proteins and polysaccharides. Our intent is to provide a level of understanding that can be used to underpin future research directions. This will help to resolve existing issues that limit organoleptic and nutritional qualities of polyphenol-rich foods and drinks. It will also allow a better understanding of the functional consequences of these interactions on food/biological systems. The methods used to study non-covalent and covalent interactions are described, and the limiting factors of each method are emphasized. The biochemical mechanisms of interaction between polyphenols and macromolecules are also described. In processed food, non-covalent polyphenol/macromolecule interactions are largely due to weak associations, and result from a combination of hydrogen bonds and hydrophobic interactions. The biochemical mechanisms for covalent interactions involve oxidation of phenolic compounds, whether enzymatically mediated or not, with the formation of o-quinones or o-semi-quinones, or the cleavage of procyanidin interflavanic bonds in acid medium with the formation of carbocations. The effects of factors such as polyphenol structure, macromolecule structure, relative concentrations of both polyphenol and macromolecule, solvent composition, ionic strength, temperature, and pH are discussed.

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Interactions between polyphenols and polysaccharides: Mechanisms and consequences in food processing and digestion. a b s t r a c t Background: Interactions between intracellular polyphenols and plant cell-walls have received little attention, due to analytical limitations. It was difficult until recently to analyse the most implicated polyphenols, which are proanthocyanidins (aka condensed tannins), and because these weak interactions were too low for quantification. They are becoming recognized as a factor to understand extractability, functional and health effects of polyphenols. Scope and approach: New approaches that have been used since the turn of the century are binding isotherms and isothermal titration calorimetry. They allow to investigate specifically these interactions, quantify the affinities between cell-walls and polyphenols as well as the impact of fruit maturation or processing, and the consequences on the finished beverages and food. This review will highlight results on this topic since 2001. Key findings and conclusions: The most common polyphenols are phenolic acids and oligo or polymeric flavanols (proanthocyanidins), located inside the vacuole in intact plant cells. The proanthocyanidins bind spontaneously to the plant cell-wall polysaccharides through plant tissue disruption, for example during grinding, mastication or thermal treatments, etc. The highest affinity is observed with pectins, which may help explain some of the effects of maturation on polyphenol extractability, e.g. in wine making. Presence of proanthocyanidins together with the cell-walls in the lower gut further impacts on the production of colonic metabolites. This has profound consequences on the extractability and bioavailability of the polyphenols, on the functional characteristics of extracted polysaccharides, and on the fermentation kinetics of dietary fibers and polyphenols.

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Apple pomace, a by-product of the cider production, has been studied as a potential source of polyphenols, compounds of great interest for the industry. Ultrasound has been used to improve extraction efficiency in terms of time needed and total polyphenol content. A preliminary study has been first investigated to optimize ethanol proportion of aqueous extractant (50%, v/v) and solid/liquid ratio (<15%, w/v). A response surface methodology has then been used to maximize total polyphenol content of extracts and investigate influence of parameters involved in extraction procedures for both total polyphenols content and composition of extracts. Optimal settings reached from a central composite design were applied for ultrasound-assisted extraction and were compared to conventional procedure: yields were increased by more than 20%. Ultrasound-assisted polyphenols extraction from apple pomace appears to be a relevant, rapid, sustainable alternative to conventional procedure, and that scale up of the process is possible.