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.

The interactions between procyanidins and pectic compounds are of importance in food chemistry. Procyanidins with low (9) and high (30) average degrees of polymerization (DP9 and DP30) were extracted from two cider apple varieties. Commercial apple and citrus pectins, as well as three pectin subfractions (homogalacturonans, partially methylated homogalacturonans with degree of methylation 30 and 70) at 30 mM galacturonic acid equivalent, were titrated with the two procyanidin fractions (at 30 mM (-)-epicatechin equivalent) by isothermal titration calorimetry and UV-vis spectrophotometry. Slightly stronger affinities were recorded between commercial apple or citrus pectins and procyanidins of DP30 (Ka = 1460 and 1225 M(-1) respectively, expressed per monomer units) compared to procyanidins of DP9 (Ka = 1240 and 1085 M(-1), respectively), but stoichiometry and absorbance maxima differed between apple and citrus pectins. It was proposed that methylated homogalacturonans interacted with procyanidins DP30 mainly through hydrophobic interactions. The stronger association was obtained with the longer procyanidin molecules interacting with highly methylated pectins.

Interactions between seven hairy regions of pectins, rhamnogalacturonans II and arabinogalactan-proteins and procyanidins with different average degrees of polymerization, low (DP9) and high (DP30), were investigated by isothermal titration calorimetry and absorption analysis to study the impact of neutral sugar side chains of pectins on these associations. Associations between pectic fractions and procyanidins involved hydrophobic interactions and hydrogen bonds. No difference in association constants between various hairy regions and procyanidins DP9 was found. Nevertheless, arabinan chains showed lower association constants, and hairy regions of pectins with only monomeric side chains showed higher association with procyanidin DP30. Only very low affinities were obtained with rhamnogalacturonans II and arabinogalactan-proteins. Aggregation could be observed only with the procyanidins of DP30 and the protein-rich arabinogalactan-protein. Associations were obtained at both degrees of polymerization of the procyanidins, but differed depending on neutral sugar composition and the structure of pectic fractions.

Procyanidins can bind cell wall material in raw product, and it could be supposed that the same mechanism of retention of procyanidins by apple cell walls takes place in cooked products. To evaluate the influence of cell wall composition and disassembly during cooking on the cell walls' capacity to interact with procyanidins, four cell wall materials differing in their protein contents and physical characteristics were prepared: cell wall with proteins, cell wall devoid of protein, and two processed cell walls differing by their drying method. Protein contents varied from 23 to 99 mg/g and surface areas from 1.26 to 3.16 m(2)/g. Apple procyanidins with an average polymerization degree of 8.7 were used. The adsorption of apple procyanidins on solid cell wall material was quantified using the Langmuir isotherm formulation. The protein contents in cell wall material had no effect on procyanidin/cell wall interactions, whereas modification of the cell wall material by boiling, which reduces pectin content, and drying decreased the apparent affinity and increased the apparent saturation levels when constants were expressed relative to cell wall weight. However, boiling and drying increased apparent saturation levels and had no effect on apparent affinity when the same data were expressed per surface units. Isothermal titration calorimetry indicated strong affinity (K(a) = 1.4 × 10(4) M(-1)) between pectins solubilized by boiling and procyanidins. This study higllights the impact of highly methylated pectins and drying, that is, composition and structure of cell wall in the cell wall/procyanidin interactions.