Non-extractable Polyphenols in Fruit: Distribution, Changes, and Potential Health Effects
Different fruits contain significant amounts of non-extractable polyphenols (NEPP). NEPP are strongly associated with cell wall materials including pectin, hemicellulose, and lignin, which are important components of dietary fibre. The content of NEPP and the proportions they represent of total polyphenols vary considerably from fruit to fruit. Furthermore, drying and thermal processing affect the association of polyphenols to cell wall fibres, resulting in different proportions of NEPP. After intake, the fibre components of fruit, with their associated NEPP, reach the colon nearly intact. There, the NEPP–fibre complex may have health-promoting effects. In combination with the fibre, the associated NEPP: (1) bind bile acids, leading to excretion of the acids and a reduction in cholesterol; and (2) stimulate colonic fermentation, resulting in increased levels of bioaccessible polyphenols, phenolic metabolites, short-chain fatty acids, and beneficial bacterial species. The NEPP and dietary fibre may act synergistically in these effects, which are related to gastrointestinal and cardiovascular health. Additionally, the slow and continuous circulation of phenolic metabolites and the enhanced bioavailability by the accompanying prebiotic fibre may be unique benefits of NEPP. Thus, NEPP play important roles in the health benefits derived from eating fruit.