Bound Forms of Phenolic Compounds and their Bioavailability

Abstract

In this review, the properties of bound (unextractable) phenolic compound forms in food structure, protein-phenolic and carbohydrate-phenolic relationships, metabolism and bioavailability in digestion are discussed. Phenolic compounds in foods are determined by conventional methods using organic solvents. However, bound forms which remain in the extraction residue and constitute a significant portion of the total phenolic compounds are ignored. Therefore, the total phenolic content of foods, especially those with high phenolic content, has not been accurately determined in the past. Bound phenolic compounds which account for 20-60% of the total phenolic content of fruit, vegetables, cereals and legumes are now taken into account in determining the phenolic composition. Bound phenolics can be covalently attached to the cell wall by structures such as cellulose, pectin, protein by ester, ether or acetal bonds. Due to their aromatic rings and hydroxyl groups, phenolic compounds have the ability to bind to the cell wall and to structures such as protein, carbohydrate, lipid by hydrophobic and hydrophilic interactions, hydrogen and covalent bonds. There are many factors in the interaction of phenolics with macromolecules such as molecular weight, degree of polymerization, and number of aromatic groups. This complex is known to have many positive effects on human health. The bioavailability of phenolic compounds in foods depends on their release, absorption and passage into the bloodstream during digestive or intestinal fermentations. In particular, polyphenols in certain foods bind to macromolecules such as proteins, carbohydrates, lipids in the cell wall structure and greatly affect the bioavailability of the gastrointestinal tract. Due to the difficult digestion of cell wall materials, bound phenolic compounds can reach the colon without alteration in the gastrointestinal tract.