Plant phenolic compounds are physiologically active substances widely distributed in the plant matrix, acting as signals or plant defense mechanisms in protective (pericarp, etc.) and nutritive (germ, etc.) tissues of cereals, fruits and vegetables, providing physical and chemical barriers to pathogen invasion, deterring insect and animal attack, and having physiological functions such as antibacterial and antioxidant properties, second only to cellulose, hemicellulose and Lignin. Phenolic compounds are secondary plant metabolites (e.g., phenolic acids, flavonoids, etc.) formed by the substitution of one or more hydroxyl groups (methoxy) for hydrogen atoms on the benzene ring of aromatic hydrocarbons.
Phenolic compounds exist in different forms in plants, either as free phenolic compounds or as bound phenolic compounds formed by interaction (conjugation) with cellulose, proteins, lignin, etc. through ester, ether and carbon-carbon double bonds, or as plant substrates due to ionic bonding with dietary fibers, or embedded in cells.
The bioactivity of phenolic compounds in different forms in plants needs to be reflected by their bioavailability (release (bioaccessibility), absorption, metabolism and circulation) for human health effects. Bioavailability is the degree to which a bioactive component is absorbed into the circulatory system and thus becomes bioactive in the target tissues and organs. Bioavailability, in turn, refers to the amount of bioactive compounds that may be absorbed by the intestine, and thus bioavailability is strictly dependent on bioavailability (release).
The bound phenolic compounds present in plants are formed by combining macromolecules (proteins, carbohydrates, etc.) with phenolic compounds, which are broken down and released by the action of microorganisms in the large intestine (colon). The released bound phenolic compounds pass through the epithelial cells of the intestine and enter the blood circulation, where they are carried by the blood to various organs for action, and therefore the bound phenolic compounds are highly bioavailable.
Esterified, glycoside and insoluble bound phenolic compounds extracted from tea oil showed higher antioxidant activity than free phenolic compounds in DPPH, ABTS, FRAP and ORAC assays, and the antioxidant activity increased with increasing content.
Phenolic compounds play a defensive role in plants and are considered to have broad-spectrum antimicrobial properties, not only slowing down (or even stopping) the growth of pathogenic bacteria, but also limiting the production of toxins by pathogenic bacteria.
Inflammation that may cause disease in the body is triggered by pro-inflammatory factors (endogenous peptides) produced by cellular immune responses, including nitric oxide (NO), TNF-α and interleukins. The presence of bound phenolic compounds in plants can prevent inflammation in several ways: by reducing pro-inflammatory cytokines and tumor necrosis cells; by enhancing the expression of anti-inflammatory factors (IL-10); and by modulating inflammatory signal pathways (NF-kB and MAPK) in such a way that they become one of the effective anti-inflammatory substances.
Naturally bound phenolic compounds have inhibitory effects on α-amylase and α-glucosidase, etc., which can reduce the absorption of simple sugars in the intestine and thus achieve blood sugar control.