Reactivity of flavanols: Their fate in physical food processing and recent advances in their analysis by depolymerization

Abstract

Flavanols, a subgroup of polyphenols, are secondary metabolites with antioxidant properties naturally produced in various plants (e.g., green tea, cocoa, grapes, and apples); they are a major polyphenol class in human foods and beverages, and have recognized effect on maintaining human health. Therefore, it is necessary to evaluate their changes (i.e., oxidation, polymerization, degradation, and epimerization) during various physical processing (i.e., heating, drying, mechanical shearing, high-pressure, ultrasound, and radiation) to improve the nutritional value of food products. However, the roles of flavanols, in particular for their polymerized forms, are often underestimated, for a large part because of analytical challenges: they are difficult to extract quantitatively, and their quantification demands chemical reactions. This review examines the existing data on the effects of different physical processing techniques on the content of flavanols and highlights the changes in epimerization and degree of polymerization, as well as some of the latest acidolysis methods for proanthocyanidin characterization and quantification. More and more evidence show that physical processing can affect content but also modify the structure of flavanols by promoting a series of internal reactions. The most important reactivity of flavanols in processing includes oxidative coupling and rearrangements, chain cleavage, structural rearrangements (e.g., polymerization, degradation, and epimerization), and addition to other macromolecules, that is, proteins and polysaccharides. Some acidolysis methods for the analysis of polymeric proanthocyanidins have been updated, which has contributed to complete analysis of proanthocyanidin structures in particular regarding their proportion of A-type proanthocyanidins and their degree of polymerization in various plants. However, future research is also needed to better extract and characterize high-polymer proanthocyanidins, whether in their native or modified forms.

Directed Accumulation of Nitrogen Metabolites through Processing Endows Wuyi Rock Tea with Singular Qualities

“…Flavonol glycosides and flavonoid glycosides were also promoted through the acceleration of oxidative reactions of flavonols and flavonoids with glycosyl groups [ 18 ] which further resulted in the increase of TF and the reduction of SS ( Figure 2 A,C). Subsequently, hydrothermal reactions played a dominant role in the pan-firing stage under the conditions of high temperature and moisture [ 12 , 19 ]. During this stage, the concentrations of TF were reduced through flavonoid glycoside hydrolysis ( Figure 2 A) [ 12 ], while those of SS and FAA increased via the hydrolysis of cellulose and protein, respectively ( Figure 2 C,D) [ 20 ].…”

“…The trend in TP changes corresponded closely with changes in EGCG, ECG, EGC and GC ( Figure 2 H–K), indicating these compounds function as the dominant catechins contributing TP levels in processed tea. It has also been reported that the bioavailability of EGCG, ECG and EGC may be higher than that of GC and GCG, which are respective epimerization products of EGC and EGCG in high-temperature roasting [ 12 , 19 ].…”

Potential Application of Tea Polyphenols to the Prevention of COVID-19 Infection: Based on the Gut-Lung Axis

“…For example, compared with catechin monomers, its oligomers have higher antibacterial properties, which may be attributed to the polymers having more phenolic hydroxyl groups and benzene ring structures and having a more substantial binding ability with proteins ( 48 ). However, compared with catechin oligomers, catechin polymers and polymers have weaker bacteriostatic effects, which may be because the molecular weight of catechin polymers increases with the increase of the degree of polymerization, making it challenging to penetrate bacterial cell membranes ( 49 ). In addition, affected by the steric hindrance effect of macromolecules, the activity of the phenolic hydroxyl group is weakened, resulting in a decrease in the antibacterial ability.…”

Interactions between Salivary Proteins and Dietary Polyphenols: Potential Consequences on Gastrointestinal Digestive Events

“…Among reactions involving polyphenols, oxidation is the one occurring the most frequently during plant raw material transformation . From a chemical point of view, oxidation of polyphenols can be summarized by the ability of the phenolic nucleus to lose an electron or a hydrogen atom leading to the formation of reactive species.…”