6.7.1. Tannins

Proanthocyanidins are mixtures of oligomers and polymers of flavan-3-ol linked through carbon bonds, mainly C4 → C8. Tannins are divided into two groups, hydrolysable tannins and condensed tannins or proanthocyanidins (PAC). Hydrolysable tannins are derived from gallic acid and include gallotannins and ellagitannins. PACs are mixtures of oligomers and polymers of flavan-3-ol. Depending on the interflavan carbon–carbon bond, PACs could be A-type or B-type; depending on the degree of polymerization (DP), they are known as oligomeric (≤10) and polymeric proanthocyanidins (>10). In almond, most PACs are polymeric [130]. In addition, flavan-3-ol composition should be considered to determine PACs. The intrinsic complexity and diversity of almond proanthocyanidins, as well as a lack of available standards, pose analytical challenges [107].

Information about tannins is very limited and only a few studies quantify these compounds [81,107], even though PACs are the most abundant polyphenols in almond kernel, followed by flavonoids and phenolic acids. No references have been found about the presence of soluble tannins. Almond proanthocyanidins consist mainly of epicatechin and catechin, with lesser amounts of epiafzelechin. In the opinion of Bolling et al. [81,107], the cis–transconfiguration, A-/B-type ratios and flavan-3-ol types of almond PACs have not been adequately characterized.

PACs consisting exclusively of epicatechin are procyanidins (PCs). PACs containing epiafzelechin as subunits are named propelargonidins (PPs). When subunits are epigallocatechin, they are named prodelphinidins (PDs) [124].

Gu et al. [124] presented the concentrations of monomers, dimers and trimers separately because these low-molecular weight PACs oligomers (DP ≤ 3) could be absorbed intact in the gastrointestinal tract; meanwhile, PACs with DP > 3 appear not to be absorbed directly from the gastrointestinal lumen.

Table 6 summarizes the variability in tannins found in the reviewed papers published.
