*3.6. In Vitro Digestibility*

As presented in Figure 5, the influences of three types of PPC on the RDS decrease (by 6.06% of PPC1, 9.93% of PPC2, and 13.68% of PPC3) and SDS increase (by 13.41% of PPC1, 21.90% of PPC2, and 20.18% of PPC3) were all significant (*p* < 0.05). PPC3 also showed a prominent effect in the improvement of RS content, with 16.35%. These results indicated that PPC could effectively hinder the digestibility of potato starch, and the inhibition impact was closely associated with the DP of PPC. In general, the replacement of PoS with PPC3 had the strongest suppression of starch digestibility among the three.

**Figure 5.** Influence of PPC on in vitro digestibility of potato starch. RDS, rapidly digesting starch; SDS, slowly digesting starch; and RS, resistant starch. PoS, potato starch and PPC, polymeric proanthocyanidin. Different colors of lowercase letters stand for significant differences (*p* < 0.05).

For the following probable reasons, on the one hand, PPC could bind to the amino acid residues and non-competitively inhibit the catalysis of α-amylase and α-glucosidase, mainly through hydrophobic interaction, thus reducing the activity of enzymes [56]. As the DP value of PPC increases, the molecule hydrophobicity tends to be enhanced, which is advantageous to the formation of multiple interaction sites between PPC and amylase [9,11]. Zhou et al. [57] explained that the affinities for α-amylase varies among procyanidins with different DPs, mainly due to their different stabilization on the surface near to the catalytic core of α-amylase. In addition, the low swelling degree was unfavorable to the interaction of enzymes with the starch particles, thereby reducing the content of RDS. On the other hand, much of the literature has showed that starch in the presence of polyphenols might form a nonordered crystal structure with a slow digestion characteristic [5,58]. However, large-molecular-weight proanthocyanidin was likely to provide more hydroxyl groups and hydrophobic domains to favor the interactions with starch molecules. Based on this, by adding candidates of PPC, it was expected to produce natural anti-diabetic drugs or chronically digestive starch that are conductive to postprandial glucose control.
