Inhibition of starch digestion by gallic acid and alkyl gallates

As phenolic compounds, alkyl gallates may inhibit the activity of digestive enzymes for starch. Furthermore, their alkyl chains may facilitate starch inclusion complexation and results in an increased resistant starch (RS) content or slowly digestible starch (SDS) content, which may further retard starch digestion. The significance of such inhibition is that the rate of starch hydrolysis into glucose is reduced, thereby preventing hyperglycemia and related metabolic diseases. This study examined the inhibitory effects on in vitro enzymatic digestion of starch by gallic acid and five alkyl gallates of varying alkyl chain lengths, i.e., butyl gallate, octyl gallate, dodecyl gallate, hexadecyl gallate, and octadecyl gallate. Raw and cooked potato starch (PS) and high-amylose maize starch (HAMS) were tested. For both types of raw starch, gallic acid and all the alkyl gallates significantly (p < 0.05) increased RS content except for octadecyl gallate. In the case of cooked starch, the RS contents were markedly decreased (p < 0.05) as compared to those in raw starch, because gelatinization caused an overall greater susceptibility to enzymatic digestion. The reduction in RS content was less in cooked HAMS than that in cooked PS, due to a higher gelatinization temperature range of HAMS. The aforementioned effect of gallates on RS content was also found for cooked PS, while cooked HAMS experienced increased RS only with gallic acid and butyl gallate. Overall, for the digestion of both starches, regardless of raw or cooked, gallic acid and alkyl gallates with shorter chains demonstrated the strongest inhibitory effects. Our findings indicate that shorter alkyl gallates are effective in inhibiting enzymatic digestion of starch and therefore may have potential in modulating glycemic response.