The effect of pH on the chemical and structural interactions between apple polyphenol and starch derived from rice and maize.

To date, how pH affects starch-polyphenol mixtures has not been thoroughly investigated. This study explored the impact of combining apple polyphenol (AP) with both normal rice starch (NRS) and normal maize starch (NMS) across a range of pH conditions. NRS-AP mixture particle sizes across a pH range of 3-8 varied from 169.9 ± 5.4 to 187.5 ± 6.9 μm, while for NMS-AP particles, these sizes ranged from 161.8 ± 8.0 to 176.0 ± 4.9 μm, indicating that the aggregation of starch-AP was inhibited under low pH condition. The melting enthalpy (△H) values of the NRS-AP mixture across a pH range of 3-8 were 8.50 ± 0.06-9.56 ± 0.12 J/g, while the corresponding value for the NMS-AP mixture was 5.77 ± 0.05-6.21 ± 0.08 J/g. FTIR analyses revealed that the degree of order of these starch-AP mixtures significantly decreased under low pH conditions. XRD analysis further revealed that both NRS-AP and NMS-AP mixtures exhibited V-type structures, and relative crystallinity levels decreased significantly under low pH conditions. Together, these results indicate that low pH values inhibit the recrystallization of NRS-AP and NMS-AP mixtures. Overall, these findings provide additional evidence regarding the interactions between AP and specific starches under a range of pH conditions.