Ca²⁺ and degree of methylation on pectin-proanthocyanidin interactions: Structural, thermal, and rheological insights

Pectins and proanthocyanidins (PA) are known to undergo extensive intermolecular interactions. Nevertheless, the distinct interaction patterns between PA and pectins of varying methylation levels and the regulatory function of endogenous calcium have not been systematically clarified. This study was therefore conducted to reveal the combined effects of the degree of methylation and calcium on their interaction and related physicochemical properties. The non-covalent binding and covalent grafting strength between pectin and PA are higher for low-methoxyl pectin. Benefiting from its high content of residual bound calcium (1226.9 mg/kg), it forms local egg-box-like structures, further promoting non-covalent binding capacity (114.0 mg/g) and covalent grafting rate (3.1%). Correspondingly, the low-methoxyl pectin-PA non-covalent complexes exhibited significantly higher particle size (1579 nm), polydispersity index (0.48), zeta potential (-11.2 mV), and thermal stability (246.6 °C). Its rheological properties were significantly higher than those of the high-/medium-methoxyl complexes and pure pectin. Calcium cross-linking similarly improved the performance of the low-methoxyl pectin-proanthocyanidin covalent complex (CPLM). Nevertheless, thermal induction caused pectin backbone cleavage and PA oxidation, resulting in overall performance inferior to that of non-covalent complexes. These findings provide direct guidance for the precision application of pectin based on the degree of methylation, the improvement of polyphenol bioavailability in food matrices, and the construction of stable systems for thermally processed foods.