Calcium cation triggers and accelerates the gelation of high methoxy pectin

The gelation of high methoxyl (HM) pectin has been found to be affected by Ca2+, but the acting mechanism of the Ca2+ in the gelation is still unclear. In order to explore the role of Ca2+ in the gelation of HM pectin, aqueous systems composed of HM pectin and CaCl2 were investigated in comparison with alginate-CaCl2 systems through a Ca2+-concentrating gelling process (CCGP) that is our newly developed gelation method based on water evaporation. A HM pectin solution (1%) formed gels in the presence of Ca2+ (2.72 μmol/mL) during water evaporation manifesting the participation of Ca2+ in the CCGP of HM pectin. Additionally, the same amount of Ca2+ induced more dramatic changes in the flow index n, consistency index k, and loss angle δ of the HM pectin samples than those of the alginate samples suggesting that the molecular interactions of HM pectin are likely strengthened by Ca2+. After similar percentages of water were evaporated, the HM pectin-Ca2+ sample showed a δ value of 29° while the δ value of the Ca2+-free HM pectin sample was 65°, indicating that Ca2+ triggers and accelerates the gelation of HM pectin. The absence of an optimal concentration ratio of HM pectin to Ca2+ and the high Ca2+ content released from the HM pectin gel suggest that the structuring of HM pectin molecules by Ca2+ is realized through a non-specific binding and an electrostatic screening effect facilitating their hydrophobic interactions during CCGP to form gels.