Molar mass influence on pectin-Ca2+ adsorption capacity, interaction energy and associated functionality: Gel microstructure and stiffness

The role of molar mass in determining Ca2+-ion maximum adsorption capacity of polygalacturonic acid (PGA) and low methylesterified citrus pectin (CP) as well as their interaction energy was established through equilibrium dialysis followed by colorimetric determination of free ions. Modeling of the moles of Ca2+-ions bound per mole of galacturonic acid or non-methylesterified carboxyl groups against the equilibrium Ca2+-ion concentration based on the Langmuir adsorption isotherm model, revealed a decrease in maximum adsorption capacity and interaction energy with decreasing molar mass. In this context, monogalacturonic acid exhibited negligible Ca2+-binding potential. At a given molar mass, higher Ca2+-binding was observed for PGA due to a higher number of consecutive non-methylesterified galacturonic acid residues (very low residual degree of methylesterification, DM < 4%) compared to CP (DM 32%). Microstructure and rheology analysis complemented the above results, as evidenced by the higher G', (indicating stiffer systems, based on shear modulus) and a more intertwined network for the high molar mass samples. Extending this to gelling applications, which are based on pectin-cation interactions, revealed that gelation is still possible when PGA/CP systems are subjected to limited depolymerisation while substantial modifications in molar mass lead to a decrease in the elastic behaviour of systems (less stiff). This was evident from the gel-microstructure, physical appearance and gelstiffness results. The PGA and CP of distinct molar masses used in this study were generated through enzymatic depolymerisation (using tomato polygalacturonase) of mother PGA and CP samples, and structurally characterised to confirm modifications in only the molar mass, prior to use.