Interactions between citrus pectin and Zn2+ or Ca2+ and associated in vitro Zn2+ bioaccessibility as affected by degree of methylesterification and blockiness

Abstract

Insight into the effect of the degree and pattern of methylesterification of citrus pectin on the Ca2+ and Zn2+ adsorption capacity and the associated Zn2+ bioaccessibility was established. Pectins with comparable methylesterification degrees (DM) but different patterns of methylester distribution (blockwise or random) were generated. Blockwise distributions of methylesters were produced through controlled enzymatic means (using carrot pectin methylesterase) while random patterns were obtained through alkaline demethylesterification (using NaOH). The pattern of methylesterification was estimated as the absolute degree of blockiness (DBabs), which is the ratio of non-methylesterified units present in blocks to the total number of galacturonic acid units. Determination and modeling (based on the Langmuir adsorption model) of the adsorption isotherms of these structurally modified pectins for Zn2+ or Ca2+ allowed quantification of their maximum cation binding capacity and interaction energy. Decreasing pectin DM and increasing DBabs were shown to promote the Zn2+ or Ca2+ binding capacity of pectin, with the maximum binding capacity being mainly determined by the DM and interaction energy by DBabs. The influence of cation type was evident, with higher maximum binding capacities and interaction energies exhibited for Zn2+ compared to Ca2+, mainly due to the higher electronegativity of the former. Increased Zn2+ binding to pectin was associated with lower Zn2+ bioaccessibility and no influence of low levels of added Ca2+ was observed. However, enzymes and bile salts added during in vitro simulated digestion were shown to bind substantial amounts of Zn2+ ions.