Mechanism of gel formation by low methoxyl pectins

Abstract

SummaryFor stable gel formation, low methoxyl pectin (LMP) should contain at least 50% of the galacturonic acid (GA) in the free carboxylic form. This is achieved by precipitating acid and sodium hydroxide deesterified LMP at pH 0.5, 1.5 or 3.0, and ammonia deesterified LMP at pH 0.5 or 1.5, but not at pH 4.5. With such LMP, depending on the concentration of calcium added and methoxyl content, the sol becomes gel. The changes are classified into five regions: (i) sol, (ii) soft gel, (iii) good gel, (iv) brittle gel and (v) coagulated gel.The mechanism of gel formation by LMP involves the reaction of 40–50% of the GA present in the free carboxylic form with calcium which causes the precipitation of 75–90% of the total LMP including the entire galacturonic acid units present in the esterified form. The LMP so precipitated, holds 50–60 g of water per gram and forms a stable gel. Higher concentrations of calcium than the optimum increases the reactivity to 80–90% which results in strong cross linkages, incipient precipitation of LMP and loss of gel state.