Stability of pectins during storage

Low methoxyl pectins (LMP) prepared by HCl, NaOH and NH₃ deesterification procedures were stored with or without adding Na₂CO₃ as buffer at room temperature (25–30°C) or exposed to different relative humidities (r.h.) at 37°C. Losses of methoxyl, molecular weight and gelling power were observed in all the pectin samples. During storage, NaOH and NH₃ deesterified LMP were more stable than HCl deesterified LMP with respect to gelling power. The monolayer values ( V _m) of 0.5 to 0.7 g per 100 g of LMP showed that even when stored at 11% r.h., the moisture in LMP was present in the multilayer region which was conducive to methoxyl loss.
NH₃ deesterified LMP precipitated at pH 0.5 and 1.5 after saponification, formed good gels similar to the LMP precipitated at pH 0.5, dried and mixed with buffer. NH₃ deesterified LMP precipitated at pH 3.0 and 4.5, on the contrary, formed only coagulated gels.
The loss of gelling power of LMP during storage was due to depolymerization besides gradual transformation of increasing portions of pectinic acid molecules into pectic acid molecules. High methoxyl pectins with or without added sodium carbonate or citrate as buffer stored at 62 and 75% r.h. showed methoxyl loss. Between r.h. of < 0.1 and 49%, methoxyl loss was more in the buffered samples than in the control.