Physical characterisation of high amylose maize starch and acylated high amylose maize starches

Abstract

The particle size, water sorption properties and molecular mobility of high amylose maize starch (HAMS) and high amylose maize starch acylated with acetate (HAMSA), propionate (HAMSP) and butyrate (HAMSB) were investigated. Acylation increased the mean particle size (D4,3) and lowered the specific gravity (G) of the starch granules with an inverse relationship between the length of the fatty acid chain and particle size. Acylation of HAMS with fatty acids lowered the monolayer moisture content with the trend being HAMSB<HAMSA<HAMSP<HAMS, showing that the decrease is affected by factors other than the length of the fatty acid chain. Measurement of molecular mobility of the starch granules by NMR spectroscopy with Carr–Purcell–Meiboom–Gill (CMPG) experiments showed that T2 long was reduced in acylated starches and that drying and storage of the starch granules further reduced T2 long. Analysis of the Free Induction Decay (FID) focussing on the short components of T2 (correlated to the solid matrix), indicated that drying and subsequent storage resulted in alterations of starch at 0.33aw and that these changes were reduced with acylation. In vitro enzymatic digestibility of heated starch dispersions by bacterial α-amylase was increased by acylation (HAMS<HAMSB<HAMSP≤HAMSA) showing that the trend was not related to the length of the fatty acid chain. Digestibility was enhanced with an increase in particle size, or decrease in G, and inversely proportional to the total T2 signal. It is suggested that both external surface area and an internal network of pores and channels collectively influence the digestibility of starch.