Impact of heat-moisture treatment of pulse starches on their thermal, rheological and digestibility properties

Heat-moisture treatment [HMT] changes the physicochemical properties of starches by facilitating starch chain interactions within the amorphous and crystalline domains. The influence of HMT on the structure and properties of cereal and tuber starches that contain pure A- and B-type crystallites is well documented. However, similar studies on C-type pulse starches that contain varying proportions of both A- and B-type crystallites is fragmentary. Furthermore, the extent to which amylose [AM] chains influence structural changes on HMT is still dispute. The objectives of this study was therefore, to unravel the structural changes (at molecular and supramolecular levels) that occur in fababean[FB], blackbean[BB] and pintobean[PB] starches on HMT (~23% moisture) at 80°, 100° and 120°C, and their impact on physicochemical properties.

Structural changes on HMT were monitored by microscopy, HPAEC-PAD, ATR-FTIR, WAXS, ¹³C-CPMAS/NMR, DSC, RVA and susceptibility towards acid and enzyme hydrolysis. The study showed that native starches, did not differ significantly with respect to amylopectin [AP] chain length distribution [CLD], double helical content and chemical composition. However, they differed significantly with respect to crystalline stability, radial orientation of crystallites, polymorphic composition, molecular order, thermal stability, granular swelling, amylose-leaching and gelatinization properties. In all starches, HMT increased crystallinity (FB>BB>PB), gelatinization temperatures (FB~PB~BB), gelatinization temperature range (FB>BB>PB), slowly digestible starch[SDS] (FB~BB>PB) and resistant starch[RS] (FB>PB>BB) contents, and decreased the amount of B-type crystallites (FB~BB~PB). The extent of acid hydrolysis, APCLD and gelatinization enthalpy remained unchanged in all starches on HMT. The results of this study showed that the structural reorganization of starch chains during HMT at different temperatures was greatly influenced by: 1)changes to starch chain mobility, 2)AM-AP interactions and 3)crystalline stability of the native granules. The increase in thermal stability, SDS and RS, and the decrease in peak viscosity and set-back on HMT would be of interest to food processors and nutritionists.