The effect of heat-moisture treatment on the structure and physicochemical properties of tuber and root starches

Starch from the tubers potato (Solanum tuberosum), taro (Alocassia indica), new cocoyam (Xanthosoma sagitifolium), true yam (Dioscorea alata), and root cassava,(Manihot esculenta) crops was isolated and its morphology, composition and physicochemical properties were investigated before and after heat-moisture treatment (HMT) [100°C, for 10h at a moisture content of 30%). Native starch granules ranged in diameter from 3.0-110 μm and were round to oval to polygonal with smooth surfaces. The total amylose content ranged from 22.4 - 29.3%, of which 10.1 - 15.5% was complexed by native lipid. The phosphorus content ranged from 0.01 - 0.1%. The X-ray pattern of potato and true yam was of the "B " type. Whereas, that of new cocoyam and taro was of the "A" type. Cassava exhibited a mixed "A+B" type X-ray pattern. The relative crystallinity, swelling factor, amylose leaching, gelatinization temperature range and the enthalpy of gelatinization of the native starches ranged from 30-46%, 22-54%, 5-23%, 13-19°C and 12-18 J/g, respectively. Susceptibility of native starches towards hydrolysis by 2.2N HCI and porcine pancreatic ∝-amylase were 60-86% (after 12 days), and 4-62% (after 72h), respectively. Retrogradation was most pronounced in the "B" type starches. Granule morphology remained unchanged after HMT. The X-ray pattern of the "B" type starches was altered ("B"→ "A+B") on HMT. However, that of the other starches remained unchanged. HMT decreased swelling factor, amylose leaching, gelatinization enthalpy and susceptibility towards acid hydrolysis, but increased gelatinization temperatures and enzyme susceptibility. Extent of retrogradation and relative crystallinity decreased on HMT of true yam and potato starches, but remained unchanged in the other starches. The foregoing data showed that changes in physicochemical properties on HMT are influenced by the interplay of crystallite disruption, starch chain associations and disruption of double helices in the amorphous regions