Impact of annealing on the molecular structure and properties of waxy starches of different botanical origins

Annealing (ANN) changes the properties of starch by facilitating improved alignment and registration of double helices within the crystalline lamellae and growth and perfection of existing crystallites. This reorganization has been shown to impart crystalline stability, thereby altering starch properties. The mechanism of ANN has been investigated mainly on starches with amylose content in the range of 25-35%. There is dearth of information on the mechanism by which double helices within the crystalline domains of waxy starches attain crystalline perfection on ANN. Therefore, the objective of this study was to unravel the impact of ANN on the molecular structure and properties of waxy corn [WC], waxy barley [WB], waxy rice [WR] and waxy potato [WP] starches. The morphology, structure and properties of native and one-step annealed WC, WR, WB and WP starches were examined by scanning electron microscopy (SEM), HPAEC-PAD, ATR-FTIR, WAXS, ¹³C CP/MAS NMR, DSC, RVA, granular swelling and susceptibility towards acid and enzyme hydrolysis. SEM showed the presence of pores only on the granule surfaces of native WC and WB starches. The average amylopectin (AP) chain length distribution and the proportion of long chains ranged from 15.7 to 17.1 and 25.6 to 30.1, respectively. The X-ray pattern of WP was of the ‘B’ type, whereas other starches exhibited the ‘A’ type pattern. The relative crystallinity (RC), double helical content (DHC), gelatinization temperature range (GTR), enthalpy of gelatinization (ΔH), molecular order at the vicinity of the granular surface and extent of granular swelling ranged from, 33.5 to 42.4%, 35.9 to 51.3%, 10.0 to 13.8 ^oC, 11.3 to 17.5 J/g, 0.797 to 1.014 (1047/1022 ratio) and 20.8 to 31.9% (at 80 ^oC), respectively. WR showed the highest susceptibility towards amylotic enzymes while WP showed the lowest susceptibility. All starches showed a similar acid hydrolysis pattern. In all starches, granular morphology remained unchanged on ANN. Gelatinization enthalpy, X-ray pattern, relative crystallinity, molecular order remained unchanged, whereas granular swelling decreased (WR>WB>WC>WP) in all starches upon ANN. Annealing increased the gelatinization transition temperatures and decreased the GTR in all starches. Peak viscosity, breakdown viscosity and setback viscosity increased in WP, but decreased in other starches (WR>WB>WC) on ANN. Susceptibility towards amylolysis increased in WC and WB, but decreased in WP and WR on ANN. Acid hydrolysis decreased in WP, but remained unchanged in other starches on ANN. The results of this study showed that waxy starches from different botanical origins differed with respect to the extent of structural changes within the amorphous and crystalline domains on ANN.