The susceptibility of large and small granules of waxy, normal and high-amylose genotypes of barley and corn starches toward amylolysis at sub-gelatinization temperatures

Abstract Starches from waxy (CDC Candle), normal (CDC McGwire) and high-amylose (SH 99250) genotypes of hull-less barley were isolated and fractionated into large and small granules. The unfractionated and fractionated barley starches were then characterized toward their composition, morphology and structure, and compared with commercial corn starches of similar genotypes. The effect of starch properties on degree of hydrolysis (DH) at 55 °C for 1 h followed by at 30 °C for 72 h using granular starch hydrolyzing enzymes (mixture of α-amylase and glucoamylase) was also evaluated. Morphological changes of starches before and after hydrolysis were visualized by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). The proportion (% weight) of small granules in high-amylose (HA) genotypes of barley (46.5%) and corn (32.4%) starches were higher than in the waxy (WX) and normal (NM) genotypes. Apparent amylose (AM) content of corn genotypes ranged from 1 to 1.1% (WX), 21 to 26% (NM), and 66 to 70% (HA). Whereas, the corresponding ranges for barley genotypes were 4–6% (WX), 17–24% (NM), and 35–40% (HA). HA corn starch exhibited a mixed ‘A + V’-type X-ray pattern, whereas the other genotypes of corn and barley starches exhibited an ‘A’-type X-ray pattern. The relative crystallinity (RC) of the genotypes of corn and barley starches followed the order: WX > NM > HA. The AM content and RC of small granules in corn and barley starches were lower than in large granules. Regardless of genotypes and starch sources, smaller granules had a higher gelatinization temperature range (15.5–42.8 °C) and a lower enthalpy of gelatinization (9.7–21.2 J/g) than those of larger granules (11.1–41.9 °C and 13.4–23.9 J/g, respectively). Compared to large granules, the small granules were initially hydrolyzed to a greater extent (18% higher). However after 72 h, hydrolysis was 10% higher in the large granules. SEM and CLSM images revealed that the distribution of surface pores and internal channels varied between genotypes of corn and barley starches. Although small variations exist between the NM corn and NM barley starches with respect to their proximate composition and amylose content, the observed large variations in the extent of amylolysis at the initial stages of hydrolysis is indicative that the molecular architecture and granule porosity influence amylolysis.