Effect of genotype, malt pretreatment and cooking on in vitro protein digestibility and protein fractions of corn

Twelve corn genotypes, designated as: 12, 14, 22, 35, 36, 37, 41, 42, 43, 91, 113 and Mugtama 45 were used in this study. Investigation showed that corn had 4.3–6.7% moisture, 1.0–2.0% ash, 1.3–2.2% crude fibre, 4.9–6.2% fat, 11.3–16.9% crude protein, 74.7–81.1% carbohydrate, 256–436 mg/100 g phytate, 12.6–16.9% (2 h) in vitro protein digestibility (IVPD) before cooking and 10.4–13.7% IVPD after cooking. The percentages of the protein fractions: albumin + globulin, prolamin (zein), G1-glutelins, G2-glutelins, G3-glutelins and insoluble protein were in the ranges of 16.8–22.7, 31.9–50.3, 4.9–11.3, 3.0–6.9, 10.8–21.9 and 6.0–16.2%, respectively. The genotypes 91, 22 and 43 were rich in the globulin+albumin fraction and consequently could be considered as having higher nutritional quality than other genotypes. Corn seeds of cultivar Giza 2 were germinated for 6 days to obtained 2-, 4- and 6-day old malts. Corn malt was added in concentrations of 4, 8 and 12% to corn flour. The mixtures were incubated with shaking for 2 h. The protein solubility fractions for cooked and uncooked flour indicated that the albumin+globulin fractions increased significantly (P⩽0.05) by increasing concentration and age of the malt, accompanied by a decrease in the prolamin zein fraction. Cooking significantly (P⩽0.05) decreased the albumin+globulin fraction but the rate of reduction was comparatively lower with increasing age and concentration of the malt. For all treatments, the G1-glutelin and residual protein increased significantly (P⩽0.05) while G2-glutelin and G3-glutelin decreased.