The effect of citric acid concentration, reaction temperature and time on the esterification of some varieties of yam flour in Nigeria

Citric acid esterification of two accessions of white yam (Dioscorea rotundata), Aro, Efuru and Yellow yam (D. cayanensis) flours were carried out with four different concentrations of citric acid (10%, 20%, 30%, 40%w/v/100 g sample) under varying reaction temperature and time (110 °C for 2 h, 110 °C for 3 h, 120 °C for 2 h and120 °C for 3 h). The degree of substitution (DS), reaction efficiency (RE) and alpha—amylase digestibility of the products were determined. The DS obtained was 0.011–0.178. The reaction efficiency at 110 °C for 2 h, 110 °C for 3 h, 120 °C for 2 h and 120 °C for 3 h across all samples were (11.33–34.05%), (12.96–52.14%), (26.67–39.94%) and (32.01–65.11%) respectively. For esterification at 120 °C for 3 h, the reaction efficiencies were higher than 50% for almost all samples at a citric acid concentration of 10–30%w/v. However, at 40%w/v, the reaction efficiency decreased to less than 50%. Alpha-amylase digestibility reduced as the concentration of citric acid, temperature and reaction time of the esterification process increased. The least reduction in digestibility (1.15%) was obtained at 110 °C for 2 h at 10% citric acid concentration, while the highest reduction in digestibility (93.42%) was obtained at 120 °C for 3 h at 30% citric acid concentration. From the results obtained, it can be concluded that the concentration of citric acid, reaction temperature and time correlate positively with the degree of substitution and alpha-amylase resistivity, however steric effect reduced the reaction efficiency as the concentration of citric acid dosages increased.