Improved cold starch hydrolysis with urea addition and heat treatment at subgelatinization temperature

We explore how the presence of urea can influence the kinetics of amylolysis, with a long-term objective of developing practical and energy efficient bioconversion protocols. In this study, triticale and corn starches were hydrolyzed by a granular starch hydrolyzing enzyme with or without addition of urea and a pre-heating treatment at subgelatinization temperature. Differential scanning calorimetry showed that the gelatinization parameters of triticale and corn starches were negatively correlated with the urea concentration in the starch suspension. Addition of urea did not significantly affect starch amylolysis by the granular starch hydrolyzing enzyme at 30 °C. However when pre-heating at a higher yet sub-gelatinization temperature (50 °C for triticale and 61 °C for corn, 5 °C below the onset of starch gelatinization) for 30 min, the presence of urea greatly facilitated the amylolysis of both tricticale and corn starches. Scanning electron microcopy revealed starch granule mophological changes to a porous structure in residual starch granules/fragments rich in resistant starch. This means that the amylolysis pattern in the presence of urea was fundamentally changed, and urea disrupts starch hydrogen bonds effectively with heating treatment at a sub-gelatinization temperature. This treatment combination increased both starch hydrolysis rate and extent. Since extra energy was not necessary to gelatinize starch, this method may benefit starch and bio-enthanol industries to reduce the costs of starch hydrolysis.