Effect of steam jet cooking on the destruction of corn starches

Abstract

Steam jet cooking has been used for years to prepare aqueous starch dispersions for food application. The steam jet cooking generates high shear stress to starch. The objective of this research is to study the effect of shear stress on structure of corn starch granules by steam jet cooking. A laboratory scale steam jet cooker has been established with flow rate about 1L/min. Three kinds of corn starch, waxy, regular, and high amylose were used. Starch slurries (5% w/w) were cooked by steam jet cooker at temperature 100°C (SJ100), 120°C (SJ120) and 135°C (SJ135) compared with hot water boiling at 90°C 30min (HB). The insoluble particles of cooked starches were investigated by particle size analyzer, scanning electron microscope (SEM) and damage starch assay kit. There was a significant decrease in percent yield of insoluble particles of cooked dispersions for all starches in the order HB > SJ100 > SJ120 > SJ 135.The data also showed that SJ100 has higher destruction than HB, although the temperature of heat treatment was similar. The particle size of cooked starches was much larger than uncooked starches, and the particle size of HB was larger than SJ100 for all starches. This may be due to the time of cooking, 30min for HB vs. a few sec for SJ100. Percentage damaged starch of cooked dispersions for all starches became higher with increasing of cooking temperature. Although, the time of heat treatment was much shorter for SJ100 than HB, the damaged starch was higher as well. Those date all revealed the effect of shear stress from steam cooking. On SEM observation, the damaged granules showed sponge like structure for the starch dispersions cooked by HB. The starches heated by steam jet cooking were fractured into small fragments.