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ABSTRACT
The effect of processing conditions in an excess steam jet cooker on the degradation of waxy maize starch was studied. The temperature of the steam, the flow rate of the starch slurry, and the concentration of starch were determined to influence the extent of degradation. The viscosity of concentrated solutions of the jet-cooked product and the intrinsic viscosity of dilute solutions were used as measures of the extent of molecular degradation. The viscosity decreased at higher reaction tem
peratures, and at higher steam-to-starch ratios. Multiple passes through the jet cooker decreased the viscosity dramatically for the first two passes, but little additional change was observed for further passes. The results show that mechanical and thermal degradation effects are both important in the jet cooking of waxy maize starch, although the primary effect is due to mechanical degradation.
Steam jet cooking of starch is an effective means of rapidly forming an aqueous starch solution. A slurry of granular starch is brought into contact with high-pressure steam, leading to the gelatinization, disruption, and solubilization of the granules. Jet cookers have been used traditionally to process starch for paper coating (Klein and Brogly 1981) and more recently to form starch-lipid composites (Fanta and Eskins 1995). Previous studies have demonstrated that the processing conditions in the jet cooker will affect the viscosity and molecular weight of the cooked product. This study examines a broader range of processing conditions to relate product differences to specific processing variables.
Dintzis and Fanta (1996) studied the jet cooking of four different starches under two different cooking conditions. At low steam flow rates, all of the steam is condensed as it heats the incoming slurry; this is referred to as gentle, or thermal jet, cooking. At higher steam flow rates, or excess steam cooking, the temperature remains higher in the jet cooker and the flow is turbulent. They showed that the intrinsic viscosities for waxy maize starch, normal maize starch, and waxy rice starch were much lower for samples prepared by excess steam cooking than by gentle cooking. However, a high-amylose maize starch showed only a small difference between the two treatments. The viscosity of 10 wt% cooked starch samples also decreased with increasingly severe cooking conditions. For the...