Hydroxypropylation of tapioca starch

The hydroxypropylated starch was synthesized by the reaction of tapioca starch and propylene oxide in the presence of sodium hydroxide. Two types of the reaction medium, water and the mixture of water and ethanol, were used. Tapioca starch 40% based on the dry starch weight was used for all reactions. In case of the reaction in water, propylene oxide 5-20%, sodium sulfate 8-15%, sodium hydroxide 0.5-2.0% based on the dry starch weight, the reaction time 6-24 hours and the temperature 30-50℃ were varied. When the mixture of water and ethanol was used, no sodium sulfate was added. Various percentages of ethanol in water, 30-70%, were attempted. The degree of substitution of each product was determined by colourimetric method. It was found that the degree of substitution increased when the concentration of sodium sulfate, sodium hydroxide, propylene oxide, reaction time and temperature increased. The best condition of the reaction in water that gave the highest degree of substitution, 0.0927, was sodium sulfate 15%, sodium hydroxide 1.5%, propylene oxide 9% based on the dry starch weight, the reaction time 12 hours and the temperature 40℃. However, the reaction in 70% ethanol in water provided higher degree of substitution, 0.2057. The 1H-NMR spectroscopy was also applied for measuring the degree of substitution. The physical properties of the hydroxypropylated tapioca starch with highest degree of substitution were investigated and compared with the native tapioca starch. It was revealed that its gelatinization temperature was lower and the viscosity was higher. In addition, the granule morphology of the hydroxypropylated starch was nearly the same as the native tapioca starch.