Potato starch has been acetylated in a counter rotating twin strew extruder using vinylacetate and sodium hydroxide. The desired starch acetylation reaction is accompanied by an undesired parallel base catalysed hydrolysis reaction of vinylacetate and a consecutive hydrolysis reaction oi. the acetylated starch. Also deacetylation may occur. During the extrusion process sodium hydroxide, vinylacetate, granular potato starch and water were supplied to the extruder. Conversions up to 100 % could be achieved. The degree of substitution could be varied from 0.05 to 0.2. Selectivities from vinylacetate towards starch of up to 80% could be achieved. From reaction kinetics and experiments it could be concluded that the deacetylation reaction could be neglected. The degree of gelatinisation appeared to be a major key parameter. Increasing the gelatinisation in an early stage of the extrusion process, at high starch concentrations and temperatures, increases the selectivity and degree of substitution. The gelatinisation degree can be increased by applying high shear and temperatures to the starch granule and by using screws with modified geometries. [References: 23]
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