the invention relates to the use of biodegradable graft copolymers producible in the presence of empirically determined quantities of water from polyvinyl alcohols with a degree of saponification of 88 to 99 . 5 mol . % by an acid catalysed acetalisation and / or transacetylisation reaction with carbohydrates containing carbonyl groups or presumptive carbonyl groups , in which the degree of acetqalisation , expressed in carbonyl equivalents per 100 vinyl alcohol equivalents , is between 0 . 2 and 20 , and preferably between 1 and 10 , as a plasticiser for the thermoplaticisation of starch and / or starch mixtures .