Starch can be gelatinised more efficiently by a thermomechanical treatment, if a dialdehyde polysaccharide with a relatively low degree of oxidation is added to the starch. This thermomechanical treatment can be carried out at a temperature below 115 DEG C., or even below 100 DEG C., and with a high throughput rate. With this treatment a granulate or other semi-finished product is obtained that can be shaped to produce an article that has excellent dimensional stability in water. By adding dialdehyde polysaccharide with a low degree of oxidation to starch it is possible to blow transparent films based on thermoplastic starch which have improved mechanical properties.