Table 1 displays the swelling capacity q and the solubility S at 92 C. It was attempted to determine q also at ambient temperature, but the amount of swelling was too low to obtain reliable results with the available sample size. Our results show that q in hot water is dramatically reduced on heating through the M transition. This, together with the microscopic observations discussed above, indicates that heating with 16% water in the DSC may provoke similar effects as a pronounced heat-moisture treatment (Jacobs & Delcour, 1998). S is also reduced by the thermal treatment, but increases somewhat when heating through the M2 transition. We observed a distinct medium effect on q which is due to the polyelectrolyte effect exerted by the phosphate ester groups in potato starch. This effect is screened by the ions in simulated tap water. The sensitivity of q to the medium diminished as the heat treatment advanced. This is likely a consequence of the increasing degree of physical cross-linking (Flory, 1953), but a removal of the
phosphate groups by thermal de-esterification could play a role as well