Cooked potatoes have a wide range of food applications, but the mechanism by which softening occurs on heating is not clearly understood. Heating potato parenchyma tissue results in two independent, concurrent events; weakening of the binding between cells and swelling of intra-cellular starch. Potato plants containing starches with a range of high amylose contents and reduced swelling properties were available. This provided the opportunity to separate cooking effects of inter-cellular pectin from swelling of intra-cellular starch. Their individual contribution to the separation of cells and the softening of cooked potato tissue was established by studying the influence of heat on the material properties of a range of starch-modified potatoes. For all potato lines studied, the strength of the heated tissue decreased markedly following 30 minutes at 80degreesC or 5 minutes at 100degreesC. Microscopy of the line in which there was minimal starch swelling, indicated that the cells of the cooked tissue principally contained fluid, in contrast to the controls in which the cells were filled with swollen starch on cooking. Since all the lines followed the same trend with regard to the thermal weakening of the tissue, we conclude that weakening of potato tissue on cooking is primarily controlled by thermal degradation of the middle lamella.