A number of our recent projects have been concerned with the role of cell walls in fruit and vegetable texture. We have worked on onions, tomatoes, broccoli, celery, apples, beets and beansprouts. However from an applied point of view we have been particularly concerned with potatoes, both cooked and processed. The texture of cooked potatoes and all commercial potato products depends on the extent of cell separation that occurs during cooking. In dicot crops like potatoes, intercellular adhesion is mediated by pectins and thermal degradation of pectic galacturonan during the cooking process allows cell separation. Because calcium ions can cross-link pectins it would seem logical that calcium content might be a factor in the control of cell adhesion. That was one reason why we developed the use of EELS elemental imaging to assess calcium localisation, but so far we have not found any connection between texture and calcium content. Other aspects of pectin structure and cross-linking would appear to be more important, and there are indications of a correlation with the nitrogen content of the middle lamella between cells. However in potatoes the dominant factor is the intercellular separation force induced during cooking by starch swelling inside the cells. In freshly harvested potatoes, texture correlates well with starch content although after prolonged storage this correlation tends to break down.

Cooked potato tissue. Right: little cell separation. Left: extensive cell separation due to high starch content