In potato tubers patatin is the most abundant protein, it is a 43 kDa glycoprotein with a lipid acyl hydrolase activity. Next, different classes of potato protease inhibitors are present. The content and biological activity of patatin and a fraction of potato protease inhibitors of molecular size 20-22 kDa were monitored as a function of storage time of whole potato tubers. It was observed that the amount of buffer-extractable protein decreased gradually during storage of whole potatoes of the cultivars Bintje and Desiree whereas, for Elkana, after an initial decrease it increased after approximately 25 weeks.Patatin can be divided into two mass isomers, that each can be divided into various isoforms with a slightly differing primary sequence (genetic variants). The isoforms appeared to be of highly homogenous character, therefore, patatin can be studied as a single protein species.Isolated patatin at room temperature is a highly structured molecule at both secondary and tertiary level as indicated by fluorescence, circular dichroism and Fourier transform infrared spectroscopy. Patatin unfolds partly upon heating or lowering the pH. At low pH, when the starting conformation is already irreversibly unfolded to a certain extent, only minor changes occur upon heating. The unfolding of patatin coincides with its precipitation in the potato fruit juice. The acid or heat precipitation of patatin when present in this juice may be enhanced by so far unknown components.The thermal aggregation of patatin was studied by dynamic lightscattering and chromatographic analysis of the proportions of non-aggregated and aggregated patatin as a function of incubation time and temperature. The aggregation of patatin requires the unfolding of the protein and can accurately be described quantitatively with a two-step model. The course of aggregation suggested a mechanism of slow coagulation, limited by both reaction and diffusion.