| Abstract: | Tubers of the potato cultivars Pentland Dell and Record were stored for a period of up to 40 weeks at 5°C and 10°C over four consecutive storage seasons (1989–1990 to 1992–1993). Reconditioning treatments at 20°C were also performed at early, mid and late stages in storage. To assess the turnover of proteins during storage, tubers were analysed for free amino acid and soluble protein content. The net direction of nitrogen flow was dependent on the state of dormancy of the tuber and hence storage duration. An increase in free amino acid content commonly occurred during the latter part of storage, caused by an upturn of proteinase activity on the break of dormancy. This increased enzyme activity was probably due to de novo synthesis of proteinases, in particular of a 47 kDa aspartic proteinase which was purified to homogeneity and shown to have a marked substrate-specificity for endogenous tuber proteins such as patatin. The rate of protein turnover increased with storage temperature, although the net direction of nitrogen flow was temperature independent. However, when nitrogen flow exhibited a distinct direction, such as the protein breakdown in Pentland Dell during late storage, this was enhanced by higher temperatures. The accumulation of free amino acids in Pentland Dell at 10°C corresponded with a deterioration of processing quality that could not be accounted for by an upturn in reducing sugar content. Despite an excess of free amino acids with respect to reducing sugars, amino acids had a probable syn-ergistic influence on fry colour over the later stages of storage resulting in a darker colour per unit reducing sugar than in early storage. Reconditioning treatments were ineffective as a means of lowering the free amino acid pool size, these treatments operating solely through the decrease in reducing sugar content. |
