Changes in sugar contents and acid invertase in potato tubers during low-temperature storage

Low-temperature storage of potato tubers causes an increase in sugars; this phenomenon is known as ‘low-temperature sweetening’ (Isherwood 1973, Burton 1989). The sugar content and the activity of acid invertase in the potato tubers of several Japanese cultivars during storage were investigated. When potatoes were stored at 4 ºC, three types of changes in the sugar content were observed among cultivars, which defined as type-1, type-2, and type-3. During storage, the reducing sugars were increased much higher in type-1, but the increasing trend also was observed in type-2 and it was 4- to 6-fold lower than that of type-1. However, in type-3, the sucrose was increased rather than increase of reducing sugars. On the other hand, the activity of vacuolar acid invertase was increased in the type-1, in contrast, in the type-2 and type-3 cultivars, the activities were very low during storage at 4 ºC. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of acid invertase showed that the transcripts of the enzyme were accumulated in the type-1 whereas the changes were not occurred in the type-3 during storage at 4 ºC. These results suggest that the activity of vacuolar acid invertase is related to the type of changes in the sugar content during low-temperature storage among potato cultivars. The sugars and amino acids contents in the potato tubers and their effects on the acrylamide formation during frying were also investigated. A strong correlation between the reducing sugar content in tubers and acrylamide level in chips was found. Storage temperatures lower than 8 ºC increased the reducing sugar content markedly in potato tubers, resulting in an increase in acrylamide level in chips after frying.