Starch isolated from ND 860-2 potatoes, which were resistant to chillsweetening, had higher amylose and lower amylopectin as well as a higher crystallinity as compared to starch isolated from Norchip po-'tatoes, which were susceptible to low temperature sweetening. ND 860-2 starch exhibited greater resistance to cr-amylase attack and amorphous swelling as well as higher gelatinization temperatures and onset swelling temperatures than starch isolated from Norchip. Examination of starch-granule structure using bright field light micros-CODV indicated crvstalline concentric rings in ND 860-2 not seen in N&hip. Scannini electron microscopy 07 granules incubated with (Yamylase indicated more intact ND 860-2 starch granules compared to Norchip. These data strongly suggest that starch granule composition is a factor differentiating the low-temperature sweetening sensitive cultivar from the resistant potato seedling.
The contribution of sucrose, a nonreducing sugar, to nonenzymatic browning in potato chips was investigated using a model system of buffered sugars and glycine applied to filter paper discs that were then heated in oil. It was found by fiber optic calorimetry that sucrose and the amino acid produced darkening comparable to that of reducing sugars. It is postulated that sucrose enters the reaction by thermal hydrolysis to yield glucose and fructose. Addition of glucose and glycine to potato slices by vacuum infiltration resulted in increased darkening after frying but the sugar proved to be the limiting factor in nonenzymatic browning of potato chips, emphasizing the importance of sucrose in this reaction.
During storage at 4°C and 12"C, a potato cultivar susceptible to chillsweetening (Norchip) accumulated significantly (P10.05) higher levels of sucrose, fructose and glucose than a potato selection resistant to chill-sweetening (ND 860-2). ND 860-2 tubers exhibited a significantly (P10.05) higher respiration rate throughout storage, reflected in higher activities of phosphofructokinase (PFK), glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PdDH). Storage significantly (Plo.05, reduced res$rat& rate for both cultivars. G6PDH showed no significant (P>O.O5) difference in specific activity or V,, between 4y"C and 1pC for either cultivar. However, Km decreased at 4°C for both cultivars, possibly due to build up of substrate.
Sugar extraction and chip preparationA composite chip sample for each cultivar/storage treatment was prepared from 8 representative tubers that were peeled and then sliced into 3 mm cross sections using an automatic slicer. Tuber slices were fried in pure vegetable oil at 140°C to a moisture content of 2%. Agtron (model M30A) color readings were obtained for all chip samples; color scores of less than 50 were deemed unacceptable (Coffin et al., 1987). From these same 8 representative tubers, a 100 g sample of pared tissue was placed in a Braun juicer (Braun Canada Limited, Mississauga, ON). Juice was collected and an equal volume of methanol added. Sugar analysis was performed in triplicate for each treatment according to the HPLC method of Wilson et al. (1981). Analyses were done in triplicate for each treatment.
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