Everyone who grows or stores potatoes for chips or fries knows how challenging it is to deliver tubers that consistently produce light-colored fried products that meet processor and consumer expectations. Many factors contribute to dark color formation, including heat and water stress during tuber development, immaturity or over maturity of the crop at harvest and low temperatures during storage.
Tuber responses to low temperatures that lead to sugar accumulation and dark color formation in fried products are referred to as "cold-induced sweetening." Breeding programs have made steady progress toward maintaining tuber processing quality under a range of growth and storage conditions by developing varieties that are less sensitive to in-season environmental stress and low temperature storage.
Yet few tubers for chip or fry processing are stored at less than 45-48 degrees F because of the risk associated with dark color formation. Instead, tubers are stored at relatively warm temperatures, even though these temperatures are more favorable for the growth and spread of pathogens that cause rot. As described in several recent publications (Bhaskar et al., 2010; Ye et al., 2010; Wu et al., 2011; Liu et al., 2011), sharply reducing the activity of a single enzyme, acid invertase, greatly increases the resistance of potato tubers to cold-induced sweetening and improves the processing quality of existing cultivars stored at temperatures of 39-42 degrees.
Cold Hard Facts
Acid invertase has been implicated for many years as an important factor in cold-induced sweetening and in defects such as sugar-end defect. Acid invertase cleaves one molecule of sucrose into one molecule of glucose and one molecule of fructose. These two reducing sugars react with other compounds in tubers during frying to produce dark-colored pigments and acrylamide. Heath concerns related to acrylamide in food have been recently raised. Efforts are under way to minimize the acrylamide content of processed potato products.
The critical role that acid invertase plays in the accumulation of reducing sugars in cold-stored tubers was highlighted in recent research reports by us and by two other groups. Each group used molecular tools to decrease the amount of acid invertase to a few percent of that found in conventional tubers. When these modified tubers were stored at temperatures as low as 39 degrees F, dramatic improvements in processing quality were observed. This is illustrated in Figure 1, where the dark color of chips made from tubers of Atlantic and MegaChip stand in stark contrast to the light color of chips from genetically modified, low-invertase Atlantic and MegaChip.
Similar results have been obtained with Dakota Pearl, Snowden, Katahdin, E-Potato 3 (a Chinese line) and Ranger Russet when genetic modifications have strongly decreased the amount of acid invertase. In initial greenhouse trials and small-plot field evaluations, no differences were observed between the low-invertase lines and the parent cultivars. Hence, these data indicate it is feasible to improve the low-temperature storage capability of existing cultivars without compromising agronomic traits such as yield, specific gravity and disease resistance.
The observed improvements in chip and fry color correspond to equally dramatic reductions in tuber-reducing sugar content. Figure 2 illustrates the large reduction in tuber glucose observed in our low invertase lines of Atlantic and MegaChip compared to the cultivar checks. Because reducing sugars contribute to acrylamide formation as well as dark color formation, decreasing acid invertase activity can make significant improvements in both of these areas simultaneously. The data so far are encouraging in this regard. When chips were made from low invertase Katahdin (Bhaskar et al., 2010) or Atlantic (Bhaskar et al., 2010) stored at 39 degrees F or fries were made from low invertase Ranger Russet (Ye et al., 2010) stored at 42 degrees F, acrylamide content of the finished products was reduced by 80-90 percent compared to the standard cultivars.
In these recent research reports, the critical importance of acid invertase in reducing sugar accumulation was demonstrated using genetic modification as a research tool. We also showed that some wild species relatives of cultivated potato have very low amounts of acid invertase (Bhaskar et al., 2010). This raises the possibility that the low-invertase trait could be incorporated into cultivated potato through conventional targeted breeding. Although this approach is likely to require more time to implement than direct genetic modification, it avoids the complications that could be associated with introducing a genetically modified variety.