POTATO GENETICS, CYTOGENETICS, DISEASE RESISTANCE, AND PRE-BREEDING UTILIZING WILD AND CULTIVATED SPECIES
Location: Vegetable Crops Research Unit
Title: Potato Processing from Low Temperature Storage
Submitted to: Potato Grower
Publication Type: Trade Journal
Publication Acceptance Date: July 15, 2011
Publication Date: September 1, 2011
Citation: Bethke, P.C., Jiang, J. 2011. Potato Processing from Low Temperature Storage. Potato Grower. 40:20-21.
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. Potato breeding programs have made steady progress toward maintaining tuber 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 commercially at less than 45-48°F because of the risk associated with dark color formation. Instead, tubers are stored at relatively warm temperatures that are more favorable for the growth and spread of pathogens that cause rot. As described in four recent publications, sharply reducing the activity of a single enzyme greatly increases the resistance of potato tubers to low temperature sweetening and improves the processing quality of existing cultivars.
Acid invertase is the enzyme that cleaves one molecule of sucrose into one molecule of glucose and one molecule of fructose. Glucose and fructose are both reducing sugars, and they react with other compounds in tubers during frying to produce dark-colored pigments and acrylamide. Heath concerns related to acrylamide in food have been raised recently, and efforts are underway to minimize the acrylamide content of processed potato products. Acid invertase has been implicated for many years as an important component of tuber responses leading to reducing sugar accumulation in cold-induced sweetening and in defects such as sugar end defect.
Recent research reports from three separate groups have highlighted the critical role that acid invertase plays in reducing sugar accumulation. Molecular tools were used to reduce the amount of acid invertase to a few percent of that found in conventional tubers. When these modified tubers were stored at low temperatures, as low as 39°F, dramatic improvements in processing quality were observed. This is illustrated in Figure 1, where tubers from cultivar Atlantic and genetically modified Atlantic were stored at 38°F for one month prior to frying. Chips from the standard Atlantic tubers were dark and commercially unacceptable, whereas chips from the low-invertase Atlantic were light-colored and commercially acceptable. Similar results have been obtained with Dakota Pearl, Snowden, MegaChip, Katahdin, E-Potato 3 (a Chinese line) and Ranger Russet when similar genetic modifications have been used to strongly decrease 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 that 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. 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 chip were made from cold-stored, low invertase Atlantic or fries were made from Ranger Russet stored at 42°F, acrylamide content of the finished products was reduced by 80-90% 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. One report showed that some wild species relatives of cultivated potato also have very low amounts of acid invertase. This raises the possibility that the low-invertase trait could be incorporated into 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.