Effect of planting and vine-kill timing on sugars, specific gravity and skin set in processing potato cultivars

Quality and storability of potato tubers harvested for storage are affected by their chemical, physiological and physical maturity. The sucrose concentration in potato tubers is indicative of the chemical maturity of the crop and of the potential processing quality of the crop after storage. High reducing sugar concentrations result in undesirable discoloration of fried potato products. Sucrose does not directly contribute to the discoloration of tuber tissue upon frying, but it influences reducing sugar concentrations during storage. Physiologically mature tubers have maximized their dry matter content, resulting in high specific gravities that are desirable for most aspects of potato processing. We examined the effect of different planting and vine-kill dates on the sucrose and glucose concentrations and specific gravity of five processing potato cultivars grown at Hancock, WI, during 2002 and 2003. Although planting date usually had no effect on sugar content at harvest, sucrose and glucose content decreased with earlier planting date at vine-kill in one of two years. Greater sucrose and glucose concentrations and specific gravities were found at harvest with later vine-kill dates. Of particular concern for processing, stem-end glucose concentrations consistently exceeded bud-end glucose concentrations for all cultivars, regardless of the cultural parameters implemented. Physically immature tubers have poor skin set and are prone to skinning and mechanical damage during harvest, which renders them more vulnerable to dehydration and infection by rotting pathogens in storage. Skin set of 'Russet Burbank' tubers in 2003 improved with late vine-kill timing. Our data indicate that chemical maturity does not necessarily correlate with either physiological or physical maturity in processing cultivars, thus rendering the use of cultural practices to improve tuber maturity at harvest problematic.