IN MOST FRUITS AND VEGETABLES, such as apples or carrots, it is desirable to have high levels of sugars. But when we talk about sugar content in potatoes, it is generally in a negative way-we would prefer to have a negligible amount present, especially for those destined for the processing market. But even if you are storing fresh-market potatoes, we cannot completely ignore potential ways sugar can accumulate if any part of the crop goes to a dehydrator, french fry processor or sold to a fresh-cut fry retail restaurant.
There are three dominant sugars in a potato: sucrose, glucose and fructose. The primary reason these sugar concentrations are a concern is due to the reaction of reducing sugars-glucose and fructose-with free amino acids in the presence of heat (frying) to form a darkened color of fried products. Although sucrose is not directly involved in the non-enzymatic darkening reaction, it is a pool or source for reducing sugar accumulation.
Cultivars differ in their genetic potential to accumulate starch, sucrose and reducing sugars. That accumulation will also depend upon temperature and storage conditions. In general, the lower the storage temperature, the greater the reducing sugar accumulation. Some cultivars can be stored at cooler temperatures and still have acceptable sugar levels and fry color because genetically they have very low levels of the sucrose-breaking enzyme acid invertase. For instance, Russet Burbank and Umatilla Russet can be stored at 45 degrees and still maintain processing quality, whereas Western Russet and Ranger Russet need to be held at warmer temperatures (48 degrees) for acceptable frying quality. And even better yet, Premier Russet can be stored at 42 degrees and still have very acceptable fry color and sugar levels.
Although Russet Norkotah potatoes are not typically used to make fried products, they are utilized for dehydrated products and for fresh-cut fries. The lower specific gravity and tendency to rapidly accumulate reducing sugars in storage does not make it a quality cultivar for frying. Disregarding the texture and specific gravity issues, Russet Norkotah may have acceptable glucose levels at harvest appropriate for some processed products.
Comparing Russet Norkotah to Russet Burbank at 45 degree storage, glucose accumulation is much greater with Russet Norkotah, leading to unacceptable levels very quickly. These glucose levels would be much higher if the potatoes were stored cooler such as 39 to 42 degrees. Extremely high glucose levels are of concern for producing dehydrated products and for restaurants purchasing potatoes for fresh-cut fries.
Recent attention has been given to the potential issue of acrylamide levels in consumable food products such as cereal, bread, cookies and fried potato products. Higher acrylamide concentrations are seen in carbohydrate rich foods that are baked or fried. In general, the higher the reducing sugar concentration, the greater probability for acrylamide formation. Lower reducing sugar concentrations generally equate to lower acrylamide levels. The possibility of acrylamide provides another reason to be knowledgeable on sugar metabolism in potatoes and the impact on consumer quality.