David H. Picha and Malkeet S. Padda

Sweet potatoes are regarded as one of the most nutritious vegetable crops. They are known to be an excellent source of vitamin A (orange-flesh types) and dietary fiber and contain significant amounts of vitamin C, vitamin E, vitamin B9 (folate) and various minerals. In a recent study, the Center for Science in the Public Interest, a nonprofit, independent organization seeking to improve the public’s health, ranked the sweet potato No. 1 of all vegetables in nutritional content. For many consumers, the decision to purchase sweet potatoes is based the product’s nutritional value and perceived health benefits, in addition to flavor.

To help enhance consumption of sweet potatoes in both domestic and export markets, postharvest laboratory research in the AgCenter recently has focused on quantifying additional health-beneficial properties of sweet potatoes, including their antioxidant content and potential nutraceutical value – contributions to health beyond simple nutrition.

Phenolic Acids in Sweet Potatoes

Phenolic acids are known to be important, naturally occurring antioxidants, which function as scavengers of damaging free-radical molecules inside the cell. In humans, the damage caused by free radicals has been implicated in the aging process and in the development of cancer, cataracts and heart disease. Phenolic compounds are important secondary metabolites in sweet potatoes. These compounds are involved in a number of biosynthetic processes, including the maintenance of cell wall integrity and chemical defense against pathogen at tack. Coupled with high vitamin content and other metabolites, phenolic compounds can make a significant contribution to the nutraceutical value of sweet potatoes.

A rapid and sensitive analytical procedure was developed to quantify individual phenolic acids in sweet potato tissue using high-performance liquid chromatography. Using this methodology, the major phenolic acids in sweet potatoes were identified and the amounts were quantified in fresh root tissue and various edible preparations. Antioxidant activity in sweet potato tissue was determined using the compound Trolox to measure the antioxidant level.

Principal Phenolic Acids in Sweet Potatoes

Chlorogenic acid, caffeic acid and three isomers of the dicaffeoylquinic acids were identified as the principal phenolic acids in sweet potato root and leaf tissue. Among these, chlorogenic acid was the major phenolic acid except in young, immature leaves where 3,5-dicaffeoylquinic acid was the predominant phenolic acid.

Many factors were found to influence the concentration of phenolic compounds and antioxidant activity in sweet potatoes. These factors include growth stage of the tissue, type of tissue, genotype, postharvest care (curing, storage temperature, duration) and processing method.

Small roots at the initial stages of growth contained the highest amounts of phenolic compounds. The cortex tissue just beneath the skin tended to contain higher amounts of phenolic acids than the inner pith tissue, although the results were usually not statistically significant. Antioxidant activity in root pith tissue ranged from a high of 9.2 milligrams of Trolox equivalents per gram of sweet potato dry weight in very small-sized roots to 1.2 milligrams per gram in large-sized roots.

Sweet potato leaf tissue had a significantly higher phenolic acid content and antioxidant activity than root tissue. Young immature leaves had the highest phenolic acid content. Antioxidant activity in leaf tissue ranged from a high of 99.6 milligrams Trolox equivalents per gram of sweet potato dry weight in young leaf tissue to 21.0 milligrams per gram in old leaves and 9.6 milligrams per gram in leaf stems.

Sweet potato roots may be prepared for consumption in different ways. Microwaving, conventional oven baking and boiling are the most widely used preparation methods in homes and in food-service establishments. The method of heat processing and tissue location was found to influence the phenolic acid composition and antioxidant activity of Beauregard sweet potatoes. Heat processing (microwaving, boiling or baking) reduced total phenolic content and antioxidant activity in the skin tissue, but not in the cortex or pith tissue. Compared with microwaving and boiling, conventional oven baking resulted in a higher loss of antioxidant activity in skin tissue. Although the sweet potato may be considered as a rich source of antioxidants, distribution of phenolic acids in the root varies according to tissue location. Skin tissue typically contains the highest amount of phenolic compounds and antioxidant activity, followed by the cortex (tissue just beneath skin) and pith (inner flesh tissue).

Minimally processing sweet potatoes into fresh-cut products may also influence their phenolic content and antioxidant potential. In a comparison of different styles of cut, sliced sweet potatoes had a significantly higher total phenolic content than shredded. Storage of the fresh-cut products for several days resulted in an increase in total phenolic content. A holding temperature of 41 degrees resulted in a higher total phenolic content than at 32 degrees.

Implications for Industry

Results of these studies may have useful implications in developing new products and functional foods from sweet potatoes. For example, because of their high phenolic content and antioxidant activity, small, immature sweet potato roots and leaves may be useful sources of naturally occurring nutraceutical compounds for functional food manufacturers. The high phenolic content in skin tissue may also have implications in utilization of waste peelings from processed sweet potatoes. Also, these tissues may be concentrated sources of caffeoylquinic acids, which are useful to the pharmaceutical industry.

Consumers are placing increasing importance on the nutritional quality of a food product in making their purchasing decisions. The nutraceutical value and antioxidant content are important components of nutritional quality. Sweet potatoes may be important sources of phenolic acid nutraceutical compounds and antioxidants. Research to quantify the favorable nutritional and nutraceutical value of sweet potatoes will help provide important product-composition information for marketers to use in stimulating interest in sweet potato consumption. This information also will be useful to food processors interested in developing new value-added sweet potato products.

David H. Picha, Professor, and Malkeet S. Padda, former Graduate Research Assistant, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.

(This article was published in the spring 2009 issue of Louisiana Agriculture.)