Glycemic Index of Sweet Potato as Affected by Cooking Methods

Abstract

Understanding the effect of cooking on glucose availability will aid in the recommendation for including sweet potatoes as a regular component in American diets. Heating breaks down starch granules to allow amylopectin and amy-lose to be more readily digested by pancreatic amylase, which theoretically should increase the glycemic index of sweet potato. Twelve volunteers consumed 25 g of available carbohydrate from Beauregard sweet potato skin and flesh sepa-rately that were subjected to conventional cooking methods: baking at 163 °C for 1 hour; microwaving for five minutes in a 1000 watt microwave; dehydrating at 60°C for 16 hours; and steaming at 100°C for 45 minutes. Available carbohydrate was determined by difference from proximate analysis of protein, lipid, total dietary fiber, moisture, and ash. Fasted par-ticipants measured blood glucose levels at 0, 30, 60, 90, and 120 minutes after consuming 25 g of carbohydrate from test foods or glucose. Glycemic indices calculated from these methods for steamed, baked and microwaved sweet potato flesh were 63 ± 3.6, 64 ± 4.3 and 66 ± 5.7, respectively, indicative of a moderate glycemic index food. However, dehydrated and raw sweet potato flesh had a low glycemic index (41 ± 4.0 and 32 ± 3.0, respectively). Steamed skin, baked skin, and dehydrated flesh did not have a statistically different glycemic index (P> 0.05) from that of raw sweet potatoes. A second experiment confirmed the low glycemic index of raw sweet potato, especially the skin, and showed that a commercial ex-tract of the sweet potato cortex, Caiapo, tended to lower the glycemic index of white potato to a level that was not differ-ent from the raw sweet potato peel. The physiological mechanism for the lower glycemic index was not due to a greater release or a greater clearance of insulin during the glycemic response. Depending on cooking methods, " Beauregard " sweet potato flesh and skin may be considered low and medium glycemic index foods, which may prove beneficial for diabetic or insulin-resistant consumers.