ARS | Publication request: Glycemic index of sweetpotato as affected by cooking methods

Submitted to: Open Nutrition Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 6, 2011
Publication Date: January 1, 2012
Repository URL: http://handle.nal.usda.gov/10113/54010
Citation: Allen, J.C., Corbitt, A.D., Maloney, K.P., Butt, M.S., Truong, V.D. 2012. Glycemic index of sweetpotato as affected by cooking methods. Open Nutrition Journal. 6:1-11.

Interpretive Summary: Glycemic index is a ranking of carbohydrate containing foods according to their immediate effects on blood sugar levels after ingestion. Understanding the effect of cooking on glucose availability will aid in the recommendation for including sweet potatoes as a regular component in American diets. This study aimed to investigate the change in glycemic index after cooking sweet potatoes under conventional domestic methods. Twelve volunteers consumed 25 g of available carbohydrate from Beauregard sweetpotato skin and flesh separately that were subjected to baking, microwaving, steaming and dehydrating. Blood glucose levels of fasted participants were measured 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 sweetpotato flesh were 63 ± 8.4, 64 ± 10 and 66 ± 13.3, respectively, indicative of a moderate glycemic index food. However, dehydrated and raw sweetpotato flesh had a low glycemic index (40 ± 8.2 and 28 ± 7.3, respectively). Steamed skin, baked skin, and dehydrated flesh did not have a statistically different glycemic index from that of raw sweetpotatoes. Depending on cooking methods, Beauregard sweetpotato flesh and skin may be considered medium and low glycemic index foods, which may prove beneficial for diabetic or insulin-resistant consumers.

Technical 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 amylose 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 separately 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 participants 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 extract of the sweet potato cortex, Caiapo, tended to lower the glycemic index of white potato to a level that was not different 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.