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.

Potentially valuable proteins can be extracted from sweet potato peel, a waste product of sweet potato processing.

Sweet potato proteins have been shown to possess antioxidant and antidiabetic properties in vivo. The ability of a protein to exhibit systemic effects is somewhat unusual as proteins are typically susceptible to digestive enzymes. This study was undertaken to better understand how digestive enzymes affect sweet potato proteins. Two fractions of industrially processed sweet potato peel, containing 6.8% and 8.5% protein and 80.5% and 83.3% carbohydrate, were used as a source of protein. Sweet potato proteins were incubated with pepsin, trypsin, and chymotrypsin and protein breakdown was visualized with SDS-PAGE. After pepsin digestion, samples were assayed for amylase inhibitory activity. Sporamin, the major storage protein in sweet potatoes, which functions as a trypsin inhibitor as well, exhibited resistance to pepsin, trypsin, and chymotrypsin. Sporamin from blanched peel of orange sweet potatoes was less resistant to pepsin digestion than sporamin from outer peel and from extract of the white-skinned Caiapo sweet potato. Trypsin inhibitory activity remained after simulated gastric digestion, with the Caiapo potato protein and peel samples exhibiting higher inhibitory activity compared to the blanched peel sample. Amylase and chymotrypsin inhibitory activity was not present in any of the samples after digestion.