| North Carolina is the leading producer of sweetpotatoes in the United States. In 2009, 21% of the 679,780,240 pounds of sweetpotatoes that were produced in North Carolina were processed before entering the market. Generally, one of the first steps in processing is removal of the peel. The peel is considered a waste product, but contains potentially valuable compounds, such as proteins. The first objective of this research was to optimize a process for extracting proteins from industrial peel waste. Material from the primary peeling of raw roots and from a secondary peeling after blanching was obtained from a local sweetpotato processing facility. Peel was mixed with saline solvent to dissolve proteins, and then the mixture was centrifuged to separate non-soluble material. After the proteins were precipitated from the supernatant with CaCl 2, centrifugation was used to obtain a protein pellet. Linear segmented and quadratic models were used to optimize peel to solvent ratio, NaCl concentration, and CaCl2 concentration. More proteins could be extracted from secondary peelings than primary peelings. The highest recovery, 32.0%, was obtained by mixing 1 g of secondary peelings with 59.7 mL of 0.025 mM NaCl and then precipitating with 6.8 mM CaCl2. The protein banding pattern and glycosylation characteristics of the extract were similar to Caiapo, a commercial anti-diabetic supplement containing sweetpotato proteins.;In order to exhibit systemic effects, a protein must survive gastric and duodenal digestion, and then be absorbed. Sweetpotato proteins have been reported to possess anti-diabetic, antioxidant, and anti-proliferative properties, but the mechanism by which the proteins evade digestion is unknown. The second objective of this research was to determine the susceptibility of sweetpotato proteins to digestive enzymes. Caiapo, an extract from primary peelings, and an extract from secondary peelings were incubated with pepsin, trypsin, and chymotrypsin. Samples were removed throughout the digestion procedure and protein breakdown was visualized with SDS-PAGE. Samples were also assayed for amylase activity and amylase inhibitory activity after incubation with pepsin. Proteins were present in all of the samples that were resistant to digestion by pepsin, trypsin, and chymotrypsin. The extract from secondary peelings exhibited lower resistance to pepsin than Caiapo and the extract from primary peelings. Compact structure is most likely responsible for the noted resistance to digestion, since the amino acid sequence of the major storage protein in sweetpotatoes, sporamin, showed numerous potential cleavage sites. In addition, heat treatment, which would cause denaturation, increased susceptibility of the protein to digestion. Trypsin inhibitors remained active after simulated gastric digestion, with the Caiapo and extract from primary peelings exhibiting higher inhibitory activity compared to the extract from secondary peelings. Active amylase and chymotrypsin inhibitors were not found in any of the samples after digestion. Modified glucose tolerance tests in rats confirmed the lack of digestion-resistant amylase inhibitors, and showed that sweetpotato proteins do not alter maltose digestion or glucose absorption. |
