In vitro starch digestibility, α-amylase and α-glucosidase inhibitory capacities of raw and processed forms of three varieties of Livingstone potato ( Plectranthus esculentus )

The starch digestibility, α-amylase and α-glucosidase inhibitory capacities of the flours from raw and processed (boiled and fried) tubers of three varieties (riyom, beebot and langaat) of P. esculenta were investigated using standard techniques. The range of the chemical constituents of the flours (expressed in g/100 g, dry weight) were: lipids (0.20 to 17.27), ash (1.26 to 2.87), proteins (7.55 to 11.60), total starch (47.18 to 64.70), digestible starch (39.34 to 59.48), resistant starch (RS) (3.06 to 7.84) and rapidly digestible starch (RDS) (16.89 to 30.67). White bread contained significantly higher TS, DS and RDS but lower RS than the flours. The IC50 values of the aqueous extracts of the flours ranged from 257.50 to 1515.94 μg/ml and 128.89 to 641.90 μg/ml for the α-amylase and α-glucosidase assays respectively. Frying of the P. esculenta tubers potentiates the activities of α-amylase and α-glucosidase enzymes compared with the raw or boiled forms. Interest in functional foods has increased in recent times due to their roles in ameliorating a lot of metabolic disorders, one of which is type 2 diabetes. Recent therapeutic targets for the control of postprandial hyperglycemia involve inhibition of α-amylase and α-glucosidase activities that play key roles in the digestion of dietary carbohydrates. However, the undesirable side effects associated with the synthetic α-amylase and α-glucosidase inhibitors have led to increased search for these inhibitors from plant based foods. Livingstone potato is one of such plants that has captivated the interest of researchers due to their demonstrated pharmacological relevance. However, Livingstone potato tuber is usually eaten following processing and there are no reports on the effect of processing on its starch digestibility. Therefore, the findings of this study which showed that methanol extracts of the tubers showed stronger α-glucosidase inhibition than α-amylase inhibition could particularly be of interest to the health care and nutraceutical industries as exploitation of this finding could circumvent the adverse side effects associated with synthetic antidiabetic drugs.