Diabetes mellitus is a chronic metabolic disease caused by insufficient insulin production by the pancreas or when the body loses its ability to utilise insulin effectively or both. This leads to an accumulation of glucose in the blood of diabetic people which is detrimental for their health in the long term. Due to an increase in prevalence, the disease is becoming a growing concern to health authorities worldwide, especially in developing regions where inadequate health care systems and poor socio-economic conditions exacerbates the situation. A potential way of preventing diabetes is to limit starch digestibility to control blood glucose levels. Sorghum and maize are important food cereals in many regions of the world and they contain various phenolic compounds, particularly flavonoids which can inhibit starch hydrolysing enzymes like α-amylase. Therefore these cereals could have potential anti-diabetic properties.
In this study, various extracts prepared from bran samples of white maize and white and red non-tannin sorghums were analysed for inhibitory activity against porcine pancreatic α-amylase using the Megazyme Ceralpha α-amylase assay kit. It was necessary to provide a basis for an understanding of the amylase enzyme inhibitory properties of the brans in relation to their phenolic content and therefore, their potential anti-diabetic properties. The total phenolic content of white maize and red and white non-tannin sorghum bran methanolic extracts was therefore determined, using the Folin Ciocalteu assay. The profile and concentration of flavonoids in extracts from the bran samples was determined using high performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS). Red non-tannin sorghum bran and its extracts had higher inhibitory activity against porcine pancreatic α-amylase than bran and bran extracts from white maize and white non-tannin sorghum. Unextracted bran samples also inhibited the enzyme, indicating that the bran components inhibiting the enzyme did not need extraction and could exert inhibitory effects in situ. The bran of the red non-tannin sorghum varieties had significantly (p<0.05) higher levels of total phenolics (3.35 – 4.13 g CE/100 g) than that of the white maize (1.07-1.20 g CE/ 100 g) and white non-tannin sorghum varieties (0.99-1.15 g CE/ 100 g) as shown by results from the Folin Ciocalteu assay . Results from HPLC analysis showed that extracts from red sorghum varieties had significantly (p<0.05) higher levels of total flavonoids (166.8-269.8 mg/100 g) than extracts from white maize (18.7-24.8 mg/ 100 g) and white non-tannin sorghum (64.9-69.9 mg/100 g). Acidified organic bran extracts had higher total phenolics than non-acidified organic and water extracts. Results from LC-MS analysis showed that the acidified methanol extract from red non-tannin sorghum bran had the highest concentration of flavonoids with flavones (apigenin and luteolin) and flavanones (eriodictyol and naringenin) detected as the two main groups of flavonoids. In agreement with total phenolic and flavonoid content, this extract also had the highest α-amylase inhibitory activity. The water extract of the red non-tannin sorghum (Mr BUSTER), was the only water extract of all the grains that contained flavanones like eriodictyol, and was also the only water extract that showed inhibition against α-amylase. These observations indicate that the flavone and flavanone compounds identified in the extracts are important for inhibition of the α-amylase enzyme.
Nutraceutical-type preparations from red non-tannin sorghum bran could have applications in foods as anti-diabetic agents by inhibiting α-amylase activity and thus controlling postprandial glucose levels in people suffering from diabetes.