Impact of processing on the carbohydrate quality and digestibility of pearl millet (pennisetum glaucum)

Pearl millet (Pennisetum glaucum) is a climate resilient cereal consumed widely in traditional diets in rural regions of Africa. It is a source of complex carbohydrates, and has a high content of protein and minerals compared to maize, rice and sorghum. Previous research has suggested beneficial effects of consumption of complex carbohydrates. However, there is limited research on the impact of processing on carbohydrate quality and digestibility of traditional millet products.
In this study, the impact of domestic processing on the carbohydrate composition and digestion pearl millet was investigated. Nigerian pearl millet was milled into flour particles ranging from 250 to 1000 µM in size and flours were used to prepare unfermented cooked porridge (ibyer) and fermented porridge (ibyer-i-angen) up to 72 h fermentation. The protocols for carbohydrate analysis and starch digestion used in this study were optimised using millet flour and validated with cooked samples.
The optimisation of carbohydrate analysis and starch digestion indicated that agitation is a key factor for the extraction of soluble sugars and effective digestion of millet starch. Some commercial enzymes used in carbohydrate digestion contained high levels of glucose and other sugars that can could interfere with the digestion and measurement of starch. The soluble sugar was low in millet flour (2.61%) and increased 2 folds after cooking and 6 folds after fermentation for 72 hours. This suggests the carbohydrate hydrolase activities in flour and fermentation medium. The starch content of millet was not altered by cooking (68.80%) but decreased to 35.88% following 72 h of fermentation. Digestion of starch using optimised static digestion protocol showed that the gastric phase significantly improved the hydrolysis of millet starch (p<0.05), increasing starch digestion from 15.27 to 47.07% in millet flour. The starch digestibility of ibyer was around 56.01% and increased to 69.39 and 98.62% after 12 and 72 h of fermentation of ibyer-i-angen. Millet samples contained both rapidly digestible starch (RDS) and resistant starch (RS). The RDS and RS contents were 41.37 and 43.98% for (ibyer), 52.78 and 30.98% for 12 h and 64.96 and 18.35% for 24 h fermented ibyer-i-angen. The relatively high RS of ibyer was is in part explained by the high susceptibility of millet starch to retrogradation suggested by differential scanning calorimetry (DSC) of ibyer. The estimated GI of millet was low (<55) for ibyer, and intermediate (<69) 12 h and 24 h and high (>70) for 48 and 72 h fermented ibyer-i-angen. In conclusion, domestic processing had a significant impact on carbohydrate quality and digestion of millet with low glycemic potential. The modulation of starch resistance by processing could be exploited to produce ibyer and ibyer-i-angen to address specific nutritional requirements in both over and undernourished populations.