Yield of ethanol from enzyme-hydrolyzed yam (Dioscorea rotundata) and cocoyam (Xanthosoma sagittifolium) flours

Abstract

Fresh whole yam tubers and cocoyam corms were separately processed into flours by washing, peeling, blanching, slicing,drying and milling. The flours were enzyme-hydrolyzed by mixing 500g of flour with 2Lof water followed by treatment with a combination of bacterial alpha amylase, limit dextrinase and fungal alpha amylase and heating to 92

o C at pH 6.30. The hydrolysis process was maintained until iodine test for starch was negative. The hydrolyzed slurry was cooled to 20 o C and yeast was inoculated. The slurry was allowed to ferment for 5 days during which the pH, specific gravity (sg), total titratable acidity (TTA), and % sugar content were respectively measured. The yield of dry chips and flour (30.6% and 25.3% respectively) were about half of the yield for same items from cocoyams (64.1% and 54.2%). The sugar content (for yam/cocoyam slurries) decreased from 19.05/16.8%wt (on day 1) to 1.72/1.58%wt (on day 5), the ethanol content increased from 0%wt/vol on day 1 to 5 / 4.99%wt/vol on day 5. The acid produced by fermentation reduced the pH from 5.80 to 4.01 and 5.82 to 4.20 for the yam and cocoyam slurries respectively on day 5. The correlaton coefficient of the regressed sg vs TTA data was -1, sg vs ethanol content was -1 and sugar vs ethanol was -1 for both media. The yield of flour from yam was 25.4% and 54.2% for cocoyam. The yield of ethanol from the 500g of flour was 130g for yam and 117g for cocoyam. Raw cocoyams offer a greater potential for flour and ethanol production than yams