Characterization of glucose syrup and ethanol from starch extracted from selected cassava and maize varieties grown in Uganda

Abstract

The research aimed to extract starch from the selected cassava and maize varieties, evaluate enzymatic and acid hydrolysis processes for glucose syrup production, and assess ethanol production. Characterization of the resulting glucose syrup and ethanol was also performed. A total of four (4) cassava varieties and four (4) maize varieties were used in this study. These included two local varieties of cassava (Mukumba and Mwezi mukaaga), two improved cassava varieties (TME 14 and NAROCASS 1), two open pollinated maize varieties (Longe 4H and Longe 5H) and two hybrid maize varieties (H 5355 and H 2115). Cassava was harvested at twelve (12) months while maize was harvested at four (4) months (120 days) after planting. Cassava and maize starch were extracted in three independent replicates using the wet and alkaline extraction methods, respectively. Glucose syrup was produced using acid and enzyme hydrolysis. Glucose syrups were fermented using Young’s dried active yeast (Saccharomyces cerevisiae) to obtain ethanol. There was no significant difference in the starch yield of the local and improved varieties for either cassava or maize. H 2115 produced the highest yield of 50.83% among the maize varieties while Mwezi mukaaga had the highest yield (29.1%) among the cassava varieties. Selection of the varieties for starch characterization was based on starch yield. Swelling power and solubility for all the starch varieties increased with increasing temperatures with Mwezi mukaaga having the highest swelling power (18.23 g/g) at 90°C and TME 14 having the highest solubility (2.45%) at 90°C. The content of amylose of cassava starch was higher than that of maize with Mwezi mukaaga having highest amylose content of 24.25%. Maize had higher resistant starch content than cassava with H2115 having the highest resistant starch content of 0.82%. The gelatinization temperatures for all starch varieties were not significantly different (p>0.05). Cassava varieties were much lighter than the maize varieties. The cassava starch granules were round, truncated with larger particle sizes (17.40 and 17.95 μm) as compared to the maize starch granules which were round, irregular and polygonal with smaller particle sizes (11.05 and 9.10 μm). Acid hydrolysis significantly produced higher (p>0.05) sugar concentration for both crop types as compared to enzyme hydrolysis. Among maize varieties, H 2115 had highest sugar content of 46.1 g/100 g (acid hydrolysis) and H 5355, 40.3 g/100 g (enzyme hydrolysis), while among cassava varieties, TME 14 had the highest sugar content of 31.2 g/100 g (acid hydrolysis) and Mukumba, 12.41 g/100 g (enzyme hydrolysis). Enzyme hydrolyzed samples showed highest sugar consumption by the yeast hence a higher alcohol (ABV) content and ultimately a better fermentation efficiency. Enzyme hydrolyzed glucose syrups had a higher ethanol content than the acid hydrolyzed syrups, with no difference between maize and cassava varieties in each treatment. These results propose that maize and cassava are good sources of starch, glucose syrup and ethanol with the maize varieties used in this study giving higher sugar concentrations than the cassava varieties.