Ethanol from sweet sorghum in a year-round production cycle

Four sweet sorghum varieties, M81E, Sugar Drip, Top 76-6, and Umbrella, were sowed early in the years 2011 and 2012 in two experimental plots located in the Guatemalan Pacific lowlands within the sugarcane-producing region. Three harvests were done, a first cut and two consecutive ratoon crops. Stalk biomass, stalk sugar content, and proportions of individual sugars in the extracted juice were measured. Ethanol production employing simultaneous sugar extraction from stalk particles and fermentation by a commercial yeast strain was carried out in 2011 samples from the first harvest and second ratoon crop. Fresh stalk biomass productivity was the same for the first harvest and the first ratoon crop, but it was dependent on variety and site. The second ratoon crop diminished drastically without exception. Considering only two harvests per year, Top 76-6 had the best overall productivity with a rounded average of 47 Mg/ha/harvest, equivalent to 94 Mg/ha/year, and the higher sugar productivity, equal to 5,285 kg/ha/harvest or approximately 11 Mg/ha/year. Proportions of individual sugars at harvest varied among varieties, sites, and year, and three sample groups were identified by hierarchical clustering. An inverse correlation was found between stalk sugar content and the ratio between hexoses and sucrose at harvest, implying that the smaller the ratio, the higher the sugar content. Ethanol production was between 200 and 250 g/ethanol/kg of dry stem for Sugar Drip, Top 76-6, and Umbrella varieties for first harvest samples. The data for the second ratoon crop was lower for all varieties and sites. Ethanol productivity was higher for Umbrella and Top 76-6 and equal to approximately 2,500 L/ha/harvest.