Comparative Analysis of Grain Sorghum and Corn as Raw Materials for Bioethanol Production using Very High Gravity Fermentation

Global warming and the resulting climate change are increasing the extent of marginal agricultural zones and reducing the availability of conventional feedstocks for bioethanol production, particularly corn. Therefore, it is necessary to find economically justifiable grain crops that can withstand arid climates better than corn. Grain sorghum can be a good choice because it is naturally drought-resistant and can be used as a feedstock for bioethanol production. In Ukraine, the economic viability of sorghum is supported by its increasing market availability and a lower cost of cultivation compared to corn, often resulting in a 15–20% reduction in raw material expenses in arid southern regions. The aim of this study is a comparative assessment of the technological characteristics of mash and wort from grain sorghum and corn, prepared using VHG technology, with a focus on viscosity, enzymatic efficiency, and the content of volatile by-products that affect the quality of raw distillate.

An early-maturing grain sorghum hybrid, the “Brigga” variety, was selected for comparison with the traditional raw material, corn of the hybrid variety “DKS 3730”. Wort from corn and grain sorghum with an increased concentration of dry matter (greater than 270 g/L) was prepared for ethanol fermentation. This was achieved through the hydroenzymatic treatment of ground grain raw materials, suitable enzyme preparations, and auxiliary components. The dynamic viscosity of corn and sorghum mash was measured using a rotational viscometer. Fermentation of the wort was carried out on a fermentation stand using dry yeast intended for bioethanol production. On completion of yeast fermentation, the fermented wort was subjected to distillation to determine ethanol concentration and the content of by-products in the raw distillates by chromatographic analysis.

The grains of corn and sorghum varieties have similar starch levels, ranging from approximately 64% to 78%. However, the differences occur in their protein and non-starch polysaccharide content. These differences can alter the density of the wort and affect the quality of fermentation.

When appropriate enzymes were applied, wort from grain sorghum and corn contained approximately 14% vol. ethanol after 72 hours of fermentation. The bioethanol production yield from sorghum and corn feedstock reached 1.52–1.54 g/(l•h). Raw sorghum wort distillates showed 11.1% lower levels of volatile by-products than corn. The proportions of unwanted by-products and total ethanol production were similar for both grains.

Studies show that sorghum wort, despite its higher hemicellulose and β-glucan content, maintains a low viscosity. Due to this low density, it provides a higher dry matter concentration during VHG fermentation. However, the thinner consistency of sorghum mash makes it better for high-density bioethanol because it can be loaded with more grain and still result in decent fermentation.

Grain sorghum, due to its composition, has an advantage over corn in the production of bioethanol using Very High-Gravity technology. Understanding technological approaches to preparing sorghum wort will help achieve post-fermentation ethanol concentrations exceeding 18% v/v, which will positively affect the economic efficiency of biofuel production.