Assessment of granular starch hydrolysis enzyme on ethanol yield from partially swollen sorghum starch and analysis of extracted protein properties

Abstract

Conventional bioethanol production from starch-based crops involves high-temperature cooking, which is energy-intensive and degrades the protein quality of distiller's grains (DG), a valuable co-product. This study addresses the critical gap of reducing the energy demand and protein degradation by comparing conventional high-temperature processing with granular starch hydrolyzing enzyme (GSHE) fermentation at low temperatures. Specifically, the novelty lies in optimizing partial starch swelling treatments (50 °C, 60 °C, 70 °C) to enhance ethanol yields while preserving DG protein quality.

Using sorghum varieties (normal and waxy) as a model system, we conducted experiments combining low-temperature starch swelling and GSHE fermentation to evaluate their impacts on ethanol yield and protein properties. Waxy sorghum exhibited higher ethanol fermentation efficiency than normal sorghum. Partial starch swelling significantly improved ethanol yield without compromising DG protein quality. GSHE fermentation with starch swelling at 70 °C for 30 min achieved the highest ethanol concentration (12.02 % v/v) and yield (92.74 %) for waxy sorghum. Protein digestibility remained high for both waxy (85.39 %) and normal sorghum (85.21 %) even at higher swelling temperatures. Surface hydrophobicity of DG proteins increased with temperature, particularly at 95 °C during conventional processing. Notably, partial starch swelling improved the lightness (L∗ values) of sorghum proteins, indicating better quality. Molecular characterization further revealed the specific effects of processing on protein properties.

This research highlights the potential of low-temperature starch swelling combined with GSHE fermentation to enhance ethanol production efficiency and protein quality in DG, offering a sustainable alternative to conventional bioethanol processes.