Sorghum Protein Extraction by Sonication and Its Relationship to Ethanol Fermentation

Cereal Chem. 85(6):837–842 The objectives of this research were to develop a rapid method for extracting proteins from mashed and nonmashed sorghum meal using sonication (ultrasound), and to determine the relationships between the levels of extractable proteins and ethanol fermentation properties. Nine grain sorghum hybrids with a broad range of ethanol fermentation efficiencies were used. Proteins were extracted in an alkaline borate buffer using sonication and characterized and quantified by size-exclusion HPLC. A 30-sec sonication treatment extracted a lower level of proteins from nonmashed sorghum meal than extracting the proteins for 24 hr with buffer only (no sonication). However, more protein was extracted by sonication from the mashed samples than from the buffer-only 24-hr extraction. In addition, sonication extracted more polymeric proteins from both the mashed and nonmashed samples compared with the buffer-only extraction method. Confocal laser-scanning microscopy images showed that the web-like protein microstructures were disrupted during sonication. The results showed that there were strong relationships between extractable proteins and fermentation parameters. Ethanol yield increased and conversion efficiency improved significantly as the amount of extractable proteins from sonication of mashed samples increased. The absolute amount of polymeric proteins extracted through sonication were also highly related to ethanol fermentation. Thus, the SE-HPLC area of proteins extracted from mashed sorghum using sonication could be used as an indicator for predicting fermentation quality of sorghum. Sorghum (Sorghum bicolor L. Moench) is a drought-resistant and low-input cereal grain grown throughout the world, and interest in using it for bioindustrial applications is now growing in the United States (Farrell et al 2006). Although currently only ≈2.5% of fuel ethanol is produced from grain sorghum, annual consumption of sorghum by the ethanol industry is steadily increasing from 11.25% in 2004 to 15% in 2005 and 26% in 2006 (Renewable Fuels Association 2005, 2006, 2007). Researchers and ethanol producers have shown that grain sorghum is a viable feedstock (technically acceptable, fits the infrastructure, and can be economically viable) for ethanol, and could make a larger contribution to the nation’s fuel ethanol requirements. Starch and protein are the two major components in sorghum grain. Recent research has shown that starch content is a good indicator of ethanol yield in the dry-grind process but starch content itself could not explain conversion efficiency well (Wu et al 2007). Sorghum varies in protein content from 6 to 18%, with 70–90% of the total protein belonging to the storage proteins (kafirins) (Lookhart et al 2000). According to previous research with 68 sorghum hybrids, a strong negative correlation was observed between ethanol yield and protein content (R 2 = 0.60, P < 0.01) (unpublished data), which is similar to data reported for soft wheat cultivars (Swanston et al 2007). However, multiple linear regression, including both starch and protein content as predictors, verified that protein content did not significantly contribute to ethanol yield (P = 0.395). The effect of protein content on conversion efficiency was statistically significant (P = 0.015) but represented only 8.6% of variation in efficiency (unpublished data).