Impact of sorghum proteins on ethanol fermentation and investigation of novel methods to evaluate fermentation quality

Abstract

Sorghum has been considered one of the best species dedicated to biofuel production because of its drought tolerance, low fertilizer or pesticide input, established production systems, and genetic diversity. The mission of this research was to better understand the relationship among "genetic-structure-function-conversion." The main focus of this research was to study the impact of sorghum proteins on ethanol fermentation and to investigate novel methods for evaluation of sorghum fermentation quality. Changes of sorghum protein in digestibility, solubility, and microstructure during mashing were characterized. Sorghum proteins tended to form highly extended, strong web-like microstructures during mashing. The degree of protein cross-linking differed among samples. Formation of web-like microstructures due to cross-linking reduced conversion efficiency. A rapid method for extracting proteins from mashed and nonmashed sorghum meal using sonication (ultrasound) was developed, with which the relationships between the levels of extractable proteins and ethanol fermentation properties were determined. There was a strong relationship between extractable proteins and fermentation parameters. Ethanol yield increased and conversion efficiency improved significantly as the amount of extractable proteins increased. The Rapid-Visco Analyzer (RVA) was used to characterize pasting properties of sorghum grains. Results showed a strong linear relationship between ethanol yield and final viscosity, as well as setback. A modified RVA procedure (10 min) with an application of α-amylase was developed to simulate the liquefaction step in dry-grind ethanol production. There was a remarkable difference in mashing properties among the sorghum samples with the normal dosage of α-amylase. The modified RVA procedure is applicable not only for characterization of mashing properties but also for prediction of tannin content and for optimization of [alpha]-amylase doses for starch liquefaction. A small-scale mashing (SSM) procedure requiring only 300 mg of samples was investigated as a possible method of predicting ethanol yield of sorghum grain. There was a strong linear correlation between completely hydrolyzed starch (CHS) from SSM and ethanol yields from both traditional and simultaneous saccharification and fermentation procedures. CHS was a better indicator for predicting ethanol yield in fermentation than total starch.