Comparison of methods for extracting kafirin proteins from sorghum distillers dried grains with solubles

Use of coproducts generated during fermentation is important to the overall economics of biofuel production. The main coproduct from grain-based ethanol production is distillers dried grains with solubles (DDGS). High in protein, DDGS is a potential source of protein for many bioindustrial applications such as adhesives and resins. The objective of this research was to characterize the composition as well as chemical and physical properties of kafirin proteins from sorghum DDGS with various extraction methods including use of acetic acid, HCl-ethanol and NaOH-ethanol under reducing conditions. Extraction conditions affected purity and thermal properties of the extracted kafirin proteins. Extraction yields of 44.2, 24.2, and 56.8% were achieved by using acetic acid, HCl-ethanol and NaOH-ethanol, respectively. Acetic acid and NaOH-ethanol produced protein with higher purity than kafirins extracted with the HCl-ethanol protocol. The acetic acid extraction protocol produced protein with the highest purity, 98.9%. Several techniques were used to evaluate structural, molecular and thermal properties of kairin extracts. FTIR showed alpha-helix dominated in all three samples, with only a small portion of beta-sheet present. Electrophoresis results showed alpha(1), alpha(2) band and beta kafirins were present in all three extracts. Glass transition peaks of the extracts were shown by DSC to be approximately 230 degrees C. Kafirin degraded at 270-290 degrees C. Size exclusion chromatography revealed that the acetic acid and HCl-ethanol based extraction methods tended to extract more high molecular weight protein than the NaOH-ethanol based method. Reversed phase high-performance liquid chromatography showed that the gamma kafirins were found only in extracts from the NaOH-ethanol extraction method.