Sorghum Protein Extraction by Sonication and Its Relationship to Ethanol Fermentation

Abstract

Cereal Chem. 85(6):837-842The 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.

Grain Proteomics of Sorghum: Improving Digestibility, Nutritional Quality and Starch Conversion to Ethanol

“…Crude protein and starch levels across the lines in the current study were similar to those attained in previous analyses (67,165,257). Starch content was shown to have a major influence on grain ethanol production, where a strong positive linear correlation was observed between total starch and ethanol yield (R²=0.736) (Fig.…”

“…4.3), as previously observed (67,93,258). Additionally, β-kafirin had significant, although relatively small impacts on digestibility as observed using a multi-variant analysis (P=0.0416),…”

“…The structure of the protein-starch matrix directly influences the rate of starch digestion, impacting on food quality traits, such as glycemic response and grain nutritional value (66). The proportion of soluble to insoluble proteins in the matrix determines the access of enzymes to starch and oligo-and polysaccharides (67). Non-kafirins provide a readily accessible source of nitrogen and carbon to the embryo during germination and also play an important secondary role in seed storage, providing structure to the protein-starch matrix (68,69).…”

“…The ratio of soluble to insoluble and residual protein reflects the extent of cross-linking among storage proteins, which impacts on important grain quality traits for food and feed, particularly flour viscosity and protein digestibility. For example, the rate of fermentation depends on the proportion of soluble to insoluble proteins in the matrix, which governs the access of enzymes to starch and polyssacharides (67). Isolation of storage proteins under non-reducing conditions produces groups of 'soluble' proteins consisting primarily of monomeric kafirins and metabolic enzymes.…”

“…Sorghum and maize prolamins, referred to as kafirins or zeins, respectively, exhibit extensive sequence homology and a similar relative distribution in the PBs (248). However, variation in the functional characteristics and degree of polymerisation of the proteins accounts for differences in grain quality traits like digestibility and ethanol conversion across maize and sorghum (65,67). Immunolocalisation studies indicate that cysteine-rich β-and γ-kafirins are located on the periphery of sorghum PBs, while the α-kafirins fill the interior of the structure along with small amounts of δ-kafirin (53).…”