Protein quality in cereals evaluated by rapid estimation of specific protein fractions
1975
The lysine content varies considerably among the different solubility classes of cereal proteins. Thus the proportion of lysine-deficient protein to total protein may serve as an index of protein nutritional value in cereals. In normal and high lysine corns an inverse correlation (-0.86) was established between total lysine content of grain and amount of zein (a protein low in lysine). Zein was precipitated from 70% ethanol-0.5% sodium acetate extracts of corn meal by addition of salt solution and was measured turbidimetrically. This simple, rapid, and inexpensive procedure was used successfully to estimate lysine in large numbers of corn samples varying in genetic background, composition and physical properties. It is a useful screening tool in breeding programmes for vitreous opaque-2 high lysine corn. Variation in lysine content of glutelin protein is a factor reducing accuracy of the method. An alcohol-soluble fraction of reduced glutelin has a low lysine content. Extracting meals with 70% ethanol-0.5% sodium acetate containing 0.1M mercaptoethanol yields this protein as well as zein. Turbidimetric analyses of total protein in this extract give an improved negative correlation with lysine content of grain. Since nutritional value of protein in other grains such as sorghum and barley is also reduced by prolamine and alcohol-soluble glutelin content, the method can be extended to them.
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