Changes in proteins of sorghum, maize and pearl millet during food processing and variability of proteins in sorghum cultivars

Abstract

Changes in the proteins of sorghum, pearl millet and maize after processing into tortillas or acidic and alkaline to were studied. Protein solubility, molecular weight (MW) distribution and pepsin in vitro digestibility were determined for processed and unprocessed samples. Proteins were extracted in four fractions: (I) salt water, (II) 60% t-butyl alcohol, (III) 60% t-butyl alcohol with 2% beta-mercaptoethanol (β-ME) and (IV) 2% sodium dodecylsulfate (SDS) with 5% β-ME and 0.0625 M Tris (pH 6.8). The four protein fractions were separated by Discontinuous SDS polyacrylamide gel electrophoresis (Disc. SDS-PAGE). Sorghum contained higher levels of fraction III proteins (37 to 49%), i.e. alcohol-soluble reduced glutelins, than pearl millet (10.1%) or maize (20 to 22%). Proteins from fraction I and II decreased while fraction III and residue proteins increased after processing. The alkaline process caused the greatest increase in fraction III proteins. This indicates that some proteins in alkali treated samples were crosslinked or formed complexes which remained in the residue. The electrophoretic patterns of protein extracts showed that processing of sorghum, pearl millet and maize affected the MW distribution and intensities of protein bands. Fraction I proteins which had a significant reduction in number and intensity of bands were more affected than the fractions II, III and IV. Pepsin in vitro protein digestibility decreased from 10 to 30% after processing for all three cereals. However, these values were lowest for processed sorghum. The significant reduction in pepsin digestibility of sorghum after cooking may be a consequence of the large amounts of crosslinked prolamins which were unaccessible to the enzyme. Proteins from 27 sorghum cultivars representing 5 subseries and 15 working groups were extracted with two solvent systems and fractionated by Disc. SDS-PAGE. Fraction I proteins were extracted in salt water and fraction II proteins were extracted in a buffer which contained 0.0620 M Tris (pH 6.8), 2% SDS and 5% β-ME. Many sorghums had several bands in common, indicating some genetic relationship among them. The number of similar proteins between 2 cultivars was used to calculate a similarity index (SI). Low similarity indexes (SI) (57 to 75%) were obtained for fraction II proteins from pairs of sorghums representing different subseries, whereas cultivars from the same working group in a subseries had higher SI values (85 to 100%). These results indicate that SDS-PAGE of fraction II proteins from sorghum is a useful technique which has some potential for cultivar identification.