Classification of Cereal Proteins

It is possible to classify proteins on several different bases. One basis is on the cereal from which they come and where in the seed they are found, another is the Osborne classification system, which is based on the solubility of the protein. Proteins can be further classified in chemical terms such as molecular weight and the presence or absence of sulfur. 

Cereal proteins when classified by the Osborne sequential extraction method yield four different classes albumins, which are water soluble, globulins, which are soluble in salt solutions, prolamins, which are soluble in alcohol-water mixtures, and glutelins, which are soluble in dilute acid or alkali. Chen and Bushuk added a fifth fraction by dividing the glutelin into two fractions, one soluble in dilute (0.05 m) acetic acid and the other insoluble in this reagent.5 

Of course these classes do not constitute a single protein but mixtures of proteins having the same solubility characteristics. Improved methods of fractionation reveal how many different components are present in each fractionation. 

Biochemical and genetic researchers divide the proteins into three groups S-poor, i.e. sulfur poor, S-rich, i.e. sulfur rich, and HMW-prolamins, i.e. high molecular weight prolamins. 

The Osborne classification, which dates from 1907, was updated at a symposium on gluten in 1996. Gianibelli el al. point out that at least 1300 peptides can be obtained from wheat endosperm proteins after disulfide bond rupture using two-dimensional fractionation.6 

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The discussion of protein solubility in Chapter 9 leads logically into the next topic because it is usually necessary to have proteins in solution in order to characterize them. Of course, some important characterization can be made by hydrolyzing the proteins (e.g., determination of the amino acid composition and sequence). What we are mainly concerned with in this chapter is the measurement of molecular composition and properties of proteins or their subimits in or close to their native state. This is required if our aim is to relate composition to fimctional properties. The classification of cereal proteins into four groups based on solubility was described in the previous chapter. This classification still proves to be valuable for researchers. However, in recent times, techniques have been developed that enable more accurate quantification of cereal protein composition. 

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