Nutritional Aspects of Cereal Proteins and Approaches to Overcome Their Deficiencies

The inferior nutritional value of cereal protein is primarily because of the high content of the storage protein prolamin. These proteins are in general characterized by a very high content of proline and glutamine and a low content of lysine and other nutritionally essential amino acids. The cereals vary with respect to prolamin and lysine content. Rice and oats have a relatively low prolamin content, around 10%, and an acceptable lysine content. Wheat and barley contain 40-45% prolamin and about 3.5% lysine, while maize and sorghum contain more than 50% prolamin in their seed protein, that results in a lysine content below 2.5%. Intensive screenings for changes in endosperm morphology, protein composition, or lysine content have led to the detection of a number of mutants with reduced prolamin content and increased content of lysine and other essential amino acids. These high-lysine and low-prolamin mutants have a considerably improved nutritional value of the seed protein. However, all the mutants found so far are also characterized by a reduced starch content and grain yield, and high-lysine varieties have only been grown to a very limited extent. The present main approaches studied to improve the nutritional value of cereal proteins are (i) the replacement of some of the prolamin with lysine-rich storage proteins present in low amounts in the seed, and (ii) the improvement of the nutritional value of prolamin by genetic engineering.