Protein Quality

The nutritional quality of maize is determined by the amino acid makeup of its protein. Maize is deficient in two essential amino acids: lysine and tryptophan, making it a poor protein food. The kernel is made up of the endosperm, the germ, the pericap, and the tip cap. The protein concentration is highest in the germ, but the quality is better in the endosperm. The germ proteins contribute significantly to essential amino acids, so maize food products without the germ, including quality protein maize (QPM) endosperm, are lower in protein quality compared to the whole kernel.

The germ contributes most of the oil, sugar, vitamins, and minerals of the kernel. The germ also has a lower leucine to isoleucine ratio, giving it a higher biological value. Biological value is defined as the amount of absorbed nitrogen needed to provide the necessary amino acids for the different metabolic functions in the body.

The high consumption of maize by the human population and the well-established lysine and tryptophan deficiencies in maize protein motivated researchers to develop the QPM to increase concentrations of these essential amino acids in its protein. Newer varieties provide higher protein content (18%) by increasing the prolamine (zein) fraction in maize endosperm. An example of QPM is one opaque-2 maize.

QPM varieties have almost double the percentages of lysine and tryptophan compared to normal maize, but are similar in overall protein content. However, the QPM varieties have a greatly reduced amount of the major storage protein, zein. The biological value of common maize is 45 percent whereas the QPM is about 80 percent. Hence the production and consumption of QPM maize in countries that use maize as their chief grain crop would have a beneficial effect on the nutritional state of the people and significant economic implications from the better use of what is produced and consumed.

Compared to normal maize, production of QPM varieties may have some disadvantages. QPM varieties have softer, floury endosperms, providing a slightly lower yield and making the plant more susceptible to storage insects. Furthermore, QPM varieties have lower zein content, which is associated with lysine deficiency and a higher imbalance of essential amino acids. Hence they are considered to be of a lower quality. They are also susceptible to weevils in storage.

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