The Effect of Storage on the Nutritional Qualities of the Proteins of Wheat, Corn and Soybeans

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Abstract
The effect of storage at approximately 78°F. (25.5°C.) and at moisture contents of 6 to 12% on the nutritive value of the proteins of wheat (Turkey Red), corn (an open-pollinated and a hybrid variety) and soybeans (Illini) was determined. The storage periods lasted from 730 to 1,020 days. The corn and wheat samples were stored both as whole kernels and as meals ground to pass a 1 mm sieve. The soybeans were stored as whole beans and as an autoclaved, defatted, and dehydrated meal. The beans stored whole were autoclaved, defatted, dehydrated and ground in the same manner before being tested. The tests of the biological availability of the proteins (N × 6.25) of the stored seeds were carried out two or three times during the storage period by the nitrogen balance procedure developed in this laboratory, using growing albino rats as subjects. The utilization of the seed proteins in digestion was measured by the coefficient of true digestibility, making due allowances for the non-dietary nitrogen (metabolic nitrogen) in the feces. The utilization in metabolism was measured by the biological value, representing the percentage of the absorbed nitrogen retained for maintenance and for growth under standardized conditions of feeding. The methods of testing used detected no consistent or significant deterioration in the digestibility or the biological value of corn and wheat proteins during storage for two to three years, either as whole kernels or as a ground meal. Soybeans stored for 1,020 days as raw whole beans deteriorated definitely and significantly in both the digestibility and the biological value of their nitrogen (protein). Under the conditions of storage imposed, the average impairment in digestibility of nitrogen amounted to 10 percentage units, while the impairment in biological value averaged 13 percentage units. The total impairment in the nutritive value of nitrogen, as measured by the net utilization values, amounted to about 17 percentage units, representing a deterioration in the nutritive value of conventional protein (N × 6.25) of 28%. Since the preheating of soybeans prior to storage prevented this deterioration largely or entirely, the reactions responsible for observed changes are probably largely enzymic in character and involved in the respiration of the seed embryo, which makes up 92% of the whole seed. The possibility of sugaramino acid involvement is discussed. Working with samples stored for 1,020 days, methionine supplmentation improved the biological value of both storage samples of soybeans, as it does that of raw or heated freshly harvested beans; in the case of the samples stored raw and as whole beans, the average increase in biological value amounted to 6 percentage units, while for the preheated sample the increase averaged 20 percentage units. These facts permit the conclusion, first, that the damage inflicted by storage on the metabolic utilization of soybean proteins relates primarily to the cystine or methionine (or both) components of soybean proteins; and, second, that the greater storage damage in the unheated beans involves also other amino acids than cystine or methionine. From the results of these and other studies it appears that the nutritive value of the proteins of cereal grains stored under conditions preventing insect infestation and mold growth (by lowering moisture content below a critical level) is not appreciably altered by seed respiration over long periods of time. This is not true, however, for the soybean, the proteins of which may suffer considerable nutritional damage after one year of storage. The distinction between the two types of seed may reside largely in the fact that the living part of the seed, the embryo, constitutes only a small part of the cereal seed but a predominant part of the legume seed.