SOME EFFECTS OF PROCESSING ON THE NUTRITIVE PROPERTIES OF PROTEINS¹

Abstract

The processing of foods is a large and important industry and has become a necessity in our modern way of life. Through processing, the excess from one harvest may be carried over to the next and perishable foods may be served out of season. I n addition, the processing of foods often makes them more acceptable to the consuming public. Flavors and palatability are often improved and in soqe cases the food value of the nutrient constituents is also increased.On the other hand, some food processing methods are deleterious. Perhaps the most widely recognized food nutrients which are destroyed to various degrees by commercial processing as well as by home cooking are the vitamins. Less recognized but just as important is the loss in protein nutritive quality. Although the deleterious effects of heat treatment on proteins have been known f o r a long time, it is only lately that this important problem has received the attention that it merits.Our interest has been centered around the effects of heat on protein nutritional quality and more specifically on the effects of heat on proteins when reducing sugars such as dextrose are present during the processing.The reaction that takes place is apparently that of the nonenzymatic browning (Maillard) reaction between the free amino groups of the protein and the reducing sugar. This reaction is important in that the storage life of many foodstuffs is shortened because of this reaction which causes darkening, flavor changes, and sometimes formula failure. There is evidence now to indicate that the nutritive value of the protein is also impaired. Likewise, during the processing of some food materials such a reaction also occurs, resulting in color and flavor changes, and more important, in nutritional changes which have only recently been recognized. METHODS Preparation of Protein-dextrose Complexes.In these experiments casein * was used as the protein. Unless otherwise stated the casein-dextrose complexes were prepared by mixing one gram of casein with one gram of dextrose' per gram of pure protein (N X 6.38) and 0.5 ml. of water per gram of casein, and autoclaving for the desired period and a t the designated temperature. Usually one-kilo batches were prepared in shalIow Pyrex pans.Animal Assay Methods. Diets were prepared from treated and untreated casein 80 that the protein content was approximately ten per cent and f a t was added to six per cent.

A Laboratory Method for the Evaluation of Changes in Protein Quality

“…Results of animal experiments of other authors with similar processed casein and meat products agree well with our results [2,5,15,19,22,25]. The results with meat demonstrate, that the often postulated control of lysine alone is not always sufficient for evaluation of changes in protein quality.…”

Section D. Nutritive value of milk and milk products

“…Some loss of nutritive value due to heating has again been demonstrated in animal experiments for casein (637,646,657), and for lactose-containing, but not for lactose-free, lactalbumin (637). Losses were accentuated when these proteins were autoclaved with reducing sugars (637,658,659). Inactivation of aminoacids other than lysine when casein is refluxed with lactose has been postulated (637), and subsequently confirmed in a study of the rate of enzymic hydrolysis (660).…”

“…Inactivation of aminoacids other than lysine when casein is refluxed with lactose has been postulated (637), and subsequently confirmed in a study of the rate of enzymic hydrolysis (660). In heated glucose-casein mixtures loss of lysine, arginine and tryptophan was demonstrated by microbiological assay after acid hydrolysis (659,661) and of arginine, lysine, methionine, histidine and threonine by rat test (659). The greater damage to the protein indicated in the biological experiment may have been due to lesser availability to the rat of amino-acid-sugar complexes that would have been broken down in the hydrolysis that preceded the microbiological assay (cf.…”

Amino Acid Supplements, Lysine Supplementation of a Breakfast Cereal and Milk Combination

“…Many prepared breakfast cereals are composed of wheat, and whether supplementation of these foods with lysine would improve their nutritive value when fed with milk and sugar, as normally eaten, is of interest. This may be of special importance, as Baldwin, Lowry, and Thiessen (3) showed that much of the lysine content of proteins may be rendered unavailable to the body as a result of heat treatment of food, such as occurs in the manufacture of toasted cereals.…”