Nonenzyme browning and its effect on protein nutrition

Abstract

The nonenzyme browning involves the thermal decomposition of sugars, the caramelization, the decomposition of oxi‐acids, the so called “Maillard reaction” between amino acids and carbohydrates, the reaction between oxidized fats and proteins, and those alterations which take place by the alkaline treatment of proteins. The Maillard reaction is of secondary importance in the case of foodstuffs and fodders with low carbohydrate contents (meats, meat meal, fish meal). By the heat treatment, the sulphur‐containing amino acids of proteins (cystine, methionine) are damaged primarily because of oxidation, but the decrease in the amount of threonine, serine, tryptophan, and lysine is observable too. According to the formation of enzyme resistant cross‐links, the in vitro and in vivo digestibility of protein decreases after the heat treatment and the communication with oxidized fats. Besides the amino acids mentioned, the possibility of enzymatic break‐off of leucine and isoleucine is reduced too. In the course of the heating of proteins the occurance of racemization has to be considered too (formation of alloisoleucine). The basic mechanism of the reaction between sugars and simple amino acids is already essentially explained: amino‐acids break off after the formation glycosilamines and Amadori products but they are linked irreversibly to some, partly unsaturated decomposition products of sugars, types of 6 and 3 carbon atoms. The decrease in the biological usability of amino acids starts already with the Amadori products. The reactivity of the single amino acids depends on the number of carbon atoms, on the basicity, and on the polarity of the amino acid molecule. The especially highly reactive amino acids of proteins are (1) the essential lysine (because of its 6‐NH₂ group), (2) other types of basic amino acids, and (3) tryptophan (because of the lability of the indole ring), methionine, cystine and threonine. In the Maillard reaction of tryptophan the —NH— group of the indole ring is involved too. The Maillard reaction is highly influenced by the pH of foodstuffs or other agents. The reduction of pH which may be performed by the increase of fermentation in the baking industry, lessens the decomposition of lysine and tryptophan in proteins. The raise of pH in basic domain enhances the Maillard reaction up to a maximal value but a decrease may be observed when the pH is raised further on. In foodstuffs and in other solid protein‐carbohydrate systems the increase of the moisture content generally enhances the Maillard reaction, the sensibility of the single amino acids to the changes in the moisture content is different. In the case of the alkaline treatment of proteins, we must reckon not only with the decomposition of single amino acids, first of all that of cystine by beta‐elimination, but with the formation of some amino acid derivatives as lysinoalanine, lanthionine, and in ornithinoalanine too. Presently lysinoalanine is of toxicological importance as proved by experiences on rats. The isomerization of essential amino acids may appear too during the alkaline treatment of proteins and this bears a reducing effect on the nutritive’ value.