Alkali treated food proteins, amino acid

Amino Acid Racemization in Alkali-Treated Food Proteins—Chemistry, Toxicology, and Nutritional... 

In this paper we describe seme of the factors which influence racemization of amino acid residues in food proteins and discuss toxicological and nutritional consequences of feeding alkali-treated food proteins. 

Masters, P. M. and Friedman, M. (1979). Racemization of amino acids in alkali-treated food proteins. J. Agric. 

Masters, P. M. and Friedman, M. (1980). Amino acid racemization in alkali-treated food proteins—chemistry, toxicology, and nutritional consequences. In Chemical Deterioration of Proteins", J. R. Whitaker and M. Fu3imaki, Eds. ACS Symposium Series, Washington, 0. C. 123, 165-194. 

It may also be surprising how easily this racemization may occur. Friedman and Liardon (126) studied the racemization kinetics for various amino acid residues in alkali-treated soybean proteins. They report that the racemization of serine, when exposed to 0.1M NaOH at 75°C, is nearly complete after just 60 minutes. However, caution must be used when examining apparent racemization rates for protein-bound amino acids. Liardon et al. (127) have also reported that the classic acid hydrolysis, employed to liberate constituent amino acids, causes amino acids to racemize to various degrees. This will necessarily result in D-isomer determinations that are biased high. Widely applicable correction factors are not possible since the racemization behavior of free amino acids is different from that of amino acid residues in proteins (which can be further affected by sequence). Of course, this is not a problem for free amino acid isomer determinations since the acid hydrolysis is unnecessary. Liardon et al. also describe an isotopic labeling/mass spectrometric method for determining true racemization rates unbiased by the acid hydrolysis. For an extensive and excellent review of the nutritional implications of the racemization of amino acids in foods, the reader is directed to a review article written by Man and Bada (128). 

M Friedman, R Liardon. Racemization kinetics of amino acid residues in alkali-treated soybean proteins. J Agric Food Chem 33 666-672, 1985. 

These observations cause concern about the nutritional quality and safety of alkali-treated foods. Chemical changes that govern formation of unnatural amino acids during alkali treatment of proteins need to be studied and explained. The nutritional and toxicological significance of these changes need to be defined. Finally, appropriate strategies to minimize or prevent these reactions need to be developed. 

Nutritional and Physiological Effects of Alkali-Treated Proteins. The first effect of the alkaline treatment of food proteins is a reduction in the nutritive value of the protein due to the decrease in (a) the availability of the essential amino acids chemically modified (cystine, lysine, isoleucine) and in (b) the digestibility of the protein because of the presence of cross-links (lysinoalanine, lanthionine, and ornithinoalanine) and of unnatural amino acids (ornithine, alloisoleucine, / -aminoalanine, and D-amino acids). The racemization reaction occurring during alkaline treatments has an effect on the nitrogen digestibility and the use of the amino acids involved. 

Alkali treatment of proteins is becoming more common in the food industry and may result in several undesirable reactions. When cystine is treated with calcium hydroxide, it is transformed into amino-acrylic acid, hydrogen sulfide, free sulfur, and 2-methyl thia-zoIidine-2,4-dicarboxyIic acid as follows ... 

For example, the respective values at pH 10.6 are 0.262, 0.494, and 1.04 mole per cent (ratio of about 1 2 4) at pH 11.2 the values are 0.420, 0.780, and 1.32 mole per cent and at pH 12.5 (pH of 1% protein solution in 0.IN NaOH), the respective values are 0.762, 0.780, and 2.62 mole per cent. (Note that the value of casein approaches that of gluten at this pH). The observed differences in lysinoalanine content of the three proteins at different pH values are not surprising since the amino acid composition, sequence, protein conformation, molecular weights of protein chains, initial formation of intra- versus intermolecular crosslinks may all influence the chemical reactivity of a particular protein with alkali. Therefore, it is not surprising to find differences in lysinoalanine content in different proteins treated under similar conditions. These observations could have practical benefits since, for example, the lower lysinoalanine content of casein compared to lactalbumin treated under the same conditions suggests that casein is preferable to lactalbumin in foods requiring alkali-treatment. 

Other chemicals of possible concern for health and safety foimd in yeast proteins include tyramine (0—2.25 mg/g) and histamine (0.2—2.8 mg/g), formed by decarboxjiation of the corresponding amino acids (38). These compoimds are also found in other fermented (including piclded) foods. Their presence in yeast extracts used as condiments contributes very Htde to human intake. Likewise, the nephrotoxic compoimd lysinoalanine has been identified in alkali-treated yeast extracts, at a level of 0.12 mg/g. However, the chemical occurs at similar low concentrations in almost all heat- and alkaU-treated foods. 

Effects of Alkali. Although alkali had been used to treat certain foods for many years, only recently has it been used widely by the texturized protein industry. Alkali-mediated degradation of proteins has long been known (13, 39-44). Some of the main initial reactions are apparently / -eliminations of cystines and substituted serines and threonines. The products (or their intermediates) then alkylate various other amino acid side chains to form substances like lanthionine and lysino-alanine [N -(DL-2-amino-2-carboxyethyl)-L-lysine]. Possible toxicities are currently under investigation (45, 46), but nutritional losses could also be important. 

Food proteins are often treated with alkali to improve their functional properties. For example, soy protein is treated with alkali and heat during extrusion to produce textured fibers for use as meat analogues and extenders which are widely marketed for human consumption (7). Several studies have shown that soy protein treated with alkali contains significant amounts of racemlzed amino acids (8-10). 

Liardon, R. and Hurrell, R. F. (1983). Amino acid racemization in heated and alkali-treated proteins. J. Agric. Food Chem., 31, 432-437. ... 

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