Food alkali-treated

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. 

The nutritional and physiological effects of the alkali-treated food proteins have been studied extensively. Efforts have been concentrated mainly on the effects of lysinoalanine. 

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. 

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. 

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. 

The formation of lysinoalanine in home-cooked food has been reported recently for conditions of preparation well outside the alkaline pH range (18). Is lysinoalanine ubiquitous in the heat- and alkali-treated preparations of man s daily dietary intake, and has it been so for ages If the answer to this question is yes, man must have developed (induced ), since the days of having learned to tame the fire, enzymes or enzyme systems for the effective metabolic utilization of lysinoalanine. The early application of peptides with radiolabeled lysinoalanine becomes a more urgent one. 

Karayianis, N.I. J.T. MacGregor L.P. Bjelfanes. Lysinoalanine formation in alkali-treated proteins and model peptides. Food Cosmetics Tox. 1979,17, 585—590. 

It is Important to know the separate effects of racemization and crosslinking for several proteins, especially those important in food systems. Therefore, the purpose of this study was to isolate racemization from crosslinking, examine the effects of racemization on i vitro digestibility of alkali-treated zein and in vivo accumulation of alkali-trated protein by Isolated rat jejunum. 

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. 

Karaylannis, N. I., MacGregor, J. T. and Bjeldanes, L. F. (1979a). Lysinoalanine formation In alkali-treated proteins and model compounds. Food Cosmet. Toxicol., 17, 585-590. 

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

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. 

Casado F.J., Sdnchez A.H., Rejano L., Montano A. D-Amino acid formation in sterilized alkali-treated olives. Journal of Agricultural and Food Chemistry, 55 3503-3507 (2007). 

Availability. Not all the vitamins in foods are in ab)sorb-able form. Eor example, (1) niacin in many cereals is bound to a protein and cannot be at)sorE)ed through the intestinal wall, unless the food is treated with an alkali to release the vitamin from the inaccessible complex (2) fat-soluble vitamins may fail to be at)sork)ed if the digestion of fat is impaired and (3) vitamin B-12 requires a factor produced in the stomach (the intrinsic factor) for its ab)sorption. 

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. 

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