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tion include high pH, temperature, and time of exposure. Since protein concentration did not affect LAL formation, intra- rather than intermolecular cross-links are formed in gluten proteins. Hydrolysis of gluten by pronase followed by alkali exposure of the resulting peptides formed less LAL as compared with the undigested protein (72). The maximum amount of LAL formed when wheat gluten was treated with 10% sodium carbonate (Na2CO3) by weight at pH 10.5 at 100 C for 60 min was 22 mol/kg (73). Addition of malic acid to the Chinese wheat noodles treated with 5% Na2CO3 before drying resulted in the reduction of LAL content to 14.4 mg/kg. Treatment of instant noodles with malic acid completely prevented LAL formation, presumably because the acid protonates the reactive -NH2 groups of lysine side chains to the nonreactive -NH + form. The 3 LAL content of Finnish whole grain cereal powder ranged up to 265 mg/kg; apple porridge powder, up to 241 mg/kg; and rice porridge powder, up to 19 mg/kg (74). Exposure of barley to sodium hydroxide induced the formation of 1.16 g of LAL/kg of the grain (75). Since barley contains about 10% protein whose lysine content is between 2% and 3% (76), about one-half of the lysine seems to have participated in LAL formation. Liming of corn meal during the preparation of tortillas involves exposure to calcium hydroxide and heat. Studies by Sanderson et al. (77) showed that (i) up to 1339 g of LAL/g of protein was present in corn our exposed to high concentrations of sodium hydroxide; and (ii) the LAL content of tortillas was 810 mg/kg protein and of commercial masa, 200 mg/kg. In related studies, we found that the LAL content of white dent corn tortillas before baking was 0.16 g/16 g N and after baking, 0.36 g/16 g N (78). This result shows that the baking process induces the formation of LAL. Additional studies showed that alkali-treated high-lysine corn protein isolate treated with sodium hydroxide contained 1.42 g LAL/16 g N. The corresponding value for calcium hydroxide treated samples was 1.51 g LAL/16 g N. These observations suggest that consumers of tortillas prepared from more nutritious high-lysine corn may ingest high amounts of LAL. Our studies also showed that cystine content of the high-lysine corn protein was nearly lost as a result of both treatments. Legumes
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Exposing soy proteins to alkaline conditions (pH 8 14) for various time periods (10 480 min) and temperatures (25 95 C at 10 C intervals) destroyed all of the cystine and part of the arginine, lysine, serine, and threonine residues at the higher pHs and temperatures (34, 45). These losses were accompanied by the appearance of LAL and unknown ninhydrin-positive compounds. LAL formation was suppressed by protein acylation of amino groups with acetic and succinic anhydrides and by addition of SH-containing compounds, copper salts, and glucose. Free and protein-bound LAL was stable to acid but not to basic conditions used for protein hydrolysis.
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LAL concentrations of less than 500 g/g in mildly treated rapeseed products were similar to those present in casein, soy, and other food products (79). The results demonstrate the need for careful control of reaction conditions during protein extraction at high pH in order to minimize LAL formation. Dairy Products
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All milk proteins have a high lysine content, which can react with the dehydroalanine derived from phosphoserine side chains to form LAL. Since casein isolation on an industrial scale involves isoelectric precipitation followed by neutralization at alkaline pH (80, 81), and since heat is widely used to pasteurize and process milk (82), the effect of alkali and heat on the formation of LAL and other unusual amino acids has been widely studied in order to de ne its signi cance for food quality and nutrition. The LAL content of raw and pasteurized milk (in mg/kg protein) ranged up to 15; of whey protein concentrate, up to 145; of ultra-high-temperature (UHT)-heated milk, up to 400; of autoclaved milk, up to 880; of sodium caseinate, up to 1530; and of calcium caseinate, up to 1560 (10, 83 92). The LAL content of 54 Finnish milk protein concentrates and isolates ranged from 0 to 143 mg/kg of protein (74). The corresponding values for milk-based infant formulas, baby foods, and formula diets exceed 300 mg/kg in some cases. Commercial milk products contain signi cant amounts of LAL. The LAL content of processed cheeses with added caseinates ranged from 50 to 1070 mg/kg protein (74, 93, 94). The mean value for imitation mozzarella cheeses of 54 mg/kg was about 50 times greater than the corresponding value for the natural cheeses. The LAL content of pasteurized milk (in mg/kg protein) was 0.44; of natural mozzarella cheeses, 0.4 to 4; and of processed and imitation mozzarella cheeses, 15 to 421. These observations are the basis for the suggestion that the LAL content appears to be a good indicator to distinguish natural from imitation mozzarella cheeses (39). Because peptide bonds in cheese proteins slowly hydrolyze during storage due to the action of proteolytic enzymes (94a), and since cleavage of peptide bonds affects both LAL formation and racemization (37, 95, 96), comparisons of LAL content should be made with cheeses of the same storage history. Infant Formulas
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The LAL content of commercial powdered infant formulas ranges up to 920 ppm and of liquid formulas up to 2120 ppm (Table 6.1.1). The amino acid content of 13 liquid and powdered milk-based infant formulas produced in Italy showed a marked difference in LAL content between liquid and powdered samples (97). The powders contained low concentrations of LAL, whereas the LAL content of the liquid samples ranged up to 1032 mg/kg protein. LAL content was a sensitive index of heat damage, and correlated with other indices such as hydroxymethylfurfural content and reduction in
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