Lysine browning reaction

Of all amino acids involved in the browning reaction, lysine with its c-amino group is especially susceptible to side reaction and crosslinking and becomes unavailable. 

Since then, many workers have demonstrated the deleterious effect of the browning reaction on the availability of lysine in foods by heating or baking as reviewed by Reynolds (20) and recently by Dworschak (13). 

Of course, the influence of glass transition-associated mobility on rates of reaction below Ts depends on the type of reaction involved. The nonenzy-matic browning reaction between xylose and lysine in a carboxymethylcellu-lose/lactose matrix has been reported to essentially cease below Tg, perhaps because translational motion of the reactants is needed in order for this reaction to occur to an appreciable extent (28). 

Amino acid analysis confirmed that Maillard browning reactions occurred during the frying process, since basic amino acids such as lysine and arginine decreased significantly, whereas all others decreased only slightly (Table I). 

The nonenzymatic browning reactions of fructose and fructose-lysine aqueous model systems were investigated at 100°C between pH 4.0 and pH 12.0 by measuring the loss of reactants and monitoring the pattern of UV-absorbance and brown color development [2]. At all the pH values tested, the loss of fructose was lower in the presence than in the absence of lysine. The promoting effect of pH was clear on the browning development and was in agreement with the earlier studies [4,197],... 

Komthong, P. et al. Effect of high hydrostatic pressure combined with pH and temperature on glucose/ fructose-leucine/lysine/glutamate browning reactions, J. Fac. Agric. Kyushu Univ., 48, 135, 2003. 

Complex mixtures are produced by non-enzymatic browning reactions between thermally oxidized lipids and amines, amino acids and proteins (see Chapter 11.B.4). Interactions between aldehydes, epoxides, hydroxy ketones, and dicarbonyls with proteins cause browning that has been related with losses of lysine, histidine, and methionine. Schiff base formation results in polymerization to form brown macromolecules. Interactions between epoxyalkenals formed at elevated temperatures and reactive groups of proteins produce protein pyrroles polymers and volatile heterocyclic compounds. Much of the published research in this complex chemical area was based on model systems. More stmctural information is needed however with real foods subjected to frying conditions. 

The chemical and enzymatic browning reactions of plant polyphenols and their effects on amino acids and proteins are reviewed. A model system of casein and oxidizing caffeic acid has been studied in more detail. The effects of pH, time, caffeic acid level and the presence or not of tyrosinase on the decrease of FDNB-reactive lysine are described. The chemical loss of lysine, methionine and tryptophan and the change in the bioavailability of these amino acids to rats has been evaluated in two systems pH 7.0 with tyrosinase and pH 10.0 without tyrosinase. At pH 10.0, reactive lysine was more reduced. At pH 7.0 plus tyrosinase methionine was more extensively oxidized to its sulphoxide. Tryptophan was not chemically reduced under either condition. At pH 10.0 there was a decrease in the protein digestibility which was responsible for a corresponding reduction in tryptophan availability and partly responsible for lower methionine availability. Metabolic transit of casein labelled with tritiated lysine treated under the same conditions indicated that the lower lysine availability in rats was due to a lower digestibility of the lysine-caffeoquinone complexes. 

ROS can modify amino acid side chains, with histidine, tryptophan, cysteine, proline, arginine, and lysine among those most susceptible to attack (Brown and Kelly 1994). As a result, carbonyl groups are generated, and these carbonyl concentrations can be measured directly in plasma by using atomic absorption spectroscopy, fluorescence spectroscopy, or HPLC following reaction with 2,4-dinitrophenylhydrazine. 

The factors affecting the Maillard reaction include temperature, time, moisture content, concentration, pH, and nature of the reactants. - It has been shown that, out of 21 amino acids, glycine, lysine, tryptophan, and tyrosine provide the most intense browning when exposed to five saccharides, especially a-lactose. The Maillard reaction is also responsible for the decreased availability of lysine in proteinaceous foods. 

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