In the Maillard reaction, reducing sugars (pentoses > hexoses) condensate with amino acids, producing a mixture of insoluble dark-brown polymeric pigments, termed melanoidins. Aldoses and ketoses react with aliphatic primary and secondary amines of amino acids and proteins to form ^-glycosides, which readily dehydrate to Schiff's base by the Maillard reaction (Figure 19.4). This is the basis for the well-known nonenzymatic browning reaction. In the early stages of the reaction, premel-anoidins are formed, which are water-soluble mixtures of carbonyl and aromatic substances. The Maillard reaction of amino acids with sugars occurs during a variety of food processes, cooking, and in storage. Some products formed are volatile with strong odors. The development of Maillard reactions can result in the losses of essential amino acids (lysine and methionine) and undesired discoloration and off-flavor in food. Many Maillard reaction products, especially those with xylose or tryptophan in combination, strongly inhibit mutagenicity in model systems.
Premelanoidins have been shown to inhibit growth, cause liver damage, and interrupt reproduction in laboratory animals. Maillard products of fructose-glycine and fructose-arginine increase the mutagenicity of 3-amino-1,4-dimethyl 5H-pyridol-(4,3-b)indole. Antimutagenic effects seem to correlate well with antioxidant effects. However, mutagenicity of benzo(a)pyrene is moderately inhibited by such products. Some products of the Maillard reaction have been shown to induce allergic reactions.
Various Glycation Products
Various Glycation Products
FIGURE 19.4 Maillard reaction of amines with aldoses.
Maillard reactions of sugars and amino acids and proteins lead to a cascade of reactions. The reaction end products have been observed in collagen-rich tissues in vivo and in vitro, and are associated with stiffening of artery walls and joints with aging. Plasma glucose reacts with hemoglobins via the Maillard reaction, which along with glycation of lens protein may contribute to complications of diabetes. The Maillard reaction can be prevented by adding carbonyl groups of reducing sugars and regulating the temperature, pH, and water content.
Amadori products result from early Maillard reactions. The products create brown pigments, giving the characteristic color of some cooked foods such as bread crust, as well as volatile compounds that give various odors such as roasting aromas. More than 2000 volatile compounds have been identified. The proportions and the amounts of different Maillard products depend on processing time, temperature, water activity, and pH, resulting in a variety of flavors and colors. Under- or overcooking can spoil the flavor of a meal, depending on the degree of Maillard products formed. Both stored and cooked foods contain Maillard products. The reaction can and does occur at room temperature, and many Maillard compounds are found in uncooked foods, though usually at lower concentrations than in cooked foods.
There has been a recent growing interest in studying the Maillard reaction in vivo, particularly in relation to diabetes and aging. It is thought that the cross-linking between long-lived proteins such as collagen and free sugars, especially fructose, which has a high cross-linking potential, produces glycation end products of Maillard reaction, which contribute to tissue degeneration. There is no reason to believe that Maillard reaction products consumed in food participate in any way in the body's own internal cross-linking reactions that contribute to aging, because the Maillard reaction products are produced as part of normal cellular metabolism and via a separate biochemical pathway.
Products from Maillard reactions have been found to possess antioxidant activity. Maillard reaction products are known to inhibit oxidative degradation of natural organic compounds. Not much is known about the structures of such Maillard reactions products or the mechanisms of their formation. They are found in most cooked foods and have a characteristic brown color. These antioxidant compounds have been found to be formed from histidine and glucose or arginine and xylose, with the amount produced depending on reaction time, initial pH, and molar ratio of reactants used.
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