Lesson 25 : Polycyclic Aromatic Hydrocarbons and Other Processing Products
Products of the maillard reaction
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 N-glycosides, which readily dehydrate to Schiff’s base by the Maillard reaction.
This is the basis for the well known nonenzymatic browning reaction. The Maillard reaction of amino acids with sugars occurs during a variety of food processes, cooking, and in storage. The development of Maillard reactions can result in the losses of essential amino acids (lysine and methionine) and undesired discoloration and off lavor in food. Many Maillard reaction products, especially those with xylose or tryptophan in combination, strongly inhibit mutagenicity in model systems. Some products of the Maillard reaction have been shown to induce allergic reactions.
The reaction end products 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.
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.
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.