M AILLARD R EACTION P RODUCTS

12.5.3 M AILLARD R EACTION P RODUCTS

The Maillard reaction is a nonenzymatic browning reaction that involves the condensation between a reducing sugar and a free -amino group derived from

amino acid, peptide, or protein. 83 The reaction contributes to the overall develop- ment of brown color and flavor during the heat treatment of foods. The initial

condensation forms a labile glycosyl–amino product, which rearranges to Amadori or Heyns compounds and subsequently degrades to form deoxysones and Strecker degradation products, which are precursors for higher molecular weight polymers, referred to as melanoidins. Little is known about the structure of melanoidins, with no single melanoidin presently isolated for identification. Melanoidins have an effect similar to that of nondigestible fiber, in terms of reducing calcium

absorption. 84 Nonsoluble melanoidins produced from the final stage of the Maillard reaction bind to calcium, thereby decreasing solubility and thus hindering para- cellular absorption in the small intestine. 85,86 The calcium-sequestering potential of melanoidins has been described to be related to the number of acidic donor groups in the food pigment derived from both model browning reactions and coffee

brew. 87 As is the case with other examples of nondigestible fibers, the melanoi- din–calcium ion complex that reaches the large intestine may be transformed to reverse the reduced bioavailabilities of divalent ions if colonic activities related to the complexes compensate for reduced bioavailability in the small intestine. Other studies have indicated that smaller molecular and soluble premelanoidins may also bind to calcium and retain the solubility of this ion, thereby making it available for absorption, as evidenced by increased urinary calcium excretion. 84

In addition to having melanoidins sequester calcium ions, which results in reduced solubility for paracellular absorption, melanoindins may also influence

enterocyte metabolism. Studies using Caco-2 cells have indicated that enterocyte metabolism is not affected by the presence of the Maillard reaction product (MRP),

but in fact may act to facilitate the transport of soluble calcium. 85 Potential enhance- ment of calcium absorption in the large intestine by MRPs is again most likely due to the fermentation of browning products. It has been proposed that the effect of MRPs on microflora present in the large intestine is comparable to other forms of nondigestible carbohydrates, since MRPs can stimulate the nonspecific growth of

anaerobic bacteria, 88 such as lactobacilli, and associated increase in lactic acid concentration, which will contribute to a lowering of gut pH. 86 Moreover, simulated upper-gut digestion using both peptic and pancreatic activity has not led to a pro- duction of low molecular weight products, thus reflecting the relative nondigestibility of MRPs. Despite the potential for MRPs to reduce calcium absorption, due to the in vitro evidence for sequestering activity, there is less indication that the net effect of MRPs to decrease calcium absorption occurs in vivo. Although apparent calcium absorption may indeed be decreased in the small intestine, some degree of compen- sation resulting from an increased apparent calcium absorption in the large intestine is important in evaluating the overall effect of MRPs on calcium bioavailability.

Carbohydrates and Mineral Metabolism

Moreover, since consumption of MRP is also associated with an increase in calcium urinary excretion, there is no evidence for reduced change in calcium retention.