Nonenzymatic browning products

This book is based on the symposium that was designed to assess the current perspective and future direction of research on the nutritional bioavailability of zinc. Inhibitors suspected of interfering with the absorption of zinc are some factors that infiuence bioavailability. Phytates, dietary fibers, proteins, nonenzymatic browning products, and certain micronutrients are among these substances. These inhibitors are covered in various chapters. 

M29. Monnier, V. M., and Cerami, A., Detection of nonenzymatic browning products in the human lens. Biochim. Biophys. Acta. 760, 97-103 (1983). 

Mills F.D., Weisleder D. and Hodge J.E. (1970) 2,3-Dihydro-3,5-dihydroxy-6-methyl-4//-pyran-4-one, a novel nonenzymatic browning product. Tetrahedron Lett. 15, 1243-6. 

As part of extensive studies lasting over 30 years on the structures of chromophores involved in nonenzymatic browning reactions, two intensely orange, previously unknown, compounds have been identified (2R,8aR)-l and ZS, 8aR)-4-(2-furyl)-7-[(2-furyl)methylidene]-2-hydroxy-2//,7//,8a//-pyrano[2,3-3]pyran-3-one <1998CAR215>. Additional studies on the single Maillard reaction products of these compounds have also been reported <1998JFA3912>. 

Our study also investigated the effect of water activity (a ) on the kinetics of the formation of pyrazines. water activity is defined as the ratio of partial pressure of water in a food to the vapor pressure of pure water at a given temperature. Nonfat dry milk (NFEM) was chosen as a model system for this study since NFEM and lactose/casein systems which had undergone nonenzymatic browning were found to contain pyrazines (21. 22). The current study investigates the effect of increasing product over the range of 0.32 to 0.85 on the rate of formation of pyrazines. 

These nonenzymatic reactions are responsible for numerous changes on food properties and may impair food safety. Although these reactions are of great importance in the production of aroma, taste and color, they are often accompanied by a reduction of the nutritive value of different foods and by the formation of toxic compounds harmful for human health (Ledl and Schleicher, 1990). Results of nonenzymatic browning can be either desirable or undesirable. The brown crust formation on bread is desirable the brown discoloration of evaporated and sterilized milk is undesirable. For products in which the browning reaction is favorable, the resulting color and flavor characteristics are generally experienced as pleasant. In other products, color and flavor may become quite unpleasant. 

Over time, diagrams were developed relating water activity with enzyme activity, dormancy of stored seed, loss of dry product crispness by moisture absorption, pigments and vitamins degradation, nonenzymatic browning, and fat oxidation. Response curves generally are not linear, and readers working with food or feed formulations are referred to the technical literature about their products. 

Heterocyclic compounds are primarily formed through nonenzymatic browning reactions. Recent studies of deep-fat fried food flavors led to the Identification of pyrazlnes, pyridlnes, thlazole, oxazoles and cyclic polysulfides which had long-chain alkyl substitutions on the heterocyclic ring. The Involvement of lipid or lipid decomposition products in the formation of these compounds could account for the long-chain alkyl substitutions. ... 

Nonenzymatic browning (NEB) results from a series of reactions starting with an amino-carbonyl condensation. It represents an important cause of quality loss in foods, and has also been reported as a cause of damage of pharmaceutical products (Kumar and Banker, 1994). Its mechanism and kinetics have been extensively studied in real foods and in model systems. These studies have shown that the rate of the NEB reaction is strongly dependent on the material composition, temperature, water content, water activity, and pH (Hodge, 1953 Labuza et al., 1970 Labuza and Baisier, 1992). 

In foods, we often have what may be called reaction cascades, i.e., a whole series of reactions, partly consecutive, partly parallel, with bifurcations and with more than one reaction pathway leading to the same product. Examples are nonenzymatic browning or Maillard reactions, as well as several changes occurring during heat treatment. Chain reactions may be involved as well, as in the formation of hydroperoxides during the autoxidation of fats ... 

Pyrroles are common products in the reaction of different lipid oxidation products with primary amino groups of amines, amino acid, and protein (Zamora et al., 2000). For example, when squid microsomes were oxidized with iron and ascorbate, TEARS increased simultaneously with the -value (yellowness) and pyrrole compounds, concomitantly with a decrease in free amines. Off-color formation in squid muscle could be due to the nonenzymatic browning reactions occurring between aldehydic lipid oxidation products and the amines on phospholipids head groups... 

Sulfur dioxide is undesirable if the concentration is above a few parts per million. The delicate color of the fruit is impaired and in some cases the metal of the can is corroded. Direct consumption cane sugars produced by the sulfitation process are usually undesirable for canning but the supply is limited to one section of the country. Sulfur dioxide is used in processing beet sugars but not as a bleach as was the practice earlier in the industry. It is now recognized that a few parts per million of sulfur dioxide added before the evaporation of the thin juice act as an inhibitor to the nonenzymatic browning and result in a sugar with less than 1 p.p.m. of sulfur. In the same manner the starch-conversion producers have been able to eliminate the objectionable hydroxymethyl furfural, an undesirable side reaction product in starch conversion. 

It must be remembered that compounds formed by enzymatic processes can react subsequently in a nonenzymatic transformation. Additionally, by examining the chemical transformations involved in the production of compounds via nonenzymatic browning reactions, several routes can be involved in the production of the same compounds from different precursors (2337). 

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