Browning reaction systems

Even in an excess of ligands capable of stabilizing low oxidation state transition metal ions in aqueous systems, one may often observe the reduction of the central ion of a catalyst complex to the metallic state. In many cases this leads to a loss of catalytic activity, however, in certain systems an active and selective catalyst mixture is formed. Such is the case when a solution of RhCU in water methanol = 1 1 is refluxed in the presence of three equivalents of TPPTS. Evaporation to dryness gives a brown solid which is an active catalyst for the hydrogenation of a wide range of olefins in aqueous solution or in two-phase reaction systems. This solid contains a mixture of Rh(I)-phosphine complexes, TPPTS oxide and colloidal rhodium. Patin and co-workers developed a preparative scale method for biphasic hydrogenation of olefins [61], some of the substrates and products are shown on Scheme 3.3. The reaction is strongly influenced by steric effects. 

From the reaction of iron atoms with a solution of 1,5-COD in methylcyclohexane, brown crystals of Fe(l,5-COD)2 can be isolated in 40% yield (81). The thermal instability (rdec = -30°C) of this substance presents special problems in isolation of a pure product however, subsequent reactions can often be performed on solutions freed from iron metal by low-temperature filtration. The yellow-brown cobalt system, Co(l,5-COD)2, may be prepared similarly (134). 

Hodge, J. E. 1953. Chemistry of browning reactions in model systems. J. Agri. Food Chem. 1, 928-943. 

Clearly, a number of chromogenic systems are available and may be varied between laboratories. The horseradish peroxidase-DAB system is probably the most widely favored as the brown reaction product contrasts well against a wide range of counterstains and mountants. DAB is not only alcohol resistant but can be visualized in the electron microscope. Osmification can produce a more intense dark brown-black color and a similar effect is achieved by post-treatment with nickel sulphate or cobalt chloride but such enhancement is seldom necessary. The reaction product is relatively stable with fading occurring only after years in routine storage. 

Heterocyclic compounds are primarily formed through non-enzymatic browning reactions. Recent studies of deep-fat fried food flavors led to the identification of pyrazines, pyridines, thiazole, 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. Model systems were used to study the participation of lipids in the formation of pyrazines, pyridines, thiophenes and cyclic polysulfides. 

A maltol-ammonia browning reaction produced thirteen pyrazines, two pyrroles, two oxazoles, and one pyridine (12). The major products of this system were 2-ethyl-3-hydroxy-6-methylpyridine and 2-ethyl-3,6-dimethylpyrazine. It is difficult to construct possible formation mechanisms for these compounds from maltol and ammonia. All the carbon atoms must come from maltol. It is possible, then, that maltol degrades into smaller carbon units and that these fragments recombine to form larger carbon units, producing these compounds. Recently, the formation of thiophenones and thiophenes from the reaction of 2,5-dimethyl-4-hydroxy-3(2H)-furanone and cysteine or cystine was reported (13. 14). All these reaction mixtures were reported to possess a cooked meat-like flavor. 

Many 2-alkylpyridines were isolated and identified in roasted lamb fat (45). 2-Alkylpyridines were proposed to form from the corresponding unsaturated n-fatty aldehyde reacting with ammonia upon heat treatment. Ammonia, arising from glycine, can react with nonanal, arising from beef fat, produced 2-butylpyridine in a beef fat/glycine browning model system (6). The same phenomenon was observed in the formation of 2-pentylpyridine from 1-decanal. Maltol produced 2-ethyl-3-hydroxy-6-methylpyridine as a major product in a reaction with ammonia (12). 

Although a maximum rate of formation of pyrazines occurred at approximately aw 0.75 in the NFEM sanples in this study, one cannot assume that this would be the case in all types of foods or reaction systems. Addition of glycerol and hydrophilic polymers to sugar/amino acid systems has been demonstrated to shift the aw at which visible browning will occur (29, 34). It is more likely that the rate of pyrazine formation increases up to a maximum at an intermediate aw range and then decreases again, but not necessarily at aw 0.75. 

The precipitated brown / -TiCl3 undergoes, in certain spots of its surface, an alkylation reaction with the organoaluminium compound present in the reaction system, which results in the formation of active species with a Ti-C bond (or with a Ti-H bond in the case of a metal hydride activator (in these spots, scheme (8) shows the activation of a catalyst precursor such as TiCl3 with AIR3 as the catalyst activator ... 

E. J. Mulders, Volatile components from the non-enzymic browning reaction of the cys-teine/cystine-ribose system, Z. Lebensm. Unters. Forsch, 1973, 152, 193-201. 

L. Vandewalle and A. Huyghebaert, The antioxidant activity of the non-enzymatic browning reaction in sugar-protein systems, Med. Fac. Landbouww. Rijksuniv. Gent., 1980, 45, 1277-1286, via Food Sci. Technol. Abstr., 1983, 15, 2A132. 

K. Eichner, The influence of water content on non-enzymic browning reactions in dehydrated food and model systems and the inhibition of fat oxidation by browning intermediates, in Water Relations of Foods, R.B. Duckworth (ed), Academic Press, London, 1975, 417 134. 

The purpose of this paper is to review the numerous papers published on flavors, tastes and odors resulting from the browning reaction. Investigations of model systems which have been observed under laboratory conditions are considered and their possible significance in basic and industrial processes will be discussed. Speculation on the possible correlation between model system results and specific processed food items will be presented. Results of recent work in our laboratory on flavor notes developed upon heating ribose with various amino acids will be discussed. 

Flavor notes of products of amino compounds-sugars reaction were reviewed with emphasis on amino-ribose system. Meat-like flavor was imparted by browning reaction products of camosine-, citrul-line-, histidine-, glutamic-, 2-pyrrolidone-5-carboxylic-, methionine-, cysteine-, cysteic-, and taurine-ribose. Recent advancements in nitrogen-, oxygen- and sulfur hetercyclics and lipid browning were presented. 

---