Reactions, non-enzymatic browning

Microbial growth, enzymatic reactions, non-enzymatic browning (reaction between carbonyl and amino compounds), and lipid oxidation are the major deterioration mechanisms that limit the stability of low moisture (o intermediate moisture foods (o.6o < <0.85) and biological materials. 

Numerous review articles (1-20) and a number of books (21-26) have been devoted to the Maillard reaction, viz., the non-enzymatic browning reactions involving amino acids and reducing sugars. 

Volatile compounds formed by non-enzymatic browning reactions are of great importance for the sensory properties of heat-treated foods. 

Non-enzymatic browning reactions play a central role in the formation of food aroma and flavor, especially in heat-treated foods. The purpose of this work is to present sensory data, scattered in the literature, for volatile non-enzymatic browning reaction products and related compounds. The compilation has no pretensions to completeness and only a small part of the extensive patent literature has been covered. Anyhow, it is felt that a compilation of this kind, which has not been available hitherto, would be useful to workers in the field. 

The volatile compounds formed by the Maillard reaction are only one group of flavor compounds in foods. Schutte (1) presents a brief summary of the major classes and their modes of formation from precursors. Some of them can be formed by different pathways. An example is the furans, which can be formed by non-enzymatic browning reaction but also by biotransformation. 

The early scientific discoveries relating to heat induced aroma development can be traced to the work of Louis—Camille Maillard at the University of Nancy during the period of 1912 to 1936 (2). He published at least 8 papers on the subject of the reaction of sugars with amino acids. The Maillard Reaction, or so-called, non enzymatic browning reaction chemistry, has become the focus on a great amount of scientific work (3). 

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. 

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. 

Rinderknecht, H. and Jurd, L. (1958). A novel non-enzymatic browning reaction. Nature 181, 1268-1269. 

Soy protein products undergo the typical non-enzymatic browning reaction (Maillard reaction) in the presence of reducing sugars. The course of this reaction is similar to that of other proteins which have been studied in some detail. This subject has been reviewed recently by Feeney, Blankenhorn, and Dixon (49) as a special aspect of the carbonyl-amine reactions of proteins. 

Kim, N.K. and Harris, N.D. Antioxidant effect of non-enzymatic browning reaction products on linoleic acid, in Trends in Food Science, pp. 19-23 (1989) (edited by A.H. Ghee), Singapore Institute of Food Science and Technology, Singapore. 

Complex mixtures are produced by non-enzymatic browning reactions between thermally oxidized lipids and amines, amino acids and proteins (see Chapter 11.B.4). Interactions between aldehydes, epoxides, hydroxy ketones, and dicarbonyls with proteins cause browning that has been related with losses of lysine, histidine, and methionine. Schiff base formation results in polymerization to form brown macromolecules. Interactions between epoxyalkenals formed at elevated temperatures and reactive groups of proteins produce protein pyrroles polymers and volatile heterocyclic compounds. Much of the published research in this complex chemical area was based on model systems. More stmctural information is needed however with real foods subjected to frying conditions. 

Chapter 12 on Frying contains new material on complex interaction compounds produced by non-enzymatic browning reactions between thermally oxidized lipids and amines, amino acids and proteins. This chapter... 

Hodge, J.E. Rist, C.E. The Amadori rearrangement under new conditions and its significance for non-enzymatic browning reactions. J. Amer. Chem. Soc. 1953b, 75, 316-322. 

Heyns rearrangement products are also intermediates in the non-enzymatic browning reaction of food and, similar to the Amadori rearrangement products, can enter the Maillard reaction cascade as mentioned above (Sect. 2.3). Heyns and co-workers have conducted investigations in this context [112]. 

Saccharides are highly reactive substances that are transformed into many different products, even without the participation of other reaction partners, during food storage and processing. The most common and important reactions of carbohydrates are their reactions with amino compounds that are known as non-enzymatic browning reactions, which also involve reactions of carbohydrates in the absence of amino compounds. Reactions of carbohydrates with amino compounds (proteins, amino acids) are the MaiUard reactions. The products of these reactions are important flavour-active compounds and are yellow, brown or black pigments in many foods and compounds with beneficial properties (e.g. antioxidants), but can also be substances with antinutritional or even toxic effects. 

Alditols occurring in foods as natural components are the result of biochemical reactions, but they can also arise as products of Cannizzaro reaction or other non-enzymatic browning reactions. Many synthetic alditols are used as food additives (sweeteners see Section 11.3.2.2). 

Gluconic and mannonic acids (as well as glycohc, glyceric, lactic and pyruvic acids) are present in relatively large amounts as natural components in many plant materials, as well as in those foods in which non-enzymatic browning reactions take place. Table 4.10 illustrates the composition and content of aldonic, deoxyaldonic and some other acids in chicory root and malt. 

All derivatives of furan-2-carbaldehyde are reactive substances. They are easily oxidised by oxygen to the corresponding acids, which, together with the appropriate alcohols, are also formed from furan-2-carbaldehyde derivatives by Cannizzaro reaction. Derivatives of furan-2-carbaldehyde condense together or with other carbonyl compounds and enter into non-enzymatic browning reactions. Oxidation of 5-hydroxymethylfuran-2-carbaldehyde, for example, yields 5-hydroxymethylfuran-2-carboxylic acid (4-198). Condensation... 

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