Many different types of organic reactions lead to the production of brown pigments at moderate temperatures. In spite of many reviews of the subject, there has been no comprehensive organization of the reactions. In this review some relationships are shown to exist among the carbonyl-amino, the nonamino, and the oxidative types of browning. Recent findings have provided the basis for an integration of the several isolated partial theories of browning (Maillard, sugar fission, ascorbic acid, furaldehyde) heretofore proposed. The significance of the occurrence of the Amadori rearrangement in the Maillard reaction is stressed, and a mechanism for browning in sugar-amine systems based upon the rearrangement is outlined. Attention is directed to the little-studied but important role of dehydrogenated reductones in both enzymatic and nonenzymatic browning reactions. Investigations of browning reactions in model systems during the past 3 years are reviewed, with the pertinent older studies, and the results of most of these are shown to fit into the proposed scheme of reactions. A classified directory to the major part of 201 references on browning in nitrogenous model systems (1 940 to March 1953) is included. THE CHEMISTRY OF BROWNING RE-ACTIONS in nitrogenous model systems has been adequately reviewed to the year 1950 by Danehy and Pigman (33).

The Amadori rearrangement of glycosylamine derivatives to l-desoxy-l-amino-2-ketose derivatives occurs (1) slowly in the solid state on storage at 25°and (2) rapidly in hot alcoholic solution in the presence of compounds containing active methylenic hydrogen atoms. These new conditions gave crystalline l-desoxy-l-amino-2-ketose derivatives from glycosyl derivatives of secondary alkylamines, of primary and secondary aralkylamines, and of a primary aromatic amine. 1-Desoxy-lpiperidino-D-fructose in aqueous solution with amino acids produced brown substances much more rapidly at 25°than did D-glucose or N-D-glucosylpiperidine at the same />H. The other desoxyaminoketoses also gave rapid browning reactions with amino acids. Since the Amadori rearrangement occurs spontaneously in N-glycosides in the dry state, and the products of the rearrangement undergo rapid browning reactions with amino acids, a mechanism for non-enzymatic browning in sugar-amine systems based on the Amadori rearrangement is proposed. I. The Amadori RearrangementThe isomerization of N-substituted glycosylamines (N-glycosides) to l-desoxy-l-amino-2-ketose (isoglucosamine) derivatives is known as the Amadori rearrangement. Amadori3 showed that, depending on the manner of heating of D-glucose with a primary arylamine, two different isomers, one labile and one stable, could be isolated. Kuhn, with Dansi,4 Weygand6 and Birkofer,6 identified Amadori's more labile isomer as the N-glucoside (I, R = aryl, R' = H) and the more stable isomer as an isoglucosamine derivative (II). Whereas both Amadori and Kuhn mentioned reaction conditions which did not involve the addition of acids, Weygand,7 in a critical review of the rearrangement conditions, was unable to obtain isoglucosamine derivatives consistently from N-D-glucosylarylamines unless catalytic amounts of acid were present.The Amadori rearrangement has been demonstrated to be general for glycosyl derivatives of primary aromatic amines6•7; however, it has been reported not to occur for glucosyl derivatives of piperidine,6•8 dibenzylamine,6 and alkylamines in general.9 Since we have recently shown10 that Kuhn and Birkofer's "dibenzylamine-N-glucoside''6 was actually 1-desoxy-l-dibenzylamino-D-fructose (II, R and R' = CeHsCHo--), the restriction of the occurrence of the rearrangement to only primary arylamine-N-glycosides11 is not valid. We have now found that the Amadori rearrangement occurs for glycosyl derivatives of piperidine, morpholine, diethanolamine and /3-phenylethylamine under new conditions that appear generally applicable to all types of glycosylamines.

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