Sugar Fragmentation in the Maillard Reaction Cascade:  Formation of Short-Chain Carboxylic Acids by a New Oxidative α-Dicarbonyl Cleavage Pathway

Abstract

The formation of short-chain carboxylic acids was studied in Maillard model systems (90 degrees C, pH 6-10) with emphasis on the role of oxygen and water. The total amount of acetic acid formed did not depend on the reaction atmosphere. In the presence of labeled dioxygen or water (18O2, H2 17O), labeled oxygen was partially incorporated into acetic acid. Thermal treatment of 1-deoxy-d-erythro-2,3-hexodiulose (1) and 3-deoxy-d-erythro-hexos-2-ulose in the presence of 17O-enriched water under alkaline conditions led to acetic and formic acid, respectively, as indicated by 17O NMR spectroscopy. The suggested mechanism involves an oxidative alpha-dicarbonyl cleavage leading to an intermediary mixed acid anhydride that releases the acids, e.g., acetic and erythronic acid, from 1. Similarly, glyceric and lactic acids were formed from 1-deoxy-3,4-hexodiuloses, corroborated by complementary analytical techniques. This paper provides for the first time evidence for the direct formation of acids from C6-alpha-dicarbonyls by an oxidative mechanism and incorporation of a 17OH group into the carboxylic moiety. The experimental data obtained support the coexistence of at least two newly described reaction mechanisms leading to carboxylic acids, i.e., (i) a hydrolytic beta-dicarbonyl cleavage as a major pathway and (ii) an alternative minor pathway via oxidative alpha-dicarbonyl cleavage induced by oxidizing species.

Basic Chemistry and Process Conditions for Reaction Flavours with Particular Focus on Maillard‐Type Reactions

“…The authors also evidenced that ␤-dicarbonyl cleavage, which can be seen as an acyloin cleavage or a reverse Claisen-type reaction, represents a general cleavage route for diacylcarbinol intermediates of the Maillard reaction under aqueous conditions and that the frequently reported hydrolytic ␣-dicarbonyl cleavage can be ruled out as a sugar fragmentation mechanism. Furthermore, a new sugar fragmentation pathway has been suggested to occur under oxidative conditions (Davidek et al, 2006b) by oxidative ␣-dicarbonyl cleavage with a Baeyer-Villiger-type rearrangement as key steps. The sugar degradation pathway will mainly depend on the reaction conditions ( Fig.…”

“…A1 and A2, oxidative ␣-dicarbonyl cleavage; B1 and B2, hydrolytic ␤-dicarbonyl cleavage by nucleophilic attack of OH − at the C-2 and C-4 carbonyl group, respectively (adapted fromDavidek et al, 2006b).…”

Formation of carboxylic acids during degradation of monosaccharides

“…Another possible way is direct oxidation of lactaldehyde. The formation of l-lactic acid in pentose and hexose mixtures can also be explained by oxidative α-dicarbonyl cleavage of 1-deoxy-3,4-diuloses providing d-glyceric acid (hexoses) and glycollic acid (pentoses) as the counterparts (for hexoses Figure 2, pathway B2) (Davídek et al 2006b). …”

“…The direct formation of lower aldonic acids by decomposition of unstable 1-and 2-hydroperoxides with the simultaneous production of formic acid is assumed to be also involved during sugar oxidation (Davídek et al 1990). Moreover, the recently described mechanism based on oxidative α-dicarbonyl cleavage of 1-deoxy-d-erythro-hexo-2,3-diulose (Figure 2, pathway B1) (Davídek et al 2006b) may be also considered for the formation of a certain portion of C 4 -aldonic acid with acetic acid as a counterpart.…”

“…B1 and B2, oxidative β-dicarbonyl cleavage. Arrow thickness corresponds with the extent of reaction (adopted from Davídek et al 2006b) …”