Czech J. Food Sci. Vol. 25, No. 3: 119–130 Formation of α-Hydroxycarbonyl and α-Dicarbonyl Compounds during Degradation of Monosaccharides Ondřej NovoTNÝ, Karel CEJPEK and Jan VELÍšEK Department of Food Chemistry and Analysis, Faculty of Food and Biochemical Technology, Institute of Chemical Technology in Prague, Prague, Czech Republic Abstract Novotný O., Cejpek K., Velíšek J. (2007): Formation of α-hydroxycarbonyl and α-dicarbonyl com- pounds during degradation of monosaccharides. Czech J. Food Sci., 25: 119–130. The formation of α-hydroxycarbonyl and α-dicarbonyl compounds from monosaccharides (glucose, fructose, arabinose, glyceraldehyde, and 1,3-dihydroxyacetone) was studied in three different model systems comprising an aqueous and alkaline solution of potassium peroxodisulfate (K2S2O8), and a solution of sodium hydroxide, respectively. In total, six α-hydroxycarbonyl (in the form of O-ethyloximes) and six α-dicarbonyl compounds (as quinoxaline derivatives) were identified by GC/MS and quantified. Acetol, glycolaldehyde, 1,3-dihydroxyacetone, methylglyoxal, and glyoxal were the most abundant low molecular weight carbonyls. Within the model systems studied, the yield of α-hydroxycarbonyl and α-dicarbonyl compounds was 0.32–4.90% (n/n) and 0.35–9.81% (n/n), respectively. The yield of α-dicarbonyls was higher than that of α-hydroxycarbonyls only in aqueous solution of K2S2O8 and in the other two model systems an inverse ratio of these two carbonyl types was found. For the first time, ethylglyoxal was identified as a sugar deg- radation product and several mechanisms explaining its formation were proposed. The achieved data indicated that low molecular weight α-hydroxycarbonyl and α-dicarbonyl compounds are predominantly formed by a direct retro- aldol reaction and α- and β-dicarbonyl cleavage. It was evident that some compounds were produced from the sugar fragmentation products. Thus, isomerisation, reduction of dicarbonyls by formaldehyde (cross-Cannizzaro reaction), and mutual disproportionation are possible reaction pathways participating in the formation of α-hydroxycarbonyl compounds. Oxidation and disproportionation of α-hydroxycarbonyl precursors as well as the aldol condensation of low molecular weight carbonyl species (followed by subsequent reactions) play an important role in the formation of several α-dicarbonyl compounds. Keywords: monosaccharides; α-hydroxycarbonyl compounds; α-dicarbonyl compounds; retro-aldol reaction; per- oxodisulfate; acetol; 1,3-dihydroxyacetone; glyceraldehyde; glycolaldehyde; acetoin; 1-hydroxybutan-2-one; lactaldehyde; glyoxal; methylglyoxal; ethylglyoxal; butane-2,3-dione; 1-hydroxybutane-2,3-dione; pentane- 2,3-dione The transformation of reducing sugars repre- α-dicarbonyl compounds, oxoacids, furan and sents a complex of reactions, where many sugar- pyran derivatives) are involved (Angyal 2001). derived carbonyl intermediates (such as aliphatic Fragmentation is considered to be one of the es- aldehydes and ketones, α-hydroxycarbonyl and sential transformations of sugars in foods. During Supported by the Ministry of Education, Youth and Sports of the Czech Republic, Projects COST OC 927 and MSM 6046137305. 119 Vol. 25, No. 3: 119–130 Czech J. Food Sci. the reaction, highly reactive low molecular weight of Weenen and Apeldoorn (1996), which have α-hydroxycarbonyl and α-dicarbonyl compounds been speculated that acetaldehyde (the Strecker are formed. These carbonyls undergo different aldehyde of alanine) is involved in the process of subsequent reactions providing a great number of pentane-2,3-dione formation. secondary products; many of them are coloured, Generally, three mechanisms have been sug- flavour- and/or biologically-active compounds. gested to proceed during sugar fragmentation: It is well known that reducing sugars undergo retro-aldolisation and α- and β-dicarbonyl cleav- fragmentation predominantly in alkaline media, age (Weenen et al. 1993). Reducing sugars, the although it was observed also in weakly acidic solu- corresponding endiols and deoxyuloses, imines tions to some extent (Theander & Westerlund derived from amino acids and related Amadori and 1980). Shaw et al. (1968) identified acetol and three Heyns rearrangement products are the precursors hydroxybutanones during a base-catalysed degra- (Weenen 1998). Since enolisation is not restricted dation of fructose. In addition, hexane-2,5-dione, to any part of the molecule and water elimina- furans, alicyclic compounds and carboxylic acids tion is not restricted in amount, the spectrum of were found indicating some subsequent reactions primary cleavage products is large. For instance, of primary formed carbonyls. Many model stud- 1-deoxy-2,3-dioxo and 1-deoxy-2,4-dioxo species ies showed that fission of sugar chain is signifi- undergo retro-aldolisation and β-cleavage to form cantly enhanced by amines (primary or secondary methylglyoxal and acetol, respectively (Weenen amines, amino acids and proteins). Weenen and 1998). Oxidative degradation via unstable hy- Apeldoorn (1996) investigated formation of droperoxides was also considered as a possible four α-dicarbonyl fragments during degradation pathway leading to fission of carbohydrate skeleton of glucose, fructose, xylose, 3-deoxy-d-erythro- (Vuorinen 1985). The cleavage is often followed hexos-2-ulose and fructosylalanine either alone by many subsequent reactions such as hydration, or with alanine or cyclohexylamine in phosphate elimination of water, oxidation, Cannizzaro reac- buffer solutions (pH 8, 100°C/1 h). The extent of tion, aldolisation or disproportionation giving rise α-dicarbonyls formation considerably depended to a variety of secondary products. For example, on the reaction partners used and increased in the model studies indicated that hydroxyacetone and order: without amine < alanine < cyclohexylamine. 1-hydroxybutan-2-one were the precursors of The formation of α-dicarbonyl fragments (glyoxal, five alicyclic compounds, which were observed in methylglyoxal, butane-2,3-dione) from mono- and fructose cleavage experiments (Shaw et al. 1968). disaccharides under caramelisation and Maillard Huyghues-Despointes and Yaylayan (1996) reaction conditions was studied by Hollnagel found 40 compounds after heating of methylgly- and Kroh (1998). It was found that under the oxal (100°C/8 min). Propane-1,2-diol (18%), acetic reaction conditions used monosaccharides form acid (30%), acetol (7%), and 2,4-dimethyl-1,3-di- higher amount of dicarbonyls than disaccharides oxolane (7%) were major products. It was found and glucose forms more dicarbonyl compounds that glyceraldehyde, together with methylglyoxal, then fructose. It was suggested that fructose tends are even better precursors of 5-hydroxymethyl- to yield cyclic products rather then fragmenta- furan-2-carbaldehyde then monosaccharides and tion products. Several 13C-labelling studies with disaccharides (Cämmerer & Wedzicha 1999). selectively enriched sugars and amino acids were The formation of various phenols and other aro- performed to verify the origin of reactive C2, C3, matic compounds from sugar fragments was also C4 and C5 fragments (Huyghues-Despointes & reported. Theander and Westerlund (1980) Yaylayan 1996; Yaylayan & Keyhani 1999, 2000; reported five phenols identified in a slightly acidic Yaylayan et al. 2000; Weenen 1998). For example, solution of erythrose. glucose/alanine model studies have indicated that The presented work is focused on degradation butane-2,3-dione is formed by a single pathway of sugars in three different media: in an aqueous involving only glucose carbon atoms, whereas and alkaline solution (0.3M NaOH) of potassium pentane-2,3-dione is formed by two pathways, peroxodisulfate (K2S2O8) and in 0.05M solution one involving glucose C-atoms only (10%) and of sodium hydroxide. Peroxodisulfate represents the other (90%) through participation of C1-C2 a multifunctional agent being simultaneously an alanine atoms and C3 unit of glucose (Yaylayan & acidulant, oxidising agent, and radical initiator. Keyhani 1999). These findings corroborate results In aqueous solutions, it decomposes providing 120 Czech J. Food Sci. Vol. 25, No. 3: 119–130 2– – disulphate (S2O7 ), hydrogen sulphate (HSO4 ), chloride were added to 10 ml of cooled reaction triplet oxygen, hydrogen peroxide, sulfate radi- mixture. The pH value was adjusted to 7.5 with –● cal anion (SO4 ), peroxodisulfate radical anion 1M and 0.1M sodium hydroxide and the mixture 2– ● ● (S2O8 ), and hydroxyl radical (HO ). It is known was held for 2 h at 40°C. The pH value was then that sulfate radical anion reacts with water pro- readjusted to 6.0 with 0.1M hydrochloric acid ducing hydroxyl radical and that the rate of this and the mixture was extracted with three 20 ml reaction accelerates with increasing pH (Hayon et portions of diethyl ether. Each extract was dried al. 1972). Hence, the degradation of sugars should over anhydrous sodium sulfate. The combined be initiated by free radicals more extensively in extracts were concentrated to approximately 2 ml the alkaline solution of peroxodisulfate. The al- and analysed by GC/MS. Calibration solutions kaline medium itself significantly enhances the were prepared in water with addition of particular sugar fragmentation. The objective of this study internal standard, derivatised by the same proce- was to identify and quantify low molecular weight dure as described above and analysed by GC/MS. carbonyl fragments arising from sugars in our Following characteristic ions were used for quan-