Competitive Formation of 2,3-Butanedione and Pyrazines through Intervention of Added Cysteine during Thermal Processing of Alanine-Xylose Amadori Compounds

Abstract

The intervention of cysteine (Cys) on the formation of 2,3-butanedione and pyrazines was evaluated during the thermal processing of the alanine-xylose Amadori compound (AX-ARP). With the involvement of Cys, the competitive formation of 2,3-butanedione and pyrazines was induced. The formation of 2,3-butanedione in the AX-ARP/Cys model was suppressed due to the inhibitory effect of the precursors of 2,3-butanedione like deoxypentosones, while the added Cys in the AX-ARP/Cys model competed with the recovered alanine (Ala) to capture glyoxal and methylglyoxal to make up for the absence of pyrazines in the AX-ARP model at an initial pH value of 7. The content of pyrazines increased from 0 up to 16.48 μg/L (120 °C, 120 min). Exogenous Cys itself showed lower reactivity with 2,3-butanedione through the Strecker degradation reaction; while the pH was increased to 8, the degradative products of Cys were facilitated to consume the residual 2,3-butanedione giving rise to the formation of 2,4,5trimethylthiazole at 120 °C. It was the degradative products of Cys that accelerated the reaction for consumption of 2,3-butanedione rather than Cys itself. Additionally, the inhibitory effect of Cys on 2,3-butanedione formation was weakened under a basic condition, while the promotional effect on the formation of pyrazines was further boosted. With more Cys participating in the process of AX-ARP thermal degradation, the formation of 2,3-butanedione was further inhibited, while the yields of pyrazines were increased.

Cysteine-Induced pH-Dependent Formation of Thiols and Sulfides or 2-Acetylthiazole and Pyrazines during Thermal Treatment of N-(1-Deoxy-d-xylulos-1-yl)-alanine

“…When purified ARP was heated alone, as mentioned above, the content of pyrazines was not detected or it was in a very minute amount, but with the addition of Cys, the formation of pyrazines was detected at pH 7 and above (Table ), which was consistent with the study of Shu et al They suggested that the amino group of Cys was more reactive at pH 7.1 to form pyrazines in the 2,5-dimethyl-4-hydroxy-3­(2H)-furanone and Cys reaction system. It was observed that the content of pyrazines in the presence of Cys was higher than that formed from the ARP heated alone (Table ), which was due to the fact that Cys could compete with the regenerated alanine to capture α-dicarbonyl compounds leading to the increase of pyrazines . In addition, Cys could undergo thermal degradation to release ammonia, which subsequently could react with some reactive intermediates such as glycolaldehyde generated from the degradation of ARP to form more α-aminoketones, and thus promote the formation of pyrazines .…”

“…Volatile compounds were identified by comparing their retention indices (RI) relative to n -alkane (C 7 –C 30 ) standards under identical GC–MS conditions with KI values obtained from the NIST database. The volatile substances detected in different systems were quantified according to the previous study . Different concentrations of the chemical standards and the fixed concentration of internal standards were mixed to construct standard curves.…”

“…It was observed that the content of pyrazines in the presence of Cys was higher than that formed from the ARP heated alone (Table 1), which was due to the fact that Cys could compete with the regenerated alanine to capture α-dicarbonyl compounds leading to the increase of pyrazines. 10 In addition, Cys could undergo thermal degradation to release ammonia, which subsequently could react with some reactive intermediates such as glycolaldehyde generated from the degradation of ARP to form more αaminoketones, and thus promote the formation of pyrazines. 28 As shown in Figure 2, compared with the initial pH, the final pH decreased after heat treatment, which was mainly because of the depletion of the ARP and the formation of organic acids from the oxidation of aldehydes.…”

“…It should be noted that Cys can participate in flavor formation during the thermal degradation of ARP, which would commonly result in an imbalance of flavor. In our previous study, the involvement of Cys was found to suppress the generation of 2,3-butanedione but enhance the formation of pyrazines in the alanine-xylose ARP/Cys model . Furthermore, due to its unique active nucleophilic thiol group (−SH), the introduction of Cys results in lots of volatile sulfur compounds, for example, thiols and sulfides, eliciting obvious odors resembling a meaty aroma .…”

“…In our previous study, the involvement of Cys was found to suppress the generation of 2,3-butanedione but enhance the formation of pyrazines in the alanine-xylose ARP/Cys model. 10 Furthermore, due to its unique active nucleophilic thiol group (−SH), the introduction of Cys results in lots of volatile sulfur compounds, for example, thiols and sulfides, eliciting obvious odors resembling a meaty aroma. 11 It has been pointed out by Zhan et al 12 that, as a result of the thermal reaction of model mixtures containing the riboseglycine ARP and Cys, the main sulfur-containing substances such as thiazoles and thiophenes were formed.…”