Formation Mechanism And Regulation Method Of Thermal Process Flavor Of Methionine-glucose Amadori Compound

Maillard reaction products（MRPs）,considered as the main components of commercial savory flavorings,are usually characterized by strong flavor.But the aroma loss of MRPs was noticeable during the processing or storage,which resulted in the instability of aroma enhancement.Therefore,the improvement of aroma stability of Maillard flavorings was an important research content in the aroma enhancement of food,and also an urgent scientific issue in the field of flavor.Developing stable flavor precursors to form fresh flavor during thermal processing is an effective strategy to improve the stability of aroma enhancement.As important flavor precursors,Maillard reaction intermediates are considered to have superior aroma potential and excellent stability characteristics,which can make up for the defects of traditional Maillard flavorings,and therefore are proposed to be the potential substitutes of MRPs.Although the Maillard reaction intermediates show strong ability in flavor formation during heat treatment,there are some defects of Maillard reaction intermediates such as the low yields in aqueous medium,as well as the fewer types of formed flavor substances and the variation of flavor profile compared to that of MRPs.To solve these issues,the effect of different preparation methods on the formation contents of Maillard reaction intermediates and the formation pattern of flavor substances during the heat treatment of Maillard reaction intermediates were investigated,and the missing flavor components as well as their missing mechanism was clarified.Whereafter,the way for controlled formation of characteristic flavor components of Maillard reaction intermediates was researched to attempt to provide a basis for the formation of expected flavor and its application in food.In views of the low contents of Maillard reaction intermediates prepared in aqueous medium,a binary methionine/glucose（Met/Glc）mixture was established to investigate the effect of vacuum dehydration,dry heat or aqueous heat treatment at 95°C on the contents of methionine/glucose-derived Amadori rearrangement product（MG-ARP）.It was found that the dry heat treatment and vacuum dehydration promoted the chemical balance to transfer towards the formation of MG-ARP by reducing the water content of the reaction model,and the contents of MG-ARP were improved by 6.0 and 5.7 times,respectively.It was also confirmed that the vacuum dehydration treatment could limit the MG-ARP degradation or the formation of by-products,which was a more effective method for the preparation of MG-ARP.The prepared MG-ARP was further identified by mass spectrometry and nuclear magnetic resonance,which verified the prepared product was the target compound N-（1-deoxy-D-fructos-1-yl）-methionine,and there were four forms of cyclic isomerism in aqueous medium.The transformation between the Met,MG-ARP,3-deoxyglucosone（3-DG）,1-deoxyglucosone（1-DG）and browning products during the vacuum dehydration treatment at different temperatures were studied.The results revealed that increasing the dehydration temperature under vacuum promoted the formation of MG-ARP more than the degradation of MG-ARP by combining with the analysis of kinetic models.Adjustment of the weak alkaline condition and the equimolar carbonyl and ammonia was beneficial to the MG-ARP formation but limited the massive degradation of MG-ARP.Therefore,the optimum preparation parameters of MG-ARP are as follows:equimolar Met with Glc,initial p H value of 7.5,and dehydration under vacuum at 95°C for 100 min.The Met/Glc and MG-ARP reaction models were established to analyze the differences in the species and contents of flavor components formed under different reaction temperatures.The results showed that MG-ARP had a superior ability of rapid formation of processed flavor.Moreover,the advantages in the species and contents of flavor compounds formed by MG-ARP model were more obvious under heat treatment with low temperature.However,the fewer species of flavor compounds were also found in the thermal products of MG-ARP compared to that of Met/Glc,reflecting as the fact that some pyrazine compounds were absent.The flavor profiles of MG-ARP and Met/Glc models prepared at different temperatures were analyzed by principal component analysis（PCA）.It was found that the flavor profiles of MG-ARP formed in the temperature of 100 or 120°C were similar to that of Met/Glc in the temperature of 120or 130°C,respectively,indicating that MG-ARP had obvious advantages in the formation of processed flavor especially under low temperature.The differences in the species and contents of pyrazines formed in the Met/Glc and MG-ARP models was systematically studied.Quantitative analyses of pyrazines and methional revealed that MG-ARP generated more methional compared to Met/Glc,whereas lower content and fewer species of pyrazines were observed in the MG-ARP model,which indicated that theα-aminocarbonyls condensation was the rate-limiting reaction in pyrazine formation during the degradation of MG-ARP.Comparing to the availability ofα-dicarbonyl compounds generated from the Met/Glc model,methylglyoxal（MGO）was considerably effectiveα-dicarbonyl compound for the formation of pyrazines during MG-ARP degradation,but the asynchrony on the formation of glyoxal（GO）and recovered Met as well as the low content of diacetyl（DA）were unfavorable for the formation of corresponding pyrazines.Furthermore,the rapid drop of p H during the heat treatment of MG-ARP could limited the condensation ofα-aminocarbonyls for pyrazine formation by rendering them less nucleophilic.Therefore,the inhibitory effect caused by the insufficient interaction of precursors and rapid drops in p H limited the formation of pyrazines during MG-ARP degradation.Increasing reaction temperature could reduce the negative inhibitory effect by promoting the content of precursors.In order to regulate the formation of pyrazines during the thermal degradation of MG-ARP,the effect of additional precursors（Glc,Met,GO,and MGO）on the formation of pyrazines of MG-ARP model was investigated.The results revealed that the additional Met could not only be used as a precursor to alleviate the asynchronous formation between short-chainedα-dicarbonyl compounds and recovered Met during MG-ARP degradation for reducing the negative inhibitory effect of pyrazine formation,but also effectively promoted the retro-aldolization reaction of C₆-α-dicarbonyl compounds to GO and MGO,which finally improved the species and content of produced pyrazines.Although the addition of Glc to MG-ARP was favorable for the formation of GO and MGO,Glc was more likely to compete againstα-dicarbonyl compounds to react with the recovered Met,which subsequently exacerbated the asynchrony of the formed precursors and amplified the negative effect of pyrazine production.Compared with the control group of MG-ARP,the addition of GO and MGO had no significant effect on the content of formed Met,suggesting that the highly reactive GO and MGO was hard to interact with recovered Met due to the delayed recovery of Met from MG-ARP degradation.The potential effect of additional GO and MGO on the degradation of MG-ARP limited the formation of pyrazines.In order to clarify the effect of additional Met on the color development during the thermal degradation of MG-ARP,the resulting color formation and the changes in the concentrations of MG-ARP,Met,andα-dicarbonyl compounds varying with reaction time were investigated under different reaction conditions.It was found that additional Met had little effect on the degradation rate of MG-ARP but got more involved in subsequent reactions and resulted in a decrease in the contents of C₆-α-dicarbonyl compounds.During the MG-ARP degradation,the formation of GO and MGO as key color precursors was facilitated by additional Met,through the retro-aldolization reaction of C₆-α-dicarbonyl compounds,and therefore contributing to the acceleration of browning formation.This mechanism has been further confirmed in a buffer system,and this effect of Met addition was related to the special thioether group contained in Met and the reaction temperature,which would provide a theoretical basis for effective improvement of food color.The recovered Met from the degradation of MG-ARP was partially oxidized to methionine sulfoxide（Met SO）and methionine sulfoxide（Met SO₂）.The effect of the addition of Met and oxidized Met products on pyrazine yields of the MG-ARP model were investigated.The pyrazine contents in the MG-ARP/Met and MG-ARP/Met SO models were found to be lower compared to those in the MG-ARP/Met SO₂ model,and the inefficiency of pyrazine formation in the MG-ARP/Met model was proposed due to the fact that Met oxidation competitively inhibited the oxidation of dihydropyrazines for pyrazine formation in spite of relatively high methylglyoxal（MGO）content.The models of MGO mixed respectively with Met,Met SO,or Met SO₂ were established for further investigation of the mechanism for the involvement of Met oxidation in pyrazine formation.It was observed that the aldolization or carbonyl-amine reaction of Met SO with MGO was another important reason for the inhibition of pyrazine formation,except for the competitive inhibition of oxidative formation of Met SO on dihydropyrazine oxidation,and the adduct of MGO-Met SO was identified by MS/MS.These results also accounted for the phenomenon of low pyrazine yields but high yields of long-chain substituted pyrazines,which were converted from dihydropyrazines with the aldehyde involvement.