| The non-enzymic browning reaction includes Maillard reaction, caramelization and ascorbic acid browning among which Maillard reaction is the most commo n browning reaction during storage or processing of food. The Maillard reaction refers to complex interactions between carbonyl compounds(reducing sugar) and a mino compounds(amine, amino acid, peptide and protein). The Maillard reaction generates volatile compounds and melanoidins which has significant influence in flavor and color of food. Studying of non-enzymic browning reactions will be helpful to control and let the process react towards the direction people wished. Researching part of non-enzymic browning reaction products is meaningless while studying all of products is impossible as the complexity of processes and variety coupled with light concentration of products. C hemical kinetics is a branch of physical chemistry which studies the rate and reaction mechanisms of chemical processes. C hemical kinetic study has always been a basic method to investigate mechanisms and predict changes of the non-enzymic browning reaction, thus allowing people to control the reaction.Besides reducing sugars, ascorbic acid is another compound with potential dicarbonyl group which attracts much attention in non-enzymic browning reaction. The purpose of this study was to quantitatively determine the residual concentration of reactants, UV- absorbance, browning intensity and the concentration of volatiles, then describe separately their rate of consumption and generation, varying with time and temperature in L-ascorbic acid/L- glutamic and L-ascorbic acid/L-aspartic model systems. The major content and results are as follo ws:1. Single factor and orthogonal array design methods were applied to determine which factors affect liquid extraction efficiency of non-enzymic browning systems, such as L-ascorbic acid/L-glutamic model systems. Based on the peak area of pyrazine compounds, the optimum conditions of HS-SPME were 75 μ m CAR/PDMS fiber for the a 30 min extraction at 50 ℃ in 7 m L non-enzymic browning reaction liquid, with Na C l(1.4 g) added as electrolytes. Pyrazine compounds produced by non-enzymic browning had its best effect of extraction and sensitivity. The maximum relative standard deviation was 5.03%, the minimum value was 0.31%, the average was 1.90%. The method has a good repeatability, and can be used to extract pyrazine compounds formed by non-enzymic browning reaction and volatiles produced by non-sulfur-containing amino acids.2. We also investigated the effect of molar ratio on the residual concentration of reactants, UV-absorbance, browning intensity and volatiles in L-ascorbic acid/L-glutamic model systems. The results showed that ascorbic acid would begin self-pyrolysis reaction at first, and then glutamic reacted with its decomposition products. The rate of UV-absorbance increased as the concentration of amino acid increased while browning intensity remained the same, which is different from traditional non-enzymic browning model systems. The concentration of pyrazines was highest when there was an excess of glutamic acid, and it was lowest when there was an excess of ascorbic acid. In the end, the concentration of pyrazines leveled out in the excessive ascorbic acid model system.3. The changes of the UV-absorbance and browning intensity were studied, using a UV spectrophotometer to measure the absorbance at 420 nm and 294 nm. The results showed that absorbance increased with increasing time and temperature during the reaction and the increasing rate of absorbance followed zero order reaction kinetics. The Ea of UV-absorbance and browning intensity were 79.95 k J/mol and 90.09 k J/mol under L-ascorbic acid/L-glutamic model system while 94.59 k J/mol and 93.95 k J/mol under another system. With Pearson correlations coefficients above 0.95, there was a very strong correlation degree between absorbances at 420 nm and 294 nm, absorbances at 294 nm and residue ASA concentration or absorbances at 420 nm and residue ASA concentration.4. The analysis of volatiles produced by non-enzymic browning reaction were studied through GC-MS and GC by using the HS-SPME and the 3-Cl-2,5-dimethylpyrazine as internal standard substance. The results showed that model system of ascorbic acid/glutamic and ascorbic acid/aspartic generated alkylpyrazines as the main product. The volatiles needed a certain time to generate at low temperatures, then its number and concentration increased with increasing time and temperature. The concentration of volatiles produced by ascorbic acid/glutamic model system reached a maximum within a short time and then leveled out at 150℃, while the concentration growth was maintained in ascorbic acid/ aspartic model system. |
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