Study On Non-enzymatic Browning Reaction Of L-ascorbic Acid In Sub-critical Water

The non-enzymatic browning(NEB)reaction based on L-ascorbic acid(ASA)was generally carried out in the conventional reaction system at atmospheric pressure with temperature lower than 150℃.The results showed that the formation types and browning kinetics of volatile substances were greatly affected by the reaction system.There are unique physical and chemical properties about sub-critical water(SCW),and it is often used in natural product extraction,organic catalysis and other fields.It is of great practical significance to explore the formation mechanism and chemical behavior of NEB reaction volatile substances of ASA in SCW system,which is also a key scientific problem to be solved when applying this system to food industry.In this thesis,the effects of various reaction factors in SCW on the species,distribution and formation mechanism of volatile substances,as well as on the thermal and kinetic chemical behavior of browning reaction process were investigated.The main research contents are as follows:1.The self-degrading non-enzymatic browning reaction of ASA in SCW.The effects of different reaction factors,including reaction time,reaction temperature,system pH and concentration of ASA on the self-degradation of ASA in SCW system were firstly investigated.The study included the effects of the reaction factors on the generation of volatiles,degradation kinetics,browning and degradation mechanism.The results showed that the various reaction parameters had different degrees of influence on the self-degradation of ASA,and the most significant influence was the pH of the reaction system.The distributions and contents of volatile products(VPs)were determined by the pH of the SCW system.The major product under the acidic conditions was furfural,while the main products under the basic conditions were the furan derivatives as well as the cyclopentanone compounds.The self-degradation kinetics of ASA and the brown products(BPs)generation in SCW showed that the degradation of ASA was in accordance with the pseudo first order reaction kinetics,and the brown products generation was closely related to the hydrogen ion concentration in SCW.The acidic conditions(pH=5.0)and the lower concentration of ASA(0.03 mol/L)were more favorable to the self-degradation of ASA.The average apparent activation energy(E_a)of ASA in SCW was 15.77,31.70 and 47.53 kJ/mol at pH 5.0,7.0 and 9.5,respectively.Based on the experimental results,the possible mechanism for the generation of volatiles from ASA self-degradation in SCW as well as brown products was proposed.2.The self-degradation reaction of amino acids in SCW.The self-degradation behavior of the several common amino acids with representative structures was investigated in SCW system,and the effect of solution pH on the self-degradation of amino acids to generated volatile substances and the mechanism of amino acid degradation was explored.The results showed that these amino acid were almost not degraded in SCW system because of their simple structures and good thermal stability of glycine,L-alanine,L-arginine and L-aspartic acid.L-serine and L-threonine can be decarboxylated in SCW system to produce intermediate aminoacetaldehyde matrix,and then various pyrazine derivatives,including 2,3,5-trimethylpyrazine,3,5-dimethyl-2-(3-methylbutyl)pyrazine and 3,6-dimethyl-2-ethylpyrazine,among others to be generated.L-phenylalanine in SCW system mainly generated benzaldehyde,phenylacetaldehyde,acetophenone and 7-ethyl-1,3,5-cycloheptatriene.L-cysteine self-degradation would form the key intermediate mercaptoacetaldehyde,and then through interaction with various small molecule aldehydes to generate thiophene,thiazole products.Based on the generation of volatiles from each amino acid in SCW system,the possible generation mechanism of volatiles from each amino acid in SCW system was proposed.3.ASA/glycine Maillard reaction in SCWThe effects of the reaction factors on the volatiles formed by the ASA/glycine Maillard reaction in the SCW system were investigated,which the reaction parameters includes the reaction temperature and pH.The results showed that the volatiles generated from the reaction of ASA and glycine Maillard in the SCW system were mainly pyrazines,pyrroles and pyridines.The effects of reaction temperature on concentration of pyrazines were less obvious,while higher temperature would promote the formation of pyrroles and pyridines as well as other substances.For pyrroles,they were only detected at the reaction temperature of 210℃,which indicated that temperature had a strong influence on formation of pyrroles.The alkaline environment would promote the formation of nitrogen-containing heterocyclic volatiles.Based on the experimental results,the mechanism for the formation of pyrazine,pyrrole and pyridine volatiles from the reaction of ASA with glycine Maillard in SCW system was proposed.4.ASA/L-cysteine Maillard reaction in SCW.The effects of reaction factors on the volatiles generated by the ASA/L-cysteine Maillard reaction in the SCW system were investigated,which the reaction parameters includes the reaction temperature and pH.The results showed that the volatiles produced by the ASA/L-cysteine Maillard reaction were mainly thiophenes,pyrazines and polysulfides.Among of them,thiophene derivatives were formed at all pH conditions.Polysulfides were more suitable for formation under the acidic conditions.And the formation of pyrazine derivatives was major under weakly alkaline conditions.The possible mechanisms of formation of some volatiles were investigated according to the experimental results.