Study Of The Browning Mechanism Of Black Garlic Based On Fructose-Histidine Model System

Black garlic is a kind of deep-processed product of garlic,which is produced by heating fresh garlic at controlled temperature and humidity.Black garlic removes the stink of garlic and has stronger physiological activities such as antioxidant activity.The black garlic cloves are black,soft and porous,sour and sweet,and dose not have spicy taste as fres garlic.During the processing of black garlic,the color of garlic cloves gradually changed from white to brown,and finally to black.The color change is the most intuitive sensory change during the processing of black garlic,and brown formation is also an important indicator of black garlic maturity.But the browning mechanism of black garlic is still unclear,and the relationship between browning mechanism and quality formation of black garlic remains to be further studied.Studing the browning mechanism of black garlic and clarifing the relationship between browning and quality have important guiding significance for optimizing processing technology of black garlic and ensuring the quality.In this thesis,a fructose-histidine model was established.By studying the key substances,investigating the correlation between browning intensity and key substances,exploring the effect of reactant concentration,initial pH and temperature on the browning,isolating the browning products and analyzing their structure,the browning mechanism was explored.The main research results are as follows:?1?The establishment of fructose-histidine browning model.It was found that amino acid can promote the formation of reducing sugars and accelerate the browning reaction.The reducing sugar formation and browning intensity was9.9 and 11.4 times that of control group respectively.The results showed that,fructose was the major reducing sugar and histidine had the most significant effect on browning,52.4 times that of control group.Based on the findings,fructose-histidine model was established.?2?Correlation analysis of browning intensity and key substances in Maillard reaction.During the browning process,fructose content and histidine content decreased with time and fitted well with the first order kinetic equation.The browning intensity and melanoidin content rose in the first stage and then decrease,and the maximum values occured at 18 d and24 d respectively.The browning intensity fitted with zero order kinetic equation in ascent stage as well as decline stage,and the melanoidin content in ascent stage fitted with zero order kinetic equation.The acetic acid content and 5-HMF content increased with time and piecewise fitted with zero order kinetic equation.The acetic acid formation rate constant in earlier stage was bigger than that in later stage,and the 5-HMF formation rate constant reversed.Correlation analysis showed that the melanoidin content and acetic acid content were positively correlated with browning intensity,and the fructose content and histidine content were negatively correlated with browning intensity.There was no significant correlation between 5-HMF content and browning intensity.?3?The effects of reactant concentration,initial pH and temperature on the browning intensity and key substances in Maillard reaction.As fructose content and histidine content increased,the browning intensity,fructose and histidine consumptions,and acetic acid content increased.5-HMF content increased with fructose content and decreased with histidine content.The effect degree of fructose concentration on the browning intensity and key substances was as follows:fructose consumption>browning intensity>5-HMF content>acetic acid content>histidine consumption.The effect degree of histidine concentration on the browning intensity and key substances was as follows:fructose consumption>browning intensity>histidine consumption>acetic acid content>5-HMF content.The effect degree of fructose and histidine concentration on the browning intensity and key substances was as follows:browning intensity>histidine consumption=5-HMF content>fructose consumption>acetic acid content.Initial pH can affect the intensity and shape of ultraviolet and fluorescence spectra of browning model.As the initial pH increased,the browning intensity,fructose and histidine consumptions,and acetic acid content increased,while 5-HMF content decreased.The effect degree of initial pH on the browning intensity and key substances was as follows:browning intensity>fructose consumption>acetic acid content>histidine consumption>5-HMF content.As the temperature increased,the browning intensity,fructose and histidine consumptions,acetic acid and 5-HMF content increased.The effect degree of heating temperature on the browning intensity and key substances was as follows:fructose consumption>histidine consumption>browning intensity>acetic acid content>5-HMF content.?4?Isolation and identification of intermediate product in fructose-histidine Maillard reaction.A method of semi-preparative HPLC was developed for the key product from the fructose-histidine Maillard reaction products.Structural analysis was proformed through two-dimensional NMR.A nitro heterocyclic compound was obtained,and which was identified as2-acetylpyrido[3,4-d]imidazole.?5?Structure analysis of fructose-histidine model melanoidin.The molecular weight,elemental composition and other structure characteristics of fructose-histidine model melanoidin were studied using gel exclusion chromatography,PDA,elementary analysis,infrared spectrum and other methods.The results showed that,the molecular weight of soluble fructose-histidine model melanoidin was 400-4,617 Da,and the ratio of carbon to nitrogen was 4?1.Melanoidin absorb heat and degrade upon 200^oC.Melanoidin with different molecular weights had similar UV-visible spectra,and the absorbance of melanoidin with molecular weight of 1,000-3,500 Da was the highest.Scanning electron microscope pictures showed that the particle size of soluble melanoidin increased with molecular weighr,and increased to 2?m as the molecular weight increased to3,500 Da.The Fourier infrared spectrum showed that,as the molecular weight increased,the absorbance of soluble melanoidin at 1371 cm^-1 decreased,and increased at 1209 cm^-1 and1034 cm^-1.The results of element analysis showed that the content of carbon and nitrogen increased with molecular weight increased,while the content of oxygen decreased.These results provide a reference for the structure analysis of melanoidin.