| Coffee,as a traditional and historical beverage,is popular in our daily life due to its aroma and refreshing effect.However,the aroma intensity of coffee brew has been reported to have a rapid decrease,inducing the deterioration of aroma profile and aroma quality.In this research,the coffee brew was selected as the research object.The changing rules of aroma were studied during coffee storage and 2-furfurylthiol(2-FFT)was screened as the instable key aroma compound.A quantification method of 2-FFT was also established.Based on this,the aroma binding and releasing rules were studied.After that,the dominant binding precursor was screened and the reversible reaction mechanism between 2-FFT and binding precursor was studied.For irreversible loss,the 2-FFT decreased by Fenton reaction was studied.Based on the acknowledge of 2-FFT binding mechanism,aroma releasers were selected to inhibit the binding reaction path to release the key unstable aroma compounds which may help to enhance the “roasted-sulfur” note and improve the coffee aroma quality.To optimize the preparation condition of coffee,the coffee beans roasting temperature,ground coffee/water ratio and extraction time were optimized based on color determination,off-flavor detection,solid content analysis and sensory evaluation.The optimum condition was that coffee beans should be roasted under 225°C for 15 min,and then the ground coffee was presented with water in the ratio of 1/20(g/mL).The coffee extraction time was 4 min.The aroma increasing and decreasing rules were studied in open and sealed system.The results showed that physical diffusion and chemical changes would be seriously presented in 15 min.During this time,the largest decrease went to 2-furfurylthiol,guaiacol,dimethyl trisulfide,and propionaldehyde.They decreased by 98.7%,62.1%,45.0% and 73.0% respectively.However,in the sealed system,these decreases were relative less.But 2-FFT and guaiacol still decreased by 100% and 39.4% respectively.By the chemical reaction kinetic study of guaiacol and 2-FFT,the decrease kinetics activation energy of guaiacol was 51.2 kJ/mol and the decrease kinetics activation energy of 2-FFT was 26.8 kJ/mol.Thus,2-FFT was more unstable to decrease during coffee storage.Considering the decrease percentage,the instability and the contribution to aroma profile,2-FFT was selected as the key instable aroma for further study.Due to the instability of 2-furfurylthiol and sensitivity to the coffee matrix,it is hard to quantify 2-FFT in coffee brew.So,a special quantification method of 2-FFT in brewed coffee was established.By the addition of cysteine and the use of vacuum evaporation,a coffee blank model was prefabricated.This model has similar emulsion components as regular coffee brew and 2-FFT was removed,so it's suitable to build standard curve.The 2-FFT quantification method was established based on the standard curve made by this blank model.The method evaluation was conducted and it showed this method has a recovery rate of 92.9~99.9%.The linearity ranges from 0.6 ?g/L to 249.0 ?g/L,the limit of quantification is 0.5 ?g/L.This method works well for the quantification of free and total 2-FFT in coffee brew.To find the dominant decrease mechanism of 2-FFT,the hypothesis of 2-FFT binding and releasing behavior was proposed,that is the decrease of 2-FFT was composed of reversible bound 2-FFT and irreversible decreased 2-FFT.The reversible bound 2-FFT could be released to free form by cysteine.The irreversible decreased 2-FFT could not be released again.The formula for calculation the amount of 2-FFT decreased by reversible and irreversible approaches was established and the decreased amounts of 2-FFT by these two routes were presented.The result showed that the reversible bound 2-FFT was the dominant approach for free 2-FFT lost.The reversible bound 2-FFT occupied over 85% amount in all decreased 2-FFT.To further collect the reversible bound rules between 2-FFT and coffee matrix,the storage pH,temperature and time effect on 2-FFT binding and releasing capacity was studied.The results indicated that the increase of storage time and temperature could strengthen the binding reaction between coffee matrix and 2-FFT.However,for storage pH,the results were more complex: at low pH condition,the bound 2-FFT was in a low level and the binding capacity of coffee was lower.However,at high pH condition,the aroma binding capacity of coffee was significantly increasing,especially in the ranges of pH 3 to 6.To further confirm the dominant aroma binder in coffee brew,different nonvolatile compounds including hydroxyhydroquinone(HHQ),coffee lipids,melanoidins and carbohydrates were screened and separated from coffee brew.The effect of these nonvolatile compounds on coffee characteristic aromas were studied and the results showed that coffee lipids were significantly negatively correlated with the headspace concentration of oil soluble aromas.HHQ and melanoidin in low molecule weight were both negatively related with 2-FFT.However,HHQ showed a higher correlated coefficient with 2-FFT decreasing.Through the study of pH and cysteine addition in coffee brew,the mechanism of HHQ binding 2-FFT reaction was studied.It is proposed that the 2-FFT was bound by quinones which was the oxidation products of HHQ.This explanation was further verified by the comparison of reaction activity between HHQ,HHQ oxidation products and the oxidation products with sodium sulfite and showed that the oxidation products of HHQ had a higher aroma binding activity.The 2-FFT decreased via free radical approach was also studied in Fenton reaction system.It was showed that the ascorbic acid could promote the oxidation of 2-FFT when the ascorbic acid was lower than 0.03 g/L.However,when the concentration was higher than 0.03 g/L,the 2-FFT oxidation process could be inhabited.Based on the study of 2-FFT binding mechanism,five potential aroma releasers were chosen to release 2-FFT.Cysteine and ascorbic acid showed a higher 2-FFT releasing efficiency.The using dosage of cysteine was optimized due to the off-flavor threshold determination and the ascorbic acid dosage was optimized based on the 2-FFT remaining amount after coffee storage.Finally,0.05 g/L ascorbic acid and 0.045 g/L cysteine were used in coffee brew to release aroma.The aroma profile and sensory test were conducted before and after cysteine and ascorbic added.It showed that aroma releasers could stabilize 2-FFT,methyl furfuryl disulfide,dimethyl trisulfide,4-vivylguaiacol,4-ethylguaiacol and 3-ethyl-2,5-dimethyl-pyrazine.Correspondingly,the nutty,sulfured,chocolaty aroma and aroma intensity were significantly increased.Based on partial least squares analysis,the correlation between aroma compounds and sensory notes were studied.It showed that nutty was positive contributed by the group of pyrazine,such as 2,6-dimethylpyrazine.Sulfured note was positive correlated with 2-FFT,dimethyl trisulfide.Chocolaty was contributed by 2,6-dimethylpyrazine and 2-FFT and smoky was positive correlated with pyrazines and phenols.By electric nose analysis,it was showed that the coffee added by aroma releasers showed a higher similarity of aroma profile to fresh coffee brew compared to the control sample.Overall,by cysteine and ascorbic acid addition,2-FFT decreasing paths were controlled-released and the 2-FFT rapid degradation problem was improved which benefit to the improvement of aroma quality of coffee brew. |
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