Study On The Changes Of Glycosidic Aroma Precursors And β-glycosidase Activity In Citrus And Their Correlations

Oranges are the largest consumption fruit in the world with rich nutrition and high commercial value. However, the aroma active compounds lost seriously during the process and storage of oranges, which results in the quailty deterioration of the products. It is found that glycoside hydrolase can contribute to the hydrolysis of the glycosidic aroma precursors to increase citrus aromas. The prsent study focused on the varying pattern of beta-glycosidase activity and bound volatile compounds and free volatiles from peel and pulp during oranges growth process through the determination of beta-glycosidase activity and aroma active compounds during various growth periods. Then SPSS software was used to assess the correlation of beta-glycosidase activity and bound volatile compounds. The main research results were listed as follows:(1) Response surface method is used to ascertain the determination condition of beta-glycosidase enzymes activity. Additive amount of substrate p-NPG of4mL(0.01mol/L), the response time of49.59min, reaction temperature of40.85℃, the largest detected enzyme activity of beta-glycosidase was8.96×10-3U/g based on the above conditions. In this condition, the experiment value of beta-glycosidase activity was close to the model predicted values. Considering the practical operation feasibility, the measurement conditions were improved as follows, p-NPG addition of4mL(0,01mol/L), the response time of49.6min, reaction temperature of40.9℃.(2) The enzymology properties of beta-glycosidase in oranges were studied, then the changes of beta-glycosidase activity from peel and pulp during various growth periods were determined. The research results were listed as follows:The michaelis constant of beta-glycosidase Km=132.021mol/L, maximum reaction velocity Vmax=0.163U/g. The optimum reaction temperature of beta-glycosidase was around40℃, and beta-glycosidase showed relatively high thermal stability at30℃. High and low temperature can inhibit the enzyme activity. Beta-glycosidase enzyme was sensitive to the changes of pH value. PH value between5and6, enzyme activity was larger, and the enzyme activity decreased continuously with pH rise and fall. Moreover, strongly acidic and alkaline environments can make enzyme inactivation.(3) The research results about metal ions activator and inhibitor showed that the enzyme activity of beta-glycosidase from oranges was activated by four kinds of metal ions including Mn2+, Ca2+, Mg2+, Ba2+, where the activation effect of the Mn2+was most obvious. The effection of three kinds of metal ions including Co2+, Na+, K+on beta-glycosidase activity was not significant. But Zn2+, Fe2+, Cu2+, Ag+, Hg2+, Fe3+had different degree of inhibition to enzyme activity, in which the inhibitory effect of Ag+, Hg2*, Fe3+were most significantly. Moreover, the Fe3+could lesd the inactivation of beta-glycosidase.(4) Diversity and changes of aroma compounds in orange peel and pulp from different growth phase were studied using electronic nose and GC-MS. The electronic nose analysis results showed that the aroma difference between original period and other growth period was relative more pbvious, but the differences among other growth periods were not much obvious. The differences of pulp aroma among all growth periods were found, but the differences were not obvious. This showed that comprehensive difference of aroma type was not obvious during orange growth periods, and that aroma signal strength varyed.(5) The content and species of bound volatile compounds from orange peel in each period were different. Species and content had a downward trend as a whole, the bound volatile compounds of highest content in orange peel during original period, but the content in medium and late maturity dropping to around10%of original period. Bonding aroma compound p-Vinylguaiacol was checked out from peel in five consecutive months. Benzyl alcohol was also detected from peel in four months.There were differences in content and species of free volatiles from orange peel in each period, both species and content showing a rising trend on the whole. There were large variations in content of free volatiles in orange peel, free volatiles with few species and little content in original period, but achieving maximum in the late ripe and the content being5.4times of original period.(6) The content and species of bound volatile compounds from orange pulp in each period were also different. Species and content had a downward trend as a whole. But the content of bound volatile compounds and free volatile compounds in pulp was less than that in peel in every growth period. The content of bound volatile compounds of pulp in original period was highest, but the content dropped a lot In the advanced and mature period. Vanillin and p-vinylguaiacol were checked out in pulp bound volatile compounds in five consecutive months.There were differences in the content and species of free volatiles in pulp at each period, the content of free volatiles in pulp showed a downward trend on the whole. There were large variations in content of free volatiles in orange pulp, and the content of free volatile compounds droped to the minimum value in mature period.(7) Informed of correlation analysis by SPSS software, there were significantly negative correlation between beta-glycosidase activity and the total content of bound volatile compounds in orange peel and pulp during growth process. Moreover, p-vinylguaiacol from peel, vanillin and p-vinylguaiacol from pulp were negatively correlated to beta-glycosidase activity, respectively.