Enzymatic Browning Factors And Control Techniques Of Pear Juice In Different Varieties

Pear is one of the China's major fruits, with excellent nutrition and health value and variety of resources. Pear juice is one of the mail pear processed products, and also one main pear processing export product. Enzymatic browning ofen accur during pear juice processing process, which impairs the quality and marketability of pear juice product, so depth study of pear juice enzymatic browning factors and control technique could provide theoretical and practical significance for elucidating the pear juice enzymatic browning rule, promoting the pear juice processing technology, and improving the pear juice product quality. In this paper, with some large pear varieties of china as raw materials, enzymatic browning characteristics and factors of pear juice in different varieties, enzymatic properties of oxidases related to enzymatic browning from Yali pear, and enzymatic browning control technique of Yali pear juice was studied. Primarily study results and conclusions were as follows:Characteristics of juicing and enzymatic browning in different pear varieties: Characteristics for juicing in the tested seven pear varieties were different, Hwangkumbae had a higher juicing ratio and ratio of soluble sugar to titrable acid, so its juicing property was best. The enzymatic browning degrees of pear fruit in different varieties differed, Huangguan pear and Whangkeumbae juice enzymatic browning was lighter, color values of pear juice were 78.4% and 73.5% respectively, Sydney and Suisho juice enzymatic browning was heavier.Enzymatic browning factors of pear juice in different varieties: The major enzymes related to enzymatic browning of pear juice in different varieties were different, for example, there were significantly negative correlations between PPO activity and color values of Changba pear, Hosui pear, Whangkeumbae and Huangguan pear juice, color values of Suisho and Yali pear juice were mainly related to APX activity; and there were big differences in activities of enzymes related to enzymatic browning in pear fruits of different varieties. Total phenols contents in pear fruits of different varieties varied. The phenolic compouds in pear fruits mainly were (+)-catechin, chlorogenic acid and coumarin, contents of phenolic compounds also varied with pear varieties, for example, except coumarin, other kinds of phenolic acids contents were higher in Changba pear, coumarin content of Suisho was significantly higher than other pears, phloretin was tested from Huangguan pear. pH values of pear fruit in different varieties were unequal, color values of pear juice showed a significant negative correlation with pH values of pear fruit.Enzymatic properties of oxidases related to enzymatic browning from Yali pear: The optimum pH and temperature of three oxidases was 5.6, 4.0, 7.0, and 20, 40, 50°C, respectively. PPO was relative thermostable, POD thermolabile, and APX most thermostable. Appropriate heat treatment and acidic condition could inhibit activities of the three enzymes. The Km and Vm values of PPO for catechol were 0.22 M and 1111 U/ml min; the Km and Vmax values of POD for guaiacol were 0.14 M and 1429 U/ml min, and 0.095 mM and 476 U/ml min for H2O2.; the Km and Vmax values of APX for ascorbic acid were 0.41 mM and 455 U/ml min, and 0.083 mM and 208 U/ml min for H2O2. Ascorbic acid had a strongest inhibition of POD, effectiveness of citric acid on APX was strongest than other enzymes , L-cysteine exhibited the strongest inhibitory effect on PPO, phytic acid could not inhibit POD activity effectively, so the inhibitory effects on each enzyme of the four inhibitors differed.Study on antibrowning technology of Yali pear juice: In Yali pear juice processing process, color value of pear juice fluctuated changed and descented steeply from grind to squeezing phase, so Yali pear juice enzymatic browning mainly occured in squeezing phase. Total phenols content decreased gradually during the processing process, especially in squeezing phase. POD and PPO activities first increased and then decreased, the activities of them increased obviously after squeezing phase. APX activity overall had a gradual decline during the processing process. Using Box - Behnken response surface analysis, it is concluded that the best composite antibrowning agent for Yali pear juice was 0.025% Vc, 0.0065% L-Cys, 0.77% PA and 0.40% CaCl2, which were proved to be effective by experimental studies.