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1.
苹果中多酚氧化酶的性质   总被引:6,自引:0,他引:6       下载免费PDF全文
苹果中的多酚氧化酶是引起苹果及苹果酒褐变的重要原因之一 ,用乳化剂及酚结合剂从苹果中提取多酚氧化酶 (PPO) ,研究其酶学性质 :不同底物的Km、Vm,反应的最适温度、pH及两种底物同时存在时的酶学效应 .结果表明 :4 甲基儿茶酚为最适底物 ,其最适温度 30℃ ,最适 pH4 .5 ,不同浓度咖啡酸对酶的影响不同 ,儿茶素对不同浓度绿原酸的酶促反应影响也不同 .研究几种效应物对酶活力的影响表明 :偏重亚硫酸钠、半胱氨酸、抗坏血酸为强烈抑制剂 ,羧酸类对酶具有抑制效应 ,肉桂酸比同一结构形式的苯甲酸抑制效果强  相似文献   

2.
Model solutions containing chlorogenic acid (CG) and (-)-epicatechin (EP), and phenolics from nine apple cv., were oxidized with apple polyphenoloxidase (PPO). Browning was determined by absorbance at 380–700 nm, and transformed into CIE L*, a* and b*. Multilinear correlations were established between initial and degraded amounts of each phenolic. In model solutions of CG and EP, color values and A400 correlated strongly with initial and oxidized CG and EP. Both phenolics contributed to decrease in L* and increase in A400. Increase ii CG increased a*, but EP had little effect. The imnact of increased EP on b* was two to three times higher than for CG: In each phenol class (hydroxycinnamic (HD), flavan-3-ol (FA), flavonol (FO) and dihydrochalcone (DC) derivatives), degraded amounts were increased or decreased by increasing amounts of other classes. Besides major phenolics, HD (mainly CG) and FA (mainly EP and procyanidin B2), FO and DC, appeared to affect color development.  相似文献   

3.
Enzymatic Browning in Apple Pulps   总被引:3,自引:0,他引:3  
Enzymatic browning in Golden Delicious apple pulp was studied as related to degree of ripeness and temperature (3.5–31°C). Green apple pulp showed the highest rate of browning. This was attributable to differences in ascorbic acid (AA) content and polyphenoloxidase activity in young fruits. The rate of browning determined by CIE L* measurements followed complex temperature dependent kinetics, represented by a multiple linear effects with log time. Equal changes in L* parameter yielded straight lines in a log temperature vs log time plot. Inhibition with AA caused an initial slow rate of browning and a well defined break point associated with exhaustion of antioxidant properties of the AA. The greater the AA concentration the longer the initial period.  相似文献   

4.
Enzymatic Browning Inhibition in Cloudy Apple Juice by Electrodialysis   总被引:3,自引:0,他引:3  
Cloudy apple juice (pH 3.8) was acidified to pH 2.7, at 16–24°C and at a constant current density of 40 mA/cm2, in an electrodialysis (ED) unit composed of an AB electrocell with three compartments, a bipolar membrane (Neosepta BP-1) and two ion-exchange membranes (anion- and cation-exchange). The treatment inhibited 81% of the activity of polyphenoloxidase and slowed enzymatic browning, indicated by a decrease in indice “a” and an increase in “L” on the Hunter Lab colorimeter scale. During ED treatments, conductivity and K+ concentration in apple juice decreased by, respectively, 21% and 86%.  相似文献   

5.
    
Osmotic dehydration (OD) permits the preservation of foods via a decrease in water content and an increase in solute concentration. Osmotically dehydrated fruits such as apple are suitable for the manufacture of desserts, cakes, salads, yoghurts, etc. Different microstructural and engineering aspects of OD are already known, but its effects on enzymatic activity are still unknown. This study analyses the activity of polyphenoloxidase (PPO) in fresh Granny Smith apples and the effect that OD by immersion in sucrose‐saturated syrup has on this activity. The low PPO activity found in the edible parenchyma of osmotically dehydrated apples is attributed to penetration by the osmotic agent and flooding of the intercellular spaces, which produces a low moisture content and a limited O2 concentration in the immediate environment of the enzyme. These results show that OD prevents enzyme–substrate interaction. Thus the low PPO activity would reduce browning of this type of product. Copyright © 2005 Society of Chemical Industry  相似文献   

6.
浑浊苹果汁加工中苹果PPO热失活条件的研究   总被引:16,自引:1,他引:16  
研究了热处理苹果汁(加或不加防褐变剂)、苹果浆使其中的PPO完全及部分失活所需要的温度和时间。结果为:在90℃处理苹果汁15s可使其中的PPO完全失活,而对于苹果浆则需要90℃处理20s以上。加了防褐变剂的苹果汁经90℃处理10s可使其中的PPO完全失活;此外,经80℃处理30s以上虽不能使其中的PPO完全失活,但添加的防褐变剂及果汁自身的pH值可抑制残余的PPO从而防止酶促褐变的发生。70℃处理苹果汁40s不能防止其中的酶促褐变。  相似文献   

7.
The effect of different thermal and chemical pretreatments on quality and enzyme activities of smoked mushroom was investigated. Mushrooms were blanched (water and steam) and dipped in different concentrations of sulphites (SO2), hydrogen peroxide (H2O2), ethylene‐di‐amine tetra‐acetic acid (EDTA) and citric acid for 10 min before smoking. Enzyme activities, colour characteristics, microbiological and sensory examinations were carried out every 2 weeks up to 8 weeks of storage in refrigerator at 4 °C. Results could be concluded that smoked mushroom pretreated with SO2, H2O2 and steam blanching had the best colour values, the better score for all sensory characteristics and lower non‐enzymatic browning compared with other pretreatments. The most effective pretreatment against total aerobic bacteria and yeast & moulds were citric acid, EDTA and steam, and then smoking of mushroom can be attributed to the reduction of microbial counts. The most effective pretreatments on quality and safety of smoked mushrooms were H2O2 and steam. It can be concluded that thermal and chemical treatments, rather than smoking of mushroom, reduce enzyme activities and are suitable to preserve mushrooms.  相似文献   

8.
鲜切马铃薯褐变抑制效果研究   总被引:15,自引:1,他引:15  
本文采用含硫化合物L-半胱氨酸、酸化剂苯甲酸、酶底物竞争性抑制剂肉桂酸、防腐剂山梨酸;螯合剂β-环糊精等褐变抑制剂对鲜切马铃薯褐变的抑制效果进行了研究。通过褐变度与多酚氧化酶活性相关分析和比较确定了不同抗褐变剂实际效果。结果表明,L-半胱氨酸、肉桂酸、苯甲酸对鲜切马铃薯具有良好的抗褐变效果。  相似文献   

9.
采用制作丙酮粉的方法提取了牛蒡中的多酚氧化酶(PPO),并对其酶学特性进行了研究。实验结果表明,牛蒡多酚氧化酶最适温度为40℃,最适pH为6.0,在40℃下热稳定性较好,而在70℃加热3min即可使其完全失活。牛蒡PPO的米氏常数为0.1337mol/L,最大反应速率Vmax为7.387U/min。通过响应面实验设计的方法对防止牛蒡酶促褐变的配方进行了优化,结果表明当复合护色液中EDTA、L-半胱氨酸和柠檬酸亚锡二钠的护色剂的浓度都为0.04%(w/v)时,牛蒡的褐变度最小。  相似文献   

10.
用分光光度计法在波长420nm下,以邻苯二酚为底物研究了兰州百合鳞茎中多酚氧化酶(Polyphenoloxidase,PPO)的部分特性。实验发现百合PPO的活性在4~25℃时,随着温度的升高而升高,25℃时PPO活性达到最大值,之后PPO活性随着温度的升高而下降;百合PPO有两个最适pH,分别为4.0和6.0,当pH低于4.0、高于6.0时活性均较低。抗坏血酸、异抗坏血酸钠、半胱氨酸在实验用的五种抑制剂中对PPO的抑制作用最强。  相似文献   

11.
研究了热烫温度、热烫时间、热烫固液比以及热烫液加酸量对香蕉热处理过程中PPO酶活以及香蕉浆褐变的影响。结果表明,香蕉热处理的优化条件为:热烫温度为100℃、热烫时间为6min、热烫固液比为1∶5(w∶v)和柠檬酸浓度为0.6%。在此条件下进行验证实验,测得香蕉浆中的残留PPO酶活为16U,香蕉浆液的褐变度仅0.072,对控制香蕉在加工过程中的褐变有明显效果。   相似文献   

12.
Browning and changes in the composition of sliced and whole carambola (Averrhoa carambola L.) fruit during storage were investigated. Susceptibility to browning after slicing, packaging and storage for 4 wk at 4.4°C varied considerably between four cultivars and five selections. There was no difference in browning susceptibility between fruit harvested at mature green or breaker stages of maturity. Freshly sliced carambola browned only slightly when exposed to air, but packaged slices that had been stored for 2 or more wk at 4.4°C browned rapidly (within 6 hr) when exposed to air. Whole fruit stored at 4.4°C for up to 6 wk, then sliced, showed much less susceptibility to browning. Ascorbic acid decreased and polyphenoloxidase activity increased in carambola slices during storage, but less in whole fruit. Treating slices with 1.0 or 2.5% citric acid + 0.25% ascorbic acid (in water) prior to packaging was very effective in limiting browning.  相似文献   

13.
Inhibition of enzymatic browning and decay on cut surfaces of Golden Delicious apple using ascorbic acid, cysteine, sodium chloride, calcium chloride, citric acid and sodium ascorbate alone or in combinations was investigated at 4 and 10 °C for a storage period of 0, 7 and 14 days, in an attempt to find the most effective treatment. Apple segments immersed in ascorbic acid and citric acid alone showed visual traces of browning after 7 days storage at 4 °C. After 14 days storage, only ascorbic acid and ascorbic acid plus sodium chloride had moderate browning, while all other treatments were severely affected. However, at 10 °C, only ascorbic acid was effective in reducing the level of browning, although its effect was minimal after 14 days storage. Browning was more severe at 10 °C than 4 °C in all solutions. The browning measurement (a* value) became increasingly positive from 7‐ to 14‐day storage. Microbial decay was absent in all treatments within 7 days at 4 and 10 °C. However, three test solutions showed microbial decay after 14 days storage at 10 °C in addition to the control solution, which showed decay at both 4 and 10 °C storage temperatures.  相似文献   

14.
15.
When endive polyphenoloxidase (PPO) was incubated with a crude papaya latex extract, it rapidly lost its activity. Inactivation was ascribed to thermostable nonenzymatic factors of low molecular weight. These factors were partially purified by a two step protocol including gel filtration chromatography on Biogel P2 and ion exchange chromatography using DEAE Sephadex A25. The PPO-inactivation rate was first order, when either inactivating agent or proton concentration was evaluated. Inactivation could be partially reversed by CuSO4, which suggested that the inactivating factor(s) bound to the copper site of the enzyme. On a more rapid time scale than inactivation, papaya latex extract acted also as a weak noncompetitive PPO inhibitor.  相似文献   

16.
17.
Activation of a crude polyphenoloxidase (PPO) preparation extracted from Williams pears was investigated. Comparison between several activation agents led to the hypothesis of a limited conformational change involved in the activation process. Using ammonium sulphate precipitation followed by hydrophobic and ion exchange chromatography, two PPO fractions, F1 and F2, were 124- and 36-fold purified. F1 contained two forms characterised by isoelectric point equal to 4·2 and 4·5 while F2 contained two other forms associated with isoelectric point of 3·8 and 4·0. The molecular mass determined by gel filtration and confirmed after SDS-page was unique (c 43 kDa). F1 and F2 showed similar apparent Km values, in the 5–11 mM range for the three main phenolic substrates in pear cortex. Chlorogenic acid appeared to be a better substrate than catechins for pear PPO. © 1997 SCI.  相似文献   

18.
    
Abstract: The catalytic oxidation of phenolic substrates by polyphenoloxidase (PPO) causes pericarp browning of postharvest rambutan fruit. In the present study, PPO and its endogenous substrates were extracted from rambutan pericarp tissues (RPT). The substrate extracts were sequentially partitioned with ethyl acetate and n-butanol. The analysis of total phenolic content showed that the most phenolic compounds were distributed in ethyl acetate fraction. By high-performance liquid chromatography (HPLC), (−)-epicatechin (EC) and proanthocyanidin A2 (PA2) were identified from this fraction. After reacting with rambutan PPO, EC turned brown rapidly within 10 min, indicating that it was a significant endogenous substrate. Although PA2 could also be oxidized by the PPO, it turned brown very slowly. In addition, because EC and PA2 were continually catalyzed into browning products by PPO during storage of the fruit at 4 and 25 °C, their contents in RPT gradually declined with the extended storage time. It was further observed that both substrate contents in rambutan fruit storing at 25 °C decreased more rapidly than that storing at 4 °C, suggesting that low temperature inhibited the catalytic oxidation of substrates so as to slow down pericarp browning. Practical Application: Pericarp browning is a serious problem to storage and transport of harvested rambutan fruit. A generally accepted opinion on the browning mechanism is the oxidation of phenolic substrates by PPO. Ascertaining PPO substrates will effectively help us to control enzymatic reaction by chemical methods so as to delay or even prevent pericarp browning of harvested rambutan fruit.  相似文献   

19.
  总被引:2,自引:0,他引:2  
ABSTRACT: Polyphenoloxidase (PPO) activity in western rock lobster (WRL) hemolymph results in blackening, or melanosis , of the tissues after processing. The impact of processing temperatures on WRL PPO activity was evaluated under steady state and non-steady state temperature conditions. Baseline PPO activity and total PPO activity were determined spectrophotometrically. PPO activity showed a heat-induced net activation effect between 60 and 80 °C. Deactivation of the enzyme was not significant until temperatures reached 90 °C. During heating, a balance existed between heat-induced activation and deactivation of PPO. Therefore, maximization of PPO deactivation and minimization of melanosis formation in processed WRL would require the internal temperature of processed lobsters to exceed 90 °C.  相似文献   

20.
苹果酒的酶促与非酶促氧化褐变研究   总被引:3,自引:0,他引:3  
通过对苹果、苹果酒中酚类物质、多酚氧化酶(PPO)的测定表明,氧化褐变贯穿苹果酒生产的始终;酶促氧化在苹果酒生产加工前期占优势,非酶促氧化则一直存在。以(-)-表儿茶素为例,实验中比较了这2种氧化褐变,酶促氧化反应速率是非酶促氧化的约800倍,酶促褐变程度(1.5731/120min)也远大于非酶促褐变(0.4742/316h),且两者的氧化产物也有所不同。苹果酒的氧化褐变是酶促氧化与非酶促氧化综合作用的结果。  相似文献   

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