The Key Technologies In Processing Of Green Raisin

Green raisin is the featured product in China. Thompson Seedless is the main feed stockof green raisin, while browning is the main problem in process. This paper had concentratedon two complex problems, the biochemical characteristics of polyphenol oxidase (PPO) andthe stability of chlorophyll, which were concerned with the browning of the raisin inprocessing and storage, The mainly obtained results were listed as the following,(1) Purification and characterization of PPO from Thompson seedless grape. The enzymeextraction from grape was carried out using McIlvaine buffer. Sedimented with ammoniumsulfate at30%-90%saturation, purifying with Sephadex G-100column chromatography. Thepolyacrylamide gel electrophoresis showed the enzyme was purified to a single line after theabove treatment. The molecular weight of the enzyme was identified as47.7KD.(2) The biochemical characteristics of PPO. The effect of reaction condition, inhibitor,sugar concentration on PPO activity and thermal stability of PPO were studied. Optimum pHand temperature for grape PPO activity were pH=6.0and T=25℃; PPO showed activityusing the following substances: Catechol, D, L-DOPA, Kmand Vmaxvalues were45mM,34.6mM and500OD/min,384.6OD/min, respectively; the most effective inhibitor wasL-cysteine, followed in decreasing order by sodium metabisulfite, ascorbic acid, EDTA, NaCl,citric acid. It was discovered that metal ions of Mg²⁺and Cu²⁺increased, while Zn²⁺and K+reduced the PPO activity. Sucrose has undoubtedly major inhibition capacity; while fructoseand glucose showed lower inhibition. The enzyme was heat-stable between10to25oC, andshowed significant activity loss at temperatures higher than40oC and completely inactivationat70oC for10min. Thermal inactivation of PPO showed a first-order kinetic with anactivation energy (Ea) of146.1±10.8kJ/mol at pH6.0.(3) The stability of chlorophyll extracted form Thompson seedless grape. The use of phaseseparation between hexane and N, N-dimethylformamide (DMF) permits obtaining achloroplast pigment extract free of Thompson seedless grape. The stability of chlorophyllwasn't chang at low temperature, in weak acidic and alkaline conditions,but it would becomeyellow when the temperature was higher than60oC；The central magnesium atom is easilyremoved in acidic conditions (pH<4), replacing it with hydrogen and forming the unappealingolive-brown pigments, pheophytins. Under neutral condition, glucose and sucrose wouldn'tchang the stability of chlorophyll,but the relative content of chlorophyll in fructose solution would decrease; Adding metal ions such as K+,Ca²⁺,Zn²⁺to chlorophyll can protect itsstability, but its original green wouldn't completely recover.According the Response surface (RSM) analysis on the data of Box-Behnken design, theoptimal conditions for the stability of chlorophyll were set as temperature of38.3℃, pH7.44,fructose concentration of21.4%.(4) Chlorophyll was extracted from Thompson seedless grape at different temperature, pH,sugar, sugar concentration and metal ions, and its stability was studied. Chlorophyll was morestable in20℃and nature pH than in other pH; With the rising of temperature and thechange of pH, the degradation of chlorophyll was more likely to happen. The relative contentchlorophyll wasn't affected by the glucose concentration and adding glucose to pulp was helpto the stability of the chlorophyll at pH3, but adding glucose, sucrose, or fructose to pulp canimprove the stability of the chlorophyll at pH4. The addition of metal ions such was K+,Zn²⁺,Cu²⁺,Mg²⁺helpful to increase the stability of chlorophyll at pH4, but adding to metalion can decrease at pH3.According the Response surface (RSM) analysis on the data of Box-Behnken design, theoptimal conditions for the stability of chlorophyll were set as temperature of23.3℃, pH4.53,fructose concentration28.1%, and glucose concentration42.3%.(5) As a pretreatment method, using steam heating and Zinc chloride dipping together inthe drying of grape can protect the colour, and inhibited the browning of raisin.