Hollow fiber ultrafiltration was successfully applied to obtain a clear, amber-colored pear juice. For the three hollow fiber membrane cartridges tested (50,000, 30,000, and 10,000 dalton molecular weight cut-off), the process parameters were optimized and found to be similar. The permeate flux increased with increased transmembrane pressure and then declined. Flux reached a maximum at an average transmembrane pressure of 157 kPa with an average feed stream velocity of 0.15 meters/set at 5O'C. Higher flux was obtained at higher temperatures within the temperature limitations of the membrane. Flux decreased linearly with the logarithm of the concentration.
Stability, substrate and inhibitor specificity and electrophoretic properties of a crude polyphenol oxidase (PPO) preparation extracted from d'Anjou pears (Pyrus communis L.) were investigated. The pH optimum of PPO occurs at 7.0. Heat inactivation of PPO followed first order kinetics and approximately 50% of PPO activity was inactivated after heating for 11.7, 6.25, 2.24 and 1.1 min at temperatures of 70", 75", 80" and 85"C, respectively. Crude PPO extract was active towards o-dihydroxyphenols, but inactive towards monophenols. Disc electrophoresis revealed eight active isozymes toward catechol, 4-methyl catechol, chlorogenic acid, caffeic acid, dopamine, d-catechin and DL-dopa. Similar electrophoretic patterns were observed with all substrates. No differences in the electrophoretic patterns were observed between fresh PPO preparations and preparations which had been frozen or dialzyed. Pear PPO was relatively sensitive to most inhibitors tested with the exception of sodium chloride. Of the inhibitors studied L-cysteine and diethyldithiocarbamate were the most effective against pear PPO.
MATERIALS & METHODSTwo purified fractions of strawberry polyphenoloxidase (PPO) were used in model systems containing D-catechin alone or in combination with either pelargonin or cyanin. Km values using D-catechin were 0.5 mM and 0.41 mM and Vmax 82,700 and 18,800 nmoles Oz/min/ mg protein, respectively, for each isozyme. Reaction of PPO with Dcatechin led to the formation of yellow-brown pigments with maximum absorbance at 390 nm. Little PPO activity could be detected using cyanin as a substrate and no activity with pelargonin. PPO and D-catechin together caused the loss of 50 to 60% of the anthocyanin pigments after 24 hr at room temperature with the formation of a precipitate.
When polyphenoloxidase (PPO) was exposed to sulfite prior to susbs&ate addition, inhibition was irreversible. Trials to regenerate PPO activity, using extensive dialysis, column chromatography, and addition of copper salts were not successful. Increased concentrations of sulfite and pH levels less than 5 enhanced the inhibition of PPO by sulfite. At pH 4, concentrations greater than 0.04 mg/mL completely inhibited 1,000 units of PPO activity almost instantaneously. This suggested that the HS03-molecule was the main component in the sulfite system inhibiting PPO. Column chromatography, extensive dialysis, and gel electrophoresis did not demonstrate 35S02 bound to purified pear PPO protein. Formation of extra protein bands of sulfite inhibited purified pear PPO fractions on gel electrophoresis was demonstrated. This and other evidence suggested that the major mode of direct irreversible inhibition of PPO was modification of the protein structure, with retention of its molecular unity.
Mixing strawberries ground under liquid nitrogen with polyvinylovrrolidone alone or combined with Amberlite XAD4 in oH 4.5 buffer Gelded stable polyphenoloxidase (PPO) extracts. Temperature stability of the enzyme varied with substrate and followed first order kinetics' of inactivation. pH optima were 5.5 with catch01 and 4.5 with 4-methylcatechol. Maximum activity was with D-catechin, followed by 4-methylcatechol and pyrogallol with no cresolase activity. Inhibition occurred with diethyldithiocarbamate, potassium metabisulfite, KCN and dithiothrcitol. Benzenesulfinic acid, cysteine and ascorbic acid blocked the browning reaction without enzyme inhibition. Extraction with Dowex AG 2-X8 resulted in browning. No loss in activity after 95 days occurred with quick-frozen fruit stored under nitrogen at -40°C.
Pear juice concentrates were prepared from juices treated with O-2 mM cysteine and color changes of the concentrates were observed during storage at 1" , 21") and 38°C for 6 months. Initial browning of the concentrates were eliminated by the cysteine treatment of the pear juice. Rate of browning of the concnetrates increased with increasing storage temperature and decreasing cysteine concentration. Cysteine concentration of the concentrates decreased during storage and the rate of decrease was a function of storage temperature. Cysteine appeared to retard the Maillard reaction in the concentrates. No deleterious changes in flavor intensity were noted in the stored cysteine-treated concentrates.
Sublethal amounts of 2,4-D [(2,4-dichlorophenoxy) acetic acid] were applied to several vegetable crops in order to determine effect on yield and quality in a simulated drift experiment. Tomato (Lycopersicon esculentum Mill.) and root crops were most sensitive to 2,4-D; as little as 2.1 g/ha of acid distorted tomato fruit shape and elongated radish (Raphanus sativus L.) roots. All root crops were rendered unmarketable by 10.4 g/ha and gross yields were reduced by exposure to 104 g/ha. Yield of peppers (Capsicum frutescens L. var. grossum) was increased by exposure to 2.1 g/ha and severely depressed by 104 g/ha. Cucumber (Cucumis sativus L.) fruit shape was distorted at 11 g/ha and yield was reduced slightly. Lettuce (Lactuca sativa L.), onion (Allium cepa L.), and cabbage (Brassica oleracea L. var. capitata) were least sensitive to 2,4-D. Exposure to 20.8 g/ha did not reduce yields of lettuce or cabbage. Onion yields were reduced by application of 104 g/ha. Bush bean (Phaseolus vulgaris L.) yield was decreased by exposure to 22 g/ha, but potato (Solarium tuberosum L.) yield increased at 16 g/ha. Herbicide residues in the crop foliage were proportional to the degree of exposure. Residue analysis combined with foliar symptoms may be useful in predicting crop damage following 2,4-D drift.
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