| Fresh-cut fruits and vegetables, characterized by convenience, health, safety and fresh, have been more and more important in the fruit and vegetable processing industry. It will be reduced commodity value and shelf life of fresh-cut fruits and vegetables because of browning lead by mechanical processing. In this paper, browning degree, total phenols, free phenols, total anthocyanins, polyphenol oxidase (PPO), peroxidase (POD), phenylalanine ammonia lyase (PAL) activity and their correlation during the storage of fresh-cut purple sweet potato were investigated. The characterization of PPO and POD from purple sweet potato was studied. The browning mechanism of fresh-cut fruit and vegetables was elucidated. To control browning, a safe, economic and suitable method to control browning was screened through the single factor and response surface experiment. The main results showed as follows:1. The phenols of fresh-cut purple sweet potato were identified by combination of ultraviolet spectrophotometry and high performance liquid chromatography (HPLC). It showed the main phenols were chlorogenic acid, caffeic acid and ferulic acid, which contents were94.30±6.05,5.40±1.45,17.54±2.70mg/100g. Chlorogenic acid was substrate of purple potato’s PPO and POD, and caffeic acid was the substrate purple potato’s POD.2. The physiological and biochemical changes of fresh-cut purple sweet potato during storage were measured. The results showed that:during the storage, the browning degree, b*and malondialdehyde (MDA) were in a general trend of increasing, while total anthocyanins and a*were in a general trend of decreasing. Total phenolics content rose then decreased. The activities of PPO, POD, PAL and the content of free phenolic, Chlorogenic Acid showed significantly different between20℃and4℃. Low storage temperature could inhibit the increase of activities of PPO, POD, PAL, reduce accumulation of MDA, delay the degradation of anthocyanins, reduce browning degree and maintain the quality of fresh-cut purple sweet potato. Activities of PPO, POD, PAL, the surface color of a*, b*, C*and the content of MDA, Chlorogenic Acid, free phenols, total anthocyanins were significantly related to the browning of fresh-cut purple sweet potato. The mechanism of browning of fresh-cut purple sweet potato was:PPO, POD and PAL activities of fresh-cut purple sweet potato were rapidly increased after cutting, the increasing PAL activity was promoted the content of phenols, which provided enough substrates for browning; the tissues’s cells were broken after cutting, which made membrane system damage. Thereafter the breakdown of regional localization of enzymes and substrate, then the free phenols were oxidized into quinone catalyzed by PPO and POD, finally polymerized into browning complex, and the cut surface browning occurred.3. The characteristics of PPO in purple sweet potato were investigated. The results showed that the maximum absorption wavelength of PPO was405nm. The pattern of PPO isoenzyme had4bands in peel, while1band in pulp. The optimum temperature of PPO from purple sweet potato was55℃. The PPO became very unstable above50℃, the enzyme would be completely inactivated at90℃for3min. pH value has great influence on PPO activity, the optimum pH was6.5, while there was a shoulder peak at4.0. PPO activity was almost lost at pH9.0. The combining capability of PPO from fresh-cut purple sweet potato to various phenolic substrates were in the order of Chlorogenic Acid> Caffeic Acid> Pyrocatechol. However, the optimum phenolic substrate of PPO from purple sweet potato was Chlorogenic Acid. Among all the inhibitors, NaHSO3, L-Cysteine and ascorbic acid could completely inhibit the PPO activity when their concentration were0.02%; citric acid, sodium chloride and EDTA suppress the PPO activity, with increasing concentrations, inhibition effect had not significant increase; ultrasonic treatment could inhibit PPO activity, the optimum treatment was120W for4min.4. The characteristics of the POD in purple sweet potato were investigated. The results showed that the pattern of POD isoenzyme had4bands. Band1and3were catholicity, band2and4were peel specialized. The optimum temperature of PPO from purple sweet potato was55℃. The POD became very unstable above70℃, the enzyme after treated at80℃and90℃for6and3min, respectively, would be inactivated completely. The optimum pH and concentration of H2O2were6.0and12mmol/L, respectively. The combining capability of POD to various phenolic substrates were in the order of Gallic Acid> Caffeic Acid> Guaiacol> Chlorogenic Acid> Pyrocatechol> Pyrogallic Acid. However, the POD showed the highest activity when combined with Gallic Acid. Among all the inhibitors, L-Cysteine and ascorbic acid could completely inhibit the POD activity when their concentration were0.04%and0.06%, respectively; However, citric acid, sodium chloride, sodium sulfite and EDTA had no obviously inhibiting effect at test concentration on POD activity.5. Response surface methodology was applied to optimize the anti-browning process of fresh-cut purple sweet potato. Based on the results of single factor (ultrasonic power, the concentration of L-Cysteine, citric acid and ascorbic acid) and the response surface experiment, the regreeion equation of fresh-cut purple sweet potato anti-browning conditions which was Y=6.29-0.15A+0.02B+0.097C-0.057D-0.013AB+0.055AC-0.028AD+0.083BC+0.11BD-0.15CD+0.25A2+0.039B2+0.059C2+0.054D2were established by the statistical method, the regression coefficient was0.899, indicating that the response surface model regression was fit well. The optimum anti-browning conditions of fresh-cut purple sweet potato were as follows:ultrasonic power was130W, the concentration of L-Cysteine, citric acid and ascorbic acid were0.03%,0.20%and0.06%, respectively. Under the optimal conditions, the actual value (6.10±0.09) of browning degree of fresh-cut purple sweet potato which stored at4℃for15days, was close to the calculated value (6.04), which proved that this model can well predict the relationship between browning degree and anti-browning conditions of fresh-cut purple sweet potato.6. Fresh-cut purple sweet potato was treated by the optimized anti-browning conditions. The results showed that treatment group could slow down the declining of a*, C*and the rising of b*,△E, and which could maintain the color of fresh-cut purple potato. The browning degree of purple sweet potato in treatment group were obvious lower than control (p<0.05), and the activities of PPO, POD had the same tendency, which showed that the optimized anti-browning treatment could effectively slower the browning and physiological metabolism of purple sweet potato, and maintain its quality. |
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