| Fresh-cut fruits and vegetables are generally refered to the products that remain in a fresh state after they are processed, but the discoloration always takes place, being thus regarded as one of the important factors to the shelf life of fresh-cut fruits and vegetables. Both chufa (Eleocharis tuberosa(Roxb.)Roem.et. Schult) and lotus root (Nelumbo nucifern guertn) serve as appropriate fresh-cut materials; However, certain discolorations have been reported during processing and storing. Yellowing is often observed in fresh-cut chufa during storage, and for fresh-cut lotus root, variance in browning on the opposite sections has also frequently been noticed. The present study aims to investigate yellowing mechanism of fresh-cut chufa and browning orientation of fresh-cut lotus root during storage. Certain regulations for browning orientation of fresh-cut lotus root are also discussed.It may not be some specific enzymatic activities that result in the yellowing of fresh-cut chufa; certain considerations are discussed. reasons as follows: (1) None of the phenols interrelated with browning are found in fresh-cut chufa during storage expect at the initial stage; (2) As the key enzymes for the enzymatic browning reaction, both PPO and POD remain very low in their activities and exhibit little change during storage; (3) The yellowing of fresh-cut chufa could not be prevented by employing ascorbic acid, which has been reported as most widely used for browning control.Further researches have shown that there are three distinct absorption peaks between 200-400 nm in the methanol extract of the yellowed parts from the fresh-cut chufa. Exact chemical identification suggests that the yellowed matter may be flavone or mixture of flavone and anthraquinone. Meanwhile, five microbes of one fungus and four bacteria separated from the yellowed tissue of the fresh-cut chufa have been cultured. Morphologic identification shows that the only fungus falls into Geotrichum, while other four bacteria belong respectively to Salinococcus, Streptococcus, Flavobacterium and Staphylococcus. The microbes in Geotrichum, Streptococcus and Staphylococcus have been proved to play a leading role in causing fresh-cut chufa yellow seriously.Many experiments have substantiated that fresh-cut lotus root represents an obvious browning orientation in that the end-ward sections became brown earlier and more seriously than the head-ward ones. This phenomenon is regarded as induced by the lotus root itself other than the external factors. Furthermore, distinct difference was observed in the water contents of the opposite sections as well as that found in other factors including PAL, PPO, POD activities and the level of membrane lipid peroxidation. It indicated that the water imbalance of the opposite sections of fresh-cut lotus root acted as a key factor in causing browning orientation. A high positive correlation existed between the water content and the activities of browning-related enzymes such as PPO and POD. The activities of these enzymes rose as the tissues lost moisture. As for PAL activity, the trend was similar. Accordingly, the water imbalance of the opposite sections of fresh-cut lotus root might result from the diverse water potential and membrane permeability.Purified PPO was obtained from the fresh-cut lotus root by ammonium sulfate fractionation, DEAE-Sepharose and Phenyl Sepharose 6 Fast Flow columns chromatography. The enzyme was found to be homogenous by SDS-PAGE. The molecular weight of the fresh-cut lotus root PPO as estimated by SDS-PAGE ranged from 65.9 kD to 66.1 kD according to Sephadex G-100 gel fractionation. The purified enzyme had an optimal pH of 6.0 for activity. It remained rather stable when kept in pH 5.6~ pH 6.5 citrate/phosphate or phosphate buffer and at 4℃. Its optimum reaction temperature was 20℃and stable below 40℃. However, the activity decreased dramatically when temperature rose above 60℃. Ascorbic acid, Erythorbic acid, L-cysteine and sodium sulfite could almost completely inhibit PPO activity, while EDTA-Na, citrate acid, NaCl and CaCl2 showed slight inhibition. FeSO4, FeCl3 and CuSO4 could remarkably activate the fresh-cut lotus root PPO activity, especially at a high concentration.Partly purified POD was obtained from the fresh-cut lotus root by ammonium sulfate fractionation, Sephadex G-75 and DEAE-Sepharose columns chromatography. The purified enzyme had an optimal pH of 6.0 for activity. It appeared very stable at pH 4.4, 6.5 and 9.0 respectively when kept at 4℃. The optimum reaction temperature was 30℃, and the thermal stability of it was low. The activity decreased rapidly with the rising temperature. Ascorbic acid, L-cysteine and sodium sulfite almost completely inhibited the enzyme activity, Fe2+ and Fe3+ also appeared some inhibition to POD, whereas citrate acid, NaCl and CaCl2 had only slight inhibition. CuSO4 had a remarkable activating effect on the fresh-cut lotus root POD activity.Ascorbic acid could not be expected to reverse the browning orientation of the fresh-cut lotus root, although it was certainly effective in delaying the browning phenomenon. Even worse, the tissue showed purple stains on the epidermis after treated with ascorbic acid. It had been proved that 0.4 % AA treatment could most efficiently inhibit the browning process. However, chitosan coating treatment was shown to be effective not only to postpone the browning but also to regulate the orientation of browning with an optimal concentration of 1.5 %. It indicated that such a chitosan coating had meliorated the water imbalance of the opposite sections of the fresh-cut lotus root. Simultaneously, PAL, PPO and POD activities had been inhibited; thereinto PPO activity was nearly entirely inhibited by 1.5 % chitosan, whereas POD activity was merely partly inhibited. Negative correlations existed between the enzyme activity and the chitosan concentration. LOX activity also decreased, leading the decrease in the superoxide anion free radicles and MDA. However, the membrane permeability of the fresh-cut lotus root would increase and the tissue decay could be induced by chitosan coating treatment.The reason why the membrane permeability of fresh-cut lotus root increased and the tissue decay was induced by chitosan coating treatment might rest with the action of edible acetic acid as the chitosan solution was composed of both edible acetic acid and chitosan. This edible acetic acid changed the pH value, which served as a key factor for microbe growth. In the meantime, the difference of water content between the opposite sections was reduced due to the increasing of membrane permeability, which was induced by the acetic acid. As a result, the diversity of the enzyme activity that induced water imbalance was also eliminated. The browning orientation was almost completely removed by 1.8 % acetic acid concentration. Similarly, PAL, PPO and POD activities were all inhibited but with diverse degrees. The membrane lipid peroxidation was also prevented. Consequently, the edible acetic acid in the chitosan coating was the real crux for regulating the browning orientation of the fresh-cut lotus root. However, acetic acid in high concentration can lead a redder chroma in tissue color, which deserves further attention. |
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