| Agaricus Bisporus, an edible fungus which is riched in nutrition, tastes delicious and is a kind of food with health-care function. With the change of people's idea of comsumption, the market demand of fresh mushroom is increasing gradually. However, respiration and metabolism of the post-harvest mushroom is abnormally strong, and the consumption of nutrition causes sensory quality deteriorating rapidly. Among all changes, browning is one of the most important factors which declines sensory quality of mushroom. Thus, in this dissertation we studied the factors causing browning, the possible browning-mechanism during storage in low temperature, as well as the browning-control technology of mushroom.The results were as follows:1. Studies of the factors causing mushroom browningCompared with the non-inoculation group, the effect of the microorganism on mushroom surface on browning was studied by inoculating the bacteria separated from the mushroom surface in the sterilizing mushroom surface. The results showed that the microorganism wasn't the main factor that caused browning. Using browning degree and internal whiteness of mushroom as reference indexes, effects of storage temperature, packaging methods and oxygen concentration on mushroom browning were studied. It was found that the degree of browning was ralated to storage temperature. Mushrooms stored at (1±0.5)℃expressed lower browning degree and higher internal whiteness than other temperatures. When temperature was below 0℃, the condensing moisture on the surface of mushrooms got frozen, resulting in mushroom browning or yellowing. The package made by PVC material used to seal the mushroom reduced mushroom's moisture loss effectively and retarded browning during storage. 5% of oxygen concentration could delay browning on mushroom effectively. Mushroom treated in such way remained good internal whiteness quality and appearance quality after 23d storing at (1±0.5)℃.2. Study of the mechanism of mushroom browningMushroom browning began from the stipe during storage. With the extension of storage time, browning area gradually expanded. To measure changes of mushrooms stipe browning-releted indicators during storage, the key indicators related to enzymatic browning were found. The total phenols content was related to changes in both mushroom browning degree (BD) and internal whiteness (HW value), however, VC content and deoxidize sugar content were irrelevant to the changes in BD and HW, which showed that the main reason of browning was enzymatic oxidation of phenolic substrates. BD increased gradually during mushroom storage and internal whiteness decreased. PPO activity first increased gradually and then declined slightly after 17d. POD activity decreased after reaching peak value 5d later. CAT activity continued to decline during whole period of storage, while SOD activity increased first and then decreased. Analysis showed that POD, SOD and CAT were the main cell protective enzymes during the aging process of mushroom while PPO played an important role in catalyzing oxidation of phenols compounds. During storage PPO activity was significantly correlated to BD and HW (P<0.05). The stipe cell ultrastructure of mushrooms from different stages of storage was observed by electron microscope, and it could be proved that the completely broken membrance system which maintened PPO and phenolic regional distribution was the direct cause to lead its tissue browning.3. Study of the effect of ozone treatment on mushroom browningEffect of ozone treating time on mushroom browning was studied by comparing non-treatment against ozone treating for 5min, 10min and 15min. The results showed that ozone treatment would not slow down the rise in mushroom BD. On the contrary it led to more severve browning than the non-treatment. The longer the ozone treating time was, the higher the membrane permeability was and the faster the total phenols content of mushroom stipe declined, the more serious the browning was.4. The choice of vacuum pre-cooling temperatureUsing browning degree, weight loss rate and stipe PPO activity as reference indexes and storing in the (1±0.5)℃, (90±5)% RH for 17d as fixed conditions, the relation between vacuum pre-cooling and mushroom browning was investigated by comparing the non-treatment against different final pre-cooling temperatures of 3℃, 5℃and 7℃. The results showed that the vacuum pre-cooling could effectively inhibit stipe PPO activity and reduce browning. The effect of the pre-cooling mushroom to 5℃was better than others, whose PPO activity and the browning were the lowest after 17d storage. So 5℃was chosen as the final vacuum pre-cooling temperature to conduct the following experiment.5. Effect of vacuum pre-cooling and modified atmosphere packaging on mushroom browning and storage qualityThe mushrooms treated with vacuum pre-cooling to 5℃and packed with modified atmosphere package (group A: 5% O2 + 95% N2) were experimented immediately to prevent the browning, comparing with mushrooms without vacuum pre-cooling and packed by PE material. The results showed that vacuum pre-cooling combined with package containing 5% O2 could retard the declining of whiteness of mushroom stipe and keep lower stipe cell membrane permeability, higher PPO activity, total phenols content and POD activity. The mushrooms expressed a good internal quality and appearance under (1±0.5)℃, (90±5)% RH storage condition after 25d.
|
PDF Full Text Request