Study On The Preservation Technology And Endogenous Formaldehyde Content Control Method Of Shiitake Mushroom

Skiitake mushroom(Lentinus edodes), a valuble edible fungi for its delicious taste, valuble nutrients and various healthy functions, can be used as both food and medicine. China is the biggest country in the world of production, export and consumption of skiitake mushroom. Recent years, the developed countries limited or forbad our country from exporting skiitake mushroom which brought about a heavy loss to our skiitake mushroom industry. So, it's necessary to research perservation technology and control endogenous formaldehyde content. The changes of the physiology, biochemistry, endogenous formaldehyde content, y-glutamyl transpeptidase (GGT) and S-alkyl-L-cysteine sufoxidelyase (C-S lyase) at different sites in shiitake mushrooms were studied. At the same time, the effects of active modified atmosphere packaging (MAP) and fresh keeping agents on the postharvest quality, endogenous formaldehyde content, GGT and C-S lyase activity of shiitake mushrooms were also investigated. The major results obtained in this study were as follows:The results showed, during the storage period, the Vc content in pileus and upper part of stipe were higher than those in epidermis and lower half of stipe. The reducing sugar content was lower than those in epidermis and lower half of stipe. The SOD activity in pileus and upper part of stipe showed a rising trend, but it decreased in epidermis and lower half of stipe. The CAT activity in pileus was the highest and it was lowest in lower half of stipe. The content of O2, H2O2and MDA in pileus and upper part of stipe were lower than those in epidermis and lower half of stipe. These suggested that the antioxidation and anti-aging abilities of pileus and upper part of stipe were better than that of epidermis and lower half of stipe. The PPO activity in epidermis was the highest. The PAL activity in pileus was the highest in the early7days, so the total phenolic content in pileus was the highest. There was a sharp increase (P<0.01) of PAL activity in epidermis at the7th day, and the total phenolic content in epidermis was the second highest. The trend of GGT and C-S lyse activity and the endogenous formaldehyde content in epidermis and pileus were similar, and significantly(P<0.01) higher than that of upper and lower part of stipe. These results showed no correlation between the endogenous formaldehyde content and browning degree of skiitake mushrooms. The low moisture content and high cellulose content of stipe was responsible for lower enzyme activity and endogenous formaldehyde content.The shiitake mushrooms stored at a low temperature (4℃), the gas components were2%O2+7%CO2(M1),2%O2+10%CO2(M2),2%O2+13%CO2(M3), respectively. The results showed MAP was good for skiitake mushroom preservation. MAP was effective in delaying the decrease in firmness, holding moisture content, controlling the respiration rate, browning degree and pH value of shiitake mushrooms, inhibiting the increasing of malondinaldehyde(MDA) and reducing sugar content, and controlling the decomposition of protein, vitamin C, and soluble sugar. During the early storage, M2and M3treatmented groups kept an lower relative membrane permeability, PPO activity and total phenolic content. When the storage was over nine days, the relative membrane permeability of M2and M3treatmented groups exceeded the control group(up to 54.69%and57.48%, respectively), the PPO activity increased, the total phenolic content decreased and enzymatic browning increased. These indicated that skiitake mushrooms was more susceptible to CO2concentration, and excessive CO2concentration was harmful for mushrooms preservation. Endogenous formaldehyde content in shiitake mushrooms has positive correlation with GGT and C-S lyase activity (correlation coefficients were0.858and0.748, respectively). MAP could inhibit activity of enzymes and decrease the endogenous formaldehyde content. The mixture gas of2%O2+10%-13%CO2in packing showed the best effect for preservation and control of endogenous formaldehyde content of skiitake mushroom.When the grope seed extraction(GSE), glutamic acid and glutathione were used as fresh-keeping agents, we found that glutamic acid group showed the best sensory quality and GSE group was the second. GSE was more effective in controlling the endogenous formaldehyde content of skiitake mushrooms than glutamic acid and glutathione. The results also showed:(1)Fresh-keeping agents delayed the decrease in firmness, held moisture content, controlled the respiration rate, inhibited the decomposition of protein and the rancidity of shiitake mushrooms. The10g/L of GSE was the best choice for delaying the decrease in firmness, holding moisture content and inhibiting the decomposition of protein.(2)The GGT and C-S lyase activity of Glutamic acid and glutathione groups showed similar tendency with endogenous formaldehyde content. Same phenomenon was also found in CK and GSE groups. GGT and C-S lyase showed the best activity for controlling the endogenous formaldehyde content;(3)GSE did not react with formaldehyde to control the endogenous formaldehyde content of skiitake mushroom. Further studies are necessary to do on the mechanism.