Metabolic Mechanism And Regulations Reasearch Of Endogenous Formaldehyde Content In Lentinus Edodes

In recent years, the issue of endogenous formaldehyde in Lentinus edodes has caused widespread concern. Formaldehyde is a metabolite in shiitake mushrooms, y-glutamyl transpeptidase (GGT) and S-alkyl-L-cysteine sulfoxidelyase (C-S lyase) are the two key metabolic enzymes in formaldehyde formation pathway. In microorganisms, formaldehyde dehydrogenase (FADH) is a main enzyme for formaldehyde detoxification which is ultimately dependent upon the concentration of GSH. In present study, the contents of endogenous formaldehyde、GSH、amino acid、reducing sugar and some enzymes activities related with endogenous formaldehyde metabolism were investigated in the Lentinus edodes growth stage and postharvest storage, and microwave new production technology was used to regulate formaldehyde content in dried mushrooms. The major findings are as follows:At different growth development stages, the content of reducing sugar as energic substances was falling with the fruit body developing; The content of soluble protein increased first and then decreased, which indicated the metabolism rate of the fruits was not consistent. The endogenous formaldehyde content which was higher in the stalks than that in the caps increased significantly from the first mature stage, basically achieved stable at the seventh mature stage, decreased a little at the ninth mature stage. GGT activity and C-S lyase activity in the fresh mushrooms had the similar tendency, which were higher in the stalks than those in the caps and reached the highest level at the fifth mature stage. These results showed that the accumulation of formaldehyde may resulted from the increase of GGT and C-S lyase activity. The contents of GSH、 GSSG、T-GSH in the caps were higher than those in the stalks. During the growth period, the content of T-GSH almost remained the same, the content of GSH increased gradually and the content of GSSG decreased gradually. FADH activity in stalks was higner than that in caps, which reached the highest level at seventh mature stage. It indicated that the shiitake mushrooms have their own repair mechanisms, formaldehyde increased FADH activity and GSH content in order to eliminate itself in the mushrooms. There were18kinds of amino acids in both caps and stalks, however, essential amino-acid including methionine (Met) and tryptophan (Trp) were not existed in mushrooms. Glutamic acid (Glu) possessed the first site in the contents of total amino acids. During growth development of the mushrooms, the contents of most of the amino acids increased first and then decreased. The content of the total amino acids reached the highest level at seventh growth stage which was best for harvest.During the post-harvest storage period, the content of reducing sugar and soluble protein decreased gradually, which indicated that reducing sugar was existed as energetic substances and the metabolism rate of the fruits was falling. Formaldehyde is the metabolite in Lentinus edodes. In post-harvest storage, the formaldehyde content in mushrooms increased first and then decreased, which was lower in caps than that in stalks. After drying at different storage times, the formaldehyde content in dried mushrooms decreased. GGT activity and C-S lyase activity in the fresh mushrooms had the same tendency as formaldehyde. These results showed that the increase of formaldehyde content was correlated with the activity of GGT and C-S lyase. The contents of GSH、GSSG and T-GSH in caps were higher than those in stalks. During the post-harvest storage period, the content of GSH decreased gradually, the content of GSSG increased, the content of T-GSH almost remained the same. FADH activity increased first and then decreased, which was higher in stalks than that in caps. These results indicated that the increase of FADH activity may result from the accumulation of formaldehyde in mushrooms. Because of participation in formaldehyde detoxification, GSH were consumed and converted into GSSG. The contents of all amino acids in caps were higher than those in stalks. The contents of most amino acids decreased in the post-harvest storage period because of participation in mushrooms metabolism. have Different tendencies of Glutamic acid (Glu) and cystine (Cys) from others may resulte from participation of GSH in formaldehyde metabolism.After microwave pretreatment, the content of formaldehyde in mushrooms increased with increasing microwave time, the content of protein decreased, while the content of reducing sugar remained the same compared to the control. At the same time, GGT、C-S lyase activity were decreased as well as the content of GSH、GSSG and T-GSH. However, FADH activity increased first and then decreased. After microwave pretreatment, the changes of the cell microstructure were observed by transmission electron microscopy (TEM). Mushroom cells kept integrated, however, the outer structure and permeability were influenced. These results showed that non-thermal effects of microwave affected enzyme activity and metabolism of the mushrooms. Dry the mushrooms which were microwaved for different time in the hot-air oven. Formaldehyde content in dried mushrooms decreased with increasing microwave time. Because the decrease of GG、C-S lyase activity resulted in the less production of formaldehyde in mushrooms during drying process. Strengthen of cell permeability also promoted formaldehyde volatilization in drying process. Therefore, microwave treatment before drying process controlled formaldehyde content in mushrooms.In order to study the relationship between the production of formaldehyde and Characteristic flavor substances in mushrooms, lenthionine and1,2,4,5-tetrathiepane in the dried mushrooms were measured. The content of1,2,4,5-tetrathiepane decreased with increasing microwave time, while the content of lenthionine increased first and then decreased. It has been reported that1,2,4,5-tetrathiepane and lenthionine are produced during drying process. Therefore, the changes of1,2,4,5-tetrathiepane and lenthionine content in dried mushrooms were related with GGT、C-S lyase activity in drying process. The relationship between formaldehyde and the two odor compounds was analyzed. There was a positive correlation between the amount of1,2,4,5-tetrathiepane and the formaldehyde (r=0.83, p=0.04), the equations of y=0.1195x+2.6885, which supports the postulation that the endogenous formaldehyde produced along with the odor formation in shiitake mushroom.