| Oyster is the largest cultured shellfish in China, which is distributed in the coastal areas of China. In recent years, aquaculture industry has been developed in the coastal areas of China, these coastal areas have become an important oyster production and export base. Oyster meat contains rich nutrients, such as vitamins, calcium, zinc and free fatty acids. In the composition of oyster’s fatty acid, C20:5n-3 and C22:6n-3 is the main content, accounting for 25% ~ 36% in the total content of the free fatty acid. But some studies have found that these free fatty acids are not absolutely safe for human body, and the detection of shellfish toxins mouse bioassy often cause false positive results.In this study, oyster was chosen as the research object and we choose gas chromatography analysis technology. we selected boron trifluoride methyl esterification to make the fatty acid derived, and 100% fixed liquid cyanide propyl silicone capillary column was used to monitor the content of oyster samples in different storage temperature.Objective: We investigated the changes of free fatty acid content in oyster samples with storage temperature and time, in order to explore the origin of false positive results in mouse bioassay of fatty soluble shellfish toxin.The main results are as follows:(1) The analysis of HPLC-MS/MS showed no shellfish toxin in the oyster samples, which means that it was negative samples from the beginning to the end. Then, according to GB/T 5009.212-2008, the mouse bioassay experiments were conducted. After the storage of five month, there was no one mouse died, but mice were killed and false positive results appeared when we took 2.5 times said sample volume of mice.(2) The composition of free fatty acids in oyster samples was analyzed using GC-FID method. We used Amberlyst-26(A-26) anion exchange resin adsorption, fluoride and boron esterification. The inner wall of chromatographic column is 100% cyanide propyl Polysiloxane Stationary There were 20 kinds of free fatty acids in oyster samples though GC analysis. They are C14:0、C15:0、C16:0、C16:1、C17:0、C18:0、C18:1n-9t、C18:1n-9c、C18:2n-6t、C18:2n-6c、C20:0、C18:3n-6、C20:1、C18:3n-3、C20:2、C22:0、C20:3n-6、C20:4n-6、C20:5n-3 and C22:6n3 espectively. The content of C20:5n-3 in free fatty acid is the biggest, followed by C22:6n-3.(3) We explored 0-4 ℃,-10 ℃ and-20 ℃ storage conditions downstream from fatty acids content variation with time. The results showed that the-10 ℃ and-20 ℃ storage samples total content of free fatty acids changes over time increased at first and decreased after, and then slowly increase, 0 to-4 ℃, total free fatty acid content increased with the change of storage time;-20 ℃ samples free fatty acid content was significantly higher than that of the – 10 ℃ in the same storage time.(4) We explored the changes of the content of toxic free fatty acid with time under 0-4 ℃,-10 ℃ and-20 ℃ storage conditions. The results showed that: under- 10 ℃ and-20 ℃ storage low toxic free fatty acid C18:2n-6t increased at first and then a sharp decline is close to zero and C18:2n-6c almost unchanged, C22:6n-3 grew slowly. High toxic free fatty acid C20:5n-3 increased dramatically, C18:3n-3 grew slowly, and the change of C20:4n-6 was not obvious. From second months of storage, the content of free fatty acid in oyster preserved by-20 ℃ was significantly higher than that in-10 ℃.The content of low toxicity free fatty acids C22:6n-3 in oyster increased slowly before the 5th days and then grew dramatically. The change of C18:2n-6t and C18:2n-6c was not obvious. The change of high toxic free fatty acid C20:5n-3 in oyster was the same as C22:6n-3, but the content of C20:5n-3 was significantly higher than C22:6n-3. C18:3n-3 grew slowly. The change of C20:4n-6 was not obvious.(5) The study found that the content of C20:5n-3 increased respectively for 67.90 ug/g, 136.79 ug/g at-10 ℃ and-20 ℃ after five months. Unlike the results of literature which had been reported : the content of C20:5n-3 can reach 389 ug/g at-10 ℃ after 4 months.This paper argues that this may be related to the origin and harvest season of oyster. |
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