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Quality Control of Culture Media for Food Microbiology

First, quality control of culture media for food microbiology in preparation:

(1) Container: the flask, plate, test tube and other equipment for preparing and repacking the culture medium shall be neutral, free of acid and alkali inhibitor residues, and the bottom of the plate shall be flat, so as to avoid the different thickness of agar and affect the drug sensitivity test results.  

(2) Ingredient source: all kinds of ingredients are reliable and do not contain substances that inhibit the growth of the target bacteria. Special requirements of the culture medium, such as glucose oxidation fermentation test (O / F test), in addition to adding glucose, can not contain other sugars, and indicators can not use alcohol solution to avoid false-positive in the O / F test. The water for medium preparation shall be distilled water.  

(3) PH value adjustment: adjust the pH value according to different requirements of the medium, and control it within ± 0.2 of the required range. It should be noted that the pH value of the medium decreased by 0.1-0.2 after autoclaving, so the pH value should be 0.1-0.2 higher than the actual need.  

(4) Sterilization: different sterilization methods shall be selected according to the different components and preparation quantity of the culture medium, so as to achieve the sterilization effect without damaging the components of the culture medium. In general, the stable medium can be sterilized under high pressure, i.e. 121 ℃ for 15 minutes, while the medium containing sugar should be sterilized at 108 ℃ to prevent the destruction of sugar. Materials that are not resistant to high heat, such as serum and milk, can be sterilized by interval sterilization.  

(5) Subpackage: determine the subpackage quantity according to the purpose and requirements of use. The plates and test tubes used for subpackage of the culture medium shall be clean and free of acid and alkali residues; when preparing the plate culture medium, the operating platform shall be level to avoid the different thickness of the agar plate and ensure aseptic operation.  

Second, quality control of culture media for food microbiology after preparation;
  
(1) The appearance of medium: it includes color, transparency, precipitation and coagulation. If cracks are found on the surface of the medium or it is separated from the edge of the dish, it means that the medium is dehydrated and must be discarded.  

(2) Sterility test: sterility test shall be carried out for each batch of prepared medium. The culture medium sterilized first and then repacked can be tested by sampling method. The selective culture gene contains inhibitory substances, which can inhibit many microorganisms. Therefore, 10 times of sterile liquid medium can be added to dilute the inhibitory substances in order to detect the contaminating bacteria. Even after the sterility test, check the visible colony on each plate when inoculating.  


(3) Performance test: before using each batch of newly made or newly purchased culture medium, it is necessary to take the known nature of the stock bacteria for the performance tests. According to different purposes, the medium can be divided into enrichment medium, isolation medium and identification medium.  

① Enrichment medium: inoculate a small number of bacteria that are difficult to grow, and observe the enrichment within a certain period of time. The smaller the minimum inoculation concentration that bacteria can grow, the better the performance of the enrichment medium.  

② Isolation medium: the target bacteria should grow well and the non-target bacteria should be inhibited. Generally, the inoculation amount of the target bacteria required to grow should not be too much. The better method is to adjust the test bacteria to a bacterial suspension with a turbidity of 0.5 MC and then smear it on the culture medium with a standard inoculation ring of 0.001ml to observe the growth of the colonies.  
  
③ Identification medium: strains with typical characteristics should be selected for the performance tests.    


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