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Microbiology Test

Common Issues and Solutions in Food Microbiology Testing Operations

By food microbiology testing, we can determine the food processing environment and food hygiene situation, and correctly evaluate the degree of bacterial contamination in food. So, what are the common issues in food microbiology testing and how to solve them?

When testing the total number of colonies in food microbiology testing, can we just use one dilution?

No, we cannot. It is required to select 2-3 suitable dilutions for each sample for testing. Even if the number of colonies in a sample is determined, it is recommended to test one more dilution as it can verify the testing results. There are risks if only one dilution is tested. Also, if strictly following the national standard requirements, this type of testing does not meet the standards, and some strict external audit departments may issue non-compliance issues. Therefore, it is recommended to strictly follow the national standards in testing.

How should we count and report the results of food microbiology testing on smear samples?

The counting of food microbiology exam on smear samples is quite complicated. Let’s use an example to explain! If the sample area to be smeared is 25 cm, and the sampled smear liquid is 10 mL and only 1 mL of the sample liquid is taken for testing, it means that the testing is conducted for a 10-1 dilution of the original sample. The total colony count on the plate multiplied by 10 represents the number of bacteria in the sample. The reported result is the number of bacteria per unit area. For example, if the sample area is 25 cm and the counted colony on the plate is 13, the reported result should be 13*10 CFU/25cm.

What are the precautions in the configuration of culture media in food microbiology exam?

  • In the processing of microbiology test in food industry, the sterilization temperature should be strictly controlled. Sterilization should be conducted according to the requirements, especially for culture media with a high sugar content. The temperature should not be too high, otherwise, the sugar will be caramelized, affecting the quality.

  • Agar based medium cannot be melted repeatedly. Repeated melting will destroy the nutritional ingredients in the culture media.

  • Culture media cannot be sterilized repeatedly. Repeated sterilization will also cause damage to the nutritional ingredients.

  • After sterilization of the agar-based culture media, it needs to be shaken well.

Should we sterilize the remaining culture media in food microbiology exam for reuse?

There are two situations:

  • For the first situation, if the culture media is melted again after it has solidified, according to the guidelines of food microbiology testing, the unused culture media cannot be solidified and kept for use next time. In this case, the culture media can only be discarded, and according to the national standards, the disposal also needs to comply with relevant laws and regulations.

  • For the second situation, the culture media that has not been used up after the first sterilization can be saved after the agar-based culture media solidifies. However, the storage conditions need to avoid light and keep dry, to ensure that its composition will not change. When reused, it only needs to be melted according to the requirements and should not be sterilized again. If the agar-based culture media has been opened but not used up after the first sterilization, reuse is not recommended because there are risks involved. However, for liquid culture media, there is no melting process involved. A culture medium that has been first sterilized, opened but not used up can be saved according to the national standard requirements. When used, the composition of the culture medium needs to be stable and must not be sterilized twice. If it is not possible to ensure whether the culture medium meets the requirements, it should be discarded.

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