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Freeze-dried mixed cultures as samples for proficiency tests and collaborative studies in food microbiology
Abstract:
A method is presented for preparing samples of freeze-dried mixtures of microorganisms for proficiency tests and collaborative studies. The samples may include most microorganisms that are found in routine analysis in food laboratories. Transport of samples during 48 h did not decrease the number of microorganisms, nor was the variability among samples significantly affected by transport. The standard deviation of counts after 5 weeks of storage varied from 0.04 (Staphylococcus aureus) to 0.17 (Clostridium perfringens) log unit. Storage of samples for 10 weeks decreased the number of viable organisms by 0.02-0.43 log unit. Variability among samples increased for Providencia alcalifaciens and Bacillus cereus after 10 weeks of storage. No significant increase was found for the other organisms.
Insights
Freeze-dried microorganism samples are suitable for proficiency testing, showing stability during transport and short-term storage. Long-term storage impacts viability and variability for certain species, but overall stability is good for routine food laboratory analysis.
Area of Science:
- Microbiology
- Food Safety
- Analytical Chemistry
Background:
- Proficiency testing and collaborative studies are crucial for ensuring accuracy in food laboratory analysis.
- Standardized sample preparation methods are needed for reliable inter-laboratory comparisons.
- The stability of microbial samples during transport and storage is a key factor in study validity.
Purpose of the Study:
- To present a method for preparing freeze-dried microorganism mixtures for proficiency tests.
- To evaluate the stability of these samples during transport and storage.
- To assess the impact of storage duration on microbial viability and sample variability.
Main Methods:
- Preparation of freeze-dried mixtures of various microorganisms relevant to food analysis.
- Transport of samples for 48 hours to assess stability.
- Storage of samples for up to 10 weeks to evaluate long-term viability and variability.
Main Results:
- Transport did not significantly decrease microorganism numbers or affect sample variability.
- After 5 weeks of storage, standard deviations in counts ranged from 0.04 to 0.17 log units.
- Storage for 10 weeks caused a slight decrease (0.02-0.43 log units) in viable organisms, with increased variability for specific species (Providencia alcalifaciens, Bacillus cereus).
Conclusions:
- Freeze-dried microorganism samples are stable for transport and short-term storage in proficiency testing.
- The method provides reliable samples for routine food laboratory analysis.
- Long-term storage requires consideration of potential impacts on specific microbial species.