Freeze-dried mixed cultures as samples for proficiency tests and collaborative studies in food microbiology.
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.
D005516:Food Microbiology / Q000592:standards*