The use of reference materials in quality assurance programmes in food microbiology laboratories

Nine different reference materials (RMs) for use in food and water microbiology have been developed with the support of the European Commission (EC). The production process of RMs is based on spray drying bacteria suspended in milk. The highly contaminated milk powder (HCMP) obtained is mixed with sterile milk powder to achieve the desired level of contamination and is subsequently filled into gelatine capsules. The HCMP may need to be stabilised by storage for more than a year before a stable RM can be prepared. The HCMP are mixed with sterile milk powder using a pestle and mortar in order to produce homogeneous RMs. For routine use of RMs Shewhart control charts can be produced. Based on log 10 transformed counts, control limits are calculated. Rules for the interpretation of results facilitate the detection of out of control situations. Besides RMs there are also CRMs (Certified Reference Materials) that are certified by the EC Community Bureau of Reference (BCR) and are intended for occasional use. Based on the BCR certificate, user tables are produced presenting the 95% confidence limits for the number of capsules likely to be examined in practice. Also power analysis is made to indicate the minimum difference between the certified value and the observed geometric mean value in relation to the number of capsules examined.