Properties of Bacillus cereus spores in reference materials prepared from artificially contaminated spray dried milk

A reference material for Bacillus cereus was developed based on spray drying of milk artificially contaminated with B. cereus spores. Various properties of the B. cereus spores in the milk powder were determined. The stability of the materials was good with no detectable decrease in the contamination level during 1 1/2 years storage at -20 degrees C or 4 weeks at 22, 30 or 37 degrees C. The homogeneity of the material was found acceptable for use as a reference material. Heat treatments (10 min at 70 or 80 degrees C) and addition of lysozyme to the enumeration medium did not influence the number of spores counted. The germination of the spores depended on the type of medium in which the milk powder was reconstituted, and on the storage period of the material. The suitability of the material was confirmed in a collaborative study. From the results obtained it was concluded that the material developed meets the general requirements set for reference materials and can therefore be used for, among others, testing laboratory performance.