SUITABILITY OF REFERENCE MATERIALS FOR MI CROBIOLOGICAL COLLABORATIVESTUDIES

Information about the accuracy and the standard of proficiency depends upon the availability of suitable reference materials. Samples of mil kpowder, eggpowder and bowl chopped meat (raw frankfurter type meat em ulsion) were examined. The distribution of microorganisms (characteris tic) within a homogenized and processed single sample follows a random distribution, described by the POISSON-model. The use of the tested m aterials guarantees to keep an acceptable standard of proficiency in l aboratories. To determine the precision of a bacterialogical method it is necessary to investigate several samples of a lot under the same c onditions (repeatabilty) as well as under various conditions (reproduc ibility). The variation between the single samples of a lot has the mo st important effect on the total variance. Therefore perfect homogenei ty cannot be assumed. Using powdry materials carries difficulty of hig h levels of contamination along with a low portion of aerobic spore-fo rming bacteria is rare. Another disadvantage of powders is the low sta bility over long time storage. For that reason they should be used whe n still in a fresh condition. On the contrary bowl chopped meat can be easily manufactured and stored. Also sufficiently high levels of cont amination are present. However the transport of deep frozen samples of bowl chopped meat is more complicated than the transport of spray dri ed samples. There were still no well-published results on the stabilit y of variance of bowl chopped meat. In spite of all these disadvantage s and with regard to these limitations milk- and eggpowder as well bow l chopped meat can be used as reference materials.