AS 4659.2:2015 pdf free.Guide to determining the equivalence of food microbiology test methods Part 2: Quantitative tests.
4.3 Select test organisms
4.3.1 For non-selective methods
All samples should be naturally contaminated samples.
4.3.2 For selective c/uatl!I!ative methods
A minimum of live strains of the target organism should be selected. The reference culture prescribed as a positive control should be one of the live strains. The other strains may be selected from the following list (in order of preference):
(a) Strains of the target organism isolated by the laboratory from previous samples of the matrix under examination.
(b) Strains of the target organism isolated by reference laboratories or industry sources as representative of the strains found in the type of matrix under examination.
(c) Strains of the target organism traceable to culture collections.
NOTl-: Commercial preparations of known concentrations may he used.
The strains selected should encompass the variation that may be expected to be found within the target organism in the matrix under examination.
The identity of the strains chosen should be determined by appropriate means (e.g. biochemical, physiological, serological, molecular) before performing further work. Consultation with a reference laboratory or an expert may be necessary. The source and identity of the strains should be recorded.
NOTE: It is possible. by the selection of test strains, to bias the results of the study. Care should be taken to select representative strains which do not possess characteristics which are likely to lead to a biased result being obtained.
4.4 Select samples of the matrix
Five typical samples of the matrix should be selected. They should represent as far as possible the range of variation found within the defined matrix. The choice of samples will depend upon the matrix, but may consist of batches with high or low background contamination, samples with different pH values, fat, protein or moisture levels. The aim of the selection is to choose samples which will represent the range of variation expected to be found in the defined matrix. Care should he taken to select samples with characteristics which are expected to yield homogeneous results in the test being validated. Naturally contaminated samples, if available, should be used instead of artificially contaminated samples. Care should be taken to determine the identity, level and homogeneity of contamination.
The significant characteristics of the product should be measured and the results recorded.
4.5 Preparation (selective quantitative method)
For artificially inoculated samples, the strains chosen should be prepared for use by culturing the strains oithe test organism in a non-selective medium (e.g. tryptone soya agar, nutrient agar) to early stationary phase (e.g. 18 hour culture) at the optimum growth temperature. A cocktail of strains may he prepared by combining approximately equal numbers of’ each strain grown as a pure culture. The cocktail of strains should be added to each matrix sample at two levels:
(a) Approximately 10 times the lower limit of detection of the reference method.
(b) Approximately within one log10 of the upper limit of detection of the alternate method or the highest expected level of the target organism.AS 4659.2 pdf download.