Top-down Evaluation of Uncertainties in Quantitative Assays for Microorganisms

On the basis of Microbiology of the food chain-Estimation of measurement uncertainty for quantitative determinations (ISO 19036) and Guidelines for the estimation of measurement uncertainty of food microbiological quantitative detection (RB/T 228-2023), the top-down method was used to design an experimental scheme for uncertainty evaluation in three different quantitative assays by using raw and fermented milk as samples. The uncertainty evaluation for total colony number included technical uncertainty (u_tech), matrix uncertainty (u_matrix) and Poisson distribution uncertainty (u_poisson); when the result was 9.4 × 10⁴ CFU/mL, the extended uncertainty was 0.31 (lg (CFU/mL)). The uncertainty evaluation for maximum probable number (MPN) included u_tech, u_matrix, and uncertainty based on the most probable number (u_MPN); when the result was 24 MPN/g, the extended uncertainty was 0.94 (lg (MPN/g)). The uncertainty evaluation for total colony number that needs to confirm target microorganisms included u_tech, u_matrix, u_poisson, and confirmatory uncertainty (u_conf); when the result was 22 CFU/g, the extended uncertainty was 0.29 (lg (CFU/g)).