Comments to EURACHEM/CITAC guide “Measurement uncertainty arising from sampling”

The EURACHEM/CITAC Guide “Measurement uncertainty arising from sampling” deals with the design and analysis of experiments for the evaluation of the sampling and analytical standard deviation when a defined sampling and analytical method is used for the determination of the concentration, expressed as mass fraction (mg/kg), of an analyte in a specified material. The Guide recommends reporting the relative expanded uncertainty and using it directly, i.e. it implicitly assumes that the standard deviation is proportional to the mass fraction even in case the experimental data do not support this assumption. Example A1 (and some of the other examples of the Guide) demonstrates that this can result in extreme levels of underestimation or overestimation of the uncertainty of measurement results. Hence, such recommendations should be avoided!     

Evaluation of three applications of a semi-automated most-probable-number method for the assessment of microbiological parameters in dairy products

Several rapid and automated microbiological methods have been introduced by food control laboratories because they allow microbiological quality (e.g. contamination with spoilage microorganisms and/or pathogens) of products to be checked or hygiene monitoring undertaken during production. This study evaluated three applications of the TEMPO^® system; an automated method based on the most-probable-number technique, performed in parallel with the relevant ISO standard method for comparison. Escherichia coli certified reference material and soft cheese samples contaminated artificially with E. coli were used throughout the study. Performance characteristics including precision, bias and limit of detection of E. coli (EC), coliforms (TC) and viable aerobic mesophilic microflora (TVC) were determined with particular attention given to low-level contamination, which typically occur in manufacturing and retail as well as storage and consumption at home. TEMPO^® EC was more precise than TEMPO^® TC whilst TEMPO^® TVC showed the most variation. Moreover, higher numbers of E. coli were obtained with TEMPO^® EC than TEMPO^® TC. Reliability of the system depended on the specificity of detection of the targeted group of microorganisms as well on the method. Although, methods like TEMPO^® offer convenience, they often lack the necessary clarity in terms of operational details, measurement principles and data processing necessary for reliable routine use. Installation of an ‘expert modus’ to facilitate access to original data would significantly increase commercial confidence in the application and use of these systems.