Preparation of Reference Material for Proficiency Test for Enumeration of Coliforms in Cheese Matrix

Marcelo Luiz Lima Brandão,Valéria de Mello Medeiros,Armi Wanderley da Nóbrega,Carla de Oliveira Rosas,Rodrigo Rollin Pinheiro,Juliana de Castro Beltrão da Costa,Marcus Henrique Campino de La Cruz,Silvia Maria Lopes Bricio,Paola Cardarelli-Leite

doi:10.4236/detection.2013.11002

Abstract

It is widely accepted that quantitative reference materials (RM), are indispensable tools for verification of the precision and accuracy of analytical measurements. The RM can be used by food microbiology laboratories, as part of their quality assurance programmes, to achieve their quality control. In Brazil, Anvisa RDC No. 12/01 specifies the enumeration of coliforms as one of the parameters for evaluating cheese quality. The aim of this study was to produce a quantitative RM for proficiency testing (PT) for use in the testing of enumeration of coliforms in cheese matrixes. A sample of an ultra-filtered cheese with a coliforms count of <3.0 MPN/g and a total n? of viable aerobes of 1.2 × 103 CFU/g was used as the matrix to produce the RM. The ultra-filtered cheese matrix was distributed in flasks, contaminated with a specific concentration of an Escherichia coli strain and submitted to freeze-drying. Sucrose was used as the cryo-protector. The RM produced was considered sufficiently homogeneous and stable at ≤ ?70?C during the entire study period (348 days). The material was also considered sufficiently stable at 4?C for six days, but instable at 30?C and 35?C for the same period. At ?20?C the RM was sufficiently stable for 161 days. It was concluded that the material showed all the necessary requirements for a quality RM to be used as PT items and could be transported to the laboratories taking part in a PT at up to 4?C for up to 6 days, since the results indicated maintenance of the cell concentrations during this period. This is the first study to describe a methodology for producing RM containing coliforms in a cheese matrix.

Highlights

Food safety and quality are two of the most important factors determining the consumer acceptance and purchase dynamics of a product
It is known that proficiency testing (PT) samples should resemble as closely as possible to routine samples analyzed by participant laboratories, it is a common practice to produce lyophilized samples in order to improve analyte stability [911,20]
The freeze-drying process was considered satisfactory, since the integrity of the product was not compromised over time as demonstrated by the stability profile shown by the reference material

Summary

Introduction

Food safety and quality are two of the most important factors determining the consumer acceptance and purchase dynamics of a product. Official food control laboratories are required to use validated methods wherever possible. For this reason, analytical methods must be subjected to internal and/or external validation studies [1]. The RM can be used by food microbiology laboratories, as part of their quality assurance programmes, to achieve their quality control, method validation, staff training control and evaluation of the laboratory’s performance [3]. They are employed in laboratory accreditation, as well as in the establishment of traceability in the framework of internationally agreed standards [1]. The use and traceability of RM, as well as the laboratory participation in proficiency testing (PT) schemes by interlaboratory comparisons are requirements of the aforementioned standard [4]

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Results