Sources of contamination of raw milk butter with Escherichia coli during processing

Introduction
In Wallonia, raw milk butter manufacture is frequenta. A hygienic indicator for this process is E. coli, with criteria established by Regulation (EC) No 2073/2005b. Five samples of butter are considered at the end of manufacture. If levels of E. coli are < 10 cfu/g, batch is considered as satisfying. If a maximum of two samples have levels between 10 and 100 cfu/g, batch is acceptable. In other circumstances, batch is unsatisfactory. This study aimed at monitoring butter manufacture in dairy farms facing recurrent spoilage by E. coli to identify key steps responsible for this contamination.
Materials and Methods
Six Walloon farms were considered, and visited three times each. Surface samples were collected using Petrifilm Rapid E. coli/coliforms or swabs, in triplicates, including pipes before and after cream separator, cream separator itself, containers for cream maturation and churn. Intermediate and end products were also considered, in five replicates, namely whole raw milk, skimmed milk, ripened cream and butter. Product samples were diluted 10-fold in Buffered Peptone Water and spread on Petrifilm. All Petrifilms were incubated at 30 °C for 18 to 24 h. Statistical treatments were performed using Minitab19. Correlations between two variables were considered using linear regression. Effect of factors on milk quality was assessed using one-way ANOVA.
Discussion
Globally, out of 18 butter batches, 14 (78%) were unsatisfactory, but generally with levels < 3 log10 cfu/g. Identified critical points of contamination included whole milk supply pipe and milk inlet to cream separator. Milking equipment impacted significantly milk quality; milking robots being associated with recurrent contaminations of intermediate and end products. Frequent and effective disinfection practices must be recommended. Producers should also limit time lapse between milking and cooling. As last resort, starters could be added to cream for faster and more intense maturation.