Microbial biodiversity, quality and shelf life of microfiltered and pasteurized extended shelf life (ESL) milk from Germany, Austria and Switzerland

Information on factors limiting the shelf life of extended shelf life (ESL) milk produced by microfiltration and subsequent pasteurization is very limited. In this study, three different batches of ESL milk were analyzed at different stages of the production process and during storage at 4 °C, 8 °C and 10 °C in order to evaluate the changes in bacterial cell counts, microbial diversity and enzymatic quality. Additionally, detailed biodiversity analyses of 250 retail ESL milk packages produced by five manufacturers in Germany, Austria and Switzerland were performed at the end of shelf life. It was observed that microfiltration decreased the microbial loads by 5--6 log10 units to lower than 1 CFU/mL. However, bacterial counts at the end of shelf life were extremely variable and ranged between b1 and 8 log10 CFU/mL. 8{%} of all samples showed spoilage indicated by cell counts higher than 6 log10 CFU/mL. The main spoilage groups of bacteria were Gram-negative post-process recontaminants (Acinetobacter, Chryseobacterium, Psychrobacter, Sphingomonas) and the spore formers Paenibacillus and Bacillus cereus,while other spore formers and Microbacteriumspp. did not reach spoilage levels. Paenibacillus spp. and B. cereus apparently influenced enzymatic spoilage, as indicated by increased free fatty acid production, pH 4.6 --- soluble peptide fractions and off-flavors. In some cases, enzymatic spoilage was observed although microbial counts werewell below 6 log10 CFU/mL. Thirteen B. cereus isolates were characterized for their toxin profiles and psychrotolerance. Hbl, nhe,and cytK toxin genes were detected in ten, thirteen, and four isolates, respectively,whereas the ces gene was always absent. Interestingly, only three of the thirteen isolates could be allocated to psychrotolerant genotypes, as indicated by the major cold shock cspA gene signature. Generally, large discrepancies in microbial loads and biodiversity were observed at the end of shelf life, even among packages of the same production batch.We suggest that such unexpected differences may be due to very low cell counts after ESL treatment, causing stochastic variations of initial species distributions in individual packages. Thiswould result in the development of significantly different bacterial populations during cold storage, including the occasional development of high numbers of pathogenic species such as B. cereus or Acinetobacter.     «

Information on factors limiting the shelf life of extended shelf life (ESL) milk produced by microfiltration and subsequent pasteurization is very limited. In this study, three different batches of ESL milk were analyzed at different stages of the production process and during storage at 4 °C, 8 °C and 10 °C in order to evaluate the changes in bacterial cell counts, microbial diversity and enzymatic quality. Additionally, detailed biodiversity analyses of 250 retail ESL milk packages produced by...     »