Effect of refrigeration and probiotic adjunct on pathogenic and spoilage microorganisms in raw milk for direct human consumption

We analyzed the microbiological quality and safety of raw milk from vending machine and assessed the evolution of its microbiota during the refrigeration at 7°C. We also monitored the survival of E. coli O157:H7 strain artificially added to the raw milk during the refrigeration and investigated the possibility to use the probiotic Lactobacillus rhamnosus GG strain to control pathogenic and spoilage bacteria in raw milk. Total aerobic mesophilic microbiota of raw milk exceeded the limit allowed by the current European legislation and increased of about two log units after 48 hr of refrigeration at 7°C. The spiked E. coli O157:H7 strain multiplied slowly in the refrigerated raw milk increasing of about 1 log unit, whereas the probiotic strain was able to slightly lower the populations of Pseudomonas spp., Listeria spp. and Staphylococcus aureus but was unable to inhibit the growth of E. coli O157:H7. Practical applications: This study allowed us to confirm the microbiological threat associated to the direct consumption of raw milk. Ad hoc strategies to improve the overall microbiological quality of raw milk from vending machines within the whole chain (from farm to consumption) are needed. A protective and probiotic adjunct able to control spoilage and pathogenic bacteria in raw milk with an adequate microbiological quality (at least compliant with the current legislation) may be a valuable mitigation strategy, but probiotic strains more effective than L. rhamnosus GG should be used.