Raw milk for the direct human consumption inoculated with the probiotic Lactobacillus rhamnosus GG

Although highly debated, raw milk for direct human consumption is expanding in many countries, being sold through vending machines. Moreover, the current legislation requires the refrigeration of vending machine raw milk at a temperature comprised between 0 °C and 4 °C also during the storage at home. Nevertheless, not all consumers boil raw milk before consumption, and often administer raw milk even to children, and many of them do not use an insulated bag to carry the raw milk at home. In addition, household refrigerators usually have temperatures above 4 °C. In this study, we investigated the possibility to use the probiotic Lactobacillus rhamnosus GG strain, to control the microbiological quality and safety of raw milk for the direct human consumption. This probiotic was chosen since it is commonly used for the production of probiotic foods and supplements due to its recognized beneficial effects on consumer health, and because it produces antimicrobial peptides which inhibit both Gram-negative and Gram-positive bacteria. L. rhamnosus GG was added (final concentration in milk of ca. 7.5 log cfu/mL) to raw milk spiked and not with an Escherichia coli O157:H7 strain (final concentration in milk of ca.4 log cfu/ml). Three trials were performed in three different days (on three different raw milk samples). Microbial population changes in conditions mimicking milk transportation and storage at home, i.e. during the refrigerated storage of milk at 7 °C, were investigated. The probiotic L. rhamnosus GG survived during the 48 hours of cold storage at 7 °C remaining at levels similar to those normally encountered in probiotic products, and acidified only slightly the raw milk, whose pH decreased from ca. 6.8 to ca. 6.4 after 48 hours of refrigerated storage. Moreover, it performed a certain antagonistic activity against some groups of microorganisms. As for Pseudomonas spp., the raw milk revealed a high population (ca. 5.4 log cfu/mL) of these spoilage bacteria (due to the lipases, thermostable proteases and lecithinase they release in raw milk) which increased more than 1.5 log units/mL during 48h of refrigeration at 7 °C, consistent with their psychrotophic nature. In particular they remained constant during the first 6 hours of incubation in raw milk with and without the probiotic adjunct, probably due to the bacteriostatic and/or bactericidal action of the endogenous enzymes of raw milk and/or metabolites of the microbiota. After 48 hours of cold storage the amount of pseudomonads in raw milk with the probiotic adjunct was ca. 0.92 log cfu/ml lower than in raw milk without the probiotic (P<0.05). Similarly, lower amounts of ca. 0.5 and ca. 0.9 log cfu/mL were reached in raw milk with the probiotic, after 48 hours of refrigeration, for the populations of Listeria spp. and of Staphylococcus aureus, respectively (P<0.05). Such a positive effect was not found in the probiotic raw milk artificially contaminated with approximately the E. coli O157:H7 strain maybe because the presence of the pathogen in the heterogeneous microbial ecosystem of the raw milk negatively affected the performances of the probiotic L. rhamnosus GG strain. Using protective and probiotic adjuncts may be a valuable strategy to control spoilage and pathogenic bacteria in raw milk with an adequate microbiological quality (at least compliant with the current legislation), but probiotic strains more effective than L. rhamnosus GG should be used.