EFFECTS OF CITRUS ESSENTIAL OILS ON THE MICROBIOLOGICAL SAFETY OF PRIMO SALE CHEESE

In the last years, the consumers demanding foods with no chemical preservatives for food conservation, determined an interest by the food industries for the use of natural biopreservatives. Several essential oils from various plants and fruits have been characterized for their antibacterial activities in order to select new biopreservatives. The aim of this work was to evaluate the organoleptic effect of citrus essential oils to be applied in the processing of sheeps’ milk “Primo Sale” cheese and the inhibitory effect on the main bacterial pathogens of dairy interest. In order to better evaluate the antibacterial effect, the first approach was based on pasteurized milk, in view of the future application in raw milk cheese production. Three industrial citrus essential oils (EOs) extracted by cold pressing of the citrus peels were tested: Orange [Citrus sinensis (L.) Osbeck] Lemon [Citrus limon (L.) Osbeck] and Tangerine [Citrus reticulata Blanco]; sheep bulk milk was pasteurized for 30 min at 60°C. Each EO was tested at two different concentrations 100 and 200 µl/l of milk. For each dose two cheese makings were made: one production was not added with pathogenic bacteria to evaluate the organoleptic characteristics, while the second production was added with 30 CFU/mL of Listeria monocytogenes 24B0 and Salmonella typhimurium 50431 and 103 CFU/mL of Escherichia coli C45/1245 and Staphylococcus aureus PSS52. All cheeses were obtained by coagulation with 0.3 mL/L of microbial rennet (Fromase® 220 TL, DSM Bright Science Brighter Living, Heerlen, UK) and acidification by means of the starter culture Lactococcus lactis CAG4 and CAG37 10 mL/L to reach a final concentration of 106 CFU/ml in milk. Four cheese makings without EOs were used as control trials and prepared as follows: only rennet; rennet plus pathogenic bacteria; rennet plus starters; and rennet plus pathogenic and starters. Cheese samples were homogenised with a stomacher (BagMixer® 400, Interscience, Saint Nom, France). Microbial suspensions were plated and incubated as follows: Lactic acid bacteria cocci on M17 agar, incubated anaerobically at 30°C for 48 h; L. monocytogenes on Listeria selective agar base incubated at 37°C for 48 h; Escherichia coli and S. typhimurium were both detected on Hektoen enteric agar incubated at 37°C for 24 h; St. aureus on Baird Parker and incubated at 37°C for 48 h. Although in vitro tests showed a certain inhibition against especially, the Gram positive bacteria, but also towards Salmonella spp. none of the two Gram negative pathogens was inhibited in cheeses. Optimal results were registered against L. monocytogenes which was inhibited of almost 3 Log cycles by all EOs at both concentrations tested. St. aureus was not inhibited by orange EO, while 1-2 Log cycles lower than control (only pathogens) were found in EO cheeses. The inhibitory effect could also be due to the competitions for nutrients due to the most rapid growth of Lc. lactis. However, the difference registered among orange and the other two citrus EO trials highlighted the in vivo antibacterial activity of lemon and mandarin EOs. Sensory tests are being prepared to evaluate the appreciation by judges.