Inactivation Of Microorganisms In Skim Milk And Shredded Mozzarella Cheese Using High Pressure Carbon Dioxide And Nitrous Oxide

Inactivation of microorganisms with high-pressure carbon dioxide (HP-CO2) is emerging as an innovative process for the sterilization of biological materials. However, its application in milk processing poses a challenge since CO2-induced reduction in pH may lead to casein precipitation. High-pressure nitrous oxide (HP-N2O) has been suggested as an alternate choice for the processing of fluid milk. Agitated supercritical carbon dioxide (Sc-CO2) at 10.3 MPa and 35°C with 100 ppm peracetic acid (PAA) resulted in a complete 8- and 5-log10 inactivation of Escherichia coli and Bacillus atrophaeus spores in thin milk-films after 15 and 40 min, respectively. The treatment also resulted in partial milk-protein coagulation (55%) and thus possible applications of this approach may be in those processes where curd formation from sterile milk is beneficial. The HP-CO2 treatment at 10.3 MPa with 50 ppm PAA resulted in 2.6-, 5.4- and 9.2-log10 reductions of E. coli in agitated bulk milk after 120 min at 5, 15 and 25°C, respectively, whereas a 0.7-log10 reduction of B. atrophaeus spores was obtained at 25°C. The Fermi model was used to describe the inactivation kinetics of E. coli and B. atrophaeus. This strategy should be attractive for low-temperature ([LESS-THAN OR EQUAL TO]25°C) pasteurization of fluid milk. A 20-min treatment of skim milk with added nisin (150 IU/mL) using HP-N2O (15.2 MPa and 65°C) resulted in 8- and 8.6-log10 reductions of E. coli and Listeria innocua, respectively. ! Meanwhile, a 2.5-log10 inactivation of B. atrophaeus spores was obtained when lysozyme (50 [MICRO SIGN]g/mL) was also added and the temperature was increased to 85°C. There were no significant changes in the physico-chemical properties of the treated milk and no sub-lethally injured cells were detected following the treatment. Agitated Sc-CO2 at 9.8 MPa and 35°C with 100 ppm PAA synergistically resulted in the inactivation of major microbial groups in shredded Mozzarella cheese after 30-min of treatment. A >5-log10 reduction in the populations of E. coli, L. innocua, yeasts & molds and the total bacterial counts along with a 4-log10 reduction of Geobacillus stearothermophilus spores was achieved during storage for 21 days at 25°C.