Pulsed Light Based Treatments As A Non-Thermal Strategy For Microbial Control On Cheese Surface

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Cheese products made from pasteurized milk often undergo post-pasteurization contamination when subjected to cutting, slicing, or packaging, either at the processing plant or in retail environments. Post-processing cross-contamination has significant health and economic consequences when leading to either hospitalizations or losses due to microbial spoilage. Pulsed Light (PL) treatment, consisting of short, high-energy light pulses, known to effectively inactivate microorganisms on surfaces, was evaluated as a solution to address surface decontamination of cheeses, either as a stand-alone treatment, or in combination with the antimicrobials nisin and natamycin. The effect of PL on color change, oxidative stability and onset of mold growth was also investigated. Slices of white cheddar and processed cheese were spot inoculated with Pseudomonas fluorescens 1150, Escherichia coli ATCC 25922 and Listeria innocua FSL C2-008, at a concentration of either 5 or 7 log CFU/slice. The inoculated samples were exposed to PL doses of 1.2 to 13.4 J/cm2, directly or through UV-transparent packaging. For combination treatments, cheese slices were dipped into a 2.5% Nisaplin (nisin) or a 50 ppm Natamax (natamycin) solution prior to inoculation. The antimicrobial treatments were tested both before and after PL application. The survivors were recovered and enumerated by standard plate counting (SPC). When survivor counts fell below the SPC detection limit, the most probable number (MPN) technique was used. Experiments were performed in triplicate and data was analyzed using a general linear model. Color change, oxidative stability, and onset of molding were monitored periodically on non-inoculated cheddar cheese samples stored at 6 [MASCULINE ORDINAL INDICATOR]C for one month. Color measurements were taken before and after PL treatment, and expressed as CIELAB values. Development of lipid peroxides was monitored colorimetricaly as a measure of oxidative stability, and the onset of molding was assessed visually on a daily basis. PL treatment alone was most effective against E. coli, achieving a maximum reduction of 5.4 ± 0.1 log CFU, at a dose of 13.2 J/cm2. For P. fluorescens, a maximum reduction of 3.7 ± 0.8 log CFU was obtained while a 3.4 ± 0.2 log CFU maximum reduction was achieved for L. innocua. The packaging, inoculum level and cheese type had no effect on L. innocua and P. fluorescens inactivation levels, while E. coli's response was more variable, depending on treatment conditions. PL combination treatments with antimicrobials showed that the presence of natamycin in cheese may interfere with PL, while a synergistic effect between PL and nisin was observed against Listeria, and only when nisin was applied after the PL treatment. PL was also found to be effective in extending the shelf life of cheese. Treatment of cheddar cheese slices with 9 PL pulses, or 10.1 J/cm2, delayed the onset of molding by a week, and slowed down the rate of molding after that. In terms of the effect of PL on cheese quality, no significant color change or increase in lipid peroxides was observed at a dose of 10.1 J/cm2 after one month of refrigerated storage. Overall, this work suggests that PL has strong potential for decontamination of cheese surface, and thus for improving the safety and extending the shelf life of cheese.
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Pulsed Light; Cheese; Food safety
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Union Local
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Moraru, Carmen I
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Wells, Martin Timothy
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Food Science and Technology
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M.S., Food Science and Technology
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Master of Science
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