The antimicrobial effects of long-wave ultra-violet light and furocoumarins on some micro-organisms that occur in cheese brines

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Listeria monocytogenes and Escherichia coli O157:H7 have been reported to survive in the brines used to store cheeses like Feta or Teleme, but such brines cannot be heat sterilized as the yeasts and lactic acid bacteria essential for normal cheese maturation are present as well. Long-wave UV light (UVA∼365 nm), acting in conjunction with photosensitizing compounds (e.g. furocoumarins like psoralen) might have more limited microbiocidal properties, so that, perhaps, pathogens could be eliminated from the cheese brine but not the desired yeasts and lactic acid bateria. In laboratory trials, UVA (intensity 45 W m−2, exposure time 60 s) with psoralen (5 mg l−1) was active againstListeria innocua —chosen to mimic the behaviour of L. monocytogenes—Escherichia coli O157:H7 andStaphylococcus aureus in a physiologically neutral solution, but E. coli O157:H7 (99% reduction in viable cell count) and L. innocua (99·8% reduction) were slightly less sensitive than S. aureus (99·99% reduction). Yeasts from Feta cheese brines were less affected by the same UVA/furocoumarin system—Debaryomyces hansenii (97·5% reduction) and Yarrowia lipolytica (82·7% reduction), as were typical lactic acid bacteria, namely Lactobacillus paracasei subsp. paracasei (97·8% reduction) and Lactobacillus plantarum (91·9% reduction). A UVA exposure time of 100 s with psoralen (5 mg l−1) was lethal to the ‘pathogens’ but, against the desirable species, onlyYarrowia lipolytica (97·4% reduction) readily survived the same treatment. It was concluded that the UVA/furocoumarin system was microbicidal but not, at least in the form under test, sufficiently selective in its action for use with cheese brines where certain of the microfloras need to be retained.