Inactivation of Listeria monocytogenes/Pseudomonas biofilms by peracid sanitizers

The ability of peracetic acid and peroctanoic acid sanitizers to inactivate mixed-culture biofilms of a Pseudomonas sp. and Listeria monocytogenes on stainless steel was investigated. Types of biofilms tested included a 4-h a ttachment of the mixed-cell suspension and a 48-h biofilm of mixed culture formed in skim milk or tryptic soy broth. Biofilm-containing coupons were i mmersed in solutions of hypochlorite, peracetic acid, and peroctanoic acid either with or without organic challenge. Organic challenge consisted of ei ther coating the biofilms with milk that were then allowed to dry, or addin g milk to the sanitizing solution to achieve a 5% concentration. Surviving cells were enumerated by pouring differential agar directly on the treated surfaces. The peracid sanitizers were more effective than chlorine for inac tivating biofilm in the presence of organic challenge. The 48-h mixed-cultu re biofilm grown in milk was reduced to less than 3 CFU/cm(2) by 160 ppm of peracid sanitizer after 1 min of exposure. Peroctanoic acid was more effec tive than peracetic acid against biofilm cells under conditions of organic challenge. Pseudomonas and L. monocytogenes were inactivated to similar lev els by the sanitizer treatments, even though Pseudomonas predominated in th e initial biofilm population.