Efficacy of chitosan, carvacrol, and a hydrogen peroxide-based biocide against foodborne microorganisms in suspension and adhered to stainless steel

The ability of natural compounds to inactivate foodborne organisms adhered to surfaces was investigated with the ultimate aim of replacing synthetic biocides by more environmentally friendly, natural alternatives. The antimicrobial efficacy of 0.5, 1.0, and 2.0% chitosan and Spor-Klenz RTU (a commercial biocide based on hydrogen peroxide and peroxyacetic acid) and 0.5, 1.25, and 2.0 mM carvacrol was determined at 20 degreesC against Listeria monocytogenes, Salmonella enterica serovar Typhimurium, Staphylococcus aureus, and Saccharomyces cerevisiae adhered to stainless steel disks. Treatment with up to 2.0% chitosan reduced the viable cell count in the microbial films of the four test organisms by 2.4, 1.8, 2.3, and 0.9 log CFU/test surface (t.s.), respectively. By contrast, planktonic counts of the same organisms were reduced by 0.8 to 1.7 log CFU/ml at 2.0% chitosan. Treatment with 2 mM carvacrol reduced the viable counts of adhered listeriae, salmonellae, and yeasts by 2 to 3 log CFU/t.s. but S. aureus counts were reduced by only 0.9 log CFU/t.s. The efficacy of any single compound was species specific. In the case of microbial films prepared using listeriae and salmonellae, Spor-Klenz RTU was most biocidal, followed by carvacrol and then chitosan. However, dried films of S. aureus were most sensitive to chitosan and relatively resistant to carvacrol and Spor-Klenz RTU. By contrast, yeast films were most sensitive to carvacrol and least sensitive to chitosan. It was concluded that carvacrol and chitosan may have potential for use as natural biocides although optimization of conditions would be necessary.