Inactivation of MS2 Bacteriophage, Murine Norovirus-1, Salmonella spp., and Enterococcus faecium on Strawberries by Using Gaseous Ozone

Introduction:  Sufficiently mild processing technologies need to be investigated to improve the safety of berry products. Ozone technology has been used as a sanitation tool in the food industry; however, limited publications are available on the inactivation of foodborne bacteria and viruses on fresh produce by using gaseous ozone.

Purpose : This study was an evaluation of the effect of gaseous ozone on foodborne pathogens and their surrogates on strawberries.

Methods: Fresh strawberries (25 g) were inoculated with MS2, MNV-1, Salmonella spp. and Entercoccus faecium, separately, and treated with gaseous ozone at concentrations of 1% (ca. 15 g/m³) and 6% (ca. 80 g/m³) for 5 and 30 min. Oxygen was used to flush away residual ozone. Ozone (6%) for 5 min and nitrogen flushing were applied to frozen samples. After treatment, samples were transferred into a filter-stomach bag containing either 50 ml virus elution buffer or 225 ml buffered peptone water to recover and enumerate viruses and bacteria, respectively. All experiments were conducted in triplicates.

Results: On fresh strawberries, 1%/5 min, 1%/30 min and 6%/5 min ozone treatments resulted in MS2 reductions of 1.78, 2.10 and 2.23 log₁₀ units, respectively. MNV-1 showed reductions of 1.12, 1.58 and 1.14 log₁₀ units, respectively. The highest reductions were achieved using 6%/30 min (3.30 log₁₀ for MS2 and 1.76 log₁₀ for MNV-1). Reductions of Salmonella spp. ranged from 1.00 to 2.06 log₁₀ units. Enterococcus faecium appeared to be more resistant (P < 0.05), producing reductions ranging from 0.45 to 1.52 log₁₀ units. For frozen strawberries, treatment with ozone resulted in log₁₀ reductions of 1.60, 0.72, 0.67 and 1.77 of Salmonella spp., E. faecium, MNV-1, and MS2, respectively.

Significance: This is the first study showing the inactivation effect of gaseous ozone against both viral and bacterial strains on frozen strawberries.