Inactivation of Spoilage Bacteria in Package by Dielectric Barrier Discharge Atmospheric Cold Plasma—Treatment Time Effects

The objective was to investigate the effect of treatment time of dielectric barrier discharge atmospheric cold plasma (DBD-ACP) on inactivation of spoilage bacteria, Pseudomonas fluorescens and Macrococcus caseolyticus. P. fluorescens and M. caseolyticus were isolated from spoiled chicken carcasses and suspended in liquid media in packages before the samples were treated with DBD-ACP at 55 kV for different times. Ozone concentrations in package headspace and pH changes in bacterial suspensions were measured to assess plasma formation and antimicrobial activity inside packages. Colony formation was used to indicate the inactivation efficacy. Ozone concentrations reached 1000 ppm after a 3-min treatment. pH dropped by more than 0.25 units in bacterial suspensions immediately after a 6-min treatment and 0.70 units after the 6-min treatment and 24-h storage at 4 °C. Populations of both bacteria significantly reduced as the DBD-ACP treatment time increased. The treatment for less than 1.5 min resulted in more than 1 log reductions in both bacteria, which were similar to each other. However, treatments for additional 1.5 min reduced P. fluorescens and M. caseolyticus populations by more than 4 and 2.5 log cycles, respectively. The difference in bacterial kills between the two types of bacteria extended to the 6-min treatments. Our results demonstrate that treatment times significantly affect the inactivation of spoilage bacteria by DBD-ACP system. After the treatment times are extended for more than 1.5 min, the in-package DBD-ACP treatment has more effects on Gram-negative spoilage bacteria P. fluorescens than Gram-positive spoilage bacteria M. caseolyticus.