Effect of non-thermal plasma technology on microbial inactivation and total phenolic content of a model liquid food system and black pepper grains

The objectives of this study were to investigate the effects of cold plasma technology on the growth and survival rates of vegetative cells and spores, and total phenolic content of black pepper grains. Plasma treatment was carried out using a non-thermal plasma jet system operating at 20 kHz using atmospheric air at a flow of 11 L/min. A model liquid food system and black pepper grains were both inoculated with Bacillus subtilis vegetative cells and spores. The samples were treated at 15 and 30 kV for treatment times of 3-20 min. The plate count method was used to determine colony-forming units for selected storage times at 4 degrees C i.e. at 1, 24 and 48 h post treatment. The highest log reduction was observed at 24 h post treatment, i.e. 2.92 log reduction. A 1 log reduction was achieved in the case of black pepper inoculated with spores for all selected storage times. No significant differences in total phenolic content were observed between treated and non-treated black pepper samples (p > 0.05). Optical emission spectroscopy was used to detect reactive species likely to be responsible for cell death. Atomic oxygen, atomic nitrogen, hydroxyl radicals, nitrite oxide and nitrate were detected in light emitted from the plasma. Cell membrane damage caused by non-thermal plasma technology was observed using scanning electron microscopy. This study concludes that cold plasma technology has potential for industry application in food processing to reduce microbial loads in liquid food systems and dried foods with limited impact on food quality.