Inactivation of yeast and bacteria using combinations of radio frequency electric fields and ultraviolet light

Abstract

Combining two nonthermal processes can increase inactivation of microorganisms in juice. Radio frequency electric fields (RFEF) is a nonthermal technology that is more effective at inactivating yeasts than bacteria, while ultraviolet (UV) light is the opposite. Apple juice was inoculated with Escherichia coli K‐12 and Saccharomyces cerevisiae and processed with RFEF, UV, RFEF + UV, or UV + RFEF at 40 and 50°C. The RFEF conditions were 15 kV/cm, 170 μs, and 46 J/ml. The UV conditions were 254 nm, 12 s, and 3.3 J/ml. At 40°C, RFEF inactivated 4.0 log S. cerevisiae and 1.5 log E. coli, while UV inactivated 0.8 log S. cerevisiae and 4.9 log E. coli. The combined processes inactivated 4.9 log S. cerevisiae and 5.8 log E. coli. Neither the order of the processes, nor temperature, significantly effected inactivation (p < .05). These results may assist in designing nonthermal pasteurization systems for liquid foods. Practical applications Pasteurizing juice nonthermally, without heat, maintains the utmost quality. Radio frequency electric fields (RFEF) is a nonthermal technology that has been shown more effective at killing yeasts than bacteria, while nonthermal ultraviolet (UV) light has been demonstrated better at killing bacteria than yeasts. The present study is the first to combine RFEF and UV to kill bacteria and yeast. Apple juice inoculated with Escherichia coli and Saccharomyces cerevisiae was nonthermally processed with RFEF, UV, or a combination of RFEF and UV. At 40°C, the combined process killed 99.999% of S. cerevisiae and E. coli. The energy of the combined process, 49 J/ml, was comparable to that of the most efficient commercial process currently available. Combining the nonthermal RFEF and UV processes capitalizes on the strength of each individual process to inactivate yeasts and bacteria and will result in safe and delicious juice.