Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 9, 2010
Publication Date: October 29, 2010
Citation: Ukuku, D.O., Geveke, D.J., Zhang, H.Q. 2010. Behavior of radio frequency electric fields injured Escherichia coli in nutrient and non nutrient media during storage. International Journal of Food Microbiology. 8(3&4):170-174. Interpretive Summary: Information on conditions required for resuscitation of Radio Frequency Electric Fields (RFEF) damaged E. coli cells is limited. In other to understand this phenomenon, Apple juice inoculated with Escherichia coli K-12 was treated with RFEF. Surviving populations containing the injured cells were added to different media with or without magnesium and calcium chloride and then stored at 5 and 23C for 24 h. The effect of media pH, storage temperatures and presence of magnesium and calcium chloride on the resuscitation of injured cells was tested after storage. The initial population of injured E. coli cells in all liquid media decreased during storage at 5C. In samples containing magnesium and calcium chloride, injured cells recovered and growth occurred in samples stored at 23C for 24 h. For example, the injured E. coli cells in apple juice took longer time to repair and re-grow than in nutrient broth. In the other media stored at 23C for 24h, the added salts extended survival of injured cells during storage. The results of this study suggest that cold storage of RFEF treated apple juice would kill the remaining surviving injured E. coli cells. Therefore, RFEF treatment and cold storage of treated apple juice is an effective alternative to heat pasteurization.
Technical Abstract: Information on conditions required for resuscitation of Radio Frequency Electric Fields (RFEF) damaged E. coli cells is limited. Apple juice inoculated with Escherichia coli K-12 at 7.8 log CFU/ml was treated with RFEF at 20 kHz, 15 kV/cm for 170 micro second at 55C with a flow rate of 540 ml/min. An aliquot (1 ml) of treated juice was added to 9 ml phosphate buffer saline (PBS, pH 7.2), 0.1% peptone water (PW, pH 7.2), deionized distilled water (ddH2O, pH 6.9), tryptone soy broth (TSB, pH 7.1) and apple juice (AJ, pH 3.8) and stored at 5 and 23C. Periodically (at 0, 3, 6, 18 and 24 h), 0.1 ml of samples listed above were plated on tryptic soy agar (TSA) amended with 0, 1.5 and 3.0 mM/ml calcium chloride and magnesium chloride, and MacConkey agar plates to investigate the behavior of RFEF injured E. coli cells. The initial population of injured E. coli cells in control ddH2O decreased from 3 log to 1.3 log CFU/ml during storage at 5 and 23C for 24 h. Addition of calcium and magnesium ions to PW, ddH2O and PBS did not encouraged recovery of injured cells but maintained higher percentage of injured populations. These ions increased E. coli populations in TSB and AJ by 0.8 and 0.4 log, respectively at 24 h. The results of this study suggest that immediate storage of RFEF treated apple juice at 5C for 24 h would inactivate the remaining surviving RFEF injured E. coli cells. Therefore, RFEF treatment and cold storage of treated apple juice would provide an alternative to heat processing for a safe and fresh tasting apple juice.