Survival of sublethally injured Listeria in model soup after nonisothermal heat and high-pressure treatments

Abstract

Food processors and consumers are interested in mildly processed refrigerated foods due to their increased quality, added convenience and enhanced nutrition. However, microbial responses (survival, injury and regrowth) to mild food preservation technologies need to be assessed thoroughly. This work investigated Listeria innocua responses to high-frequency agitating retort (convective), static retort (conductive) heating (at 62, 65, 68 °C) and combined high pressure–temperature (P/T) treatments (at 400/685 MPa and 20–50 °C). After processing and cold storage at 4 and 8 °C, injured and uninjured cells in the surviving population were quantified with differential plating on nonselective and selective agar media. Both convective and conductive heat treatments initially resulted in 1.5–5.0 log CFU/ml survivors, whereas combined P/T resulted in conductive heat > combined P/T. Following 3-week storage at 4 °C, highest cell regrowth was observed with conductive heat > combined P/T > convective heat. Under temperature abuse conditions (8 °C), survivors mostly resuscitated and regrew further. Observing higher injury rates and lower regrowth at 4 °C for convectively heated cells was apparently due to more effective accumulation of heat damage on bacterial population dispersed within the soup. Although pressure-injured cells generally regrew further during cold storage, combined P/T treatments showed a synergistic effect on suppressing bacterial recovery at increasing treatment intensities. Results from this study can be utilized in designing future studies on further understanding bacterial response differences to novel treatments with heat and/or high pressure.