Combined effects of heat, nisin and acidification on the inactivation of Clostridium sporogenes spores in carrot-alginate particles: From kinetics to process validation

Combined effects of mild temperatures, acidification and nisin on the thermal resistance of Clostridium sporogenes ATCC 11437 spores were assessed. Inoculated carrot-alginate particles were used as a solid-food model for the validation of the spore inactivation during the flow of a solid–liquid food system through the holding tube of an aseptic processing unit. Inactivation kinetics was studied in a water bath with the spores inoculated into carrot-alginate particles and in Sorensen's phosphate buffer. For temperatures of 70–90 °C, D-values in the buffer were 24.9–5.7 min, much lower than those evaluated for the particles (115.1–22.2 min). Statistical analyses showed significant synergistic effects of temperature and pH on spore inactivation for both media. Acidification reduced the heat resistance of the spores by reducing the D-values. Nisin was not significantly effective at the lower concentrations (up to 750 IU/g). The combination of 90 °C, pH: 4.5 and 500 IU/g nisin resulted in a ten-fold decrease of the D-value for spores inoculated in the particles (from 111.1 to 10.6 min). Microbial validation tests were conducted using a pilot-scale aseptic processing unit with a mixture of carrot cubes (10%) and carrier liquid of 2%-carboxymethylcellulose solution (90%). Spore-inoculated carrot-alginate particles (initial counts of 106 CFU/g, obtained after come-up-time pre-heat) with pH 3.5 and 2000 IU/g nisin were processed at 90 °C in the aseptic processing unit. Microbial analysis showed no spore survivors in the particles after passing through the holding tube (5.2–6.0 min of residence time). The proposed combination of these hurdles significantly enhanced the spore inactivation rate ( D 90=1.17 min) as compared to that for thermal treatment only ( D 90=19.6 min).