Viability of bacterial spores surviving heat-treatment is lost by further incubation at temperature and pH not suitable for growth

Abstract

Spores from 21 strains from different genera were heat-treated and stored in different sets of process conditions (4 temperatures and 3 pH levels) defined to prevent growth. In these conditions, spores surviving the heat treatment progressively lost viability during storage. Different inactivation curve shapes (linear, shoulder and tailing) and different sensitivities to storage were observed. B. coagulans showed the fastest inactivation kinetics, with more than 4-log reduction of spore population within 24 h after heating and G. stearothermophilus displayed slower inactivation kinetics, whereas all the anaerobic strains studied (M. thermoacetica and Thermoanaerobacterium spp.) proved resistant to storage conditions, with no destruction detected during 90 days in most cases. Inactivation rates were relatively unaffected by sub-lethal pH but sharply accelerated by temperature: Inactivation became faster as temperature increased (in the 8 °C–55 °C temperature range), with growth blocked by low pH in sub-lethal temperatures.There were changes in surviving spore numbers after the heat-treatment phase. This has implications and applications in canned food industries, as the probability of a retorted sample testing as non-stable, meaning possible spoilage, may decrease with time. In simple terms, a batch of low-acid canned food that tests as non-shelf-stable after an incubation test i.e. positive growth conditions, may later become negative if stored at room temperature (below the minimal growth temperature for thermophilic spores), which may change the marketability of the batch.