Bacterial spores isolated from ingredients, intermediate and final products obtained from dairies: thermal resistance in milk

Abstract

The ingredients used for the development and production of milk-based products pose the risk of the introduction of new, emerging spore-formers producing highly thermoresistant spores. Therefore, the aim of this study was to examine the heat resistance of spores isolated from dehydrated ingredients, intermediate and final products. Furthermore, the influence of the heating medium (milk or phosphate buffer) on the heat resistance was determined in order to assess which medium is best to use in the context of dairy processing. Sixteen spore-forming strains from seven different species (Bacillus amyloliquefaciens, Bacillus flexus, Bacillus subtilis, Bacillus thermoamylovorans, Bacillus smithii, Geobacillus pallidus, Geobacillus stearothermophilus) producing thermoresistant spores were selected after their isolation from diverse food products obtained from local dairies such as cocoa powder, milk powder, spices, and dessert products. Spores produced from the chosen strains were tested regarding their heat resistance at 110, 120, and 125 °C for 30 min. Highly thermoresistant spores surviving a heat treatment for 30 min at 125 °C in milk were produced by B. amyloliquefaciens and G. stearothermophilus. The inactivation of the spores was higher in milk than in phosphate buffer. This study highlights that not only raw milk but also dehydrated ingredients are important sources of thermoresistant spores in dairy processing. Since the inactivation of spores in milk and phosphate buffer turned out to be significantly different, milk rather than laboratory media or buffers should be employed as the model heating system when designing thermal processes for new milk-based products.