The growth potential and thermal resistance of bacterial spores under conditions relevant for ambient acid dairy-based products

The spoilage of acid foods, such as fruit juices and canned fruits and vegetables, caused by spore-forming bacteria has been well studied in literature. However, there is a lack of knowledge on the spoilage potential of spore-forming bacteria in ambient/shelf-stable acid dairy-based products. The present study investigated the germination and growth potential of spores from sixteen different bacterial species commonly present in milk and dairy or non-dairy ingredients. Experimental conditions were selected as relevant to acid dairy-based products (pH 4.6, the presence of 0.4% and 0.8% lactic acid (LA)) when stored at normal (25 °C) and elevated ambient temperatures (40 °C) for a period of 12 weeks. As a consequence of the observed growth, the spore heat resistance of four strains of Alicyclobacillus acidoterrestris was evaluated in a yoghurt-based medium. The growth experiments indicated that the spores of seven bacterial species (A. acidoterrestris, B. megaterium, B. subtilis, B. coagulans, C. beijerinckii, C. butyricum, and C. tyrobutyricum) out of sixteen exhibited the ability to germinate and grow at a pH of 4.6 and temperatures of 25 °C and/or 40 °C. However, none of the spores from 16 bacterial species grew at pH 4.6 in the presence of lactic acid (0.4% and 0.8%) at 25 °C, while only A. acidoterrestris spores grew at 40 °C regardless of the presence of lactic acid in the growth medium. The thermal inactivation kinetics of A. acidoterrestris spores revealed that all four strains possess high thermal resistance profiles in yoghurt-based medium at pH 4.6, indicating that traditional pasteurization treatment may not be sufficiently effective in controlling their growth. This study highlights the significance of A. acidoterrestris as a potential causative agent of spoilage in ambient acid dairy-fruit products at elevated ambient temperatures and the need for further research on effective control measures for this species.