Abstract
Acid and heat inactivation in orange and apple juices of Salmonella enterica serovar Typhimurium Colección Española de Cultivos Tipo (i.e., Spanish Type Culture Collection) 443 (CECT 443) (Salmonella Typhimurium) and S. enterica serovar Senftenberg CECT 4384 (Salmonella Senftenberg) grown in buffered brain heart infusion (pH 7.0) and acidified brain heart infusion up to pH 4.5 with acetic, citric, lactic, and hydrochloric acids was evaluated. Acid adaptation induced an adaptive response that increased the subsequent resistance to extreme pH conditions (pH 2.5) and to heat, although the magnitude of these responses differed between the two isolates and fruit juices. The acid resistance in orange juice for acid-adapted cells (D-values of 28.3–34.5 min for Salmonella Senftenberg and 30.0–39.2 min for Salmonella Typhimurium) resulted to be about two to three times higher than that corresponding to non-acid–adapted cells. In apple juice, acid-adapted Salmonella Senftenberg cells survived better than those of Salmonella Typhimurium, obtaining mean D-values of 114.8 ± 12.3 and 41.9 ± 2.5 min, respectively. The thermotolerance of non-acid–adapted Salmonella Typhimurium in orange (D 58-value: 0.028 min) and apple juices (D 58-value: 0.10 min) was approximately double for acid-adapted cells. This cross-protection to heat was more strongly expressed in Salmonella Senftenberg. D 58-values obtained for non-acid–adapted cells in orange (0.11 min) and apple juices (0.19 min) increased approximately 10 and 5 times, respectively, after their growth in acidified media. The conditions prevailing during bacterial growth and heat treatment did not significantly influence the z-values observed (6.0 ± 0.3°C for Salmonella Typhimurium and 7.0 ± 0.3°C for Salmonella Senftenberg). The enhanced acid resistance found for both isolates could enable them to survive for prolonged time periods in the gastrointestinal tract, increasing the risk of illness. Further, it should be taken into account that microbial growth in acidified media also induces a cross-protection response against heat that should also be considered for the design of pasteurization processes for acid foods.