The aim of the present study was to evaluate the effect of oxygen availability (aerobic, hypoxic and anoxic conditions) and sub-optimal pH (6.2 and 5.5) in a structured medium (10% w/V gelatin) on ...the growth of two immobilized L. monocytogenes strains (C5, 6179) at 10 °C and their subsequent acid resistance (pH 2.0, e.g., gastric acidity). Anaerobic conditions resulted in lower bacterial population (P < 0.05) (7.8–8.2 log CFU/mL) at the end of storage than aerobic and hypoxic environment (8.5–9.0 log CFU/mL), a phenomenon that was intensified at lower pH (5.5), where no significant growth was observed for anaerobically grown cultures. Prolonged habituation of L. monocytogenes (15 days) at both pH increased its acid tolerance resulting in max. 10 times higher t4D (appx. 60 min). The combined effect though of oxygen availability and suboptimal pH on L. monocytogenes acid resistance was found to vary with the strain. Anoxically grown cultures at pH 5.5 exhibited the lowest tolerance towards lethal acid stress, with countable survivors occurring only until 20 min of exposure at pH 2.0. Elucidating the role of oxygen limiting conditions, often encountered in structured foods, on acid resistance of L. monocytogenes, would assist in assessing the capacity of L. monocytogenes originated from different food-related niches to withstand gastric acidity and possibly initiate infection.
•Oxygen limitation impaired bacterial growth in structured media of mild pH.•Oxygen availability and suboptimal pH affected acid survival of L. monocytogenes in a strain-dependent manner.•Acid resistance of L. monocytogenes decreased with decreasing oxygen availability.•Ηabituation of L. monocytogenes increased its acid tolerance regardless of oxygen availability.
Salmonella spp. is known to survive in intermediate– and low–moisture foods. Bakery products such as cream-filled brioche (aw 0.82–0.84), depending mainly on the aw of the fillings and the baking ...they receive for food preservation, may support survival of the pathogen. The study aimed to model the inactivation of osmotically adapted and non–adapted Salmonella in cream–fillings (praline and biscuit) and cream-filled brioche at different storage temperatures. All matrices were inoculated with ca. 6.0 log CFU/g of osmotically adapted and non–adapted five–strain cocktail of Salmonella (Typhimurium, Agona, Reading, and Enteritidis) and stored aerobically in 120 mL screw-capped containers at 15, 20, and 30 °C. Adaptation of Salmonella was induced in cream–fillings (praline and biscuit) with aw adjusted to 0.88, by adding sterile water to each of the original fillings (aw 0.78–0.83) and incubating at 37 °C for 1 h. Survival of Salmonella was assessed at regular time intervals throughout storage using thin layer agar method to enhance the recovery of injured cells (n = 4). Inactivation curves were fitted best with the Weibull model using the freeware GInaFit tool and the estimated δ and β values were used to calculate the time for 4D reduction–t4D. Results showed that inactivation of Salmonella increased with temperature, while osmotic adaptation enhanced its survival in a food matrix–related manner. Higher survival rates of adapted cells were observed in cream–fillings (t4D: 79.9 ± 27.1 days on biscuit and 150.3 ± 19.6 days on praline) compared to brioche (t4D: 61.3 ± 0.9 days on biscuit and 52.5 ± 4.6 days on praline) at 20 °C. Secondary (linear) modelling of t4D showed that the survival of Salmonella was affected by temperature and osmotic adaptation. Model simulation of pathogen inactivation in independent trials on cream–fillings agreed well with observed data. In conclusion, the present data could be used as a means to identify areas for improving the performance of existing models quantifying the survival of Salmonella in bakery-confectionary products with intermediate aw.
