The antimicrobial effects of natural compounds (caprylic acid, CA; citric acid, CTA; and vanillin, VNL) on the inactivation of Cronobacter sakazakii and Salmonella enterica serovar Typhimurium were examined in reconstituted infant formula. The samples were treated with: 1) CA, CTA, or VNL alone (0, 10, 20, 30, 40, 60, and 80mM); 2) a combination of CA (10 and 20mM) and CTA (15 and 30mM); and 3) a combination of CA (10 and 20mM) and VNL (15 and 30mM), at mild feeding temperatures (40°C and 45°C), and the bacterial populations were assayed periodically (0, 5, 10, and 30min). For both bacteria, the combined treatments had marked synergistic antimicrobial effects compared with the sum of the effects of each individual treatment. For example, there was no noticeable reduction (P>0.05) in the population of C. sakazakii following an individual treatment with 20mM CA or 30mM CTA for 5min at 40°C, whereas the population was reduced to undetectable levels (reduction>7.3 log CFU/ml) following treatment with a combination of CA and CTA (20 CA+30 CTA for 5min at 40°C). As the temperature increased, the bactericidal effect was stronger at all time points with a synergistic effect. In a validation assay using a low level inoculum (approximately 103CFU/ml) of desiccation-stressed bacteria in certain conditions, the combined treatments (e.g., CA 10mM+CTA 30mM for 5min at 45°C for C. sakazakii, and CA 10mM+VNL 15mM for 10min at 45°C for S. Typhimurium) completely destroyed the bacteria with no recovery of cell viability. Disintegration of the membrane and changes in the cell structure or morphology, such as plasmolysis and membrane disruption, were detected by flow cytometry and electron microscopy, respectively. These methods use antimicrobials that could be applied as food additives in infant formula, which may help to eliminate bacteria.
- Caprylic acid
- Citric acid
- Combined treatment
- Cronobacter sakazakii
- Salmonella enterica serovar Typhimurium