Purpose: To compare the effects of citric and acetic acid on the growth of B. licheniformis for pH values between 4.6 and 7, and determine if B. licheniformis growth may occur in vegetable products containing acetic rather than citric acid.
Methods: Cocktails of 3 to 5 strains of B. licheniformis were grown Tryptic Soy Broth or cucumber juice (CJ), a non-inhibitory vegetable growth medium in microtiter plate wells (200 µl volumes). Plates were incubated with lids at 30°C for 24 hr to determine growth rates by optical density. For acid concentrations ranging from 25 mM to 300 mM, growth rates were compared to protonated and total acid concentrations.
Results: For pH 4.6, the highest total concentration of citric acid tested (300 mM) was approximately 8 times less inhibitory compared to the lowest total acetic acid concentration (25 mm). For both acids, combined data for all pH and acid conditions indicated that 10 to 15 mm (fully) protonated acid was needed to inhibit growth.
Significance: The data indicate that acetic acid was significantly more effective at inhibiting B. licheniformis growth than citric acid for conditions typical of acidified vegetable products. These data could help explain why pH increase due to B. licheniformis has not been seen in pickled vegetables but has been observed in tomato products.