Abstract
A two-temperature agar diffusion bioassay is commonly used to quantify the concentration of nisin using Micrococcus luteus as the indicator microorganism. A finite element computational model based on Fick's second law of diffusion was used to predict the radius of the inhibition zone in this diffusion bioassay. The model developed was used to calculate nisin concentration profiles as a function of time and position within the agar. The minimum inhibitory concentration (MIC) of nisin against M. luteus was determined experimentally. The critical time (Tc) for growth of M. luteus within the agar diffusion bioassay was experimentally determined using incubation studies with nisin. The radius of the inhibition zone was predicted from the computational model as the location where the predicted nisin concentration at Tc was equal to MIC. The MIC was experimentally determined to be 0.156 μg mL−1, and Tc was determined to be 7 h. Good agreement (R2 = 0.984) was obtained between model-predicted and experimentally determined inhibition zone radii.
Highlights
The use of “all natural” ingredients has received considerable attention from the food industry over the past decade due to consumer preference for natural over chemical preservatives (Smid and Gorris 1999)
Nisin is an example of a natural antimicrobial agent that is effective against gram-positive bacteria
This polypeptide is produced during fermentation by strains of Lactococcus lactis subsp lactis and was granted ‘generally regarded as safe’ (GRAS) status for use in foods by the Food and Drug Administration (FDA) (Hurst 1981)
Summary
The use of “all natural” ingredients has received considerable attention from the food industry over the past decade due to consumer preference for natural over chemical preservatives (Smid and Gorris 1999). Nisin is an example of a natural antimicrobial agent that is effective against gram-positive bacteria. This polypeptide is produced during fermentation by strains of Lactococcus lactis subsp lactis and was granted ‘generally regarded as safe’ (GRAS) status for use in foods by the Food and Drug Administration (FDA) (Hurst 1981). The most widely used method for the quantification of nisin is the one-temperature agar diffusion bioassay (Pongtharangkul and Demirci 2004). In this assay, wells are bored in agar plates seeded with an indicator microorganism. The model development can enable us to rapidly construct standard curves required for antimicrobial quantification
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