Effect of lauric acid addition on the antimicrobial efficacy and water permeability of corn zein films containing nisin

The effect of the addition of lauric acid and nisin to corn zein films on the water permeability and inhibition of bacterial growth was examined using two methods. A zone of inhibition test on solid media and a log reduction assay with the film exposed to a bacterial culture (Lactobacillus plantarum) in liquid media (cell count method)for six hours were conducted. The corn zein cast films contained nisin (5.0 mg nisin/g film and lauric acid at 0, 4, and 8% (w/w). The highest level of lauric acid addition (8%) significantly lowered the film water permeability compared to the 4 and 0% levels. The zone of inhibition test on solid media showed no clear inhibitory zones for films containing lauric acid alone, but did produce clear zones for films with nisin with or without lauric acid. Zone size decreased as the level of lauric acid increased or with decreased water permeability. Cell counts in liquid media exposed to film were reduced by the addition of lauric acid alone to the film. The nisin alone and in combination with lauric acid in the film reduced cell numbers in the cell count as well as increased zone size in the zone assay. No additional cell reductions were found when lauric acid and nisin were combined in the films compared to films containing lauric acid alone. Nisin alone was not as effective in reducing cell counts as 8% lauric acid. Log reductions for the nisin only treatment were 2 logs after I h and 3 logs after 4 h. However, while the lauric acid only treatment achieved the same ultimate 5.5 log reduction, the I and 2 h reductions were 0 and 1 logs, respectively.