NisaplinR, a commercial product of nisin was obtained from Aplin & Barret Ltd. (Trowbridge, U.K.), and stored at ambient temperature. Activity of nisin was determined using the agar spot method (Janes and others 1999). Nisin preparations were serially, 2-fold diluted and 5 μL from each dilution were spotted on MRS agar (MRS broth containing 1.5% agar) pre-poured plates that were overlaid with about 108 cells of a sensitive L. plantarum NCDO 955 in 5 mL of soft MRS agar (MRS broth containing 0.8% agar). These plates were incubated at 30 ◦C for 24 h and examined for the presence
of 2mmor larger clear zones of inhibition. Arbitrary units (AU) permL (AU/mL) were calculated as the reciprocal of the highest dilution, in a 5 μL amount, which gave a clear zone of growth inhibition on a lawn of sensitive cells (Pucci and others 1988; Bhunia and others 1991). AU/mL of the nisin preparation was calculated to be 320 AU/mg dry material. The amount of Nisaplin required to give the desired 1000 AU/mg of nisin was calculated to be 0.3125 g of Nisaplin/liter.