Nisin was purchased from Sigma-Aldrich Chemical Co.
(N5764; Sigma, St. Louis, Missouri, USA) that contains 2.5%
nisin with minimum potency of 106 U/g. Nisin (100 mg) was
dissolved in 100 mL 0.02 N HCl to give the concentration of
103 U/mL. Then the solution was sterilized by autoclaving
at 121 曟 for 15 min, and kept in a refrigerator at 4 曟 until
used. All other chemicals and reagents were of the highest
commercial grade.