Abstract: The present invention is directed to a method for retaining the antimicrobial activity of the byproducts of growth of microorganisms during storage. In the method, a pasteurized low fat or skim milk substrate is provided. The substrate is inoculated with an S. lactis culture. The substrate is then fermented to cause growth of the culture and the release of metabolic byproducts, including nisin, from the culture. The fermented substrate is loaded into a container. The headspace of the container is then flushed with a quantity of an inert gas in excess of the amount required to displace all of the ambient air in the headspace. A small amount of a CO.sub.2 producing chemical, such as calcium carbonate, sodium carbonate and sodium bicarbonate, is mixed into the fermented substrate. The CO.sub.2 producing chemical has the effect of both stabilizing the antimicrobial activity of the cultured substrate and aiding in the release of nisin from the cells produced during fermentation of the substrate.