A concentrated nisin preparation is obtained from a dilute aqueous solution of nisin produced by growing in a culture medium a nisin-producing strain of Streptococcus lactis by a process in which foam is produced from the aqueous solution in the presence of a small quantity, not exceeding 0,1%, of a surface active agent especially a non-ionic surface active agent, and the foam is then collected. The culture liquid is transferred to an upright tube having a rotating plate at the top and air is passed upwardly through the tube, the foam collecting at the top of the tube being collapsed prior to collection by the rotating plate. Other inert gas such as nitrous oxide, carbon-dioxide or nitrogen may be used in place of air. The collapsed foam is treated with a salting out agent such as sodium chloride, the precipitated nisin dissolved in a solvent such as methanol and is reprecipitated by the addition of a non-solvent such as acetone. The dilute aqueous solution of nisin may be formed by growing the Streptococcus lactis in a milk culture medium precipitating a curd of casein or paracasein from the culture medium, adjusting the pH of the culture medium to a value less than 4,5 and thereafter separating from the curd the aqueous liquid containing the nisin.
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