The bacteriocin nisin is active against both food spoilage and foodborne pathogenic Gram-positive bacteria. Previous studies have demonstrated that nisin activity can be enhanced in combination with chelating agents (e.g. EDTA), certain emulsifiers (e.g. monolaurin) and certain herb extracts (e.g. carvacrol, carvone). Such combined food protection solutions offer the benefits of a multiple hurdle system, both for preventing the emergence of less sensitive organisms and reducing the required dosage of a single preservative. The present investigation investigated the combination of nisin with natural extracts of the herb rosemary (Rosmarinus officinalis).  In vitro testing demonstrated such a combination to be synergistic. The rosemary extracts enhanced both the cidal and static antibacterial activities of nisin but did not extend its spectrum of activity. The rosemary extracts contained high levels of antioxidant phenolic diterpenes and low levels of flavour-inducing compounds such as essential oils. Comparative testing with differently processed extracts identified the phenolic diterpenes carnosol and carnosic acid as the primary synergists. The synergism was demonstrated against Listeria monocytogenes and Bacillus cereus in vitro and in a range of food models (frankfurters, meat and cheese pasta sauces, chicken soup, pâté). The effect was increased by low temperature and acid conditions.

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