No abstract

The effect of addition of purified nisin Z in liposomes to cheese milk and of in situ production of nisin Z by Lactococcus lactis subsp. lactis biovar diacetylactis UL719 in the mixed starter on the inhibition of Listeria innocua in cheddar cheese was evaluated during 6 months of ripening. A cheese mixed starter culture containing Lactococcus lactis subsp. lactis biovar diacetylactis UL719 was selected for high-level nisin Z and acid production. Experimental cheddar cheeses were produced on a pilot scale, using the selected starter culture, from milk with added L. innocua (10 5 to 10 6 CFU/ml). Liposomes with purified nisin Z were prepared from proliposome H and added to cheese milk prior to renneting to give a final concentration of 300 IU/g of cheese.The nisin Z-producing strain and nisin Z-containing liposomes did not significantly affect cheese production and gross chemical composition of the cheeses. Immediately after cheese production, 3-and 1.5-log-unit reductions in viable counts of L. innocua were obtained in cheeses with encapsulated nisin and the nisinogenic starter, respectively. After 6 months, cheeses made with encapsulated nisin contained less than 10 CFU of L. innocua per g and 90% of the initial nisin activity, compared with 10 4 CFU/g and only 12% of initial activity in cheeses made with the nisinogenic starter. This study showed that encapsulation of nisin Z in liposomes can provide a powerful tool to improve nisin stability and inhibitory action in the cheese matrix while protecting the cheese starter from the detrimental action of nisin during cheese production.Nisin is a cationic polypeptide of 34 amino acids produced by Lactococcus lactis strains (18, 47). Two natural variants of nisin (nisin A and nisin Z) are known and are equally distributed among nisin-producing strains (34). These variants differ by a single substitution, at position 27, with histidine (nisin A) and asparagine (nisin Z) (34). This structural modification gives nisin Z higher solubility and diffusion characteristics which are important for food applications (12).Nisin has an inhibitory effect against a wide variety of grampositive food-borne pathogens and spoilage microorganisms (40) and can also act on several gram-negative bacteria when the integrity of their outer membranes is disrupted (23,43). The use of nisin as a food preservative dates back to 1956, when nisin was proposed to control growth and spore formation of Clostridium botulinum and Clostridium sporogenes in cheese (31). Nisin is the only bacteriocin that has been approved by the World Health Organization as a preservative in food (46), and Nisalpin, the commercial product containing 2.5% pure nisin A, is being legally used in more than 50 countries for specific food applications (10). However, the loss of nisin activity from the commercial form has been reported for several food products during storage (8,9). Moreover, the use of nisin in its free form in cheese can be expensive and results in inhibitory effects against the suitable acidifying or aroma-p...