Abstract
Nisin is a bacteriocin produced by Lactococcus lactis subsp. lactis, with antimicrobial activity against many Gram-positive bacteria, including bacterial spores. Nisin has been successfully used in numerous products, such as processed cheeses, dairy products and canned foods. However, its effectiveness in meat products is variable due to interactions with certain meat components. The aim of this project therefore was to determine a number of factors reported to reduce nisin activity in meat and meat products, evaluate the extent of interference with its efficacy and introduce multicomponent preserving systems that improve its antimicrobial action. Loss of nisin activity in meat has been partly ascribed before to the formation of a nisin- glutathione adduct. Activity is lost more quickly in raw meat than in cooked meat and this has been taken as evidence that the reaction is enzyme-mediated. Horizontal agar diffusion method and MALDI-TOF MS confirmed the formation of the nisin-glutathione adduct which was shown not to be enzyme mediated. Retention of activity in cooked meat was shown to be due to loss of free sulfhydryl groups during heat processing as a result of the reaction of glutathione with proteins and not a result of the inactivation of endogenous enzymes, as indicated by measuring the free thiol content. Microbial enzymes did not appear to play a role as similar losses were seen in raw and cooked meat extracts containing undetectable levels of microorganisms. Enumeration of viable counts indicated that increased nisin concentrations were required for the total inhibition of L. monocytogenes following incubation of nisin with glutathione. The inhibitory effect of glutathione on nisin activity was removed in the presence of ascorbic and dehydroascorbic acid, sometimes with enhancement of nisin's bactericidal effect. However, there was no evidence for a direct reaction between glutathione and dehydroascorbic acid. The addition of ascorbic acid and dehydroascorbic acid in raw meat extract as well as in raw and thermally processed meat slurry was shown to exert the same protective effect seen with laboratory media. The synergy between nisin, ascorbic and dehydroascorbic acid was more pronounced at chill temperatures. Simultaneous application of antioxidants and nisin delayed the recovery of nisin-injured L. monocytogenes cells especially at low temperatures. Antioxidants, at the levels used, had no bactericidal effect on their own, but they did retard growth by decreasing growth rate and increasing lag phase. This was probably due to antioxidants diverting energy from injuiy repair-related functions. The effectiveness of the combination of nisin with propyl gallate was completely negated in whole fat UHT milk due to interactions of nisin with fat. Overall, the findings of this study clearly show that nisin performs better when its use is supplemented with other preservation agents or methods, since its amphiphilic properties make it vulnerable to interactions with other food macromolecules resulting in reduced efficacy.