Novel Delivery Systems of Nisin to Enhance Long-term Efficacy against Foodborne Pathogen Listeria monocytogenes

Novel intervention strategies such as food grade antimicrobials are used to enhance food safety. Nisin is a widely used naturally occurring antimicrobial effective against many pathogenic and spoilage microorganisms. However, its antimicrobial efficacy in food matrices is much reduced because of interactions with food components. Novel delivery systems may overcome this problem by protecting nisin in capsules and releasing it in a controlled manner. The overall goal of this research was to develop delivery systems of nisin to improve its long-term antimicrobial effectiveness. The first objective was to develop a low-cost method to extract nisin from a commercial preparation containing ~2.5% nisin. The best extraction yield was observed at 6 mg solids/mL 50% v/v ethanol. The nisin extract, after adjusting to 70% v/v ethanol, was dissolved with 2% zein (corn prolamins) and different amounts of Tween 20, glycerol, and thymol (another naturally occurring antimicrobial) for spray drying, a practical encapsulation method. Spray-dried capsules were characterized for release profiles of nisin at different pH conditions. Spray drying inlet temperature was first studied at 75 to 120°C, and the greatest sustained release of nisin was observed for capsules produced at 105°C, which was used in rest of this study. The impacts of Tween 20 and glycerol supplemented in spray-drying formulations were studied next. Addition of either 0.05% Tween 20 or glycerol in the spray-drying solution reduced the burst release of nisin at pH 6.0. Capsules with a higher amount of Tween 20 showed more complete release of nisin at pH 8.0, while glycerol had no apparent impact. The most sustained release of nisin at pH 6.0 was observed for capsules produced with both thymol and glycerol. Various capsules were then studied for their ability to inhibit the growth of Listeria monocytogenes at pH 6.0 during incubation at 30°C. Un-encapsulated nisin and thymol showed inhibition only for 12 h, while capsules with nisin and thymol containing either low or medium level of glycerol inhibited L. monocytogenes for >96 h. Our antimicrobial delivery systems, based on food grade ingredients and practical processes, have potential for practical application to enhance microbial safety and extend the shelf-life of foods.