Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 23, 2009
Publication Date: September 23, 2009
Citation: Liu, L.S., Jin, Z.T., Coffin, D.R., Hicks, K.B. 2009. Preparation of antimicrobial membranes: coextrusion of poly(lactic acid) and Nisaplin in the presence of plasticizers. Journal of Agricultural and Food Chemistry. 57:8392-8398. Interpretive Summary: Active packaging materials have a large market of $17 billion which is growing at an annual rate of about 7%. Polylactic acid (PLA), a polymer derived from biobased monomer, is an ideal packaging material because it is a gas barrier, biodegradable, and possesses excellent mechanical properties. Therefore, the incorporation of antimicrobial reagents into PLA films to generate active PLA as a packaging material has been a hot topic in both academia and industry. PLA films are prepared either at a high melting temperature or using organic solvents. However, most antimicrobials are heat-sensitive, and the use of organic solvents is harmful to environment. In this study, PLA was first melted at 160 deg C in the presence of lactic acid or lactide, cooled to 120 deg C, and at this point the polypeptide was fed into the extruder and the films were prepared. Noticing that PLA doesn’t melt at a temperature lower than 155 deg C, and that PLA would not be a melt at 120 deg C without the presence of lactic acid or lactide, the maximal temperature at which Nisaplin® can retain its antimicrobial activity, the method highlights a new strategy to incorporate heat-sensitive substances into high melting temperature thermoplastics. The PLA films resist the growth of dangerous food-poisoning microorganisms (pathogens) because of the presence of the active antimicrobial compound. Thus, the films can function as novel antimicrobial food packaging materials. The study will help to expand the markets for PLA, a product derived from carbohydrates from plant material, and may replace the use of non-biodegradable packaging materials made from petroleum.
Technical Abstract: Nisin is a naturally occurring antimicrobial polypeptide, and is popularly used in foods and food packaging industries. Nisin is deactivated at temperatures higher than 120 deg C, and therefore can not be directly incorporated into poly(L-lactic acid) (PLA), a biomass-derived biodegradable polymer, by co-extrusion, because PLA melts at temperatures around 160 deg C. However, PLA can remain in a melt state at temperatures below the Tm in the presence of lactic acid or other plasticizers. In the present study, PLA was co-extruded with lactic acid, or lactide, or glycerol triacetate at 160 deg C. After the PLA was melted, the temperature of the barrels was reduced to 120 deg C, and then Nisaplin®, the commercial formulation of nisin, was added and the extrusion was continued. The resultant extrudates possess the capability to suppress the growth of the pathogenic bacterial L. monocytogenes, demonstrating a significant antimicrobial activity. The present study provides a simple method to produce PLA-based, antimicrobial membranes. The method can also be used for the co-extrusion of other heat-sensitive substances and thermoplastics with high melting temperature.