Covalent Immobilization of Polypeptides on Polylactic Acid Films and Their Application to Fresh Beef Preservation

To enhance the advantage of a long-term stability and low-toxicity active packaging system, two biodegradable covalent immobilized antibacterial packaging films were developed and applied to fresh beef preservation in this study. A polylactic acid (PLA) film was prepared by the extrusion-casting method. The surface of the PLA film was modified with plasma treatment to generate carboxylic acid groups, and then antibacterial agent nisin or epsilon-poly lysine (epsilon-PL) was covalently attached to the modified film surface. Physical, chemical, and antimicrobial properties of films were then characterized. Scanning electron microscopy and water contact angle images confirmed that nisin or epsilon-PL was successfully grafted onto the film surface. The values of protein loading on the nisin-g-PLA film and epsilon-PL-g-PLA film were 5.34 +/- 0.26 and 3.04 +/- 0.25 mu g of protein/cm(2) on the surface. Microbial analysis indicated that the grafted films effectively inhibit the growth of bacteria. Finally, the effects of the nisin-g-PLA film or epsilon-PL-g-PLA film on physicochemical changes and microbiological counts of fresh beef during cold storage at 4 degrees C were investigated. The total viable count of the control sample exceeded 7 logarithms of the number of colony forming units per gram (log CFU/g) after 11 days of cold storage (7.01 +/- 0.14 log CFU/g) versus 15 days for the epsilon-PL-g-PLA film (7.37 +/- 0.06 log CFU/g) and the nisin-g-PLA film (6.83 +/- 0.10 log CFU/g). The results showed that covalent immobilized antibacterial packaging films had positive impacts on the shelf life and quality of fresh beef. Therefore, a covalent immobilized antibacterial packaging system could be a novel preservative method for foods.