Performance of antimicrobial bio active zein film against Escherichia coli and listeriainnocua during storage of Olivier salad

Antimicrobial packaging is a novel preservation technology that is designed base on releaseing of active agents incorporated with the packaging polymer into the packaged food or the surrounding environment for improveing the quality and food safety. Zein, water insoluble proteinof cornof corn, reported to be thermoplastic material with high tensile strength, excellent hydrophobic film, biodegradability.Zataria multiflora Boiss. (Z) is a thyme-like plant belonging to the Laminaceae family that geographically grows wild in central and southern parts of Iran. Antimicrobial activity of Z essential oil (ZEO) has been successfully tested against foodborne pathogens. Most of the studies on antimicrobial packaging mainly focused on the initial screening of newly developed films for ZEO in laboratory media and quantifying the bacterial reductions obtained during storage for different types of packaged food products. it is really important to know the variation in antibacterial activity of the agents when incorporated into the packaging film from its original activity in order to establish the levels that need to be incorporated for effective bacteria inhibition, hence The aims of the present study were to investigate the antimicrobial potential of ZEO incorporated zein film against pathogenic microorganisms inoculated in olivier saladto provide evidence of their applicability in the design of active food packaging systems.
Z was collected in the Shiraz province of Iran and extected by using clevenger type apparatus. Chemical composition ZEO was analyzed by gas chromatography.Antibacterial activity of ZEO was studied in the liquid (direct contact) and solid media (vapour phase) against Listeriainnocua and Escherchia coli . Zein powder was dissolved in a hydro alcoholic solution to obtain 16% (w/w) films forming solutions. The solution was stirred for at 80 °C using a magnetic stirrer hotplate. Glycerol (15%) was added to the solution and stirred again for 8 min at 30°Cm, then ZEO was added to the polymer solutions in 5% and 10% with respect to polymer content and stirred for 8 min. Films were obtained by casting, the film forming solutions were extended over it using an extension bar and introduced in a drying tunnel equipped with a heat of 2500 W during 20 min. Control zein films were prepared without active agent. To evaluate the efficacy of developed zein films was tested against two bacteria. Prior to the experiment, a loop of each strain was transferred to 10 ml of TSB and incubated at 37 ˚C for 18 h to obtain early stationary phase cells and100 µl of microorganism were inoculated into tubes with 10 ml of TSB. The tubes were then incubated at 37 °C for 18 h and 4 °C for 5 days. As control, zein film without active agent was also used in every experiment. Antibacterial activity of zein films containing 10% ZEO were evaluaterd for packaing of olivier salad as a real food model for 6 days, by performing serial dilutions with peptone and subsequent plating in Palcam Listeria Selective Agar and Brilliant Green Agar for ListeriainnocuaandEscherchia colirespectively. Plates were incubated at 37 °C for 48 hours.
GC–MS analysis showed thatthe major compound of ZEO was carvacrol (45.22%). The antimicrobial effects of ZEO in the vapor phase against Escherchia coliand Listeriainnocuaby using the disk diffusion methodshowed that the25 µg of ZEO produced a visible decrease in microbial density retraction zone (). No inhibition was observed with the addition of lower amounts of the antimicrobial agents, therefore 25 µg was the minimum inhibitory concentration (MIC).Activezein films containing 5% ZEOweren’t able to inhibit the growth of the two bacteria in the vapour pase.. When the concentrations of ZEO was increased to 10%, only active films with a diameter of 80 mm (50.24 cm2)provided a retraction zone () against Listeriainnocuaand E. coli. These values were indicative that incorporation of the agent in the zein films produced weaker vapor phase inhibition than its incorporation in the paper disk.The results of this studied showedthat films with 5% ZEO caused a growth reduction of 1.17 log againstListeriainnocuaand 1.14 log against Escherchia coli, while 10% ZEO produced reductions of 2.16 and 2.64 log against Listeriainnocua, Escherchia colirespectively. There was no significantdifference betweenlog reduction values of ZEO incorporated in zein films against Listeria innocuaand Escherchia coli. And also in the liquid media with increasing the concentration of ZEO in zein films, the log reduction values were increased against both bactreia. Despite of the excellent antibactrerial effect of ZEO in zein films in direct contact, bio active zein films were not effective enough for reducing the contamination of packaged Olivier.