Abstract:
An antimicrobial packaging material for food stuffs containing from 0.05% to 1.5% by weight of a natural essential oil. The oil can be selected from primarily linalool and/or methylchavicol, but also from one or more of citral, geraniol, methyl cinnamate, methyl eugenol, 1,8-cineole, trans-a-bergamotene, carvacrol and thymol blended with one or more polymers selected from ethylene vinyl alcohol copolymer, polyacrylates, including ethyl acrylate methyl methacrylate copolymers, lonomers, nylons and other hydrophilic polymers or polymers possessing functional groups capable of partially anchoring the additives and the blender mix is coated onto the food contact face of a food grade packaging film or incorporated into a food grade packaging film. A binding agent such as polyethylene glycol is added to the blend to improve the retention of the volatile oil in the polymer during processing. This material has no regulatory limitations and, at the referred concentrations, does not form detectable off-flavors.

Description:
This application is the National Stage of International Application No. PCT/AU2005/000916 filed on Jun. 27, 2005; and this application claims priority of Application No. 2004903510 filed in Australia on Jun. 29, 2004 under 35 U.S.C. §119; the entire contents of all are hereby incorporated by reference. 
     This invention relates to a polymeric antimicrobial packaging material, using natural additives, particularly for use with food stuffs. 
     BACKGROUND TO THE INVENTION 
     Control of food spoiling micro-organisms, particularly on processed food stuffs, is an ever present concern in preserving food products. Various approaches such as aseptic packaging and pre-fill and post-fill sterilization are applied as a possible control method but they often result in undesirable changes in food quality characteristics. In addition fresh and many minimally processed foods cannot be preserved by such approaches and must rely on other methods. Washing in antimicrobial solutions is one possibility but protection during storage and distribution is still necessary. Incorporating antimicrobial agents in the food is another option but requires relatively large amounts of additive that increase cost and may lead to undesirable off-flavours. Modified atmosphere packaging is a common method to extend the shelf life, particularly of fresh produce, by reducing the rate of food deterioration achieved by modification of the initial concentrations of oxygen and carbon dioxide inside the package. However, these modified gas concentrations change with time. Also, some microorganisms can grow in the absence of oxygen and for some foodstuffs the absence of oxygen can affect freshness and flavour perception and even encourage the growth of harmful anaerobic microorganisms. Thus an antimicrobial agent is usually required. Synthetic antimicrobial agents in or on foodstuffs are usually not acceptable by consumers, as they prefer natural foods and food components. These additives may also accumulate above safe levels and affect flavour and taste. Antimicrobial agents are often active only against a selected range of microorganisms and those that are active against Gram-positive bacteria may not be active against Gram-negative bacteria and/or yeasts and/or moulds. 
     Incorporation of antimicrobial agents into packaging materials, particularly packaging films has been proposed in the past. These have generally been synthetic antimicrobial compounds. Migration of these compounds from the films into the food can be unacceptable. Generally, food safety and packaging regulations prohibit the release of (active) additives that are not approved for food content or they limit the allowable migration of such active additives from the packaging material. 
     USA patent application 2002/0051754 discloses an antimicrobial packaging polymer which incorporates quaternary ammonium and phosphonium salts. 
     There are plant extracts that possess antimicrobial activity and which are approved for use with foods. Some of them, like allyl isothiocyanate (AIT), posses a repelling smell causing unacceptable off-flavours. 
     Japanese abstract 03145406 discloses a rodent repellant which mixes an essential oil with an organic polymer including rubber and wood. 
     U.S. Pat. No. 5,688,509 discloses a controlled release insecticide for repelling insects from food, tobacco and other consumables. The active agent is incorporated in a controlled release carrier at a content above 1%. 
     WO00/69277 discloses an antimictobial agent for treating swine dysentery and diseases of hoofed animals using a combination of carvacrol an essential oil from oregano and thymol an essential oil from thyme. 
     USA application 2004/0071757 discloses an antiviral patch made by the addition of an essential oil to a pressure sensitive adhesive in concentration levels above 1%. Basil contains essential oils that exhibit activity against both Gram-positive and Gram-negative bacteria as well as against yeasts and moulds. The major active constituents of basil oil from various sources (basil varieties) are linalool, methylchavicol, citral, geraniol, methyl cinnamate, methyl eugenol, 1,8-cineole and trans-a-bergamotene. The two major components of basil, linalool and methylchavicol, possess very weak smells, not detectable at low concentrations. As the active basil oils are volatile and much of them may be lost during mixing and processing of the polymer into a packaging material, special precautions and procedures have to be followed. 
     It is an object of this invention to provide an antimicrobial packaging film, coating, container, or packaging component, such as labels, that uses a natural food acceptable agent in a food grade packaging material. 
     BRIEF DESCRIPTION OF THE INVENTION 
     To this end the present invention provides an antimicrobial packaging material for food stuffs which incorporates one or more volatile essential oil constituents in a relatively low temperature processable polymer composition in combination with a component that controls the release rate of the volatile oils. 
     By using a low temperature processable polymer composition, the loss of the volatile oil and also the risk of denaturing the oil constituents are reduced. Preferably the volatile component is mixed into a polymer component at low temperatures, or the polymer component is soaked in a solution of the volatile additive followed by evaporation of the solvent, to form a master batch which is then blended with the main polymer prior to processing and film forming. In this way higher additive retention values within the polymer are achieved. A preferred component for this process is an ethylene-vinyl acetate (EVA) copolymer which anchors and/or increases the solubility of the volatile oil components in the polymer matrix and reduces the processing temperature, resulting in a reduced evaporation of the antimicrobial additive during processing. The inclusion of a binder such as polyethylene glycol (PEG) also improves the retention rate and controls the release of the volatile oil. Other possible polymer candidates for the master batch are ethylene vinyl alcohol copolymer, polyacrylates, including ethyl acrylate methyl methacrylate copolymers, ionomers, nylons and other hydrophilic polymers or polymers possessing functional groups capable of partially anchoring the additives. A preferred film forming polymer is low density polyethylene (LDPE) blended with the EVA master batch containing the additive. These polymers are acceptable food grade packaging forming polymers. 
     An alternative to blending the master batch with another polymer is to coat a packaging film with the masterbatch. A preferred coating composition consists of the volatile essential oil constituent, such as a principal constituent of basil essential oil, incorporated in a polyacrylate polymer such as an ethyl acrylate methyl methacrylate copolymer. The coating may contain between 2 and 10% preferably 5% by weight of the volatile oil and between 98% and 90% preferably 95% by weight of the polyacrylate. To control the release of the volatile oil one or more additional coatings of the polyacrylate only, can coat the volatile oil layer. To minimize the volatilization of the volatile oil into the atmosphere through the polyethylene matrix (the outside of the package), a high gas barrier material may be laminated to that side of the packaging material. 
     The preferred concentration in the film is between 0.1% and 2.0% by weight of the total polymer structure. The concentration of the active oil required to impart antimicrobial activity for providing a sufficient shelf life is below 1.0% by weight. This concentration is low compared to prior art films. The minimum content of the additive is above 0.05% by weight. This low content saves cost in preparing the antimicrobial films. Because microbial growth and spoilage tends to occur at the surface of foodstuffs, close to the packaging film, effective control is achieved. With essential oils comprising principal constituents of herbs such as basil, thyme and oregano that are acceptable food ingredients, migration of the oils from the film is acceptable. The preferred essential oils are Linalool, methylchavicol, citral, geraniol, methyl cinnamate, methyl eugenol, 1,8-cineole, trans-a-bergamotene, carvacrol and thymol either used alone or in any combination to provide effective control against a range of microbes. 
     Sufficient volatile oil is added to the coating or blending masterbatch to account for loss of the active during processing to form the packaging film and during storage. 
    
    
     
       DETAILED DESCRIPTION OF THE INVENTION 
       Preferred embodiments of the invention are described in the following examples and the results are illustrated in the attached drawings in which: 
         FIG. 1  illustrates the normalized inhibitory action of the antimicrobial films of this invention against  E coli  compared with films containing synthetic microbial agents; 
         FIG. 2  illustrates the decrease in additive retention during accelerated storage of the antimicrobial film at 25 and 35° C.; 
         FIG. 3  illustrates the growth curve of  E coli  after addition of active agents used in the present invention; 
         FIG. 4  graphs the inhibition of  E coli , inoculated on the surface of Cheddar cheese; 
         FIG. 5  graphs the inhibition of  Listeria , inoculated on the surface of Cheddar cheese. 
     
    
    
     EXAMPLE 1 
     The polymer films containing basil essential oils were prepared as follows:
     Linalool (L260-2 from Aldrich Chemical Company) and   Methylchavicol (AUSTL 21320 from Aurora Pty Ltd) were individually blended with EVA powder into a master batch which was then mixed with LDPE to a concentration of 1.5% by weight of the polymers.   

     The mixture was extruded in a single screw extruder at temperatures not exceeding 160° C. and blown into a film. The films formed were transparent, and had a content of 0.34% of the active agent (linalool or methylchavicol). The performance of the films was tested with  Escherida coli  a gram negative bacteria and  Listeria innocua.    
     Table 1 shows the antimicrobial activity as observed on an agar disc diffusion assay (solid media) 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Treatment 
                 Conc % w/w 
                 Zone of inhibition mm 
               
               
                   
               
             
             
               
                   
                 LDPE 
                 — 
                 — no reaction 
               
               
                   
                 Linalool-LDPE 
                 0.34 
                 11.5 ± 0.19 
               
               
                   
                 Methylchavicol-LDPE 
                 0.34 
                 10.1 ± 0.18 
               
               
                   
               
             
          
         
       
     
     The performance of the films of this invention compared to films containing synthetic actives is shown in  FIG. 1  where B and D are the films of this invention, which achieved comparable activity compared to much greater concentrations of synthetic active agents such as O (neomycin) and S (triclosan). 
     The activity over time of similar films of this invention (based on linear low density polyethylene-LLDPE) is shown in Table 2 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Time of valuation 
                 Conc % w/w 
                 Zone of inhibition mm 
               
               
                   
               
             
             
               
                 After production 
                   
                   
               
               
                 LLDPE 
                 — 
                 — no reaction 
               
               
                 Linalool-LLDPE 
                 0.056 
                 11.3 ± 1.89 
               
               
                 Methylchavicol-LLDPE 
                 0.053 
                  8.8 ± 0.44 
               
               
                 After 1 year of storage 
                   
                   
               
               
                 Linalool-LLDPE 
                 0.037 
                 11.1 ± 0.11 
               
               
                 Methylchavicol-LLDPE 
                 0.028 
                  8.2 ± 0.12 
               
               
                   
               
             
          
         
       
     
       FIGS. 2 and 3  illustrate the retention of the added compound during accelerated storage conditions and shows that sufficient additive was retained to provide protection during the projected shelf life of food products. 
     Commercial Cheddar cheese was sterilized under UV light and then inoculated with  Escherida coli  or  Listeria innocua  (10 4  cfu g −1 ). The cheese was wrapped and heat sealed in plain LDPE and in films containing either linalool or methylchavicol. They were stored at 4° C. for 35 days and then a second batch was stored at 4° C. for 1 day and then at 12° C. for 15 days (temperature abuse conditions). The results of these tests are shown in  FIGS. 3 ,  4  and  5 . 
     EXAMPLE 2 
     Polymer film samples, based on various blends of LDPE, the co-polymer EVA (Ethylene Vinyl Alcohol) and the binding agent PEG (Polyethylene Glycol), containing thymol and carvacrol, either as a mixture or as a single component, were prepared from powdered LDPE (Microlene LD XJF 143/17, Qenos Pty. Ltd., Australia). Purified versions of thymol (98%) and carvacrol (98%) were obtained from Sigma-Aldrich Company Inc., Australia. The AM substances, mixed into a master batch, were incorporated into the LDPE matrix using a laboratory press. 
     Preparation of Films with Different Concentrations of am Agents 
     At the initial stage, films containing thymol or carvacrol at formulation concentrations varying from 1.0%-5.0% w/w were prepared to find the effective concentration/s of active agent retained in the sample after processing. The master batches were prepared by doping the copolymer in antimicrobial agent (in formulation concentrations of 1, 2, 3, 4, and 5% w/w respectively) dissolved in 70% v/v ethanol (EtOH). 
     With the aim of enhancing the retention of the antimicrobial agents, different film formulations containing 5% w/w antimicrobial agent (thymol or carvacrol), different concentrations of the binding agent and/or the copolymer were studied (Table 3). Except for the master batch preparation, the same procedure as described above was applied for the preparation of film samples of each formulation. 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Different formulations used for the preparation of sample films 
               
             
          
           
               
                   
                 Composition (% w/w) 
                   
               
             
          
           
               
                   
                 Antimicrobial 
                   
                   
                   
                 Preparation of 
                   
               
               
                 Formulation (F) 
                 agent 
                 Binding agent 
                 Copolymer 
                 Master batch preparation 
                 final mixture 
                 Remarks 
               
               
                   
               
             
          
           
               
                 F 1 
                 5 
                 — 
                 10 
                 Doped copolymer in 
                 Mixed master 
                 No binding agent 
               
               
                   
                   
                   
                   
                 antimicrobial agent 
                 batch with LDPE 
                   
               
               
                   
                   
                   
                   
                 dissolved in 70% v/v EtOH 
                   
                   
               
               
                 F 2 
                 5 
                 2.5 
                 5 
                 Doped copolymer in a 
                 Mixed master 
                 Copolymer % was 
               
               
                 F 3 
                 5 
                 2.5 
                 10 
                 mixture of antimicrobial 
                 batch with LDPE 
                 increased at a 
               
               
                   
                   
                   
                   
                 agent and binding agent, 
                   
                 constant level of 
               
               
                   
                   
                   
                   
                 dissolved in 70% v/v EtOH 
                   
                 binding agent 
               
               
                 F 4 
                 5 
                 2.5 
                 — 
                 Binding agent mixed with 
                 Mixed master 
                 No copolymer 
               
               
                   
                   
                   
                   
                 antimicrobial agent 
                 batch with LDPE 
                   
               
               
                   
                   
                   
                   
                 dissolved in 70% v/v EtOH 
                   
                   
               
               
                 F 5 
                 5 
                 2.5 
                 10 
                 Doped copolymer in a 
                 Mixed master 
                 Binding agent 
               
               
                   
                   
                   
                   
                 mixture of antimicrobial 
                 batch with LDPE 
                 incorporated at a 
               
               
                   
                   
                   
                   
                 agent dissolved in 70% v/v 
                 containing 
                 later stage 
               
               
                   
                   
                   
                   
                 EtOH 
                 binding agent 
               
               
                   
               
             
          
         
       
     
     Films containing mixtures of thymol and carvacrol were prepared according to the Formulation 1 (F1) method as described in above. For the preparation of the film samples two AM agent mixture concentrations, 5% or 10% w/w, were applied in the formulation. Within each formulation concentration thymol and carvacrol were mixed in the ratios: 1:1, 1:2 and 2:1 respectively. Control films were prepared by incorporating the single AM compounds at a concentration of 5% w/w. 
     Antimicrobial Activity Assay in-vitro 
     The films containing a single or combinations of antimicrobial agents were assessed for antimicrobial activity against the reference microorganisms: (i) gram negative bacteria— Esherichia coli  (UNSW 080300);  Pseudomonads aeruginosa  (UNSW 080400); (ii) gram positive bacteria— Staphylococcus aureus  (UNSW 051300);  Listeria innocua  (ACM 4984); (iii) yeast— Saccharomyces cerevisiae  (UNSW 703100) and (iv) fungi— Aspergillus niger  (UNSW 80900). The test strains:  E. coli, P. aeruginosa, S. aureus, S. cerevisiae  and  A. niger  were obtained from the culture collection, University of New South Wales (UNSW), Sydney, Australia and the Australian collection of microorganisms (ACM), Dept. of Microbiology, University of Queensland, Australia. 
                                                                               TABLE 4                   Conditions used for the growth and maintenance of test cultures                Incubation conditions                Liquid   Solid       Temperature       Reference organism   medium a     medium b     Time (h)   (C. °)                      E. coli     NB   NA   24   h   37° C.         Listeria innocua     BHIB   BA   24   h   37° C.         Pseudomonas     NB   NA   24   h   37° C.         aeruginosa                               Staphylococcus     NB   NA   24   h   37° C.         aureus                               Aspergillus niger     MEB   MA   1   week   25° C.         Saccharomyces     MEB   MA   48   h   25° C.         cerevisiae                   a NB, Nutrient Broth (Amyl, AM 131); BHIB, Brain Heart Infusion Broth (Amyl, AM 11); MEB, Malt Extract Broth (Amyl, AM 110)         b NA, Nutrient Agar (Amyl, AM 130); BA, Blood Agar Base C (Amyl, AM 24); MA, Malt Agar (Amyl, AM 109)            
Results and Discussion
 
     The point thickness of the sample films varied in a range of 70-140 μm. The calculated average thickness was approximately 110 μm. The average thickness was twice as high as the thickness of the extruded LDPE-based AM films used in example 1 for the determination of AM activity. 
     The post processing concentration of AM agent in the films is an important parameter as it directly relates to the AM activity. The post processing concentration of the AM agents, thymol and carvacrol, show a net loss during processing (Tables 5, 6 and 7) which is believed to be due to their volatile nature. The post processing retention of carvacrol was found to be better than that of thymol (Table 5). This might be attributed to the slightly higher boiling point of carvacrol (234-236° C.) than that of thymol (232° C.). 
     
       
         
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                 Formulation 
                 Post processing 
               
               
                   
                 Treatment 
                 conc. (% w/w) 
                 conc. (% w/w) 
               
               
                   
               
             
             
               
                   
                 Copolymer doped in 
                 1.0 
                 0.23 
               
               
                   
                 Thymol before 
                 2.0 
                 0.45 
               
               
                   
                 mixing with LDPE 
                 3.0 
                 0.52 
               
               
                   
                   
                 4.0 
                 0.74 
               
               
                   
                   
                 5.0 
                 1.09 
               
               
                   
                 Copolymer doped in 
                 1.0 
                 0.44 
               
               
                   
                 Carvacrol before 
                 2.0 
                 0.65 
               
               
                   
                 mixing with LDPE 
                 3.0 
                 1.02 
               
               
                   
                   
                 4.0 
                 1.42 
               
               
                   
                   
                 5.0 
                 1.82 
               
               
                   
               
             
          
         
       
     
     In order to enhance the retention of the AM agents, different film sample formulations using various combinations of a binding agent and a copolymer were tested (Table 3). In these formulations the concentration of AM agent was kept at a constant level (5% w/w) to ensure effective post processing concentrations. With the exception of formulation F4, the results (Table 6) show, increases in the post processing retention when the binding agent (F2, F3 and F5) is present in the films. Formulation F4, no use of the copolymer, showed the lowest retention by far. This result may indicate that the binding agent may only be capable of increasing the retention of the AM agents when the copolymer is present in the formulation. Thus, the increased retention seems to be a compound effect of the binding agent and the copolymer. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Post processing retention of AM agents in LLDPE based films 
               
             
          
           
               
                   
                 Post processing 
                   
                 Post processing 
               
               
                 Treatment 
                 conc. (% w/w) 
                 Treatment 
                 conc. (% w/w) 
               
               
                   
               
             
          
           
               
                 THYMOL 
                 F1 
                 0.89 
                 Carvacrol 
                 F1 
                 1.89 
               
               
                   
                 F2 
                 1.36 
                   
                 F2 
                 1.87 
               
               
                   
                 F3 
                 1.90 
                   
                 F3 
                 2.32 
               
               
                   
                 F4 
                 0.50 
                   
                 F4 
                 0.96 
               
               
                   
                 F5 
                 1.60 
                   
                 F5 
                 2.04 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Post processing retention of active agents in LDPE-based 
               
               
                 AM films containing mixtures of AM agents 
               
             
          
           
               
                   
                   
                 Target ratio 
                 Post 
                 Post 
                   
                 Actual ratio 
               
               
                   
                   
                 Thymol/ 
                 processing 
                 processing 
                   
                 Thymol/ 
               
               
                   
                 Target conc 
                 Carvacrol 
                 Conc 
                 Conc 
                 Actual total 
                 Carvacrol 
               
               
                 Treatment 
                 % w/w 
                 w/w 
                 thymol 
                 carvacrol 
                 conc 
                 w/w 
               
               
                   
               
             
          
           
               
                 carvacrol 
                 5 
                 — 
                 — 
                 1.12 
                 1.12 
                 — 
               
               
                 thymol 
                 5 
                 — 
                 1.73 
                 — 
                 1.73 
                 — 
               
               
                 Mixtures 1 
                 5 
                 1:1 
                 1.46 
                 1.62 
                 3.08 
                 1.0:1.1 
               
               
                   
                   
                 2:1 
                 1.95 
                 0.98 
                 2.93 
                 1.9:1.0 
               
               
                   
                   
                 1:2 
                 1.20 
                 2.48 
                 3.68 
                 1.0:2.0 
               
               
                 Mixtures 2 
                 10 
                 1:1 
                 2.05 
                 1.83 
                 3.88 
                 1.1:1.0 
               
               
                   
                   
                 2:1 
                 3.96 
                 2.57 
                 5.79 
                 1.5:1.0 
               
               
                   
                   
                 1:2 
                 2.27 
                 4.22 
                 6.49 
                 1.0:1.8 
               
               
                   
               
             
          
         
       
     
     The retention rates of the formulations with the mixed AM agents showed interesting results (Table 7). Although, the post processing retention of the AM agents was still less than the formulation concentration of the mixtures, the retention was substantially better when compared to the retention of the single AM agents. The retention at 5% w/w single formulation concentration as recorded in Tables 5 and 6 (F1) show losses of approximately 80% and 60% for thymol and carvacrol respectively. The 5% w/w total mixture formulations show individual losses of approximately 40% (thymol) and 35% (carvacrol). 
     Another interesting result was that the ratios resulted from the remaining concentrations of thymol and carvacrol in the films were quite similar to the formulation ratios. 
     Minimum Inhibitory Concentration (MIC) 
     Samples of LDPE-based films with different respective actual concentrations of thymol and carvacrol (Table 5) showed a dosage-depended AM activity in the agar disc diffusion assay (Table 8). Films containing thymol were found to be the most effective with the lowest MIC values of 0.51, 0.74 and 1.09% w/w against  Stap. aurues, S. cerevisiae  and  E. coli , respectively. 
     
       
         
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 AM activity of films containing various post processing concentrations 
               
               
                 of AM agent/s as observed by agar disc diffusion assay 
               
             
          
           
               
                   
                 Zone of Inhibition (2)  (mm) 
               
             
          
           
               
                   
                 Conc. 
                 
                   E. coli 
                 
                 
                   S. aurues 
                 
                 
                   S. cerevisiae 
                 
               
               
                 Treatment 
                 (% w/w) 
                 (3.25 × 10 6 ) 
                 (1.45 × 10 7 ) 
                 (4.1 × 10 6 ) 
               
               
                   
               
               
                 Control 
                   
                     — (1)   
                 — 
                 — 
               
               
                 Thymol 
                 0.23 
                 — 
                 — 
                 — 
               
               
                   
                 0.45 
                 — 
                 — 
                 — 
               
               
                   
                 0.52 
                 — 
                 6.00 b   
                 — 
               
               
                   
                 0.74 
                 — 
                 6.12 ± 0.2 b   
                 6.50 ± 0.7 b   
               
               
                   
                 1.09 
                 6.35 ± 0.2 a   
                 6.90 ± 0.8 a   
                 7.50 ± 0.7 a   
               
               
                 Carvacrol 
                 0.44 
                 — 
                 — 
                 — 
               
               
                   
                 0.65 
                 — 
                 — 
                 — 
               
               
                   
                 1.02 
                 — 
                 6.00 z   
                 6.00 z   
               
               
                   
                 1.42 
                 6.94 ± 0.9 y   
                 7.45 ± 0.7 y   
                 7.33 ± 1.5 y   
               
               
                   
                 1.82 
                 8.85 ± 0.7 x   
                 8.36 ± 0.8 x   
                 8.25 ± 1.0 x   
               
               
                   
               
               
                   (1) No inhibition 
               
               
                   (2) Means in the same column followed by same letter are not significantly (p &lt; 0.05) different in DMR test 
               
             
          
         
       
     
     The inhibitory effect was quite similar for these three microorganisms The effect increased significantly with the increase of AM agent concentration in the film. As expected,  Stap. aurues  (gram positive) was found to be the most sensitive followed by  S. cerevisiae  and  E. coli  respectively. However, none of the film samples showed any inhibition against  L. innocua  (gram positive) and  Ps. aeruginosa  (gram negative), although thymol and carvacrol have been reported to demonstrate AM activity against these two microorganisms (Cosentino et al., 1999; Walsh et al., 2003). The concentrations of more than 1% w/w should have been sufficient to show some AM activity. 
     The AM activity of the films prepared by different formulation methods varied significantly (Table 9). The addition of the binding agent to the copolymer in the master batch increased the post processing retention of AM agent in the films (Table 9). However, AM activity did not necessarily increase with the increased retention associated with the binding agent. The inhibition against  Stap. aurues, E. coli  and  S. cerevisiae  shown by the films with the binding agent is either less than or equal to that of F1 (no binding agent) films. This may be due to a reduced release of the AM agent by the binding agent. 
     Compared to the control, all sample films reduced significantly the colony diameter of  A. niger  (Table 9). Even films with very low concentration of active agent (e.g. F4) showed effective inhibition after 2 days and 1 week of incubation. It was found that there was no significant difference in inhibitory effect between the formulation methods. The results do not indicate an effect of a possible slower agent release by the binding agent. Thus, the addition of a binding agent may be effective in extending the activity shelf life of the AM film for long term inhibition. Several investigators support the hypothesis that the best antifungal activity of volatile compounds is achieved through gaseous contact as opposed to aqueous or agar contact (Guynot et al., 2003). The microatmospheric method as applied in the determination of AM activity of films against  A. niger  represents the activity of the active agent in its volatile state. The method may also be considered to mimic the headspace environment in a food packaging system where evaporation or equilibrated distribution of an active substance from the packaging material into the headspace and onto the food is a potential mechanism for application. 
                                                                                                                             TABLE 9                   Inhibition of microorganisms by AM films from different formulations                Zone of inhibition (2)  (mm)   Colony diameter (mm)                Conc.     E. coli       Stap. aurues       S. cerevisiae       A. niger  (1.3 × 10 6 )            Treatment   (% w/w)   (1.31 × 10 6 )   (1.47 × 10 6 )   (3.25 × 10 6 )   After 2 d   After 1 wk                    Control               — (1)     —   —       41.6 a, x         73.7 a, x         Thymol   F 1   0.89   7.16 ± 1.1 a     6.66 ± 0.8 a         6.63 ± 0.6 ab     10.3 b     26.5 b             F 2   1.36   6.24 ± 0.5 b     6.15 ± 0.4 a     —   16.6 b     28.2 b             F 3   1.90   6.05 ± 0.5 b     6.25 ± 0.7 a     6.97 ± 1.1 a     15.7 b     33.2 b             F 4   0.50   —   —   —   19.7 b     39.7 b             F 5   1.60   5.84 ± 0.2 b     6.13 ± 0.4 a     6.12 ± 0.2 b     13.6 b     32.5 b         Carvacrol   F 1   1.89   7.45 ± 1.0 x     7.58 ± 1.5 x     7.35 ± 1.3 x     14.2 y     33.7 y             F 2   1.87   —   5.85 ± 0.3 y     —   18.3 y     35.7 y             F 3   2.32   6.15 ± 0.4 y     6.12 ± 0.3 y     6.50 ± 0.7 x     14.3 y     40.5 y             F 4   0.96   —   —   —   13.5 y     37.7 y             F 5   2.04   6.20 ± 0.4 y     6.18 ± 0.4 y     6.33 ± 0.5 x     13.2 y     38.0 y                   (1) No inhibition         (2) Means in the same column followed by same letter are not significantly (p &lt; 0.05) different in DMR test            
AM Activity of Mixed Agents
 
     Combination of two AM agents in the polymeric substrate had a significant effect on the AM activity (Table 10) compared with the AM activity of the single agents. Except for  S. cerevisiae , a significantly higher inhibition was observed for film samples containing mixtures of AM agents than the films containing single compounds. It is interesting to note that films with mixture 11 (10% w/w total agent concentration in formulation) showed an inhibitory effect against  L. innocua . The actual concentration of AM agent/s in these films may be too low for action against  L. innocua . It was also observed that the AM efficacy may be affected by the mix ratio of the agents as is demonstrated by the results of Mixtures I ratio 1.0:2.0 compared with results of Mixtures II ratio 1.1:1.0. Although the actual total concentration is comparable in both formulations there is a significant difference in inhibitory effect for the three bacteria and  S. cerevisiae . This indicates a possible synergistic effect that depends on the AM agent concentration ratios. However, further studies on efficacy of films with AM mixtures are needed to fully explore the synergistic or antagonistic effect on AM efficacy. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 10 
               
             
             
               
                   
               
               
                 Inhibition of microorganisms by LDPE-based AM films containing mixtures of AM agents 
               
             
          
           
               
                   
                   
                 Ratio 
                   
                   
               
               
                   
                 Total 
                 (thymol: 
                 Zone of inhibition (2)  (mm) 
                 Diameter (mm) 
               
             
          
           
               
                   
                 conc. 
                 carvacrol 
                 
                   E. coli 
                 
                 
                   S. aurues 
                 
                 
                   L. innocua 
                 
                 
                   S. cerevisiae 
                 
                 
                   A. niger 
                   (3) 
                 
               
               
                 Treatment 
                 (% w/w) 
                 w/w) 
                 (1.31 × 10 6 ) 
                 (1.47 × 10 6 ) 
                 (6.9 × 10 5 ) 
                 (3.25 × 10 6 ) 
                 (1.3 × 10 6 ) 
               
               
                   
               
               
                 Carvacrol 
                 1.12 
                     — (1)   
                 6.4 ± 0.5 d   
                 6.6 ± 0.4 c   
                 — 
                 6.6 ± 0.5 b   
                 32.0 ± 1.5 b   
               
               
                 Thymol 
                 1.73 
                 — 
                 6.3 ± 0.2 d   
                 6.4 ± 0.4 c   
                 — 
                 6.4 ± 0.5 b   
                 26.5 ± 4.9 b   
               
               
                 Mixtures I 
                 3.08 
                 1.0:1.1 
                 8.0 ± 0.8 c   
                 8.4 ± 2.6 b   
                 — 
                 8.3 ± 1.5 b   
                  5.5 ± 0.7 c   
               
               
                   
                 2.93 
                 1.9:1.0 
                 7.4 ± 0.4 c   
                 7.8 ± 0.4 b   
                 — 
                 9.2 ± 1.5 b   
                 0.0 c   
               
               
                   
                 3.68 
                 1.0:2.0 
                 7.4 ± 1.0 c   
                 7.8 ± 0.4 b   
                 — 
                 7.9 ± 0.8 b   
                 0.0 c   
               
               
                 Mixtures II 
                 3.88 
                 1.1:1.0 
                 11.0 ± 0.6 ab    
                 10.9 ± 0.8 a   
                 7.8 ± 1.2 a   
                 17.5 ± 2.5 a   
                 0.0 c   
               
               
                   
                 5.79 
                 1.5:1.0 
                 11.5 ± 1.3 a   
                 10.3 ± 0.5 a   
                 8.0 ± 0.9 a   
                 19.4 ± 2.5 a   
                 0.0 c   
               
               
                   
                 6.49 
                 1.0:1.8 
                 10.5 ± 0.5 b   
                 11.3 ± 1.3 a   
                 7.8 ± 0.4 a   
                 19.9 ± 2.9 a   
                 0.0 c   
               
               
                   
               
               
                   (1) No inhibition 
               
               
                   (2) Means in the same column followed by same letter are not significantly (p &lt; 0.05) different in 
               
               
                 DMR test 
               
               
                   (3) Colony diameter (mm) after 1 week of incubation 
               
             
          
         
       
     
     Through variations in film component formulation and master batch preparation methods, the retention (post processing concentration) of thymol and carvacrol could be enhanced. The application of a binding agent in combination with a copolymer also increased the AM activity of the films as tested by the Agar assay method. Films containing a combination of thymol and carvacrol showed significantly increased retention as well as significantly higher AM activity. There are also indications of synergistic effects in association with agent concentration ratios. 
     EXAMPLE 3 
     Multi-coated Films Containing Natural Anti-microbial Agents Coating Solution 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 Ethyl acrylate methyl methacrylate copolymer 
                 25% 
                 by weight 
               
               
                 (40% by weight of dry substance in purified 
                   
                   
               
               
                 water) 
                   
                   
               
               
                 Ethanol (95% by volume) 
                 75% 
                 by weight 
               
               
                 Linalool 
                 from 1% 
                 by weight 
               
               
                   
               
             
          
         
       
     
     30-40% by weight of linalool is detected in the dried coating. 
     Films of LLDPE coated with coatings prepared as above with target concentrations of the active agent being 1% or 0.05% of the total film weight were tested for activity against  E. coli  and the results are shown in Table 11. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 11 
               
             
             
               
                   
               
               
                 AM activity of coated LLDPE films against  E. coli   
               
               
                 as observed by agar disc diffusion assay 
               
             
          
           
               
                   
                 Target 
                 Actual 
                 Zone of 
               
               
                 Method 
                 Conc./% w/w 
                 Conc./% w/w 
                 Inhibition/mm 
               
               
                   
               
               
                 Coated without 
                 — 
                 — 
                 — [1]   
               
               
                 agent 
                   
                   
                   
               
               
                 linalool 
                 1.0 
                 0.9108 
                 15.32 ± 1.45 [2]b   
               
               
                   
                 0.05 
                 0.0477 
                 12.20 ± 0.68 [3]ab   
               
               
                 methylchavicol 
                 1.0 
                 1.0259 
                 13.52 ± 0.57 ab   
               
               
                   
                 0.05 
                 0.0492 
                 11.31 ± 1.00 ab   
               
               
                   
               
               
                   [1] —, no reaction 
               
               
                   [2] Values for zone of inhibition are represented as mean ± SEM 
               
               
                   [3] The treatment with same letter within row is not statistically significant difference (p &gt; 0.05) 
               
             
          
         
       
     
     From the above results it can be seen that the present invention provides a unique antimicrobial packaging material that uses a natural antimicrobial additive in an inexpensive and yet efficacious film formulation that is acceptable with foodstuffs. Those skilled in the art will appreciate that the invention may be implemented in embodiments other than those described without departing from the core teachings of the invention.