82 Novel food packaging techniques chitosan may make it suitable for use as an edible film for low moisture applications where mould spoilage is a concern, e.g. bakery applications 5.4.2 UV irradiated nvlon A recent development is surface modification of polymers leading to antimicrobial activity, for example treatment of nylon with an excimer laser at UV frequencies(193 nm)(Ozdemir and Sadikoglu, 1998, Shearer et al 2000). This has been described as a physical modification(Appendini and tchkiss, 2002), although the actual change which leads to the induced antimicrobial activity is a chemical change: amides on the nylon surface are converted to amines, which remain bound to the polymer chains, as observed with X-ray photoemission spectroscopy (XPS). Antimicrobial nylon-6,6 is prepared by irradiating with an UV excimer laser at 193 nm for a total exposure of 1-3 J/cm*. This results in conversion of approximately 10% of the surface amides and some etching of the film surface(see Fig. 5.4). The antimicrobial effect is strongly dependent on the wavelength of the laser used, with films treated at 193 nm showing a 5 log reduction in K pneumoniae in one hour, while film treated at 248 nm had no antimicrobial effect (Ozdemir and Sadikoglu, 1998). XPS analysis of the surface of film treated at 193 nm indicated that surface amide groups were converted to amines, while film treated at 248 nm showed no such change. The mechanism of antimicrobial activity is presumably similar to that of chitosan, poly-L-lysine and other cationic polymers, involving interaction with negatively charged microbial membranes leading to membrane disruption and leakage of cellular constituents The activity of UV irradiated nylon has been tested against various bacteria ( Shearer et al, 2000). In comparisons with untreated nylon, the treated nylon resulted in slight reductions in viable cell numbers for E coli and S. aureus. Some bacterial reduction was also observed for the untreated nylon, presumably due to bacterial adsorption. The treated and untreated nylons were not significantl different for reduction of E. faecalis and P. fluorescens. At least three hours of exposure were required for a significant reduction in cell counts and, for Saureus, he activity of the treated nylon increased with increasing temperature, no effect was observed a 4C or 15oC Protein(0. 1% Bovine Serum Albumin) completely inhibited the antimicrobial activity. Shearer and colleagues(2000) compared the ntimicrobial effect of treated film to that of secondary amines (n-butyl butyl amine)in solution and found that, to obtain a significant effect, a ten fold higher concentration of soluble secondary amine was required compared to the calculated number of amines formed on the surface of the film. it is not mentioned if the increased surface roughness was factored into this calculation; increased surface roughness results in a significant increase in the absolute surface area of the film, with a consequent increase in the number of active sites The results of the antimicrobial assays of UV irradiated nylons are not definitive. The data does not clearly show that the bacteria are killed as opposed to adsorbed on the surface of the film: the increased surface area resulting fromchitosan may make it suitable for use as an edible film for low moisture applications where mould spoilage is a concern, e.g. bakery applications. 5.4.2 UV irradiated nylon A recent development is surface modification of polymers leading to antimicrobial activity, for example treatment of nylon with an excimer laser at UV frequencies (193 nm) (Ozdemir and Sadikoglu, 1998; Shearer et al., 2000). This has been described as a physical modification (Appendini and Hotchkiss, 2002), although the actual change which leads to the induced antimicrobial activity is a chemical change: amides on the nylon surface are converted to amines, which remain bound to the polymer chains, as observed with X-ray photoemission spectroscopy (XPS). Antimicrobial nylon-6,6 is prepared by irradiating with an UV excimer laser at 193 nm for a total exposure of 1-3 J/cm2 . This results in conversion of approximately 10% of the surface amides and some etching of the film surface (see Fig. 5.4). The antimicrobial effect is strongly dependent on the wavelength of the laser used, with films treated at 193 nm showing a 5 log reduction in K. pneumoniae in one hour, while film treated at 248 nm had no antimicrobial effect (Ozdemir and Sadikoglu, 1998). XPS analysis of the surface of film treated at 193 nm indicated that surface amide groups were converted to amines, while film treated at 248 nm showed no such change. The mechanism of antimicrobial activity is presumably similar to that of chitosan, poly-L-lysine and other cationic polymers, involving interaction with negatively charged microbial membranes leading to membrane disruption and leakage of cellular constituents. The activity of UV irradiated nylon has been tested against various bacteria (Shearer et al., 2000). In comparisons with untreated nylon, the treated nylon resulted in slight reductions in viable cell numbers for E. coli and S. aureus. Some bacterial reduction was also observed for the untreated nylon, presumably due to bacterial adsorption. The treated and untreated nylons were not significantly different for reduction of E. faecalis and P. fluorescens. At least three hours of exposure were required for a significant reduction in cell counts and, for S.aureus, the activity of the treated nylon increased with increasing temperature; no effect was observed a 4ºC or 15ºC. Protein (0.1% Bovine Serum Albumin) completely inhibited the antimicrobial activity. Shearer and colleagues (2000) compared the antimicrobial effect of treated film to that of secondary amines (n-butyl butyl amine) in solution and found that, to obtain a significant effect, a ten fold higher concentration of soluble secondary amine was required compared to the calculated number of amines formed on the surface of the film. It is not mentioned if the increased surface roughness was factored into this calculation; increased surface roughness results in a significant increase in the absolute surface area of the film, with a consequent increase in the number of active sites. The results of the antimicrobial assays of UV irradiated nylons are not definitive. The data does not clearly show that the bacteria are killed as opposed to adsorbed on the surface of the film; the increased surface area resulting from 82 Novel food packaging techniques