Chicken fillets subjected to UV‐C and pulsed UV light: Reduction of pathogenic and spoilage bacteria, and changes in sensory quality

Abstract We have compared the efficacy of continuous ultraviolet (UV‐C) (254 nm) and pulsed UV light in reducing the viability of Salmonella Enteritidis, Listeria monocytogenes, Staphylococcus aureus, enterohemorrhagic Escherichia coli, Pseudomonas spp., Brochothrix thermospacta, Carnobacterium divergens, and extended‐spectrum β‐lactamase producing E. coli inoculated on chicken fillet surface. Fluences from 0.05 to 3.0 J/cm2 (10 mW/cm2, from 5 to 300 s) used for UV‐C light resulted in average reductions from 1.1 to 2.8 log cfu/cm2. For pulsed UV light, fluences from 1.25 to 18.0 J/cm2 gave average reductions from 0.9 to 3.0 log cfu/cm2. A small change in the odor characterized as sunburnt and increased concentration of volatile compounds associated with burnt odor posed restrictions on the upper limit of UV treatment, however no sensory changes were observed after cooking the meat. Treatments under modified atmosphere conditions using a UV permeable top film gave similar or slightly lower bacterial reductions. Practical applications Ultraviolet (UV) light may be used for decontaminating the surface of food products and reduce viability of pathogenic and spoilage bacteria. Exposure of raw chicken fillet surface to various doses of continuous UV‐C or pulsed UV light proposed in the present work represent alternatives for microbiological improvement of this product. Chicken fillets can be treated in intact packages covered with UV permeable top film, thus avoiding recontamination of the meat. UV‐C light treatment is a low cost strategy with low maintenance, whereas pulsed UV light involves more elaborate equipment, but treatment times are short and less space is required. Both methods can be helpful for producers to manage the safety and quality of chicken fillets.

Various physical and chemical methods to reduce microbes on poultry products have been studied, such as water spraying, air chilling, ultrasound, irradiation, trisodium phosphate, and lactic acid (Capita et al., 2002;Loretz, Stephan, & Zweifel, 2010). Potential disadvantages using these methods are sensory changes, deterioration of product appearance and quality, and safety concerns. In recent years, there has been a growing interest in using ultraviolet (UV) light for decontamination of poultry. UV light is widely known for its germicidal effect by damaging nucleic acids (Kowalkski, 2009). The high energy associated with short-wavelength UV energy (UV-C), primarily at 254 nm, is absorbed by cellular RNA and DNA. This energy absorption initiates a reaction between adjacent pyrimidine bases to form dimer lesions, which in turn inhibit replication and transcription in cells (Harm, 1980;Weber, 2005).
As a means for controlling surface microorganisms on food products, regulations in conjugation with using conventional continuous UV-C light (henceforth referred to as UV-C light) in the United States are given by the U.S. Food and Drug Administration (FDA) (FDA, 2010). UV-C light can be employed in Europe, however, in Germany the use is limited to water, fruit and vegetable products, and stored hard cheeses (Anonymous, 2000). Decontamination of raw boneless, skinless chicken, or broiler breast fillets by the use of UV-C light has been reported to reduce bacterial counts of various pathogens by 0.6 to 1.7 log depending on the conditions used (Chun, Kim, Lee, Yu, & Song, 2010;Haughton et al., 2011b;Isohanni & Lyhs, 2009;Sommers, Scullen, & Sheen, 2016). High intensity pulsed UV light has been approved by the FDA up to 12 J/cm 2 (FDA, 2010). The UV energy spectrum of pulsed UV light consists of a continual broadband spectrum from deep UV to infrared light, especially rich in UV range below 400 nm, which is germicidal. In addition to creating dimer lesions, pulsed UV light has been proposed to cause cell damage and cell death by inducing damage of the cell membrane and to cause rupture of the bacteria by thermal stress (Krishnamurthy, Tewari, Irudayaraj, & Demirci, 2010;Takeshita et al., 2003;Wekhof, 2000). The use of this technology for food decontamination has previously been reviewed (Demirci & Panico, 2008;Gomez-Lopez, Ragaert, Debevere, & Devlieghere, 2007). Pathogen reduction on boneless skinless chicken breast has been reported to vary from 1.2 to 2.4 log depending on the conditions used (Keklik, Demirci, & Puri, 2010;Paskeviciute, Buchovec, & Luksiene, 2011). Several investigations have demonstrated the effectiveness of UV light on microbial reduction in vitro, and a wide range of bacterial species were reduced by 5-7 log when treated on petri dishes under different conditions (Farrell, Garvey, Cormican, Laffey, & Rowan, 2010;Gomez-Lopez, Devlieghere, Bonduelle, & Debevere, 2005;Paskeviciute et al., 2011;Rowan et al., 1999).
The objective of our investigation was to study and compare the efficacy of UV-C and pulsed UV light against pathogens and bacteria often found as natural contaminants on fresh chicken meat, of which several of the species have not previously been investigated for UV light treatment on food. To our knowledge, studies on UV light exposure of intact packages of MAP-chicken fillet for bacterial reduction have not been reported, thus we aimed at undertaking this issue using a UV permeable top film. We also aimed at determining whether the UV light treatments had adverse effects on the sensory quality of chicken fillets.

| Bacterial strains, media, and growth conditions
The bacterial strains used in this work are listed in Table 1. The strains were maintained at 2808C in their respective media supplemented with 20% glycerol (vol/vol). Rifampicin resistant (Rif R ) derivatives were prepared for all isolates by growing strains in liquid media containing 200 mg/ml rifampicin as described by Heir et al. (2010), except for the ESBL-producing E. coli strains already resistant to several types of antibiotics. Growth experiments using a Bioscreen C instrument (Labsystems) where the Optical Density (OD) at 600 nm was monitored, showed no significant difference in growth between the original strains and their Rif R mutants in their respective media and growth conditions. The different bacterial strains of each species were cultured separately.

| UV illumination experiments of chicken and agar surface inoculated with bacterial cells
Fresh skinless chicken breast fillets were purchased from local Norwegian supermarkets. The meat was cut into pieces of 10 cm 2 , and one side of the chicken was inoculated by spreading 15 mL suspension of a multi strain mix of one species (described above) to obtain bacterial levels of 10 5 -10 7 cfu/cm 2 . The inoculated chicken samples were left at room temperature to dry for 1 hr prior to UV light treatment. To assess the indigenous background flora of the chicken, uninoculated samples were also analyzed. For in vitro illumination experiments, serial 10-fold dilutions of each multi strain mix were made and plated onto the respective agar media (described below). In the UV-C light experiments, samples were treated in a custom made aluminum chamber (1.0 3 0.5 3 0.6) m 3 equipped with two UV-C lamps (UV-C Kompaktleuchte, 2x95 W, B € ARO GmbH, Leichlingen, Germany) in the ceiling. The UV-C light was emitted essentially at 253.7 nm, measured using a UVX Radiometer (Ultra-Violet Products, Ltd., Cambridge, UK) equipped with a UV-C sensor (model UVX-25, Ultra-Violet Products). Both sample distance (6 cm) from the lamps and duration of the exposures were chosen with aim to be relevant for industrial production lines. Exposures were thus at 10 mW/cm 2 , which is close to a maximum when using commercial lamps, for 5, 10, 30, 60, or 300 s, giving fluences of 0.05, 0.1, 0.3, 0.6, 3.0 J/cm 2 , respectively. For the pulsed UV light experiments, a semiautomated intense pulsed UV system instrument XeMaticA-SA1L (SteriBeam Systems GmbH, Kehl-Kork am Rhein, Germany) was used. Samples were placed in the instrument chamber at a 6.5 cm distance from the xenon lamp (19 cm), which was water cooled, had an aluminum reflector (10 cm 3 20 cm), and the spectral distribution was 200-1,100 nm, with up to 45% of the energy being in the UV-region (maximal emission at 260 nm). The fluences were set according to the manufacturers specifications, and were adjusted to 1.25 J/cm 2 (low) or 3.6 J/cm 2 (high). The lowest level of exposure would result in limited bacterial reductions, and fluences up to and above the limit value of 12 J/cm 2 , which is the maximum permitted dose by FDA (FDA, 2010), were tested. Samples were exposed either once to the low pulse, or one, three, or five times to the high pulse (3.6, 10.8, or 18.0 J/cm 2 , respectively). Three parallels of both treated

| Microbial analyses
Chicken samples were added 90 ml of peptone water and the samples were homogenized for 1 min in a stomacher (AES Smasher, AES Chemunex, Bruz, France). Serial 10-fold dilutions from each sample were prepared. Quantification of C. divergens (cfu/cm 2 ) was performed using a

| Packaging film analyses
The UV permeable top film Opalen 65 was evaluated for its ability to transmit UV light by measuring UV light at 254 nm (described above).
The extended O 2 barrier properties of the top film was evaluated by using empty packages with 100% N 2 that were initially exposed to four different UV-C and pulsed UV light treatments up to 10.8 J/cm 2 in addition to an untreated control, with five packages per treatment. The packages were analyzed for concentrations of residual oxygen at packaging and after 21 days of storage with a Dansensor Checkmate 3 (Dansensor, Ringsted, Denmark). The top films of the trays used for oxygen analysis were also evaluated for structural damages by UV light by scanning electron microscopy, where the samples were mounted on an aluminum stub using double-sided tape coated with carbon, before being coated with gold/palladium using a SC7640 auto/manual high resolution sputter coater (Quorum Technologies, Ashford, UK). An EVO-50-EP environmental scanning electron microscope (Zeiss, Cambridge, UK) was used to study the samples at a magnification of 80003.

| Preparation of chicken samples for sensory analyses
Refrigerated fresh skinless chicken breast fillets obtained from a local producer were mixed to achieve an equal number of cfu per cm 2 on the surface. One set of chicken samples were exposed to UV light in air (unpackaged chicken), and were thereafter packaged in modified atmosphere, while a parallel set of chicken samples were exposed to UV light under modified atmosphere (MAP-chicken), as described above. None of these chicken samples were inoculated with bacterial culture, and both sample sets were then stored at 48C for 6 days before being used for the sensory analyses described below. The color stability of the chicken fillets were evaluated by visual inspection of the chicken before and after UV light exposure, and after storage. control, chicken exposed to UV-C at fluence 0.1 J/cm 2 (10 s at 10 mW/cm 2 ), chicken exposed to UV-C at fluence 0.6 J/cm 2 (60 s at 10 mW/cm 2 ), chicken exposed to pulsed UV light at low intensity at fluence 1.25 J/cm 2 and chicken exposed to pulsed UV light three times at high intensity giving a fluence of 10.8 J/cm 2 . Based on a pretrial performed by the panelists, a consensus list of attributes for the profiling was developed: Smell of raw chicken (sour odor, sunburnt odor, burnt odor, metallic odor, sulfur odor, off-odor, cloying odor, and rancid odor) and odor/taste/flavor of cooked chicken (sunburnt odor, burnt odor, sour flavor, burned flavor, metallic flavor, off-flavor, cloying flavor, and rancid flavor). Both raw and cooked chicken fillet samples were evaluated. For the raw samples, the panelists were given 1/6 raw chicken fillet served at room temperature on white plastic cups coded by random three-digit numbers. The cooked samples were heated (1008C, 100% steam, 30 min) in an Electrolux Air-o-steam oven (Combi LW 6 GN 1/1 Gas) to a core temperature of 788C 6 38C. After heating, the samples rested for 5 min before each panelist were served one-fourth cooked chicken fillet in a white porcelain bowl with lid marked with a random three-digit number, that had been preheated at 658C. Samples were kept at 658C for the evaluation. The panelists had unsalted crackers and lukewarm water for rinsing the palate between samples.

| Sensory evaluations
The coded samples were evaluated in duplicate and served randomized according to sample, panelist, and replicate. Each panelist recorded their results at individual speed using an unstructured line scale with labeled endpoints ranging from no intensity (1), to high intensity (9), using the EyeQuestion Software (Logic8 BV, Elst, The Netherlands) for direct recording of data.
Changes in the quality or sensory properties of raw chicken as a result of UV light exposure were also assessed by a smaller consumer test. Twenty randomly chosen test persons were asked if they would want to use the chicken samples for dinner. In addition, they assessed the quality of the chicken on a scale ranging from very bad (1), to very good (9).

| Dynamic headspace gas chromatography mass spectrometry
The same set of raw chicken samples used in the pretrail sensory evaluation was subjected to dynamic headspace gas chromatography mass spectrometry (GC/MS) analysis. Based on variation found both in the sensory results and the GC/MS results of the pretrial, chicken samples that showed the greatest variation were further selected for analysis of volatile organic compounds. These included: untreated control, chicken exposed to UV-C light at fluence 0.60 J/cm 2 (60 s at 10 mW/cm 2 ) and pulsed UV light three times at high intensity giving a fluence of 10.8 J/ cm 2 treated in air, and pulsed UV light at low intensity at fluence 1.25 J/cm 2 treated under modified atmosphere. A gas chromatography analysis was carried out on chicken samples as previously described (Olsen, Vogt, Veberg, Ekeberg, & Nilsson, 2005 blanks and standard samples before, during and after the sample series, and the selected major compounds (80-100%) on a peak area basis were included in the data analysis.

| Statistical analysis
Bacterial reductions log cfu/cm 2 between control and UV light treated samples were calculated. Analysis of variance (ANOVA) and Tukey's multiple comparison test were used to determine statistically significant effects on the reduction by the treatments (R 3.3.2; R Core Team [2016]) using a significance level of .05. For sensory evaluation, the same analyses were performed on the descriptive sensory data from the trained panel to identify sensory attributes that discriminated between samples.

| Weibull models
For each species, a two-parameter Weibull distribution was fitted to the observed log reductions to produce predictive models of the effects of UV exposure. The chosen Weibull model is defined as: where N 0 and N denote the number of bacteria per square cm before and after UV exposure, respectively, f is the UV dose (fluence), a is the scale parameter (describes how sharply the curve drops in the beginning), and b is the shape parameter (describes the shape of the curve).
Common models were produced based on log reduction data for all the bacterial species.  Table S1.
UV-C light exposure with fluences from 0.05 to 3.0 J/cm 2 (10 mW/cm 2 , from 5 to 300 s) in air, gave the largest reduction of 2.8 log for C. divergens after the highest fluence treatment, while only 1.7 log reduction was obtained for EHEC. The lowest fluence level gave up to 2.2 log reduction for S. aureus, and EHEC was reduced the least with 1.1 log. By comparing UV-C light results using ANOVA within each species, some of the shorter treatments were considered statistically different from the treatments of longer duration for S. Enteritidis   1 Flowchart illustrating the experimental set-up. Reduction of bacteria on skinless chicken fillets using UV light treatments (a), and sensory analyses of chicken fillets treated with UV light (b). Chicken fillets inoculated with pathogens and bacteria often found as natural contaminants on fresh chicken meat were exposed to different UV light treatments in air, representing unpackaged chicken, and for two selected species on modified atmosphere packaged (MAP)-chicken. The bacterial species are listed in Table 1. Sensory analyses of chicken fillets with no added bacteria were conducted after UV light treatments of both unpackaged chicken and MAP-chicken 0.12 6 0.03% at packaging to 0.69 6 0.02% after 21 days, and were similar for the different UV light treatments and the untreated control.
Scanning electron microscopy analysis showed no structural damages to the UV treated films (not shown). The ability of the film to transmit UV light was measured as 80.5% at 254 nm, which was compensated for by increasing the UV doses accordingly in the illumination experiments.

| Weibull models describing bacterial reduction
Weibull models created to predict the log reduction patterns for the different bacterial species are shown in Figure 4 and parameters for the models are listed in Table 2. RMSE values indicating the goodness of fit, were the lowest for S. aureus exposed to UV-C light (0.20) and the highest for Pseudomonas spp. exposed to pulsed UV light (0.55). Determination coefficient (R 2 ) values ranged from 0.41 to 0.80 for UV-C light and from 0.47 to 0.89 for pulsed UV light.
Since R 2 indicates the proportion of variation in log reduction explained by the fitted Weibull model, a value approaching 1 would signify perfect predictability. Since all of the ß (shape parameter) values were less than 1, the Weibull fits of the reduction data were concave upward. The highest ß values were obtained for EHEC and S. Enteritidis (0.32 and 0.31, respectively) for pulsed UV light. The a (scale parameter) values were very small, implying concentrated distribution, as seen by how sharp the curve drops in the beginning.
There was a noticeable difference between the two UV methods, where higher a values were obtained for UV-C light than for pulsed UV light, with C. divergens as an exception. Common models based on log reduction values for all the species gave a good fit for the majority of the species, but for L. monocytogenes exposed to both UV-C and pulsed UV light, reduction was overestimated. The same was seen for EHEC exposed to UV-C light and C. divergens exposed to pulsed UV light. Denaturation of proteins in chicken has been considered to be initiated at temperatures higher than 568C (Murphy, Marks, & Marcy, 1998). Only minor elevation of the temperature was observed, 2.5-4.08C and 4.0-6.58C for UV-C light treatments at fluences 0.6 J/cm 2 and 3.0 J/cm 2 , respectively, and 0.5-2.58C and 2.5-3.58C for pulsed UV light treatments at fluences 10.8 and 18.0 J/cm 2 , respectively. The rise in surface temperature was only temporary since the surface was rapidly cooled by the low temperature of the interior of the chicken fillet.

A A A A A A AA A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A B B B B B B B B C C C C C C C C A A A A A A A A A A
FIG URE 5 Sensory analysis of (a) raw chicken fillet samples and (b) cooked chicken fillet samples. Chicken samples were exposed to continuous UV-C light at 10 mW/cm 2 for 10 s (UVC-10) and 60 s (UVC-60), giving fluences of 0.1 J/cm 2 and 0.60 J/cm 2 , respectively, and pulsed UV light to a low pulse with fluence of 1.25 J/cm 2 (PUV-L) and three times to a high pulse giving a fluence of 10.8 J/cm 2 (PUV-Hx3), both in air (O 2 ) and anaerobic (CO 2 : N 2 ) atmospheres, representing unpackaged chicken and MAP-chicken, respectively. The intensities of different odors of raw samples and odor/taste/flavor of cooked samples were registered, 1 5 no intensity and 9 5 high intensity. The letters above the columns indicate grouping according to ANOVA and Tukey multiple comparison test. Samples with the same letter are considered being equal for the specific property MCLEOD ET AL. | 9 of 15 4 | DI SCUS SION

| Effect of UV treatment on inoculated bacteria
There are large differences between the conventional continuous UV-C light and pulsed UV light with respect to wavelengths, intensities, and exposure times. In this work, we have compared the efficacy of continuous UV-C light and pulsed UV light in reducing bacteria on chicken fillet. We used multi strain mixtures of the same species and bacterial cells that were in the same state during the different treatments. In earlier studies, single strains were often used which may not show reductions representative for the species. Differences in reduction within species have been reported, and state of the cells can influence the sensitivity to UV light (Farrell et al., 2010;Haughton et al., 2011b). To avoid possible changes in sensory perception, it is desirable to maximize bacterial reduction without treating the surface of a product more than necessary. Treatment levels employed for both UV methods were practical and relevant within industrial production, from weak exposures resulting in limited bacterial reduction, up to levels exceeding the maximum permitted dose by the FDA for pulsed UV light (FDA, 2010). The fluences are not directly comparable between the two methods, since the different wavelengths in the UV spectrum have different germicidal effectiveness (Bintsis, Litopoulou-Tzanetaki, & Robinson, 2000). For UV-C exposure at 0.05 J/cm 2 , the germicidal effect was comparable to a fluence of 1.25 J/cm 2 for the pulsed UV light. UV-C light showed a higher germicidal effect when the same fluence was employed for the two methods, which can be explained by most of the energy being emitted at 254 nm, where the germicidal effect is close to the maximum (Bintsis et al., 2000).
In the range tested, a limited dose-response effect was observed, likely caused by shading effects of the irregular surface structure of the chicken fillet. The increase in reduction with increasing dose was though more apparent for the pulsed UV light. Any substance between the light source and the bacterium that absorbs light will impair the decontamination process (Gomez-Lopez et al., 2007). Even when a surface appears smooth to the naked eye, it may harbor crevices and cracks where bacteria are shielded against direct exposure, and bacteria may also be covered by protein or other organic matrices. Moreover, the average size of a bacterium is approximately 1 mm 3 2 mm, and although its spreading was carried out carefully, it is practically impossible to avoid some overlapping. A shielding effect for colonies of L. monocytogenes growing on petri dishes where the upper cells of a colony appeared to protect the lower cells has previously been described (Gomez-Lopez et al., 2005). At high fluence rates, the light should be able to penetrate deeper, but still, the efficiency of using UV light for decontamination of foods is lower than when tested on smooth surfaces. Reductions of 5-7 log achieved on agar in petri dishes was in accordance with previous reports (Farrell et al., 2010;Gomez-Lopez et al., 2005;Paskeviciute et al., 2011;Rowan et al., 1999), and the observed higher resistance of L. monocytogenes to pulsed UV light, reduced only 4 log after treatment at low fluence of 1.25 J/cm 2 , has also been reported previously (Gomez-Lopez et al., 2005;Lasagabaster & de Maranon, 2012). In general, the reductions of inoculated bacteria on chicken fillet surface observed in this study correlated well with previous findings, both for UV-C (Chun et al., 2010;Haughton et al., 2011a;Isohanni & Lyhs, 2009;Sommers et al., 2016) and for pulsed UV light (Keklik et al., 2010;Paskeviciute et al., 2011), including for C. divergens, Pseudomonas spp., and B. thermospacta, for which previous reports on UV light inactivation on food surfaces does not exist or are scarce. EHEC seemed to resist the UV-C light treatments better than ESBL-producing E. coli, and better than the other species tested as well.
The Weibull distribution is suitable for the analysis of bacterial reduction (Chen, 2007;Keklik, Demirci, Puri, & Heinemann, 2012;Martin et al., 2007;Ugarte-Romero, Feng, Martin, Cadwallader, & Robinson, 2006;van Boekel, 2002), and was previously demonstrated to be more successful than models such as the log-linear model and firstorder kinetic model (Chen, 2007;Martin et al., 2007). The model seemed to be a useful tool to describe the reduction patterns and give clues to how pathogens and spoilage bacteria on chicken fillet surfaces are likely to respond to UV light treatments.
The Weibull fits of the reduction data were concave upward, indicating that exposed cells were destroyed and that the more resistant cells or those shaded from exposure were left undamaged. Volatile organic compounds from chicken which showed an increase in concentration (pg/g) as a result of exposure to UV light. The samples included were chicken exposed to pulsed UV light at low intensity at fluence 1.25 J/cm 2 (PUV-L) treated under anaerobic (CO 2 :N 2 ) atmosphere (MAP-chicken), an untreated control (Untreated), chicken exposed to UV-C light at 10 mW/cm 2 for 60 s (UVC-60) giving a fluence of 0.60 J/cm 2 and pulsed UV light three times at high intensity (PUV-Hx3) giving a fluence of 10.8 J/cm 2 treated in air (O 2 ). The precision of replicate measurements were within 15% Salmonella Typhimurium treated with pulsed UV light were shown to give about 2 log reduction, but with double the exposure time (30 s) in comparison with unpackaged samples (15 s) (Keklik et al., 2010). The additional bacterial reduction obtained on ready packaged chicken fillet product would increase shelf life and safety. Treatment after packaging should be simple to implement at industrial packaging lines without reductions in production efficiency.

| Sensory quality of the chicken fillets
Meat exposed to UV light can develop off-flavors caused by the absorption of ozone and oxides of nitrogen, or because of photochemical effects on the lipid fractions of the meat (Bintsis et al., 2000). Lipid oxidative rancidity is regarded as the most important nonmicrobial factor responsible for meat deterioration, resulting in adverse changes in appearance, texture, odor, and flavor (Frankel, 1998). An increase in fatty aldehydes due to lipid oxidation during irradiation of poultry meat has been documented (Du, Ahn, Nam, & Sell, 2000, 2001Du, Hur, Nam, Ismail, & Ahn, 2001;Kim, Nam, & Ahn, 2002). The major fatty aldehyde hexanal is a typical volatile secondary lipid oxidation product (Beltran, Pla, Yuste, & Mor-Mur, 2003;Jayasena, Ahn, Nam, & Jo, 2013;Shi & Ho, 1994). Although we observed an increase in the concentration of hexanal, particularly for unpackaged chicken exposed to UV light, no significant effect was found on the corresponding rancidrelated sensory attributes in the professional sensory evaluation. This suggests that lipid oxidation does not have a negative impact on the perceived odor and flavor of the chicken meat at the applied UV doses.
The higher intensity of the sunburnt odor for chicken exposed to the most intense dose of pulsed UV light, does, however, seem to pose restrictions on the upper limit of treatment of unpackaged chicken. The sensory intensity value was though only 3.4, which is considered relatively low, and for lower doses relevant in industrial application, the odor should not be a problem. Detected changes in concentrations of volatile compounds correlated well with the sensory observations.
Increased levels were seen in unpackaged chicken after UV light exposure. Hydrocarbons may be generated during irradiation of poultry meat (Du, Ahn, et al., 2000, 2001Du, Hur, et al., 2001;Kim et al., 2002), where increased concentrations of propanol and butanol have been documented (Du et al., 2000Du, Hur, et al., 2001). In accordance, we detected increased levels of pentane, heptane and 1pentanol. Sulfur compounds with low odor thresholds are important to odor associated with irradiation (Angelini, Merritt, Mendelsohn, & King, 1975;Batzer & Doty, 1955;Patterson & Stevenson, 1995). Dimethyltrisulfide, although only detected in small amounts in unpackaged chicken after UV light exposure, was reported by Patterson and Stevenson (Patterson & Stevenson, 1995) to be the most potent off-odor compound in irradiated raw chicken. Other compounds that showed an increase and which character could be associated with sunburnt/irradiated odor and flavor, were 2-pentanone (roasted sweet), and 1pentanol (roasted meat) (Brewer, 2009 The color of raw or cooked poultry meat is by origin pale with a low content of the muscle pigment myoglobin. Furthermore, the color of raw meat is dependent on the oxidation state of myoglobin (Mugler & Cunningham, 1972; United States Department of Agriculture, 2013). Chicken breasts exposed to high doses of UV light was previously reported to turn darker, show more redness and a slight increasing amount of yellow coloration (Park & Ha, 2015). The color of the chicken fillets was not affected by the treatments at the doses used in our experiments, as in agreement with other reports (Chun et al., 2010;Haughton et al., 2011a). Together these results indicate that sensory and quality changes are small or negligible both after UV-C and pulsed UV light treatments.

| Advantages and disadvantages of continuous UV-C and pulsed UV treatments
Both UV-C and pulsed UV light treatments provide effective tools for reduction of microorganisms. They are rapid and efficient nonchemical, nonionizing, and nonthermal surface decontamination treatments and can be used in continuous processing. The methods have been shown as effective technologies for decontamination of stainless steel conveyors and surfaces in the production environment (Haughton et al., 2011b;Sommers, Sites, & Musgrove, 2010). They can be used on foods and synergistically with other treatments (Mukhopadhyay & Ramaswamy, 2012). The methods require little energy use, are easy to implement and require no increase in work load. UV light is safe to apply, but some precautions have to be taken to avoid exposure of workers to light and to evacuate any ozone generated by the shorter UV wavelengths (Gomez-Lopez et al., 2007). The effect of both UV-C and pulsed UV light is impaired in opaque matter, where bacteria are shielded from direct exposure such as by food surface topography, organic matter, or by other bacteria. The UV light treatments of this study did not alter the properties of the EVOH film used, as was also the case with polyethylene, polypropylene and polyvinyldichloride films (Tarek, Rasco, & Sablani, 2015). The top film used transmitted approximately 80% of the UV light, while in previous studies, films with polypropylene and polyethylene barrier layers transmitted 75% (Keklik, Demirci, & Puri, 2009) and 72% (Keklik et al., 2010), respectively, of pulsed UV light at 1.27 J/cm 2 . By using a packaging film with a high UV transmission, the chicken fillets could be packaged before the UV light treatment, thereby avoiding the risk of MCLEOD ET AL.
| 11 of 15 recontamination. Both methods would be beneficial for large scale industrial UV decontamination operations. UV-C light treatment is a low cost strategy with low maintenance (Keklik, Krishnamurthy, & Demirci, 2012). The treatment time is somewhat longer in comparison with pulsed UV light treatment, and therefore the equipment may require more space if installed over for example a conveyor belt.
Pulsed UV light provides rapid decontamination, but involves equipment that is more elaborate. The xenon flash lamps used for pulsed UV light are also more environment friendly than the mercury-vapor lamps typically used in UV-C light treatment (Gomez-Lopez et al., 2007).

| CON CLU S I ON
Despite good hygiene practices during production of fresh meat, contamination of carcasses with pathogens and spoilage bacteria cannot be completely prevented. There is pressure on the food industry for nutritious, fresh and healthy food products, to maximize the shelf life of the products, and for reducing costs and waste. Antimicrobial interventions that effectively reduce the bacterial load are feasible in slaughter and product processing. They should be safe, economic, and easy to handle. Also, interventions should not change the organoleptic quality of the food and should be widely accepted by consumers. The exposure of raw chicken fillet surface to various doses of UV-C or pulsed UV light proposed in this work represents useful alternatives for reducing the viability of pathogenic and spoilage bacteria on this product.