For the presentation of the results and their discussion, the following must be taken into account: For the UV-C-treated samples, the untreated samples served as controls, while for the PAA treatment, the water-treated samples were used as controls (to record the rinsing effect).
For the presentation of the effect of the combination of UV-C and PAA, the single-treated samples as well as the UV-C/water-combination were used as controls.
3.2. Effects of PAA and UV-C Treatments on the Microbiological Survival on Pork During Storage
The antimicrobial effects of treatments with PAA, UV-C, and their combination on pork inoculated with
Y. enterocolitica or
B. thermosphacta are presented in
Table 2. Considering the single treatments, on storage days 1 and 7 UV-C treatment resulted in a significant reduction of both bacterial species in comparison to the untreated (control) samples, whereas PAA treatment significantly reduced only
B. thermosphacta on days 1 and 7 of storage compared to the H
2O treated samples. After combined UV-C/PAA treatment compared to the UV-C/water treatment (where the PAA treatment has been replaced by a water treatment), a significant reduction of bacteria was only found for
Y. enterocolitica inoculated pork on storage day 14.
The effect of UV-C irradiation on
B. thermosphacta and
Y. enterocolitica indicates that UV-C treatment can effectively reduce bacterial counts. These results mainly agree with Reichel et al. [
11], who presented a significant reduction of both
Y. enterocolitica and
B. thermosphacta on pork on storage days 1, 7, 14, irradiated with UV-C doses of 408 mJ/cm
2 and 2040 mJ/cm
2. Isohanni et al. [
31] who treated broiler meat and skin with UV-C doses of 9.4, 18.8, and 32.9 mJ/cm
2 presented significant reductions of
Campylobacter (
C.) jejuni. Moreover, Haughton et al. [
30] found significantly lower
C. jejuni,
S. Enteritidis and
E. coli numbers on chicken meat after treatments with UV-C doses up to 192 mJ/cm
2. Reichel et al. [
11] presented in in vitro studies higher reductions of up to 4.0 log
10 cfu/mL of
Y. enterocolitica and
B. thermosphacta at lower UV-C doses of up to 30 mJ/cm
2. However, there are several reasons for the low reduction of the bacteria on pork and the high variation of the results compared to the in vitro studies. One is that UV light only reacts on surfaces not penetrating the matrix [
43] and bacteria might shield each other from the UV-C rays [
9,
44,
45]. Another reason might be the rough surface of the meat with pores and caverns, which protect the bacteria from the UV-C light and complicate the recovery of the bacteria. Furthermore, the meat proteins may lead to absorption of the UV-C rays [
11,
45,
46]. Low reduction results might also be caused by reparation of the UV-C generated DNA damages either in the light, catalyzed by the photolyase, or in the dark by several enzymes that repair DNA damages by excision [
47,
48]. To clarify the latter assumption, we analyzed how
B. thermosphacta and
Y. enterocolitica inoculated on pork, treated with doses of 408 and 2040 mJ/cm
2, and stored for 60 min in the dark or light, grew in comparison to samples analyzed directly after UV-C treatment. Growth would indicate repair of the DNA damages after UV-C treatment. However, since UV-C treated samples had significantly lower bacterial levels directly after treatment and after 60 min of light or dark storage compared to the untreated control samples, it could be suggested that photoreactivation does not effectively influence the bacterial counts (
Table 3).
Reichel et al. [
49] also found no photoreactivation effects of
B. thermosphacta and
Y. enterocolitica, which were inoculated on ham, treated with UV-C doses of 408 and 4080 mJ/cm
2, and stored in the dark and light for 60 min. Reichel et al. [
11] presented through in vitro studies that after 60 min of light storage, higher bacterial counts were determined than directly after treatment of both bacterial species with increasing UV-C doses up to 30 mJ/cm
2. These data indicate that although no photoreactivation of the bacteria was seen on pork, the storage period of 60 min was sufficient to evaluate this repair mechanism. It can be suggested that high UV-C doses, necessary for a significant treatment effect on meat, generate bigger DNA damages, resulting in reduction/overburden of the different DNA repair systems—during light repair, the photolyase enzyme system, and during dark repair, several enzymes that repair DNA damages by excision of dimers [
47,
48].
The presented effects of PAA on
B. thermosphacta on days 1 and 7 of storage indicate that PAA spray treatment can extend the shelf life of pork. This result agrees basically with the study of Bertram et al. [
27,
50], who applied 1200 ppm PAA by spraying turkey, chicken breast, and drumsticks with skin. They showed a significant decrease in
C. jejuni counts and TVC on days 6 and 12 of storage. Smith et al. [
51] found significant reductions of
C. jejuni on broiler carcasses after treatments with 100 and 200 ppm PAA by immersion or spraying. Nagel et al. [
29] presented significantly lower
S. Typhimurium and
C. jejuni counts on inoculated poultry carcasses after treatments with 400 ppm or 1000 ppm PAA in a post-chill immersion tank, whereas Ellebracht et al. [
28] or Penney et al. [
52] treated beef with 200 ppm or 180 ppm PAA, respectively, resulting in significant reductions of
E. coli and
S. Typhimurium counts (only Ellebracht et al. [
28]). In similar experiments, Cap et al. [
53] presented significant reductions of Shiga toxin-producing
E. coli. The different study conditions, the varying susceptibilities of the used bacteria, and particularly the meat matrix may have caused the different effects also within the present study. The assumption that differences in the susceptibility of bacteria play a role is also supported by Aarnisalo et al. [
54], Poimenidou et al. [
55], or Skowron et al. [
56]. They analyzed the minimal inhibitory concentrations (MICs) of 6, 12 or 6
Listeria (
L.)
monocytogenes strains, respectively, against PAA or PAA containing disinfectants, and detected differences in the MIC values. Similar to this, Bertram et al. [
27] analyzed the MICs of 25
C. jejuni and
C. coli isolates, and also found some variation in the values. Considering the effect of the meat matrix, Bertram et al. [
27] clearly showed that growth of
C. jejuni is inhibited at concentrations of 2 to 8 ppm PAA in in vitro (MIC) experiments, while 1200 ppm PAA is necessary to reduce the bacterial species on turkey and broiler skin by approximately 1.0 log
10 cfu/g skin.
The UV-C/PAA results indicate that after a significant reduction of bacteria due to UV-C application or PAA spraying, further treatment with another preservation method did not improve the treatment effects. However, studies on a combination of these two preservation methods on pork initially inoculated with
B. thermosphacta or
Y. enterocolitica have not been published yet. There are in vitro studies that have shown a reduction of the amount of
L. monocytogenes using disinfectants containing chemicals such as hydrogen peroxide or phosphonic acid in addition to PAA. However, these studies were not carried out on meat products [
54,
56]. Other studies found reductions in
Salmonella or
E. coli counts after treatment of chicken carcasses with chemicals such as lauryl ethyl arginate (LAE), acidic calcium sulfate, polylysine, or vinegar solution in combinations [
57,
58]. Sukumaran et al. [
59] treated chicken skins with PAA and bacteriophages and found beneficial effects on
Salmonella levels. The absence of significant reductions was probably due to the meat matrix, which partly also influenced the effectiveness of individual treatments with UV-C rays or PAA.
To avoid that possible treatment-related reductions in bacterial counts reach the detection limit of 2.0 log
10 cfu/cm
2 and thus could not be detected, the samples were inoculated with high bacterial counts at the beginning of the study. As contamination of meat is often lower, especially in slaughterhouses or meat processing plants that comply with hygiene standards, lower bacterial counts of 1–6 × 10
6 cfu/mL were also inoculated in further experiments. Using the lower inoculum concentration, both bacterial species showed a significant reduction in the bacterial counts after UV-C treatment compared to their control. In contrast, the PAA treatment did not result in a significant reduction of both species when using the lower inoculum concentration. After combined UV-C/PAA treatments, the bacterial counts were similar to those after UV-C/water treatment, but compared to the PAA treatment alone the
Brochothrix and
Yersinia counts were significantly reduced when using a lower inoculum concentration (
Table 4).
The water treatment caused significant reductions of
B. thermosphacta up to 0.87 log
10 cfu/cm
2 (day 1) and of
Y. enterocolitica up to 0.98 log
10 cfu/cm
2 (day 1) compared to the untreated samples. This effect was also shown by Bertram et al. [
27], who treated chicken drumsticks with a water spray with the same distance and water volume and achieved reductions for
C. jejuni of 0.77 log
10 cfu/g (day 1), 0.64 log
10 cfu/g (day 6), and 0.57 log
10 cfu/g (day 12). Nagel et al. [
29], who treated chicken carcasses 20 s in a water dip (in 1.5 L) achieved a more than 0.5 log cfu/sample reduction of
S. Typhimurium und
C. jejuni. In the present study, we considered the water treatment as control in relation to the PAA or PAA/UV treatment assuming that washing down effects, as presented also by the other studies, might be detectable due to insufficient attachment of the bacteria to the meat surface.
The results after UV-C treatments were similar at both low and high inoculum concentrations. However, the missing effect of PAA on B. thermosphacta and the significantly lower results after combined UV-C/PAA treatments compared to the PAA results are in contrast to the findings with the higher inoculum concentrations. The data with low inoculum concentrations rather show that UV-C irradiation is more effective than PAA.
That the initial reduction in bacterial counts did not increase again during further storage is probably caused by the low temperature and the inhibitory effect of the oxygen within the modified atmosphere as shown by Zhang et al. [
60], and was also demonstrated for the control samples within the present study.
3.3. Effects of PAA, UV-C Treatment, and the Combination UV-C/PAA on the Color, pH, Mb Redox form Percentages and AC Results of Inoculated Meat
The meat color is an important factor influencing the purchase behavior of the consumers, especially, if the meat is packed. This is because other sensory parameters like smell, taste, or texture cannot be assessed at retail level. A bright red color is an indicator for freshness and is influenced by the Mb content and the percentages of the Mb redox forms [
61]. With regard to the effects of the UV-C and PAA treatments alone and in combination on the color results, no significant effects could be found (
Table 5 and
Table 6).
The present results agree with those presented by Reichel et al. [
11], who also found no significant effects of UV-C treatment on the color of pork with doses of 408 mJ/cm
2 and 2040 mJ/cm
2 and subsequent MAP storage for 14 days. This is in accordance with Lyon et al. [
62], who found no significant color changes of chicken breasts after UV-C irradiation with a dose of 300 mJ/cm
2 and after a storage period of 7 days. In contrast to the present study, Stermer et al. [
15] presented significantly higher a* values of beef after UV-C irradiation with doses of 500 mJ/cm
2. Wallner-Pendleton et al. [
63] found significantly lower L* values after UV-C irradiation of broiler legs with a dose of 82.56 mJ/cm
2 at day 0 and 10 of storage. However, color values of the chicken breast were not influenced by the treatments. Park et al. [
16] also found significantly lower L* values of chicken breast meat irradiated with UV-C doses of 60–3600 mJ/cm
2, and significantly higher a* values of chicken breast meat irradiated with UV-C doses of 1800–3600 mJ/cm
2.
With regard to the PAA results, Bertram et al. [
27,
50] also found no effects of PAA on the color of turkey and broiler breast meat sprayed with 1200 ppm PAA solution. In contrast to this, ground beef patties showed significantly higher L* values if treated with 200 ppm PAA compared to the untreated samples (Quilo et al. [
25]). In a similar study, Quilo et al. [
26] found on days 0, 1, 2, 3, and 7 significantly higher a * results of the ground beef after treatments with 200 ppm PAA.
The missing effects of the combination of UV-C and PAA are comprehensible since no effects of color by using the single applications were achieved in the presented study either. Unfortunately, no comparable studies have been published. However, despite the partly contradictory results of the UV-C and PAA treatments on the color results in other studies, the present study indicates that both preservations methods alone or in combination could be used without negative impact on the color of meat.
With regard to the Mb redox form percentages, no significant effects of the UV-C and PAA alone and in combination was obtained (
Table 7).
Reichel et al. [
11] also found no impact of the UV-C treatment on the Mb redox form percentages of pork, whereas Bertram et al. [
27,
50] found no significant effect of the PAA treatment on the Mb parameters of broiler and turkey breast muscle. No other studies have been published which analyzed the Mb redox forms after combined UV-C and PAA treatments. However, the Mb results seem to be comprehensible if one considers the almost similar color results. During storage, OxyMb is oxidized to MetMb, which causes a color change from red to brown and may negatively affect the consumers’ behavior [
64].
No significant effects of UV-C and PAA alone or in combination on the pH values as well as AC results could be found in the present study (
Table 8).
The pH results agree with those of Park et al. [
16] or Reichel et al. [
11], who also found no effects of UV-C treatment on this parameter. No influence of a PAA treatment on the pH results was also seen by Bertram et al. [
27,
50] or Quilo et al. [
25]. In addition, Reichel et al. [
11] also found no significant effects on the AC after UV-C irradiation with 408 and 2040 mJ/cm
2 compared to the untreated pork samples for all days of storage (1, 7, 14). In contrast, Bertram et al. [
50] presented similar AC values of broiler meat, when treated with 1200 ppm PAA or water, the solvent of the PAA. The AC value determination quantifies the concentration of antioxidant substances like tocopherol, ascorbic acid, or glutathione within the tissue. As these substances might reduce the oxidation of molecules such as proteins and lipids and thus prevent oxidative stress within the cells [
50,
65], a reduction in AC results indicates an oxidative influence on the tissue. This can result in negative effects on the physicochemical properties of the meat such as color, Mb redox forms, or lipid peroxidation: The latter might also affect human health [
66]. However, as in the present study, neither UV-C nor PAA changed the AC values, an oxidative effect of these preservation methods could be excluded, even considering the color or Mb results presented. Therefore, it is comprehensible that the combined treatment with UV-C and PAA also had no impact on the AC as well as color or Mb results.