3.1. Antimicrobial effects of PVR edible coating on chicken breast fillets
Figure 6 shows the results of TBC counts in the coatings (C1, C2 and C2
EO) containing EO from 0.5 to 2.0%. The TBC of CBF without coating (C1) was about 4.9 log cfu/g on 1
st day and increased to 8.9 cfu/g after 12 days of storage, similar to the previous studies [
2,
33,
34,
35,
36,
37,
38,
39,
40]. The application of ECEO on the chicken with various EO concentrations (C2) significantly (p< 0.05) suppressed the growth of TBC during storage periods. During 12 days of storage, the TBC was increased in C1 by 4 logs and in C2 by 2.7 logs cfu/g. Increasing the amount of EO in the ECEO solution increased the antimicrobial effect on the TBC. However, only treatment of ECEO (2%) in the coating significantly (p<0.05) inhibited TBC at the end of 12 days storage. The coating affected the TBC growth by limiting the oxygen and water vapor transfer.
Figure 6.
Growth of TBC on the control C1 and C2 with different EO concentrations during storage of 12 days at 8°C.
Figure 6.
Growth of TBC on the control C1 and C2 with different EO concentrations during storage of 12 days at 8°C.
In our present study, ECEO coatings showed a bacteriostatic effect on the TBC. Generally, edible coatings made from wheat gluten greatly limit the water vapor transfer through the coating to the food surface [
41]. The hydrophobic coating compositions like wheat gluten-based coatings showed antimicrobial effects against common spoilage microorganisms found in poultry products. Similar to gluten, chitosan and carboxymethyl cellulose also make hydrophobic interactions. Studies showed that they had succeeded against lipid oxidation and other enzymatic spoilage but failed against microbial spoilage when used alone in the EC composition. Various EOs were used to increase the antimicrobial effectiveness of these coatings and prolonged the shelf life of poultry by inhibiting the growth of common spoilage microorganisms; psychrotrophic bacteria, lactic acid bacteria,
Pseudomonas, Campylobacter and Enterobacteriaceae [
20,
38,
42]. Most studies related to the antimicrobial effects of EOs emphasize that EOs may change cell wall permeability and intracellular alterations leading to cell death [
38,
43,
44,
45,
46]. Alma et al. [
27] observed the chemical composition and antimicrobial activity of EO of PVR against 13 bacteria and 3 yeast species. At MIC tests, EO obtained from PVR inhibited 9 of 13 bacteria and all yeasts. Also, Ghalem and Mohamed [
24] investigated the antimicrobial activity of EO of PVR against
E. coli,
Proteus and
S. aureus. EO inhibited the growth of all bacteria.
Besides the EO of PVR and wheat gluten, other edible coatings containing different EOs had antimicrobial effects on chicken breast fillets. Garavito et al. used guar gum, nisin, and oregano oil as edible coating ingredients for application on chicken breast fillets. Similar to our present study with EO of PVR, nisin and oregano oil containing samples showed a bacteriostatic effect on TBC during 16-day storage at 4°C [
3]. Bazargani-Gilani et al. [
34] investigated the antimicrobial effect of pomegranate juice and chitosan coating, which is hydrophobic like wheat gluten used in our study, enriched with
Zataria multiflora essential oil (ZEO) on chicken meat stored at 4°C. Samples containing ZEO and chitosan significantly lowered the numbers of TVC on each sampling day during 20 days of storage. Fernández-Pan et al. [
2] researched the antimicrobial efficacy of whey protein isolate (WPI) coating with oregano and clove EO on chicken breast fillets stored at 4°C. The WPI coating containing 20 g/kg oregano EO was the most effective, having 2 log reductions against aerobic mesophilic bacteria and 1 log reduction against psychrotrophic bacteria and
Enterobacteriaceae. Generally, the results of TBC agreed with studies including EO blended edible coatings conducted with chicken breast fillets [
21,
38,
40,
42]. Meat and meat products should contain no more than 7 log CFU/g of TBC; according to the Food and Agricultural Organization (FAO), the acceptable limit for TBC in poultry is below <6 log cfu/g [
2,
42,
44].
L. monocytogenes counts were performed in coated and uncoated CBF contaminated by
L. monocytogenes (
Figure 7). The EO-added coating had a higher antimicrobial effect against
L. monocytogenes than the uncoated
L. monocytogenes (UCLM) (p<0.05). The antimicrobial activity increased with increasing EO concentration, while
L. monocytogenes in coated
L. monocytogenes (CLM) reached 6.2 log cfu/g,
L. monocytogenes increased to only 3.9 log cfu/g in CLM
(2) at the end of 12 days of storage (p<0.05). The number of
L. monocytogenes in UCLM samples increased by 3.1 logs; however, that in CLM samples increased by 2.4 during the storage. Antimicrobial activity of coating increased significantly with EO addition with even CLM
(0.5) coating lowered the
L. monocytogenes count (p<0.05). The highest antimicrobial effect was seen in CLM
(2), inhibiting the growth of
L. monocytogenes. The antimicrobial activities of CLM
(1), CLM
(1.5), and CLM
(2) against
L. monocytogenes were not significantly (p>0.05) different from each other during whole storage. Previous study showed that the minimum inhibitory concentration (MIC) of EO of PVR against
L. monocytogenes was 0.25% [
47]. So all of the coated samples having EO concentrations above the MIC of 0.25% should be effective against
L. monocytogenes growth. By looking at similar growth trends of TBC (
Figure 6) and
L. monocytogenes (
Figure 7) count, one can say that
L. monocytogenes adapted well to highly competitive flora during storage. But including EO in the coating composition was significantly limiting the growth. Similar research results showed that
L. monocytogenes was susceptible to EO-added coatings.
Figure 7.
Growth of L. monocytogenes on the UCLM and CLM with different EO concentrations during storage of 12 days at 8°C.
Figure 7.
Growth of L. monocytogenes on the UCLM and CLM with different EO concentrations during storage of 12 days at 8°C.
Abbasi et al. [
10] observed the antibacterial effects of fortified nanoemulsion of the starch-based edible coating, including ZEO, on chicken meat. The results showed that uncoated control samples had reached 11.42 log cfu/g from 4 logs initial
L. monocytogenes number, where the most effective coating nanoemulsion of ZEO with cinnamaldehyde reached only 6 log cfu/g at the end of 20 days. Shekarforoush et al. [
38] found
that L. monocytogenes numbers did not change significantly (p>0.05) in ready-to-barbecue chicken meat coated with chitosan and oregano essential oil and stored at 8°C with 4.7 log cfu/g. Unlike our storage period, their results covered only the first 3 days of storage.
Adding essential oil or another antimicrobial agent increases the effectiveness of the preservation potential of edible coating against pathogenic bacteria. Raeisi et al. [
45] studied the combined effects of rosemary, cinnamon essential oils, and nisin together and found a greater inhibitory effect on
L. monocytogenes during the storage of chicken meat. Ala et al. [
42] reported that bioactive carboxymethyl cellulose coating containing
Ziziphora clinopodioides (ZEO; 0.25 and 0.5%) and
Mentha spicata (MEO; 0.5%) essential oils applied on chicken breast fillets caused
L. monocytogenes numbers to increase from 5 log cfu/g to 7.55 and 7.83 logs for negative and positive controls, respectively. In the research of Souza et al. [
48], bio nanocomposite edible films containing ginger EO had antimicrobial effects against
L. monocytogenes and other pathogens in chicken breast samples. Janes et al. [
49] studied the effect of zein propylene glycol film containing nisin and calcium propionate as antimicrobial agents on chicken meat against
L. monocytogenes during 8°C and 4°C storage. They have obtained
L. monocytogenes growth was suppressed by 5.4 logs on day 8 compared to uncoated control samples stored at 8°C. In our present study, we had a 4 log difference on day 12 just using CLM
(2) coating.
The UCST (uncoated
L. monocytogenes) samples from an initial microbial load of 4.3 log cfu/g reached 6.1 log cfu/g at 10 days of storage (
Figure 8). The coating containing EO 1.5% and higher showed significant inhibition against the growth of
S. Typhimurium (p<0.05), suppressing their growth by approximately one log. The effects of CST (coated
L. monocytogenes), CST
(0.5), and CST
(1) on the growth of
S. Typhimurium were not significantly different (p>0.05). A previous study also showed that the MIC of EO obtained from PVR against
S. Typhimurium was 1.5% (v/v) [
47]. CST samples up to 1% EO concentration showed similar results (p>0.05). These samples had results around 5 log cfu/g at day 12, where CST
(1.5) and CST
(2) were more effective than the other coated samples having 4.8 log cfu/g and 4.1 log cfu/g, respectively. As expected, the most effective coating was CST
(2), with significantly lower
S. Typhimurium numbers during storage (p<0.05).
Figure 8.
Growth of S. Typhimurium on the UCST and CST with different EO concentrations during storage of 12 days at 8°C.
Figure 8.
Growth of S. Typhimurium on the UCST and CST with different EO concentrations during storage of 12 days at 8°C.
Ala et al. [
42] have reported that TBC of uncoated samples increased to 9 log cfu/g on the 10
th day of storage, and the most effective coating with EOs increased to 4.2 log cfu/g from the initial number 3.55 log cfu/g. Carboxymethyl cellulose coatings with several essential oils decreased
S. Typhimurium by 3 logs at the end of 13 days of storage at 4°C. This direct decrease trend differing from our present study in
S. Typhimurium numbers can be explained by lower storage temperature than our storage conditions.
S. Typhimurium is a well-known bacterium with uncompetitive growth characteristics at lower refrigeration temperatures like 4°C [
42]. In our study, the
S. Typhimurium count fluctuated during storage at 8°C. Bacterial growth increased dramatically in the first 5 days, slowed down in the following days, and decreased after the 10
th day of storage. The main reason for decreasing in
S. Typhimurium count in all samples after the 10
th day of storage may be because
S. Typhimurium, as a mesophilic bacteria, could not compete with the mainly psychrophilic bacterial flora in the samples at refrigeration temperatures [
50].
Pinto et al. [
4] observed the microbial and quality properties of chicken breast fillets treated with sodium alginate edible coatings containing oregano and curcumin EOs. According to
S. Typhimurium count results, all samples containing EOs had approximately 2 logs decrease during 7-day storage at refrigerated temperatures. Unlike our results, they observed a continuous decrease in
S. Typhimurium counts at all samples during whole storage. All samples with coating had significantly lower numbers than control samples. But essential oils used and their combined blends did not have any significant difference within each other.
Goswami et al. [
51] used pea starch coating with thyme EO to research antimicrobial effects against pathogens and spoilage bacteria found in chicken breast meat. They have found that total aerobic counts increased from 4.7 to 7.1 log cfu/g during 12-day storage at 4°C in
Salmonella inoculated control samples. EO-added samples had similar results from 4.0 to 7.2 log cfu/g. On the other hand,
S. Typhimurium count results showed a decrease in control samples from 5.2 to 4.2 log cfu/g, whereas EO-added samples had a significantly higher decrease from 4.3 to 2.2 log cfu/g. Their results were similar to our TBC numbers but different from
S. Typhimurium count results. Since their storage is at 4°C, this is expected due to the
S. Typhimurium growth characteristic at lower temperatures mentioned before.
3.2. Sensory analysis of chicken breast fillets with PVR edible coating
Results of sensory analysis for raw CBF cubes according to quality parameters of; appearance, smell, texture, and general acceptance were shown in
Figure 9a. The appearance, smell, and general acceptance of CR were scored significantly better than UCR samples (p<0.05). In other words, C2
(2) coating kept sensory properties of raw CBF cubes at a higher acceptance significantly (p<0.05) than UCR except for texture.
Figure 9.
Sensory analysis scores; (a) Uncoated and coated raw chicken breast cubes, (b) uncoated and coated grilled chicken breast cubes with taste parameter at the end of 5 days storage at 8°C.
Figure 9.
Sensory analysis scores; (a) Uncoated and coated raw chicken breast cubes, (b) uncoated and coated grilled chicken breast cubes with taste parameter at the end of 5 days storage at 8°C.
Also, the results of grilled CBF cubes tested for quality parameters of; appearance, smell, texture, taste, and general acceptance were given in
Figure 9b. Again both of the samples, UCG and CG, had similar and higher scores meaning C2
(2) coating did not alter the appearance, smell, and texture of grilled chicken breasts applied (p>0.05). But the taste and general acceptance of CG samples were significantly higher than UCG samples (p<0.05). Among all parameters, the smell is crucial for a suitable edible coating application on food material. Coated raw samples had a significantly lower smell score than uncoated samples in raw chicken breast fillet samples, but it was not significantly different in grilled samples (p>0.05). In addition, the “Taste” of CG samples scored significantly higher than UCG samples (p<0.05), meaning that C2
(2) coating containing EO of PVR can be used for this food material keeping organoleptic properties at reasonable levels.
Research studies include sensory analysis for edible coating when EOs added concerning changes in organoleptic properties of food materials [
52]. Since this present study is the first research regarding the sensory evaluation of EOs obtained from
Pistacia vera L. resin, comparisons of the sensory evaluation were conducted with the antimicrobial edible coatings containing other EOs.
Panahi et al. [
53] obtained odor results similar to our study, with lower scores of uncoated controls than samples coated using sodium alginate incorporated with
Ferulago angulate. Boiss essential oil and nisin during 12 days of storage.
Bazargani-Gilani et al. [
34] investigated the pomegranate juice-added chitosan coating enriched with ZEO on chicken breast meats during 20 days of storage. Pomegranate juice-treated samples showed significantly higher scores than all control groups. Before 5 days of storage odor of samples treated with ZEO was higher than the control, but after 5 days, off-odor due to microbial spoilage occurred. The results obtained before five days of storage showed that ZEO-treated coatings improved the odor of chicken breast samples, which suits our findings for the smell.
In a study with very similar results with odor properties, Garavito et al. [
3] developed an edible coating of guar gum and isolated soy protein enriched with oregano EO. According to sensory evaluation during 10 days of storage, all sensory parameters of coated samples were kept at acceptable levels at the first 6 days of storage. The odor of uncoated samples decreased significantly lower than coated samples on day 6, like our results on the 5
th day (p<0.05).
Nouri Ala et al. [
42] formed carboxymethyl cellulose coatings that are hydrophobic similar to wheat gluten used in this study, with several plant-based EOs (ZEO and MEO) and recorded the lowest sensory scores for the uncoated chicken fillets. And also, the coating did not adversely affect the sensory characteristics of the chicken meat samples.
In another study, the taste of the chicken samples was unaffected by using chitosan film combined with oregano essential oil, which also increased the shelf-life of chicken fillets by 14 days while maintaining acceptable sensory attributes [
35].
Yousefi et al. [
54] had decreased sensory properties during storage of 16 days at 4°C for the lactoperoxidase system-alginate-coated chicken breasts. On day 0, all samples had high sensory scores of 8/10, but on day 16, only coated samples had acceptable scores. According to panelists, the results and situation of the products were unacceptable for uncoated samples at the end of the storage.
For edible coatings applied to food materials, the sensory properties are crucial in influencing consumer choice and decision. While preserving the food materials from harmful effects, the coating should keep the sensory properties “acceptable” to consumers. The present and previous studies showed that edible coatings and their bioactive components are used, so they had no adverse effect on the sensory properties of food materials coated with edible coatings.