Compound percentages of the essential oil of Thymus vulgaris: α-thujene 0.71%, α-pinène 0.49%, cis-sabinene hydrate 0.46%, camphene 0.14%, β-myrcene 0.71%, α-terpinene 0.58%, β-phelladrene 0.37%, γ-terpinene 1.92%, ρ-cymene 9.89%, carvacryl acetate 1.19%, β-linalool 1.51% (Ben Jabeur et al., 2017).

Compound percentages of the essential oil of bark of Cinnamomum zeylanicum: styrene benzene, ethenyle- (CAS) 5.5%, α-pinene 0.50%, benzaldehyde 1.10%, cyclohexane, 1-methyl-3-(1-methylet) 0.52%, benzenepropanal (CAS) B-phenylpr 1.21%, L-borneol 0.46%, trans-cinnamaldehyde 1.07%, cinnamic aldehyde 52.30%, α-longipinene tricyclo (5.4) 0.70%, α-copaene 11.42%, sativene 0.48%, trans-caryophyllene bicyclo (7.2) 0.80%, ar-curcumene 0.55%, naphtalene, 1,2,3,4,4a,5,6,8aocta 2.28%, α-muurolene naphtalene, 1 0.70% δ-cadinene 6.25%, cis-calamenene 3.61%, naphtalene, 1,2,3,4,4a,7- hexahydro 0.83%, α-calacorene 0.68%, naphtalene, 1,2,3,4,4a,7-hexahydro 0.78%, t-muurolol 1-naphtslenol 1,2,3 1.95% (Kazemi and Mokhtariniya, 2016).

Compound percentages of the essential oil of Citrus paradisi, Citrus reticulata Blanco, Citrus sinensis and Citrus aurantium L. subsp. bergamia:α-pinene 0.27%, sabinene 0.08%, myrcene 3.03%, Z,Z,Z-1,4,6,9- nonadecatetraene 0.20%, limonene 50.42%, Z-β-ocimene 0.12%, γ-terpinene 0.05%, n-octanol 0.11%, linalool 0.45%, n-nonanal 0.17%, trans-para-mentha-2,8-dien-1-ol 0.77%, P-mentha-E-2,8(9)-dien-1-ol 0.73%, 3-(methoxy)- 3-methyl-6-prop-1-en-2-cylcyclohexene 0.03%, cis-β- terpineol 0.04%, α-2,4-cyclohexadiene-1-methanol 0.08%, n-octanol 0.08%, (-)-terpinen-4-ol 0.45%, trans-isocarveol 0.13%, α-terpineol 1.19%, [1,1'-bicyclopentyl]-2-one 1.16%, decanal 1.28%, octyl-acetate 0.09%, trans-carveol 3.09%, carvomenthol 0.15%, 5-isoprorpenyl-2-methyl-2- cyclohexen-1-ol 0.81%, 2-methyl-5-(1-methylethene) 2-cyclohexen-1-one 0.50%, 3-methyl-6-(1-methylethene) 2-cyclohexen-1-one 0.07%, perillaldehyde 0.22%, undecanol 0.18%, limonen-10-ol 0.15%, perilla alcohol 0.04%, n-undecanal 0.12%, 4-vinyl- guaiacol 0.92%, α-a-cubebene 0.24%, 3,7-dimethyl 6-octen-1-ol 0.19%, α-copaene 1.49%, neryl acetate 0.06%, β-copaene 0.54%, α-, trans-bergamotene 0.06%, dodecanal 0.84%, (E)- caryophyllene 0.34%, β-copaene 0.06%, α-guaiene 0.07%, 6,10-dimethylundeca-5,9-dien-2-one 0.07%, α-humulene 1.23%, cadina-1,4-diene 0.07%, γ-muurolene 0.07%, β-copaene 1.30%, bicyclogermacrene 0.22%, α-muurolene 0.24%, (E,E)-, α-farnesene 1.67%, γ-cadinene 0.03%, δ-cadinene 2.53%, sesquisabinene 0.20%, trans-cadina-1,4-diene 0.07%, α-elemol 0.79%, (-)-spathulenol 0.54%, n-dodecanoic acid 1.28%, ethyl iso-allocholate 0.08%, epicubenol 0.22%, γ-eudesmol 0.31%, α-cadinol 0.08%, cadin-4-en-10-ol 0.99%, 3,7-dimethyl 6-octenal 0.15%, humulene 0.22%, β-sinensal 1.65%, 2,6,10-trimethyl 2,6,9,11- dodecatetraenal 0.05%, α-sinensal 3.14%, 2-pentyl-2- nonenal 0.19%, tetradecanoic acid 0.94%, nootkatone 0.28%, cryptomeridiol 0.05%, farnesyl acetone 0.05%, methyl-hexadecanoate 0.26%, 2-dodecen-1-yl(-) succinic anhydride 0.12%, n-hexadecanoic acid 5.65%, methyl ester 9,12-octadecadienoic acid 0.32%, linoleic acid 0.94%, methyl linoleate 0.19%, dodecenyl succinic anhydride 0.31% (Goyal and Kaushal, 2018).

The results of the chemical composition of Citrus peel essential oil can slightly various in literature because it depending upon the age of the plant, harvesting time, geographical and ecological conditions (Wu et al., 2013).

At the end of the feeding experiment, 6 broiler chickens were randomly selected from each group for chemical analysis. Selected broiler chickens were transported to the technological laboratory of the Department of Technology and Quality of Animal Products, where they were slaughtered humanely and technologically processed. The breast and thigh muscles with skin were separated from the carcass. These samples of breast and thigh muscles were transported in a portable refrigerator to the chemical laboratory of the Department of Food Hygiene and Safety, where the preparation of samples for chemical analysis was performed.

The thighs were boned and labeled with the TEO, STO, B, TEO + B, and SEO + B experimental groups, and in each group with a sample number from 1 to 6. Breast muscle samples were similarly labeled. Samples of the control group were marked K and with sample numbers from 1 to 6. Each sample was mixed in a Grindomix 200 laboratory grinder and weighed 50.0 g. Samples of breast and thigh muscles with skin were transferred to laboratory flasks with ground glass. Weighing of breast and thigh muscle samples was performed on an automatic scale of the Kern 440-49N type with an accuracy of d = 0.01 g.

The average dry matter content of the chicken breast muscle with skin and the statistical evaluation of the results are given in Table 2.

The average dry matter content of breast muscle with skin in the experimental groups ranged from 26.73 g.100 g^-1 with citrus fruit essential oil, followed by an average dry matter content of 26.98 g.100 g^-1 with thyme essential oil, 27.07 g.100 g^-1 with thyme essential oil + citrus fruit essential oil, 27.12 g.100 g^-1 with cinnamon essential oil nd 27.89 g.100 g^-1 with cinnamon essential oil + citrus fruit essential oil. These values of the average dry matter content in the breast muscle with the skin are lower compared to the average dry matter content in the breast muscle with the skin of the control group. The difference in the dry matter content of the pectoral muscle with the skin was not statistically significant among the groups (p >0.05). Statistical evaluation of the measured breast muscle dry matter content data expressed as the standard deviation revealed that the smallest variation was in the thyme essential oil experimental group and the highest in the thyme essential oil + citrus fruit essential oil experimental group, i.e. SD = 0.98 versus SD = 1.56.

Mosca et al. (2018) in their study report breast muscle dry matter content in broiler chicken hens and cocks with an average value of 28.21 g.100 g^-1, which is higher compared to our results. Evaris et al. (2021) report a dry matter content of breast muscle of only 23.47 g.100 g^-1 and point out that the addition of essential oil to feed reduces the dry matter content of breast muscle broiler chickens.

The average dry matter content of the chicken thigh muscle with skin and the statistical evaluation of the results are given in Table 3. The average dry matter content of the thigh muscle with skin in the experimental groups ranged from 27.33 g.100 g^-1 with cinnamon essential oil, followed by an average dry matter content of 27.42 g.100 g^-1 with thyme essential oil, 28.07 g .100 g^-1 with cinnamon essential oil + citrus fruit essential oil, 28.09 g.100 g^-1 with citrus fruit essential oil and 28.14 g.100 g^-1 with thyme essential oil + citrus fruit essential oil. These values of the average dry matter content of the skin thigh muscle are lower compared to the average dry matter content of the skin of the control group.

The difference in the dry matter content of the thigh muscle with the skin was not statistically significant among the groups (p >0.05).

Statistical evaluation of the measured data of the dry matter content of the thigh muscle with skin expressed by the standard deviation revealed that the smallest fluctuation of values was in the experimental group with thyme essential oil and the highest in the experimental group with cinnamon essential oil, i.e. SD = 0.96 vs. SD = 1.44.

Haščík et al. (2012) report higher values of dry matter content in the thigh muscle as are our data. They achieved results of 31.51 g.100 g^-1 in the control group with coccidiostats, and 30.21 g.100 g^-1 or 29.88 g.100 g^-1 in the experimental groups with pollen extract. They experimented with a hybrid combination of the Ross 308 broiler chickens. Mosca et al. (2018) report the dry matter content in the thigh muscle of broiler chickens without taking into account sex 26.3 g.100 g^-1, which is a lower value compared to our results. The stated lower dry matter value in the thigh muscle may be due to another hybrid combination of broiler chickens or the feeding of feed mixtures of a different composition than our feed mixtures. Evaris et al. (2021) report a dry matter content of the thigh muscle of only 24.14 g.100 g^-1.

The average fat content of the chicken breast muscle with skin and the statistical evaluation of the results are given in Table 4.

The average fat content of breast muscle with skin in the experimental groups ranged from 1.49 g.100 g^-1 with cinnamon essential oil + citrus fruit essential oil, followed by an average fat content of 1.56 g.100 g^-1 with citrus fruit essential oil, 1.68 g.100 g^-1 with thyme essential oil + citrus fruit essential oil, 1.78 g.100 g^-1 with thyme essential oil and 1.81 g.100 g^-1 with cinnamon essential oil. These values of the average fat content of the breast muscle with the skin are higher compared to the average fat content of the breast muscle with the skin of the control group.

The difference in the fat content of the breast muscle with the skin was not statistically significant among the groups (p >0.05). Statistical evaluation of the measured data of fat content in the breast muscle with skin expressed by the standard deviation revealed that the smallest fluctuation of values were in the experimental group with thyme essential oil and the highest in the experimental group with cinnamon essential oil + citrus fruit essential oil, i.e. SD = 0.11 vs. SD = 0.16.

A study by Zotte et al. (2020) reports a breast fat content of 1.21 g.100 g^-1, which is lower than our results, which may indicate that the addition of essential oils to feed for broiler chickens has the effect of increasing the fat content as in our results. In another study, Zampiga et al. (2019) report a fat content in breast muscle of 1.71 g.100 g^-1, which is lower compared to the results of our study after feeding of feed mixtures with thyme and cinnamon essential oils and higher compared to the group, where cinnamon essential oil + citrus fruit essential oil, thyme essential oil + citrus fruit essential oil and citrus fruit essential oil were used. These results may indicate that feeding of thyme and cinnamon essential oils tends to increase fat storage.

Kim et al. (2020) recorded a fat content of chicken breast muscle of 1.10 to 1.44%, who examined this indicator in terms of the impact of conventional farming systems and welfare systems with the application of welfare principles. Our results of the fat content of the control group and cinnamon essential oil + citrus fruit essential oil are the closest to their highest value of fat content in breast muscle.

The average fat content of the chicken thigh muscle with skin and the statistical evaluation of the results are given in Table 5.

The average fat content of the thigh muscle with skin in the experimental groups ranged from 2.59 g.100 g^-1 with cinnamon essential oil + citrus fruit essential oil, followed by an average fat content of 2.71 g.100 g^-1 with thyme essential oil, 2.83 g.100 g^-1 with cinnamon essential oil, 2.91 g.100 g^-1 with thyme essential oil + citrus fruit essential oil of 2.93 g.100 g^-1 with citrus fruit essential oil. These values of the average fat content of the thigh muscle with the skin are higher compared to the average fat content of the thigh muscle with the skin of the control group. The difference in the fat content of the original thigh muscle mass with the skin among the groups was not statistically significant (p >0.05). Statistical evaluation of the measured data of the fat content of the thigh muscle with skin expressed by the standard deviation revealed that the smallest fluctuation of values was in the experimental group with thyme essential oil and the highest in the experimental group with cinnamon essential oil, i.e. SD = 0.96 vs. SD = 1.44.

Evaris et al. (2021) report a fat content in the thigh muscle of 9.66 g.100 g^-1, which is a significant difference compared to our results.

The average cholesterol content of the chicken breast muscle with skin and the statistical evaluation of the results are given in Table 6.

The average cholesterol content of the breast muscle with skin in the experimental groups ranged from 0.028 g.100 g^-1 with cinnamon essential oil, cinnamon essential oil + citrus fruit essential oil, and thyme essential oil + citrus fruit essential oil, followed by average cholesterol content of 0.034 g .100 g^-1 with citrus fruit essential oil and 0.042 g.100 g^-1 with thyme essential oil. These values of the average cholesterol content of the breast muscle with the skin are lower compared to the control group (when using a cinnamon essential oil, cinnamon essential oil + citrus fruit essential oil, thyme essential oil + citrus fruit essential oil and when using citrus fruit essential oil) or higher (when using thyme essential oil). The difference in the cholesterol content of the breast muscle with the skin between the groups was not statistically significant (p >0.05).

Statistical evaluation of the measured cholesterol content data in the breast muscle with the skin expressed by the standard deviation revealed that the smallest fluctuation of values was in the experimental group with citrus fruit essential oil and the highest in the experimental group with thyme essential oil, i.e. SD = 0.001 vs. SD = 0.005.

In general, raw poultry meat has approximately 27 to 90 mg of cholesterol.100 g^-1 (Bragagnolo, 2009). Shang et al. (2020) report a cholesterol content in the breast muscle of 0.054 g.100 g^-1, which is a higher value of the cholesterol content in the breast muscle compared to our results. Krauze et al. (2021) note that broiler chickens fed cinnamon essential oil in feed showed better growth performance due to the beneficial effect of the product on the small intestinal microbiome, metabolism. Their study states that HDL cholesterol level was increased and levels of total cholesterol and non-esterified fatty acids as well as triacylglycerols were decreased. Their opinion on the total cholesterol also confirmed our results in an experiment with broiler chickens, which was used only cinnamon oil or cinnamon essential oil also combined with citrus fruit essential oil. Lin et al. (2003) also state cinnamon essential oil has, among other things, strong hypocholesterolemic effects. Koochaksaraie et al. (2011) believe that cinnamon aldehyde found in cinnamon essential oil is primarily responsible for the hypocholesterolemic effect of stimulating the excretion of cholesterol from the body.

In the context of citrus fruit essential oil, it is known that the active compound in sweet oranges such as tannins, saponins, flavonoids, and essential oil can inhibit the absorption of cholesterol in the intestine. The presence of tannins will stick or lining the intestine membrane thus inhibiting the absorption of cholesterol (Oluremi et al., 2007). In our experiment, the cholesterol content was reduced mainly when using citrus fruit essential oil in combination with thymus and cinnamon essential oils. Some studies have shown that bioactive compounds from plant materials, including sterols, polysaccharides, and polyphenols, exhibit cholesterol lowering effects both in vitro and in vivo. Condensed tannins belong to the family of flavonoids, which occur in plants as mixtures of flavan- 3-ol oligomers or polymers (Desrues et al., 2017). To determine the potential cholesterol-lowering activities of polyphenols, researchers usually investigate their inhibitory effects on the cholesterol solubility in mixed micelles in vitro at first. It is believed that reduction in cholesterol micellar solubility by phenolic compounds may inhibit the cholesterol absorption in the intestinal lumen and is attributed to their interactions with phosphatidylcholine or bile acid (Ogawa et al., 2015). In recent years, some research teams put forward that the direct interactions of plant functional components with cholesterol may be another possible mechanism of their hypocholesterolemic effects. For instance, β-sitosterol can be co-crystallized with cholesterol to form insoluble mixed crystals to further reduce the absorption of cholesterol in the small intestine (Christiansen et al., 2003).

There is also a study by Kriaa et al. (2019), who report that bacteria found in the intestinal microbiome use cholesterol from the intestinal contents to form their cell walls and thereby reduce the amount of exogenous cholesterol in the body. In their study, Koeth et al. (2013) showed that dietary carnitine and direct supplementation with trimethylamine-N-oxide both reduced reverse cholesterol transport (p <0.05) in mice only in case of the intact gut microbiome.

The main issue regards the molecular mechanisms, which link microflora and trimethylamine-N-oxide synthesis to reverse cholesterol transport reduction. In the intestines, trimethylamine-N-oxide is referred to diminish cholesterol uptake by reducing Niemann-Pick C1-like 1, which takes cholesterol from the lumen into the enterocytes (Altmann et al., 2004). However, the intestinal metabolism of the cholesterol cannot justify the decrease of reverse cholesterol transport. Of note, in the liver, trimethylamine- N-oxide has been referred to reduce the expression of Cyp7a1, a fundamental bile enzyme in the metabolism, and transport of cholesterol (Bronzato and Durante, 2017).

Chicken meat is recognized for several health benefits due to its high nutritional value and, in addition to its high protein content, also its low cholesterol content, low energy value, and low-fat content (Sujiwo et al., 2018).

The average cholesterol content of the chicken thigh muscle with skin and the statistical evaluation of the results are given in Table 7.

The average cholesterol content of the thigh muscle with skin in the experimental groups ranged from 0.034 g.100 g^-1 with thyme essential oil, followed by an average cholesterol content of 0.037 g.100 g^-1 with cinnamon essential oil, 0.039 g.100 g^-1 with cinnamon essential oil + citrus fruit essential oil, 0.044 g.100 g^-1 with citrus fruit essential oil up to 0.045 g.100 g^-1 with thyme essential oil + citrus fruit essential oil.

These values of average cholesterol content in the thigh muscle with skin are either lower (when using thyme essential oil), the same (when using cinnamon essential oil), or higher (when using citrus fruit essential oil, thyme essential oil + citrus fruit essential oil or cinnamon essential oil + citrus fruit essential oil).

The difference in the cholesterol content of the thigh muscle with the skin between the groups was not statistically significant (p >0.05).

Statistical evaluation of the measured data of the cholesterol content of the thigh muscle with skin expressed by the standard deviation revealed that the smallest fluctuation of values was in the experimental group with cinnamon essential oil and the highest in the experimental group with thyme essential oil + citrus fruit essential oil, i.e. SD = 0.002 vs. SD = 0.005.

Shang et al. (2020) report a cholesterol content in the thigh muscle of 0.060 g.100 g^-1, which is a higher value of cholesterol content compared to our results. The cholesterol content of raw and cooked meat and poultry products ranges from 40 to 90 mg.100 g^-1 (Honikel, 2009).

Phytobiotics are products of plant origin extracted from medicinal plants, spices, or even fungi. They have a high impact on the body due to the high content of bioactive compounds provided by secondary metabolism. Numerous studies performed on various livestock, including broiler chickens, have confirmed the multidirectional effects of phytobiotics in animal nutrition on improving health and productivity.

In addition, a positive effect on the nutritional and dietary value of products of animal origin has been revealed (Al- Yasiry et al., 2017; Kiczorowska et al., 2017). Ciftci et al. (2010) state that after feeding a diet with cinnamon essential oil, the level of cholesterol in the blood serum is reduced, as well as in the breasts and thighs of broiler chickens. These authors also confirmed that this essential oil manifests hypocholesterolemic properties. But their opinion was not confirmed in our experiment on the thigh muscle. It has been reported that the thymol and carvacrol content of thyme slows down the activity of the cholesterol-synthesizing enzyme hydroxymethyl-glutaryl coenzyme A reductase, and thereby, decreases cholesterol levels (Elson, 1995). In agreement with this finding, it has also been reported that thyme significantly decreases cholesterol and triglyceride levels in quails (Khaksar et al., 2012) and also in broiler chickens (Safa and Al- Beitawi, 2009). The reduction of cholesterol content in the thigh muscle due to thyme essential oil was also found in our experiment.

The correlation between dry matter and fat, dry matter and cholesterol, and between fat and cholesterol in the breast muscle with skin by groups are shown in Table 8.

Based on the evaluation of the correlation relationship between the two variables, i.e. different results were found between dry matter and fat, between dry matter and cholesterol, and between fat and cholesterol of breast muscle with skin. Differences in the linear relation between the observed variables were not statistically significant (p >0.05).

A strong positive linear relationship with an r value above 0.5 was found between dry matter and fat in the control group, in the cinnamon essential oil experimental group, in the citrus fruit essential oil experimental group, and the thyme essential oil + citrus fruit essential oil experimental group.

Furthermore, a strong positive linear relationship with an r value above 0.5 was found between fat and cholesterol in the experimental group with thyme essential oil and a strong negative linear relation with a value above 0.5 between fat and cholesterol in the experimental group with cinnamon essential oil + citrus fruit essential oil.

An interesting result of the correlation relation was recorded in the experimental group with citrus fruit essential oil between fat and cholesterol, which manifested itself only in a trivial positive linear relation with a value below 0.1.

Other values of the linear relation between dry matter and fat, between dry matter and cholesterol, and between fat and cholesterol showed a mean value, either positive or negative.