3.1. Cheese Composition
The moisture, protein, FDM, and acidity contents as well as pH of the examined cheese samples during ripening period of 90 days are illustrated in
Table 1. The results showed the pH values of the cheese samples were significantly changed under the effects of different coating materials (edible and commercial coatings) and ripening times (
P < 0.01).
The pH of the cheese samples increased until day 30 (except for the FM3 sample) and then gradually decreased until day 90 of ripening. The reduction in pH values during ripening is due to the metabolism of the remaining lactose to lactic acid by NSLAB [
31]. Higher levels of dissolved CO
2 in the cheese atmosphere produced buffering effects due to the lower CO
2 permeability of 1.25%. Flaxseed coating may explain the stable pH value of FM3 sample until day 30 of ripening [
32]. Changes in the pH values of the cheese samples may be the consequence of alkaline compound formation due to proteolytic degradation during the ripening period [
33]. At the end of the ripening period, the highest and lowest pH rate were detected in FM1 and control samples (5.42 versus 5.21), respectively (
P < 0.01), which contributed to the increase in NSLAB bioactivity after 90 days in the control sample and the increment in lactic acid production.
The titratable acidity of the cheese samples increased during ripening time (
P < 0.01). According to the results of the microbial analysis, the NSLAB population increased during the 90 days of ripening, which could increase the production of lactic acid and subsequently increase the titratable acidity [
34]. The greatest number of NSLAB counts on day 90 of ripening was detected in the XG sample, which could confirm the higher titratable acidity of this sample compared to the other samples.
The results showed that the moisture content of the cheese samples decreased significantly during the ripening period (
P < 0.01). These results were consistent with those reported by Buriti et al. and Kasimoglu et al. [
35,
36], who have shown that the moisture content was reduced during ripening in all the cheese samples examined in their study. It is expected that the reduced moisture may be due to synergism and osmotic flow during the ripening period. The results showed that the types of coatings used to coat Cheddar cheese had no significant effect (
P > 0.01) on the moisture content of the samples, and all the samples, with the exception of the control, reached the same levels of moisture content on day 90 of ripening. At the end of the ripening period, the highest and lowest rate of moisture were detected in the control and FM3 samples, respectively. The higher hydrophilic properties of 1.25% flaxseed mucilage coating increases water absorption, which leads to a further decrease in the moisture rate in the FM3 sample.
The fat in dry matter (FDM) values of the cheese samples increased significantly during the ripening period (
P < 0.01). This may be the result of an increase in the fat content due to the moisture loss caused by the hydrophilic nature of the coating materials, accompanied by a decrease in lipid levels due to lipolysis. All of these changes kept the amount of fat at its initial level [
19,
37]. In the control sample, the amount of FDM was higher than those coated with edible coatings at the end of ripening (
P < 0.01). This may result from the substitution of fat by moisture in the protein matrix of cheese because of the hydrophilic nature of the commercial coating, which is higher compared to those of the other edible coatings, which can accordingly result in an increase in the amount of FDM [
37] in this sample.
The protein content increased in all samples throughout the ripening period (
P < 0.01). However, there was no significant difference between the protein content of the cheese samples coated with different bio-materials (
P > 0.01). It is expected that the nature of the coating materials does not affect the protein composition of the cheese. The results of the present study are consistent with those of Henriques et al. [
38].
3.3. Proteolysis
Proteolysis rate is an indicator of the ripening degree of the cheese, which reflects the development of the texture, aroma, and flavor of the cheese [
41]. The TCA-SN/TN rate showed an increasing trend in the coated cheese during the ripening period (
P < 0.01) (
Figure 2). The results of this study are consistent with those reported by Yilmaz and Dagdemir [
34] and EL-Sisi, Mohamed Gapr & Kamaly [
42]. The changes in proteolysis rate during ripening were lowest in the control sample. Furthermore, there was a significant difference in the TCA-SN/TN rate between different samples (
P < 0.01). The lowest and highest levels of TCA-SN/TN were detected in FM1 and FM2 samples, respectively. The increased levels of TCA-SN/TN over the 90 days of ripening can be associated with an increase in the NSLAB population and, consequently, an increase in the protease enzymes, which led to a higher proteolysis rate [
43,
44].
The amount of tyrosine and tryptophan amino acids decreased in all the specimens during day 30 to 60 of ripening and then showed an increase until the end of the ripening period (
Figure 3). In the present study, the highest amounts of tyrosine and tryptophan amino acids, followed by an increased level of proteolysis, were found in the XG sample, which was consistent with the microbial analysis of this specimen and indicated the presence of the largest NSLAB population in the XG sample on day 90 of ripening. An increase in the activity of SB and an increment in the degree of proteolysis due to the activity of these bacteria in XG were seen after 30 days of ripening [
45].
The increased levels of tyrosine and tryptophan amino acids were related to decomposition of proteins into amino acids as a result of proteolysis during the ripening period. The highest amounts of tyrosine and tryptophan amino acids were detected in sample XG and the lowest value was measured in the FM1 sample. Lawrence and Gills [
45] have concluded that the increased level of accessible water enhanced the activity of microorganisms, enzymes, and proteolysis grade.
3.4. Free Fatty Acid Composition
The odor and flavor of cheese are directly affected by the free fatty acids (FFA) released during lipolysis, along with other volatile components and compounds derived from the proteolysis process [
46]. In Cheddar cheese, lipase is derived from various sources such as milk, starter, NSLAB, and rennet [
47].
The components of FFA in the coated Cheddar cheese are presented in
Table 2. A reduction in the C4:0, C6:0, C14:0, C14:1, and C18:0 (
P < 0.01), C10:0 and C20:0 (
P < 0.05), and C18:1 and C18:2 (
P > 0.01) fatty acids was observed during the ripening period. This may be contributed to the hydrolyzation of fatty acids to other compounds, such as ketones, alcohols, lactones, aldehydes, etc. [
48].
The amount of C4:0 fatty acid significantly decreased during ripening (
P < 0.01), while the amounts of C8:0, C12:0, C16:0, and C16:1 fatty acids significantly increased during this period (
P < 0.01). The amount of C4:0 fatty acid obtained from cheese was higher than that reported by Katsiari et al. [
49] (0.85 mg/100 g). It has been shown that high levels of C4:0 fatty acid in cheese imply selective lipolytic activity [
48]. The lowest and highest amounts of C4:0 fatty acid on day 90 of the ripening period were found in the FM2 and control samples, respectively. A smaller amount of C4:0 fatty acid meant a less rancid flavor, which led to higher protection of cheese by 1% flaxseed mucilage coating against fat oxidation [
50]. This fatty acid plays an important role in the organoleptic properties of cheese and improves the flavor [
51]. The same trend was detected for C6:0 and C10:0 fatty acids, which reduced from 2.15 mg/100 g to 3.30 mg/100 g on the first day of the experiment to 1.57 mg/100 g and 3.04 mg/100 g on day 90, respectively.
The lowest and highest relative levels of both C6:0 and C10:0 fatty acids were observed in FM2 and the control samples, respectively. These findings are in agreement with previous studies reported in herby pickled cheese [
26].
The concentrations of C8:0 and C20:0 fatty acids in FM1 cheese were significantly higher than those of the control and other samples coated with edible coatings. In contrast, the concentrations of C8:0 and C20:0 fatty acids of XG cheese were significantly lower than those of the control and other samples coated with edible coatings. The lower levels of C8:0 and C20:0 fatty acids in the XG sample may be due to a lower lipolysis rate in this sample (
Figure 1).
Unlike C14:0 and C14:1 fatty acids, the higher concentration of C12:0 after 90 days of ripening is in agreement with results reported by Voigt et al. [
52] in Cheddar cheese (
P < 0.01). The highest levels of C16:0, C16:1, and C18:0 (
P < 0.01), C18:1 and C18:2 (
P > 0.01) were detected in the FM2 sample. Therefore, the high level of long-chain fatty acids in FM2 cheese may be related to the lipolysis amount (
Figure 1). We found that the highest level of fatty acids belonged to C16:0 and C18:1 fatty acids, in accordance with other studies on hard cheeses [
53,
54].
3.5. Microbial Analysis
The effect of different coating materials, on the population of the NSLAB, SB, and TMAB bacteria in coated Cheddar cheese during the ripening period is shown in
Figure 4a–c.
As illustrated in
Figure 4a, the population of the NSLAB after 60 days of ripening was the highest in sample C and the lowest in FM2. This difference may be due to the reduction of oxygen penetration by commercial coating and the reduction of the relative oxygen pressure and, consequently, increased availability of the microaerophilic NSLAB in the control sample [
42,
55].
The highest increase in NSLAB population at the end of the ripening period was attributed to the samples coated with xanthan gum and the lowest increase was detected in the samples coated with 1% flaxseed mucilage. It is expected that coating, due to limiting the air penetration and the reduction of the relative oxygen pressure inside the cheese, can result in an increase in the survival of microaerophilic NSLAB [
42,
55,
56].
This property was higher in the samples coated with xanthan gum. As shown in
Figure 4b, the decrease in the number of SB counts during the ripening period can be due to the autolysis of these bacteria that resulted from the releasing of intracellular enzymes and cellular compounds including nucleic acid and glucose in the cheese matrix.
These compounds increase the survival rate of NSLAB in cheese, in accordance with other reports [
57]. The highest and lowest bioactivity of starter bacteria were detected in xanthan gum coating (log
10 7.98 CFU/g) and control (log
10 7.57 CFU/g) samples, respectively (
P < 0.05).
It seems that a xanthan coating on Cheddar cheese increases the bioactivity rate of lactic acid bacteria (starter and non-starter) in comparison with other coatings.
Figure 4c shows that the number of TMAB increased on day 90 compared to day 1 of ripening (
P < 0.01). The highest number of TMAB was detected in samples coated with xanthan gum (log
10 7.42 CFU/g) at the end of the ripening period (
P > 0.01). This may be due to the less effective non-permeability of the xanthan gum coating against bacterial growth.
3.6. Sensory Evaluation
Sensory evaluation was performed to investigate the flavor, texture, color, and cutting of the coated samples (
Table 3). The flavor of cheese is directly affected by the free fatty acids (FFA) released during lipolysis along with other volatile components and compounds derived from the proteolysis process [
46]. The results showed no significant differences between the experimental specimens and control in terms of flavor, texture, color, and cutting. These results are concordant with those reported by Cui et al. [
58], who reported that coating cheese with chitosan did not significantly affect the sensory properties of the cheese.
According to the sensory evaluation results, the use of edible coatings not only had no negative effect on the sensory properties of cheese samples, but also some of these coatings (XG, FM1, and FM2) improved the flavor of the cheese compared to the control sample.
Therefore, a coating on Cheddar cheese does not affect the amount of TMAB, which is consistent with the findings of Yilmaz and Dagdemir [
33] and Sarioglu and Oner [
57] in Kashar cheese.