Carnauba Wax Applied to the Shell Surface of Chicken Eggs Improved the Shelf Life and Internal Quality of the Eggs

  

    
Shell    surface treatment
    
Storage    time (days)
  
  

    
07
    
14
    
21
    
28
  
  

    
Weigth loss (%) 
  
  

    
Carnaúba    wax 30%
    
0.30    Aa 
    
0.37    Aa
    
0.77    Aab
    
1.28    Ab
  
  

    
Control
    
1.71    Ba 
    
2.91    Ba 
    
6.28    Bb 
    
6.56    Bb
  
  

    
Yolk    pH
  
  

    
Carnaúba    wax 30%
    
6.00    Aa
    
6.04    Aa
    
6.05    Aa
    
6.06    Aa
  
  

    
Control
    
6.19    Ba
    
6.37    Ba
    
6.35    Ba
    
6.26    Ba
  
  

    
Albumen    pH
  
  

    
Carnaúba    wax 30%
    
8.66    Ab
    
8.6    Ab
    
8.62    Ab
    
8.35    Aa
  
  

    
Control
    
9.20    Ba
    
9.33    Bb
    
9.37    Bb
    
9.29    Bab
  
  

    
Albumen percentage (%)
  
  

    
Carnaúba    wax 30%
    
60.31 
    
59.70 
    
58.02 
    
55.25
  
  

    
Control
    
60.28 
    
57.44 
    
56.12
    
56.19
  
  

    
Yolk percentage (%) 
  
  

    
Carnaúba    wax 30%
    
29.87
    
30.61
    
32.06
    
34.78
  
  

    
Control
    
29.76
    
32.81
    
33.69
    
33.62
  
  

    
Yolk    index
  
  

    
Carnaúba    wax 30%
    
0.37    Aa
    
0.33    Aa
    
0.29    Ab
    
0.22    Ac
  
  

    
Control
    
0.30    Ba
    
0.23    Bb
    
0.18    Bc
    
0.16    Bc
  
  

    
Haugh unit
  
  

    
Carnaúba    wax 30%
    
78.31    Aa
    
68.30    Ab
    
60.39    Abc
    
60.17    Ac
  
  

    
Control
    
54.22    Ba
    
50.53    Ba
    
50.53    Ba
    
49.20    Ba
  
  

    
Means with different lowercase letters within a row are    significantly different after Tukey’s test (P <0,05).

      Means    with different capital letters within a column are significantly different    after Tukey’s test (P <0,05). 
  

Table 1: Internal quality comparison of eggs from commercial laying chickens externally coated with a semi-solid product based on carnauba wax versus control groups (negative control without coating) and positive control (mineral oil), stored for 28 days at room temperature.

Effects of Coating and Storing Eggs with Carnauba Wax on Variable Weight Loss

Regardless of the storage period, commercial eggs in the control group stored at room temperature significantly lost more weight (p=0.05) compared to the treated group that received the carnauba wax coating and mineral oil treatment. The weight loss increased over the 4 weeks of storage for both the negative and positive control groups, as well as for the group treated with carnauba wax. The percentage of weight loss in the negative control group was initially 1.71% and increased to 6.56% after 4 weeks of storage. In the mineral oil-treated group, the weight loss was 0.34% in the first week, increasing to 1.69% in the last week of evaluation, while the carnauba wax-treated group showed an initial weight loss of 0.30%, increasing to 1.28%, demonstrating the effectiveness of the product in egg preservation.

Eggs sold in the Brazilian retail market are typically stored at room temperature, which varies between 25°C and 27°C. In contrast, in other countries like the United States, eggs are stored at a temperature of 4°C [14]. However, it's important to note that depending on the season and region of the country, this temperature can vary significantly, which can directly impact the quality of commercial eggs. Given the variations in storage temperatures, this study opted to use room temperature, which better reflects the reality of the Brazilian market. In Brazil, regulations require that eggs be washed before they are sold. This is done to ensure food safety and reduce the risk of bacterial contamination that may be present on the egg's surface. Egg washing, as highlighted by Gole et al. [15], can damage the shell cuticle and expose pores, resulting in moisture loss and a deterioration of internal quality. These insights may help explain the differences observed between the control group without coating and the group treated with the semi-solid product based on carnauba wax and mineral oil regarding the observed weight loss in our study.

The weight reduction of eggs over the storage period provides important information about the quality and shelf life of fresh eggs. Weight loss during storage is a natural process influenced by the movement of water and gases (carbon dioxide) through the eggshell, as highlighted by Wardy et al. [16]. Egg weight loss mainly occurs due to the transfer of moisture from the albumen to the external environment through the shell's pores. In our study, it was observed that eggs coated with carnauba wax formed an impermeable protective layer similar to the natural cuticle covering the shell of fresh eggs. This natural cuticle plays a crucial role in protecting the pores on the shell, making it difficult for water to escape and preventing the entry of bacteria through these pores, as observed by Muñoz et al. [17] and Pires et al. [18]. Based on these results, we hypothesize that the waxy coating seals these pores and reduces the evaporation process.

Pushpakumara et al. [20] evaluated mineral oil and two other variations of vegetable waxes as coatings for chicken eggs stored at room temperature (27±2°C) for five weeks. They observed a lower weight loss in coated eggs compared to the control group without any coating, highlighting the effectiveness of these coatings in egg preservation. This finding is consistent with the results obtained in our study. However, we observed an even greater efficacy in preserving eggs coated with carnauba wax compared to mineral oil, which already has well-established results. In our study, it was observed that eggs coated with carnauba wax formed an impermeable protective layer similar to the natural cuticle that covers the shell. This highlights the effectiveness of the carnauba wax-based product in providing a more robust barrier compared to mineral oil coating. Carnauba wax, derived from the leaves of the Brazilian palm tree Copernicia prunifera, is known for its highly hydrophobic nature, imparting waterproof properties. In contrast, coating with mineral oil may not offer as effective a barrier, making carnauba wax a superior choice for egg preservation.

A study conducted by Eyng et al. [21], which also used carnauba wax as a coating for chicken eggs at different concentrations (12% and 15%) and temperatures (10°C and 25°C), demonstrated that during the storage period of the study, the weight loss in eggs coated with wax, at both concentrations and temperatures, was lower compared to the uncoated group. This indicates that regardless of temperature and concentration, the waxy coating on eggshells can preserve the internal quality of the eggs. In the study by Rachtanapun et al. [22], the authors used a mixture of paraffin and beeswax (0.5%) and found that this compound had the lowest weight loss after 4 weeks of storage at 25°C.

The Effects of Coating Eggs with 30% Carnauba Wax and Storage on Egg White pH and Yolk pH

Throughout all storage days, both the yolk pH and albumen pH in the untreated control group were significantly higher (p<0.05) compared to the group coated with carnauba wax and mineral oil for both analyzed variables. However, the yolk and albumen pH in the mineral oil-treated group were significantly higher (p<0.05) than in the wax-treated group.

In our study, the yolk pH in the untreated control group ranged from 6.19 to 6.37, and the positive control group treated with mineral oil ranged from 6.13 to 6.21. Both control groups showed a larger variation in yolk pH compared to the group treated with carnauba wax, which had a very small variation, ranging from 6.0 to 6.06 over the storage days. As for albumen pH, there was a range of 9.17 to 9.56 for the untreated control group (negative control), the albumen pH of the mineral oil-coated group (positive control) ranged from 9.9 to 9.29 over the storage days, while the albumen pH of the wax-coated group varied from 8.35 to 8.75, showing lower pH levels compared to both negative and positive controls. Overall, all groups experienced an increase in pH as the storage time at room temperature increased.

Albumen pH can be used to determine the freshness of the egg white (Scott & Silversides [22]. Eggs coated with carnauba wax showed significantly lower pH values than the control group during the storage period. Albumen pH can also be used as a measure to indicate egg quality. A newly laid egg contains about 1.44 to 2.05 mg of dissolved CO2 (in the form of carbonate), resulting in a pH between 7.6 to 8.7 [22]. Thus, during the storage period, this CO2 and water are released through the pores in the eggshell, which increases the albumen pH, ranging from 8.9 to 9.4 [19,23]. These results suggest that the carnauba wax coating may delay the loss of carbon dioxide through the eggshell pores, acting as an effective gas barrier. In the study by Souza [24], the author tested a 6.25% carnauba wax emulsion compared to the untreated control group and a group of eggs coated with mineral oil. The conclusion was that eggs coated with mineral oil had a lower albumen pH (8.99) than the group treated with carnauba wax (albumen pH=9.38) after 35 days of storage at room temperature. It seems that the low concentration of carnauba wax in the emulsion couldn't protect the eggs as effectively as mineral oil did. A higher concentration of carnauba wax appears to better preserve the internal quality of the eggs, as found in our study. Pires et al. [18] also observed that at the end of the storage period, only mineral oil was able to maintain the albumen pH below the values observed in untreated eggs.

The yolk pH can vary between 6.4 to 6.9 during storage due to the release of CO2 and water through the shell. Pushpakumara et al. [19] observed in their study that the coatings used on eggs acted as a barrier and helped reduce this evaporation through the shell when compared to uncoated eggs. In our study, the effectiveness of carnauba wax was also evident, as coated eggs had a small variation in pH. Studies have also supported the presence of lower pH values in the albumen and yolk of eggs coated with coatings containing essential oils on the last day of storage [26,27].

Effects of 30% Carnauba Wax Coating and Storage on Yolk Index

There was a significant difference (p<0.05) in the yolk index of eggs from the control group and those coated with carnauba wax and mineral oil during the 4-week storage period at room temperature. There was also a significant difference (p<0.05) between eggs coated with carnauba wax and eggs coated with mineral oil. After 4 weeks of storage, the yolk index of uncoated eggs decreased from an initial value of 0.3 to 0.16, while that of eggs coated with the 30% semisolid carnauba wax product decreased from 0.37 to 0.22. In general, throughout the storage period, both the uncoated control group (negative control) and the control group treated with mineral oil (positive control), as well as the group treated with the semi-solid waxy product, showed a reduction in egg yolk index. However, it was observed that the egg yolk index in the negative control group decreased more rapidly compared to eggs coated with carnauba wax and mineral oil (positive control). Nevertheless, eggs treated with carnauba wax maintained a significantly higher yolk index (p<0.05) compared to the positive control.

The yolk index is a ratio between the height and width of the yolk and is used as a measure of egg freshness, with a higher yolk index indicating better quality [28]. During the storage period, there is a weakening of the membranes, liquefaction of the egg white, and water absorption by the yolk, resulting in a decrease in the yolk index [18,29]. The results obtained by Pires et al. [18] demonstrated that for a period of fewer than 8 weeks, the use of coating was able to preserve the yolk index for a longer time compared to uncoated eggs, which is consistent with our study.

Effects of Coating Chicken Eggs with Carnauba Wax and Storage for the Haugh Unit (HU) Variable

Changes in the Haugh unit of eggs from the positive control group (treated with mineral oil); negative control group (no treatment) and eggs coated with carnauba wax during the 4-week storage period can be seen in Table 1.

At room temperature, regardless of the storage time, the eggs in the control group had statistically lower (p<0.05) Haugh unit values compared to the eggs coated with the waxy product. Overall, the Haugh unit decreased with increasing storage periods, as was also observed in the yolk index. However, the Haugh unit of eggs in the control group decreased more rapidly than the eggs coated with the carnauba wax product. It is evident that after the first 7 days of storage, the group of coated eggs maintained a Haugh unit value of 78.31, while the control group showed a much lower value of 54.22.

As the egg is stored, various changes occur in its physical and chemical properties, leading to a gradual deterioration. The decrease in the Haugh unit is indicative of this deterioration process. The Haugh unit is a metric that assesses egg quality based on albumen height and egg weight. Normally, the Haugh unit decreases as the storage time progresses, which can be attributed to processes such as ovomucin proteolysis, breaking of disulfide bridges, or interactions between a aand β βββovomucins [20]. The reduction in HU values suggests a change in the texture of the album, making it less thick and more liquid as discussed by Pushpakumara et al. 20].

The Haugh Unit, as proposed by Yüceer and Caner [12], serves as a parameter for classifying egg quality. In our study, one week post-storage, eggs treated with carnauba wax were classified as AA (HU: 78.31), indicating high quality; however, this classification dropped to A after 14 days (HU: 68.30) and remained so until 28 days (HU: 60.17), suggesting good quality albeit slightly lower than the initial rating in the first week of room temperature storage. For eggs coated with mineral oil, the Haugh Unit in the first week of storage was (HU: 60.32), classifying the eggs as A. In the second week, the classification of the eggs dropped to B (HU: 55.75), remaining in this category until the end of the experiment at 28 days (HU: 52.31). In contrast, untreated eggs started the first week of storage with a B classification (HU: 54.2) and maintained this classification until the end of the storage period (28 days), with an HU of 49.20 (Table 1). This suggests that the carnauba wax coating had a positive impact on maintaining egg quality over the storage period, as the treated eggs retained a higher classification compared to untreated eggs, which deteriorated in quality more rapidly.

In a study conducted by Jirangrat et al. [29], it was also observed that eggs coated with an oily substance like mineral oil, stored at room temperature for 5 weeks, changed from classification AA to B, while the control group without coating changed from AA to C according to the Caner & Yüceer [30] classification, which is consistent with our findings in this study. The difference between wax-coated eggs and untreated eggs at the end of the 28-day experiment can be seen in Figures 1B and 1C, respectively. The treated eggs exhibit intact yolks and visible dense albumen, in contrast to the control group, where the dense albumen is liquefied, and the yolk appears limp and flattened, indicating signs of deterioration.

Ongoing efforts in food preservation have driven advancements in research and technology in this field. Researchers and scientists are working to develop more effective and sustainable methods for preserving food, much like the studies on eggs discussed earlier. In addition to eggs, fruits are also being coated with different materials such as natural waxes, resins, edible polymers, or combinations of these components to protect and preserve the quality of fresh fruits. According to Blum et al. [31], immersing persimmons (Diospyrus kaki) in a solution with 12.5% carnauba wax helped maintain ascorbic acid levels and tissue firmness, allowing for storage for up to 49 days. Mota et al. [32] found that the use of carnauba wax effectively reduced fresh mass loss and wilting in yellow passion fruits (Passiflora edulis) stored at room temperature. These studies highlight the potential of wax coatings, such as carnauba wax, in preserving the quality and extending the shelf life of various fresh produce items, which is an important aspect of food preservation and reducing food waste.

Our waxy coating product (Figure 1A) appears to have great potential for the egg industry and retail establishments that face challenges in providing refrigeration systems for eggs. This coating material was designed to be environmentally friendly, safe, highly water-resistant, with good mechanical properties, and it offers a cost-effective solution. The use of this technology presents two distinct approaches. The first involves providing the product directly to organic farms and small producers, allowing them to treat the eggs before offering them to end consumers. Alternatively, consumers themselves can purchase fresh eggs and perform the treatment at home. Another option would be to transform the semi-solid product into a liquid formulation, enabling application by spraying directly onto the eggs, following a common practice with mineral oil in some farms.

    

    

    Figure 1: Internal quality of coated and uncoated eggs after 28 days of storage at room temperature. (A): Semi-solid product based on carnauba wax formulated for surface coating of eggshells. (B): Egg treated with carnauba wax: intact yolk and visible dense albumen; (C): Uncoated control egg: the dense albumen is liquefied, and the yolk is flaccid and flat.