Approaches of Egg Decontamination for Sustainable Food Safety

Abstract

Eggs are a rich source of protein, minerals, lipids, and vitamins. Eggs are an essential source of bacterial microflora. Controlling antimicrobial resistance and reducing food loss and waste are essential for a sustainable future. To prevent spoilage and to preserve eggs, a variety of techniques, including thermal and non-thermal, are often used. This paper explores the decontamination methods for egg preservation that have been applied. In previous studies, the initial contamination of the eggs varied from 2 to 9 log CFU per egg. Either thermal or non-thermal techniques resulted in reduced concentrations of Salmonella enteritidis, Salmonella typhimurium, and Escherichia coli, respectively, on the surface of the egg that ranged 0.62–5.9 log, 1.27–4.9 log, and 0.06–6.39 log, respectively, for the former, and being 1.2–7.8 log, 5.0–7.8 log, and 6.5–6.6 log, respectively, for the latter. Thermal approaches were more effective than the non-thermal approaches. Some of these methods had negative consequences on the egg’s functionality, while combination methods, such as thermoultrasonifcation (ozone-UV radiation or heat-ozone), mitigated these effects. Other decontamination methods require further investigation, particularly the potential for scaling up for commercial usage and the associated costs. In conclusion, decontamination methods are required to extend shelf life of eggs, and to decrease consumer risks associated with foodborne pathogens.

1. Introduction

A key component of a sustainable future is food security, which complements food safety [1]. Currently, foods of high quality, biological safety, lack of additives, and little processing are increasing in demand from consumers. Worldwide, hens are bred to provide non-fertile eggs for human consumption as the egg is recognized as a source of high-quality protein with great nutritional value [2,3]. Even though the eggshell is a natural barrier, quality deterioration can still occur, especially when eggs are incorrectly kept and exposed to possible microbial contamination, which can cause significant financial losses for the egg business [4]. Microorganisms can contaminate the egg during collection, storage, and shipping. Contact with contaminated feces can also cause contamination, and the contamination can pass into the inside of the egg [5]. It has been estimated that over 10% of eggs produced in the hen house are uncollectable or break before being used [6]. An investment in lowering the amount of food lost or wasted will be just as useful as an investment in new food production capacity. As a result, decreasing food losses from farm to consumer will be an essential component of the future solution to feed 10 billion people sustainably [1]. Hence, improving shell quality is the main factor in reducing the number of cracked eggs, resulting in significant savings for the industry. Nutrition, hen age, genotype, housing system, and other factors have been demonstrated to be involved in shell quality improvement. These factors are not completely effective in preventing shell egg contamination. In this manner, eggs must undergo some treatments that extend their shelf life and decrease the contamination risk by foodborne microorganisms as contaminated shell eggs are responsible for more than 75% of the 1.2 million Salmonellosis cases and infections that occur each year [7]. Indeed, extending the shelf life of eggs is a challenge; however, laying hens’ nutritional studies have proved that by nutritional manipulation of the animals, it is possible, besides other quality improvements. Pasteurization of the eggshell with infrared radiation, hot water, or hot air has been displayed to be effective in inactivating Salmonella [8]. However, such thermal treatments cause the denaturation of protein and forming a gel network of albumen around the inner shell surface, this obviously affects the rheological properties of yolk and albumen proteins, resulting in disagreeable textural properties and, as a result, a reduction in consumer acceptability [9]. Several non-thermal technologies, such as modified atmosphere packaging, UV, ozonation, and cold storage, have been used to reduce deterioration in fresh product quality and to extend shelf life [10,11].

This study was designed to summarize the methods for egg decontamination and a variety of techniques, such as thermal and non-thermal, that were used to prevent spoilage and preserve eggs.

2. How the Egg Can Protect Itself

Each component of the egg acts as a barrier to external influences to protect it (Figure 1), to reach as a sustainable food for humans. The chicken egg is very popular and consists of the shell (9–12%), albumen (60%), and yolk (30–33%) [12]. Every egg compartment has both physical and chemical antimicrobial defense mechanisms to prevent contaminating microbes from entering and growing. The shell and underlying membrane serve as the first line of defense against microorganism entry. The shell of the egg is a rigid structure made largely of calcium carbonate with an organic matrix. Internally, there are two shell membranes, and the inner one that acts as a barrier when an organism penetrates the shell. Shell membranes are effective filters composed of antibacterial glycoprotein fibers that may protect against the penetration [13]. The integrity of these barriers (egg white, shell membranes, shell, and cuticle) is necessary to prevent microbial proliferation and penetration. In addition to the physical barrier, egg white is a vital line of defense against invading bacteria owing to it represents a not favorable environment for the microbial development. Egg albumen is not a suitable growth medium and discourages the growth of various microorganisms, as egg albumen has a pH of 7.6–8.5 (a newly laid egg [14]), a low level of simple nitrogenous constituent, apoprotein that binds riboflavin, lysozyme that hydrolyzes bacterial peptidoglycan, ovotransferrin that chelates iron, and avidin that binds biotin [15,16]. Unfortunately, many microorganisms can invade and cause egg spoilage despite physical barriers and other antimicrobial factors.

The egg’s innate antibacterial defenses are unlikely to keep it from spoiling on its own. When these defenses work together, they can create a significant barrier to microbial invasion and deterioration. This barrier can be strengthened even further by proper egg handling and processing. When eggs are kept at room temperature, they undergo changes that might lead to deterioration. Eggs and their products must go through several techniques of avoiding egg spoilage in order to increase their shelf life and decrease consumer hazards associated with food-borne diseases, such as Salmonella [17].

3. Causes of Egg Spoilage and Methods of Preventing It

Outbreaks of food-borne diseases involving salmonellosis and campylobacteriosis have been attributed to the consumption of eggs and egg products [18]. The egg can be-come microbially contaminated in two ways: vertically and horizontally. When eggs get contaminated while they are still being formed, whether in the ovary or oviduct, this is known as vertical (trans-ovarian) contamination [19]. Horizontal transmission happens after the egg has been deposited and exposed to germs post-lay, in which case the bacteria penetrate through the shell [20,21]. Salmonella enterica serovars represent the greatest risk to the safety of eggs and have been implicated in numerous egg recalls [22]. This is attributed to the fact that Salmonella has the genes needed to survive in the harsh conditions of egg white, and also can duplicate in egg yolk [23,24].

Causes of egg spoilage are summarized in Figure 2 according to previous studies [5,8,13]. Reduced susceptibility to egg spoiling and the incidence of contaminated eggs can be achieved by minimizing microbial contamination on the eggshell. During storage, microbial and non-microbial spoilage of eggs can arise; however, egg spoilage can be reduced by applying the necessary practical approaches, and either direct methods or indirect factors (Figure 2).

4. Direct Methods of Preventing Egg Spoilage

Direct methods of preventing egg spoilage were divided into two methods: (1) non-thermal and (2) thermal methods. Different methods of egg preservation have been used, each with different results and restrictions. The first line of defense to prevent change of the egg’s interior components is to decontaminate eggshells. While washing eggshells may reduce the amount of microbes on the surface, it can also damage the cuticle layer and leave residue, which makes it easier for microbes to enter the egg’s contents [25,26]. Disinfectants should have a number of critical qualities, including no corrosive effects, minimal negative effects on public health, and environmental friendliness. The findings of Knape et al. [27], Chousalkar et al. [28], and Al-Ajeeli et al. [29] revealed that all treatments that use chemical substances significantly reduced Salmonella species, whereas Salmonella Infantis penetration did not differ significantly between cleaned and unwashed eggs after washing with hydroxide and hypochlorite. Several sanitizers, including organic acids and chlorine-based sanitizers, have been utilized or mentioned in egg-washing procedures [30,31,32]. In the process of washing eggs, using electrolyzed water (EW) is a promising approach. EW is produced by electrolyzing NaCl in water [33], and its application for the prevention of foodborne diseases has already been documented [34]. Its bactericidal actions can be attributed to three properties: pH, reactive oxygen, and chlorine species, such hypochlorous acid and oxidation-reduction potential. Metabolic flux and ATP generation are altered by high oxidation-reduction potential. It prevents the oxidation of glucose, hinders the synthesis of proteins, and prevents oxygen intake and oxidative phosphorylation, which results in the leakage of certain macromolecules [35], and damages cell membranes [36]. The major components of EW are hypochlorous acid, which generates a hydroxyl radical that affects several pathogens [37]. Since bacteria typically cause proteins denaturation. Due to their low cost and environmental impact, edible coatings, such as chitosan and propolis films, have been widely employed to package food products over the past ten years [38]. Several studies have been completed on coating of egg to increase its shelf life and maintain its quality [39]. Chitosan, used as a cationic biopolymer, was found to have exceptional film-forming abilities and great antibacterial activity against different pathogens, including strains of bacteria, yeast, and fungi [40]. In addition, propolis, a resinous substance, from buds, leaves, stems, and fissures, and combined with beeswax, performs a significant role in the control of the eggshell surface microbiome due to its strong biological properties, such as its antibacterial, antiviral, and antioxidant activities [41,42,43]. Eggs can be decontaminated using a variety of non-thermal methods, such as using organic materials, which had favorable significant impacts on the interior quality of eggs, extending of shelf life, and decreased fungal spores development [44,45]. Modern technologies that do not employ heat to inhibit microbial growth have gained interest due to the detrimental effects that high temperatures can have on egg quality. Several of these non-thermal technologies, including high hydrostatic pressure, ultrasound, pulsed light, cold plasma, and ozonation, have recently been proposed for use in the egg industry [46]. Results of Ragni et al. [47], Dasan et al. [48] and Moritz et al. [49] indicated that atmospheric plasma either gas or pressure significantly decreased in different Salmonella species, with no adverse influences on egg quality. It is well-established that Cold Plasma has a germicidal action [50]. When this non-thermal technique is used, microorganisms suffer fatal damage. It has been established that certain plasma components, including charged particles, UV light, and reactive oxygen and nitrogen species, are responsible for this antibacterial activity [51]. Plasma exposes microorganisms to a powerful barrage of radicals that primarily harm the cell surface and prevent it from repairing itself in a timely manner. As a result, cell death and cell wall rupture take place. Furthermore, similar to electrical pulses, the administration of plasma causes the cell membrane to produce pores, which results in the leakage of intracellular substances [51]. Ozone has the ability to inactivate bacteria, fungus, viruses, and protozoa and their spores. It has been proposed that microbial cells contain a number of vulnerable regions to the effects of ozone, including the unsaturated lipids found in the cell membrane. Damage to the membrane encourages component release and results in microbial death [52]. Moreover, it has been demonstrated that using ozone at concentrations of 2 and 4 ppm, respectively, was efficient in maintaining the functional properties and inner quality of fresh eggs throughout storage, destroyed fungal colonies without damaging the cuticle, and significantly reduced Salmonella [3,53,54]. Similarly, pulsed light fluence was found to reduce cells of Salmonella on the eggshell without opposing influences on the egg albumen quality, and sensory and functional properties, and Escherichia coli was completely inactivated [55,56]. Due to the protection of the egg cuticle layer and the ability to increase shelf life without residue issues, pulsed light appears to be an effective decontamination approach when utilized for egg preservation. Through photochemical and photo-thermal effects, pulsed light could successfully inactivate bacteria on the surfaces of food. The high-powered illumination is absorbed by microbial DNA, causing structural alterations in its physical and chemical makeup that eventually result in genetic information loss, unpaired replication, gene transcription, and cell death [46,57]. Furthermore, pulsed light has no harmful on internal egg quality [58]. Al-Ajeeli et al. [29] reported that the application of H₂O₂+UV treatment to shell eggs is a new technology with significant implications for the preservation of egg safety and quality. Each treatment reduced S. enteritidis to below the detection n threshold (200 CFU /egg). Additionally, Sert et al. [59] found that utilizing a 35 kHz ultrasonic wave for 5, 15, and 30 min at 30 °C significantly improved egg quality. The reduction in Salmonella was 1.45 and 0.62 log CFU/egg by using neutral electrolyzed water and 2% citric acid, respectively. On the other hand, citric acid 2% solution damaged the cuticle and exposed eggshell pores [34]. Rodriguez-Romo and Yousef [60] found that the combination of ozone and UV radiation caused significant inactivation of Salmonella. Using vacuum-packing is a highly effective method for egg preservation, as Aygun and Sert [61] indicated that over the storage period, vacuum packed eggs reduced microbial contamination levels and the egg shelf life was increased by at least 42 days. Similarly, using biological fumigation decreased the number of microorganisms on the eggshell surface and fumigated eggs were found to have significantly higher egg quality than non-fumigated eggs over a range of storage periods [62]. Despite the unquestionable advantages that thermal approaches provide to food safety through microbial membrane damage, a procedure like pasteurization can denature egg proteins and subsequently alter its functional characteristics [63,64]. Eggs lose some of their ability to foam and emulsify when heated over 60 °C [65]. Egg white proteins may coagulate more readily at temperatures higher than 60 °C. This claim is confirmed by the findings of a study conducted by Uysal et al. [66] who found that raising the pasteurization temperature caused denaturation, decreased protein solubility, and loss of egg functional characteristics. Additionally, these authors noted that heat altered the lysozyme and ovalbumin protein compositions. Pasteurization is required to extend the shelf life of eggs and their products, and to decrease consumer risks associated with foodborne pathogens, for instance, heat-resistant Salmonella. As a result, the widely used method for preventing food deterioration is thermal pasteurization, which includes hot water immersion [67], steam method [68], microwave [69], using thermoultrasonication [70] and freeze-drying [71]. Finally, a number of efficient non-thermal and thermal methods of preventing egg spoiling are summarized in

Table 1. The non-thermal methods of preventing egg spoiling (with types of contamination).

Method/Chemical Substances	Types of Contamination	Effect on Egg Properties and Decontamination	Reference
Using organic substances
Application of peracetic acid with different concentrations.	Eggs were artificially and naturally contaminated with fungi strains	- Peracetic acid reduced the development of fungi spores and hyphae slightly compromised the cuticle structure. - Highest concentration of peracetic acid solution induced micro fragmentation and compromised egg quality.	Soares et al. [45]
Using an atmospheric plasma
Using an atmospheric pressure plasma jet with varying frequencies under various experimental conditions	Salmonella enteritidis (S. enteritidis) was experimentally infected in eggshells at a concentration of 7 log CFU/egg.	- No deleterious impact on egg quality. - Reduction level of S. enteritidis concentration on egg surface ranged from 2 to 5 log CFU/egg	Dasan et al. [48]
Using atmospheric gas plasma	Experimentally inoculated with S. typhimurium and S. enteritidis (5.5–6.5 log CFU/eggshell)	- No significant harmful impacts of the gas plasma were detected on traits of egg quality. - After 90 min S. typhimurium significantly decreased by 3.5 Log CFU/eggshell. Maximum declines for S. enteritidis ranged from 3.8 to 4.5 log CFU/eggshell	Ragni et al. [47]
Using atmospheric pressure plasma	artificially contaminated with S. enteritidis 8 log CFU/ml	Reduction of 99.63% of the initial S. enteritidis population	Stolz et al. [72]
Using indirect and direct cold atmospheric plasma under different experimental conditions	Initially contamination with 8 log CFU Salmonella cells. (Salmonella enterica serovar Typhimurium)	- No significant differences in internal egg quality. - Salmonella population on egg surface was decreased below the detection limit (2 log CFU cells/egg) after 25 min of indirect treatment and 10 min of direct treatment. - Highest inactivation of Salmonella number was more than 6 log CFU/egg for humid and about 3.90 ± 0.35 log CFU/egg for dry gas	Georgescu et al. [73]
Semidirect cold atmospheric pressure plasma for 300 s.	S. enteritidis on egg shell average of 4.14 log CFU (±4.04 log CFU)	- Atmospheric pressure plasma did not significantly affect internal egg. - Significant reduction in S. enteritidis was 4.1 log CFU	Moritz et al. [49]
Using ozone
Different ozone concentrations in combination with different exposure periods.	S. enteritidis contaminated shell eggs at two levels 2–4 log CFU and 5–6 log CFU/shell	- Inactivation of 2–4 log CFU S. enteritidis/eggshell was reached. - Salmonellae were significantly decreased up to 6 log for the high contamination level.	Braun et al. [53]
The eggs were exposed 60 ppm of O3 for 120 or 300 min	Eggs were contaminated with fungi	- The treatment of O3 for 120 min destroyed fungal colonies and did not injury the cuticle excessively.	Soares et al. [3]
Using pulsed light
Using various levels of pulsed light fluencies.	Inoculated eggs with S. enterica serovar Typhimurium CECT 4156 (6.0 ± 0.2 log CFU/eggshell)	- Eggs exposure to 2.1 J/cm2 and up to 10.5 J/cm2 decreased Salmonella cells on the eggshells. - No significant harmful influence on the egg albumen quality, sensory, and functional properties was observed.	Lasagabaster et al. [55]
Using different levels of pulsed light fluencies.	S. enteritidis was 6.3 log CFU/washed egg and 4.5 log CFU/unwashed egg.	Maximum decontamination was 3.6 and 1.8 log CFU/egg for unwashed and washed eggs respectively.	Hierro et al. [74]
Using different methods: pulsed light, coating, and washing on shelf life of eggs and the interior qualities of the eggs during storage for 6 weeks at 25 °C.	(Escherichia coli ATCC 8739 strain) 9 log CFU/ml	- Increasing storage time had a negative effect on internal egg quality and weight loss - Maximum inactivation of E.coli (3.77 log CFU/egg) was achieved at pulsed light fluence of 1.32 J/cm2. - Eggshell decontamination by pulsed light ranged from 3.16 to 3.40 log CFU/egg	Wang et al. [56]
Using pulsed light on eggs inoculated with S. enteritidis.	S. enteritidis approximately 8 log CFU/ml	- No visual injury to the egg. - A maximum log reduction of 5.3 logs/cm2 of Salmonella was detected with the treatment of 20-s and the dose of 0.1 J/cm2.	Keklik et al. [75]
Using chemical substances
Using different disinfection procedures under different experimental conditions Chlorine, Chlorhexidine, and Lugol’s solution	The initial contamination of the eggs was about 7.5 log CFU (S. enteritidis) in the shell and membranes but not in the egg content.	- The reductions in log CFU caused by disinfection ranged from 0.28 to 1.86. - Disinfection with chlorhexidine, ethanol, Lugol’s solution, quarternary ammonium solutions, or flaming after dipping in ethanol failed to reach complete decontamination of the shell and membranes.	Himathongkham et al. [76]
Using spraying Sanitizing treatments consisted of distilled deionized water, iodine-based disinfectant, and chlorine sanitizer.	S. typhimurium and S. enteritidis inoculated on eggshell surfaces under simulated industry egg processing conditions S. typhimurium populations (3.48:5.29 log CFU/egg for no spray) S. enteritidis populations (4.95:7.85 log CFU/egg for no spray)	- All treatments significantly reduced Salmonella spp. population on the shell. - S. typhimurium populations were 1.27:2.65, 1.85:2.23, and 1.9:2.45 for deionized water, iodine-based disinfectant, and chlorine spray, respectively. - S. enteritidis populations were 3.59:3.99, 3.51:4.57, and 3.75:4.28 for deionized water, iodine-based disinfectant and chlorine spray, respectively.	Knape et al. [27]
Evaluation of spraying different sanitizing treatments alone or in combination with ultra violet Eggs were taken at 0, 7, and 14 days of storage at 4 °C.	S. enteritidis counts of 4.3, 3.2, and 3.2 log CFU/egg of S. enteritidis for the 3 replicate trials, respectively,	- Appling H2O2+UV treatment on shell eggs is a novel technology with significant implications for egg quality and safety preservation. - No differences in customers’ preferences for overall flavor among the four treatments tested. - All treatments lowered SE below the detection limit (2.3 log CFU/egg).	Al-Ajeeli et al. [29]
Washing by chemical substances
Washing eggs by a hydroxide and hypochlorite based solution followed by a compatible sanitizer at 32 °C.	Salmonella Infantis: approximately 3 and 5 log colony forming units (CFU) per ml	- Washing increased cuticle cover - Shell thickness of unwashed and washed eggs was significantly different. - Salmonella Infantis penetration did not differ significantly between cleaned and unwashed eggs.	Chousalkar et al. [28]
Various methods
Using neutral electrolyzed water and 0.9% NaCl solution or 2% citric acid solution for 1 min	Eggs were artificially contaminated with E. coli or Salmonella (a concentration of 6 log CFU/mL)	- Citric acid 2% solution damaged the cuticle and exposed eggshell pores. - Salmonella was reduced by 0.62 log CFU/egg, 1.45 log CFU/egg for 2% citric acid, and neutral electrolyzed water, respectively. - E. coli was reduced by 6.39 log CFU/egg for neutral electrolyzed water vs.0.06 log CFU/egg for 2% citric acid.	Medina-Gudiño et al. [34]
Combined method ozone (O3) and UV
Using several methods, including gaseous ozone (O3) and UV light (UV:1500 to 2500 mW/cm2) for one minute, followed by ozone at 5 lb/in2 gauge for one minute.	Eggshell externally contaminated with Salmonella (5.9 to 4.60 log CFU/g eggshell)	- Treating egg shells with ozone (O3) or UV light significantly reduced Salmonella by 5.9 and 4.6 log units, respectively. - Salmonella was inactivated on eggshell in a short period and at low temperature with the combination of ozone and UV radiation.	Rodriguez-Romo and Yousef [60]
At 37.7 °C, the contaminated eggs were washed with either pH 9 or pH 11 wash water. Both cleaned and unwashed eggs were chilled either quickly or slowly.	Eggshell were inoculated with a double mutant of S. enteritidis (resistant to both streptomycin and nalidixic acid)	Keeping wash water at pH 11and 37.7 °C then fast chilling of washed eggs to 7.2 °C decreased S. enteritidis on the surface of eggs.	Catalano and Knabel [77]
Biological fumigation
Applying biological fumigation of fungal volatile organic compounds (M. cinnamomi).	16 strains of the tested microorganisms were inactivated after exposing of fungal volatile organic components	- Using M. cinnamomi as a biological control agent reduced microorganisms present on the eggshell surface by fumigation. - Egg quality was not affected by using fumigation. - Fumigated eggs had significantly better egg quality than non-fumigated eggs under different storage periods	Suwannarach et al. [62]
Range	3–9 log CFU	0.62–5.9 log S. enteritidis,1.27–4.9 log S. typhimurium and 0.06–6.39 log E. coli

,

Table 2. The non-thermal methods of preventing egg spoiling (without types of contamination).

Method/Chemical Substances	Effect on Egg Properties	Reference
Using organic substances
Using two levels of concentration aqueous solution of citric acid. The egg quality was assessed after 7, 14, 21, and 28 days (storage period)	- Enhancement of egg quality - Decrease egg weight loss, air cell and intensive water transport from albumen to yolk. - Increase structural albumen and vitelline membrane resistance.	Drabik et al. [44]
Coating eggs with food grade mineral oil and stored at different storage conditions of six weeks	- Coated shell eggs had a significantly lower weight loss, pH of both the yolk and albumen over time than uncoated shell eggs. - Significant differences were noted between visual sensory properties of coated and uncoated under storage conditions. - Coated shell eggs stored at the three conditions had a prolonged shelf life.	Mudau [78]
Coating shell egg with chitosan: The eggs were stored at ambient laboratory controlled conditions (around 25 °C with 70–75% relative humidity for 6 weeks)	- Storage duration and coating had desirable significant effects on internal egg quality. - All coated shell eggs, shell had higher pierce and strength, which resulted in longer shelf life. - The coatings of lysozyme-chitosan might be a feasible substitute for preserving the interior quality of fresh eggs during long-term storage.	Yuceer and Caner [4]
Using various coatings (whey protein concentrate, whey protein isolate, shellac, and zein) under different storage periods	The coatings enhanced shell strength and functional characteristics, and they might be a feasible substitute method for preserving egg interior quality during long-term storage.	Caner and Yüceer [79]
Using ozone
Using gaseous ozone at different concentrations with different exposure times during storage for 6 weeks at 24 °C.	- Ozone at 6 ppm negatively affected on eggshell quality. - Ozone concentrations of 2 and 4 ppm were effective in maintaining the interior quality and functional qualities of fresh eggs during storage.	Yüceer et al. [54]
Various methods
Using ultrasonic wave 35 kHz for 5, 15, and 30 min at 30 °C and the control (no ultrasound), and treated eggs were storage for 10 days at 5 °C, and for 10 d at 22 °C.	- The ultrasonic treatment considerably increased egg quality. -The total mesophilic aerobic bacteria values of albumen and yolk declined as the ultrasonic treatment period from 5 to 30 min. - The sensory characteristics of egg shells were enhanced by ultrasonic treatment.	Sert et al. [59]
Combined method ozone (O3) and UV
Using a vacuum packaging machine (1, 14, 28, 42 days).	- After 42 days at 22 °C, vacuum packed eggs had better internal egg quality. - Vacuum packed eggs decreased microbial contamination levels over the storage period. - Vacuum packing extended the egg shelf life to at least 42 days.	Aygun and Sert [61]
Different storage period under different storage temperature	Storage of eggs at temperatures ranging from 0.6 to 2.2 °C minimizes quality deterioration during refrigerated storage and maximizes internal egg quality retention.	Shin et al. [80]

,

Table 3. The thermal methods of preventing egg spoiling (with types of contamination).

Method	Types of Contamination	Effect on Egg Properties/Decontamination	Reference
Hot water immersion
Eggs pasteurized for 25 min in a 57 °C circulating water bath	Salmonella enteritidis (6 –7 log CFU)	- Pasteurization processes didn’t adversely affect the internal egg quality except albumen viscosity and turbidity. - Reductions in S. enteritidis of about 3 log cycles	Hou et al. [81]
Dipping eggs in hot water for three seconds	The initial contamination of eggs was about 7.5 log CFU (S. enteritidis) in the shell and membranes	Dipping eggs in hot water for three seconds completely destroyed S. enteritidis in shells and membranes, but occasionally caused the eggs to crack.	Himathongkham et al. [76]
Hot water immersion (at 57 °C and 58 °C for different dwell time in a water bath (0–85 min)	Six pooled strains of S. enteritidis (initially at 6·8 log CFU/g) inoculated near the center of the yolk	- Immersion heating at 57 °C or 58 °C for 35 min augmented the Haugh unit (HU) values and decreased the clarity of albumen. - Shell pasteurization processes did not harmfully affect the internal egg quality. - Immersion heating at 58 ° C for 57.5 min was effective in eliminating S. enteritidis (3.3–5.5 log cycles reduction)	Schuman et al. [82]
Hot water immersion at 56.7 °C for 60 min and 55.6 °C for 100 min	Eggshell was inoculated with a composite of heat resistant S. enteritidis approximately 9 log CFU/ml	- There were desirable significant effects for internal egg quality. - Hot water immersion inactivated heat-resistant S. enteritidis in eggshell by 4.5 log.	Geveke et al. [83]
Three thermal treatments, hot water immersion, hot water spraying and hot air, alone and in combination with different radio frequencies under different experimental conditions	The initial population of S. typhimurium in the eggshell was approximately 6.5 log CFU/ml	- Increased the length of thermal treatments had significant effect on HU scores - Combination of radio frequencies with the different thermal treatments has not significantly affected HU scores - Hot water immersion and spraying declined the Salmonella by 5: 5.2 log after 65 min and 70 min, respectively. - Combination with different radio frequencies remarkably and clearly reduced the processing time.	Yang and Geveke [67]
Steam method
Hot-air oven (at 55 °C and 180 min) with a forced-air circulating fan to pasteurize internally inoculated eggshell	S. enteritidis (6 –7 log CFU)	- No significant differences for HU scores, pH yolk index, and color between fresh and pasteurized eggs. - Albumen viscosity, and turbidity showed significant differences. - A 5-log reduction in S. enteritidis was achieved after 180 min of hot-air treatment.	Hou et al. [81]
A hot air (a treatment of 2 shots of 8 s at 600 °C) with an interval of 30 s of cold air	S. enteritidis cells contaminated eggshells ranged between 4 and 5 log CFU/eggshell	- No significant changes in the tested egg quality traits were noted (shell color, albumen turbidity, pH and the cuticle assessment). - The treatment of hot air declined the S. enteritidis load on eggshells of up to 1.9 log.	Pasquali et al. [84]
Hot air using steam generators with 600 °C for 8 s and followed by cold air (20–25 °C) for 32 s	E. coli (induced to possess nalidixic acidresistance), S. enteritidis (streptomycin-resistant strain), or L. monocytogenes	- No harmful influences of the hot air treatment (yolk index, albumen pH, albumen turbidity, eggshell color, and cuticle). - The reduction in S. enteritidis load on eggshells of untreated and treated eggs was 1.9 and 0.1 log CFU/eggshell, respectively. -The reduction in L. monocytogenes (1.2 log CFU/eggshell).	Manfreda et al. [85]
Hot air oven, 55 °C, 2 h Water bath, 57 °C, 15 min Microwave oven 9:15 s	An inoculum of S. typhimurium containing 7 log CFU was injected into the center of yolk of each egg	- No significant differences in the internal egg quality among studied groups. During 15 days of storage at ambient temperatures, the pasteurization procedure had little or no influence on the characteristics of eggs. - Dry and wet heat reduced S. typhimurium counts by 2.1 log and 2.0 log CFU/mL yolk, respectively, but microwave heating reduced S. typhimurium counts by just 1.2 log CFU/mL yolk.	Shenga et al. [86]
Steam is applied to the eggs as they go through a thermal trap, a partly enclosed room filled with steam	Salmonella enterica serovar Typhimurium 8 log CFU per egg	- No harmful influences on HU, yolk and albumen pH, and albumen whip were observed. - With a short treatment of a few seconds in the thermal trap prototyped completely inactivated Salmonella (>7.8 log CFU reduction).	Zion et al. [68]
Using the microwave technology
Using the microwave technology where frequency (from 300 MHz to 300 GHz, the wavelength (from 1 mm to 1 m)	Salmonella enteritidis in both the high (5 log CFU/g) and low (2 log CFU/g)	-There was no difference in water activities and albumen pH. - There were significant alterations in yolk pH. - A 2-log reduction in S. enteritidis in both the low (2 log CFU/g) and high (5 log CFU/g) inoculum.	Lakins et al. [69]
Combination or combined methods
Water-bath heating for 25 min at 57 °C followed by hot-air heating for 60 min at 55 °C	S. enteritidis (6 –7 log CFU)	- The overall functionality of pasteurized eggshell is acceptable under the studied heating conditions. - Reductions in S. enteritidis of about 7 log cycles.	Hou et al. [81]
The combination of Radio-frequency heating (60 MHz) for 3.5 min in water at 35 °C.	E. coli in the whole egg was measured at about 7.5 log CFU/ml	- E. coli counts were decreased (ranged from 6.5–6.6 log). - The combination of radio-frequency and hot water was faster than utilizing only hot water.	Geveke et al. [87]
Reduction range	2–9 log CFU	1.2–7.8 log S. enteritidis, 5–7.8 and 6.5–6.6 E. coli

and

Table 4. The thermal methods of preventing egg spoiling (without types of contamination).

Method	Effect on Egg Properties	Reference
Using the microwave technology
Pasteurization by using microwave energy for heating fresh in eggshell, egg white, and yolk under different frequencies	The shell membrane and eggshell exhibited very good transparency to microwaves in their dielectric properties.	Dev et al. [88]
Combination or combined methods
Using different methods and extended storage: (1) Immersion in hot water bath, heating followed by gaseous ozone; (2) Pasteurization; (3) Extended storage (at 4 °C, as opposed to 25 °C: 0–8 weeks.	Detrimental effects on quality markers and damaging to albumen were more severe in heat-pasteurized eggs than those treated with the ozone-based process. - Both unprocessed and processed eggs maintained superior quality when stored at 4 °C, as opposed to 25 °C.	Perry et al. [89]
Using different radiofrequency frequencies under different experimental conditions	- The shell membrane and eggshell exhibited very good transparency to radio waves in their dielectric properties. - Radiofrequency heating showed negative effects on foam stability, viscosity, coagulation and turbidity with increasing the heating rate.	Kannan et al. [90]
Hot air 180 °C for 8 s Hot water 95 °C for 10 s, Infra-red 210 °C for 30 s, Steam 100 °C for 2 s.	- The treatments displayed no visible injury to the shell or visible signs of white coagulation. -Significant reductions in Salmonella numbers (6 log without harmfully injuring the interior contents).	James et al. [8]
Using Pasteurization by microwave at 300 W for 40–45 min, and at 250 W for the same period.	- Microwave pasteurised eggs showed much reduced foaming ability than unpasteurized eggs, and increased yolk pH and HU values due to protein coagulation. - There was no significant variation in foam stability or albumin pH.	Mudau [78]
Using thermoultrasonication
Using thermoultrasonication	- No significant differences were detected in egg properties between untreated and treated eggs. - It is effective only on bacteria that are present on the egg surface and does not affect those inside.	Cabeza et al. [70]
Freeze-drying or cryodesiccation	Provides transportability, uniformity, ease of use, and stable microbiological quality.	de Souza Aquino et al. [71]

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5. Indirect Factors Affecting Egg Components as Barriers to External Influences and Contamination

The eggshell cleanliness and shell strength are the most essential exterior egg quality characteristics. The quality of eggs is impacted by a variety of factors, the most important of which are the hens’ age, genotype, diet, and housing system [91]. The housing system has the greatest impact on eggshell cleanliness, while genotype and laying hen age have the most impact on eggshell strength [92,93].

Age and genotype: One of the major internal variables influencing egg quality is the age of the flock [94]. The age of the chickens affects the egg shape index, which declines with age [95]. Molnár et al. [96] revealed that starting at the age of 60 weeks, the egg shape index and Haugh units declined each week. Increased age of birds leads to the deterioration of shell quality [97]. The thickness of the eggshell diminishes with age, according to studies by Bozkurt and Tekerli [98]. Age has an impact on the albumen index, according to Bozkurt and Tekerli’s [98] findings; eggs from younger chickens had a higher albumen index than eggs from older hens. The quality of the interior components is also affected by the age of the chickens; eggs produced at the start of the laying cycle had greater Haugh units, albumen, and yolk index [98]. The composition of eggs (dry matter, fat, etc.) is altered to some extent by genotype, implying that inherited inclinations have a substantial impact on egg quality [99]. Eggshell weight and strength are mostly determined by the genotype of modern hybrid breeds, according to Ledvinka [100]. It is well known that eggs from white egg laying hybrids have higher eggshell quality than brown egg hybrids [101]. Eggshell thickness is associated with the eggshell formation length and is more affected by genotype than the weight of eggshell, making it a more reliable pointer of eggshell quality than the weight of eggshell. Both Ledvinka et al. [102] and Leyendecker et al. [103] described differences in eggshell thickness between white and brown hybrids.

Housing system: Several studies on the influence of housing systems on eggshell quality parameters have been conducted, including conventional cages, enriched cages, litter, and free-range systems. Eggs produced from hens housed in cage systems had higher albumen indices, Haugh unit values, and yolk indices than those from the floor system [104]. According to Singh et al. [105], conventional cage eggs had higher albumen heights than eggs from floor chickens. In cages, fewer cracked eggs have been produced [106,107]. Mertens et al. [108] investigated the impacts of various housing systems (free-range, conventional cages, aviary and enriched cages) on eggshell quality and found that aviary eggs had the strongest shells and free-range eggs had the weakest. Furthermore, Tumova et al. [109] discovered that eggshells produced in cage housing systems were stronger (4744 g/cm²) than those produced in the litter (4651 g/cm²). The ultra-structural features of the shells presumably support eggshell strength in cage eggs. Cage housing systems produce eggs with stronger eggshell, which might be related to the crystals size and orientation as the major determinants of shell thickness and strength [109]. It might be assumed that the thin shells mainly in litter system are created from larger crystals, which result to lower eggshell strength [110]. Finally, the floor housing system increases the risk to obtained dirty eggs, which increases the chance of microbes spreading and making it unsafe.

Nutrition: There are many factors involved in the eggshell formation and calcium utilization, such as ascorbic acid, vitamin D, zinc, protein (including amino acids), fat, calcium lactate, and calcium gluconate. Since calcium metabolism requires vitamin D, a lack of vitamin D results in poor eggshell quality, primarily due to a decrease in eggshell weight [111]. Trace elements, such as zinc, copper, and manganese, have been shown to positively affect eggshell quality. They influence calcite crystal growth during eggshell formation and eggshell mechanical properties. The quantity of energy, crude protein, amino acids, linoleic acid, and total fat consumed by hens can have an impact on egg quality [112]. Numerous studies have demonstrated that considerable increases in egg weight owing to increasing dietary nutrient concentration are mostly caused by increased yolk weight [113]. High albumen weight is also associated with dietary energy intake, which favors protein synthesis [112]. The dietary supplementation with plant components improved egg quality [114,115]. Abo Ghanima et al. [116] illustrated that shell thickness and Haugh unit were improved by the addition of essential oils in the layers diet compared with the control, because of these oils had an effect on the metabolic activity of the beneficial bacteria colonies within the intestine, leading to affirmative influences on mineral absorption rate especially calcium.

Heat stress: Heat stress leads to a disturbance in the acid-base balance and low ability to transport calcium across the duodenum into the blood, resulting in poor eggshell quality [117]. Ionized calcium, the accessible form of blood calcium that may be used by the shell gland to build the eggshell, may bind to proteins because of the increased blood pH due to an increase in bicarbonate concentration [118]. Shortly after the hen is exposed to heat stress, this may lead to decreased eggshell quality [119]. Franco-Jiminez et al. [120] demonstrated that when chickens were subjected to two weeks continuous heat stress, both eggshell weights and thicknesses dropped concurrently. Numerous tests have demonstrated a decrease in eggshell quality in chickens exposed to both cyclic and continuous heat stress [117]. This could be because of a shift in blood chemistry that lowers blood ionized calcium levels, or a reduction in calcium absorption in the small intestine [121].

Health status: Infections in the hen’s reproductive system can indirectly or directly affect eggshell quality. Certain illnesses use an ascending infection to infect the oviduct and ovary. Due to low egg quality typically results from a confluence of circumstances, diagnosing the reason or causes of decreased egg quality is frequently challenging. Infectious bronchitis, Newcastle disease virus, and egg drop syndrome can cause an increase in the incidence of shell-less eggs and eggshell pigmentation loss [122]. Generally, egg quality degrades in in the presence of any illness.

Storage and shipping conditions: The storage way of chicken eggs has a significant impact on the quality of the eggs. Ambient temperature, humidity, and gaseous exchange surrounding the eggs are some factors that affect egg quality [123]. Internal egg quality measures and viability often decline with longer storage times in quails [124]. Storage time can also have an impact on eggshell quality. Eggshell strength increases for four days after oviposition, with little alteration in the quality of eggshell from the fourth to eighth day after lay. Nevertheless, by the 11th to 13th day, eggshell strength has slightly decreased [78]. During shipping, unloading and loading ways, how the egg cartons are secured within the transport truck, the driver and the type of transport vehicle can all affect shell breakage, which ranges from 0.16 to 2.65% [108].

6. Conclusions

Although the ways discussed in this review, both thermal and non-thermal, were capable of reducing microbial numbers on eggshell thermal approaches were found to be more effective. Recently, the egg sector has expanded in using non-thermal technologies, such as high hydrostatic pressure, ultrasound, pulsed light, cold plasma, ozonation, and its combination that have mitigated the adverse influences of heat on the functional properties of egg whites. Pulsed light and ozone, or its combinations, caused alteration in physical and chemical makeup, microbial membrane damage with no harmful effect on internal egg quality. In addition, plasma techniques, such as charged particles, UV light, and electrical pulses, are responsible for antibacterial activity and destroy the cell surface. Decontamination methods are required to extend shelf life of eggs, and to decrease consumer risks associated with foodborne pathogens.

7. Future Perspectives and Research Needs

The egg industry heavily focuses on providing sustainable and safe shell eggs and egg products for consumers. The need for improvement and application of preventive methods along the egg supply chain either at the farm level or during the processing steps is crucial for a better control of the foodborne outbreaks. In this regard at the preharvest stage more research should further focus on reducing the vertical and horizontal transmission of pathogens through the establishment of new chicken strains resistant to Salmonella spp. infection or possessing enhanced cuticle quality and eggshell thickness to increase egg safety through genetic selection programs. Moreover, further study is required for development and implementation of sustainable egg decontamination approaches and extended shelf life, thus, going forward: (1) Improving the color and taste of edible coatings for enhancing consumer acceptability. (2) Association of new technologies with conventional antimicrobial agents as a feasible, sustainable, and multi-hurdle decontamination approaches. (3) Investigating more about how non-thermal technologies affect the sensory traits of eggs and egg products. (4) Exploring optimum conditions that can ensure consumer safety and egg quality for the non-thermal technologies. (5) Investigation of bacteriophage application as a processing aid to reduce and detect salmonella.