Preservation of egg and egg products from microbial spoilage

Eggs are a highly nutritious food that contains proteins, minerals, fats, iron, phosphorus vitamins (A, B, D, E, and K) needed by human beings. The fully mixed egg contains about 65% water, 12% proteins, and 11% fat. These various nutrient contents present in eggs make it an excellent source for bacterial microflora, including pathogenic bacteria. Thus, various preservation methods are used to eliminate the growth of spoilage-causing microorganisms.

Eggs are a low-carb food, providing less than 1 gram of carbohydrate in one large egg. They have a tiny amount of sugar and no fiber. Eggs are a good source of high-quality, complete protein. Most of it is found in the egg white: There are 4 to 5 grams protein, 17 calories, and virtually no fat in a single large egg white. Egg whites are also a good source of leucine, an amino acid that may help with weight loss.

Eggs provide important vitamins and minerals. They contain vitamin D (important for the absorption of calcium), phosphorus, vitamin A (for healthy vision, skin, and cell growth), and two B-complex vitamins that your body needs to convert food into energy. Eggs are also a very good source of riboflavin, selenium, and choline. Choline supports memory and mood.

Eggs also contain antioxidants such as lutein and zeaxanthin, which are beneficial for eye health and may protect against age-related macular degeneration.

While eggs do contain saturated fat, they also provide both polyunsaturated fat and monounsaturated fat, which are considered "good" fats because they have been shown to be helpful in lowering your LDL or "bad" cholesterol and boosting heart health. The American Heart Association recommends limiting saturated fats to about 13 grams per day if you typically consume about 2,000 calories daily.

Research on moderate egg consumption in two large prospective cohort studies (nearly 40,000 men and over 80,000 women) found that up to one egg per day is not associated with increased heart disease risk in healthy individuals. However, people who have difficulty controlling their total and LDL cholesterol may also want to be cautious about eating egg yolks and instead choose foods made with egg whites. The same is true for people with diabetes.

Given their high nutritional value and potential health benefits, eggs are a valuable food to include in a balanced diet. However, eggs are also highly perishable and susceptible to microbial contamination. Therefore, proper handling and preservation methods are essential to ensure the safety and quality of eggs and egg products.

Egg cleaning methods are the first step to prevent microbial spoilage of eggs and egg products. Egg cleaning methods aim to remove dirt, fecal matter, and other contaminants from the surface of the eggshell without damaging the cuticle or the internal quality of the egg. Egg cleaning methods can be classified into two categories: dry and wet cleaning.

Dry cleaning

Dry cleaning is a mechanical method that involves brushing, scraping, sanding, or vacuuming the surface of the eggshell to remove any adhering material. Dry cleaning is preferred for eggs that have little or no visible dirt and are intended for long-term storage. Dry cleaning does not require water or chemicals, and thus reduces the risk of introducing moisture or contaminants into the egg. Dry cleaning also preserves the natural cuticle of the eggshell, which acts as a barrier against microbial invasion. However, dry cleaning may not be effective for removing all types of dirt and may cause cracks or abrasions on the eggshell if done too roughly.

Wet cleaning

Wet cleaning is a method that involves spraying, washing, or immersing the eggs in water or a solution of water and chemicals to remove dirt and microorganisms from the eggshell surface. Wet cleaning is more effective than dry cleaning for removing organic matter and reducing bacterial load on the eggshell. Wet cleaning also improves the appearance and quality of the eggs by removing stains and odors. However, wet cleaning requires careful control of the water temperature, pressure, pH, and sanitizing agents to avoid damaging the cuticle or causing thermal shock to the egg. Wet cleaning also increases the moisture content and permeability of the eggshell, which may facilitate microbial growth and spoilage if not properly dried and stored.

Some examples of wet cleaning methods are:

• Water spraying: This method involves spraying eggs with water at a temperature of at least 35°C (95°F) and a pressure of 1.4 to 2.8 kg/cm2 (20 to 40 psi) for about 3 seconds. The water should be potable and free of chlorine or other chemicals that may affect the egg quality. The eggs should be dried immediately after spraying to prevent moisture absorption.
• Water washing: This method involves washing eggs in a continuous or batch washer with water at a temperature of 43 to 49°C (110 to 120°F) and a pressure of 2.8 to 4.2 kg/cm2 (40 to 60 psi) for about 30 seconds. The water should be changed frequently and treated with a sanitizing agent such as chlorine (50 to 200 ppm), quaternary ammonium compounds (100 to 200 ppm), or hydrogen peroxide (100 to 500 ppm) to reduce microbial contamination. The eggs should be rinsed with potable water and dried thoroughly after washing.
• Immersion: This method involves immersing eggs in a tank or a basket with water at a temperature of 43 to 49°C (110 to 120°F) and a sanitizing agent such as chlorine (50 to 200 ppm), quaternary ammonium compounds (100 to 200 ppm), or hydrogen peroxide (100 to 500 ppm) for about 5 minutes. The water should be agitated gently to remove dirt and microorganisms from the eggshell surface. The eggs should be drained and dried well after immersion.

Use of preservatives

Preservatives are substances that are added to foods or applied on their surfaces to inhibit or reduce the growth of microorganisms that cause spoilage or foodborne illnesses. Preservatives can be natural or synthetic, and they can act by various mechanisms, such as lowering the pH, reducing the water activity, disrupting the cell membrane, or interfering with the metabolism of the microbes. Some of the preservatives that are used for egg and egg products are:

1. Immersion in liquids: This method involves submerging the eggs in solutions that contain preservatives such as lime water, sodium silicate (water glass), borates, permanganates, benzoates, salicylates, formats, etc. These solutions can create a protective layer on the eggshell that prevents the loss of moisture and carbon dioxide, and also blocks the entry of oxygen and bacteria. Immersion in liquids can extend the shelf life of eggs for 2-3 months at normal temperature.
2. Coating: This method involves applying a thin layer of substances such as oil, wax, dimethylolurea, chitosan, starch cassava, yam, whey protein concentrate, soy protein isolate, etc. on the surface of the eggshell. These substances can also prevent the loss of moisture and carbon dioxide, and also inhibit the growth of mold and bacteria on the eggshell. Coating can extend the shelf life of eggs for 3-4 months at normal temperature.
3. Natural preservatives: These are substances that are derived from natural sources such as plants or animals, and have antimicrobial properties. Some examples of natural preservatives that are used for egg and egg products are nisin and lysozyme. Nisin is a bacteriocin produced by Lactococcus lactis subsp. lactis that can inhibit the growth of Gram-positive bacteria such as Listeria monocytogenes and Clostridium botulinum. Nisin is added to liquid eggs before pasteurization to enhance their safety and quality. Lysozyme is an enzyme found in hen eggs that can hydrolyze the peptidoglycan layer of bacterial cell walls, especially Gram-positive bacteria such as Bacillus cereus and Staphylococcus aureus. Lysozyme is incorporated into coating materials used to seal eggs to extend their shelf life.

Thermal processing methods involve applying heat to eggs or egg products to reduce or eliminate microbial contamination and extend their shelf life. The main thermal processing methods are:

• Pasteurization: This is the process of heating eggs or egg products to a specific temperature and time to kill pathogenic bacteria such as Salmonella. Pasteurization can be done by immersing eggs in hot water (62°C for 3 min or 64°C for 2 min) or by spraying hot water on the egg surface. Pasteurization can also be applied to liquid eggs before or after packaging, using different time-temperature combinations (e.g., 60°C for 3.5 min or 71.1°C for 3.5 s). Pasteurization preserves the quality and functionality of eggs and egg products and prevents spoilage and foodborne illnesses.
• Steam treatment: This is a method of decontaminating the egg surface by using a steam gun that sprays hot steam on the shell. The steam treatment can reduce the number of Salmonella enteritidis and Salmonella typhimurium on the eggshell without affecting the egg quality. The steam treatment can be done at 60°C for 8 s followed by cold air treatment at 20-25°C for 32 s.
• Thermostabilization: This is a method of dipping eggs into hot water to reduce the evaporation of moisture and gases from the egg. The thermostabilization can slightly coagulate the outermost part of the egg albumen, which forms a protective layer that prevents the loss of moisture and gases and the penetration of oxygen and bacteria. The thermostabilization can be done at 57-58°C for 10 min or at 60°C for 5 min.
• Preservation by drying: This is a method of removing moisture from liquid eggs to reduce their water activity and inhibit microbial growth. Before drying, liquid eggs are usually treated to remove glucose, which can cause browning and loss of functionality during drying. Drying can be done by spray drying, where liquid eggs are sprayed into a current of dry, heated air, or by roller or drum drying, where liquid eggs are passed over a heated drum. The moisture content of dried eggs can be reduced to 1% or less, which allows them to be stored at room temperature for long periods.

Low temperature methods are based on the principle of slowing down the metabolic activity and growth of microorganisms by reducing the temperature of the eggs or egg products. There are two main low temperature methods: chilling and freezing.

a. Chilling

Chilling is the most common method used for egg preservation. During chilling, the temperature and relative humidity play an important role. Eggs are stored at 4°C for short-term storage (2-3 weeks) with a relative humidity of 60-70%. The egg is stored at -1.7 ° to -0.55°C and relative humidity of 70-80 % and it can be stored for 6 months.

Chilling helps to maintain the quality and safety of eggs by preventing moisture loss, carbon dioxide loss, albumen thinning, yolk flattening, microbial growth and spoilage. However, chilling may also cause some undesirable changes such as increased pH, decreased foaming ability, increased susceptibility to cracking and reduced flavor.

b. Freezing

Freezing is another method of preserving eggs or egg products by lowering their temperature below their freezing point. Freezing can extend the shelf life of eggs up to a year or more. For freezing, first, the eggs are selected by candling and then rinsed with 200 to 500 ppm chloride. The eggs are stored at -17.8 to -20.5°C with a relative humidity of 85-90%.

Freezing can preserve the nutritional and functional properties of eggs or egg products by inhibiting microbial growth, enzymatic activity and chemical reactions. However, freezing may also cause some quality deterioration such as ice crystal formation, protein denaturation, lipid oxidation and color change.

Freezing can be applied to whole eggs, egg yolks, egg whites or blended eggs. Whole eggs are frozen after breaking and mixing the yolk and white. Egg yolks are frozen after adding salt or sugar to prevent gelation. Egg whites are frozen without any additives. Blended eggs are frozen after adding salt, sugar or other ingredients to improve their functionality.

1. Ultrasound method

Ultrasound is a high-power sound wave at frequencies between 16 kHz and 100 MHz that can be used to reduce microbial contamination in egg and egg products. Ultrasound can cause cavitation, which is the formation and collapse of gas bubbles in the liquid, resulting in mechanical and thermal effects that can damage the cell membranes of microorganisms.

Ultrasound can be applied to shell eggs or liquid eggs to enhance their functional properties and extend their shelf life. For shell eggs, ultrasound treatment with different power levels (200 W, 300 W, 450 W) and treatment times (2 min and 5 min) was evaluated for preserving eggs at 24 °C for 6 weeks. The results showed that ultrasound treatment improved the foaming and emulsifying properties of eggs, reduced the total bacterial count and Salmonella enteritidis, and maintained the quality parameters such as Haugh unit, yolk index, and albumen pH.

For liquid eggs, ultrasound can be combined with other preservation methods such as pasteurization, freezing, or high-pressure processing to achieve a synergistic effect. For example, ultrasound combined with pasteurization can reduce the thermal damage to egg proteins and improve the gelation properties of liquid eggs. Ultrasound combined with freezing can increase the ice nucleation rate and reduce the ice crystal size, resulting in better texture and water-holding capacity of frozen-thawed eggs. Ultrasound combined with high-pressure processing can enhance the microbial inactivation and prevent the browning reaction of liquid eggs.

Ultrasound is a novel and innovative processing method for egg and egg products preservation that has many advantages such as being non-thermal, non-chemical, energy-efficient, and environmentally friendly. However, more research is needed to optimize the ultrasound parameters and evaluate the sensory and nutritional quality of ultrasound-treated eggs.

Salmonella enteritidis is a common pathogen that can cause foodborne illness from consuming contaminated eggs. It can survive the conventional pasteurization process and can grow during storage. Therefore, alternative methods are needed to inactivate this bacterium in shell eggs without affecting the quality and functionality of the egg products.

One of the methods that has been studied is the microwave heating. Microwave heating is a process that uses electromagnetic waves to generate heat within the food by inducing molecular vibrations and rotations. The advantages of microwave heating are that it is fast, uniform, selective, and energy-efficient.

However, microwave heating also has some challenges and limitations. For example, it can cause hot spots, cold spots, and non-uniform heating due to the differences in dielectric properties of food components. It can also cause thermal degradation of nutrients and functional properties of egg proteins. Moreover, it can affect the appearance and texture of the egg products by causing coagulation, browning, and shrinkage.

Therefore, to optimize the microwave heating process for reducing Salmonella enteritidis in shell eggs, several factors need to be considered, such as:

• The power level and exposure time of the microwave
• The initial temperature and size of the eggs
• The position and orientation of the eggs in the microwave oven
• The presence of water or other substances to enhance heat transfer
• The post-microwave cooling and storage conditions

According to some studies, microwave heating can achieve a 5-log reduction of Salmonella enteritidis in shell eggs within 2 to 4 minutes at a power level of 800 to 1000 W. However, this also causes significant changes in the egg quality, such as:

• The yolk becomes firm and rubbery
• The white becomes opaque and gel-like
• The shell becomes brittle and cracked
• The color becomes darker and brownish
• The odor becomes sulfurous and unpleasant

To minimize these quality changes, some strategies have been suggested, such as:

• Reducing the power level and increasing the exposure time
• Preheating the eggs to 45°C before microwave heating
• Placing the eggs in a water bath or a plastic bag during microwave heating
• Rotating or stirring the eggs during or after microwave heating
• Cooling the eggs rapidly after microwave heating

In conclusion, microwave heating is a potential method for reducing Salmonella enteritidis in shell eggs, but it requires careful optimization and control to ensure the safety and quality of the egg products.

Freeze-drying or cryodesiccation is a method of preserving liquid eggs that involves rapid freezing and dehydration. The liquid eggs are first pasteurized to eliminate any microorganisms and then frozen at very low temperatures (−50 to −60°C) in a vacuum chamber. The frozen water in the eggs sublimates, meaning it changes directly from solid to gas without passing through the liquid phase. This process removes about 95% of the water content from the eggs, leaving behind a dry and porous powder.

The advantages of freeze-drying liquid eggs are:

• It preserves the nutritional quality, flavor, color, and texture of the eggs.
• It reduces the weight and volume of the eggs, making them easier to store and transport.
• It extends the shelf life of the eggs up to several years at room temperature without refrigeration.
• It prevents the growth of spoilage-causing microorganisms and enzymatic reactions in the eggs.
• It allows for easy reconstitution of the eggs by adding water when needed.

The disadvantages of freeze-drying liquid eggs are:

• It is a costly and energy-intensive process that requires specialized equipment and facilities.
• It may cause some loss of volatile compounds and vitamins in the eggs during sublimation.
• It may result in some changes in the functional properties of the egg proteins due to denaturation or aggregation.

Freeze-drying or cryodesiccation is a suitable method for preserving liquid eggs that can be used for various applications such as baking, confectionery, sauces, soups, and instant foods. Freeze-dried eggs have a high demand in the food industry as well as in emergency situations where refrigeration is not available.

• PEF is a non-thermal food preservation technology that uses short pulses (1-100 µs) of high electric fields (10-80 kV/cm) to inactivate microbial populations in foods .
• PEF can be applied to liquid whole eggs, egg yolk, egg white and egg powder to reduce the activity of various microorganisms such as Salmonella, Listeria, Escherichia coli, Bacillus cereus and molds .
• PEF has minimal or no detrimental effect on the quality attributes of egg products such as color, flavor, texture, functionality and nutritional value .
• PEF can also enhance the antioxidant activity and bioavailability of some bioactive compounds present in egg products such as carotenoids and peptides.
• PEF requires less energy and water than thermal pasteurization and can contribute to the sustainability of food production.
• PEF processing parameters such as electric field intensity, pulse duration, pulse frequency, treatment time, temperature and pH need to be optimized for different egg products to achieve the desired microbial reduction and quality preservation .

Ozone (O3) is a triatomic form of oxygen that has been gaining space in food processing due to its high sanitizing power and is also known as a highly reactive antimicrobial agent. Ozone can be used to disinfect the surface of eggs and also to treat the liquid egg products.

Ozone can be applied to eggs in different ways, such as:

• Spraying ozonated water on the egg surface
• Immersing eggs in ozonated water
• Exposing eggs to gaseous ozone
• Injecting ozone into liquid egg products

The advantages of using ozone for egg preservation are:

• Ozone can effectively reduce the microbial load on the egg surface and in the liquid egg products, including spoilage and pathogenic microorganisms such as Salmonella, Escherichia coli, Listeria monocytogenes, and Bacillus cereus.
• Ozone can also inhibit the enzymatic activity and lipid oxidation in eggs, thus preserving the nutritional and functional properties of eggs.
• Ozone can extend the shelf life of eggs and egg products by several weeks or months, depending on the concentration, exposure time, and storage conditions.
• Ozone is environmentally friendly and leaves no harmful residues in eggs or egg products, as it decomposes into oxygen and water.

The challenges of using ozone for egg preservation are:

• Ozone can cause changes in the color, flavor, and texture of eggs and egg products if used at high concentrations or for long exposure times.
• Ozone can also affect the functional properties of eggs and egg products, such as foaming, emulsifying, and gelation abilities, by modifying the protein structure and interactions.
• Ozone can be toxic to humans and animals if inhaled at high concentrations or for long periods of time, so proper safety measures are required when using ozone in food processing.

Therefore, ozone is a promising method for preserving eggs and egg products from microbial spoilage, but it requires optimization of the process parameters and evaluation of the quality attributes of the treated products.

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