Microbial spoilage of egg and egg products
Eggs are a highly nutritious food that contains proteins, minerals, fats, iron, phosphorus vitamins (A, B, D, E, and K). They are also a source of choline, lutein, zeaxanthin, and selenium, which have various health benefits for the brain, eyes, heart, and immune system.
Eggs are a versatile and affordable food that can be prepared in many ways and used in various dishes. They are also a good source of energy, as they provide about 62.5 calories per medium egg.
The most common type of egg consumed by humans is the chicken egg. However, other types of eggs such as duck, quail, goose, and turkey eggs are also edible and have similar nutritional profiles.
The chicken egg consists of three main parts: the yolk, the albumen (or white), and the shell. Each part has different nutrients and functions.
- The yolk is the yellow part of the egg that contains about 50% of the protein and most of the fat and cholesterol. It also has lipoproteins, lipovitellins, lipovitellinin, vitamin A, vitamin D, vitamin E, vitamin K, iron, phosphorus, and choline.
- The albumen is the clear part of the egg that surrounds the yolk. It contains about 12% of the protein and most of the water. It also has ovalbumin, conalbumin, avidin, ovotransferrin, ovomucoid, ovomucin, lysozyme, riboflavin (vitamin B2), niacin (vitamin B3), pantothenic acid (vitamin B5), biotin (vitamin B7), folate (vitamin B9), vitamin B12, magnesium, potassium, sodium, and zinc.
- The shell is the hard outer layer of the egg that protects the contents from external contamination. It is made mostly of calcium carbonate on an organic matrix. It has pores that allow gas exchange between the egg and the environment. It also has two shell membranes that act as barriers against bacteria and other microorganisms.
The nutritional value of eggs can vary depending on factors such as the size of the egg, the breed of the hen, the feed of the hen, and the method of cooking. However, in general, eggs are considered a complete protein source that provides all nine essential amino acids that humans cannot synthesize.
Eggs can be part of a balanced diet that supports various aspects of health. However, some people may need to limit their intake of eggs or egg yolks due to medical conditions such as high cholesterol or diabetes. Therefore, it is advisable to consult a doctor or a nutritionist before making any changes to one`s diet.
Eggs are a highly nutritious food that contains proteins, minerals, fats, iron, phosphorus vitamins (A, B, D, E, and K). They also contain choline, biotin, iodine, selenium, and antioxidants such as lutein and zeaxanthin.
The eggs most commonly consumed by humans are the eggs of hens, ducks, and quails. The chicken’s egg, in particular, is very popular and consists of the yolk (30-33%), albumen (60%), and the shell (9-12%).
Egg yolk is about 50% of proteins of which the main ones are lipoproteins, lipovitellins, and lipovitellinin. It provides three-fourths of the calories, all of the fat-soluble vitamins (A, D, E and K), and all of the choline, lutein, and zeaxanthin. The yolk also provides most of the phosphorus, iron and folate and almost half of the protein and riboflavin.
Egg albumen is about 12% protein of which are ovalbumin (54%), Conalbumin (13%), Avidin (0.05%), ovotransferrin (10%), ovomucoid (11%), ovomucin (1.5-3%), and lysozyme (3.5%). The white (albumen) provides more than half of the total protein and riboflavin.
The shell of the egg is a rigid structure made largely of calcium carbonate on an organic matrix. Internally, there are two shell membranes and the inner one that acts as a barrier when an organism penetrates the shell.
Duck eggs are larger than chicken eggs and have a higher fat content. They have a richer flavor and are often used in baking. Duck eggs also have more protein, calcium, iron, potassium, and vitamin B12 than chicken eggs.
Quail eggs are smaller than chicken eggs and have a milder flavor. They have more protein, iron, riboflavin, and vitamin B12 than chicken eggs. Quail eggs also have less cholesterol and saturated fat than chicken eggs.
Other types of edible eggs include goose eggs, turkey eggs, ostrich eggs, emu eggs, guinea fowl eggs, pheasant eggs, and gull eggs. These eggs vary in size, color, taste, and nutritional value depending on the species of bird.
Eggs are usually sterile when they are laid by healthy hens, but they can become contaminated by various sources and causes after oviposition. Some of the common sources and causes of egg contamination are:
- Fecal matter of the chicken: Chickens can carry Salmonella bacteria in their intestines and shed them in their feces. These bacteria can contaminate the eggs when they come in contact with the shell or the cloaca during or after laying .
- Dirt or materials used for the cage or nest: Eggs can be exposed to dirt, dust, litter, feathers, or bedding materials that may harbor bacteria or mold spores. These contaminants can adhere to the shell or penetrate through the pores or cracks .
- Water used during washing of eggs: Eggs are often washed to remove visible dirt and reduce microbial load on the shell. However, if the water is not clean or properly chlorinated, it can introduce bacteria or mold spores into the eggs through the pores or cracks .
- Equipment used during processing and handling: Eggs are subjected to various processes such as grading, candling, cracking, pasteurizing, drying, or freezing. These processes involve equipment that can be a source of contamination if they are not properly cleaned and sanitized .
- Materials used to pack the eggs: Eggs are packed in cartons, trays, boxes, or bags that can be made of paper, plastic, foam, or wood. These materials can be contaminated with bacteria or mold spores during production, storage, or transportation .
- Temperature used during storage: Eggs are highly perishable and should be stored at low temperatures (40°F or below) to prevent microbial growth and spoilage. However, if eggs are exposed to high temperatures (above 90°F) or temperature fluctuations during storage, distribution, or retail, they can become more susceptible to contamination and spoilage .
- Presence of eggshell cracks or micro-cracks: Eggshells are not completely impermeable and have thousands of pores that allow gas exchange. However, these pores also provide an entry point for bacteria or mold spores to reach the egg contents. Moreover, eggshells can have cracks or micro-cracks that are not visible to the naked eye but can facilitate bacterial penetration .
These sources and causes of egg contamination can result in both external and internal spoilage of eggs by various microorganisms.
Eggs are perishable and must be stored properly to prevent spoilage and foodborne illness. Spoilage of eggs during storage can be caused by both microbial and non-microbial factors. Microbial spoilage occurs when bacteria, molds, or yeasts contaminate the egg and cause changes in its appearance, odor, flavor, or texture. Non-microbial spoilage occurs when physical or chemical changes affect the quality of the egg, such as loss of moisture, carbon dioxide, or nutrients.
Microbial spoilage of eggs can happen at any stage from production to consumption. The main sources of contamination are:
- The fecal matter of the hen or other animals
- The dirt or materials used for the cage or nest
- The water used during washing of eggs
- The equipment used during processing and handling
- The materials used to pack the eggs
- The temperature used during storage
- The presence of eggshell cracks or micro-cracks
The eggshell is the first line of defense against microbial invasion. It has pores that allow gas exchange but also provide entry points for microorganisms. The eggshell also has a thin protective layer called the cuticle that prevents moisture loss and bacterial attachment. However, the cuticle can be damaged by washing, handling, or aging, making the egg more susceptible to spoilage.
The egg white (albumen) is the second line of defense. It has a high water content (88%) and a low pH (7.6-9.5) that inhibit bacterial growth. It also contains antimicrobial proteins such as lysozyme, ovotransferrin, and ovomucin that can destroy or inhibit bacteria. However, the egg white becomes thinner and more alkaline as the egg ages, losing its protective properties.
The egg yolk is the most nutritious part of the egg and the most vulnerable to spoilage. It has a high fat content (32%) and a low water activity (0.86) that favor bacterial growth. It also contains iron and sulfur that can react with bacteria to produce off-odors and colors. The yolk is separated from the white by a membrane that prevents mixing and contamination. However, the membrane becomes weaker as the egg ages, allowing bacteria to reach the yolk.
The most common bacteria involved in egg spoilage are Gram-negative rods such as Pseudomonas, Proteus, Alcaligenes, Enterobacter, Serratia, and Citrobacter spp. These bacteria can produce enzymes that degrade the egg proteins and lipids, resulting in rotten eggs with foul smells and colors. Some bacteria can also produce gas that causes the egg to float or swell.
Other bacteria that can cause egg spoilage are Salmonella, Escherichia coli, Staphylococcus aureus, Bacillus cereus, Clostridium perfringens, and Listeria monocytogenes. These bacteria can cause foodborne illness if the eggs are consumed raw or undercooked.
Molds and yeasts can also cause egg spoilage, especially if the eggs are stored in humid conditions. The most common molds are Penicillium, Cladosporium, Alternaria, Aspergillus, and Rhizopus spp. These molds can produce spores that penetrate the shell and grow on the surface or inside the egg. They can cause discoloration, softening, or rotting of the egg.
Yeasts are less common than molds but can also cause spoilage of eggs. The most common yeasts are Candida, Saccharomyces, Torulopsis, and Rhodotorula spp. These yeasts can ferment the sugars in the egg and produce alcohol and carbon dioxide. They can cause bubbling, foaming, or souring of the egg.
Non-microbial spoilage of eggs is mainly caused by physical or chemical changes that occur during storage. These changes include:
- Loss of moisture: Water evaporates from the egg through the pores of the shell, resulting in weight loss and shrinkage of the contents.
- Loss of carbon dioxide: Carbon dioxide diffuses from the egg through the pores of the shell, resulting in an increase in pH and alkalinity of the white.
- Loss of nutrients: Nutrients such as vitamins and minerals leach from the egg through the pores of the shell or degrade over time.
- Mixing of contents: The white and yolk become less viscous and more fluid as the egg ages, allowing them to mix and form a homogeneous mass.
- Mottling of yolk: The yolk becomes discolored and mottled due to oxidation of iron and sulfur compounds.
- Development of air cell: An air space forms at the large end of the egg as the contents contract and pull away from the shell.
These changes affect the appearance, texture, flavor, and functional properties of the egg. However, they do not necessarily indicate spoilage or unsafe consumption. Eggs that have undergone non-microbial spoilage can still be used for cooking or baking, as long as they are not contaminated by microorganisms.
Prevention of spoilage
Spoilage of eggs during storage can be prevented or reduced by applying the following measures:
- Buy eggs only from reputable sources that follow good hygiene and quality standards.
- Check the eggs for cleanliness, cracks, and freshness before buying. Avoid eggs that are dirty, cracked, or have an off-odor.
- Store eggs in their original carton in the refrigerator at 40°F (4°C) or below. Do not wash or wipe the eggs before storing, as this can damage the cuticle and increase the risk of contamination.
- Use eggs within 3 weeks of purchase for best quality. Use hard-cooked eggs within 1 week after cooking. Use frozen eggs within 1 year.
- Cook eggs thoroughly to destroy any harmful bacteria that may be present. Do not eat raw or undercooked eggs or foods that contain them.
- Refrigerate leftover cooked egg dishes and use within 3 to 4 days. When refrigerating a large amount of a hot egg-containing dish, divide it into several shallow containers so it will cool quickly.
- Wash hands, utensils, equipment, and work surfaces with hot, soapy water before and after handling eggs or egg-containing foods.
By following these tips, you can ensure the safety and quality of eggs and egg products during storage.
Apart from microbial spoilage, eggs can also undergo non-microbial spoilage due to physical and chemical changes during storage. These include :
- Loss of moisture and weight: The water content and gases (such as carbon dioxide) get evaporated through the pores of eggshells, resulting in a decrease in weight and volume. This can affect the appearance and quality of eggs.
- Thinning of albumen: Due to the evaporation of water content, the albumen becomes thinner and less viscous. This reduces the ability of the egg white to form stable foams and gels, which are important for some culinary applications.
- Breaking of yolk membrane: The yolk membrane becomes weaker and more prone to rupture during storage. This causes the yolk to mix with the albumen, resulting in a mottled appearance and a loss of nutritional value. The yolk also becomes more susceptible to microbial spoilage when exposed to the albumen.
- Increase in pH: The loss of carbon dioxide through the pores of the shell leads to an increase in the pH of the albumen from acidic (6.0-6.5) to alkaline (7.6-9.5). This affects the functional properties of the egg proteins, such as solubility, coagulation, and denaturation. It also makes the egg more vulnerable to bacterial growth.
- Formation of air cell: The loss of carbon dioxide and water content through the shell creates an air space between the shell membranes at the large end of the egg. This is called the air cell. The size of the air cell increases with storage time and temperature. The air cell can be used as an indicator of egg freshness, as fresh eggs have smaller air cells than older eggs.
- Deterioration of shell quality: The shell quality can be affected by factors such as washing, handling, packaging, and storage conditions. The shell can become stained, cracked, or micro-cracked, which can compromise its protective function and allow microbial contamination. The shell can also lose its color and gloss due to chemical reactions with oxygen and light.
Non-microbial spoilage of eggs can be prevented or minimized by proper handling, washing, packaging, and storage techniques. Eggs should be stored in a clean refrigerator at a temperature of 40° F or below and used within 3 weeks for best quality. Eggs should also be stored in their original carton to prevent moisture loss and odor absorption.
To avoid physical damage to the shell, eggs should be handled gently and not stacked too high or too tightly. Eggs should also be washed with potable water at a temperature above 90°F and dried immediately before packing. Washing removes dirt and fecal matter from the shell surface, but it also removes the natural protective coating (cuticle) that prevents bacterial penetration. Therefore, washed eggs should be treated with an edible oil or wax to restore their cuticle.
Eggs can also be treated with various methods to extend their shelf life and prevent spoilage. These include pasteurization, fumigation, freezing, drying, irradiation, and coating . These methods aim to reduce or eliminate microbial contamination, inhibit enzymatic activity, or create a barrier against moisture loss and gas exchange . However, some of these methods may also affect the sensory and functional properties of eggs . Therefore, consumers should check the labels for information on how the eggs have been treated and how to store them properly.
Eggs are highly nutritious and versatile foods, but they are also very susceptible to microbial spoilage. Microbial spoilage of eggs can cause off-odors, off-flavors, discoloration, gas production, and even foodborne illness. Therefore, it is important to understand how eggs can be contaminated by microorganisms and how they can defend themselves against microbial invasion.
Eggs have several natural defense mechanisms that protect them from microbial spoilage. The first line of defense is the shell, which is a hard and porous structure made of calcium carbonate. The shell has about 17,000 tiny pores that allow air and moisture to pass through, but also serve as potential entry points for microorganisms. To prevent this, the shell has a thin outer coating called the cuticle, which blocks the pores and helps to preserve freshness and prevent contamination.
The second line of defense is the shell membranes, which are two transparent protein layers that lie between the shell and the albumen (egg white). The shell membranes are tough and resistant to bacterial penetration. They also have antimicrobial properties, such as lysozyme, an enzyme that can break down the cell walls of some bacteria.
The third line of defense is the albumen, which is the clear and viscous liquid that surrounds the yolk. The albumen is composed of about 90% water and 10% protein, mainly ovalbumin. The albumen has a low pH (around 9) and a high viscosity, which make it an unfavorable environment for microbial growth. The albumen also contains several antimicrobial proteins, such as ovotransferrin, which binds iron and deprives bacteria of this essential nutrient; avidin, which binds biotin and inhibits bacterial metabolism; and ovomucoid and ovoinhibitor, which inhibit proteases and prevent bacterial digestion of proteins .
The fourth line of defense is the vitelline membrane, which is a thin and transparent membrane that encloses the yolk. The vitelline membrane acts as a physical barrier that prevents the mixing of the yolk and the albumen. The yolk is the most nutritious part of the egg, containing proteins, fats, vitamins, minerals, and cholesterol. The yolk also has a low pH (around 6) and a high osmotic pressure, which inhibit bacterial growth. However, the yolk is also rich in iron and lipids, which can support the growth of some bacteria if they manage to reach it .
Despite these defense mechanisms, eggs can still be contaminated by microorganisms from various sources, such as fecal matter from the hen or the environment; dirt or materials used for the cage or nest; water used for washing or cooling; equipment used for processing or handling; packaging materials; temperature fluctuations during storage; or cracks or micro-cracks in the shell . Some of these microorganisms can resist or overcome the defense mechanisms of eggs and cause spoilage or infection. These include bacteria such as Salmonella spp., Pseudomonas spp., Proteus spp., Alcaligenes spp., Enterobacter spp., Serratia spp., Stenotrophomonas spp., Cloaca spp., Acinetobacter spp., Moraxella spp., Citrobacter spp., Flavobacterium spp., Cytophaga spp.; molds such as Penicillium spp., Cladosporium spp., Alternaria spp.; and yeasts such as Candida spp., Rhodotorula spp. .
Some of these microorganisms can cause visible changes in eggs, such as gas production (e.g., by Pseudomonas), yellow pigmentation of the shell membrane (e.g., by Flavobacterium or Cytophaga), green discoloration of the yolk (e.g., by Pseudomonas), black rotting of the albumen (e.g., by Proteus), pink discoloration of the albumen (e.g., by Serratia), pin-spot molding on the shell (e.g., by Penicillium), or fungal rotting of the whole egg (e.g., by Cladosporium) . Other microorganisms can cause invisible changes in eggs, such as loss of freshness, flavor, or nutritional value, or production of toxins or pathogens that can cause foodborne illness (e.g., by Salmonella) .
Therefore, it is essential to prevent or reduce microbial spoilage of eggs by applying good hygienic practices during production, processing, handling, and storage of eggs and egg products. These include keeping the hens and their environment clean and healthy; washing and sanitizing the eggs and the equipment; cooling and refrigerating the eggs promptly; avoiding cracks or damages to the shell; using appropriate packaging materials; and pasteurizing or cooking the eggs or egg products before consumption .
The microorganisms that cause egg spoilage can be classified into bacteria, molds, and yeasts. Most of them are aerobic and psychrotrophic, meaning that they can grow in the presence of oxygen and at low temperatures. Some of them can also produce enzymes that degrade the egg proteins and lipids, resulting in off-odors, colors, and flavors .
The bacteria that cause egg spoilage can be divided into two groups: those that contaminate the eggshell and those that penetrate the eggshell and reach the egg contents.
The bacteria that contaminate the eggshell are mainly Gram-positive cocci and bacilli, such as Staphylococcus, Streptococcus, Aerococcus, Bacillus, and Micrococcus. These bacteria can form biofilms on the eggshell surface and resist washing and disinfection . They can also cause yellow pigmentation of the shell membrane due to the production of carotenoids by Flavobacterium or Cytophaga species.
The bacteria that penetrate the eggshell and reach the egg contents are mainly Gram-negative rods, such as Pseudomonas, Proteus, Alcaligenes, Enterobacter, Serratia, Stenotrophomonas, Cloaca, Acinetobacter, Moraxella, and Citrobacter. These bacteria can produce sulfides, amines, ammonia, and other volatile compounds that cause rotten smells and tastes in eggs . They can also cause green or black discoloration of the egg yolk or albumen due to the production of hydrogen sulfide or iron sulfide by Proteus or Pseudomonas species.
Some bacteria can also cause foodborne illnesses by contaminating eggs. The most notorious ones are Salmonella enterica serovar Enteritidis and Typhimurium, which can cause salmonellosis in humans. These bacteria can infect the ovaries of hens and contaminate the eggs internally before they are laid. They can also survive pasteurization and refrigeration if the eggs are not properly handled .
Other bacteria that can cause foodborne illnesses by contaminating eggs are Escherichia coli O157:H7, Campylobacter jejuni, Listeria monocytogenes, Staphylococcus aureus, Clostridium perfringens, and Bacillus cereus. These bacteria can contaminate the eggs externally through fecal matter, soil, water, equipment, or human contact. They can also grow rapidly in eggs if they are stored at warm temperatures or for long periods .
The molds that cause egg spoilage are mainly filamentous fungi that belong to the genera Penicillium, Cladosporium, Alternaria, Aspergillus, Mucor, Rhizopus, and Fusarium. These molds can grow on the eggshell surface or inside the egg through cracks or pores. They can produce spores that spread easily through air or contact .
The molds that grow on the eggshell surface can cause pin-spot molding or fungal rotting. Pin-spot molding is characterized by small black or green spots on the shell that indicate early mold growth. Fungal rotting is characterized by large patches of mold growth that cover most of the shell surface and indicate advanced spoilage.
The molds that grow inside the egg can cause internal molding or mycotic infection. Internal molding is characterized by fuzzy growths of mold on the yolk or albumen that indicate moderate spoilage. Mycotic infection is characterized by liquefaction of the yolk or albumen and formation of gas bubbles that indicate severe spoilage.
Some molds can also produce mycotoxins that are harmful to humans and animals. The most common ones are aflatoxins produced by Aspergillus species and ochratoxins produced by Penicillium species. These mycotoxins can cause liver damage, kidney damage, immunosuppression, and cancer if ingested in large amounts .
The yeasts that cause egg spoilage are mainly unicellular fungi that belong to the genera Candida, Saccharomyces, Rhodotorula, and Cryptococcus. These yeasts can grow on the eggshell surface or inside the egg through cracks or pores. They can produce carbon dioxide and ethanol that cause fermentation and spoilage of eggs .
The yeasts that grow on the eggshell surface can cause surface yeast spoilage. This is characterized by a white or pink film on the shell that indicates mild spoilage.
The yeasts that grow inside the egg can cause internal yeast spoilage. This is characterized by a sour smell and taste, a cloudy appearance, and a slimy texture of the egg contents that indicate moderate to severe spoilage.
Some yeasts can also cause foodborne illnesses by contaminating eggs. The most common ones are Candida albicans and Cryptococcus neoformans, which can cause candidiasis and cryptococcosis in humans, respectively. These yeasts can infect the mucous membranes, skin, or internal organs of humans and cause symptoms such as itching, burning, inflammation, fever, headache, and meningitis .
Egg products are eggs that have been processed to remove the shell and are sold in liquid, frozen, or dried forms. They are widely used in the food industry for their functional properties such as foaming, binding, gelling, emulsifying, and thickening. Egg products are also used to make various food products such as sauces, pasta, biscuits, cakes, processed meats, fish products, milk products, etc.
Egg products are susceptible to microbial spoilage if they are not properly handled, processed, and stored. Microbial spoilage of egg products can result in off-odors, off-flavors, discoloration, gas production, and loss of functional properties. Microbial spoilage can also pose a health risk to consumers if pathogenic microorganisms are present.
The main factors that influence the microbial spoilage of egg products are:
- The quality and hygiene of the raw eggs
- The type and extent of processing
- The addition of preservatives or additives
- The packaging and storage conditions
- The exposure to contamination during handling and distribution
The quality and hygiene of the raw eggs are important to prevent the introduction of spoilage microorganisms into the egg products. Eggs should be collected, washed, graded, and stored under sanitary conditions. Eggs should also be free from cracks or defects that can allow the entry of microorganisms into the egg contents.
The type and extent of processing affect the microbial stability of egg products. Processing methods such as pasteurization, drying, freezing, and irradiation can reduce or eliminate the microbial load of egg products. However, some microorganisms may survive or grow after processing if the conditions are favorable. For example, psychrotrophic bacteria such as Pseudomonas spp. can grow in pasteurized liquid egg products stored at refrigeration temperatures . Dried egg products are not susceptible to microbial spoilage due to low water activity , but they can be recontaminated by moisture or microorganisms during reconstitution or handling.
The addition of preservatives or additives can enhance the shelf life and quality of egg products by inhibiting the growth of spoilage microorganisms. Some common preservatives or additives used in egg products are salt, sugar, citric acid, sodium benzoate, potassium sorbate, nisin, lysozyme, etc . However, some preservatives or additives may also affect the functional properties or sensory attributes of egg products.
The packaging and storage conditions are crucial to prevent the microbial spoilage of egg products. Egg products should be packaged in airtight containers that prevent oxygen and moisture ingress and protect from light exposure. Egg products should also be stored at appropriate temperatures that prevent the growth of spoilage microorganisms. For example, liquid egg products should be stored at 4°C or below for up to 10 days, frozen egg products should be stored at -18°C or below for up to 12 months, and dried egg products should be stored at 10°C or below for up to 24 months.
The exposure to contamination during handling and distribution can compromise the microbial quality of egg products. Egg products should be handled with care and hygiene to avoid cross-contamination with other foods or sources of microorganisms. Egg products should also be transported and distributed under controlled temperature and humidity conditions to prevent microbial spoilage.
In conclusion, microbial spoilage of egg products is a major concern for the food industry as it affects the quality, safety, and functionality of these products. Therefore, proper hygiene practices, processing methods, preservation techniques, packaging materials, storage conditions, and handling procedures should be followed to ensure the microbial stability of egg products.
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