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.
1. Egg cleaning
The egg is sterile when laid, they are contaminated after they are laid so the first method to limit microbial contamination is egg surface cleaning.
The egg is cleaned by two methods
- Dry cleaning: In this method, the dirt and fecal matter attached to the surface of eggs are removed mechanically.
- Wet cleaning: In this method, the water of at least 35°C is sprayed on the egg to removed surface dirt.
2. Use of preservatives
a. Immersion on liquids
- The eggs are immersed in lime water for 24 hours to get an effective result and the eggs can be kept for 2-3 months at normal temperature by this preservation method.
- Immersion in the water glass, a 10% solution of sodium silicate has been a successful method of home preservation.
- Other chemical preservatives used in which eggs are immersed are borates, permanganates, benzoates, salicylates, formats, etc.
- The chemical preservatives such as a solution of hypochlorites, acids, formalin, quaternary ammonium compounds, and detergent-sanitizer combinations have been used for eggshell surface sterilization.
- Different substances are applied on the surface of the shells of the eggs that form a thin coating
- This coating substance helps to prevent loss of moisture and CO2 gases
- These substances also help to prevent oxygen and bacterial penetration.
- Dimethylolurea has been found an effective coating that inhibits mold growth.
- Oil and wax are mainly used to coat the surface of the shells of eggs.
- Other biodegradable coating materials used are chitosan, starch cassava, and yam, whey protein concentrate, soy protein isolate, etc.
c. Natural preservatives
- Nisin is a multifactorial bacteriocin produced by Lactococcus lactis subsp. Lactis and is added to liquid eggs before the pasteurization.
- Lysozyme (muramidase or N-acetylmuramic hydrolase) is an antimicrobial enzyme naturally found in the hen egg. This lysozyme is incorporated into coating materials used to seal eggs to extend the shelf–life of fresh eggs.
3. Thermal processing
a. Pasteurization of eggs
- The eggs are pasteurized by immersing in hot water for a specific time (62°C for 3 min or 64°C for 2 min).
- The Salmonella typhimurium is killed after the pasteurization
- Pasteurization also helps to preserve eggs by maintaining the quality of egg and also kill microorganisms present on the surface of eggs shells.
- Pasteurization is also applicable for egg yolk, egg white, and whole egg liquid sterilization.
b. Steam method
- Steam treatment is used to decontaminate egg surface by using a steam gun.
- Whole eggs are heat-treated by using steam generators at 60°C for 8 seconds followed by cold air treatment of 20-25°C for 32 seconds.
- This method is effective against Salmonella enteritidis and Salmonella typhimurium in eggshells without affecting the egg quality.
- In this method, the eggs are dipped into hot water to reduce the evaporation of moisture and gases from the egg.
- The outermost part of egg albumen is slightly coagulated which helps in the prevention of loss of moisture and gases.
d. Preservation by drying
- Before drying of liquid eggs, the glucose is removed.
- Dried eggs are produced by using:
Spray drying methods where the liquid is sprayed into a current of dry, heated air.
Roller or drum method where a liquid egg is passed over a heated drum.
- The moisture of the egg is decreased to 1 % by this process that helps to reduce microbial contamination.
4. Use of low temperature
- This 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.
- 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%.
- Ultrasonic is a high-power sound wave at frequencies between 16 kHz and 100 MHz.
- In this method, the sonic wave is passed through the egg liquid and the changes occur in the pressure which leads to cavitation, which causes gas bubbles in the liquid causing a bactericidal effect.
6. Microwave method
- Microwaves used in the food industry for heating are of frequencies ranges from 300 MHz to 300 GHz.
- Microwave heating is a method in which electromagnetic waves are used to generate heat in food.
- Electromagnetic waves can reduce Salmonella enteritidis, which is often found in shell eggs
7. Freeze-drying or cryodesiccation
- In this process, the liquid eggs that are previously pasteurized are rapidly frozen at −50 to −60°C followed by dehydration.
- The water contained in the egg passes directly from the solid state to the vapor state by sublimation.
8. High-pressure process (HPP)
- It is a non – thermal pasteurization process in which food is subjected to high pressure in the region of 3300 -600 MPa for about 10 minutes.
- The application of 600 Mpa for a 2 minutes cycle in boiled eggs was able to extend the shelf life of these products during refrigeration.
- HHP at a pressure between 200 and 350 MPa can be used for liquid eggs as did not cause detectable protein denaturation.
9. Pulse Electric Field (PEF)
- The pulsed electric field is one of the non-thermal food preservation technologies in which food is subjected to short pulses (1-100 µs) of high electric fields with a duration of nano to milliseconds and intensity of 10 – 80 kV/cm to foods placed between two electrodes.
- The application of PEF in egg products has been found to control spoilage or pathogenic microorganisms.
- 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.
- O3 between 4 and 6 mg.L−1 could be used to maintain the internal quality of the eggs and extend their shelf life.
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