Non-thermal egg processing to extend shelf life
Non-thermal technologies, including high hydrostatic pressure, pulsed electric fields, ultrasound, pulsed light, cold plasma technology and ozonation, are effective methods for controlling microbial growth and to preserve the quality of fresh eggs and their derived products. The effect of non-thermal technologies on the sensory attributes of eggs and egg products is constantly improving.
The egg is a growth medium for various micro-organisms owing to the presence of water, lipids and proteins. Thus, inadequate food safety management can result in serious public health problems. The mandatory high temperatures used during egg pasteurisation can be detrimental to the functional properties, nutritional value and sensory qualities of eggs.
Therefore, to extend egg shelf life, it is necessary to use novel non-thermal technologies, such as high hydrostatic pressure, pulsed electric fields, ultrasound, pulsed light, cold plasma technology or ozonation, which have minimal impact on the original properties of food.
High hydrostatic pressure technology
With high hydrostatic pressure technology, the food is compressed at high pressure inside a machine comprising a cylinder with sealed closures. The machine has a high-pressure intensifier pump, a monitoring system and a product-handling device. This device applies water pressure to food which damages the cell membrane and proteins, and disrupts cell function, thus deactivating harmful micro-organisms.
The high hydrostatic pressure technology also affects the spores of micro-organisms and blocks their ability to return to the vegetative state. Research has shown that applying high hydrostatic pressure technology for 3-17 minutes reduces Salmonella enterica serovar Enteritidis counts in liquid eggs.
However, this method affects egg colour, increases viscosity and may induce reversible modifications in egg white and yolk proteins. In duck eggs, the application of high pressure for 10 minutes aggregates egg white proteins and reduces the solubility of yolk proteins.
The use of high hydrostatic pressure technology in combination with a mild thermal treatment, reduces egg microbial count and preserves the functional properties of eggs but increases egg yolk viscosity. Combining high hydrostatic pressure with 2% triethyl citrate improves the quality of processed eggs and reduces the microbial load, as well as the processing temperature required for thermal pasteurisation.
Pulsed electric field technology
Pulsed electric field technology is a non-thermal processing method used to treat liquid food products, such as eggs. The pulsed electric field system consists of a treatment chamber, a pulse generator, a fluid-handling system and monitoring devices. With this method, electric fields are applied to eggs in the treatment chamber for short time periods.
The use of pulsed electric field technology disrupts micro-organisms’ cell membrane and causes leakage of intracellular compounds and cellular lysis. Research has shown that pulsed electric field technology inactivates micro-organism spores and reduces Salmonella enterica serovar Enteritidis counts in eggs. A combination of pulsed electric field technology together with heating to to 60°C reduces E. Coli counts in eggs. This technology, plus heat treatment with lemon essential oil, also reduces the population of Salmonella enterica and Listeria monocytogenes in eggs.
Pulsed electric field technology preserves egg lipoproteins and egg colour, and improves foaming capacity and water-soluble protein content compared to thermal pasteurisation. Although if the processing time exceeds 600 microseconds, the soluble protein content decreases while insoluble proteins, mainly lysozyme, ovalbumin and ovotransferrin, aggregate.
Ultrasound technology
With ultrasound technology, sound waves at frequencies of 20 kHz or above generate energy. The ultrasonic waves travel through a liquid medium and create cavitation bubbles which release energy and create high-temperature areas. During treatment, the bubbles burst and causing enzyme destruction and cellular death. The application of ultrasound technology reduces Salmonella enterica serovar Enteritidis in liquid whole eggs after 30 minutes of treatment.
Combining ultrasound technology with antimicrobial enzyme lysozyme inactivates Salmonella typhimurium in liquid whole eggs. Furthermore, using ultrasound technology for 30 minutes removes E. Coli completely from raw eggshells and increases eggshell strength, albumen and yolk heights, as well as Haugh units. However, this treatment decreases yolk width and albumen luminosity. Ultrasound can also be used to evaluate the freshness of poultry raw eggs in storage.
Pulsed light technology
Pulsed light technology uses the emission of high voltage electrical pulses, comprising ultraviolet, visible and near-infrared wavelength light, for a period of a few seconds. The emission of light pulses rich in ultraviolet photons breaks organic molecular bonds, changes the DNA structure and inactivates micro-organisms in eggs.
The use of pulsed light technology reduces the microbial population in eggs and egg products, such as powdered eggs, hard-boiled peeled eggs and raw eggshells. Treating eggs with pulsed light removes E. Coli and Salmonella enterica serovar Enteritidis and has no negative impact on the functional properties of the egg whites, such as foaming ability and stability.
Cold plasma technology
Cold plasma technology creates pores in the cell membrane which causes the leakage of intracellular compounds and cell death. Cold plasma technology enhances the microbiological safety of fresh eggs by reducing the Salmonella enterica serovar Enteritidis population. Cold plasma technology can be used in combination with essential oils to remove Salmonella from eggshells.
Ozonation technology
Ozone is generated through corona discharge, ultraviolet radiation and electrolysis. Ozone inactivates bacteria and their spores, fungi, viruses and protozoa, by damaging the cell membrane. Exposing fresh eggs to 6 ppm ozone for 4 minutes reduces Enterobacteriaceae, Staphylococci, yeast and mould populations. In addition, ozonation technology decreases Salmonella enterica serovar Enteritidis in raw eggshells and has no negative impact on weight, pH of the yolk and albumin, and the colour of treated eggs.
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