Abstract
Electron-beam or E-Beam Radiation is a non-thermal method of food processing. It does not alter the physical properties of food products when an appropriate dose is applied. There is a rising global demand for fresh produce, ensuring its safety and quality and robust processing methods are developed to meet customer requirements. Food irradiation is one of the extensively researched processing technologies which has been considered safe and effective for the preservation of food by International Organizations such as FAO, WHO, USFDA and Codex Alimentarius. The E- beam irradiation technology generates high-energy electrons using compact equipment called linear accelerators. E-beam irradiation is also known as electronic pasteurization or cold pasteurization. E-beam irradiation is widely explored in the food industry, particularly in the processing of milk, meat, fruits and vegetables, including the food packaging industry. Hence, this article delivers an insight into the application of E- beam irradiation technology in food processing.
Introduction
Radiation is energy that moves from one place to another in a form that can be described as waves or particles. Irradiation is the process by which an object is exposed to radiation or ionizing energy. Based on the energy of the radiated particles, radiation is classified into two types: Ionizing and Non-ionizing radiation. The former is a high-energy radiation that produces ions upon interaction with the molecules, so that the molecules become charged or ionized. It could be due to the presence of unstable atoms with excess energy. They release this excess energy in the form of radiation and return to a stable state. The latter is a form of radiation with less energy than ionizing radiation. Food irradiation (the application of ionizing radiation to food) is a technology that improves the safety and extends the shelf life of foods by reducing or eliminating microorganisms and insects. Like pasteurizing milk and canning fruits and vegetables, irradiation can make food safer for the consumer.
Gamma rays and E-beams are two types of ionizing radiation commonly employed in food processing and are used to produce the desired changes in food products. These technologies differ in how they generate the radiation, their energies, depth of penetration and the dose level. Gamma rays are emitted by radioactive elements like Cobalt 60 and Caesium 137. The gamma rays have a more profound penetrating ability. At present, the dose of radiation recommended by the FAO/WHO Codex Alimentarius Commission for use in food irradiation should not exceed 10 kGy.
An E-beam is a stream of high-energy electrons which are negatively charged. E- beam irradiation is a non-thermal process that is used to extend the shelf life and ensure the safety of the food products and does not alter the characteristics of food when an appropriate dose is applied. It is also known as cold pasteurization or electronic pasteurization, as the food is processed under low temperatures, which can significantly inactivate the microorganisms in food, thereby maintaining its nutritional quality and characteristics. E-beam irradiation does not leave any chemical residues.
Although E-beam irradiation technologies employ ionizing radiation, the radiation is not produced by radioactive materials. Rather, it is generated by specialized equipment called industrial electron accelerators. These accelerators have switch-on/switch-off technologies that can be turned off when not in use. In contrast, radioactive sources, such as cobalt-60 and cesium-137 cannot be switched off and generate gamma radiation continuously, since they are undergoing natural radioactive decay. The ability to switch the radiation source on and off has major implications in terms of operating costs, worker safety and the carbon footprint of an E-Beam food processing facility.
The E-beams are of high intensity and have good penetrability. The advantage of using this technology is the faster operation time, no nuclear waste and it can be applied in a bidirectional manner, such that the irradiation can come into contact with food products from the top and bottom of the sample. Utilizing this technology helps to deliver safe and wholesome foods when the proper dose is applied. E-beam technology yields the highest returns if high-quality products are treated at lower doses. However, it has a limitation of poor penetration power, i.e., it can penetrate to a depth of 2.5 to 5 cm. In addition, it consumes high power and needs proper maintenance. (Annamalai et al., 2020)
Working Principle of E-Beam Irradiation Technology
The process of exposing the food material to a beam of electrons produced from commercial electricity in a vacuum environment is called E-beam Irradiation (Figure 1). It is a process of generating electrons from a cathode. In 1933, van de Graff proposed the first electron accelerator, wherein, a stream of electrons from a heated cathode was accelerated through an evacuated tube between the cathode and anode with an energy input of 4 Mev (Sridhar & Bhat, 2008). The electrons are fired from an electron gun (Figure. 2), creating a beam of electrons.
The E-beam is carried across a radiofrequency wavelength in a linear accelerator, which has positive as well as negatively charged cavities that increases the speed of the beam travelling across the radiofrequency wavelength through the accelerator. The speed of an electron through the linear accelerator is increased to 99.99% speed of light at energies not to exceed 10 MeV (10 million electron volts), which breaks the molecular or atomic bonds releasing free electrons and ions that react with other charged molecules, to release secondary ions, (Clemmons et al., 2015).
The beam of excited electrons passes through a scanning magnet as the pulsed beam exits the linear accelerator; at this time, the E-beam pulse is sized and oriented in a specific pattern. The scanning magnet scans the pulsed beam through a 48-inch wide scan horn. The scanned E- beam exits the LINAC (linear accelerator) system through the scan horn’s window, after which the beam cones out until the beam of electrons is applied over the food product. As the accelerated electrons pass through the food, ions are released in interaction with the molecules within the food.
Effects of E-Beam Irradiation
The release of high-energy electrons from the accelerator produces direct and indirect ionization effects. The direct effect is caused when the electrons interact with the molecules within the food, which causes the ionization of adjacent molecules until the excess energy dissipates. The DNA is the primary target, which on ionization causes the breakage of the double strands of DNA, thereby inactivating the organisms. This technology causes the inactivation of pathogens and ensures food safety and the inactivation of spoilage organisms, thereby extending the shelf life.
E-beam also causes the splitting of water molecules, generating highly reactive free radicals, like hydrogen peroxide, hydrated electrons and protons. These free radicals also cause the breakage of DNA double strands. Studies have shown that in frozen foods, the damage is mostly due to direct effects, as under frozen conditions, the free radicals cannot diffuse to adjacent molecules. And the effect of e-beam processing does not increase the temperature of the food product. Hence, it is a non-thermal food processing method.
Role of E- beam irradiation in Food Processing
a) E-beam Application in Seafood Products
E-beam Irradiation process has been applied in the seafood industry. Based on the dose applied, E-beam can be used to carry out pasteurization and sterilization of the product without changing the sensory properties and temperature of the food. It has been used to inactivate microorganisms and extend the shelf life of sea foods. The sensitivity of microbes to the radiation is used to estimate the dose level required to inactivate the potential microorganisms in the food material.
The D value of klebsiella pneumoniae in the frozen seafood ranged from 0.116 to 0.277 kGy. It has been reported that a radiation dose of 1.5kGy inactivated K. pneumoniae in fish and a dose of 2-6 kGy could inactivate pathogens like, Salmonella, Campylobacter, E.coli O157:H7, etc. It was found that 2 kGy irradiation dose could control the microbial load in refrigerated smoked salmon, also a 1-3kGy range of dose inactivated psychrophilic spoilage organisms in refrigerated fish. E-beam processing causes some physical and chemical changes in seafood and it has been shown to have a significant effect on the colour of the product. Ozone is formed during the processing by e-beams; oxymyoglobin is converted to metmyoglobin upon oxidation, resulting in a brown colour.
b) E-beam Application in Meat & Poultry Industry
The red meat and poultry regulation that is approved by FDA has laid down irradiation guidelines, which involves setting up of a maximum limit for the dosage of radiation required for meat and poultry. FDA approved an irradiation dose of not more than 4.5 kGy for fresh meat and poultry and about 7.0 kGy for frozen meat products, which is sufficient to kill or inactivate the microorganisms in meat, (Lewis et al., 2002). It was observed that using high energy (10 MeV) of E-beam irradiation to meat and egg reduced the microbial load and also inactivated the avian influenza virus. (Lung et al., 2015)
c) E-beam application in Wastewater Treatment and Food Waste Management
E- beam Irradiation Technology is being developed for the treatment of sewage sludge. It results in a desired final product without the requirement of catalysts, activators and other additives during the reaction process. E-Beam is applied for wastewater treatment in Florida. The chemical decomposition of halogenated hydrocarbons by the E-beam irradiation process draws attention towards water treatment. In drinking water and wastewater, about 1.0-2.0MeV energy of the electrons is used for irradiation. According to the results of the disinfection of Yazd municipal wastewater by E- beam after a biological step, irradiation with the dose of 2 to 3 kGy can remove more than 90% of coliforms and about 50% of COD and BO. (Emami-Meibodi et al., 2016)
About one-third of the food produced is wasted during processing and harvest. Landfills, incineration and anaerobic digestion are some of the methods used for municipal waste management. The valuable contents of food get entrapped in food waste and are often discarded. The E-beam process helps to break down the wastes and extract valuable products from the food, hence providing vapourization of food wastes. The E-beam process has chain scission property that breaks down bulky polymers to low molecular weight materials. It is an effective approach for the decontamination of food waste. The input of electrons and the formation of free radicals are effective for the inactivation of microorganisms.
d) E-beam Application in Biodegradable Packaging
Biodegradable packaging decreases the environmental impacts of packaging; they are degraded by microorganisms and mineralized into carbon dioxide and water. The raw material for biodegradable plastics includes Polyacetic acid (PLA), which gives clarity, rigidity, thermal resistance and strength. Electron-beam processing of PLA has in the past, usually been carried out at beam energies ranging from tens of keV up to several MeV. E-beam treatment does not introduce any additional agent and thus preserves the original chemical composition of the polymer. (Laput et al., 2022)
The PLA has poor mechanical properties, which are improved by e-beam processing. The e-beam causes a crosslinking reaction between adjacent polymer chains resembling vulcanization, resulting in improved thermal and mechanical properties as well as the introduction of the “memory effect” in polymeric materials after controlled irradiation, (Sabharwal, 2013). E- beam radiation shows a depth-dependent reduction of molecular weight of polymers. For a 1.5 MeV beam delivering 50 kGy of radiation, the molecular weight of PLA is not affected beyond a depth of 5.3 mm and the molecular weight of PLGA is not affected beyond a depth of 5.4 mm. (Leonard et al., 2009)
e) E-beam Application in Package Sterilization
E- beam sterilization of packaging material is a proven, state-of-the-art technology that outperforms any legislative and customer requirements. Studies have shown that depending on the level of microorganisms, a dose of 5–7kGy was found to be effective against yeast, mould and spores and no viable microorganism was found after treatment, (Ansari & Datta, 2003). However, appropriate packaging material has to be selected to withstand the E-beam radiation. Recently, Tetrapak has adopted the use of low-energy e-beam technology for sterilizing the packaging materials, (Shayanfar & Pillai, 2015). This technology has helped to avoid the use of chemicals for sterilization, like hydrogen peroxide. The combination of E- beam surface decontamination and aseptic packaging reduces the need to treat or sterilize finished products. (Berejka, 2009)
Conclusion
E-beam Irradiation is a non-thermal process and also an extensively researched technology. It has potential applications in the Food Industry for ensuring microbiological safety, thereby protecting public health from foodborne diseases and maintenance of quality without causing many changes in the sensory and organoleptic properties of food. It is also known as cold pasteurization, as the process does not generate heat or cause a rise in the temperature of the food product. This process releases high-energy E-beams of a particular dosage to radiate the food material, thereby causing damage to the DNA and proteins of the microbial cell. It produces ionizing radiation. While penetration is lower compared to that of gamma rays, it does not lead to any nuclear waste. Further, it reduces the use of chemicals for the sterilization of materials.
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