Food Preservation by Irradiation PDF 2016

CHAPTER 6 Food Preservation by Irradiation 6.1 Introduction The increasing foodborne disease outbreaks and rising consumer response to foodborne illnesses have led to the development of new preservation methods to eliminate or reduce foodborne pathogens, such as Salmonella in poultry, Escher­ ich...

CHAPTER 6 Food Preservation by Irradiation 6.1 Introduction The increasing foodborne disease outbreaks and rising consumer response to foodborne illnesses have led to the development of new preservation methods to eliminate or reduce foodborne pathogens, such as Salmonella in poultry, Escher­ ichia coli O157:H7 in hamburger, Listeria in packaged meats, and Vibrio in shellfish. Irradiation has the potential to enhance food safety for both fresh and raw foods. Food irradiation refers to the treatment of foods by exposing them to ionizing radiation. For example, irradiation can kill harmful bacteria and other micro­ organisms in meat, poultry, seafood, and spices, extend shelf life of fresh fruits and vegetables, and control sprouting of potatoes and onions. It is a safe process that has been approved by the Food and Drug Administration (FDA). It may be referred to as a “cold pasteurization” process, as it does not significantly raise the temperature of the treated foods. Safe food handling and good manufacturing practices (GMPs) are required for irradiated foods just as for other foods if consumers are to enjoy the benefits of this technology. Radiation may be defined as the emission and propagation of energy through space or through a material. The shorter wavelengths of radiation are most effective on microorganisms without raising temperature. The irradiation does not show damaging effects of heat on food quality. Irradiation can destroy molds, yeasts, bacterial cells, spores, worms, insects, and larvae in foods. But irradiation cannot destroy toxins and enzymes in a food, and thus differs from heat treatment. 6.2 Characteristics of Radiations In the electromagnetic spectrum, energy exists as waves and the intensity of the energy increases as the waves get shorter, which is illustrated in Figure 6.1. Radiation has a wide range of energy forms in the electromagnetic spectrum. The spectrum has two major divisions: nonionizing radiation and ionizing radiation. Radiation that has enough energy to move atoms in a molecule or cause them to vibrate, but not enough to remove electrons and cause any change in atomic Food Microbiology: Principles into Practice, First Edition. Osman Erkmen and T. Faruk Bozoglu. © 2016 John Wiley & Sons, Ltd. Published 2016 by John Wiley & Sons, Ltd. 106  Food Preservation by Irradiation 107 Figure 6.1 Spectrum of different types of electromagnetic radiation. Source: Blacus, https://en.wikipedia.org/wiki/Electromagnetic_spectrum#/media/File:Electromagnetic-Spectrum.svg, used under CC-BY-SA 3.0, https://creativecommons.org/licenses/by-sa/3.0/ structure of molecules, is referred to as nonionizing radiation or long waves (such as ultraviolet light and microwaves). Radiations that fall within the ionizing radiations are short waves, gamma rays, β-rays, X-rays, and cosmetic rays. Ionizing radiations have enough energy to remove tightly bound electrons from atoms, thus creating an ion pair (negative and positive charges). This property can be used to generate electric power, to kill cancer cells, and in many manufacturing processes. Ion formation or ionization does not make an atom radioactive. For radioactivity, the nucleus of an atom should be disrupted by higher energy. The antimicrobial effect of radiation increases as the dose increases. The antimicrobial efficiency decreases in the absence of oxygen (due to reduced oxidizing reactions), low aw (due to reduced free radical formation with less water), and freezing (due to reduced availability of water). The radiation dose (level of treatment) is defined as the quantity of energy absorbed by food materials during exposure. The dose of ionizing radiation absorbed by irradiated material is measured in terms of gray (Gy) or rad. The energy absorption depends on mass, density, and thickness of food. Food irradiation doses are generally characterized as low (less than 1 kGy), medium (1–10 kGy), and high (>10 kGy). The energy level used for food irradiation is extremely low (from 0.1 to 1.0 kGy), which would be equivalent to a heat energy of 0.024–0.24 °C. The Codex Alimentarius Com­ mission recommended 10 Gy as the maximum energy level or dose of ionizing 108 Chapter 6 Figure 6.2 Effects of different types of electromagnetic radiation on molecules. radiation. Therefore, 10 kGy of ionizing energy is equivalent to a heat energy of 10 J g 1 and the heat capacity of water is 4.2 J g 1 °C 1, that is, 10/4.2 = 2.4 °C. Thus, it is a cold method of food preservation. Source: https://en.wikipedia.org/ wiki/File:EM-spectrum.png CC BY-SA 3.0; https://en.wikipedia.org/wiki/Wikipedia: Text_of_Creative_Commons_Attribution-ShareAlike_3.0_Unported_License 6.2.1 Nonionizing Radiations Nonionizing radiation is any type of electromagnetic radiation that does not carry enough energy per quantum to ionize atoms or molecules, that is, cannot remove an electron from an atom or a molecule. Instead of producing charged ions when passing through matter, the nonionizing radiations have sufficient energy only for excitation and the movement of an electron to a higher energy state (Figure 6.2). Ultraviolet light, sound waves, visible light, infrared light, microwaves, radio waves, and radio frequency are all examples of nonionizing radiation. The light from the Sun that reaches the Earth is largely composed of nonionizing radiation but most of it is filtered out by the Earth’s atmosphere. Nonionizing radiation has a considerable health risk to people if it is not properly controlled. Effects of nonionizing radiation on biological systems are given in Table 6.1. When nonionizing radiations are used in food processing, the hazard sign must be indicated on the door of the processing room (Figure 6.3). Ultraviolet Radiation Low-pressure mercury vapor discharge lamps are used for the generation of ultraviolet (UV) light: 80% of UV emission is at a wavelength of 254 nm. Wave­ length below 200 nm is absorbed by oxygen in the air producing ozone that is Figure 6.3 Nonionizing hazard sign.  Food Preservation by Irradiation 109 Table 6.1 Effects of nonionizing radiations on biological systems. Type of radiation Source Wavelength Frequency Biological effects Ultraviolet Sunlight, UV lasers 318–400 nm 750–950 THz Eye: cataract. light Skin: pigmentation, erythema. Visible Lasers, sunlight, fire, LEDs 400–780 nm 385–750 THz Damages the eyes light and skin. Infrared Heat lamps, IR lasers 780 nm to 300 GHz to Skin and eyes radiation 1 mm 385 THz absorbs heat. Skin burn. Eye: retinal and corneal burn, and cataract. Microwave Radar, some mobile phones, 1 mm to 1–300 GHz Absorbed by skin, Wi-Fi, motion detectors, 33 cm heating of tissue and telecommunication tissue damage. Radio Mobile phones, FM, AM, 33 cm to 100 kHz to Heating of tissue and frequency television 3 km 1 GHz tissue damage. harmful to biological material. The output of lamps falls off over time and they need to be monitored regularly. UV light is a powerful bactericidal agent at the wavelength of 240–280 nm. The great microbiological efficiency of UV is due to the damage to nucleic acids. The cross-linking of thymine dimers of the DNA prevents repair and reproduction that can cause inactivation of microorganisms. The energy supplied by UV radiation excites electrons in molecules from their ground state into higher energy orbitals making the molecules more reactive. These electromagnetic fields cause ion shifts on cellular membranes, changes in permeability, functional disturbances, rupture of cell structures, and failure of critical metabolic processes. These changes result in cell injury or death. The greatest lethality is shown at around 260 nm wavelength. This corresponds to a strong absorption of rays by nucleic acid bases and proteins, which leads to photochemical changes and cell death. Passage through 5 cm of clear water reduces the intensity of UV radiation by two-thirds, and 90% radiation is absorbed by a layer of milk only 0.1 mm thick. The very low penetration and the difficulty in exposing all points of the food surface are major technological problems, which limit the feasibility of UV radiation in food preservation. The low penetrability limits application of UV radiation in food industry to disinfection of air, surface area of equipment, and water, where it may catalyze oxidative changes. Due to low penetration power, it is used to inactivate microorganisms on the surface of foods (such as meat, fish, bread, milk, beer, fruit juices, baked cakes, and packed 110 Chapter 6 sliced bread), in the ripening rooms (such as cheese and dry sausages), and disinfection of water, air (such as aseptic filling room), and equipment. Generally, the resistance of microorganisms to UV irradiation follows this pattern: Gram-negative bacteria < Gram-positive bacteria  yeasts < bacterial spores < mold spores  viruses. UV resistance of various microorganisms depends on their pigment formation. Cocci that form colored colonies are more resistant to UV than colorless colonies. Dark conidia of certain molds are highly UV resistant. Bacterial cells are most easily killed by UV, while bacterial endospores and mold spores are much more resistant. UV light can cause burns on the skin and cataracts of the eyes. UV light produces free radicals that induce cellular damage, which can be carcinogenic. UV light also induces melanin production from melanocyte cells to cause sun tanning of skin. UV radiation initiates the formation of vitamin D on the skin. Plastic (polycarbonate) sunglasses generally absorb UV radiation. UV overexposure to the eyes causes snow blindness, which is a risk particularly on the sea or when there is snow on the ground. Microwave Radiation The microwave region of the electromagnetic spectrum occupies frequencies between 109 and 1012 Hz, and has a relatively low energy. Two frequencies are used in food processing: 2450 and 915 MHz. Microwaves act indirectly on microorganisms through the generation of heat. When electrically neutral foods are placed in a microwave electromagnetic field, the charged asymmetric mol­ ecules are derived. During this process, each asymmetric molecule attempts to align itself with the rapidly changing alternating current field. As the molecules oscillate on their axes while attempting to go to the proper positive and negative poles, intermolecular friction is created that produces heat. Microwaves are used for the destruction of molds in bread, beer, wines, and potato chips. Microwaves can be applied to destroy molds and yeasts on meat prior to use for burger, fruits, and vegetables to blanch and soft bakery goods and moist pasta to pasteurize. Foods are packed before microwave radiation. Microwave heating may reduce process times, energy, and water usage in some food processing areas. Microwave energy is suited for heating of food or to control pests in the food industry. Microwave heating has applications in home food preparation. The formation of heterogeneous temperatures within a food might create problems such as insufficient heating of some parts of foods, thereby allowing survival of pathogenic microorganisms. The geometry of the food, its thermal resistance, physical and dielectric properties, its mass, the power input, and the radiation frequency influence the interaction of electromagnetic waves with a food. These factors can cause differential microwave heating of different food constituents. This thermal nonuniformity limits the commercial application of microwave heating in the microbial inactivation.  Food Preservation by Irradiation 111 6.2.2 Ionizing Radiations Ionizing radiation has a frequency greater than 1018 Hz and carries sufficient energy to eject electrons from molecules. The following types of radiation sources are permitted for food irradiation processing according to the Codex General Standard of Codex Alimentarius Commission: (i) The radionuclide cobalt-60 or cesium-137. (ii) High-energy electrons. Electrons are generated by a machine at a maximum energy of 10 million electron volts (MeV). These are a form of β-rays. (iii) X-rays generated by a machine at a maximum energy of 5 MeV. Radiations do not have enough energy to affect the neutrons in the nuclei of food molecules; therefore, they are not able to induce radioactivity in the food. Again, none of these sources produces radioactive isotopes in foods. The ionizing rays have sufficient energy to convert electrons in food molecules to ions (electri­ cally charged particles) and free radicals (reactive compounds with unpaired electrons), including high-energy oxygen radicals, which kill or damage micro­ organisms. The potential application of ionizing radiation in food products is based on the effective inhibition of DNA synthesis. The right doses do not cause serious effects on the food. Irradiation dosage is a function of the energy of the radiation source and the time of exposure. The relative sensitivity of microorganisms to irradiation dose is a function of their size and water content. Irradiation doses are directly related to the extent of killing of bacterial pathogens. However, D10 values (irradiation doses required to cause a 1 log reduction of cells) applied on foods depend on density, antioxidant level, moisture content, and other food components. External factors, such as temperature and the presence or absence of oxygen, also influence the effects of radiation dose. The ionizing radiation used in food irradiation is limited to high-energy electromagnetic radiation (gamma rays of 60 Co or X-rays). They are chosen because (i) they produce desired effects on microorganisms, (ii) they do not induce radioactivity in foods or packaging materials, (iii) they are available in quantities and at costs that allow commercial use of the process, and (iv) the penetration of X-rays or gamma rays into matter occurs in different ways. The practical usable depth of gamma rays in water-equivalent material is 3.9 cm and in X-rays is 23.0 cm. Gamma Rays Gamma rays are electromagnetic radiations emitted from the nucleus of ele­ ments, such as 60 Co and 137 Cs. High-energy gamma rays have high penetration power and may be considered effective and economical for use in food proc­ essing. Gamma rays have the potential for effective and economical use in food preservation. Cobalt-60 is predominantly used in food irradiation due to its more ready and economical availability and excellent penetration power. It has a half-life of 5.3 years. It continuously emits gamma rays, and thus energy can last even when it is not used. Cesium-137 can also be used but a large amount is required. 112 Chapter 6 X-Rays X-rays have good penetration power, but they are not effectively used in food industry. X-rays are produced by bombardment of a heavy metal with high- velocity electrons within an evacuated tube. For foods, irradiation with energy up to 5 MeV can be used without any induced radioactivity. Beta Rays Beta rays may be defined as a stream of electrons emitted from radioactive substances. Beta rays have very little penetration power through foods (about 2.5 cm); they cannot penetrate inside metal cans and are ineffective to use in food preservation compared with X-rays. 6.3 Mechanisms of Microbial Inactivation by Irradiation When foods or microorganisms are exposed to ionizing radiation, atoms and molecules of foods absorb energy. This strips electrons from them and produces negative and positive ion pairs. The released electrons are highly energized and thus can remove electrons from other atoms and convert them to ions. This energization and ionization can adversely affect the normal characteristics of biological systems. Effects of radiation on biological materials are direct and indirect. The direct effect is caused by the removal of electrons as a result of energy deposition by radiation on target molecules, such as DNA. The indirect effects occur with reactive diffusible free radical formation due to the radiolysis of water, such as hydroxyl radical (OH ), hydrated electron (e ), H atom, hydrogen peroxide (H2O2), and hydrogen radical (H ). H2O2 and OH are strong oxidizing agents, while H radical is a strong reducing agent. The H+ and OH radicals are highly reactive, and cause oxidation and reduction on materials as well as the breakdown of carbon–carbon bonds, bonds of other molecules, and single and double strands in DNA at the sugar–phosphate bonds. In addition, the radicals can change the bases, such as thymine to dihydroxydihydrothymine. The principal targets of irradiation are nucleic acids and cell membrane lipids. Ionizing radiation causes damage to the cell membrane and other structures (sublethal injury). Alteration in membrane lipids, particularly poly­ unsaturated lipids, leads to perturbation of membranes and effects on various membrane functions, such as permeability. The activity of membrane enzymes may also be affected. Chromosomes of bacteria are very sensitive to free radicals and lethal damage occurs. Ionization radiations change the DNA structure of cells by disrupting certain bonds, which prevents replication and functions of DNA.  Food Preservation by Irradiation 113 The ability of bacteria to repair cell damage provides resistance to radiations. This ability of bacteria varies considerably. Irradiations can cause mutations in some microbial cells leading to either a possible increase in pathogenicity by producing toxins or loss of some metabolic abilities. The death rate of micro­ organisms by irradiation follows similar straight line as the thermal destruction curve. 6.4 Factors Affecting Inactivation of Microorganisms by Irradiation Many factors play an important role in the inactivation of microorganisms by irradiations. Some of them are type of radiation, types and species of micro­ organisms, composition of foods, oxygen content, physical state of foods, and age of microorganisms, among others. Type of Radiation Gamma rays have the potential for effective and economical use in food preser­ vation compared with X-rays and β-rays. Cobalt-60 is predominantly used as a gamma-ray source in food irradiation due to its ready and economical availability. The antimicrobial efficiency of ionizing radiation increases as the dose is increased. Types and Species of Microorganisms Microorganisms greatly differ in their sensitivity to irradiation (Table 6.2). Differ­ ent types and species of microorganisms have different sensitivities to irradiation. The major food spoilage and many common foodborne pathogens of different species are generally sensitive to irradiation and can be inactivated by low and medium doses of radiation between 1 and 7 kGy. Molds are more sensitive to irradiation than yeasts, yeasts are more sensitive to irradiation than bacteria, and Table 6.2 Doses of irradiation needed to kill different living forms. Microorganisms Dose (kGy) Microorganisms Dose (kGy) Bacillus cereus 0.17–1.6 Listeria 0.2–1.0 Clostridium perfringens 0.59–0.83 Vibrio 0.03–0.12 Clostridium sporogenes 1.5–2.2 Lactobacillus 0.3–0.9 Yersinia enterocolitica 0.04–0.21 Animal cells 0.001–0.1 Campylobacter 0.08–0.16 Insects 0.01–1.0 Aeromonas hydrophila 0.14–0.19 Bacterial cells 0.5–10 Escherichia coli 0.30–0.55 Bacterial spores 10–50 Staphylococcus aureus 0.26–0.6 Viruses 10–20 Salmonella 0.31–1.3 Molds and yeasts 1–3 114 Chapter 6 bacteria are more sensitive to irradiation than viruses. Gram-positive bacteria are more resistant to irradiation than Gram-negative and cocci are more resistant to irradiation than rods. Lethal dose levels for insects and different microorganisms are as follows: insects, about 1 kGy; molds, yeasts, and bacterial cells, 0.5–10 kGy; bacterial spores, 10–50 kGy; and viruses, 10–200 kGy. Higher doses to destroy spores (above 10 kGy) are not used in foods except on spices and vegetable seasonings. Spore formers are more resistant than non-spore formers, with the exception of Micrococcus radiodurans, Enterococcus faecium R53, and homofermentative Lactobacillus. Spores of Bacillus pumilus and Clostridium botulinum are most resistant to irradiation than other spores. Generally, Clostridium spores are more resistant than Bacillus spores. Irradiation readily kills most non-spore­ forming bacteria and parasites in foods. In general, Salmonella and Listeria are more resistant to irradiation than E. coli and Staphylococcus. The bacteria most sensitive to radiation belong to the genera Pseudomonas, Yersinia, Vibrio, Heli­ cobacter, Aeromonas, Campylobacter, and Flavobacterium. Pathogenic protozoa (such as Toxoplasma and Cyclospora) and parasites (such as Trichinella, Cyclospora, Cryptosporidium, and Toxoplasma gondii) in fresh foods can be inactivated by irradiation at 0.4–0.8 kGy. Enteric viruses and toxins of molds are extremely resistant to irradiation and cannot be effectively eliminated at approved doses of irradiation (10 kGy). Irradiation doses allowed for foods range from