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Effects of Gamma Radiation for Microbiological Control in Eggs
Chapter 17
Effects of Gamma Radiation for Microbiological Control in Eggs Marcia Nalesso Costa Harder* and Valter Arthur** *Technology College of Piracicaba—FATEC-Piracicaba/CEETEPS, Piracicaba, São Paulo, Brazil; **Nuclear Energy Center in Agriculture, Piracicaba, São Paulo, Brazil
INTRODUCTION A primary food safety concern with eggs is the presence of pathogens, in particular Salmonella spp. in raw eggs. Based on risk assessments conducted in the United States, it is estimated that 1 in 20,000 eggs are contaminated with Salmonella enteritidis (Ebel and Schlosser, 2000) which is a low incidence rate (0.005%). However, when a total of 1479 billion eggs produced worldwide each year is taken into consideration (4.93 billion egg-laying hens × 300 eggs/hen), the projection indicates that almost 74 million eggs with S. enteritidis could possibly enter the food supply. To prevent bacterial growth, eggs need to be refrigerated at a temperature of 7.2°C (45°F) or below, but above freezing. Consumers throughout the world purchase shelled eggs as a raw product. In many countries (United States, Canada, Australia, Japan, and Scandinavia are the exceptions), shelled eggs are never refrigerated. The shelled eggs are simply kept at room temperature until consumed providing ample opportunity for microorganisms to proliferate, especially when ambient temperatures are warm. If eggs are cooked to a temperature of 74°C (165°F), the Salmonella is killed. It is the improperly cooked egg that is contaminated with Salmonella that leads to food poisoning creating a public health issue. Thus, alternative methods of preserving food that minimizes the opportunity for food poisoning outbreaks leading to improvements in food safety is of paramount importance. There are several methods available to the food industry to extend shelf life and ensure the quality and safety of the food including refrigeration, freezing, drying, fermentation, and addition of preservatives. All of these methods prevent or inhibit the growth of microorganisms. Other methods inactivate microorganisms such as pasteurization, sterilization, and irradiation. In many countries, liquid egg products are required by regulation to be pasteurized. Although heat pasteurization is effective in deactivating microorganisms, functional properties, such as foaming of egg white and emulsification of egg yolk, may be impaired (Cunningham, 1986). In addition, pasteurization of frozen egg products requires the thawing and subsequent refreezing of the pasteurized egg product leading to energy inefficiencies. With respect to irradiation, the most versatile process is the use of ionizing radiation which can be performed on both frozen and nonfrozen products (Andrews et al., 1998; Santos et al., 2003). This chapter provides an overview of the current knowledge on the microbial control of raw eggs as well as egg products through the action of ionizing radiation.
BACKGROUND INFORMATION ON RADIATION Radiation is defined as either the transmission or emission of energy through a material medium or space in the form of particles or waves. Radiation is categorized as either nonionizing or ionizing. Nonionizing radiation does not have enough energy to completely remove an electron from a molecule or atom and is ordinarily not harmful to living organisms. It consists of lower ultraviolet, visible light, infrared, microwaves, radio waves, or lower energy electromagnetic waves emitted by power supplies or receivers for television or radio. In contrast, ionizing radiation does have the energy to liberate electrons from molecules and atoms transforming them into ions. Therefore, ionizing radiation consist of not only ions and atoms, but also subatomic particles as well as electromagnetic waves on the high-energy end of the electromagnetic spectrum. The energy levels of ionizing radiation, which exceed 10 electron volts (eV), can be harmful to living organisms, breaking chemical bonds and damaging DNA leading to cellular death (Sandvik, 1958). Sources of ionizing radiation include X-rays from medical devices and particles (e.g., neutrons, positrons, muons, and mesons) of secondary cosmic rays Egg Innovations and Strategies for Improvements. http://dx.doi.org/10.1016/B978-0-12-800879-9.00017-2 Copyright © 2017 Elsevier Inc. All rights reserved.
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FIGURE 17.1 The electromagnetic spectrum showing the range of electromagnetic radiation frequencies or wavelengths extending from the longest radio waves to the shorter, higher frequency, and higher energy gamma rays.
that come from primary cosmic rays interacting with the atmosphere of the Earth. Another source of ionizing radiation is from radioactive materials that emit either alpha (α), beta (β), or gamma (γ) radiation which consist of the helium nuclei, positrons or electrons, or photons, respectively (Fig. 17.1). It is the gamma radiation that is used in food to kill microbes through a process known as irradiation. A nonthermal process, a maximum irradiation dose of no more than 45 kilograys (kGy) is used to reduce microbial contamination of food without affecting its nutritional quality. One kGy is the absorption of 1 J of ionizing radiation by 1 kg of matter. The shelf life of the irradiated food product is prolonged, and food safety for humans is enhanced. There have been many studies conducted on the effects of gamma radiation on foods, especially those vulnerable to microbial contamination such as eggs.
IRRADIATION OF FOOD There are over 40 countries worldwide that currently permit the irradiation of food (Kume et al., 2009). The process of irradiation involves the inactivation of microorganisms (mainly bacteria, fungi, and yeast) which in the case of fruits and vegetables delays ripening (Iemma et al., 1999). Irradiation has been the subject of increasing attention during the last decades due to the following distinct advantages it offers over conventional methods of food preservation: (1) food can be processed after packaging; (2) it can be preserved in a fresh state; and (3) the perishable item can be kept for an extended period of time. For eggs, meats, fruits, fruit juices, and vegetables, the most promising treatment is a combination of radiation and other treatments because the sterilization with irradiation alone may require high doses of radiation and consequently promote enzyme inactivation responsible for sensory changes during storage of these products. Like other food processing techniques, irradiation can cause changes in the chemical composition and nutritional value. The nature and extent of these changes depend essentially on the type, variety, and composition of the food, the radiation dose received, and the environmental conditions during and after irradiation (Wiendl, 1984). There are three different irradiation methods (radappertization, radicidation, or radurization) used to inactivate microorganisms based on the severity of the process (Jay et al., 2005). Radappertization is the most severe of the three irradiation methods. With radappertization or sterilization of food, a dose of irradiation is applied that decreases the activity and number of living microbes (excludes viruses) to such a low level, that there is no recognized method for detection. Doses required for radappertization are generally between 25 and 45 kGy (World Health Organization, 1994; Satin, 1997). All foods—including eggs without shells in the form of egg white, yolk, or whole egg—subjected to radappertization must be parceled in hermetically sealed packets so there is no recontamination of the product to the environment. Radappertization is popular for use in meat products such as chicken fillets and turkey breast. The National Aeronautics and Space Administration, the space agency of the United States, uses radappertization to prepare irradiated food for consumption of astronauts during space flights. The irradiated food products have no microbial viability, even at room temperature, provided the package is kept intact. All irradiated foods must have expiration dates regardless of whether the package is kept intact or not because prolonged storage causes chemical and physical changes in these products. Radicidation, similar to pasteurization, is the treatment of food with a sufficient dose of ionizing radiation to inactivate nonspore forming bacteria in a way that the microorganisms are not detected by bacteriological methods normally used on processed foods. Doses required for radicidation are generally between 2 and 8 kGy (World Health Organization, 1994; Satin, 1997). Examples of foods where radicidation is applied include juices, fresh meats, fresh pasta, and eggs.
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Radurization is the least severe of the three processes of irradiation with dosages in the range of 0.4–2.5 kGy. Radurization disinfects or sanitizes food and extends shelf life by causing a reduction in the count of viable spoilage microorganisms. Examples of where radurization is used in foods include the following: (1) preventing the sprouting of bulbs and tubers, (2) preventing the deterioration of fruits and vegetables by fungi, (3) killing parasites, insects, and mites that infest food, and (4) slowing down the ripening of fruits. The delay in ripening and the shelf-life extension in fruits like bananas is a great advantage as this fruit can ripen quickly without treatment. Use of radurization to delay the fruit ripening process provides time for food distribution and exportation. The efficacy of the radiation treatments in inactivating microorganisms is dependent on several factors. Food contaminated with higher numbers of microorganisms requires a higher radiation dose. The composition of food is another influential factor. Microorganisms present in a rich culture media consisting of proteins are more resistant to irradiation than microbes present in an aqueous media. The presence of oxygen makes microorganisms less resistant to radiation. The physical status of food has influence on the ability of radiation to inactivate microorganisms. Frozen or dehydrated food are more resistant to radiation as compared to thawed or hydrated food. There is also a large range in resistance to radiation among microorganisms. Generally, microorganisms with more complex DNA have greater sensitivity to irradiation. As microorganisms go through different stages of growth, their vulnerability to radiation changes. When bacteria are maturing and not undergoing cellular division (referred to as the lag phase), they are more resistant to radiation than the log phase when bacteria are multiplying (Fellows, 2006; Franco and Landgraf, 2008).
IRRADIATION OF EGGS The biggest concern in the egg industry today, with respect to raw egg, is the presence of pathogens, especially the large family of Gram-negative bacteria of Enterobacteriaceae. One Enterobacteriaceae of particular interest is Salmonella which can gain entrance into the egg prior to shell deposition when the hen is forming her egg (Okamura et al., 2001). During oviposition or egg laying, microorganisms, including Salmonella, can also invade the egg through the pores of the shell (Messens et al., 2005). In addition to the hen, humans can also introduce pathogens into eggs during collection, processing, distribution, and preparation. These microorganisms can be inactivated if cooked properly using adequate heat. However, some consumers eat raw eggs in eggnog or protein drinks or consume undercooked eggs (e.g., eggs fried sunny-side up) increasing susceptibility to food poisoning. Rather than purchase precooked eggs, consumers prefer to purchase raw eggs in shell for preparation at home. Although used in hospitals and restaurants, further processed pasteurized liquid products available at some retail units are less popular for home use perhaps due to tradition, price, and perceptions of quality. Hot pasteurization can be used on liquid, but not solid, foods for purposes of microbial decontamination. The use of irradiation is an alternative option in decreasing pathogens, such as Salmonella spp., when using heat is impractical or undesirable for food preservation. The effects of irradiation on microbial deactivation and the functional and sensory characteristics of treated eggs have been evaluated in liquid egg (both frozen and nonfrozen), powdered egg white and yolk components, fresh whole egg with shell intact, and cooked egg. The process used to irradiate eggs to reduce pathogenic microorganisms and parasites may be likened to an alternative cold pasteurization method without heat production (Loaharanu, 1997). Radiation results in electron collisions that produce ionization leading to chemical and biological changes in eggs. Electrons can be introduced into eggs directly from an electron accelerator or indirectly by photons generated by a radioactive source (McKeown and Drewell, 1996). For example, gamma rays from the radioactive isotope of cobalt 60 (Co60) is commonly used. The Co60 has a half-life of 5.263 years and is synthetically made from the cobalt isotope of Co59 through neutron activation. The Co60 decays through beta decay (0.314 million eV) to the nickel 60 (Ni60). The nucleus of the Ni60 emits two gamma rays with energies of 1.173 and 1.332 million eV. As the gamma rays generated by a Co60 source have far greater power penetration than electron beams (Diehl, 1995), gamma rays are applied to thicker, large volume foods, whereas the electron beam is used in surface radiation (Satin, 2002). To test the effectiveness of gamma irradiation, five isolates of S. enteritidis were inoculated on the surface and inside of whole shell eggs. After inoculation, the eggs with shell intact were subjected to gamma irradiation doses of 0, 0.5, 1.0, and 1.5 kGy. Liquid whole egg was also inoculated with two isolates of S. enteritidis before irradiation with 0, 0.25, 0.50, 0.75, and 1.00 kGy. A minimum dose of 0.5 kGy was sufficient to eliminate S. enteritidis from the surface of eggs, but a higher dose of 1.5 kGy was needed to eliminate the bacteria from whole egg with shell intact as well as the liquid whole egg. There was no adverse effect on yolk color and thermolability of egg white proteins at any level of irradiation employed (Serrano et al., 1997). Using a Co60 source, liquid egg white was irradiated with 0, 0.432, 0.576, 0.720, and 0.864 milliradian and then frozen for later measurements of physical and functional characteristics. Evaluation of thawed irradiated egg white demonstrated
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a reduction in viscosity and a small decrease in the proportion of egg white solids. Upon further examination of electrophoretic protein patterns, the proportion of ovomucin, ovalbumin, and conalbumin decreased with a concomitant increase in the proportion of globulins as a result of irradiating the egg white (Ball and Gardner, 1968). In the dissertation of Harder (2009), similar results occurred where irradiated egg white experienced damage to the protein, ovomucoid. Irradiation doses used in the study of Ball and Gardner (1968) caused molecular degradation of proteins which impacted functional properties. Specifically, the angel food cake prepared from irradiated egg white required longer beating times and was of poorer quality due to texture and reduced volumes. The irradiated egg white foams were notably less stable. Interestingly, increasing the time in frozen storage improved the functional properties of irradiated egg white as indicated by cake volume and beating times of cake batter. Compared to the controls, the irradiated eggs had a different appearance and odor. Specifically, the irradiated eggs both before and after freezing had a dull greenish-yellow color and an odor similar to a mixture of sulfur and ammonium. The thawing of the irradiated egg white caused two separate layers to form. The bottom two-thirds layer was thicker than the upper layer of the thawed irradiated egg white. Once the two layers were blended together, no further physical separation of egg white occurred (Ball and Gardner, 1968). Irradiation doses of 1–4 kGy applied to frozen liquid egg white or yolk did not affect integrity of egg white proteins; however, the low dose irradiation caused a decrease in the viscosity of frozen yolk. In general, the functional properties of the liquid egg products, including emulsification, gelling, and foaming, were not affected by the irradiation treatments. In fact, use of egg white protein that was irradiated when frozen displayed enhanced functional properties, such as increased angel food cake volume, and the liquid egg yolk that was irradiated when frozen demonstrated increased stability and stiffness when used in mayonnaise. These results of Ma et al. (1993), along with research demonstrating that eggs irradiated with 2 or 3 kGy reduced bacterial contamination, especially Salmonella spp., to nondetectable levels (Tellez et al., 1995), support use of 1–4 kGy of irradiation to preserve frozen liquid egg products. Gamma radiation from Co60 was applied to powdered samples of egg white, yolk, and whole egg at doses that ranged from 0 to 25 kGy. The irradiated samples were rehydrated and viscosity was measured. Irradiation doses above 5 kGy resulted in no change in the viscosity of the egg white. However, yolk or whole egg showed changes in rheological properties as radiation dose increased which were most likely due to lipid degradation and the accumulation of lipid hyperoxides. In addition, the color of the irradiated yolk powder faded in response to increasing radiation dose (Ferreira and Del Mastro, 1998). Carotenoids (the yellow to orange fat soluble pigments) from eggs are more sensitive to irradiation than carotenoids from other sources like dry plants (Katusin-Razem et al., 1989; Harder, 2009; Fig. 17.2). These results suggest that powdered egg white as compared to yolk proteins are less susceptible to irradiation induced degradation. Solid egg yolk and solid whole egg were subjected to irradiation doses up to 10 kGy using Co60 gamma radiation. Although irradiation caused no detectable changes in vitamins B1 (thiamine) or B2 (riboflavin) or egg acidity, decomposition of triglycerides and amino acids was evident. The generation of free radicals from oxidized lipids during irradiation exposure most likely contributed to the amino acid damage. Irradiation of the solid egg yolk and whole egg also caused a decrease in carotenoids and vitamin A. However, removal of oxygen during irradiation minimized the damage to egg solid protein and lipids. Scrambled eggs and mayonnaise made from egg product subjected to 3 kGy of irradiation in air or up to 5 kGy of irradiation without oxygen were indistinguishable from nonirradiated control egg products (Katusin-Razem et al., 1989).
FIGURE 17.2 Eggs samples subjected to 20 kGy of irradiation as compared to the control eggs not subjected to irradiation. The carotenoids and vitamin A content of yolk is reduced as a result of irradiation resulting in paler yolks.
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Liquid egg, frozen egg, egg yolk powder, egg white powder, or whole egg powder were each inoculated with S. enteritidis followed by irradiation doses of 0, 2, 3, or 3.5 kGy. The highest dosage of radiation (3.5 kGy) caused a 5 log cycle reduction of S. enteritidis. A decrease in yolk color occurred in all irradiated liquid and frozen egg samples (Froehlich, 2004). The irradiation of whole eggs with shell intact has also been explored as a means of deactivating Salmonella spp. as a replacement to thermal pasteurization of shelled eggs. Fresh eggs, with shell intact, were inoculated with 108 colony forming units of S. enteritidis and then exposed to gamma radiation doses of 0, 1, 2, or 3 kGy. No viable bacterial cells were detected in the shell as well as the internal eggshell membranes with the higher doses of 2 and 3 kGy. The lower dose of irradiation (1 kGy) resulted in a 3.9 log reduction of S. enteritidis on the shell and a 95% reduction of this pathogen on the shell membranes. However, all levels of irradiation tested in this study caused a 50% deterioration in egg white quality as measured by Haugh units. The yolk color faded in the whole shell eggs treated with 2 and 3 kGy of irradiation. These results suggest that 1 kGy reduced and 2 or more kGy of irradiation eliminated S. enteritidis but not without deleteriously affecting egg white quality and yolk color (Tellez et al., 1995). Using a linear accelerator, fresh eggs with shell intact were subjected to 0, 2.5, 5, or 10 kGy of gamma irradiation. After treatment, the shells were broken and the yolk and egg white separated from one another, and the egg products stored up to 14 days at 4°C (39.2°F). Egg white viscosity was decreased and turbidity increased by exposure to irradiation. A low dose of irradiation (2.5 kGy) did not affect foaming capacity or foam stability, but higher doses of radiation greater than 5 kGy resulted in poor foaming characteristics. As the radiation dose increased, the sulfur-containing volatiles also increased, but storage in the presence of oxygen eliminated the odor. When irradiated egg white was cooked, the greenness value increased and the yellowness and lightness values decreased in a dose-dependent manner. The texture of the cooked egg white as evaluated through chewiness, resilience, hardness, adhesiveness, and cohesiveness also increased as the radiation dose was increased, but changes were small. Therefore, irradiation doses of 2.5 kGy or lower on shelled eggs had little impact on the functional and physiochemical properties of liquid egg white suggesting that irradiation technology applied to eggs with shell intact has potential for commercial application (Min et al., 2012). Cooked fried eggs were sterilized in vacuumed packed oxygen impermeable nylon bags using 30 kGy of gamma irradiation; inoculated with 106–107 colony forming units/g with each of four pathogens; subjected to 0, 1, 2, or 3 kGy of irradiation; and stored at 10, 20, or 30°C (50, 68, or 86°F) where microbiological evaluations were performed at 0, 8, and 24 h. A dose of 3 kGy eliminated Staphylococcus aureus, Listeria ivanovii, Salmonella typhimurium, and Escherichia coli in fried eggs suggesting that irradiation was a useful tool in enhancing the food safety of eggs used in composite meals (Jo et al., 2005). A by-product of irradiation of egg yolk is cholesterol oxidation products such as cholestane-triol, cholesterol-α-epoxide, 7-hydroxylcholesterol, 7-ketocholesterol, and 25-hydroxycholesterol. Dried yolk powder stored for 4 years and then subjected to ultraviolet radiation for 3 weeks showed a dramatic increase in cholesterol oxidation products (Table 17.1, van de Bovenkamp et al., 1988). In addition, a 1 kGy dose of ionizing radiation also induced formation of cholesterol oxidation products in spray dried egg powder. Subjecting the egg powder to higher irradiation doses up to 6 kGy further increased the cholesterol oxidation products (Lebovics et al., 1992). These oxidized products of cholesterol cause more injuries to human arterial cells as compared to pure cholesterol leading to coronary heart disease and atherosclerosis. Moreover, any cholesterol-containing foods that have been exposed to air and heat during processing or that has been stored at ambient temperatures may contain cholesterol oxidation products. For example, spray dried egg powder stored for 3 months that
TABLE 17.1 The Effect of 3 Weeks of Exposure to Ultraviolet Irradiation on the Concentration of Cholesterol Oxidation Products of Egg Yolk Powder that had been Stored for 4 Years at Ambient Temperature Cholesterol Oxidation Product
No Irradiation (µg/g of egg yolk)
Ultraviolet Radiation (µg/g of egg yolk)
Cholestane-trio
0.5
62.0
Cholesterol-α-epoxide
25.2
2522.0
7-Hydroxylcholesterol
78.0
507.0
7-Ketocholesterol
4.2
200.0
25-Hydroxycholesterol
13.9
860.0
Source: Adapted from van de Bovenkamp, P., Kosmeijer-Schuil, T.G., Katan, M.B., 1988. Quantification of oxysterols in Dutch foods: egg products and mixed diets. Lipids 23, 1079–1085.
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was never subjected to irradiation had the same level of 7-hydroxylcholesterol as a fresh sample of egg powder irradiated with 6 kGy of ionizing radiation (Lebovics et al., 1992). Methods for reducing the accumulation of cholesterol oxidation products in foods need to be identified (Hur et al., 2007).
OTHER USES OF GAMMA IRRADIATION IN FOOD Structural alteration of proteins through use of gamma irradiation is being investigated as a means of reducing food allergies. Common food allergies in humans include milk β-lactoglobulin, shrimp tropomyosin, and egg albumin. Subjecting food to ionizing radiation changes the antigenicity of food by altering the physical and chemical structure of proteins leading to distortion of the protein’s secondary and tertiary structures. Specifically, the epitope area of the food allergen can be modified or destroyed by gamma irradiation so that antibodies to the allergen should never be produced by the individual consuming the irradiated food (Byun et al., 2002; Harder, 2009).
LEGISLATION According to the Codex Alimentarius Commission (1984), irradiation of foods are permitted using one of the following sources of irradiation: (1) gamma rays in which the radionucleotide is Co60 with maximum power of 1.332 million eV and a half-life of 5.263 years or cesium 137 (Ce137) with a maximum energy of 0.0662 million eV and a half-life of 30 years; (2) X-rays generated by machines operating with power up to 5 million eV; or (3) electrons generated by machines operating at 10 million eV or below. The doses for irradiated foods are determined in accordance with the laws on food irradiation provided by each country. Most countries consider 10 kGy as the maximum dose to be used to irradiate food. The current resolution in Brazil dictate that: (1) the minimum absorbed dose should be sufficient to achieve the intended purpose; and (2) the maximum absorbed dose should be less than that which would compromise the functional properties and sensory attributes of the food (Brazil, 2001). Some countries require that irradiated foods be labeled with the phrase “food treated by irradiation” or that the Radura (derived from radurization) symbol be printed on the packaging label (Fig. 17.3). This symbol is used internationally to indicate that a food product has been subjected to irradiation. The Radura logo can vary in color and graphics depending on the country. The logo represents a plant within a circle and is most commonly colored green. The circumference composing the top half of the circle has dashed lines. There are varying regulations among countries in the labeling requirements of irradiated food. In the European Union, use of the Radura symbol is not common. Instead, food packages use labels indicating irradiation in the respective language of the home country. In addition, information on each irradiated ingredients in the final product must be included on the label. This rule applies to restaurant food as well. In the United States, the Food and Drug Administration requires that irradiated foods sold in stores include the Radura symbol as well as the statement “treated by irradiation” or “treated with radiation” on the label. Irradiated processed foods or foods sold in restaurants do not require labeling in the United States.
FIGURE 17.3 The Radura symbol is placed on irradiated food packages in some countries of the world. The Radura symbol originated from and was copyrighted by an irradiation food processing facility located in Wageningen, The Netherlands in the 1960s. Jan Leemhorst, then president of the company called Gammaster, recommended its use as an international label to be placed on irradiated food as long as manufacturers implemented appropriate quality parameters. The Atomic Energy of South Africa also used the Radura symbol, but instead of using the term “irradiation,” they used “radurized” on the food label. The Radura symbol is listed in the Codex Alimentarius Standard on Labelling of Prepackaged Food (CODEX-STAN 1, 2010. General standard for the labelling of prepackaged foods. http://www.codexalimentarius.net/download/standards/32/CXS_001e.pdf)
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CONSUMER PERCEPTION Public perception that irradiated food is a radiological or biohazard has resulted in these processed foods not be widely adopted by food manufacturers and the consuming public. Those individuals knowledgeable of the fact that irradiation poses no health risk to humans who consume irradiated food are frustrated by the requirement that a label be placed on irradiated food packages. The fact that food wastage due to spoilage can be minimized because of the extended shelf life of irradiated food leads to an environmentally friendly technology that most consumers often give little thought to. Increasing demand for food in developing countries with rapidly growing hungry populations can be partially alleviated by preventing food spoilage through the use of gamma irradiation.
STRATEGIES FOR IMPROVEMENT Though food irradiation is considered by the experts to be a clean and efficient technology capable of deactivating foodborne pathogens, its use as an appropriate food safety measure needs to better communicated to the consuming public to avoid misperceptions. For eggs, there is the potential of using irradiation as a nonthermal method to eliminate food pathogens without altering the characteristics of the eggs as may occur in heat treatments. A sector that demands further study is implementing technology during irradiation of eggs that improves sensory properties, and the development of new irradiated egg products.
ANALYTICAL METHODS Yolk color: A Roche fan is used to evaluate the color of the egg yolk. After breaking the shell and pouring the interior contents of the egg onto a glass plate, the color of the yolk is matched with 1 of the 10 colored bands of the fan. The ranges for color score are from very pale with a score of 1 to a score of 10 which is a very intense orange-yellow color (Sun et al., 2012). Egg white and yolk color: A colorimeter is used to determine the Commission Internationale de l’Eclairage L* (lightness), a*[redness (+)/greenness (−)], and b* (yellowness) values of egg white (Min et al., 2012) and yolk (Harder et al., 2007). Viscosity, turbidity, and foaming properties of liquid egg white: A viscometer is used to determine viscosity of liquid egg white. Turbidity is measured using a spectrophotometer. Specifically, the egg white is diluted 20 times with deionized water. The optical density of the diluted sample is measured at a wavelength of 320 nm against a blank containing deionized water (Xiong, 1992). For the determination of foaming capacity and foam stability, liquid egg white (100 g) is whipped with a wire whip for 5 min generating a foam which is then transferred into graduated cylinder that has been preweighed. The weight and the volume of the foam are determined to calculate specific density as grams per millimeter of foam as an indicator of foaming capacity. For foam stability, the foam in the graduated cylinder sits for 30 min at ambient temperature. The volume of drainage in milliliters is determined (Min et al., 2005). Volatile analysis of egg white: Gas chromatography that has a mass selective detector determines the quantity and quality of volatile compounds in liquid egg white (Min et al., 2012). Texture of cooked egg white: Cooked egg white sticks are analyzed for texture properties using a texture profile analyzer with a 5 kg loading cell and a cylinder plunger with a 38 mm wide diameter. The curve generated from the texture profile analysis determines cohesiveness, hardness, chewiness, springiness, and gumminess (Min et al., 2012). Haugh unit: An indicator of albumen quality is the Haugh unit. The weight of an individual egg is weighed using a scale with 0.01 g sensitivity. After cracking the shell, the yolk and albumen are poured onto a level, flat surface. The thick albumen height is measured using a gauge which is sensitive to 0.01 mm. The following formula is used to calculate Haugh unit: Haugh unit = 100 × log (H + 7.57 − 1.7 × W0.37), where H is the height of the albumen in millimeters and W is the egg weight in grams (Haugh, 1937).
CONCLUSIONS l
Irradiation is a clean and efficient technology capable of deactivating foodborne pathogens and extending shelf life so that food is not wasted due to spoilage. l Irradiation at an appropriate dose eliminates foodborne pathogens in frozen and nonfrozen liquid egg, powdered egg white and yolk components, fresh whole egg with shell intact, and cooked egg.
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l
Whole eggs with shell intact inoculated with S. enteritidis and then treated with gamma irradiation showed that 1 kGy reduced and 2 or more kGy of irradiation eliminated the microorganisms but not without deleteriously affecting egg white quality and yolk color. l One of the important factors needing further study is to improve on the sensory aspect of the irradiated egg. l Public perception that irradiated food is a radiological or biohazard has resulted in these processed foods not be widely adopted by food manufacturers and the consuming public. l To avoid misperceptions, improved communication is needed with the public on food irradiation being an environmentally friendly technology that reduces food wastage and improves food safety.
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AVI Publishing Company, Westport, CT, United States, p. 243. Diehl, J.F., 1995. Safety of Irradiated Foods, second ed. Marcel Dekker, New York, NY, United States. Ebel, E., Schlosser, W., 2000. Estimating the annual fraction of eggs contaminated with Salmonella enteritidis in the United States. Int. J. Food Microbiol. 61, 51–62. Fellows, P.J., 2006. Food Processing Technology: Principles and Practice, second ed. Artmed, Porto Alegre, Brazil. Ferreira, L.F.S., Del Mastro, N.L., 1998. Rheological changes in irradiated chicken eggs. Radiat. Phys. Chem. 52, 59–62. Franco, B.D.G.M., Landgraf, M., 2008. Food Microbiology. Atheneum, São Paulo, Brazil, (in Portuguese). Froehlich, A., 2004. Irradiation of liquid egg, frozen egg, yolk and white powder: reducing the population of Salmonella enteritidis and sensory and physicochemical aspects. Thesis (Doctor of Food Microbiology), Faculty of Pharmaceutical Sciences, University of São Paulo, São Paulo, Brazil. Harder, M.N.C., 2009. Effect of gamma radiation on allergenic protein hen eggs. Thesis (Doctor of Science), Center for Nuclear Energy in Agriculture, University of São Paulo, Piracicaba, Brazil. Harder, M.N.C., Canniatti-Brazaca, S.G., Arthur, V., 2007. Quantitative evaluation by a digital colorimeter of the color of the egg of laying hens fed with annatto (Bixa orellana). Portuguese J. Vet. Sci. 102, 339–342. Haugh, R.R., 1937. The Haugh unit for measuring egg quality. US Egg Poult. Mag. 43, 522–573. Hur, S.J., Park, G.B., Joo, S.T., 2007. Formation of cholesterol oxidation products (COPs) in animal products. Food Control 18, 939–947. Iemma, J., Alcarde, A.R., Domarco, R.E., Spoto, M.H.F., Blumer, L., Matraia, C., 1999. Gamma radiation on the conservation of natural orange juice. Scientia Agricola 56, 1193–1198, (in Portuguese). Jay, J.M., Loessner, M.J., Golden, D.A., 2005. Modern Food Microbiology, seventh ed. Springer, New York, NY, United States. Jo, C., Lee, N.Y., Kang, H., Hong, S., Kim, Y., Kim, H.J., Byun, M.W., 2005. Radio-sensitivity of pathogens in inoculated prepared foods of animal origin. Food Microbiol. 22, 329–336. Katusin-Razem, B., Razem, D., Matic, S., Mihokovic, V., Kostromin-Soos, N., Milanovic, N., 1989. Chemical and organoleptic properties of irradiated dried whole egg and egg yolk. J. Food Prot. 52, 781–786. Kume, T., Furuta, M., Todoriki, S., Uenoyama, N., Kobayashi, Y., 2009. Status of food irradiation. Radiat. Phys. Chem. 78, 222–226. Lebovics, V.K., Gaál, Ö., Somogyi, L., Farkas, J., 1992. Cholesterol oxides in gamma-irradiated spray-dried egg powder. J. Sci. Food Agric. 60, 251–254. Loaharanu, P., 1997. Irradiation as a cold pasteurization process of food. Annu. Rev. Nutr. 17, 255–275. Ma, C.-Y., Harwalkar, V.R., Poste, L.M., Sahasrabudhe, M.R., 1993. Effect of gamma irradiation on the physicochemical and functional properties of frozen liquid egg products. Food Res. Int. 26, 247–254. McKeown, J., Drewell, N.H., 1996. Physical mechanisms of irradiation technologies and their characteristics effects. In: McMurray, C.H. (Ed.), Detection Methods for Irradiated Foods. The Royal Society of Chemistry Information Services, Cambridge, United Kingdom, pp. 3–13. Messens, W., Grijspeerdt, K., Herman, L., 2005. Eggshell penetration by Salmonella: a review. Worlds Poult. Sci. J. 61, 71–85. Min, B.R., Nam, K.C., Lee, E.J., Ko, G.Y., Trampel, D.W., Ahn, D.U., 2005. Effect of irradiating shell eggs on quality attributes and functional properties of yolk and white. Poult. Sci. 84, 1791–1796. Min, B.R., Nam, K.C., Jo, C., Ahn, D.U., 2012. Irradiation of shell egg on the physicochemical and functional properties of liquid egg white. Poult. Sci. 91, 2649–2657. Okamura, M., Kamijima, Y., Miyamoto, T., Tani, H., Sasai, K., Baba, E., 2001. Differences among six Salmonella serovars in abilities to colonize reproductive organs and to contaminate eggs in laying hens. Avian Dis. 45, 61–69. Sandvik, O., 1958. The effect of chronic gamma radiation on the hatchability of chicken eggs. Hereditas 44, 407–414.
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Santos, A.F., Vizeu, D.M., Destro, M.T., Franco, B.D.G.M., Landgraf, M., 2003. Determination of gamma radiation doses to reduce Salmonella spp. in chicken meat. Food Sci. Technol. 23, 200–205. Satin, M., 1997. Radiation of Food. Editorial Acribia, Zaragoza, Spain. Satin, M., 2002. Use of irradiation for microbial decontamination of meat: situation and perspectives. Meat Sci. 62, 277–283. Serrano, L.E., Murano, E.A., Shenoy, K., Olson, D.G., 1997. D values of Salmonella enteritidis isolates and quality attributes of shell eggs and liquid whole eggs treated with irradiation. Poult. Sci. 76, 202–205. Sun, H., Lee, E.J., Samaraweera, H., Persia, M., Ragheb, H.S., Ahn, D.U., 2012. Effects of increasing concentrations of corn distillers dried grains with solubles on the egg production and internal quality of eggs. Poult. Sci. 91, 3236–3246. Tellez, I.G., Trejo, R.M., Sanchez, R.E., Ceniceros, R.M., Luna, Q.P., Zazua, P., Hargis, B.M., 1995. Effect of gamma irradiation on commercial eggs experimentally inoculated with Salmonella enteritidis. Radiat. Phys. Chem. 46, 789–792. van de Bovenkamp, P., Kosmeijer-Schuil, T.G., Katan, M.B., 1988. Quantification of oxysterols in Dutch foods: egg products and mixed diets. Lipids 23, 1079–1085. Wiendl, F.M., 1984. The wholesomeness of irradiated foods. Bull. Braz. Soc. Sci. Food Technol. 18, 48–56, (in Portuguese). World Health Organization, 1994. Food safety and nutritional adequacy of irradiated foods. Geneva, Italy. Xiong, Y.L., 1992. Thermally induced interactions and gelation of combined myofibrillar protein from white and red broiler muscles. J. Food Sci. 57, 581–585.
Effects of Gamma Radiation for Microbiological Control in Eggs Marcia Nalesso Costa Harder* and Valter Arthur** *Technology College of Piracicaba—FATEC-Piracicaba/CEETEPS, Piracicaba, São Paulo, Brazil; **Nuclear Energy Center in Agriculture, Piracicaba, São Paulo, Brazil
INTRODUCTION A primary food safety concern with eggs is the presence of pathogens, in particular Salmonella spp. in raw eggs. Based on risk assessments conducted in the United States, it is estimated that 1 in 20,000 eggs are contaminated with Salmonella enteritidis (Ebel and Schlosser, 2000) which is a low incidence rate (0.005%). However, when a total of 1479 billion eggs produced worldwide each year is taken into consideration (4.93 billion egg-laying hens × 300 eggs/hen), the projection indicates that almost 74 million eggs with S. enteritidis could possibly enter the food supply. To prevent bacterial growth, eggs need to be refrigerated at a temperature of 7.2°C (45°F) or below, but above freezing. Consumers throughout the world purchase shelled eggs as a raw product. In many countries (United States, Canada, Australia, Japan, and Scandinavia are the exceptions), shelled eggs are never refrigerated. The shelled eggs are simply kept at room temperature until consumed providing ample opportunity for microorganisms to proliferate, especially when ambient temperatures are warm. If eggs are cooked to a temperature of 74°C (165°F), the Salmonella is killed. It is the improperly cooked egg that is contaminated with Salmonella that leads to food poisoning creating a public health issue. Thus, alternative methods of preserving food that minimizes the opportunity for food poisoning outbreaks leading to improvements in food safety is of paramount importance. There are several methods available to the food industry to extend shelf life and ensure the quality and safety of the food including refrigeration, freezing, drying, fermentation, and addition of preservatives. All of these methods prevent or inhibit the growth of microorganisms. Other methods inactivate microorganisms such as pasteurization, sterilization, and irradiation. In many countries, liquid egg products are required by regulation to be pasteurized. Although heat pasteurization is effective in deactivating microorganisms, functional properties, such as foaming of egg white and emulsification of egg yolk, may be impaired (Cunningham, 1986). In addition, pasteurization of frozen egg products requires the thawing and subsequent refreezing of the pasteurized egg product leading to energy inefficiencies. With respect to irradiation, the most versatile process is the use of ionizing radiation which can be performed on both frozen and nonfrozen products (Andrews et al., 1998; Santos et al., 2003). This chapter provides an overview of the current knowledge on the microbial control of raw eggs as well as egg products through the action of ionizing radiation.
BACKGROUND INFORMATION ON RADIATION Radiation is defined as either the transmission or emission of energy through a material medium or space in the form of particles or waves. Radiation is categorized as either nonionizing or ionizing. Nonionizing radiation does not have enough energy to completely remove an electron from a molecule or atom and is ordinarily not harmful to living organisms. It consists of lower ultraviolet, visible light, infrared, microwaves, radio waves, or lower energy electromagnetic waves emitted by power supplies or receivers for television or radio. In contrast, ionizing radiation does have the energy to liberate electrons from molecules and atoms transforming them into ions. Therefore, ionizing radiation consist of not only ions and atoms, but also subatomic particles as well as electromagnetic waves on the high-energy end of the electromagnetic spectrum. The energy levels of ionizing radiation, which exceed 10 electron volts (eV), can be harmful to living organisms, breaking chemical bonds and damaging DNA leading to cellular death (Sandvik, 1958). Sources of ionizing radiation include X-rays from medical devices and particles (e.g., neutrons, positrons, muons, and mesons) of secondary cosmic rays Egg Innovations and Strategies for Improvements. http://dx.doi.org/10.1016/B978-0-12-800879-9.00017-2 Copyright © 2017 Elsevier Inc. All rights reserved.
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FIGURE 17.1 The electromagnetic spectrum showing the range of electromagnetic radiation frequencies or wavelengths extending from the longest radio waves to the shorter, higher frequency, and higher energy gamma rays.
that come from primary cosmic rays interacting with the atmosphere of the Earth. Another source of ionizing radiation is from radioactive materials that emit either alpha (α), beta (β), or gamma (γ) radiation which consist of the helium nuclei, positrons or electrons, or photons, respectively (Fig. 17.1). It is the gamma radiation that is used in food to kill microbes through a process known as irradiation. A nonthermal process, a maximum irradiation dose of no more than 45 kilograys (kGy) is used to reduce microbial contamination of food without affecting its nutritional quality. One kGy is the absorption of 1 J of ionizing radiation by 1 kg of matter. The shelf life of the irradiated food product is prolonged, and food safety for humans is enhanced. There have been many studies conducted on the effects of gamma radiation on foods, especially those vulnerable to microbial contamination such as eggs.
IRRADIATION OF FOOD There are over 40 countries worldwide that currently permit the irradiation of food (Kume et al., 2009). The process of irradiation involves the inactivation of microorganisms (mainly bacteria, fungi, and yeast) which in the case of fruits and vegetables delays ripening (Iemma et al., 1999). Irradiation has been the subject of increasing attention during the last decades due to the following distinct advantages it offers over conventional methods of food preservation: (1) food can be processed after packaging; (2) it can be preserved in a fresh state; and (3) the perishable item can be kept for an extended period of time. For eggs, meats, fruits, fruit juices, and vegetables, the most promising treatment is a combination of radiation and other treatments because the sterilization with irradiation alone may require high doses of radiation and consequently promote enzyme inactivation responsible for sensory changes during storage of these products. Like other food processing techniques, irradiation can cause changes in the chemical composition and nutritional value. The nature and extent of these changes depend essentially on the type, variety, and composition of the food, the radiation dose received, and the environmental conditions during and after irradiation (Wiendl, 1984). There are three different irradiation methods (radappertization, radicidation, or radurization) used to inactivate microorganisms based on the severity of the process (Jay et al., 2005). Radappertization is the most severe of the three irradiation methods. With radappertization or sterilization of food, a dose of irradiation is applied that decreases the activity and number of living microbes (excludes viruses) to such a low level, that there is no recognized method for detection. Doses required for radappertization are generally between 25 and 45 kGy (World Health Organization, 1994; Satin, 1997). All foods—including eggs without shells in the form of egg white, yolk, or whole egg—subjected to radappertization must be parceled in hermetically sealed packets so there is no recontamination of the product to the environment. Radappertization is popular for use in meat products such as chicken fillets and turkey breast. The National Aeronautics and Space Administration, the space agency of the United States, uses radappertization to prepare irradiated food for consumption of astronauts during space flights. The irradiated food products have no microbial viability, even at room temperature, provided the package is kept intact. All irradiated foods must have expiration dates regardless of whether the package is kept intact or not because prolonged storage causes chemical and physical changes in these products. Radicidation, similar to pasteurization, is the treatment of food with a sufficient dose of ionizing radiation to inactivate nonspore forming bacteria in a way that the microorganisms are not detected by bacteriological methods normally used on processed foods. Doses required for radicidation are generally between 2 and 8 kGy (World Health Organization, 1994; Satin, 1997). Examples of foods where radicidation is applied include juices, fresh meats, fresh pasta, and eggs.
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Radurization is the least severe of the three processes of irradiation with dosages in the range of 0.4–2.5 kGy. Radurization disinfects or sanitizes food and extends shelf life by causing a reduction in the count of viable spoilage microorganisms. Examples of where radurization is used in foods include the following: (1) preventing the sprouting of bulbs and tubers, (2) preventing the deterioration of fruits and vegetables by fungi, (3) killing parasites, insects, and mites that infest food, and (4) slowing down the ripening of fruits. The delay in ripening and the shelf-life extension in fruits like bananas is a great advantage as this fruit can ripen quickly without treatment. Use of radurization to delay the fruit ripening process provides time for food distribution and exportation. The efficacy of the radiation treatments in inactivating microorganisms is dependent on several factors. Food contaminated with higher numbers of microorganisms requires a higher radiation dose. The composition of food is another influential factor. Microorganisms present in a rich culture media consisting of proteins are more resistant to irradiation than microbes present in an aqueous media. The presence of oxygen makes microorganisms less resistant to radiation. The physical status of food has influence on the ability of radiation to inactivate microorganisms. Frozen or dehydrated food are more resistant to radiation as compared to thawed or hydrated food. There is also a large range in resistance to radiation among microorganisms. Generally, microorganisms with more complex DNA have greater sensitivity to irradiation. As microorganisms go through different stages of growth, their vulnerability to radiation changes. When bacteria are maturing and not undergoing cellular division (referred to as the lag phase), they are more resistant to radiation than the log phase when bacteria are multiplying (Fellows, 2006; Franco and Landgraf, 2008).
IRRADIATION OF EGGS The biggest concern in the egg industry today, with respect to raw egg, is the presence of pathogens, especially the large family of Gram-negative bacteria of Enterobacteriaceae. One Enterobacteriaceae of particular interest is Salmonella which can gain entrance into the egg prior to shell deposition when the hen is forming her egg (Okamura et al., 2001). During oviposition or egg laying, microorganisms, including Salmonella, can also invade the egg through the pores of the shell (Messens et al., 2005). In addition to the hen, humans can also introduce pathogens into eggs during collection, processing, distribution, and preparation. These microorganisms can be inactivated if cooked properly using adequate heat. However, some consumers eat raw eggs in eggnog or protein drinks or consume undercooked eggs (e.g., eggs fried sunny-side up) increasing susceptibility to food poisoning. Rather than purchase precooked eggs, consumers prefer to purchase raw eggs in shell for preparation at home. Although used in hospitals and restaurants, further processed pasteurized liquid products available at some retail units are less popular for home use perhaps due to tradition, price, and perceptions of quality. Hot pasteurization can be used on liquid, but not solid, foods for purposes of microbial decontamination. The use of irradiation is an alternative option in decreasing pathogens, such as Salmonella spp., when using heat is impractical or undesirable for food preservation. The effects of irradiation on microbial deactivation and the functional and sensory characteristics of treated eggs have been evaluated in liquid egg (both frozen and nonfrozen), powdered egg white and yolk components, fresh whole egg with shell intact, and cooked egg. The process used to irradiate eggs to reduce pathogenic microorganisms and parasites may be likened to an alternative cold pasteurization method without heat production (Loaharanu, 1997). Radiation results in electron collisions that produce ionization leading to chemical and biological changes in eggs. Electrons can be introduced into eggs directly from an electron accelerator or indirectly by photons generated by a radioactive source (McKeown and Drewell, 1996). For example, gamma rays from the radioactive isotope of cobalt 60 (Co60) is commonly used. The Co60 has a half-life of 5.263 years and is synthetically made from the cobalt isotope of Co59 through neutron activation. The Co60 decays through beta decay (0.314 million eV) to the nickel 60 (Ni60). The nucleus of the Ni60 emits two gamma rays with energies of 1.173 and 1.332 million eV. As the gamma rays generated by a Co60 source have far greater power penetration than electron beams (Diehl, 1995), gamma rays are applied to thicker, large volume foods, whereas the electron beam is used in surface radiation (Satin, 2002). To test the effectiveness of gamma irradiation, five isolates of S. enteritidis were inoculated on the surface and inside of whole shell eggs. After inoculation, the eggs with shell intact were subjected to gamma irradiation doses of 0, 0.5, 1.0, and 1.5 kGy. Liquid whole egg was also inoculated with two isolates of S. enteritidis before irradiation with 0, 0.25, 0.50, 0.75, and 1.00 kGy. A minimum dose of 0.5 kGy was sufficient to eliminate S. enteritidis from the surface of eggs, but a higher dose of 1.5 kGy was needed to eliminate the bacteria from whole egg with shell intact as well as the liquid whole egg. There was no adverse effect on yolk color and thermolability of egg white proteins at any level of irradiation employed (Serrano et al., 1997). Using a Co60 source, liquid egg white was irradiated with 0, 0.432, 0.576, 0.720, and 0.864 milliradian and then frozen for later measurements of physical and functional characteristics. Evaluation of thawed irradiated egg white demonstrated
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a reduction in viscosity and a small decrease in the proportion of egg white solids. Upon further examination of electrophoretic protein patterns, the proportion of ovomucin, ovalbumin, and conalbumin decreased with a concomitant increase in the proportion of globulins as a result of irradiating the egg white (Ball and Gardner, 1968). In the dissertation of Harder (2009), similar results occurred where irradiated egg white experienced damage to the protein, ovomucoid. Irradiation doses used in the study of Ball and Gardner (1968) caused molecular degradation of proteins which impacted functional properties. Specifically, the angel food cake prepared from irradiated egg white required longer beating times and was of poorer quality due to texture and reduced volumes. The irradiated egg white foams were notably less stable. Interestingly, increasing the time in frozen storage improved the functional properties of irradiated egg white as indicated by cake volume and beating times of cake batter. Compared to the controls, the irradiated eggs had a different appearance and odor. Specifically, the irradiated eggs both before and after freezing had a dull greenish-yellow color and an odor similar to a mixture of sulfur and ammonium. The thawing of the irradiated egg white caused two separate layers to form. The bottom two-thirds layer was thicker than the upper layer of the thawed irradiated egg white. Once the two layers were blended together, no further physical separation of egg white occurred (Ball and Gardner, 1968). Irradiation doses of 1–4 kGy applied to frozen liquid egg white or yolk did not affect integrity of egg white proteins; however, the low dose irradiation caused a decrease in the viscosity of frozen yolk. In general, the functional properties of the liquid egg products, including emulsification, gelling, and foaming, were not affected by the irradiation treatments. In fact, use of egg white protein that was irradiated when frozen displayed enhanced functional properties, such as increased angel food cake volume, and the liquid egg yolk that was irradiated when frozen demonstrated increased stability and stiffness when used in mayonnaise. These results of Ma et al. (1993), along with research demonstrating that eggs irradiated with 2 or 3 kGy reduced bacterial contamination, especially Salmonella spp., to nondetectable levels (Tellez et al., 1995), support use of 1–4 kGy of irradiation to preserve frozen liquid egg products. Gamma radiation from Co60 was applied to powdered samples of egg white, yolk, and whole egg at doses that ranged from 0 to 25 kGy. The irradiated samples were rehydrated and viscosity was measured. Irradiation doses above 5 kGy resulted in no change in the viscosity of the egg white. However, yolk or whole egg showed changes in rheological properties as radiation dose increased which were most likely due to lipid degradation and the accumulation of lipid hyperoxides. In addition, the color of the irradiated yolk powder faded in response to increasing radiation dose (Ferreira and Del Mastro, 1998). Carotenoids (the yellow to orange fat soluble pigments) from eggs are more sensitive to irradiation than carotenoids from other sources like dry plants (Katusin-Razem et al., 1989; Harder, 2009; Fig. 17.2). These results suggest that powdered egg white as compared to yolk proteins are less susceptible to irradiation induced degradation. Solid egg yolk and solid whole egg were subjected to irradiation doses up to 10 kGy using Co60 gamma radiation. Although irradiation caused no detectable changes in vitamins B1 (thiamine) or B2 (riboflavin) or egg acidity, decomposition of triglycerides and amino acids was evident. The generation of free radicals from oxidized lipids during irradiation exposure most likely contributed to the amino acid damage. Irradiation of the solid egg yolk and whole egg also caused a decrease in carotenoids and vitamin A. However, removal of oxygen during irradiation minimized the damage to egg solid protein and lipids. Scrambled eggs and mayonnaise made from egg product subjected to 3 kGy of irradiation in air or up to 5 kGy of irradiation without oxygen were indistinguishable from nonirradiated control egg products (Katusin-Razem et al., 1989).
FIGURE 17.2 Eggs samples subjected to 20 kGy of irradiation as compared to the control eggs not subjected to irradiation. The carotenoids and vitamin A content of yolk is reduced as a result of irradiation resulting in paler yolks.
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Liquid egg, frozen egg, egg yolk powder, egg white powder, or whole egg powder were each inoculated with S. enteritidis followed by irradiation doses of 0, 2, 3, or 3.5 kGy. The highest dosage of radiation (3.5 kGy) caused a 5 log cycle reduction of S. enteritidis. A decrease in yolk color occurred in all irradiated liquid and frozen egg samples (Froehlich, 2004). The irradiation of whole eggs with shell intact has also been explored as a means of deactivating Salmonella spp. as a replacement to thermal pasteurization of shelled eggs. Fresh eggs, with shell intact, were inoculated with 108 colony forming units of S. enteritidis and then exposed to gamma radiation doses of 0, 1, 2, or 3 kGy. No viable bacterial cells were detected in the shell as well as the internal eggshell membranes with the higher doses of 2 and 3 kGy. The lower dose of irradiation (1 kGy) resulted in a 3.9 log reduction of S. enteritidis on the shell and a 95% reduction of this pathogen on the shell membranes. However, all levels of irradiation tested in this study caused a 50% deterioration in egg white quality as measured by Haugh units. The yolk color faded in the whole shell eggs treated with 2 and 3 kGy of irradiation. These results suggest that 1 kGy reduced and 2 or more kGy of irradiation eliminated S. enteritidis but not without deleteriously affecting egg white quality and yolk color (Tellez et al., 1995). Using a linear accelerator, fresh eggs with shell intact were subjected to 0, 2.5, 5, or 10 kGy of gamma irradiation. After treatment, the shells were broken and the yolk and egg white separated from one another, and the egg products stored up to 14 days at 4°C (39.2°F). Egg white viscosity was decreased and turbidity increased by exposure to irradiation. A low dose of irradiation (2.5 kGy) did not affect foaming capacity or foam stability, but higher doses of radiation greater than 5 kGy resulted in poor foaming characteristics. As the radiation dose increased, the sulfur-containing volatiles also increased, but storage in the presence of oxygen eliminated the odor. When irradiated egg white was cooked, the greenness value increased and the yellowness and lightness values decreased in a dose-dependent manner. The texture of the cooked egg white as evaluated through chewiness, resilience, hardness, adhesiveness, and cohesiveness also increased as the radiation dose was increased, but changes were small. Therefore, irradiation doses of 2.5 kGy or lower on shelled eggs had little impact on the functional and physiochemical properties of liquid egg white suggesting that irradiation technology applied to eggs with shell intact has potential for commercial application (Min et al., 2012). Cooked fried eggs were sterilized in vacuumed packed oxygen impermeable nylon bags using 30 kGy of gamma irradiation; inoculated with 106–107 colony forming units/g with each of four pathogens; subjected to 0, 1, 2, or 3 kGy of irradiation; and stored at 10, 20, or 30°C (50, 68, or 86°F) where microbiological evaluations were performed at 0, 8, and 24 h. A dose of 3 kGy eliminated Staphylococcus aureus, Listeria ivanovii, Salmonella typhimurium, and Escherichia coli in fried eggs suggesting that irradiation was a useful tool in enhancing the food safety of eggs used in composite meals (Jo et al., 2005). A by-product of irradiation of egg yolk is cholesterol oxidation products such as cholestane-triol, cholesterol-α-epoxide, 7-hydroxylcholesterol, 7-ketocholesterol, and 25-hydroxycholesterol. Dried yolk powder stored for 4 years and then subjected to ultraviolet radiation for 3 weeks showed a dramatic increase in cholesterol oxidation products (Table 17.1, van de Bovenkamp et al., 1988). In addition, a 1 kGy dose of ionizing radiation also induced formation of cholesterol oxidation products in spray dried egg powder. Subjecting the egg powder to higher irradiation doses up to 6 kGy further increased the cholesterol oxidation products (Lebovics et al., 1992). These oxidized products of cholesterol cause more injuries to human arterial cells as compared to pure cholesterol leading to coronary heart disease and atherosclerosis. Moreover, any cholesterol-containing foods that have been exposed to air and heat during processing or that has been stored at ambient temperatures may contain cholesterol oxidation products. For example, spray dried egg powder stored for 3 months that
TABLE 17.1 The Effect of 3 Weeks of Exposure to Ultraviolet Irradiation on the Concentration of Cholesterol Oxidation Products of Egg Yolk Powder that had been Stored for 4 Years at Ambient Temperature Cholesterol Oxidation Product
No Irradiation (µg/g of egg yolk)
Ultraviolet Radiation (µg/g of egg yolk)
Cholestane-trio
0.5
62.0
Cholesterol-α-epoxide
25.2
2522.0
7-Hydroxylcholesterol
78.0
507.0
7-Ketocholesterol
4.2
200.0
25-Hydroxycholesterol
13.9
860.0
Source: Adapted from van de Bovenkamp, P., Kosmeijer-Schuil, T.G., Katan, M.B., 1988. Quantification of oxysterols in Dutch foods: egg products and mixed diets. Lipids 23, 1079–1085.
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was never subjected to irradiation had the same level of 7-hydroxylcholesterol as a fresh sample of egg powder irradiated with 6 kGy of ionizing radiation (Lebovics et al., 1992). Methods for reducing the accumulation of cholesterol oxidation products in foods need to be identified (Hur et al., 2007).
OTHER USES OF GAMMA IRRADIATION IN FOOD Structural alteration of proteins through use of gamma irradiation is being investigated as a means of reducing food allergies. Common food allergies in humans include milk β-lactoglobulin, shrimp tropomyosin, and egg albumin. Subjecting food to ionizing radiation changes the antigenicity of food by altering the physical and chemical structure of proteins leading to distortion of the protein’s secondary and tertiary structures. Specifically, the epitope area of the food allergen can be modified or destroyed by gamma irradiation so that antibodies to the allergen should never be produced by the individual consuming the irradiated food (Byun et al., 2002; Harder, 2009).
LEGISLATION According to the Codex Alimentarius Commission (1984), irradiation of foods are permitted using one of the following sources of irradiation: (1) gamma rays in which the radionucleotide is Co60 with maximum power of 1.332 million eV and a half-life of 5.263 years or cesium 137 (Ce137) with a maximum energy of 0.0662 million eV and a half-life of 30 years; (2) X-rays generated by machines operating with power up to 5 million eV; or (3) electrons generated by machines operating at 10 million eV or below. The doses for irradiated foods are determined in accordance with the laws on food irradiation provided by each country. Most countries consider 10 kGy as the maximum dose to be used to irradiate food. The current resolution in Brazil dictate that: (1) the minimum absorbed dose should be sufficient to achieve the intended purpose; and (2) the maximum absorbed dose should be less than that which would compromise the functional properties and sensory attributes of the food (Brazil, 2001). Some countries require that irradiated foods be labeled with the phrase “food treated by irradiation” or that the Radura (derived from radurization) symbol be printed on the packaging label (Fig. 17.3). This symbol is used internationally to indicate that a food product has been subjected to irradiation. The Radura logo can vary in color and graphics depending on the country. The logo represents a plant within a circle and is most commonly colored green. The circumference composing the top half of the circle has dashed lines. There are varying regulations among countries in the labeling requirements of irradiated food. In the European Union, use of the Radura symbol is not common. Instead, food packages use labels indicating irradiation in the respective language of the home country. In addition, information on each irradiated ingredients in the final product must be included on the label. This rule applies to restaurant food as well. In the United States, the Food and Drug Administration requires that irradiated foods sold in stores include the Radura symbol as well as the statement “treated by irradiation” or “treated with radiation” on the label. Irradiated processed foods or foods sold in restaurants do not require labeling in the United States.
FIGURE 17.3 The Radura symbol is placed on irradiated food packages in some countries of the world. The Radura symbol originated from and was copyrighted by an irradiation food processing facility located in Wageningen, The Netherlands in the 1960s. Jan Leemhorst, then president of the company called Gammaster, recommended its use as an international label to be placed on irradiated food as long as manufacturers implemented appropriate quality parameters. The Atomic Energy of South Africa also used the Radura symbol, but instead of using the term “irradiation,” they used “radurized” on the food label. The Radura symbol is listed in the Codex Alimentarius Standard on Labelling of Prepackaged Food (CODEX-STAN 1, 2010. General standard for the labelling of prepackaged foods. http://www.codexalimentarius.net/download/standards/32/CXS_001e.pdf)
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CONSUMER PERCEPTION Public perception that irradiated food is a radiological or biohazard has resulted in these processed foods not be widely adopted by food manufacturers and the consuming public. Those individuals knowledgeable of the fact that irradiation poses no health risk to humans who consume irradiated food are frustrated by the requirement that a label be placed on irradiated food packages. The fact that food wastage due to spoilage can be minimized because of the extended shelf life of irradiated food leads to an environmentally friendly technology that most consumers often give little thought to. Increasing demand for food in developing countries with rapidly growing hungry populations can be partially alleviated by preventing food spoilage through the use of gamma irradiation.
STRATEGIES FOR IMPROVEMENT Though food irradiation is considered by the experts to be a clean and efficient technology capable of deactivating foodborne pathogens, its use as an appropriate food safety measure needs to better communicated to the consuming public to avoid misperceptions. For eggs, there is the potential of using irradiation as a nonthermal method to eliminate food pathogens without altering the characteristics of the eggs as may occur in heat treatments. A sector that demands further study is implementing technology during irradiation of eggs that improves sensory properties, and the development of new irradiated egg products.
ANALYTICAL METHODS Yolk color: A Roche fan is used to evaluate the color of the egg yolk. After breaking the shell and pouring the interior contents of the egg onto a glass plate, the color of the yolk is matched with 1 of the 10 colored bands of the fan. The ranges for color score are from very pale with a score of 1 to a score of 10 which is a very intense orange-yellow color (Sun et al., 2012). Egg white and yolk color: A colorimeter is used to determine the Commission Internationale de l’Eclairage L* (lightness), a*[redness (+)/greenness (−)], and b* (yellowness) values of egg white (Min et al., 2012) and yolk (Harder et al., 2007). Viscosity, turbidity, and foaming properties of liquid egg white: A viscometer is used to determine viscosity of liquid egg white. Turbidity is measured using a spectrophotometer. Specifically, the egg white is diluted 20 times with deionized water. The optical density of the diluted sample is measured at a wavelength of 320 nm against a blank containing deionized water (Xiong, 1992). For the determination of foaming capacity and foam stability, liquid egg white (100 g) is whipped with a wire whip for 5 min generating a foam which is then transferred into graduated cylinder that has been preweighed. The weight and the volume of the foam are determined to calculate specific density as grams per millimeter of foam as an indicator of foaming capacity. For foam stability, the foam in the graduated cylinder sits for 30 min at ambient temperature. The volume of drainage in milliliters is determined (Min et al., 2005). Volatile analysis of egg white: Gas chromatography that has a mass selective detector determines the quantity and quality of volatile compounds in liquid egg white (Min et al., 2012). Texture of cooked egg white: Cooked egg white sticks are analyzed for texture properties using a texture profile analyzer with a 5 kg loading cell and a cylinder plunger with a 38 mm wide diameter. The curve generated from the texture profile analysis determines cohesiveness, hardness, chewiness, springiness, and gumminess (Min et al., 2012). Haugh unit: An indicator of albumen quality is the Haugh unit. The weight of an individual egg is weighed using a scale with 0.01 g sensitivity. After cracking the shell, the yolk and albumen are poured onto a level, flat surface. The thick albumen height is measured using a gauge which is sensitive to 0.01 mm. The following formula is used to calculate Haugh unit: Haugh unit = 100 × log (H + 7.57 − 1.7 × W0.37), where H is the height of the albumen in millimeters and W is the egg weight in grams (Haugh, 1937).
CONCLUSIONS l
Irradiation is a clean and efficient technology capable of deactivating foodborne pathogens and extending shelf life so that food is not wasted due to spoilage. l Irradiation at an appropriate dose eliminates foodborne pathogens in frozen and nonfrozen liquid egg, powdered egg white and yolk components, fresh whole egg with shell intact, and cooked egg.
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l
Whole eggs with shell intact inoculated with S. enteritidis and then treated with gamma irradiation showed that 1 kGy reduced and 2 or more kGy of irradiation eliminated the microorganisms but not without deleteriously affecting egg white quality and yolk color. l One of the important factors needing further study is to improve on the sensory aspect of the irradiated egg. l Public perception that irradiated food is a radiological or biohazard has resulted in these processed foods not be widely adopted by food manufacturers and the consuming public. l To avoid misperceptions, improved communication is needed with the public on food irradiation being an environmentally friendly technology that reduces food wastage and improves food safety.
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