New Food Engineering

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Nova Science Publishers, Inc.

Copyright © 2008 by Nova Science Publishers, Inc.

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LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA New food engineering research trends / Alan P. Urwaye, editor. p. cm.

Includes index.

ISBN-13: 978-1-60692-828-8

1. Food industry and trade--Research. I. Urwaye, Alan P.

TP370.8.N49 2007

664--dc22 2007028954

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ONTENTS

Preface vii

Chapter 1 Ionizing Irradiation of Foods 1

Albert Ibarz

Chapter 2 Fruits and Vegetables Dehydration in Tray Dryers 45

Dionissios P. Margarisand Adrian-Gabriel Ghiaus

Chapter 3 Ultrasound in Fruit Processing 103

Sueli Rodrigues and Fabiano A.N. Fernandes

Chapter 4 Optimisation of the Conversion of Ergosterol in

Mushrooms to Vitamin D, and Its Bioavailability 137

Conrad O. Perera and Viraj J. Jasinghe

Chapter 5 Protein Hydrolysis with Enzyme Recycle

by Membrane Ultrafiltration 169

Antonio Guadix, Emilia M. Guadix and Carlos A. Prieto

Chapter 6 The Development of the Processing of Yuba

(Protein-Lipid Film) 195

Li Zaigui, Shen Qunand Lin Qing

Chapter 7 Far-Infrared Heating in Paddy Drying Process 225

Naret Meeso

Chapter 8 A Novel Two-Stage Dynamic Packaging for Respiring

Produce: Concepts and Mathematics 257

Tobias Thiele and Benno Kunz

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REFACE

This new book presents new research in the growing field of food engineering which refers to the engineering aspects of food production and processing. Food engineering includes, but is not limited to, the application of agricultural engineering and chemical engineering principles to food materials. Genetic engineering of plants and animals is not normally the work of a food engineer.

Food engineering is a very wide field of activities. Among its domain of knowledge and action are: Design of machinery and processes to produce foods Design and implementation of food safety and preservation measures in the production of foods Biotechnological processes of food production Choice and design of food packaging materials Quality control of food production

Chapter 1 - Irradiation, like other types of food treatments, is a method used to make food safer for the consumer and to increase its useful life in good conditions. In this chapter the interaction of ionizing radiation with matter and the sources of production of ionizing radiation are described. The biological effects caused by this type of radiation are also described. Likewise, the application of ionizing radiation in the food industry is described as well as the effects that it has on most food components. The inhibitory effect on micro-organisms is described, as well as the effects on different kinds of foods such as meat, poultry, fish and shellfish, eggs and egg-derived products, tubers and bulbs, seeds, legumes, dry fruits, spices, seasonings and herbs, and for quarantine treatment. Finally, a short description of food treatment plants, dosimeters and certain current normative aspects of the ionizing radiation used are given.

Chapter 2 - Dehydration involves simultaneous transfer of heat, mass and momentum in which heat penetrates into the product and moisture is removed by evaporation into an unsaturated gas phase. Owing to the complexity of the process, no generalized theory currently exists to explain the mechanism of internal moisture movement. In this Chapter, the investigation of momentum, heat and mass transfer phenomena, in both laboratory and large scale convective drying systems (suitable for dehydration of thermolabile products) by means of experimental measurements and numerical simulationare presented.

The air flow inside complex geometry spaces, such as drying rooms containing hundreds of trays arranged in rows and columns, is analyzed by solution of 3-D momentum turbulent flow equations for different room configurations. Laboratory measurement data, concerning the space velocity distribution and the pressure field of the air flow over one tray, are provided and used for validation of turbulence models. The results of the flow investigation

lead to practical suggestions for the improvement of the air flow uniformity inside the drying space which is very important for the quality of the product.

A novel numerical code, DrySAC, able to predict the unsteady-state processes taking place in a complex drying system, was developed. Unlike other attempts to predict drying processes, DrySAC takes into account not only the drying process itself, but also the behavior of the other system equipment and the interaction between them. Drying curves, evolution of the air state parameters in characteristic points of the system and product properties are predicted during the drying of various fruits and vegetables and. As a practical validation of the code, the predicted values compared with the measured data taken in-situ showed very good agreement. When a dryer configuration is given, the numerical DrySAC code can be used for optimization of the process parameters when a dryer configuration is given. For the most of the studied cases, an air recirculation ratio of around 75 % has proved to be the optimum, giving a minimum drying time. The code can be used both for evaluation of existing dryers and for optimum design of the new units with valuable impacts in increasing the efficiency of the systems and in reduction of energy consumption.

Aiming to overcome the lack of experimental data in the open literature, a laboratory drying unit was constructed and is under operation for testing and monitoring the dehydration of agricultural products. Using this facility, experimental drying curves are set up for the drying of horticulture products under controlled conditions of the drying air parameters, which are gathered by means of a data acquisition system. The laboratory experimental results are useful for the validation of numerical models which further are an essential tool for optimization and increasing the efficiency of the drying process. Drying of agricultural products remains an open research field mainly because of their delicate and hard to be established, properties.

Chapter 3 - Power ultrasound has been successfully employed in the chemical industry, polymer and plastic industry for many years and its use has been growing in the food industry. Power ultrasound can produce chemical, mechanical or physical effects on the processes or products where it is applied. Taking advantage of one of the effects or their combination, power ultrasound has been used in the food industry in drying, freezing, extraction processes and enzyme inactivation. The use of ultrasound in ambient fluids is well known to cause a number of physical effects (turbulence, particle agglomeration, microstreaming and biological cell rupture) as well as chemical effects (free radical formation). These effects arise mainly from the phenomenon known as cavitation.

Herein a brief review of the use of ultrasound in the food industry is presented and the main applications are discussed. A comprehensive discussion on the effects of ultrasound in the tissue structure of fruits is presented along with photomicrographs of melons submitted to ultrasound.

A detailed discussion is presented concerning the use of ultrasonic waves in drying, where an ultrasonic pre-treatment can be used prior to air-drying. The methods involved in the ultrasonic pre-treatment are presented along with the results obtained for several fruits such as melons, bananas, pineapples, papayas and other. Mathematical models that can be used to simulate the process are presented. Optimization of the drying process is also discussed for ultrasonic pre-treatment and ultrasound-assisted osmotic dehydration.

Chapter 4 - The conversion of ergosterol in mushrooms to vitamin D2 by exposure to

ultra violet (UV) light was studied under different UV lamps (UV-A, UV-B, and UV-C) and was found to be significantly different (p<0.05). Analysis of ergosterol content in different

tissues of Shiitake mushrooms showed a significant difference (p < 0.01) in its distribution. Thus, the conversion of ergosterol in whole mushrooms to vitamin D2, by exposure to UV

light was significantly affected (p < 0.01) by the orientation of the mushroom tissues to the UV source. The conversion of ergosterol to vitamin D2 was about four times higher when

gills were exposed to UV light compared to when the outer caps were exposed to the same. The highest vitamin D2 content (184.22 ± 5.71 μg/g DM) was observed in Oyster

mushrooms exposed to UV-B light at 35 oC and around 80% moisture. On the other hand, under the same conditions of UV-exposure, the lowest vitamin D2 content (22.90 ± 2.68 μg/g

DM) was observed in Button mushrooms.

The kinetics of conversion of ergosterol to vitamin D2 showed that Oyster mushrooms

(Pleurotus ostreatus) had the highest conversion rate followed by Shiitake (Lentinula edodes) and Abalone (Pleurotus cystidus), whereas the lowest conversion rate was observed in Button mushrooms (Agaricus bisporus). Both initial moisture content and temperature of UV exposure influenced the conversion of ergosterol. The conversion of ergosterol to vitamin D2

followed zero-order kinetics, where the rate constant varied with temperature according to the Arrhenius equation (K0 = 7.32 s

-1

; Ea = 51.5 kJ mol -1

).

For the bioavailability of vitamin D, thirty male Wistar rats were fed for one weekwith a diet deficient in vitamin D. After the first week, six rats were randomly selected and sacrificed for analysis of initial Bone Mineral Density (BMD), and serum level of 25-hydroxyvitamin D [(25(OH)D]. The remaining animals were divided into two groups of 12. One group received 1 μg of vitamin D2/day from UV-exposed mushrooms for a period of four

weeksuntil sacrificed. The other group was fed the same amount of mushrooms that was not exposed to UV light, and was use as the control. At the end of four weeks, the mean serum 25(OH)D level of the experimental group was 129.42 ± 22.00 nmol/L, whereas, it was only 6.06 ± 1.09 nmol/L in the control group. The Femur BMD and the serum calcium concentration of the experimental group of animals were significantly higher (p < 0.01) than the control group. It may be concluded from the results that vitamin D2 from UV-exposed

mushrooms is well absorbed and metabolised in this modelanimal system.

Chapter 5 - Enzymatic hydrolysis allows to improve the functional, nutritional and immunological properties of proteins. For instance, protein hydrolysates are used in the food industry as ingredients in hypoallergenic formulae and clinical nutrition.

Conventional batch hydrolysis of proteins has been traditionally used to obtain protein hydrolysates due its simple operation. However, there are several disadvantages associated to this method, the high enzyme consumption being the main one. Among the solutions assayed to enhance the yield of the process, enzyme immobilisation onto highly activated supports allows to work continuously and reuse the enzyme. Since there are loss of enzyme activity and constrains for the diffusion into the support, the feasibility of this technique is limited. Continuous reaction and simultaneous separation of products from the reaction mixture can be achieved in a continuous membrane recycle reactor. Here, the low molecular weight peptides generated permeate through an ultrafiltration module with the appropriate molecular weight cut-off, while the enzyme (which acts in soluble form) is continuously recycled to the reaction tank. As important drawbacks, permeate flux decline due to membrane fouling and frequent purges are required to eliminate non-reacting substrate which involves severe difficulties in the control process.

In order to profit from the advantaged of both batch and continuous membrane recycle reactor, the objective of this research work was to design and optimise a reactor for the production of low antigenicity protein hydrolysates. The operation proposed comprises 3 consecutive steps: a) hydrolysis in a stirred tank reactor; b) ultrafiltration of the reaction mixture through a membrane with full retention of the enzyme; c) enzyme recycling, in which the retentate is returned to the tank reactor for a new hydrolysis.

The materials employed were a whey protein concentrate as substrate, a subtilisin from

Bacillus licheniformis as protease and an 8 kDa flat polyethersulfone membrane. The pH-stat

method was used to monitor the hydrolysis reaction. The molecular weight profile and the antigenicity of the hydrolysates were determined by HPLC and ELISA, respectively.

Regarding the process kinetics, zero-order for the substrate and second order for the enzyme deactivation were identified. Process optimisation involved the calculation of the optimum number of enzyme uses that minimised the enzyme consumption, subject to a required productivity. The performance of the system was compared at several temperatures to that of the conventional stirred tank reactor. Significant enzyme savings were achieved, which demonstrate the viability of this approach.

Chapter 6 - Yuba is a kind of protein-lipid film formed from soymilk under continuous heating, so it is also called to be “Tofupi” or “Tofuyi”, which means “tofu sheets or tofu shirts”. Yuba is one of the most famous traditional foods which has a history of over 2000 years and is very popular now in China. The annual yield of yuba is over 200,000 ton in China. The film can be consumed directly as an ingredient of soups or be used as a sheet for wrapping and shaping meats or vegetables into various forms with different tastes. Now freezing yuba is also used as salad for sashimi in Japan and Korea.

Yuba has first been found from the supernatant film of heated soymilk for Tofu making and was usually dried to 8% moisture content for storage. It can be stored for more than three months to six months. It contains about 55% protein and 25% lipid, which are important nutrients for monks to whom meat was prohibited. As its fine taste and unique texture, yuba spread fast in all of the country through the rede of people lived in temples.

Chapter 7 - Far-infrared heating is applied in two paddy drying processes, namely, single-stage and multi-single-stage drying processes. The single-single-stage drying process is the combination of far-infrared radiation and hot-air convection in fluidized-bed drying, and the multi-stage drying process consists of hot-air convective fluidized-bed drying, far-infrared heating, tempering and ambient air ventilation. The effect of far-infrared heating in paddy drying process on moisture content, grain temperature and milling qualities (e.g. head rice yield and whiteness) is investigated together with the microstructure of rice kernels and the pasting behavior of rice flours. Moreover, the mathematical model of far-infrared heating, which is the set of coupled heat and mass transfer equations, is developed to describe the paddy drying. This model assumed that the absorbed infrared energy completely converts to heating within the superficial layer under the surface of paddy grain, and heat is transferred into the deeper layer via conduction. Validation of the developed models is made by comparing predicted and experimental data for the average moisture content and the grain temperature of paddy.

Chapter 8 - Even when applying optimal storage conditions, off-flavors occur in packages with respiring commodities. In the case of fresh-cut produce mixes, compromises due to different optimal storage conditions have to be made and off-flavors are more likely to occur. Additionally unwanted temperature changes during the cold-chain can not always be

avoided, which leads to excessive respiration and anaerobic conditions and off-flavors are formed due to fermentation. In this work a two-stage, dynamic packaging concept is presented, where the undesired off-flavors can evaporate at the end of the shelf life by shifting the gas exchange from permeation to diffusion. This can be realized by removing an adhesive film strip from the package by the customer. A mathematical model is developed to describe the gas composition inside this kind of two-stage packages by combination of model terms for permeation, diffusion and respiration. To estimate the effect of diffusion, the gas exchange of a model package with perforations is determined and a coefficient is calculated by non-linear regression analysis. This coefficient can be used for the model to describe the gas exchange after opening the perforations.

To prove the effectiveness of the two-stage, dynamic concept, a test series with fresh-cut chicory endive was carried out. The results showed a significant decrease of lactic acid (67% at 7°C, 36% at 20°C) and ethanol (56 % at 20°C) and the end of the shelf-life compared to the traditional concept of modified atmosphere packaging. By storage experiments with temperature changes from 7°C to 20°C for 4 and 8 hours it was shown that the gas composition changed for the remaining shelf life and ethanol was detected within the packages.

Editor: Alan P. Urwaye, pp. 1-43 © 2008 Nova Science Publishers, Inc.

Chapter 1

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ONIZING

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Albert Ibarz

Food Technology Department University of Lleida (Spain)

A

Irradiation, like other types of food treatments, is a method used to make food safer for the consumer and to increase its useful life in good conditions. In this chapter the interaction of ionizing radiation with matter and the sources of production of ionizing radiation are described. The biological effects caused by this type of radiation are also described. Likewise, the application of ionizing radiation in the food industry is described as well as the effects that it has on most food components. The inhibitory effect on micro-organisms is described, as well as the effects on different kinds of foods such as meat, poultry, fish and shellfish, eggs and egg-derived products, tubers and bulbs, seeds, legumes, dry fruits, spices, seasonings and herbs, and for quarantine treatment. Finally, a short description of food treatment plants, dosimeters and certain current normative aspects of the ionizing radiation used are given.

1.

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Irradiation, like other types of food treatment, is a method used for treating foods in order to make them safer for the consumer and to increase the period for which they can be kept in good condition. In other words, it is used for making food safe and prolonging preservation times. It is a method that does not attempt to replace conventional treatments, but rather one that can be used as a complement to these treatments.

The dictionary of the Real Academia Española (Spanish Royal Academy) defines the verb radiate as “production of radiation by means of waves or particles”, while irradiate is “emit rays of light, heat or other energy from a body, or subject something to radiation”. In other dictionaries (Larouse, van Nostrand) radiation is defined as “the emission and propagation of energy in the form of waves through space or a natural medium”. Common usage of the term radiation is to refer to the waves or rays in the electromagnetic spectrum.

Irradiation from a body should not be confused with its being radioactive, as a body is

radioactive when its atoms split spontaneously.

The irradiation of foods is not a new treatment technology, as its roots can be traced back to the late 19th century; although it is not until the 1940’s that the term irradiation appears. Three clearly differentiated stages or periods in the history of food irradiation can be distinguished: the period from 1890 to 1940 represents the beginnings of the physics of irradiation and of the different sources used, all tied to the first treatments of food with radiation. The period from 1940 to 1970 corresponds to a stage of intensive research and development in the application of radiation in food treatment, and to the study of the healthiness of irradiated foods. Since 1970 a series of regulations for the safe control and application of irradiation have appeared.

From a historical viewpoint, radiation can be dated back to 1895 with the discovery of X-rays by von Roentgen. In 1896 Becquerel discovered radioactivity, and in the same year there was an initial proposal for the application of ionizing radiation in the preservation of foods in Germany. In 1898 Thompson discovered the nature of cathode rays, observing that they were electrons, and in that same year the effects of radiation on micro-organisms were already observed. In 1902-1903 Rutherford and Soddy published a theory on radioactive decay, and Marie Curie published her thesis on the nature of alpha, beta and gamma radiation. It was in 1904 when studies were published on the bactericidal effect of ionizing radiation, while in the following year, when Einstein published his theory of relativity, in Great Britain a patent was issued for the use of ionizing radiation to eliminate bacteria from foods. At the same time, in the USA, another patent was issued for the mixing of foods with radioactive matter aimed at prolonging food preservation. The period between 1905 and 1920 corresponds to a stage of basic research into the nature and the physical, chemical and biological effects of ionizing radiation. In this period studies appeared on the processing of strawberries with radiation, and a USA patent was issued for the multi-tube processing of food with X-rays. In the years 1920-30, important developments were obtained in the design of electron accelerating machines, and studies were published on the lethal effects that X-rays exerted on Trichinella spiralis in raw pork. Additionally, publications on the effects of ionizing radiation on enzymes appeared for the first time, a bioassay being performed on rodents in order to determine the possible toxicity of the irradiated foods. In 1930 a French patent was issued for the use of ionizing radiation in food preservation. In the 1940’s in the Massachusetts Institute of Technology (MIT) the viability of the preservation of minced beef by treatment with X-rays was demonstrated. In the 1950’s food irradiation programs began in the USA and in the United Kingdom, while of special note was the declaration of “Atoms for Peace” (1953) which Eisenhower delivered in the General Assembly of the United Nations, for the pacific use of atomic energy, including its application in food preservation. At the end of the same decade the USSR approved the irradiation of potatoes and grain, while in Germany a licence was granted for the treatment of spices with radiation. Furthermore, in the USA, irradiation was classified as an “additive”. In the 1960’s in Canada the irradiation of potatoes was approved, but in the Federal Republic of Germany the irradiation of food was prohibited, while the Food and Drug Administration (FDA) of the USA approved the irradiation of wheat, flour, potatoes and bacon, although it was repealed for this latter foodstuff in 1968. In 1969 in Spain the irradiation of potatoes and animal feed was initiated, as was the irradiation of mushrooms and frozen meat in the Netherlands. In the 1960’s NASA adopted irradiation as a method of food sterilization for astronauts, in Japan the irradiation of potatoes was initiated on an industrial

scale, while in Spain the irradiation of onions was initiated. In 1976 the Joint FAO/IAEA/WHO Expert Committee of the Food and Agriculture Organization (FAO), the World Health Organization (WHO) and the International Agency for Atomic Energy (IAEA) certified the wholesomeness of various irradiated foods (potatoes, wheat, chicken, papayas and strawberries), recommending that the irradiation of foods be classified as a physical process. In 1980 the Joint FAO/IAEA/WHO Expert Committee on the Wholesomeness of Irradiated Foods (JECFI) declared that the irradiation of foods at doses of up to 10 kGy did not constitute any danger, causing neither nutritional nor microbiological problems. It was in the 1980’s when the Codex Alimentarius Commission accepted as a worldwide norm the conclusions elaborated by the Committee of Experts, and this was also the decade in which an International Consultative Group for Food Irradiation (ICGFI) was established under the patronage of FAO/WHO/IAEA to evaluate the developments in food irradiation. In 1985 the final regulations of the USA and Canada on the irradiation of foods were established, and the FDA approved the irradiation of pork at low doses in order to control Trichinella. Likewise, the FDA approved irradiation to control insects, to delay the ripening of fruits and vegetables, spices and dehydrated enzymes. It was also in that decade when the European Community prepared the first draft to harmonize the legislation of the different member states with regard to irradiated foods. In the 1990’s the FDA approved the irradiation of poultry for the control of Salmonella, and in 1992 the WHO reaffirmed that irradiated foods were safe. In 1996 there were 40 countries authorizing the irradiation of at least one foodstuff, while 28 countries applied the irradiation of food on a commercial basis. In 1999 a European Union directive approved the irradiation of spices, herbs and seasonings, while the FDA presented a petition to unblock the irradiation of pre-prepared foods, due to lysteriosis epidemics that could be caused by this kind of food. In 2000 the FDA unblocked irradiation in order to be able to control Salmonella in eggshells and to decontaminate seeds.

Because of the problems implied in the irradiation of foods and all of the regulations concerning it, the ICGFI has published different Good Practice Codes on irradiation (table 1).

The irradiation of foods is a technique which has been given an unfortunate name, as the treatment of irradiation processing has been related to nuclear energy. This has meant that many times the irradiated food has been mistaken for being radioactive. However, these are completely different terms, as the former is treatment with radiation, while the latter refers to foods with radioactive potential. For this reason this kind of processing must demonstrate that food treated with irradiation is safe, much more so than with any other processing technique, even though there are numerous scientific tests that corroborate its healthiness. This kind of treatment has been attacked because it produces physical and chemical changes in the foods, but if one thinks of thermal treatments these produce very important alterations, and are still accepted by the consumer. Perhaps if human beings had not eaten cooked food and were at the dawn of thermal treatment this might have the same detractors as irradiation has, as the changes it produces in foods are much more intense than those produced by irradiation, in the form that this treatment is currently proposed.

Table 1. Codes of Good Irradiation Practice (ICGFI)

Code of Good Irradiation Practice for Insect Disinfestation of Cereal Grains (ICGFI, Document 3), IAEA, Vienna, 1991

Code of Good Irradiation Practice for Prepackaged Meat and Poultry (for control de pathogens and/or extended shelf-life)(ICGFI, Document 4), IAEA, Vienna, 1991

Code of Good Irradiation Practice for Control of Pathogens and Other Microflora in Spices, Herbs and Others Vegetable Seasonings (ICGFI, Document 5), IAEA, Vienna, 1991

Code of Good Irradiation Practice for Shelf-life Extension Bananas, Mangos and Papayas (ICGFI, Document 6), IAEA, Vienna, 1991

Code of Good Irradiation Practice for Insect Disinfestation of Fresh Fruits (as a quarantine treatment) (ICGFI, Document 7), IAEA, Vienna, 1991

Code of Good Irradiation Practice for Sprout Inhibition of Bulb y Tuber Crops (ICGFI, Document 8), IAEA, Vienna, 1991

Code of Good Irradiation Practice for Insect Disinfestatcion of Dried Fish and Salted and Dried Fish (ICGFI, Document 9), IAEA, Vienna, 1991

Code o f Good Irradiation Practice for Control of Microflora in Fish, Frog Legs and Shrimps (ICGFI, Document 10), IAEA, Vienna, 1991

Code o f Good Irradiation Practice for the Control of Pothogenic Microorganisms in Poultry Feed (ICGFI, Document 19), IAEA, Vienna, 1995

Code of Good Irradiation Practice for Insect Disinfestation of Driede Fruits and Tree Nuts (ICGFI, Document 20), IAEA, Vienna, 1995

Source: Molins, 2001.

Processed foods are those that reach the consumers, and it is the consumers who are the most concerned with their healthiness and safety. With regard to irradiated foods, as early as 1925 studies were initiated into their safety, and currently there are large numbers of publications which have been carried out on this subject. Thayer (1994) and Diehl and Josephson (1994) have performed reviews on the subject of healthiness and safety from radiological, microbiological and toxicological viewpoints and the nutritional suitability of irradiated foods.

The Joint FAO/WHO/IAEA Expert Committee has examined 100 compounds of irradiated meat from cow, pig and chicken, declaring that these foods treated such are healthy and safe. Their declaration of 1980 states: “the irradiation of any food product at an average

general dose of 10 kGy presents no toxicological risk; therefore it is not necessary to carry out more toxicological trials on the foods treated in this way” (WHO, 1981).

Another problem that certain groups impute to the irradiation of foods is that it can present a potential genetic toxicity. In this respect studies on meat and poultry (Renner et al., 1982; Phillips et al., 1980) have been carried out which conclude that there is no genetic toxicity, the researchers having detected no abnormal effects on the X or Y chromosomes. The possible mutagenicity of meat and poultry treated with radiation has also been studied (Fruin et al., 1980), having found no mutagenic activity at all in any of the samples of meat studied.

With regard to the unwillingness to accept irradiated foods, Satin (2000) presents what he calls the “Salmonella Russian roulette”. Currently it is possible to find on the market untreated fresh milk and pasteurised or sterilized milk, and if the consumer does not look at the label there is a certain possibility of acquiring untreated fresh milk, which may well contain the bacteria Salmonella, resulting in a high possibility of contracting salmonellosis.

However, in this case there is the option of acquiring thermally treated milk, which overcomes this problem. However, when the consumer wishes to buy chicken, this is sold raw, and then it is certainly a game of Russian roulette whether or not the chicken contains the aforementioned bacterium, with the problems that it could represent. The irradiation of chicken eliminates this problem, as it directly affects the Salmonella, eliminating it from the food, and that is why the opportunity should exist on the market to choose between raw and irradiation treated chicken, as otherwise the acquisition of chicken is little more than a lottery.

Irradiation is the food treatment technique that has been most studied for more than a hundred years, there being a huge number of documents which corroborate that irradiation is a safe, practical and beneficial process. Nevertheless, there is a current opposed to the treatment of foods by irradiation, denying the reality of studied facts, substituting this scientific reality for ungrounded suspicion.

This rejection experimented by irradiation is not the only rejection in the history of food processing. Pasteurisation itself, which nowadays is a conventional treatment and one assimilated by the consumer, also needed some time before being generally applied, due to its detractors, not only in its early days, but rather in some cases it took more than a century before being applied. Even today it is not available in some countries.

A noteworthy case of the application of pasteurisation is the treatment of milk, which nowadays is a general treatment in the majority of countries; nevertheless the case of Scotland should be pointed out, as this was one of the last European countries to publish legislation making pasteurisation obligatory, which occurred in 1983. In this country the incidence of salmonellosis was the highest in Europe, until the normative of obligatory pasteurisation was passed, and one year after its passing this incidence had become one of the lowest.

Pasteurisation is a thermal treatment which is easily applied to liquid foods, as the heat is evenly distributed by mixing or shaking the fluid. However, in solid foods this is not possible, as suitable thermal treatment would imply that the least heated part of the solid would reach a suitable temperature to destroy the pathogenic micro-organism. That, however, would mean that the solid would be cooked, and this is not the intention. Another form of thermal treatment would be to make a mixture of the different parts of the solid, which could only be done by triturating it, but this would destroy the structure of the food and the result would be a completely different product.

Therefore, some kind of treatment for solids should be sought which does not destroy their characteristics. This kind of treatment could be irradiation, but at the outset there were difficulties, as suitable sources of radiation were not available. At present this problem has been solved and suitable installations are available for carrying out the food irradiation treatments, aimed at obtaining foods which are healthy and safe.

The milk industry presents an interesting parallelism between pasteurisation and irradiation. The difficulties associated with the introduction of milk pasteurisation have already been mentioned, although in the 1930’s the irradiation of milk with ultraviolet rays (UV) was already employed. This kind of treatment, besides affecting pathogenic micro-organisms, presents an additional beneficial effect, as it increases Vitamin D content. The milk treated with UV rays was well accepted, and even the lots of the Red Cross for prisoners of war contained irradiated milk.

2.

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R

The term ionizing radiation is given to the series of emissions of subatomic particles and electromagnetic radiation of nuclear or atomic origin, which on interacting with matter are capable of ionizing it. In other words, it is radiation which acts on matter, making it lose electrons, which leads to the production of ions.

Radiation is a form of energy, and every person receives natural radiation from the sun and other natural components of the environment. In the same way as with other forms of radiant energy the radiation waves used to treat the foods form part of the electromagnetic spectrum (figure 1).

Figure 1. Radiation spectrum.

Irradiation has mostly been used in the field of medicine, as is the case of X-rays, and nuclear radiation in the detection and treatment of diseases, the sterilization of medical equipment, apparatus, pharmaceutical products and the production of sterilized foods for special hospital diets.

The emission of ionizing radiation is a common characteristic of many unstable atoms. These atoms described as radioactive transform themselves to become stable atoms, which is achieved by freeing energy in the form of radiation. The kind of radiation freed by the radioactive atoms can be one of four different types:

• Alpha particles (α): helium atoms, which contain two protons and two neutrons. • Beta particles (β): electrons or positrons deriving from transformation in the nucleus. • Gamma radiation (γ): electromagnetic radiation from the most energetic extreme of

the radiation spectrum. • Neutrons: chargeless particles.

Radioactive activity is the speed at which the transformations are produced in a radioactive substance, and measures the number of atoms which disintegrate in the unit of time. The unit of radioactive activity in the International System is the Becquerel (Bq), defined as a disintegration by second, although sometimes the Curie (Ci) is used, which is the activity existing in a gram of the 226Ra atom.

1 Bq = 1 disintegration/second 1 Ci = 3.7·1010 disintegrations/second

The Ci represents a considerable activity, while the Bq is a small unit. Radioactive substances with an activity below 100 Bq/g, or natural solid substances with an activity below 500 Bq/g, are considered to be harmless. Any human being possesses an activity of approximately 4000 Bq due to 40K.

2.1. Interaction of Radiation with Matter

Radiation which affects matter experiments different kinds of interactions, depending on the nature of the radiation and the type of matter. Radiation which affects matter can cause two kinds of phenomena, atomic excitation or ionization. The former produces a thermal effect, while the latter causes the formation of ions, which is why radiation is sometimes classified as thermal or ionizing, depending on the kind of interaction that it causes in the matter on which it acts.

α radiation consists of particles with two protons and two neutrons, hence they are charged and are furthermore considered heavy particles. Therefore this is radiation with a limited power of penetration, of a few centimetres through the air or of a few microns in any tissue, and thus is not able to penetrate the skin. However, it can produce a high concentration of ions, which makes these particles very dangerous, as they can cause grave cellular damage.

β radiation consists of electrons, which is a charged particle, and the interaction it experiments with matter responds to Coulomb’s law of electrical charges, in the same way as for the α particles. The power of penetration of electrons is greater than that of α particles, being a few metres through air, capable of traversing human skin, although not the subcutaneous tissue.

γ radiation consists of high energy photons, which can be absorbed by matter according to three types of processes: the photoelectric effect, the Compton effect and the production of electron-positron pairs (e--e+). Depending on the energy of the incident radiation one type or other of interaction will occur, although the final effect of all of them is the production of charged particles. In the photoelectric effect (figure 2a) all the incident energy is absorbed by the atoms of the matter, and is used to expel an electron form the atom’s shell; this process requires energy absorption of up to 500 keV. The Compton effect (figure 2b) implies the incidence of the photon, producing an elastic collision with an electron from the atom’s shell, which provides it with enough kinetic energy to be separated from the atom, while the photon leaves in a different direction with less energy and a greater wavelength (lower frequency). If the energy of the incident photon is high enough, on interaction with the atoms a pair of electron-positron particles is produced (figure 2c). In order for the Compton Effect to occur the incident radiation must possess energy of between 500 keV and 10 MeV. In the process of electron-positron pair formation the incident energy is absorbed inside the electric field of the nucleus, the amount of incident energy required being greater than 1.02 MeV. The positron formed possesses a very short life and disappears (positron annihilation) with the appearance of two photons of 0.51 MeV of energy. γ radiation possesses a power of penetration estimated at various hundreds of metres in the air, and is capable of traversing the human body, metal sheets and up to several centimetres of lead.

Figure 2. (a) Ionization process. (b) Compton Effect. (c) Pair formation.

Lastly, neutrons are chargeless particles, thus they can only interact with the atomic nuclei. Because the nuclei occupy a very small part of the total volume of matter the probability of the neutrons interacting with the matter is low, which means that these particles possess a large penetration capacity.

Therefore, to summarize, the capacity for the penetration of matter of these types of radiation is different. In the case of α particles this capacity is limited as, for example, they can be retained by sheets of paper. β particles have somewhat more penetration, being able to traverse a sheet of paper, although the human body detains them. γ radiation is more penetrating, traversing the human body and sheets of different metals, although it is detained by lead sheets. As neutrons do not possess an electrical charge they are very penetrating, traversing all the materials traversed by the previously mentioned radiation types, but they can be retained by concrete walls of sufficient thickness.

In any case, the ionizing radiation that interacts with matter undergoes a certain attenuation, which basically depends on two factors, one geometric and the other material. The geometric factor is that when increasing the distance between the source of radiation and the target matter the radiation becomes increasingly weak, with the attenuation inversely proportional to the square of the distance between the source and the object. The second factor is due to the fact that the attenuation depends on the type of radiation and its energy level, and on the type and composition of the matter. In this case the attenuation shows an exponential decay with the distance travelled by the radiation inside the matter.

2.2. Absorbed Radiation Dose

When matter receives radiation, the incident energy of the radiation can cause ionization and/or excitation of the matter’s atoms, although other effects may also appear such as different photoelectric effects, Compton effects, the formation of electron-positron pairs etc. Furthermore part of the incident radiation may not interact with the matter, traversing it

without producing any effect. Thus it is necessary to measure the energy absorbed by the matter, as it is this which can cause ionization.

The absorbed dose (D) is the amount of energy absorbed (E) per unit mass (m) of the matter during the time that it is exposed to the radiation:

m

E

D

=

The dose is controlled by the intensity of the radiation and by the time that the matter is exposed to this radiation. In the International System, the absorbed dose is measured in Gray (Gy), which is one joule of energy absorbed per kilogram of mass of the matter. Sometimes a historical unit known as the rad (“radiation absorbed dose”) is still used, which corresponds to the energy of 10-2 J absorbed by each kg of irradiated matter.

1 Gy = 1J/kg 1 Gy = 100 rad

These units of measurement are very small and often multiples are used. Thus, for example, the kGy is normally used, and to give an idea of the energy level it possesses, 1 kGy is equal to the amount of energy needed for a kg of water to increase its temperature by 0.25ºC. Therefore, this type of process is a “cold” or “non-thermal” method of food treatment.

Another important variable is the speed with which a body absorbs the radiation, known as absorbed radiation rate, which is defined as the variation of the absorbed dose with regard to the exposure time:

t

d

D

d

D

&

=

This is an important variable, as the final effect of the radiation does not only depend on the amount absorbed but also on the time that the matter is exposed to the radiation.

In addition to the amount of radiation absorbed by the matter (absorbed dose), the type of radiation should also be taken into account and its potential for causing biological damage. For this the equivalent dose (H) is determined, its unit of measurement in the International System being the Sievert (Sv), which as with the Gray is the relation between the energy absorbed from one jule for each kilogram of mass, but taking into account the kind of radiation:

R

F

D

H

=

·

where the equivalent dose is the absorbed dose multiplied by a weighting factor FR, which

depends on the type of radiation. In order to have an idea of the effect produced by the different types of radiation, if a power of penetration of 1 is given to α rays, β rays would have a value of 100 and γ rays a value of 10,000. The weighting factor values for the different types of radiation are shown in table 2.

Table 2. Weighting Factor FR

FR Radiation type

1 X-rays; β rays; γ rays; electrons and positrons 5 Protons

5 a 20 Neutrons, according your energy > 20 α radiation; heavy nucleus

In the same way as with the absorbed dose, for the equivalent dose a historical unit known as the rem (“Roentgen equivalent man”) has been used, which is equivalent to 10-2 J for each kg of matter:

1 Sv = 1J/kg 1 Sv = 100 rem

Another variable used in the measurement of radiations is the effective dose (E), which takes into account the risk of developing cancers or hereditary effects and is measured in Sv. This effective dose is a weighted sum of the average doses received by the different tissues and organs of the human body:

∑

=

H

F

E

·

where Hi is the equivalent dose for an organ i, while Fi is the weighting factor of this organ,

its value depending on the organ considered.

Furthermore, it is important to take into account the dose that a person can accumulate over time, and for that purpose the committed dose is determined, which is the dose accumulated over a certain period of time.

To achieve a better understanding of the possible danger of radiation, it should be mentioned that a dose is lethal when its value exceeds 4 Sv, and that for doses of up to 0.25 Sv no harmful effects have been observed. By means of illustration, the radiation that can be received in an X-ray of the thorax shows a value of 0.02 mSv, for a CT head scan the value is 3 mSv, and the average annual dose per person in Spain is approximately 3.5 mSv, adding all the natural and artificial contributions, while the worldwide average is 2.5 mSv. The annual average dose received by the Spanish population due to the nuclear industry is in the order of 0.015 mSv, which is equivalent to what a person would receive when undertaking a 3-hour flight, due to cosmic radiation.

2.3. Sources of Ionizing Radiation

The sources of ionizing radiation are not only artificial, as there are also natural sources. It is normal to find radioactivity and radiation in nature. The presence of ionizing radiation in our world and in the whole universe is normal, and as such a very important part of Earth’s natural radiation is due to cosmic radiation which is of extraneous origin. It is thought that

every second in the order of 2x1018 particles of very high energy reach the Earth, most of which are protons (86%) and α particles (12%), in addition to neutrons and other particles. Cosmic radiation represents about 10% of the radiation that a person receives. The dose of radiation received in this way depends on the altitude, as the atmosphere absorbs part of it, which means that at higher altitudes the radiation is greater, hence when travelling by plane the radiation is more than that received at sea level. It is estimated that at an altitude of some 10,000 m, which is a normal altitude of transatlantic flights, a dose of some 5 mSv is received; at the summit of a mountain at 6,700m the dose is 1 mSv; in cities located at 2,000m the dose is 1 mSv; while at sea level it is usually in the order of 0.03 mSv (UNSCEAR, 1988). Moreover, this radiation is due to electrically charged particles and due to the Earth’s magnetic field they are diverted, so that in the equatorial zone less radiation is received than at the poles. Therefore, dose depends on both terrestrial latitude and longitude. In the case of Spain every hour people are traversed by 105 cosmic rays of neutrons and 4·105 secondary cosmic rays (CSN, 1992a,b). Furthermore, the cosmic radiation that reaches us from outer space can interact with different atmospheric components to produce radioactive substances. Cosmic radiation is, on average, about 10% of the total dose received.

Most of the dose received is due to the radiation which comes from the Earth itself. This is due to the fact that in the subsoil there are large amounts of radioactive elements, such as uranium and thorium, among others. This radiation means that the whole planet is impregnated with radioactivity, including in the human body. It is estimated that every hour 2·108 of γ radiation is received from the soil. Due to this cause the average radioactive content in Spain of different materials is estimated at being 3,000 Bq for an adult human being, 1,000 Bq for 1 kilogram of coffee, and 25,000 Bq for 25kg of fertilizer. These figures are much higher for radioactive residues from medical and industrial applications. It is estimated that for 1 kg of low activity residues the activity is 106 Bq, for those of average activity it is 108 Bq, and for high activity residues it is 1013 Bq.

Radiation which comes from the natural decay of uranium is important, as it provokes the appearance of the gas radon, which passes through cracks and pores in the soil, mixing with the air. The decay of radon produces radioactive compounds which remain attached to the particles of dust contained in the air and reach the lungs, with an estimated disintegration in every person every hour of some 30,000 atoms, with the emission of α and β particles and γ rays.

Furthermore, it should also be borne in mind that natural radionuclides are also taken in with the ingestion of food. Noteworthy among these is 40K, which the human body contains to such an amount that it is estimated that every hour some 15·106 atoms disintegrate, emitting high energy β particles and in some cases producing γ rays (NRPB, 1986).

Besides these natural sources there are also those which are due to the processes which man carries out in his medical and industrial applications, which could be called artificial sources. Of particular note is the ionizing radiation deriving from medical activity, due to its use in diagnosis and the treatment of diseases. This radiation covers the range from X-rays to nuclear radiation. But artificial radiation does not only come from medical applications, as in industry and in daily activity there are numerous examples, such as luminous watches, smoke detectors, radiation to define the structure of welding, and many other cases.

The production of electrical energy in nuclear installations is another source of artificial ionizing radiation, although in thermal power stations the combustion of coal also produces

natural radionuclides. In Spain the dose received due to this type of activity is less than 0.001 mSv, although the people who work in these power stations can receive greater doses which nevertheless do not reach 0.01 mSv per year.

Finally, mention should also go to the radiation deriving from nuclear weapons testing and accidents like that of Chernobyl, which also contribute to the dose received by the human population. This is estimated at some 0.01 mSv per year.

3.

B

E

I

R

The biological effects that ionizing radiation produces depend on the type of interaction that occurs with the matter on which it acts. The absorption of radiation by living organisms depends on the kind and quantity of the radiation, as well as the structure and the kind of absorbing matter. Thus different kinds of effects may be shown, although in any case the incident radiation is an energy bearer, energy which is transferred to the absorbing medium either directly or indirectly, according to the mechanisms of excitation or ionization. When the absorbed radiation produces the effect of excitation of the matter’s atoms and molecules it can cause molecular changes if enough energy is absorbed, and if this is greater than that of the atomic bonds. If the ionization process is involved the effect is more important, as changes are always produced in the atoms, and it is capable of causing alterations in the structure of the molecules on which the radiation has fallen.

The ionization induced in live tissues by exposure to radiation is usually quantified by the so-called lineal energy transfer (LET), which is the amount of energy yielded per unit distance travelled by the radiation in the tissue. Radiation is classified into two categories of high and low LET. α radiation and that of neutrons are considered to be of high LET, while X-rays and β and γ radiation are considered to be of low LET.

Radiation produces different effects if the doses are high or low. For high doses of radiation the effects can be of two kinds, deterministic and stochastic. Deterministic effects are those that produce immediate effects, and show a minimum dose below which these effects do not occur, but appear immediately when the dose is greater than this minimum. Stochastic effects are those of delayed appearance and are probabilistic in nature, as is the case of cancer, which may develop some years after the exposure to the radiation. Also considered as stochastic are hereditary effects, due to genetic alterations, which appear in the descendents of the organisms which have received the dose of radiation. One thing to be borne in mind is that any organism is exposed to natural radiation, and so it is difficult to evaluate the effects of exposure to low doses of radiation, as these effects could be masked by the manifestation of conditions which could be considered normal, and which may not be due to the radiation received. For these low doses the possible effects can either be genetic or cancer.

The biological effects of radiation can act at different levels, on cells, tissues or on whole organisms. The biological damage as well as the acceptable doses can vary greatly with each case. According to the molecular complexity of the living organisms, the biological effect is produced for different doses of radiation (Urbain, 1986). Thus, for mammals the lethal dose is ranged from 0.005 to 0.01 kGy, for humans this dose is in the order of 4 Gy; for insects it is from 10 to 1000 Gy, for plants it is 1 kGy. For the bacteria, the lethal dose depends on if they

are in vegetative or sporulated form; thus, in their vegetative form the lethal dose is of 0.0-10.0 kGy, while for the sporulated forms it is ranged from 10 to 50 kGy. The most resistant organisms are the virus whose lethal dose is ranged from 10 to 200 kGy. This indicates that the greater the molecular complexity the smaller the dose required producing biological effects. Low overall doses are capable of killing a person, or else causing significant damage, while in order to completely destroy insects, larvae and eggs, the required doses are higher, and the doses needed to destroy bacteria, fungi and yeasts are much higher still.

When a cell is irradiated the radiation may act directly on the genetic material or on the macromolecules, although it may also act indirectly on the water contained in the cell, or the ionized molecules may interact with the surrounding matter. There are considered to be three different stages in the overall process, a physical stage, a chemical stage and a biochemical stage. In the first physical stage the radiation interacts with the matter, which can excite or ionize its atoms, with characteristic times of 10-15 and 10-17 s, respectively. In the chemical stage free radicals are formed, with characteristic times in the order of 10-12 s. The last is a molecular or biochemical stage, in which the free radicals recombine and can form toxic molecules. The molecules formed by direct irradiation, or radicals, and those obtained indirectly, are known as radio-induced substances. These substances can be toxic or harmful to the cell. The presence of these substances causes the cell to set in motion the so-called cell repair mechanisms, giving rise to three different possible situations. If a very large amount of toxic substances has been produced the result is death. Otherwise the cell may survive although the harm caused to the genetic material is great and does not allow the cell to reproduce; this is called reproductive failure. If the amount of genetic damage is not excessive the cell can repair part of the genetic material, allowing it to reproduce, although in this case a delay in cell division is observed, enabling the transmission of mutations to subsequent generations. Therefore, in irradiated cells two types of damage are caused, the formation of toxic substances or genetic damage. From a survival viewpoint there may be important consequences, with the loss of tissue or organ functionality, the development of cancer, sterility problems, or the transmission of mutations to offspring.

On penetrating tissues charged particles (α and β) lose energy by electric interaction with the electrons in the shells of the atoms which they strike. Due to indirect effects such as the photoelectric effect, the Compton effect and pair formation, when X and γ rays hit the tissues they end up freeing atomic electrons, which produces an end result of ionization. Due to the electric interaction of the charged particles an electron is split from the atom’s shell, which produces a positively charged atom. The separated electron can ionize other atoms. Generally, both the electron and the ionized atom are very unstable and react rapidly, giving rise to new molecules, some of which are highly reactive, being as they are free radicals. These radicals may react with each other and with other molecules, causing changes in molecules which are biologically important for cell functioning. Altogether the process lasts about one millionth of a second. The biological transformations which can occur in such a short interval may destroy or modify the cells, possibly giving rise to the appearance of genetic defects or cancer.

In the case of food irradiation the problem is quite a different one, as it is necessary to evaluate the effects of irradiation from a food viewpoint. Foodstuffs are biological material and irradiation can cause different effects, such as the destruction of insects and micro-organisms, the production of toxic substances, it may damage genetic material, or it may reduce the nutrient content of the foods. Of these four effects, on principle, the only desired

one is the first. The appearance of toxic substances is not an effect which is unique to irradiation, as the appearance of this kind of substance has also been observed in chemical and thermal food treatments (Raventós, 2003; Satin, 2000; Molins, 2001). With regard to genetic damage, it should be pointed out that from the food viewpoint reproductive viability is not important, although care should be taken to avoid causing any problem for the consumer. The destruction of nutrients not only occurs in irradiation treatments, but is also observed in other kinds of food treatments, such as in thermal processes.

4.

I

R

F

I

Food irradiation is considered as the process of applying high energy to a food with the aim of pasteurising, sterilizing or prolonging its commercial life, eliminating micro-organisms and insects.

4.1. Types of Ionizing Radiation

The sources of ionizing radiation which are applied in the food industry are X-rays, electron beams and γ radiation. Other kinds of radiation which have been used are ultraviolet rays (UV). UV rays are obtained using lamps which contain gases at different pressures, and are characterised by a low power of penetration, of only a few millimetres because their emission spectrum corresponds to rays with a wavelength considerably longer than X-rays. X-rays constitute a much more energetic electromagnetic radiation, for which their power of penetration is higher, showing a continuous spectrum of radiation with a maximum value of 5 MeV. X-rays are usually obtained by bombarding a metal plate with a high potential electronic beam (figure 3).

Figure 3. X Rays generation.

γ radiation is produced with radioactive isotopes, which in the food industry are normally the radioisotopes of 60Co and 137Cs. At present 60Co is the most commonly used for irradiating foods with γ radiation, as it is relatively straightforward to obtain and it produces

radiation with a greater power of penetration than that of 137Cs. The energy spectrum of γ radiation is not continuous, but rather is discreet, and depends on the radioisotope used. The energy proceeding from 137Cs is 0.66 MeV, while that from 60Co is 1.17 and 1.33 MeV. 60Co is produced in a nuclear reactor bombarded with neutrons granules of 59Co highly refined, and in the decay process β and γ radiation is produced. 137

Cs is obtained as a result of the fission of 235U, producing β and γ radiation (figure 4).

Figure 4. Radioactive disintegration of 60Co and 137Cs.

The source of irradiation of the isotope 60Co is obtained from 59Co, which is compressed in cylindrical tablets which are placed in 50cm long steel tubes. These tubes containing 59Co are placed in a nuclear reactor where they are bombarded constantly with neutrons, which produce radioactive 60Co, which is capable of producing a controlled emission of γ rays. 137Cs is extracted from the bars of used combustible of the nuclear reactors. This reprocessing of nuclear waste has become very controversial and its possible use as a source of irradiation in food is very improbable. Thus it appears that 60Co offers better possibilities for food processing; moreover this type of source shows a greater degree of effectiveness, greater penetration of γ rays and greater environmental safety, as it is insoluble in water.

The electron beam is a series of electrically charged particles of high energy, of up to 10 MeV. For the electrons to have a high energy level they are led to a linear accelerator which confers them with high voltages, thereby obtaining electrons with high speed, approaching that of light. The advantage of this compared to γ radiation is that the electronic beam is produced in an electric machine and can be turned on and off like a light bulb. Nevertheless, its power of penetration is low, from some 5 to 10 cm.

Table 3 shows the advantages and disadvantages of the different sources of radiation used in the irradiation of foods.

When foods are irradiated with γ rays, X-rays and electron beams, a certain degree of radioactivity can be induced in them. However, this is such a small amount that it is not distinguishable from the natural radiation possessed by the food. Hence the variation in radioactivity among different non-irradiated foods is greater than any difference existing between the same food when irradiated and non-irradiated (Stewart, 2001).

From a food viewpoint, of all the energy transfer mechanisms of ionizing radiation due to the incidence of photons, the most important is the Compton Effect. For the photoelectric effect to be produced the energy of the incident photon would have to be lower than that

provided in the normal intervals of food irradiation. On the other hand, for the formation of electronic pairs higher energy levels would be required (Stewart, 2001).

Table 3. Advantages and disadvantages for the different irradiation sources

Source type Advantages Disadvantages

γ-Rays

(60_Co 137_Cs)

- Deep penetration

- Reliability of irradiation source - Easy automation

- First category radioactive facility - Transport and storage radioactive

sources

- Activity loss of storage radioactive source

- Dose rate determined by source - Permanent irradiation emission - High cost for running and security Electron beam

(10 MeV)

- Electric source only work when switch on - Unit control possibility

- High dose rate (several kGy/s) - Absence of environmental impact - Low cost for running

- First category radioactive facility - Limited Penetration

- Need a lot of handling staff - Need automated equips X-rays

(5 MeV)

- Electric source only work when switch on - Unit control possibility

- High dose rate (several kGy/s) - Absence of environmental impact - Low cost for running

- First category radioactive facility

Source: Raventós (2003).

4.2. Irradiation Dose in Foods

As the radiation used in food treatment is electromagnetic in nature (X-rays and γ rays) or else accelerated electrons, the weighting factor FR (table 2) of the equivalent dose is 1, with

which the absorbed dose (D) and equivalent (H) coincide. Hence in this case the absorbed dose is usually employed.

For every food product the permitted doses of radiation depend on its characteristics and the aim of the treatment. This means that the dose for the elimination of insects, for pasteurisation and sterilization will be different. Hence three irradiation categories are considered according to the dose employed, either low, medium or high doses.

Low doses are those which do not exceed 1 kGy, and are used in the control of insects in grain, in the control of trichina in pork and can also inhibit the decomposition of fruits and vegetables. Average doses are those in the range from 1 to 10 kGy, and are applied in the control of pathogens in meat, poultry and fish and also retard the growth of moulds on strawberries and other fruits. High doses exceed 10 kGy, and are used to kill micro-organisms and insects in spices, and also when aiming to obtain commercially sterile foods. According to the dose of radiation the treatment usually receives different names. Thus the elimination of non-spore producing pathogenic microorganisms and parasites to an imperceptible level is called radicidation. The treatment of foods with ionizing radiation aimed at increasing their average life by reducing the number of modifying micro-organisms (pasteurisation) receives the name of radurisation, while the elimination of micro-organisms by irradiation to levels of sterilization is called radapertization. Table 4 shows the doses used to irradiate foods and the applications of each case.

Table 4. Food irradiation, dose and applications

Dose Absorbed dose

(kGy)

Application

Low < 1 kGy 0,04 – 0.10 Sprout inhibition of tubers and bulbs

0,03 – 0,20 Insects, grubs and eggs sterilization

0,50 – 1,00 Fruit and vegetables ripening process control

Medium 1 a 10

kGy

1 – 3 Insect death

1 – 7 Radicidation (pathogen elimination)

2 - 10 Radurization (pasteurization)

High 10 a 50

kGy

15 – 50 Radapertization (sterilization)

10 - 50 Spices and seasonings decontamination

4.3. Changes in Irradiated Foods

Irradiated foods are treated at low levels of radiation, which means that only chemical changes are possible, and that changes which would make them radioactive do not occur.

The large number of investigations carried out indicates that the changes produced in irradiated foods are similar to those produced by a conventional cooking treatment. The studies show that in irradiated foods toxic or mutagenic effects do not exist, and that irradiation does not produce chemical residues in the food.

Irradiation is a cold process, which means that there is only a slight temperature rise of the food during processing. There is hardly any change in the physical appearance of the irradiated foods, which do not undergo the changes in texture and colour shown by foods treated by heat pasteurisation, or by tinned and frozen foods. Certain bad tastes in meat and excessive softening of fresh peaches and nectarines have been reported.

In irradiated foods some changes do occur, although they are not as important as those that occur with conventional cooking methods.

When the high energy particles hit the matter, electrons are released from the atoms, giving rise to ions. The radiolytic products formed in this way can interact to form new compounds. A few of these reactions may give rise to strange tastes. The FDA concluded that “very few of these radiolytic compounds are unique to irradiated foods; approximately 90%

of radiolytic compounds are natural compounds of the food” (Web and Penner, 2000).

4.4. Irradiated Food Labelling

Retailed irradiated foods must bear the symbol radura (figure 5) which identifies them as such. Furthermore the sentence “treated with irradiation” must also appear. Manufacturers are permitted to add the objective of the treatment; thus, for example, it may be labelled as “treated with radiation to control deterioration”.

With non-packaged fruits and vegetables every piece must be labelled, and furthermore on the shelf containing the product and clearly visible to the consumer there must be a sign indicating that they have been treated with radiation.