Irradiation for Quality Improvement, Microbial Safety and Phytosanitation of Fresh Produce

By Rivka Barkai-Golan and Peter A. Follett

Irradiation for Quality Improvement, Microbial Safety and Phytosanitation of Fresh Produce presents the last six and a half decades of scientific information on the topic. This book emphasizes proven advantages of ionizing irradiation over the commonly used postharvest treatments for improving postharvest life of fresh fruits and vegetables to enhance their microbial safety.

This reference is intended for a wide range of scientists, researchers, and students in the fields of plant diseases and postharvest diseases of fruits and vegetables. It is a means for disease control to promote food safety and quality for the food industry and can be used in food safety and agriculture courses.

• Discusses pathogen resistance to common chemical synthetic compounds
• Presents up-to-date research and benefits of phytosanitary irradiation
• Includes comprehensive research for alternative treatments for postharvest disease control
• Provides the non-residual feature of ionizing radiation as a physical means for disease control to produce chemical free foods

• Language: English
• Publisher: Academic Press
• Release date: May 29, 2017
• ISBN: 9780128110263

Rivka Barkai-Golan

Prof. Rivka Barkai-Golan has been a senior research scientist in postharvest pathology and mycology at the Volcani Center in Beit Dagan, and a Professor of plant pathology at the Faculty of Agriculture of the Hebrew University of Jerusalem, where she has been honoured a Distinguished Lecturer. She has published more than 150 scientific publications, 3 chapters and 6 books, many of which include irradiation topics. She was the head of the Committee for irradiation research in the Volcani Center, Israel, an organizer of food irradiation conferences and the delegate to the International Conference in Geneva, 1988, by the FAO/IAEA/WHO/ITC-UNCTAD/GATT on the Acceptance, Control and Trade in Irradiated Food.

Book preview

Irradiation for Quality Improvement, Microbial Safety and Phytosanitation of Fresh Produce

Rivka Barkai-Golan

Peter A. Follett

Table of Contents

Cover image

Title page

Copyright

Preface

Acknowledgments

Chapter 1. Irradiation for Quality Improvement and Microbial Safety of Fresh Produce

Overview

Radiation Purposes—Safety and Wholesomeness of Fresh Produce

Radiation Sources and Dose Terminology

Clearances of Irradiation

Chapter 2. Ionizing Radiation for Shelf Life Extension

Suppressive Effects of Irradiation on Postharvest Pathogens

Factors Influencing Postharvest Pathogen Sensitivity to Irradiation

Suppressive Effects of Irradiation on Decay Development

The Impact of Combined Radiation Treatments on Decay Suppression of Fresh Produce

The Impact of Irradiation on the Ripening Process of Fruits

Chapter 3. Postirradiation Changes in Fruits and Vegetables

Microbiological Changes After Irradiation

Postirradiation Changes in Quality Parameters

Nutritional Changes After Irradiation—Vitamin C Content

Chemical Changes After Irradiation

Chapter 4. Irradiation Effects on Mycotoxin Accumulation

Patulin

Aflatoxins

Ochratoxin

Alternaria mycotoxins

Chapter 5. Sprout Inhibition of Tubers, Bulbs, and Roots by Ionizing Radiation

Sprout Inhibition of Potato Tubers

Sprout Inhibition of Onion Bulbs

Sprout Inhibition of Garlic Bulbs

Sprout Inhibition of Carrot Roots

Chapter 6. Irradiation for Quality Improvement of Individual Fruits

Tropical and Subtropical Fruits

Pome Fruits

Stone Fruits

Grapes

Berries

Chapter 7. Irradiation for Quality Improvement of Individual Vegetables Including Mushrooms

Solanaceae Fruits-Vegetables

Cucurbitaceae Fruit-Vegetables

Leafy Vegetables

Brassica Vegetables

Subterranean Vegetables

Mushrooms

Chapter 8. Safety of Fresh and Fresh-Cut Fruits and Vegetables Following Irradiation

Irradiation Effects on Furan Formation in Fresh-Cut Fruits and Vegetables

Irradiation Effects on Individual Fresh-Cut Fruits

Irradiation Effects on Individual Fresh-Cut Vegetables

Leafy Vegetables—General

Fresh-Cut Subterranean Vegetables

Fresh-Cut Mushrooms

Chapter 9. Benefits of Fruit and Vegetable Irradiation, Labeling and Detection of Irradiated Food, Consumer Attitude, and Future Research

Irradiation Benefits

Comparison of Irradiation With Other Technologies Intended for the Fresh Produce

Labeling of Irradiated Food

Detection of Irradiated Food

Consumer Attitude Toward Irradiated Food: The Psychological Power May Be Greater Than That of Scientific Research

Future Research and Mission

Chapter 10. Phytosanitary Irradiation of Fresh Horticultural Commodities for Market Access

Introduction

History of Insect Control Using Irradiation

Insect Radiotolerance

Methods for Developing Quarantine Irradiation Treatments

Quarantine Metrics

Regulatory Aspects of Irradiation

Regional and International Harmonization

Regional Trade

Chapter 11. Phytosanitary Irradiation: Generic Treatments

Introduction

Generic Radiation Treatments

Developing Specific Treatments for Quarantine Lepidoptera

Lowering the Dose for Specific Pests and Commodities

Generic Radiation Doses for Other Pest Groups

Commodity Quality and Unique Applications

Trade Facilitation

Chapter 12. Phytosanitary Irradiation: Combination Treatments

Introduction

Combination With Cold

Combination With Heat

Combination With Plant Essential Oils

Combination With Modified Atmospheres

Combination With Pesticides

The Future of Combination Treatments

Chapter 13. Current Issues in Phytosanitary Irradiation

Introduction

Consumer Perception

Dose and Energy Limits

Labeling

Modified Atmosphere Packaging

Country Approvals

Alternative Irradiation Equipment

The Next Steps

Appendix

References

Index

Copyright

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Preface

Food irradiation is a technology that improves the safety and extends the shelf life of foods by reducing, deactivating, or eliminating microorganisms and insects. The application of ionizing radiation using gamma rays, X-rays, or electron beam can serve many purposes, such as eliminating organisms that cause food-borne illness, destroying organisms that cause spoilage and decomposition, controlling quarantine insects to prevent their spread, inhibiting sprouting and delaying ripening, and sterilizing food for patients with impaired immune systems. Food irradiation is safe and the process has been endorsed by the World Health Organization (WHO), the Centers for Disease Control and Prevention (CDC), the US Department of Agriculture (USDA), and many other organizations around the world.

Fruits and vegetables are an important part of a healthy diet and variety is as important as quantity. The availability of a safe and diverse supply of fruits and vegetables year round is essential to our health and well-being. Several book chapters and reviews have been written on the benefits of radiation treatment of fruits and vegetables. In this book we pull together research, technological advances, and current trends from many disciplines to provide a single comprehensive source of information on the many uses of irradiation to improve the safety and supply of fruits and vegetables.

The book presents information accumulated during the past six and a half decades on the potential of ionizing irradiation as a physical treatment for shelf life extension of fruits and vegetables; the delay or retardation of the ripening and senescence processes; the elimination of sprout inhibition of tubers, bulbs, and roots of subterranean vegetables; and the elimination of human pathogenic bacteria commonly contaminating fresh and fresh-cut fruits and vegetables and involved in outbreaks of food-borne illness following consumption. A major mission of these chapters was to emphasize the scientifically proven advantages of irradiation over the commonly used treatments for improving postharvest life of fresh agricultural products and enhancing their microbial safety. The book also presents up-to-date information on the use of irradiation for phytosanitary purposes to control quarantine pests and thereby gain market access of fresh agricultural commodities.

Chapter 1 focuses on radiation sources, dose terminology, and clearances for irradiations. Chapter 2 focuses on factors influencing postharvest pathogen sensitivity to irradiation and the wish to lower the effective radiation doses via application of combined treatments with other accepted postharvest treatments. Chapter 3 focuses on postirradiation changes in fresh and fresh-cut fruits and vegetables, including microbiological changes, nutritional changes (including vitamin C content), and chemical changes in the irradiated fruits and vegetables. Chapter 4 deals with irradiation effects on mycotoxin accumulation in fresh fruits and vegetables. Chapter 5 is dedicated to irradiation effects on sprout inhibition of tubers, bulbs, and roots along with changes caused at sprout inhibition doses. Chapters 6 and 7 are dedicated to irradiation effects on individual fruits and vegetables. The fruits and vegetables discussed in these chapters are accompanied by presentation of the beneficial effects of irradiation versus the adverse or undesirable effects. Chapter 8 focuses on irradiation effects on the quality and safety of fresh-cut fruits and vegetables contaminated by human pathogenic bacteria. Chapter 9 focuses on consumer attitude toward irradiated food and the need for detecting irradiation in irradiated fresh produce or for ensuring its application. This chapter sums up the benefits of irradiation of fruits and vegetables and the subjects associated with future irradiation research on the fresh produce.

Chapter 10 presents the fundamentals of phytosanitary irradiation, the history of insect control using irradiation, research methods for development of phytosanitary treatments, and the evolution of regulatory frameworks. Chapter 11 discusses the development of generic radiation treatments to control insects, the role this has played in the growth of phytosanitary uses of irradiation worldwide, and critical topics for future research. Chapter 12 reviews options for combining irradiation with other postharvest quarantine treatments. Finally, Chapter 13 discusses current issues and next steps in the use of phytosanitary irradiation that will lead to wider commercial adoption.

This book will interest a wide range of readers including food scientists, postharvest biologists and technologists, regulatory and health officials, food processing and irradiation specialists, fruit and vegetable growers and retailers, and academicians. It is intended for both students and scientists in the field of postharvest diseases of fruits and vegetables and their control by ionizing radiation as a new physical means. It can appeal to microbiologists involved in the elimination of human pathogenic bacteria contaminating fresh and ready-to-eat fruits and vegetables that are responsible for increasing the number of outbreaks following consumption. Irradiation leads to enhanced microbial safety, wholesomeness, and sensory quality of the fresh produce. The book will inform those interested in sprout inhibition in potato tubers or onion and garlic bulbs by irradiation. It is of special interest to those involved in commodity treatment and quarantine entomology and the import and export of fresh produce.

R. Barkai-Golan

P.A. Follett

Acknowledgments

We express gratitude to Idit Sofer, the librarian of the Central Library of the Volcani Center, for her highly efficient help during the preparation of Chapters 1–9. The ideas and information presented in Chapters 10–13 are the result of conversations with many colleagues and associates including Lourdes Arevalo-Galarza, Jack Armstrong, Woody Bailey, Andrea Beam, Luis Calcaterra, Ron Eustice, Xuetong Fan, Bob Griffin, Mike Guidicipietro, Neil Heather, Yves Henon, Stanislaw Ignatowicz, Laura Jeffers, Andrew Jessup, Michael Koehn, Monique Lacroix, Nicholle Levang-Brilz, Paisan Loaharanu, Jim Moy, Suresh Pillai, Anuradha Prakash, Peter Roberts, Ralph Ross, Tatiana Rubio Cabello, Christopher Thomas, Ken Vick, Barbara Waddell, Marisa Wall, Eric Weinert, Eduardo Willink, Lyle Wong, and Larry Zettler.

R. Barkai-Golan

P.A. Follett

Chapter 1

Irradiation for Quality Improvement and Microbial Safety of Fresh Produce

Abstract

The first chapter focuses on the purposes of applying ionizing radiation to fruits and vegetables. These include the extension of their useful shelf life by inactivating postharvest pathogens or/and by delaying ripening and senescence, and inhibiting sprouting of tubers, bulbs, and roots of subterranean vegetables. Most important purposes are the improvement of microbial safety associated with human pathogenic bacteria contaminating the fresh produce and providing relevant data on wholesomeness and safety of irradiated food. Chemical and nutritional changes occurring after irradiation should be provided.

Keywords

Clearances; Dose terminology; Microbial safety; Radiation purposes; Radiation sources; Wholesomeness

Overview

With the development of pathogen resistance to some of the common chemical synthetic compounds and with the increased wish to receive fresh products free of chemical residues leading to public risk, research for alternative substances or treatments has been increased. These included the use of natural chemical compounds, the introduction of generally recognized as safe compounds, the use of biocontrol agents, the development of genetically engineered crops, the modulation of the natural host defense substances (Terry and Joyce, 2004; Charles et al., 2008) and the increased interest in physical control methods, such as cold storage, heating, modified or controlled atmosphere storage, hypobaric pressure, and ionizing radiation. The nonresidual feature of ionizing radiation as a physical means for postharvest disease control has been regarded as an important advantage in treating fresh fruits and vegetables.

Studies aimed at evaluating the possibilities of using ionizing radiation for extending the useful life of fresh fruits and vegetables via suppressing postharvest diseases and retarding physiological deterioration have been conducted since the 1950s. These studies were accompanied by investigations on the possible use of ionizing radiation as a means for extending the storage life of tuber, bulb, and root crops by sprout inhibition.

With the increased interest in minimally processed or fresh-cut fruits and vegetables, enhanced efforts were dedicated to evaluating the ability of ionizing radiation to enhance microbial safety by eliminating human pathogenic microorganisms that frequently contaminate the fresh-cut produce.

Studies on irradiation effects on fresh fruits and vegetables and later on minimally processed fruits and vegetables for improving their keeping quality aspects have been discussed along the years in a great number of reviews such as those by Sommer and Fortlage (1966), Dennison and Ahmed (1975), Brodrick and Thomas (1978), Thomas (1983, 1984, 1985, 1986a,b, 1986), Thayer (1990), Barkai-Golan (1992, 2001), Molins (2001), Groth (2007), Arvanitoyannis et al. (2009), Arvanitoyannis (2010), Niemira and Fan (2009), Cia et al. (2010), Fan (2010, 2012a,b, 2013b), Niemira (2013) and Fan and Sommers (2013a,b). A great number of other reviews have been focused on special aspects of fresh fruits and vegetables irradiation.

Early studies have already shown that the possible application of ionizing radiation for decay control may be limited by the susceptibility of the host plant tissue to irradiation, as expressed by radiation-induced damage and adverse changes in nutritional contents, color, texture, flavor, or aroma. Thus, the use of ionizing irradiation as a means for decay control will depend on the balance between pathogen sensitivity to irradiation and host resistance to its application.

To reduce the radiation dose effective for decay control and postharvest life extension, the possibility of using combined treatments of radiation with other physical treatments, mild chemical applications, or other accepted postharvest treatments has been developed.

The first part of the book brings together the variety of approaches aimed at using ionizing radiation as an alternative physical means for improving the shelf life of harvested products, including studies in various countries over the last six decades. The studies involved are aimed mainly at four directions: (1) the extension of postharvest life or shelf life directly by inactivating postharvest pathogens alone or combined with other known postharvest control means; (2) the extension of postharvest life by delaying the ripening and senescence processes, which may indirectly lead also to decay suppression in harvested fruits and vegetables; (3) the improvement or enhancement of microbial safety associated with human pathogens in minimally processed or fresh-cut fruits, vegetables, and mushrooms, a subject that gained increased interest during the last decades; (4) postharvest life extension of subterranean vegetables via sprout inhibition of tubers, bulbs, and roots.

An important advantage of ionizing radiation over chemical application is its ability to penetrate deeply into the host tissues without leaving residues. Thus, in contrast to chemicals, gamma radiation enables the control of not only surface- or wound-infecting microorganisms but also pathogens implanted within the host either as latent or as active infections. Therefore, ionizing radiation may also be considered as a therapeutic means for postharvest diseases.

The data given in part 1 of the book include, along with up-to-date information on irradiation effects on the fresh produce, also early studies because some basic or pioneer studies associated with the ability of irradiation to extend the useful shelf life of fresh fruits and vegetables by inhibition of pathological and physiological changes and enhancement of their safety and wholesomeness have been carried out in earlier investigations.

Radiation Purposes—Safety and Wholesomeness of Fresh Produce

The aim of food irradiation, similar to that of other food technologies, such as freezing and high-temperature or chemical treatments, is to maintain its quality, enhance its safety, and prolong its shelf life by eliminating microbial development and food-borne illness caused by contaminating human pathogenic microorganisms. Regarding fruits and vegetables, irradiation may act as postharvest fungicidal or fungistatic means against spoilage microorganisms and will replace chemical treatments without leaving residues in the vegetal tissues intended for consumption. Irradiation is capable of inhibiting the accumulation of human pathogenic bacteria that frequently contaminate the surface of fresh and fresh-cut fruits and vegetables and are responsible for serious outbreaks. Irradiation may also extend postharvest life by retarding the physiological activity of fresh fruits and vegetables, mainly those associated with the ripening and senescence processes, and by inhibiting sprout inhibition of potato tubers or onion and garlic bulbs during postharvest stages.

Safety and wholesomeness are basic factors for applying irradiation. Extensive research on food exposed to ionizing irradiation from different sources provided evidence that ingestion of irradiated food is safe (WHO, 1988; CAST, 1996; IAEA, 2006; EFSA, 2011). The process of irradiation includes the passage of the food items through the radiation field without having contact with radioactive substances (O’Beirne, 1989; Crawford and Ruff, 1996; Grolichova et al., 2004). Wholesomeness implies satisfactory nutritional quality and microbiological safety for consumers. Regarding wholesomeness of irradiated food, early studies have already indicated that the nutrient breakdown is considerably reduced after irradiation than after other established processes, such as heating and canning (Brodrick et al., 1985). To assess the biological safety of irradiation and provide relevant data on the wholesomeness of irradiated food, investigation of biochemical changes occurring in food exposed to irradiation has been included in many laboratories in various countries.

The new terminology of wholesomeness means safety for consumption in the widest possible sense. It includes the radiological, toxicological, and microbiological safety and the nutritional adequacy and the sensory quality of the irradiated product (Ehlermann, 2005).

Radiation Sources and Dose Terminology

Sources of ionizing radiation aimed at suppressing or inactivating pathogenic microorganisms of fruits and vegetables and those contaminating the surface of the fresh produce include gamma rays emitted by the radio-isotopes cobalt-60 (with levels of 1.17 and 1.33  MeV) or cesium-137 (with 0.662  MeV) and by high-energy electrons (e-beams) with a maximum energy level of 10  MeV. Another type of ionizing radiation that may be applied to foods is X-rays with maximum energy of 7.5  MeV (FDA, 2008). Doses of irradiation are quantified in terms of energy absorbed by the irradiated product. None of these kinds of radiation, when used for food irradiation purposes established by Codex Standard, have energy levels suitable to induce radioactivity in the irradiated food (European Food Safety Authority – EFSA Journal, 2011).

Compared to gamma rays, e-beams are characterized by a lower penetrative capacity. They can only penetrate food up to a depth of a few centimeters, which can limit the type of food that can be processed and are particularly useful for surface-contaminated products (WHO, 1988).

Regarding radiation application to foods, several terms are associated with absorbed doses (Juneja and Thayer, 2001). These include

1. Rad (used in the past)—a unit equivalent to the absorption of 100  ergs energy/g of irradiated material

2. Gray (Gy), the currently used unit of absorbed dose: 1  Gy is an energy absorption of 1  J/kg (1  J  =  10⁷  ergs; 1  krad  =  10  Gy; 1  Gy  =  100  rad  =  0.1  krad; 1  kGy  =  1000  Gy  =  100  krad).

Clearances of Irradiation

The use of irradiation for preservation of food must be approved by the US Food and Drug Administration (FDA) before being commercially applied. The FDA has already approved the use of irradiation for sprout inhibition of white potatoes in 1964 and for ripening inhibition and insect control in 1986. Since the last three decades, the amount of commercially irradiated food products has been markedly increased. As was summed up by Ehlermann (2005), the Joint Expert Committee of Food Irradiation concluded in 1980 that food irradiation is safe and acceptable for any kind of food, at least at up to an overall average dose of 10  kGy (WHO, 1988). This conclusion was adopted by Codex Alimentarius, revising its provisional standard of 1979 into the general standard of 1983, which was further modified in 2003.

A major international conference was held in Geneva, Switzerland, on December 1988 by the Food and Agriculture Organization, International Atomic Energy Agency, World Health Organization, and ITC-UNCTAD/GATT. The purpose of the conference was to establish an international document for the acceptance, control of, and trade in irradiated food. Based on a critical evaluation of available scientific data concerning the safety and wholesomeness of irradiated food, the conclusions reached were that foods irradiated up to an overall average dose of 10  kGy were nutritionally sound and safe for human consumption (WHO, 1988).

Permissions to irradiate food items may vary considerably in different countries. Lists of countries that have cleared along the years different irradiated fruits and vegetables for human consumption and the levels of clearance are given in Appendix I (Tables A.1–A.5).

Regarding clearances, important changes took place for strawberries and for lettuce and spinach.

Clearance for Strawberries Irradiation

Following the fact that strawberries are characterized by a very short postharvest life because of both physiological and pathological processes, a clearance for strawberry irradiation was given by 19 countries with a dose of 3  kGy (International Consultative Group on Food Irradiation, ICGFI, 2002). This dose was found to extend shelf life of strawberries by a factor greater than 2. Higher doses resulted in changes in fruit texture, cell wall composition, and decrease in color intensity (d’Amour et al., 1993; Yu et al., 1995).

Clearance for Lettuce and Spinach Irradiation

In 2008 an approval was given by the US FDA for the use of ionizing irradiation on fresh Iceberg lettuce and spinach at doses not exceeding 4  kGy to enhance microbial safety and extend their shelf life (FDA, 2008). The European Food Safety Authority (EFSA, 2011) came to the conclusion that in general the radiation dose needed to inactivate food-borne pathogens depended on the target pathogens, the reduction required, and the physical state of the food item rather than the food classes. Studies by Fan et al. (2012a) indicated that overall irradiation at doses of 1 and 2  kGy is feasible to enhance microbial safety of fresh-cut lettuce and of spinach with minimal effect on their quality.

Chapter 2

Ionizing Radiation for Shelf Life Extension

Abstract

Response of fungal cells to inactivation by ionizing radiation is governed by several factors of which the inherent resistance is the first. Factors affecting the sensitivity to irradiation of postharvest fruits and vegetables and their enzymatic activity have been reviewed. This chapter focuses on the suppressive effects of irradiation on decay development. The impact of irradiation alone or combined with other postharvest techniques on decay suppression and on the ripening process of fruits was discussed.

Keywords

Damage repair capacity; Ethylene involvement; Phytoalexins involvement; Shelf life extension

Suppressive Effects of Irradiation on Postharvest Pathogens

Radiation-induced morphological changes have sometimes been observed in fungi subjected to ionizing radiation. These occurred mainly after spore germination and included changes in the dimensions of germ tubes produced, the appearance of swellings in the mycelium, and the lack of cross-wall formation in species that normally form regular walls (Sommer and Fortlage, 1966).

Ionizing radiation may damage directly the genetic material of the living cell, leading to mutagenesis and eventually to cell death. It is generally agreed that nuclear DNA, which is recognized for its central role in the cell, is the most important target molecule for radiation of microorganisms. Thus, the biological effects of irradiation against pathogens are primarily the result of the DNA disruption in the cell nuclei. Among the many types of DNA modifications that contribute to cell death, most of the information is concerned with single- or double-strand breaks in the DNA, their yield, and their repair mechanism (Friesner and Britt, 2003). The potential application of ionizing radiation against postharvest pathogens of fruits and vegetables is based mainly on the fact that ionizing radiation effectively damages their DNA, thus preventing them from reproducing (Farkas, 2006).

Eukaryotic cells, such as those of molds and yeasts, have a relatively large nucleus surrounded by membrane and organized into distinct chromosomes. These nuclei represent larger targets than the genomes of prokaryotic cells of the vegetative bacteria and spores. The latter are relatively small, without a specialized nuclear membrane, and with the DNA molecule apparently freely suspended in the cytoplasm. It is not surprising, therefore, that eukaryotes are generally more sensitive to radiation than prokaryotes (Grecz et al., 1983). Exceptions are the coenocytic fungi of the Phycomycetes, such as the common postharvest genera Rhizopus and Mucor, which contain many nuclei embedded within the cytoplasm and exhibit high radiation survival.

Radiation effects on cell components other than DNA may also contribute to cell injury. Ionizing radiation may cause sublethal changes in different structures of cells, such as membranes and plastids, and lead to sublethal injury (Dickson, 2001). The exposure of fungi to ionizing radiation may result in a chain of chemical, metabolic, and physiological changes and can thus be considered as a stress treatment on fungal cells (Geweely and Nawar, 2006).

Factors Influencing Postharvest Pathogen Sensitivity to Irradiation

Inherent Resistance

Response of fungal cells to inactivation (loss of colony-forming ability) by ionizing radiation is governed by several factors, of which the inherent resistance, which is genetically controlled, is the first (Moy, 1983). Different fungal species may vary widely in their resistance to irradiation. Early studies indicated that multicellular fungal spores, such as those of Alternaria and Stemphylium species, or bicellular spores, such as pycnidiospores of Diplodia natalensis and many of the Cladosporium herbarum conidia population, are generally more resistant to gamma radiation than the unicellular spores of different fungal species (Sommer et al., 1964a,b; Barkai-Golan, 1992). The black yeast, Aureobasidium pullulans (Pullularia pullulans), is another radioresistant fungus, whose importance considerably increases after irradiation of fruits and vegetables.

The problems posed after irradiation of multicellular structures of fungi, such as mycelia and sclerotia, were mostly related to the ability of one cell in these structures to germinate and function independently of other cells (Barkai-Golan, 1992). It was also evident that some fungi have developed efficient mechanisms to protect themselves against various sources of radiation while growing in highly radioactive polluted environments (Strike and Osman, 1993).

Radiation Doses for Suppressing Fungal Growth

The direct effects of ionizing radiation on pathogen development have frequently been conducted as the first step in evaluating its potential for disease control. It was generally found that the higher the dose of radiation was applied, the greater was the destructive effect on a given fungal species. Being more resistant to radiation than the vegetative cells, spores are frequently preferred for evaluating the efficiency of irradiation on disease suppression. There are, however, a few drawbacks in using spores for dose–response studies. Moy (1983) listed them as follows:

1. A sufficient number of spores is not always available.

2. Some of the large spores are of indefinite multicellularity and are difficult to quantify on a cellular basis.

3. Sometimes more than one type of spore may be present in a culture and a uniform suspension cannot be readily obtained.

4. In many instances, it is the mycelia rather than the spores that are inactivated in fruit.

The differences in fungal response to radiation in vitro were generally exhibited by spore survival, i.e., the ability of fungal spores to form colonies. Early studies have already shown that this feature was more sensitive to radiation than the germination ability of spores (Beraha et al., 1960). The dose–response curves for spores of 17 common postharvest pathogens emphasized the wide range in their radiosensitivity, from the most sensitive species (Trichothecium roseum and Trichoderma viride) until the most resistant species (D. natalensis, Stemphylium botryosum, Rhizopus stolonifer, Alternaria citri, Alternaria alternata, and C. herbarum). For each species, however, the level of spore inactivation increased with the dose (Barkai-Golan, 1992).

Studying the relative sensitivity to gamma radiation of four postharvest pear pathogens after storage (3–4°C), Tiryaki (1990) described them from the resistant to the sensitive as follows: Botrytis cinerea  >  Alternaria tenuissima  >  Penicillium expansum  >  R. stolonifer. Radiation doses of 1000 and 3000  Gy were sufficient for decontaminating the radiosensitive species B. cinerea and P. expansum and the radioresistant species A. tenuissima and S. botryosum, respectively (Geweely and Nawar, 2006).

Studying the radiation effects on conidial germination and in vitro growth of Ceratocystis paradoxa, the major pathogen of pineapples, indicated that irradiation was capable of decreasing the percentage of spore germination, reducing germ tube elongation and reducing in colony radial growth. Irradiation effect was directly correlated with the radiation doses applied (Damayanti et al., 1990).

Studies on the direct effects of irradiation (from a Cs¹³⁷ gamma source) on B. cinerea culture showed that mycelium growth was inhibited for 23  days after a 3-kGy dose and for 32  days after a 4- to 5-kGy dose (Tiryaki, 1993). Studies on the efficiency of gamma irradiation on conidial germination of Botrytis allii showed that the germinability of fungal conidia was reduced by up to 99% after irradiation at 5  kGy. When mycelium growth was used as a criterium, young mycelia were found to be more resistant to radiation than mature mycelia (Arabi et al., 2004).

Tugay et al. (2006) evaluated spore germination and subsequent emergent hyphal growth of fungi in the presence of pure gamma radiation or mixed beta and gamma radiation of fungi isolated from a range of long-term background radiation levels. Fungal species isolated from background radiation sites showed inhibition or no response in germination when irradiated. Isolates from sites with elevated radiation showed a stimulation of spore germination. Most isolates from low–background radiation sites showed a significant reduce or no response to exposure of either source of radiation, whereas the stimulatory effect of exposure to radiation seemed to increase in magnitude with the increase in previous exposure to radiation. It was suggested that the enhanced spore germination and hyphal growth found after the exposure trials is induced by previous long-term exposure to radiation.

Radiation Doses for Suppressing Fungal Enzymatic Activity

The ability of a pathogen to produce cell wall–degrading pectolytic enzymes was found to be more radiation resistant than the potential for colony formation or the ability of spore germination. Irradiation of Penicillium digitatum conidia with sublethal doses (Barkai-Golan, 1992) resulted in a dose-dependent lag in the polygalacturonase and cellulase activities of the fungus. A lag of 24  h was obtained after a 500-Gy dose and a lag of 5  days after a 1.5-kGy dose. The inhibition of enzymatic synthesis in an irradiated conidium population was found to be directly related to the number of conidia, which remained viable after corresponding doses of gamma rays (Fig. 2.1).

Figure 2.1  Survival of Penicillium digitatum spores after irradiation (●) in relation to time until initiation of enzymatic activity (▪) and activity of cellulase (△) and polygalacturonase (▲) as expressed by percentage decrease in viscosity of carboxymethyl cellulose (CMC) and sodium polypectate (SPP), respectively. Reproduced from Barkai-Golan, R., 1992. Suppression of postharvest pathogens of fresh fruits and vegetables by ionizing radiation. In: Rosenthal, I. (Ed.), Electromagnetic Radiation in Food Science. Springer-Verlag, Berlin, Heidelberg, pp. 155–193, 209–244, with permission.

Studying the effect of gamma irradiation on the production of cell wall–degrading enzymes by Aspergillus niger, an important postharvest pathogen of apples, pears, and plums, Gherbawy (1998) found that irradiation at subinhibitory doses enhanced the production of polygalacturonase, pectin methylesterase, cellulase, and protease along with the increased production of the biomass.

Rate of Irradiation

For a given dose, the rate of application may affect both spore survival and consequent mycelial growth. Beaulieu et al. (1992, 1999) emphasized the effect of the rate of radiation application on the extension of shelf life of mushroom. They found that irradiation of white variety of mushroom at 2  kGy at both dose rates of 4.5 and 32.0  kGy/h retarded cap opening. However, mushrooms exposed to the lower dose rate retained their whiteness longer and showed a reduction in stem elongation. The rate of irradiation at a 2-kGy dose was found to enhance shelf life extension and after the ninth day the highest value of whiteness was obtained for mushrooms irradiated at a rate of 4.5  kGy/h. Analysis of the phenolic compounds revealed that these mushrooms contained more phenols than those irradiated at 32  kGy/h. The fluctuation of the precursors glutaminyl-4-hydroxyaniline was less in higher rate–treated mushrooms than in those irradiated at the lower rate. Further decrease was recorded in the higher rate–treated mushrooms. Analysis of the enzymes involved indicated that polyphenol oxidase activity in irradiated mushrooms was lower as compared with the unirradiated mushrooms. However, examination of the mushrooms’ cellular membranes by electronic microscopy revealed a better preserved integrity in those irradiated by the lower rate of irradiation. It was assumed that the browning discoloration observed in mushrooms irradiated at the high rate of application was caused both by the decompartmentation of the vascular phenol and the entry of molecular oxygen into the cytoplasm. The synergistic effect of residual active polyphenol oxidase and the molecular oxygen, in contact with the phenols, allowed an increased oxidation rate and thus a more pronounced browning in those treated with the high-rate irradiation (Beaulieu et al., 1999).

Size of Fungal Population

The number of fungal cells in the population exposed to irradiation, whether in the form of spores or mycelial cells, may greatly influence the radiation dose required to inactivate all or most of a population of identical cells.

Studying the in vitro response of Monilinia fructicola to gamma irradiation, Sommer et al. (1964a) found that the inactivation dose increased from 2 to 3  kGy when the population density was raised from 10⁴ to 10⁶  spores/mL. Working with B. cinerea–inoculated table grapes, Couey and Bramlage (1965) found that the effectiveness of a given dose was reduced with the increase in the density of spores in the inoculum. Furthermore, infection became increasingly resistant with age, probably because of the increased number of cells at a developed stage of infection. The difference in the initial number of cells in fungal populations exposed to irradiation was probably a reason for the wide range of lethal doses reported for B. cinerea or R. stolonifer by different scientists (Beraha et al., 1960; Geweely and Nawar, 2006). Barkai-Golan and Kahan (1966) showed that within a range of 1–2  Gy, the incubation period of P. digitatum– and Penicillium italicum–inoculated oranges was gradually prolonged with the reduction in spore concentration of the inoculum. Along with the delay in fungal colonization, the decrease in the initial spore concentration also resulted in a decreased rate of infection.

It seems, therefore, that for determining the effective radiation doses required for inactivating fungal spores or young fungal cells, both the inherent radiosensitivity of the fungus and the size of population on the host should be considered.

Water Content

Vegetative cells in the dehydrated or frozen state become markedly more resistant to ionizing radiation. Cellular water under these conditions is essentially immobilized and metabolism is arrested. The radiation resistance