Chapter 8 : Irradiation & Dielectric Heating
Irradiation & Dielectric Heating
Irradiation and Radiation Definition
Radiation: A type of energy traveling in the form of waves or rays (electromagnetic waves).
Irradiation: The process of exposing a substance to radiation energy.
Ionizing radiation: Capable of knocking electrons out of molecular structures, producing free radicals.
Classification of electromagnetic waves is based on frequency; shorter wavelengths correspond to greater energy.
Food Irradiation
Utilizes controlled ionizing energy (electron volts, eV) to induce molecular changes on harmful organisms and preserve food quality.
Effects on Molecular Structure: Destroys or distorts nucleic acids, breaks hydrogen bonds, oxidizes double bonds, destroys ring structures, and polymerizes molecules.
Absorbed Radiation: Measured in Gray (Gy), where 1 Gy = 1 joule/kg, representing the amount of radiation per unit mass of food.
Types of Ionizing Radiation
Electron Beam
High energy electrons accelerated to nearly the speed of light.
Energy level: up to 10 MeV; low penetration depth (~2cm); fast penetration time; high dose rate; no radioactive waste.
Gamma Ray
Produced from spontaneous degradation of radioisotopes (e.g., Co-60, Cs-137).
Energy level: 1.17-1.5 MeV; highest penetration depth (~3 feet); widely applicable and easily controlled.
X-ray
Created through collisions of accelerated electrons with dense materials (e.g., Tantalum).
Energy level: 5-7 MeV; high penetration depth (~15 inches); largely directional with slow treatment and high shielding.
Usage and Regulation of Food Irradiation
Applied in over 60 countries globally, recognized as safe for food sterilization.
Governed by international standards and agreements, such as the WTO's SPS Agreement.
Certified by organizations: International Atomic Energy Agency, Food and Agriculture Organization, World Health Organization.
Malaysian legislation supports food irradiation for improving safety and quality, implemented in 2013.
Products Subject to Irradiation
Medical Supplies: Necessary for sterilization to ensure safety.
Pharmaceuticals: Essential for maintaining product integrity.
Consumer Goods: Various non-food items that may require sterilization.
Food Products: Such as spices, packaged food, fruits, and vegetables; promising for export-oriented produce in Malaysia.
Applications of Food Irradiation
Benefits: Reduces post-harvest losses, ensures hygienic quality, extends shelf life, facilitates trade.
Dosage Categories:
Low dose (<1 kGy): Inhibit sprouting, insect disinfection.
Medium dose (1-10 kGy): Kill foodborne pathogens.
High dose (10-50 kGy): Sterilization of dried foods (e.g., spices).
Effects of Radiation
Weaken cellulose at doses >10 kGy, increase susceptibility to oxidative rancidity at doses >3-5 kGy.
Weaken structural proteins causing denaturation at doses >10 kGy.
20% loss of Thiamine at 3 kGy, though generally less impactful compared to cooking techniques.
Process Principles of Food Irradiation
Generation and Absorption of Radiation: Displacement of electrons forming free radicals which disrupt cellular processes.
Effectiveness varies by target organism sensitivity; insects and parasites are more susceptible than bacteria and viruses.
Equipment and Design Considerations
Proper design ensures maximum radiation absorption, with consideration of food product attributes (composition, size, temperature).
Continuous and batch processing modes each have unique advantages. Mobile irradiation is utilized in research.
Specifics of Irradiator Design
It includes components that enhance operational efficiency, dose uniformity, and manage processing time.
Efficiency relates to the amount of product processed per source activity, impacting operating costs and processing time.
Gamma Irradiator Structure
Composed of radiation source, biological shielding, and product handling systems (conveyor belts) for effective radiation exposure.
Cobalt-60 Characteristics
Cobalt-60 serves as the primary gamma irradiator source, with a half-life of 5.3 years, emitting gamma rays during disintegration without producing radioactive waste.
Operational Process Using Gamma Ray
Packaged food moved through radiation chamber; effective in food safety and preservation; used innovative applications like fruit juice extraction.
Electron Beam and X-Ray Machinery
Electron injector and accelerator, scanning systems, and material handling systems are key elements in these technologies.
Benefits and Barriers of Food Irradiation
Benefits
Reduces harmful microorganisms, preserves nutritional value, minimizes spoilage, and enhances food safety.
Facilitates immediate distribution post-treatment.
Barriers
Consumer skepticism and regulatory challenges primarily hinder broader acceptance.
Dielectric Heating Overview
Lower frequency spectrum of electromagnetic waves (30 Hz - 300 GHz).
Mechanism involves using the electrical properties of food (especially water) to generate heat internally through molecular friction.
Heating Mechanism
Water and ionic compounds act as electric dipoles, causing internal heating rather than relying on conduction from the surface.
Plastic, glass, and ceramics are ideal for microwave heating due to minimal absorption of microwaves.
Characteristics of Microwave Heating
Internal heating occurs at the speed of light, allowing rapid and uniform heating of food.
Clean energy process minimizes heating of the surrounding environment.
Dielectric Properties of Food
Dielectric constant and loss factor determine how effectively food converts microwave energy into heat.
High loss factor materials generate heat efficiently; penetration depth varies with material composition.
Microwave Oven Components
Key components like magnetron, isolator, waveguide, and safety features ensure efficient operation while minimizing radiation exposure.
Future Directions in Food Irradiation and Microwave Applications
Considerations include market size, consumer acceptance, regulatory frameworks, and industry growth obstacles (e.g., anti-nuclear sentiment, reliability issues).