Mark A. Smith

Electromagnetic radiation has proven to be effective for disinfection or sterilization for many applications. Effectiveness of such methods in various applications is generally a function of the energy, or wavelength, of the radiation and can vary widely. As described by Maxwell1 in his original paper of 1865, electromagnetic radiation consists of a transverse wave, with electric and magnetic field oscillations perpendicular to each other and to the direction of propagation, which occurs at the speed of light in a vacuum. Figure 10.1 shows an electromagnetic wave schematically. Two parameters are of importance in discussing electromagnetic radiation, frequency, and wavelength.

The wavelength, as noted in Figure 10.1, is the physical distance over which a wave pattern repeats, depicted as the distance from a specific point on a given wave cycle to the corresponding point on the succeeding wave cycle. The frequency of oscillation is the number of repeating cycles over a given time period. The two terms are inversely related1:

where c is the speed of light in a vacuum (299 792 458 m/s), λ is the radiant energy wavelength, and ν is the wave frequency. The frequency of oscillation is proportional to the radiant energy, a relationship elucidated by Planck2 in a seminal paper on quantum theory in 1900. Frequency and energy are connected by a proportionality constant:

where E is the radiation energy, h is Planck’s constant (6.626 × 10-34 J · s or 4.136 × 10-15 eV · s), and ν again represents the wave frequency.

Planck’s equation formed the fundamental basis of quantum theory, stating that radiant energy in the electromagnetic spectrum is actually quantized or formed of discrete and separate bundles of energy. For electromagnetic radiation, these quanta are called photons. Characteristics of the interaction between electromagnetic radiation quanta and matter are largely dependent on the photon energy.

Maxwell’s equations predicted, among other things, a spectrum of electromagnetic energy with an infinite number of frequencies, which led to the formulation of what has become known as the electromagnetic spectrum. The common depiction of this spectrum is arranged by one of the three parameters: wavelength, frequency, or energy (Figure 10.2). The electromagnetic spectrum is divided into bands or sections, each of which is named. In the lowest energy band, wavelengths are on the order of a kilometer or more. At the high-energy end of the spectrum, wavelengths are comparable in size to an atomic nucleus. Boundaries between bands on the spectrum are not clearly defined, with no precise demarcation from one type of radiation to another. Several boundaries overlap, such that a given energy radiation could be in more than one band. For example, a photon of 500 keV electromagnetic radiation could be either an X-ray or a gamma ray. The difference is one of origin and not energy, in that the X-ray is generated in the electron cloud of an atom, whereas the gamma ray arises from the nucleus.

Apart from the individual bands across the spectrum as shown in Figure 10.2, electromagnetic radiation is generally divided into two large categories, differentiated by the type of reaction that occurs between photons and the matter with which they interact. At wavelengths of several nanometers and shorter, corresponding to an energy of approximately 10 eV or higher, the photons have a sufficient energy to overcome atomic or molecular ionization energy, resulting in the ejection of an electron from atoms with which the photon interacts. Photons in this energy range are referred to as ionizing radiation, whereas those with longer wavelengths are considered
nonionizing radiation. There is also no clear demarcation in terms of wavelength (or energy) between the two divisions. Instead, radiation is classified as one or the other on the basis of the reactions it creates. Ionizing radiation is generally considered to start within the ultraviolet (UV) range and extend to higher energies3; however, these are not definitive boundaries. For example, the photoelectric effect (ejection of an electron from an atom as a result of incident photon energy) can occur in some metals (eg, sodium) at wavelengths in the visible portion of the spectrum, even though visible light is considered nonionizing.

 FIGURE 10.1 Diagram of electromagnetic radiation, consisting of electrical and magnetic oscillations in planes perpendicular to each other and to the direction in which the wave front is propagated. A wavelength is defined as the distance between successive waves, as measured from the same point in the oscillation cycle, for example, from one crest to the next crest.

The distinction between ionizing and nonionizing radiation is important in considering the effect that the type of radiation has on organisms. Photochemical reactions may occur at energies that correspond to the visible light portion of the spectrum, for example, photosynthesis, with chemical reactions resulting from ion creation occurring at higher energy. At energies lower than the visible range, the energy deposited typically results in molecular vibration or oscillation of charge carriers. The macroscopic effect of such reactions tends to be thermal as opposed to chemical. Most or all electromagnetic radiation reactions with matter can be used for disinfection or sterilization. This chapter concentrates on processes involving nonionizing radiation, with some inclusion of techniques using radiation in the UV range. Other chapters describe methods that employ ionizing radiation, from UV (see chapter 9) through X-rays and gamma rays (see chapter 29).

 FIGURE 10.2 Diagram of the electromagnetic radiation spectrum, divided into bands according to wavelength, frequency, and energy.

MICROWAVE DISINFECTION AND STERILIZATION

In 1945, a patent application submitted by Percy L. Spencer4 of Raytheon Manufacturing Company described a method for heating food that used electromagnetic energy with a wavelength of about 10 cm. This invention, the now near ubiquitous microwave oven for home use, grew out of radar technology and represented a new method for heating food.

 FIGURE 10.3 Depiction of microwave heating versus conventional radiant heating. On the left, the conventional heating system uses a heat source external to the material being treated, resulting in thermal effects beginning on the outside of the material, moving inward through conduction. On the right, microwaves penetrate the material being treated and thermal effects begin inside the material, moving both inward and outward through conduction.

Microwaves can also be used for heat treatment in an industrial setting. For disinfection or sterilization, the microbiological efficacy of the process is the same as for other processes that use heat to achieve the desired effect (see chapters 11 and 28). The difference is in the method whereby heat is generated. Various applications of microwave heating for disinfection in medical facilities are soft contact lenses, dental instruments, dentures, milk, and urinary catheters.5

In describing the relationship between microwaves and materials, it is convenient to divide all materials into three categories. They may be classified as conductors, insulators, or dielectrics. A conductor, which is a material in which electrons flow freely, allows for electricity flow in the material. Most common conductor materials are metals. When microwaves interact with conductors, the radiant energy tends to be reflected by the conductor. An insulator, conversely, is a material that inhibits the free flow of electrons and tends to not allow electricity to be conducted. Common insulator materials are plastics, ceramics, and glass. When microwaves interact with insulators, the radiant energy passes through the material and little interaction or energy deposition occurs. A dielectric material absorbs and stores energy without conducting it. In theory, it is a special class of insulator in that the energy interacts with the material, but electricity is not conducted. The microwave heating process occurs with dielectric materials, principally as interactions involving the dipole properties of the material. Because microwaves have somewhat limited penetration into the materials, microwave heating of bulky or thick material also relies on conductive heating within the material, such that the heating occurs not necessarily from the center outwards but from a point within the material to other points within the material.6

Figure 10.3 shows a simple representation of a microwave heating process as compared to a conventional electrical heater. On the left, the conventional heater creates a temperature rise in the material by an externally applied heat source, creating the thermal effect beginning at the outer surface of the material. On the right, a microwave heating system bombards the material with radiant energy, and the heating process begins within the interior of the material.

Microwave Theory

Heat is generated within a material by microwaves in a process known as dielectric heating, a physical process whereby energy from electromagnetic radiation generates vibrational energy in molecules of the insulating material (dielectric) that causes a temperature increase within the material, a process originally described in 1954.6 Other factors also contribute to the process, including uniformity of the radiation field and heat transfer driven by temperature gradients.

Because microwaves exist in the same general region of the electromagnetic spectrum as radio waves used for communications, industrial and consumer microwave equipment is general limited to specified wavelengths. In the United States, two frequencies are typically used in microwave heating processes: 915 and 2450 MHz. Both are used in commercial applications, but the latter is more commonly encountered in home-use microwave ovens. Outside of the United States, other frequencies are also used, specifically 433.92, 896, and 2375 MHz.7

Microwave heating occurs when the radiant energy interacts at a molecular level, generating friction by dipole
rotation of polar solvents and migration of dissolved ions. Dipole rotation occurs with an alternating current electric field, where the field polarity varies at the wave frequency and the molecules continually attempt to align with the polarity. Friction from this process creates heat. In microwave ovens used for heating food, water is the dipole principally responsible for the heating effect. Migration of dissolved ions creates friction in a similar manner, where heat is generated as the ions realign with the field oscillations.8 Therefore, microwave heating principally results from dipole rotation and ionic polarization. The volumetric heating rate (Q) of microwave is related to the electric field strength by

where f is the frequency of microwaves, E the strength of electric field of the wave at that location, εo the permittivity of free space (a physical constant), and ε″ the dielectric loss factor (a material property called dielectric property) representing the material’s ability to absorb that particular wavelength of electromagnetic radiation. The dielectric constant, ε″, also affects the strength of the electric field inside the object being irradiated with microwaves.7

Microwave Equipment

A simplified representation of equipment for microwave processing consists of a magnetron (a microwave cavity wherein materials are heated), waveguides that direct microwaves from the magnetron to the cavity, and a control system.9 Microwaves are generated by the magnetron, which is composed of the two parts of a diode: a cathode and an anode. Both electrodes are cylindrical and concentric, with an anode resonant cavity surrounding the cathode, as shown in Figure 10.4. A magnetic field is created by a magnet placed around the anode. Heating the cathode generates electrons with negative charge that move toward the anode, which has a positive charge. The magnetic field perturbs the path of the electrons, causing them to deviate from a straight-line trajectory. As they pass the resonator cavity, the electrons oscillate at a high frequency. The frequency of the resultant microwave matches the oscillation frequency of the electrons in the resonator cavity. These oscillations are collected by an antenna and transmitted via a wave guide to the cavity wherein material heating occurs.7

Microwave Applications

Microwave technology serves as a heat source for disinfection or sterilization purposes. Therefore, the microbiological efficacy of the method is based on the heat properties and is similar to other heat-based techniques. There have been studies that suggest microwaves could be a microbicide apart from the thermal effects, but this has been difficult to verify. Using a microwave oven designed similarly to a home kitchen appliance, studies showed inactivation of bacterial cultures, mycobacteria, viruses, and Geobacillus stearothermophilus spores, although the suggestion was that that water and higher power microwaves may be needed for sterilization.10,11,12,13,14,15 One concern is that the microwave heaters that are available commercially may have uneven energy distribution limiting treatment applications with devices (ie, hot and cold spots on solid devices), which is difficult to characterize adequately to ensure that the entire device has sterilized or disinfected.5

 FIGURE 10.4 Cross-sectional diagram of a resonant cavity magnetron, which generates microwaves from interaction between electrons and a magnetic field while moving past a series of open metal cavities. Electrons passing the cavities generate radio wave oscillations, serving as a source of microwaves. Resonant frequencies are determined by the size and shape of the resonant cavities.

Microwaves have been used in the food industry for several purposes including, tempering of frozen meat and poultry products; precooking of bacon for foodservice; sausage cooking; drying of various foods; baking of bread, biscuit, and confectionery; thawing of frozen products; blanching of vegetables; heating and sterilizing fast food, cooked meals, and cereals; and pasteurization and sterilization of various foods. For pasteurization and sterilization, the process using microwaves is dependent on thermal properties and heat generated during processing. In that way, the microbicidal effect is the same as any other heating method, a function of the temperature attained and the time the product is held at the higher temperature.7 Commercialization of the microwave heating process for
pasteurization or sterilization has proven to be problematic, with only limited success.16

In medical applications, microwaves have been studied and shown to be effective for disinfection of soft contact lenses, dental instruments, dentures, milk, and urinary catheters. For catheters, it was found that the microwave heating process tended to cause melting in certain plastics, making choice of materials a critical step. Although experience in home appliance microwave ovens suggests that disinfection of metal objects would be an issue due to arcing, with certain precautions in presentation of the objects (eg, immersed in water or controlling the application of microwave energy), it has been shown possible to use microwave heating for some metal objects.17,18,19,20,21,22,23

One use of microwaves related to medical applications is for disinfection of medical waste. This process again relies on the heating effects of microwaves to attain and maintain temperature adequate for disinfection. Medical waste is processed first by shredding the materials, depicted in Figure 10.5, which gives a more uniform and smaller particle size that does not require as high a temperature as would be required for intact materials. Shredded waste is moistened and passed through a series of microwave units for thermal processing, as shown in Figure 10.6. The small, uniform particle size and the water content ensure that microwaves heat the waste efficiently for microbial reduction, rendering it suitable for disposal as municipal solid waste in a sanitary landfill. Validation of the process can be difficult, but at least one study has been published in using biological indicators to monitor process effectiveness.24

 FIGURE 10.5 Commercial equipment for microwave disinfection of medical waste. Waste containers are emptied into the process via an automated handler and shredded into tiny particles in a system specifically designed for medical waste. Photograph courtesy of Sanitec Industries, Inc. For a color version of this art, please consult the eBook.

 FIGURE 10.6 Commercial equipment for microwave disinfection of medical waste. Shredded waste consisting of small particles is moved on a stainless steel screw conveyor, moistened with steam, and passed by a series of microwave units. Thermal treatment of each individual waste particle occurs from the inside out, assuring thorough disinfection. Treatment is verified on a regular basis by challenge testing using spores of Bacillus atrophaeus. Photograph courtesy of Sanitec Industries, Inc. For a color version of this art, please consult the eBook.

INFRARED DISINFECTION AND STERILIZATION

Moving higher in energy on the electromagnetic spectrum, just above the microwave range and below visible light is a region called the infrared. Infrared radiation was discovered more than half a century before James Clerk Maxwell1 formulated the equations describing behavior of electromagnetic radiation. Frederick William Herschel,25 a musician and composer as well as an astronomer in Great Britain, was testing filters to be used in observing sun spots when he discovered that a red filter caused a rise in temperature that was significantly larger than observed with unfiltered light. Although illumination of any surface with light will cause the object to become warm, Herschel25 found that the temperature was surprisingly higher when he passed light through a prism and held a thermometer in the area just beyond the red end of the visible light spectrum. His theory was that there was a band of light beyond the visible spectrum that produced enhanced heating effects. This radiation he referred to as calorific rays, a term that was replaced by infrared in the late 19th century.25,26