Non-Chemical Disinfestation of Food and Agricultural Commodities with Radiofrequency Power 235 solely based on the use of energy, they are naturally free of residues and therefore can s
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Trang 512
Non-Chemical Disinfestation of Food
and Agricultural Commodities with Radiofrequency Power
Manuel C Lagunas-Solar
University of California, Davis
RF Biocidics Inc., Vacaville, California
USA
1 Introduction
The presence of microbial and insect/mite pests in foods and agricultural commodities, particularly in fresh produce, dried foods, nuts, grains, seeds, nursery plants, ornamental flowers and in wood products (i.e pallets), continues to be a major factor affecting their condition for safe distribution and use in local, regional and international markets As a mean to reduce the potential of propagating non-indigenous pests, postharvest (mandatory) treatment modalities and quarantine barriers have been imposed to regulate transportation and distribution of many of these products worldwide These regulations define strategies for the detection, control, or eradication techniques for controlling quarantine insect and mite pests
Today, more than 6,500 nonnative species are already established in the United Sates and approximately 15% of these species are either economically or environmentally harmful (Pimentel, Lach, Zuñiga et al., 1999) Control or eradication practices for arthropod pests are mostly based on chemical pesticides, although host removal, adequate agricultural production practices, biological control agents, and sterile insect release are often techniques applied in place off or in conjunction with pesticides
Among the most important quarantine plant pests, various exotic fruit flies have been identified in the USA as threats to more than 250 crops On the other hand, the presence of moths in stored products represents important and unacceptable risks to many growing and expanding agricultural regions worldwide If detected, affected commodities must be processed with effective control or eradication techniques If unattended, losses in product’s quality represent unacceptable economic losses
Chemical pesticides, waxes, coatings, thermal treatments (heated air; hot water immersion), modified atmospheres, cold storage (refrigeration), and irradiation are some of the processes that have helped industry meet current challenges and demands Lately, however, new consumer preferences, trends and regulatory interventions have increased the needs for minimally processed foods with low or no residual chemicals This new trend requires that less invasive or chemical-free alternatives become available to replace or minimize the use of pesticides Furthermore, recent concerns associated with potential terrorist threats using microbial contaminants or other pests, have increased the need to develop alternatives to
Trang 6assure the safety of the food supply while minimizing economical risks associated with production and export agriculture These combined challenges are now familiar to affected governments as well as to industry and regulators worldwide
Historically, and with a few exceptions, pesticides have provided an ample spectrum of effective techniques to control pests and there is a continual industry trend to maintain and improve their use However, this practice and its effects and limitations have partially fueled the emergence of organic agriculture This in turn has prompted conventional agriculture to review its practices, its traditional processes, and to investigate new types of pesticides as well as to develop new disinfestation techniques The incorporation of fluorine
in agrochemicals to enhance stability and bioavailability is the latest attempt to increase their effectiveness while reducing their secondary impact (Jeschke, 2004) Nevertheless, their invasiveness and persistence in all environs surrounding agricultural practices continues to
be resisted by consumers and by increased limiting regulations
Past and even present industry reliance on methyl bromide fumigation for quarantine pest controls is the best and most recent example of the changing attitude that exists today with respect to invasive chemical processes The existing ban and the new restrictions on production levels have forced agriculture to look for new and better alternatives Fumigation, vacuum techniques and controlled atmospheres (CA) for insect (quarantine) control are marginally successful and restricted to long-storage commodities (i.e grains, nut products, raisins) (Bond, 2007; Calderon, 1990) For perishable fresh commodities, these techniques have failed to provide the required and timely disinfestation level Nevertheless, while somewhat successful, the needed long processing times (days or weeks) increases cost and is inadequate to fit with the logistics of marketing fresh agricultural products
The use of low-level doses of ionizing radiation (i.e food irradiation) is another effective and approved technique providing an alternative to disinfestation and disinfection of many commodities (Urbain, 1986) However, while technically useful and approved for certain applications, this approach prompts many public concerns and is usually and effectively resisted Furthermore, because irradiation facilities require a high capital investment to install and operate in order to remain economically viable, it also forces the irradiation industry to operate as major centralized facilities located near high productivity agricultural areas The seasonal nature of agriculture, however, forces the irradiation industry to meet the peak demands with excess processing capacity and to broaden off-season applications (i.e disinfection of medical supplies) to remain viable Consequently, the handling and distribution of to-be-treated food and agricultural commodities imposes new and severe logistical and cost adjustments to the user community As a result, few if any agricultural export areas rely on irradiation facilities and those operating represent a small and stagnant resource for insect control
Despite the above limitations, ionizing radiation also provide means to sterilize insects that once released in specific areas can reduce the impact of local/regional infestations
As of today, with the exception of food irradiation, few attempts to fulfill the need for new alternatives to pesticides have been investigated using single or combined physical processes If effective, these processes are inherently safer, eliminating the risks associated with the presence of pesticides in products and ultimately easing the current concerns with disposal issues, worker safety, and environmental impacts Non-chemical or residue-free alternatives also provide opportunities to yield products with attributes closer to their natural sensory and nutritional properties Furthermore, because physical processes are
Trang 7Non-Chemical Disinfestation of Food and Agricultural Commodities with Radiofrequency Power 235 solely based on the use of energy, they are naturally free of residues and therefore can serve the needs of both conventional and organic agriculture
Since 2002, research at the University of California, Davis established the use of RF power for disinfestation as well as for many novel sanitation and preservation purposes for a variety of food, non-food and agricultural commodities Since then, RF processing has been established as a novel methodology able to provide new alternatives for chemical-free disinfestation, disinfection and enzyme deactivation effects on various commodities (Lagunas-Solar, 2003; Lagunas-Solar, Zeng & Essert, 2003; Lagunas-Solar, Zeng, Essert et al 2005a; Lagunas-Solar, Cullor, Zeng, et al 2005b; Lagunas-Solar, Zeng, Essert et al 2006a) RF disinfestation, in particular, was proven as an effective, rapid, and a reliable chemical-free alternative to pesticides and capable of large-scale processing
Radiofrequency waves using designated, single frequencies are approved for industrial, scientific and medical uses by national (US Federal Communication Commission, FCC) and international organizations Currently, limited but increasing commercial use in all these areas to heat-treat and dry a variety of commodities is underway Radiofrequency power provides well-controlled, volumetric (internal) and rapid heating of a diverse variety of food and non-food commodities Appropriate food and non-food products to be processed and heated with RF power are generally known as dielectrics (poor electric conductors) and include pests, microbes, foods and non-food agricultural commodities such as soil, packaging and wood (pallets) products
Dielectric properties are directly related to the material’s chemical (molecular) composition and due to the presence and relative abundance of dipoles like water and/or induced dipoles like proteins, lipids, and carbohydrates Therefore, the material’s ability to absorb RF power and convert it to thermal power resides at the molecular level Because molecules are well distributed and organized within and on the surface of dielectric materials, the effect of absorbing RF power occurs throughout its volume and to a lesser extent on its surface (lower concentrations) where temperatures are slightly lower than its internal volume (<
1oC) For this reason, RF processing is said to be a volumetric process, comparable to microwave heating, but in contrast with any other conventional surface thermal process known today By comparison, the volumetric nature of RF processing provides with unique opportunities to reduce the needed thermal load (i.e temperature over time) required for an intended effect as heat losses by radiation are larger at the surface This volumetric property applies equally to arthropod and microbial pests as well as to the host commodity and its package
The RF disinfestation process is rapid (seconds to minutes) and proven effective when reaches lethal thermal levels (50-60oC) These levels are sufficient to provide thermal loads able to irreversibly disrupt essential and common metabolic pathways and to affect all biological stages of arthropod (and other) pests Furthermore, as the interaction of RF photons with molecules is frequency dependent, at specific frequencies insect pests exhibit a higher heating rate than the host commodity allowing a somewhat selective heating process
to be realized This selective process minimizes processing time and lowers the overall thermal load applied to the commodity thus decreasing the potential for any adverse effects
on its quality attributes
The fundamental physical concepts and the rationale behind the RF disinfestation process, including the interactive energy-transfer and conversion mechanisms (RF to thermal power) with arthropod pests are explained below
Trang 82 Physics of RF power
2.1 RF photons and the electromagnetic spectrum
Radiofrequency photons belong to the electromagnetic spectrum of radiant energy The
electromagnetic spectrum covers a very large range of wave photons with frequencies
ranging from 106 to 1020 Hz (1 Hz = 1 cycle/sec) and wavelengths from 103 to 10-12 m As
shown below in Figure 1, this range covers radiowaves (~106 to 1010 Hz), microwaves (~1010
to 1012 Hz), infrared, visible and ultraviolet radiation (~1012 to 1016 Hz) and soft, hard X rays
and gamma rays (1016 to 1020 Hz)
Fig 1 Electromagnetic spectrum (simplified)
Radiofrequency power is, however, a small segment of the radiowaves region with an
arbitrarily defined range of frequencies between ~ 1 MHz (300 m wavelengths) to 300 MHz
(1 m wavelengths) In the defined frequency range, the RF photon energy is in the 6.6 x 10-28
to 6.6 x 10-26 J/photon (or 4.1 x 10-9 to 4.1 x 10-7 eV/photon) Therefore, RF processing
involves photons of very low energy and long wavelength and therefore absorbing dipole or
induced dipole molecules can only experience excitation effects (i.e vibrational and
rotational) but will not lose electrons to cause ionization or the formation of free radicals.1
Radiofrequency waves are produced by rapid electrical oscillations and generally are able to
penetrate deep into various materials, but are reflected by electric conductors and by the
ionized layers in the upper atmosphere Like all other photons in the electromagnetic
spectrum, RF photons consists of electric and magnetic waves oscillating at right angles to
the direction of propagation (i.e transverse waves) and moving through space at the speed
of light (c = 2.998 x 108 m/sec) The combination of electric and magnetic fields originates an
electromagnetic field
The relationship between the RF photon energy and its frequency is given by Einstein’s
classical expression as:
where: E is the photon energy (Joules);
1 Chemical bond energies are in the range of 1 to 10 eV per bond Therefore, RF photons (1 to 100MHz)
carry one billionths to 100 millionths less energy than is required to break a single bond Free radicals
are extremely reactive (short lived) chemical species capable of inducing chemical reactions Their
formation is associated exclusively with sources of ionizing radiation (> 1 eV/photon)
Trang 9Non-Chemical Disinfestation of Food and Agricultural Commodities with Radiofrequency Power 237
h is the Planck’s constant (6.626 x 10-34 Joules sec or 4.136 x 10-15 eV sec); and
f is the photon frequency (Hz or cycles/sec)
This expression indicates that all photons in the electromagnetic spectrum come as discrete
quantities named “quanta” and moving at the speed of light It also indicates that photon
energy is always a multiple of Planck’s constant times its frequency (cycles/sec)
Because frequency (f in Hz) and wavelength (λ in m) of an electromagnetic wave are related
to the speed of light as
formula 1 can also be expressed as
/
indicating that photon energy E is inversely proportional to its wavelength λ
2.2 Interactions of RF photons with matter
Biological materials - including foods, microbes, arthropods and many agricultural
products, are non-magnetic in nature, therefore, only the electric field component of an
electromagnetic wave is able to interact and strongly affect the polar and induced polar
molecules in the product
In the presence of an oscillating electric field (changing polarity at a set frequency), the
interactive mechanisms of the electric field with RF active molecules (i.e dielectrics or poor
electric conductors) include: (1) reorientation of permanent dipoles (i.e water); (2) inducing
dipoles by polarization of bound charges (proteins, carbohydrates, lipids); and (3) forcing
the drift (displacement) of electronic and ionic conduction charges (mineral nutrients)
(Klauenberg & Miklavcic, 2000)
The above interactive mechanisms only act at the molecular level and thus the effects of RF
processing is based solely on the material’s chemical composition in which permanent
dipoles (i.e water) play a major role while other lower concentration non-polar or weakly
polar molecules are activated in proportion to the magnitude of the electric field Initially,
and without an electric field, polar and non-polar molecules in any material are randomly
oriented due to thermal excitation, which forces their multi-directional movement and
spatial distribution
When an electric field is applied, dipole (polar) molecules tend to re-orient and become
aligned according to the direction of the electric field in a phenomenon known as
“orientation polarization” Still, orientation is opposed by thermal excitation and therefore,
the net orientation effect is proportional to the intensity of the electric field once it
overcomes the random distribution of the active molecules in the RF field
In non-polar molecules, the electric forces separate positive and negative charges a small
distance thus inducing temporal dipoles This type of induced dipole exists only when the
electric field is present and occurs via electronic (displacement of electrons) or atomic
(displacement of charged atoms) mechanisms known collectively as “distortion
polarization”
In both cases with orientation or distortion polarization, the charges in dipoles or in induced
dipoles do not cancel and, therefore, new internal electric fields are formed Distortion
polarization is temperature dependent while orientation polarization is inversely
Trang 10proportional to temperature as RF active molecules must overcome the randomness from
thermal excitation
Furthermore, all polarization effects can only operate up to a limiting frequency after which
if frequency increases, orientation polarization effects tend to disappear as the inertial effect
of permanent polar molecules prevent reversal of their direction of motion and thus their
inertial movement (i.e momentum) cannot be overcome The RF process is thus frequency
dependent and can be optimized at certain selective frequencies matching the dielectric
properties of a material (Lagunas-Solar, Zeng & Essert, 2003)
In arthropod pests, as in all biological systems, water (free and bound) and to a lesser extend
proteins, lipids, carbohydrates are the major chemical constituents while mineral nutrients
are at trace levels Water is a natural permanent dipole but its degree of freedom depends on
its chemical environ with free (unbound) water being the most active dipole to interact with
oscillating electric fields Bound water, on the other hand, because of its binding
(coordination) with other molecules, may still be active but is somewhat restricted to
respond to electric field oscillations Proteins, including enzymes, lipids and carbohydrates
are polarizable under a voltage difference and therefore become temporal induced dipoles
able to experience electric field interactions and be actively involved in generating thermal
energy within the material Inorganic ions (i.e mineral nutrients) are always charged and
can be displaced by the electric fields and generate small electric currents which converts to
heat through resistance (Ohm’s law) Overall, although at different levels, all constituents
may be actively re-oriented or displaced generating thermal energy by combination of the
above different interactive mechanisms
Although most permanent and induced dipoles are not free to drift, displacements of
conduction charges or free ions under the influence of an electric field is a classical
phenomenon known as ionic conductivity Conduction effects (Jc in Amperes/m2) are
related directly to both conductivity (σ in Siemens/m2) and the net electric field E
(Amperes/Siemens) (Lea & Burke, 1998)
2.3 Mechanisms of RF heating
The ability to induce polarization effects in a material by an applied electric field and the
creation of new, transient electric fields and currents within the material is characterized by
a quantity noted as Ɛ and called “dielectric constant” or “permittivity” (Klauenberg &
Miklavcic, 2000) Therefore, the dielectric constant measures how easily a material is
polarized to store electric energy
However, dielectric constants are measured in relation to vacuum or air (Ɛo = 1.00000 and
1.00054, respectively) as they represent the ability of a material to store electric energy (i.e
capacitance) at a given voltage as compared to vacuum or air Therefore, relative dielectric
constants for a material are given by
where Ɛ’ is the relative dielectric constant and Êa and Ê are the applied and the net electric
field strengths (vectors), respectively
In real practice, the ratio by which each mechanism intervenes in storing electric energy is
accompanied by effective dissipation losses due to thermal excitation, inertial motions and
due to the different binding forces in lattices or accompanying the RF active chemicals
These losses force molecules to lag behind the frequency of the oscillating electric field or