all parts of the food falls within some
target range. Irradiation can be accomplished using gamma rays, X-rays and
high-energy electrons (e-beams). Gamma
rays are produced from a nuclear source
(cobalt- 60 and cesium-137) and have excellent penetration capabilities. X-rays
and electron beams are produced from
accelerators, which are powered by electricity. X-rays are electromagnetic radiation produced when energetic electrons
hit a target and are emitted by a heated
cathode whose potential may be approximately 30 to 50 kV above the target
(made of a material such as tungsten or
molybdenum). Although X-rays are photons with better penetration capabilities,
electron beams provide high efficiency
(higher dose rate) and high throughput,
and the system has switch-on/switch-off
capability. A linear accelerator consists of
a conveyor or cart system where the
product to be irradiated moves through
the electron beam at a predetermined
speed to obtain the desired dosage. This
article focuses on the recent advances in
electron beam irradiation of fresh produce achieved using an engineering
approach.
“Development of alternative pathogen
decontamination technologies would
certainly improve the safety of ready-to-eat and
fresh agricultural products.”
How Does Irradiation Work?
Living cells lose their biological function when exposed to ionizing radiation
mainly by breaking or damaging their
DNA or by interactions with active radicals such as the products of water radiolysis. One way to quantify the
effectiveness of a specific irradiation
treatment is the D10 value, the amount of
radiation necessary to achieve a 90%
(one-log) reduction of the initial population for a target pathogen in a particular
product. The D10 value varies with the
specific pathogen, food temperature,
other environmental factors and knowledge of the dose. In general, food irradiation treatments are designed for a
five-log reduction of the initial population of pathogens.
either with alanine or radiochromic film
dosimeters placed at the sample’s surface. However, these dosimeters do not
properly fit the irregular and complex
shapes of some fresh produce and can
alter the absorption of the mono-ener-getic radiation energy, possibly introducing variability in the absorbed dose. The
energy distribution in a fresh product is
strongly related to the electron’s entrance region at the surface of the product and the sample’s density.
Thus, inaccurate interpretation of the
measured dose can result in incorrect D10
values. To make things a little more complicated, there are no dosimeters available for measuring doses in the internal
regions of the produce. The engineering
approach allows for an accurate calculation of the dose distribution data within
the food product using Monte Carlo
methods.
Dose and Dosimetry
The primary physical quantity used
in dosimetry is the absorbed dose, de-
fined as the energy absorbed per unit
mass from any kind of ionizing radiation
in any target. The unit dose commonly
used is the Gray (Gy) or Joules per kilo-
gram. The older unit radian (rad) is de-
fined as 100 erg/g, and 1.0 Gy is equal to
100 rads.
Dose Calculation
Monte Carlo techniques use random
input to obtain a result. The random nature of electron beams makes them very
suitable for this purpose. Even though
several radiation transport computer
codes have been developed and adapted
for dose distribution calculations in radiation processing, they do not properly
account for the complex three-dimensional (3D) structure and non-homo-geneity of foods (e.g., density differences
including air pockets). The main difficulty in applying those codes for complex-shaped foods lies in obtaining the
actual product geometry and density values, which are critical factors in the evaluation of electron/photon interactions.
The combination of computed tomography (CT) scans, which yield the geometrical and density information of the food
item, and Monte Carlo simulation provides detailed and high-resolution dose
maps for complex-shaped foods. This
