Agriculture Reference
In-Depth Information
Infrared radiation has a wavelength longer than that of visible light, but shorter
(longer frequencies) than that of microwaves. The wavelength of infrared radiation
ranges from 780nm to 1mm. Near infrared wavelengths range from 780nm to
1400 nm, medium infrared wavelengths are between 1400 nm and 3000 nm, and far
infrared is above 3000nm. Practically, the effi ciency of infrared heating can be
increased by matching the emitted wavelength and the absorption spectrum of the
material to be heated. For example, the absorption spectrum for water has its maximum
at around 3000nm. Emission from medium-wavelength infrared is much better
absorbed by water than near infrared or short-wave infrared radiation. Bacteria have
a different cell wall composition (peptidoglycan) than plants (mainly polysaccharide,
such as cellulose). The infrared absorbance spectrum of peptidoglycan is different
from that of polysaccharide (Naumann and others 1982). Therefore, it may be possible
to develop an infrared heater that emits at certain wavelengths that are mainly absorbed
by the bacteria and other unique bacterial molecules anchored on the cell wall, result-
ing in selective heating of the bacteria. Another way of selective heating might be
achieved by using a food-grade chemical that selectively binds to bacteria and has a
unique absorption of infrared, microwave, or RF radiation. When exposed to certain
wavelength(s) of radiation, only bacteria would heated up and plant cells would not
be signifi cantly affected, thus preserving the quality of treated fresh produce.
RF Treatment
Alfalfa seeds inoculated with
Salmonella
,
E. coli
O157 : H7, and
L. monocytogenes
were exposed to RF at 39MHz and different electric fi eld intensities (Nelson and
others 2002). Although it was possible to achieve signifi cant reductions in the popula-
tions of all three pathogens without affecting seed germination, it was impossible to
obtain the desired levels (5 log) of pathogen reduction without signifi cant damage to
seed germination.
RF thermal treatment was compared with chlorine wash and hot-water dipping in
a study to improve the storability of carrot sticks (Orsat and others 2001). Carrot sticks
were heated to 60°C in less than 2min in a parallel plate RF applicator. Results
showed that the quality of the RF treated samples was greater than those treated with
chlorine (100 ppm) or hot water (by immersing carrots in boiling water until the inter-
nal temperature reached 60°C). The color and fresh taste were maintained in RF-
treated carrot samples, in contrast to the control and hot-water-treated carrots.
However, merely reducing the initial microbial load did not maintain the quality of
carrot sticks for 14 days at 6 °C.
In another study, apple fruit was initially exposed to 27.12-MHz RF energy at
12 kw for 2.75 min and then subjected to hot - water dips (48 - 50 ° C) for different dura-
tions (Hansen and others 2006). The combined heat treatment resulted in peel and
fl esh discoloration, along with reduced fi rmness and increased external and internal
damage.
Microwave
Microwave radiation has been extensively studied to disinfest fruits. For example,
microwave heating, followed by steam treatment, has been successfully applied to
