Frequencies Blast Bacteria in Fruit Juice
By Jim Core
March 27, 2003
Radio waves may be invisible, but they
enrich life in many ways. Without them, radios, televisions, cellular phones
and global positioning systems wouldn't be possible. Now, an
Agricultural Research Service scientist
is using them to make fruit juice safer.
The radio frequency electric fields (RFEF) technique inactivates bacteria in
apple juice without heating it. Although RFEF has been studied for more than 50
years as a pasteurization method, this is the first confirmed instance of a
successful inactivation of bacteria using this technique in fruit juice.
Conventional pasteurization using heat can affect the nutrient composition
and flavor of fruit and vegetable juices. The RFEF technique itself is
nonthermal because the inactivation is not produced by heat. However, when
moderate heat is applied, the combined effect is much greater than the effect
of either process used alone.
David Geveke, a chemical engineer in the ARS
Food Safety Intervention
Technologies Research Unit at the agency's
Eastern Regional Research Center at
Wyndmoor, Pa., built a specially designed treatment chamber to apply
high-intensity RFEF to apple juice. Researchers conducted experiments using
Escherichia coli K12, a harmless form of bacteria used by researchers to
study similarly behaving pathogenic strains, such as E. coli O157:H7.
Apple juice was exposed to electrical field strengths of up to 20 kilovolts
per centimeter and frequencies in the range of 15 to 70 kilohertz, using a
4-kilowatt power supply. For some perspective, lightning occurs at field
strengths of 30 to 40 kilovolts per centimeter, and 20 kilohertz is considered
to be in the upper range of human hearing. Increasing the field strength and
temperature, as well as decreasing the frequency, enhanced inactivation,
according to Geveke. E. coli in juice at 50 degrees Celsius (about 122
degrees Fahrenheit) was reduced by 99.9 percent.
RFEF could provide an alternative to pasteurization by heat. According to
Geveke, the RFEF process could be used to treat heat-sensitive products such as
fruit juices, vegetable juices and liquid egg products.
ARS is the U.S. Department of
Agriculture's chief scientific research agency.