Radio Frequencies Blast Bacteria in Fruit Juice

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.