0524-2808-MTDC: Using a Steamroom To Sterilize Pallets of Styrofoam Seedling Container Blocks: Sterilization Using Radiofrequency Ovens

Radiofrequency (RF) wave ovens can raise the temperature of styroblocks to levels that will kill pathogens. Industrial RF ovens are used for baking, curing, and drying many types of foods and materials. These ovens operate at an electrical frequency of 10 to 100 megahertz.

The material being heated is subjected to an alternating electrical field that makes the molecules inside the material rotate and move from side to side millions of times per second as they try to align themselves with the changing electrical field. This movement generates heat within the material similar to heat generated by friction. The ovens can be incorporated in a conveyor system to mechanize operations. Common microwave ovens operate on the same principle but use radio waves of about 2,450 megahertz to heat food and beverages.

PCS, Inc., in Cleveland, OH, treated styroblock containers with RF heating in a laboratory test oven. The oven operated at 40 kilowatts at a frequency of 18 megahertz using a parallel plate electrode system with variable electrode heights. The plate voltage was 12 kilovolts. Ten styroblocks were divided into two groups of five containers each. One group of styroblocks was soaked in water before treatment; the dry group was not. The styroblocks in the wet group were immersed in water for a short while, shaken to remove excess water, and placed in the RF oven. The RF field was on for 2 minutes. Blocks were removed from the oven and the surface temperatures of cells were measured with an infrared (IR) sensor. Temperatures varied somewhat, ranging from 80 to 120°F and averaging 92°F.

The RF treatment (table 1) reduced pathogens when the styroblocks were wet. It was ineffective when the styroblocks were dry. Apparently, the thin film of water on the surface of the styroblocks is heated by the RF waves, killing fungal pathogens. The RF waves themselves are not toxic to pathogens because dry treatments are ineffective.

Table 1—Test results for the radiofrequency wave oven. Styroblocks were treated either dry or wet.
Fungus	PERCENT COLONIZATION
	Wet containers		Dry containers
	Before	After	Before	After
Fusarium	54	3	73	71
Cylindrocarpon	2	0	1	1
Trichoderma	35	5	34	30
Penicillium	4	3	15	13
Other fungi	37	29	14	39
No fungi	0	60	0	0

While the results indicated that RF treatment is effective when styroblocks are wet, the high cost of the RF oven and conveyor system (more than $65,000) probably would allow only the largest nurseries to use this method.