0524-2808-MTDC: Using a Steamroom To Sterilize Pallets of Styrofoam Seedling Container Blocks: Bulk Testing Using Low- and High-Humidity Heat

Initial tests did not show whether the heat would penetrate to the innermost styroblocks if a pallet load of styroblocks (figure 4) was being sterilized. MTDC designed a test box (figure 5) for a pallet load of about 50 styroblocks. A metal storage container (7-feet wide by 13-feet long by 7-feet high) was insulated with rigid Styrofoam insulation. A residential sauna heater was mounted inside the box to heat it to 160 to 170°F. Designers fabricated a device to spill water on the sauna rocks to generate steam and raise humidity.

[photo] Forklift loading pallet of styroblocks into big oven.

Figure 4—Styroblocks loaded on a pallet were heated to 170°F. Tests were
conducted at low and high (higher than 75 percent) humidities.

[photo] Heated storage container used for sterilizing styroblocks.

Figure 5—A storage container outfitted with a commercial sauna heater
was used to sterilize styroblocks during testing.

A series of tests were conducted using different exposure times with low and high relative humidities. During three tests conducted at low relative humidities (less than 30 percent), the blocks were wetted before they were heated for 15, 30, and 60 minutes. During three additional tests at high relative humidities (higher than 75 percent), the blocks were not wetted before being heated for 15, 30, or 60 minutes.

These tests monitored styroblocks in the center of a pallet load. The blocks were thoroughly soaked with water for the tests that required wetting. Styroblocks were exposed to 160 to 170°F for the appropriate time periods.

Styroblocks tested were from the Forest Service nursery in Coeur d'Alene, ID. Samples were taken from the styroblocks before and after testing. High-humidity heat was more effective than low-humidity heat (tables 4 and 5). Fusarium and Cylindrocarpon colonization was eliminated even when the blocks were heated for just 15 minutes at high humidity.

Table 4—Results for bulk testing styroblocks in the MTDC sterilization container. Wetted styroblocks were treated for 15, 30, and 60 minutes with low relative humidities at 170°F.
Fungus	PERCENT COLONIZATION
	15 min.		30 min.		60 min.
	Low humidity		Low humidity		Low humidity
	Before	After	Before	After	Before	After
Fusarium	0	0	79	88	46	13
Cylindrocarpon	0	0	42	0	0	0
Trichoderma	0	0	42	42	8	17
Penicillium	8	83	67	96	21	17
Other fungi	42	8	13	0	92	42
No fungi	0	0	0	0	0	17

Table 5—Results for bulk testing styroblocks in the MTDC sterilization container. Styroblocks were treated for 15, 30, and 60 minutes with high relative humidities (higher than 75 percent) at 170°F.
Fungus	PERCENT COLONIZATION
	15 min.		30 min.		60 min.
	High humidity		High humidity		High humidity
	Before	After	Before	After	Before	After
Fusarium	58	0	50	0	25	0
Cylindrocarpon	0	0	0	0	0	0
Trichoderma	8	0	8	0	21	0
Penicillium	75	13	25	0	33	29
Other fungi	96	17	88	0	88	17
No fungi	0	71	0	100	0	54