Filtration to Remove Spores of Penicillium Expansum from Water
in Pome Fruit Packinghouses
The two most common disinfectants used to reduce spore levels in dump tank and flume water are chlorine (sodium hypochlorite) and sodium o-phenylphenate (SOPP).1 Chlorine is inexpensive but corrodes equipment and will not kill spores lodged in injured tissues. SOPP occasionally injures fruit if not thoroughly rinsed. Other ways to reduce spore levels include ozone and chlorine dioxide, but these methods also have limitations and are not widely used.
Physically removing decay spores from water has received little attention. In Australia, a combination of sand, cellulose cartridge, and ceramic filters has been used successfully to remove fungal spores from irrigation water.2 In Canada, dump tank water in an apple packinghouse was passed through two Jacuzzi sand filters, and levels of P. expansum were reduced 59% to 88%.4 However, the filter system was considered too erratic to provide reliable water sanitation.
In the research reported below, a commercial cartridge filter system and two sand filters were evaluated to determine how efficiently they remove decay spores from drencher and flume water.
Two commercial sand filters (model Triton TR 140, Pac Fab, Inc., El Monte, CA) were evaluated in an apple packinghouse. Each filter contained 114 kg (251 pounds) of pea gravel and 227 kg (500 pounds) of #20 silica sand and had a filter area of 0.641 m2 (6.9 sq. ft.). Flume water was pumped through the filter at 530 L (140 gal.)/min. Water entering and exiting each filter was sampled weekly from 21 November, 1986, to 11 May, 1987. Each sample was dilution plated on acidified potato dextrose agar to determine the concentration of P. expansum spores in the water.
A commercial, multistage filtering system connected to a drive-through bin drencher at several apple packinghouses was evaluated. The drenching liquid was a water suspension of DPA at 2000 parts per million (ppm) and thiabendazole (Mertect 340-F) at 528 ppm. The first stage of filtration consisted of a 0.51 mm (0.02 inch) screen and liquid cyclone units (Maxicleaner unit, Rush Consultants, Wenatchee, WA) operated at 757 L (200 gal.)/min., continually recycling the liquid from tanks containing 1500 to 2000 gal. to remove leaves, stems, and other large debris. Periodically, after 2000 to 3000 bins had been processed, the liquid in the drencher holding tank was emptied into another storage tank and the suspension pumped through a final set of filters (Zero discharge unit, Rush Consultants) at 37.9 L (10 gal.)/min. These filters consisted of four elements in step-down series: 100 and 45 µm nylon mesh filter bags, a pair of 5 µm polypropylene cartridge filters, and a terminal pair of either a 1.0 µm polypropylene or 0.45 µm glass fiber cartridge filters. Samples were taken before and after filtration on five occasions where 1.0 gm terminal filters were in use and on three occasions with 0.45 µm filters. Samples were dilution plated, incubated, and colonies of P. expansum enumerated as described above.
The average number of P. expansum spores per ml in water before and after filtration through sand filter unit #1 was 547 and 725, respectively, and the difference was statistically significant (P=0.05). For sand filter unit #2, 458 spores/ml were detected in water before and 441 spores/ml after filtration; the difference was not significant (P=0.05).
The level of spores of P. expansum in commercial drencher water before and after filtration through the four-element cartridge filter unit was 1153 and 1123 spores/ml, respectively, when the terminal filter was 1.0 µm. This represented a removal of 3% and was not significant (P=0.05). However, 92% of spores were removed from drencher solutions filtered through the four-element units when the terminal filter was 0.45 µm (617 vs. 50 spores/ml); removal was significant (P=0.03).
The cartridge filter system has potential for effective removal of decay spores and thus increased decay control. However, effectiveness is closely related to pore size of the terminal filter. The unit, when used with a commercial drencher and the 1.0 gm terminal filter, removed only 3% of P. expansum conidia. The 1.0 µm filter is nominally rated, and pore size varies considerably.
The cartridge filter system with the 0.45 µm terminal filter effectively removed spores in both laboratory and commercial tests, reducing the spore level by 92% to 99%. However, if this system is to become more useful for spore removal and decay control in packinghouse drenchers, dump tanks, and flumes, the capacity must be increased well beyond the current 10 gal./min. rate. In addition, terminal filters of 1.0 µm remove about 20% and 0.45 µm filters an even greater but unknown amount of thiabendazole, the most common fungicide used in drencher water for decay control, and fungicide concentration must be monitored and adjusted continually.
As with the sand filters, the cartridge filter units effectively increase the time that drencher suspensions can be used. Because drencher water may contain DPA, calcium chloride, and thiabendazole, reducing the amount of waste water generated (and which must be disposed of) is a positive benefit of filtration. During harvest, bottoms of bins often are covered with soil and debris.3 Filter life, and possibly filtration rate, may be increased by improved bin sanitation.
We thank the Winter Pear Control Committee for partial funding of this research. Use of trade names in this article does not imply endorsement of the products named or criticism of similar products not mentioned by Oregon State University.
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Factors affecting dispersal of Mucor piriformis in
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Robert A. Spotts and L. A. Cervantes
Mid-Columbia Agricultural Research and Extension Center, Oregon State University, Hood River
Tree Fruit Postharvest Journal 4(1):16-18