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J. SMILANICK¹, L. Margosan¹, D. A. Mlikota¹, T. C. Yuan², and B. C. Hampson³. (1) USDA-ARS, Fresno, CA 93727, (2) Air Liquide, 5230 S. East Ave, Countryside, IL 60525, (3) Cal ifornia Polytechnic State University, Food Science & Nutrition Department, San Luis Obispo, CA 93407

Water used in processing produce can carry spores of pathogens that reduce shelf life. Ozone presumably can sanitize process water, but knowledge of the doses required is needed.

Our objectives were to determine O₃ doses that kill spores and the influence of pH and O₃ concentration on O₃ potency.

O₃ was made with a corona discharge unit and dissolved in water in a contactor attached to a 1,000-L tank. O₃ concentration was monitored with an O₃ electrode and did not change during tests. Ozonated water pH was 6.4 in spore mortality tests, except when adjusted to pH 5 or 8 with H₂SO₄ or KOH in water buffered with 2 mM a₂HPO₄. Spores of Penicillium digitatum, Penicillium italicum, Penicillium expansum, Monilinia fructicola, Rhizopus stolonifier, Botrytis cinerea, and Geotrichum citri-aurantii from 2-wk-old colonies on potato dextrose agar (PDA) were collected in 2 mL of water, centrifuged, and resuspended in water at 10⁶ spores/mL. A 0.2-mL aliquot was placed on a 3-µ-pore-size filter and low-pressure vacuum dried, then 1-4 µg of O³/mL was flowed through the spores at 1.6 mL/sec. To end exposure, 3 mL of 1,000-µg/mL of calcium thiosulfate was added, followed by 5 mL of water. The filter was dried, removed, inverted, and touched to PDA, where the spores were deposited. Germination was recorded after 14-24 hr at 20°C.

Two min in 1.5 µg O₃/mL killed 95-100% of spores. None survived 3 min of contact, and this dosage, a CT product of 4.5 µg O₃/ml x min., could be used to treat packinghouse process water to control these fungi. Spores died similarly at pH 5 and 8. LD₅₀ times of P. digitatum spores in 1, 2, and 4 µg of O₃/mL were 21.6, 10.4, and 6.4 sec, respectively. LD₅₀ CTs were 0.36, 0.35, and 0.42 µg/mL x min, respectively.