Fumigation of Fruit with Acetic Acid to Prevent Postharvest Decay

Acetic acid (AA) as a vapor at low concentrations was effective in preventing fruit decay by postharvest fungi. Fumigation with 2.7 or 5.4 mg AA/liter in air at 2 and 20C reduced germination of Botrytis cinerea Pers. and Penicillium expansum Link conidia to zero after they had been dried on 0.5-cm square pieces of dialysis tubing. Decay of ‘Golden Delicious’, ‘Red Delicious’, and ‘Spartan’ apples (Malus domestica Borkh.) inoculated with 20 μl drops of conidia of B. cinerea (1.0 × 10 conidia/ml) or P. expansum (1.0 × 10 conidia/ml) was prevented by fumigating with 2.0 and 2.7 mg AA/liter, respectively. Tomatoes (Lycopersicon esculentum Mill.), grapes (Vitis vinifera L.), and kiwifruit [Actinidia deliciosa (A. Chev.) C.F. Liang et R. Ferguson var. deliciosa] inoculated with B. cinerea or navel oranges (Citrus sinensis L.) inoculated with P. italicum Wehmer did not decay when fumigated with 2.0 mg AA/liter at 5C. AA fumigation at low temperatures (1 and 5C) with 2.0 or 4.0 mg AA/liter prevented decay of ‘Spartan’ and ‘Red Delicious’ apples and ‘Anjou’ pears (Pyrus communis L.) inoculated with B. cinerea and P. expansum, respectively. ‘Spartan’ apples immersed in a suspension of P. expansum conidia (1.4 × 105 conidia/ml) and fumigated with 2.7 mg AA/liter at 5C had an average of 0.7 lesions per fruit compared to 6.1 for nontreated fruit. Increasing the relative humidity from 17% to 98% increased the effectiveness of AA fumigation at 5 and 20C. At the concentrations used in our trials, AA had no apparent phytotoxic effects on the fruit. The potential for commercial fumigation with AA to control postharvest decay of fruit and vegetables appears promising. To our knowledge, accurate data are not available on the magnitude of economic losses resulting from diseases affecting susceptible fruit, vegetable, and root crops during storage and in transit to market. However, worldwide losses in the postharvest period range from 10% to 50%, particularly in developing countries that lack sophisticated postharvest storage facilities (Jeffries and Jeger, 1990). Based on relatively conservative estimates, the amount of food lost annually could feed between 200 and 300 million people per year (Kelman, 1989). For these reasons, it is essential that losses due to postharvest decay are reduced to the lowest possible levels. Fungicides are useful but can leave residues and have been associated with potential oncogenic risks (Wilson and Wisniewski, 1989; Wilson et al., 1991). Biological controls have been developed (Janisiewicz, 1988, 1991), and recently, two natural microbial agents were granted registration in the United States. However, their commercial success remains to be demonstrated. Acetic acid (AA) is a universal metabolic intermediary and occurs in plants and animals. It is present at ≥4% in vinegar (Busta and HORTSCIENCE, VOL. 30(6), OCTOBER 1995 Received for publication 15 Feb. 1995. Accepted for publication 18 June 1995. Sumerland Research Centre Contribution no. 836. We thank Paula Haag for technical assistance. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. Foegeding, 1983). The inhibitory effect of AA on microorganisms is greater than that due to pH alone, and undissociated AA can penetrate the microbial cell to exert its toxic effect Fig. 1. Effect of acetic acid fumigation in preventing d ‘Golden Delicious’ apples were inoculated with suspension containing 1.0 × 106 conidia/ml. Th inscribed with a black marking pen, allowed to diameter). The apple on the right was fumigated (Banwart, 1981). Our goal was to determine if AA as a vapor is toxic to fungal conidia and if it could be used to prevent postharvest decay of fruit, especially decay of apples caused by Botrytis cinerea and Penicillium expansum. Materials and Methods Fumigation procedure. Four 12.7-liter fumigation chambers were constructed in our laboratory from 11-kg-capacity tin cans for solid material (Fisher Scientific, Ottawa, Canada). A chamber circulation fan was devised by modifying a 0.6-amp, 115-V, beverage circulation pump (Little Giant Pump Co., Oklahoma City, Okla.). The pump shaft casing was removed, and the motor cooling and pump impellers were exchanged. The motor was attached to the top of the chamber lid with the shaft entering the chamber through a Swagelock (Columbia Valve and Fitting, North Vancouver, B.C., Canada) bulkhead fitting with a teflon ferule to provide an airtight fit. The lid was provided with a rubber septum for AA application and a microscope slide viewing port. A strip of filter paper was attached to the bottom of the lid underneath the septum and viewing port. When we conducted a fumigation, the lids were removed and samples were placed in the chamber. The lid was tightly applied and sealed with duct tape. The circulation fan was turned on and reagent-grade glacial AA was injected via a microsyringe into the chamber through the septum onto the filter paper wick. The AA completely evaporated in 1 to 4 min depending on the volume applied and the temperature. Air circulation was maintained for 30 min to evaporate the AA and 1271 ecay on fruit inoculated with Penicillium expansum. drops (20 μl) of P. expansum conidia in a water e conidia were placed in three circles on each fruit dry, treated, and injured with a glass rod (3 mm in with acetic acid at 2.0 mg•liter–1 for 60 min at 5C. POSTHARVEST BIOLOGY & TECHNOLOGY Table 2. Percent germination of Botrytis cinerea and Penicillium expansum conidia on dialysis tubing fumigated with acetic acid at 95% relative humidity and 20C. Acetic acid Germination (%) (mg•liter–1) B. cinerea P. expansum 0.0 100 ± 0z 100 ± 0 2.7 0.0 ± 0.0 ---y 4.0 ---y ---x 5.4 0.0 ± 0.0 0.0 ± 0.0 8.0 1.7 ± 2.3 0.0 ± 0.0 10.8 0.0 ± 0.0 0.0 ± 0.0 zPercentage ± SD. yNot tested. xGermination occurred at this concentration, but the percentage was not recorded. Table 3. Number of Botrytis cinerea and Penicillium expansum colonies that grew on dialysis tubing squares after fumigation with acetic acid at 4.0 mg•liter–1 for 10 to 80 min at 20C. Time Colonies/no. square (min) B. cinereaz P. expansumy 0 (control) 3/3 3/3 10 0/6 4/6 20 0/6 0/6 40 0/6 0/6 80 0/6 0/6 zDialysis tubing squares were each inoculated with a 20-μl drop containing 1.0 × 105 B. cinerea conidia/ ml. yDialysis tubing squares were each inoculated with a 20-μl drop containing 1.0 × 106 P. expansum conidia/ml. Table 4. Average lesion diameter on fruit inoculated with 20-μl drops of Botrytis cinerea (1.0 × 105 conidia/ml) and fumigated with acetic acid for 1 h at 5C.