Using Chemicals To Stop Spoilage

J. R. Winston, H. B. Johnson, E. M. Harvey.

Chemical treatments are applied after harvest to reduce the decay of some kinds of fruits and vegetables during storage, transit, and marketing.

The treatments are directed against fungi and bacteria that contaminate the produce during harvesting, grading, and packing and against weakly parasitic organisms that do not become active until after harvest.

Suitable treatments for harvested fruits and vegetables must quickly kill or retard the development of the decay fungi. They must not injure the fruit or vegetable. They must not leave an objectionable residue or odor or affect the composition or flavor. They must not be poisonous to the consumer, and they must be economical and practicable.

The treatments may be applied by dipping in a wash tank; spraying; impregnating the wrappers, crate liners, or cartons; and fumigating in a special room or in the car or truck.

The usual method is to pass the commodity through a bath. The treating tank may be on the receiving platform of the packing house or between the washer and drier. Sometimes it has heating facilities to keep relatively insoluble compounds in solution. Sometimes the commodity is rinsed immediately upon leaving the treating tank to remove the adhering chemical.

The location of the tank in the processing line varies, depending on the mode of action of the chemicals. The chemical may be a nonvolatile, Slow. acting compound, such as the soluble borates; a slowly volatile compound such as soluble phenolic compound, or a readily volatile compound, such as chlorine.

The soluble borates borax, boric acid, a mixture of the two, and the more soluble sodium metaborate, generally are applied in 5-percent solutions. All have been widely used on oranges, grapefruit, and tangerines. The compounds are effective against green mold if the application is made before the infections have become deep-seated. The borates are also used to check stem-end rot in Florida citrus, but their effectiveness diminishes as the fruit ripens.

The borates are slow-acting. Treatment is best made soon after the fruit reaches the packing house. But usually the fruit is not treated until near the end of the packing house process, when it passes through a tank located between the washer and drier. After treatment (but before the fruit enters the drier), it is rinsed with water to remove excess boron compounds, which tend to gum up the machinery and leave an unsightly residue on the fruit. This short treatment, followed by rinsing, does not permit the toxicant to remain on the fruit for more than a minute or two. For best results, it should be left on the fruit for several hours. Since borax-treated fruit is especially apt to wilt, it is waxed after treatment.

A borax wash has also been used to a limited extent to control black rot on sweetpotatoes and decay in cantaloups. In California, sodium carbonate or soda ash, 1.25 to 2 percent, is preferred to borax for use on lemons.

Sodium ortho-phenylphenate has advantages over borax for the treatment of citrus fruits in Florida. It acts quickly and can check decay in citrus even when the application is delayed until several days after harvest, as is likely to be the case when citrus is degreened with ethylene before being packed. Without the aid of a safener, however, it is apt to cause rind injury at normal room temperatures even when it is rinsed off immediately. Even with a safener it may cause rind injury if the temperature of the treating bath is snore than lukewarm. Formaldehyde added to a water solution of the phenate or to a water-wax emulsion containing that chemical lessens the injury, but because formaldehyde evaporates quickly its safening effect is only temporary. Hexamine is a formaldehyde-ammonia reaction product with a faint odor of formaldehyde. Added to the phenate solution it practically eliminates rind injury if the temperature of the treating bath is not too high.

The sodium ortho-phenylphenate treatment is commonly applied by dipping the fruit either momentarily or for about 2 minutes in a tank containing a 1 to 1.25 percent concentration of the chemical. It is followed by a fresh-water rinse. The chemical also may be dissolved in the water phase of the wax emulsion which serves as a moderately effective safener, and applied as a flood spray. No rinse is used when the sodium ortho-phenylphenate is applied with the wax.

Sodium chloro-2-phenylphenate, a related compound, has been used to control several kinds of decay on apples and pears in the Northwest. It may be used in the washing tanks at a concentration of 0.6 percent, followed by a fresh-water rinse. Or it may be used in the rinse water at a concentration of 0.4 percent. Washing or rinsing the fruit with the chemical reduces decay, but the treatment is not entirely satisfactory; the fumes irritate the eyes, nose, and throat and contact with the bare skin may cause dermatitis.

Chlorine washes are extensively used for fruits and vegetables. We have little information on how much they reduce decay. Chlorine probably prevents the build-up of decay organisms in wash water and cooling water and so reduces the chance of infection particularly in packing sheds where dip or soaking tanks are used or where the wash water is used more than once.

Several chlorine compounds are well adapted for treating water used in the packing operations. Sodium hypochlorite is used at concentrations of 3,000 parts per million of available chlorine for tomatoes and 6,000 p. p. in. for citrus. The produce is immersed in the solution for at least 2 minutes. A new sodium hypochlorite treatment for potatoes uses a solution buffered to an acidity of 5.5 to 6.0 pH and a concentration of 100 p. p. m. of chlorine. The immersion period is only 15 seconds. Sodium hypochlorite is also used in conjunction with refrigerated water in flood-type machines for precooling bulk or packed produce. The concentration of chlorine then is 70 to 100 p. p. m. The precooling time averages about 20 to 30 minutes. This combination of chemical treatment with effective precooling is used for produce which requires rapid cooling before loading such as asparagus, celery, corn, peaches, and sweet cherries.

Chlorinated amines are sometimes used at concentrations of 40 to 300 p. p. m. and a reaction of pH 4.0 to 6.5 in wash tanks, rinse water, and cooling tanks. Solutions of this form of chlorine are more stable than sodium hypochlorite solution, and the chlorine is not lost through oxidation of organic matter. The treatment has been used successfully on many kinds of vegetables. It cannot be used on lettuce. It has been used on all kinds of citrus fruits in Texas.

J. R. WINSTON is in charge of investigations of handling, transportation, storage, and market diseases of citrus fruits and vegetables at the Horticultural Field Laboratory at Orlando, Fla.

E. M. HARVEY is in charge of handling, transportation, and storage investigations of citrus and subtropical fruits at the Horticultural Field Laboratory, Pomona, Calif.

H. B. JOHNSON is in charge of handling, transportation, and market disease investigations at the Horticultural Field Laboratory at Harlingen, Tex.