The Vapor-Heat Process

A. C. Baker.

The Mediterranean fruit fly invaded Florida in 1929 and spread rapidly across the big citrus region of the State. A campaign to wipe out the Medfly began immediately. It was the first campaign in history that was successful in eradicating a widespread insect pest.

But it was 19 months before the quarantine was lifted. Meanwhile a crop was maturing, which the country needed and on which the economy of Florida depended. A way had to be found to market the crop without risk to the rest of the country. Only embargo had been considered safe against this fly, and fruit from countries where it occurred was excluded.

What should be done? All possible information about the fly was gathered. The cold winters of the Northeastern States made those States seem safe for the shipment of fruit from protective zones the 9-mile zones surrounding the known infested zones. But the Southern States, from North Carolina and Tennessee westward, and the Pacific States, from Utah and Idaho westward, were looked upon differently. The occurrence of the fly there might mean disaster. Idaho was included as a barrier. It was decided to let no citrus from Florida enter any of those States. In order to open the markets there, a treatment had to be developed that would guarantee fruit free from any living stages of the fly.

Time was short. The new crop was hanging on the trees. To develop a process before the fruit would be ready to move seemed impossible. We had one hope. Larvae of the Mexican fruit fly, which also attacks citrus, had been killed when fruit infested with them had been heated to 110 F. or above.

We had the death points for those larvae. Larvae of the two flies were much alike. What would kill the one should kill the other.

But there was no known way to heat large loads of citrus fruit evenly. Three things we had to do : Devise a method by which carlot loads of oranges and grapefruit could be heated evenly and quickly; work out, with this method, the death points of the larvae; and develop a system that would provide a guarantee equal to that by embargo.

A year or two earlier we had bought for the Orlando, Fla., laboratory of the Department of Agriculture, for a scientific study having theoretical aspects, a large cabinet designed to provide constant temperatures and humidities. A heated mixture could be forced continuously the cabinet. We made the cabinet ready.

We loaded it with infested fruit. )riving through the fruit was a mixture of saturated vapor, air, and fine eater mist. Experiments used the mixture at 110 F., at higher and at lower temperatures. At 110 for 8 hours all larvae were killed. Our experiments continued. We needed to know what would happen to large populations of larvae, to find a source of unlimited lumbers of larvae, and to get a place where we could handle them freely without the possibility of jeopardizing the other phases of the work that were wiping out the flies and their larvae from Florida.

Hawaii was the ideal place. The Mediterranean fruit fly was abundant there in many kinds of fruit. Work could go on there without danger. A cabinet was shipped and the mortality phases were transferred to Hawaii.

Experiments had been under way at the laboratory to determine the response of different varieties of citrus to to treatment as at first laid out. The response was good: After the fruit was cooled, the experts could not distinguish treated fruit from untreated fruit. Many other kinds of fruit and vegetables that the fly was known to attack were included in the studies.

The crop was pressing us. No commercial equipment could handle carlot loads on the basis of our laboratory specifications. Engineers with commercial concerns were approached, but in vain. We tried unsuccessfully to treat carlot loads of storage fruit in various ways. The problem was to find a good way to obtain saturated vapor, which on condensing would release latent heat, a kind of hidden heat.

We were able, however, to sterilize well two cars of grapefruit. They were shipped to the New York auction. One sold at premium prices. Sterilized fruit was acceptable to the trade. But to handle the volume we needed standard equipment that would provide saturated vapor uniformly.

Arthur B. Hale, a commercial engineer, standing by and watching a laboratory run, found the solution. In a short time he had mixed water sprays and steam sprays in a metal mixing box. A thermostat to a valve on the steam line controlled the temperature. A multivane blower pulled the mixture across baffles to remove the drops of water. It forced the mixture of air, saturated vapor, and fine mist downward across a. spreader in the room and through the load of fruit stacked in the field boxes. A false slatted floor, on guiding studding, directed the spent mixture to a return duct and the mixing box. A drain in the floor carried off the condensate. Standard commercial equipment had been born.

Within a month of our test cars on the auction, sterilized fruit was rolling to market. In November authorization was given to ship to the Southern and Pacific States.

The Texas citrus industry was young in 1927 when the Mexican fruit fly appeared. Soon infestations were occurring each year late in the season. The marketing volume was small, however, and the crop could be moved early enough to avoid serious loss from infestation. Large numbers of the fly moved northward each year to Texas from large wild reservoirs in northeastern Mexico, but fruit of each season's crop was safe to ship throughout the country before the migrations came in.

Because the fly had become a problem to two countries, Mexico and the United States in 1928 joined in a study of it. A cooperative laboratory was established. Mexico provided the buildings and grounds. The United States provided the personnel and equipment. By 1929 the laboratory had developed temperature death points for the larvae of the Mexican fruit fly; information that later proved to be so useful in the work in Florida. D. L. Crawford, working in Mexico as early as 1914, had proposed that citrus be heated, but there was no known way adequately to heat it. The laboratory also had perfected traps and lures for the flies to record the incoming migrations in Texas.

Industry needed a longer marketing season and wider markets. That meant the shipment of fruit while flies were present. The laboratory in Mexico worked out the specifications against the Mexican fruit fly and sterilization was introduced with the crop of 1937-38. It made the late fruit safe. During the years the process has been improved. Time was reduced to a 4-hour holding period at 110 F. and a 6-hour approach to raise the temperature of the load. In 1948 experiments got under way on a new scheme by which the holding period would be eliminated. Fruit has been sterilized by running it up quickly to higher temperatures, then stopping. It has responded well.

The sterilization rooms also have been improved. A ventilating stack was devised through which the air and mist are driven off immediately after sterilization. Automatic temperature recorders now plot on charts the temperatures within the fruit and those of the incoming vapor mixture.

Sterilization in Texas was necessary to expansion. By 1950 half a million tons had been sterilized.

Mexican fruits subject to attack by fruit flies were long excluded from United States markets. Many still are. But with infestation of grapefruit in Texas by one Mexican species and the application of the vapor-heat process there, is was logical to admit Mexican citrus, subject to attack by the same insect, when sterilized by the same process under American supervision and guarantee. In September 1945, therefore, Mexican oranges, grapefruit, and Manila mangoes were authorized to enter the United States under those conditions. Packing plants were built for the Canadian and European trade and sterilization rooms were set up and equipped. Mexican oranges, sterilized by the vapor-heat process, began reaching American markets in 1950.

Hawaiian fruits and vegetables had been excluded for years from mainland markets because of the presence in the Islands of the Mediterranean fruit fly and the melon fly. The major agricultural industries of Hawaii, sugar and pineapples, are processing industries that the flies did not involve. But it was believed there that the opening of mainland markets would stimulate off-season production of fresh vegetables, would foster increased production of certain fruits, and ultimately would result in a profitable export trade. The specifications for Florida against the Mediterranean fruit fly were retested; specifications against the melon fly were developed; and in 1938 mainland markets were opened to Hawaiian fruits and vegetables formerly embargoed. The volume moved after sterilization, however, has been small.

In 1946 the serious oriental fruit fly invaded the Islands. It attacked almost everything, even fruits that the other two flies had left alone. Embargoes were placed on all fruits it infested. The new fly proved to be harder to kill than the other two. New specifications for the process had to be worked out, and because the treatment was more severe, tolerances had to be studied for everything involved. But by 1950 mainland markets were open to sterilized papayas, bell pepper, Italian squash, tomatoes, and fresh pineapples.

The primary aim of the vapor-heat process was to sterilize citrus fruits against fruit flies. The process, though, has been tried experimentally on other things infested by other insects. It has been tested on house plants infested by scale insects, and the results have been promising. Narcissus bulbs infested by mites and the larvae of bulb flies were heated to 111 F. for 2 hours, and the pests were eliminated. The process also increased the growth of the bulbs. Gerbera was cleaned of cyclamen mites by a 30-minute treatment. The process eliminated mites on strawberry plants. Blossoming was a little delayed but growth was excellent and fruit production was heavy and continuous. Experiments with nursery stock looked promising, but difficulty was experienced with the balls of earth in which the roots were held. Since the process influences dormancy it brought astilbe into bloom for earlier marketing.

The vapor-heat process has an unusual feature. When citrus fruit is sterilized, any that is thorn-pricked, bruised, lightly infested, or similarly injured turns brown over the injured spot and is easily culled when the fruit later runs over the graders. Much fruit, so injured, is not detectable when not sterilized. When shipped it breaks down in transit before reaching the market. Sterilization, therefore, reduces transportation losses.

A. C. BAKER is engaged in cooperative research in Mexico for the Bureau of Entomology and Plant Quarantine. The development of the vapor-heat process under his leadership in Florida in 1929 made possible the successful Plan for eradicating the Mediterranean fruit fly there while marketing the citrus crop that it attacked. Men working with him also established the specifications for that process for citrus in Texas, for fruits and vegetables in Hawaii, and for oranges, grapefruit, and Manila mangoes in Mexico.