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In vacuum fumigation, most of the air in the chamber is removed before the fumigant is introduced. It is, therefore, necessary to have a specially constructed chamber, usually made of steel, that is capable of withstanding external pressure up to one atmosphere. The installation also includes a pump able to evacuate the chamber in not more than 10 to 15 minutes, and valves and pipes for introduction and exhaustion of the fumigant. A vacuum fumigation installation is illustrated in Figure 37.
The primary object of vacuum fumigation is to hasten and improve the penetration of the fumigant into the material undergoing treatment. It was originally developed for this purpose when hydrogen cyanide (HCN) was the principal fumigant used. The advent of methyl bromide and phosphine, with their greater powers of penetration into many materials, made vacuum fumigation less important for the treatment of certain commodities.
Today, the technique is used chiefly in plant quarantine work (Richardson and Balock, 1959) and for fumigating tobacco (Tenhet, 1957) and other materials, such as compressed bales of jute bags (Monro and King, 1954) and pressed dates (Brown and Heuser, 1953a), which are difficult to penetrate at atmospheric pressure. It is also used in some food manufacturing industries for the fumigation of packaged cereals and prepared foods.
The fact that a vacuum treatment may be completed in 1.5 to 4 hours, as opposed to 12 to 24 hours for atmospheric fumigation, may commend it for use when a quick turnover of goods is necessary, as, for instance, at a busy seaport.
Vacuum fumigation cannot be used with certain tender plants, fruits and vegetables which are unable to withstand reduced pressure.
A vacuum fumigation installation is considerably more expensive than an atmospheric chamber of the same capacity. The decision as to whether the vacuum technique should be adopted should be made by weighing the possible advantages against the greater cost of installation.
The number of fumigants which may be safely and conveniently used in vacuum fumigation is strictly limited on account of various technical considerations. The materials principally employer) at present, in approximate order of importance, are:
1. Ethylene oxide/carbon dioxide mixture, which has a wide use in the food industry for treating processed and unprocessed foods. With considerably increased dosages it is also used for sterilizing food. It can also be used for sterilizing other materials, but this is outside the scope of the present manual; for general reviews of gaseous sterilization see Rauscher et al (1957); Bruch (1961).
2. Methyl bromide, as a general purpose fumigant in this field.
3. Hydrogen cyanide, formerly widely used but replaced largely by ethylene oxide and methyl bromide.
IMPORTANT. Under no conditions should phosphine, generated or dispensed in any manner, be used in vacuum fumigation. This compound is unstable at reduced pressures.
In recent years, considerable research has been undertaken on the methods of conducting vacuum fumigation for obtaining the best results with certain fumigants used on certain materials.
Basically, there are two main methods: sustained-vacuum fumigation and nearly complete restoration of the pressure, either simultaneously with the introduction of the fumigant or some time later (Page et al, 1953). The choice of one of these methods depends to some extent on the material being treated. Thus, fruit, vegetables and growing plants are usually completely ruined if they remain exposed for more than a few minutes to a pressure below 250 mm (10 in) of mercury. Seeds, grains, cereals and dry plant products are generally able to withstand these low pressures without ill effects.
SUSTAINED VACUUM FUMIGATION
The pressure in the loaded chamber is reduced to betwen 25 to 150 mm (1 to din) of mercury. The fumigant is then introduced, causing usually only a small rise in pressure. No further alteration is made to the pressure in the system until the end of the treatment which may last for 1.5 to 4 hours, at which time atmospheric pressure is restored by allowing air to enter. The fumigant/air mixture is then pumped out. The cycle of air introduction and evacuation may be repeated several times, A process referred to as "airwashing", until it is considered safe to open the door for unloading (Page et al, 1953). This method is used widely all over the world for treating tobacco, grains, flour and meals. Even in this type of vacuum treatment, gas distribution may not be entirely uniform; some chambers are equipped with recirculating systems and others with fans. Some living plants are able to tolerate sustained vacuum treatments at pressures in the region of 380 mm (15 in) of mercury. Such treatments may be useful in the control of borers or other insects inside stems or other plant tissue.
A substantial number of dormant, nonfoliated plants, roots and bulbs can be fumigated safely under a sustained vacuum of 100mm (4in) at an exposure period of 2 to 3.5 hours, as shown in Schedule F. In some cases, a higher vacuum can be used without injury.
ATMOSPHERIC PRESSURE RESTORED
Following the creation of the initial low pressure, atmospheric pressure may be restored in the chamber in several different ways, which may be summarized as follows:
1. Gradual restoration of atmospheric pressure. The required dosage of fumigant is discharged and air is then slowly introduced until a pressure just below atmospheric is reached after 2 hours in a 3-hour exposure period (Monro, 1958b).
2. Delayed restoration of atmospheric pressure. Following discharge of the fumigant, the vacuum is sustained for about 45 minutes and the air is introduced rapidly into the chamber (Brown and Heuser, 1953a, b, 1956; Monro and King, 1954).
3. Immediate restoration of atmospheric pressure. After the fumigant is discharged, atmospheric pressure is rapidly restored in the system by opening one or more valves leading into the chamber. This method, rather unscientifically described as the released vacuum or dissipated vacuum method, has been used extensively in North America for the fumigation of baled cotton (USDA, 1915).
4. Simultaneous introduction of air and fumigant. In this technique, special metering equipment is provided whereby the fumigant is introduced simultaneously with air so that 8 constant proportion of fumigant to air is maintained until the entire dosage has been introduced (Lepigre, 1949).
At the end of the treatment by any of these procedures, sir washing is carried out as described above for the sustained vacuum technique.
In a series of experiments using compressed bales of jute containing larvae of the cadelle, Tenebroides mauritanicus, it was found that the relative efficacy of these four methods, from the viewpoint of insect mortality, was in the same order as listed. In effectiveness, the sustained vacuum technique occupied an intermediate position, immediately below method 2. The results of these experiments are summarized diagrammatically in Figure 38 (Monro, 1958c).
It must be emphasized that, while the information given here may be useful as a guide for the investigation of vacuum fumigation techniques, each problem will have to be solved separately, due consideration being given to the commodity involved, the species of insects and their stages and the climatic factors in the country where the work is being done.
A detailed discussion of the structure and operation of available mechanical devices for vacuum fumigation is not given in this manual. Each manufacturer supplies the information pertaining to his product. Some information on the construction and performance standards of vacuum fumigation chambers has also been given in Sec. IV - Part 1 of Plant Protection and Quarantine Manual (USDA, 1976). In Countries where the method is used, the responsible scientific authorities would work out treatments they consider best for dealing with their own particular problems.
FIGURE 38 - Diagrammatic
comparison of mortalities of larvae of Tenebroides mauritanicus
(L.) in five methods of vacuum fumigation of compressed bales of
jute bags at 25°C. facie bale contained 300 bags and measured 50
x 50 x 60 cm.
A. Upper free space
B. On top of bale
C. Inside top bag
D. 12 cm from top surface
E. Centre of bale
F. 12 cm from lower surface
G. Underneath bale
H. Lower free space
Some suggested vacuum fumigation treatments are given in Schedule P. They are to serve mainly as a guide in the operation of small chambers of less than 25 m³ (1 000 ft³) capacity for plant quarantine purposes.
The fumigation of sacked grain was discussed together with that of stored products in Chapter 8, because the techniques for the treatment of all sacked materials are basically the same. On the other hand, the fumigation of grain stored in bulk presents special problems of gas distribution, and it is necessary to deal with this subject separately.*
Fumigants are used for disinfesting grain in most countries of the world. The chemicals used and the methods of application vary greatly. Differences in technique may be influenced by the nature of the crops and by the wide range of climatic conditions encountered. The effect of type and condition of grain on the efficacy of fumigation has been described by Harein (1959). One of the most important variables lies in the diversity of structures used for storage. The shape, size and type of construction of each particular structure create special problems in achieving and maintaining the concentrations required for the control of the insects and mites present in the grain.
The more important grain-infesting insects are cosmopolitan; they have been transported through international commerce to many parts of the world. A treatment effective in one country may therefore be successfully adapted in another if due allowance is made for the variables mentioned in the preceding paragraph. The relative amount of destruction caused by certain species varies somewhat from continent to continent or from country to country. Therefore, the most serious pests of grain may differ in their order of importance from one area of the world to another.
The dosages of the various fumigants as given in Schedules A and B are recommended as guides. Experience gained in a given country, or under specialized conditions, may indicate the need for modifications.
It was stressed in Chapter 2 that there are great variations in the susceptibility of insects to different fumigants, not only among species, but also between stages within a given species. Treatments given in this chapter should be adequate to deal with all insect species and their stages which feed and develop inside or outside the grain. Additional dosages may be needed for special conditions. All stages of the Khapra beetle, Troqoderma qranarium, other beetles of the genus Trogoderma, and the cadelle, Tenebroides mauritanicus, show exceptional resistance to ethylene oxide and to halogenated hydrocarbon fumigants, such as methyl bromide, ethylene dibromide and ethylene dichloride. When these insects are present in the grain, it is advisable to double the dosages given in Schedules A and B of fumigants containing these ingredients. On the other hand, these insects are quite susceptible to hydrogen cyanide and additional treatment is not needed with this fumigant.
* Contact insecticides are often applied to grain as it is run into storage. The use of these is often subject to rigid governmental regulations on account of the poisonous residue problem. There are wide variations in practice or in regulations in different countries. Formulations containing pyrethrins, malathion or other insecticides are permitted by some governments. A detailed discussion of these insecticides is outside the scope of this manual.
A number of different techniques are described here, some of which are peculiar to certain countries or districts. From this selection, suitable applications may be adapted or devised to solve local problems.
It should be pointed out that in some countries there are strict government regulations concerning the choice of materials that may be applied to grain for disinfestation purposes. The treatments discussed in this chapter have been practiced in some countries, but not necessarily in all. Before grain fumigation is undertaken, therefore, it is necessary to make sure that the use of the chosen material is not against the regulations of the country where the treatment is to be carried out or of any country to which the grain may be exported.
When a storage place is partly full, insects may be present not only in the bulk of the grain but also on the walls. If control is to be effected solely by fumigation, the dosage should be the same as for a full storage. As an alternative measure, contact insecticides may be applied to the walls either as a spray or as a smoke if appropriate means are used for preventing the contamination of the grain. If only the grain needs to be fumigated, the method of surface application described below may be used. If the moisture content of the grain is not high, the surface of the mass may be covered with a gas-proof sheet to prevent the vapours from diffusing upward into the empty space.
As already stated, the shape and size of a given storage unit are important considerations. Grain storage units are usually broadly classified for fumigation purposes as follows:
1. Upright (vertical) storage. In this type, the height is greater than the length or width. It is mainly found in the form of silo bins in storage units with elevators. In cross-section the bins may be almost any shape but are usually circular or rectangular.
2. Flat (horizontal) storage. One dimension, either length or width, is greater than the height. This type includes a wide variety of structures, including many temporary (sometimes called "distress") storage units. Railway freight cars (wagons) or motor vans (trucks or lorries) may be included under this heading.
3. Farm-type bins and storage units. These are usually small and often loosely constructed and their treatment requires special consideration.
In the following description of methods of grain fumigation, mention will be made of special applications suited to these three types of storage.
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