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3.2 The application of modified atmospheres based on the external addition of carbon dioxide
The processes of either lack of oxygen under nitrogen CAST or the toxic action of high concentrations of carbon dioxide in air can kill insect pests and rodents in storages and control the growth of moulds in wheat with moisture contents between 1216% mc (Banks, 1979). The toxic action of CO2 rich atmospheres is independent of CO2 concentration above 60% in air, while it declines between 60-35%, and some species are capable of surviving or are tolerant to CO2 levels immediately below 35%. Reproduction rates are however much reduced even at concentrations approaching 10% CO2 in air.
The application technique and introduction of CO2 into the structure is similar as for liquid nitrogen but after the initial purge, it has been demonstrated that there is a need for gas recirculation in order to avoid regions of inadequate CO2 concentration in the upper parts of the storage (Wilson, et al., 1980) (See Fig. 1.).
Recirculation is required in tall structures because CO2 is 1.5 times as dense as air and there is a natural downwards movement of CO2 which reduces the concentration in the headspace. Blowers rated at 3 m3.min-1 are suitable for this purpose. Recirculation should continue for 10 days (ensures headspace concentration does not fall below 40%) or preferably 14 days (CO2 maintained above 35%).
One advantage with CO2 atmospheres as compared to low oxygen in nitrogen atmospheres, is that CO2 may be applied in a "One-shot" operation in welded steel bins after gastightness specifications have been adequately fulfilled. No further gas needs to be maintained while the target regime for insecticidal activity can still be achieved (Banks, 1979; Wilson, et al, 1980).
For modified control atmospheres to have any insecticidal effect, it is necessary that certain levels of gastightness must be achieved. The specification corresponds to a hole area of about 1 cm² in the structure which maybe composed of a single leak or a number of small leaks giving an equivalent combined effect. Both wind and thermal expansion can cause extensive gas leakage during both controlled atmosphere storage and during fumigation. Wind is more important in leaky situations, while problems with thermal expansion of the headspace (Temperature effects within the bulk can be considered negligible) can occur in both poorly and well sealed silos (see Section on gastightness requirements). Retarding solar radiation by insulation or painting the external surfaces with a high reflectance paint (a matt white acrylic paint) reduces the amount of gas loss, and therefore the amount of maintenance gas that needs to be added to compensate for this loss. Moisture migration and condensation problems on the grain surface due to convection currents within the bulk are also reduced.
Controlled atmosphere storage in Australia has favoured CO2 in preference to nitrogen. This is attributed to nitrogen's low boiling point, where the onsite equipment and power supply was unable to cope with the rapid vapourization process and purge times were consequently lengthy. Even silos that attained the levels of gastightness reguired, need a continuous bleed of maintenance gas. It was difficult to fine-tune the system so that the correct bleed rate was reached without releasing the safety valves under excessive pressure and consequently losing nitrogen to the atmosphere. Alternatively, a situation could be created where the bleed rate was insufficient to compensate for the natural decay rate, thus allowing oxygen levels to rise to a point where insect survival was imminent.
From 'Alteration of storage atmospheres for the control of pests' by H.J. Banks in Grain Storage Research and its Application in Australia, CSIRO Division of Entomology ADAB Research for Development Seminar, February, 1979.
NOTE: DIMENSION IN MM
SOURCE: Wilson, A.D., Buchanan, S.A., and Sharpe, A. G. (1983), "Use and recirculation details of CO2 atmospheres" in the Int. Symp. on pract. Aspects of CA and Fum. in Grain Storages, April, 1983, Perth, Australia.
Practical aspects to CO2 application to vertical bins:
The welded steel Ascom Type 1900 tonne capacity bins are typical of those used by Bulk Handling Authorities in Australia. These are 18.2 m high (14.8 m height to eaves, 3.4 m height of cone), 13.9 m in diameter, with a total internal volume of 2418 m³. Critical areas for sealing are:
Details of purging/recirculation are found in Fig. 2. Problems of moisture migration has been discovered in some bins that had an initial grain moisture content greater than 12%, resulting in some mould growth and 'bin scalding'. To overcome this, overfilling the bins must be avoided ( 1 m below roof apex). Grain turning should be performed before the CO2 purge to redistribute any hotspots.
4. PRACTICAL ASPECTS OF CO2 APPLICATION IN HORIZONTAL STORAGES
Most conventional horizontal storages or warehouses have not been designed for the application of the CAST technique nor the efficient use of conventional grain fumigants. Some engineering modifications, as described by Ellis (1983), need to be incorporated in bulk handling facilities to accommodate this form of pest control. These modifications include;
1. Change Ventilator Design to Allow for Sealing. Existing storages normally ventilated through a number of convection type roof ventilators require the design to be modified to allow for the addition of a sealing support plate, sprayed with a sealing material. These may be removed at a later date if required.
2. Change Entry into Storage from C1 Conveyor. The overhead belt conveyor from the elevator generally through the wall into a penthouse from which the grain flow is either split or distributed on to overhead conveyors. Therefore the conveyor is elevated above the roof line and spouting (closed and sealed by a manually operated slide plate) passes through the roof to the distributing belts. Access by personnel is through a lockable, sealable man-hatch.
3. Eliminate Bird Netting by Using Sealing Plates. Ventilation has sometimes been achieved under the eaves, with nylon netting to prevent the entry of birds. This is replaced with inclined galvanised sheeting which is sealed when required.
4. Modified Sealing of Main Doors. The main doors of the storage are used for access of front end loaders during outloading. A sealing membrane is required on the hinge line. The doors are also fitted with a 'last man out hatch' fisted with rubber seal flanges so that sealing can be fully carried out from the inside, after which the operator crawls through the manhole hatch and bolts it into position.
5. Install D.l.P. Lighting. Sufficient lighting with natural light normally enters through the eaves, doors, ventilators and transluscent sheeting. In sealed storages it is necessary to install artificial D.l.P. (Dust Ignition Proof) lighting along the conveyor gallery (Light Specification:- D.l.P. HAZLUX No. 3.400 WHigh Pressure Sodium DS25B).
6. Add Girts to Allow Installation of Fans. Air convection within is a sealed storage minimised, and the use of reversible fans in each of the gable ends of the storage disperses respirable dust held in suspension.
7. Add Purlins for Strength on Roof Sheeting.
8. Eliminate any Transluscent Sheeting. The heat input is found to be high with transluscent sheeting in existing storages. When the storage is sealed, these are sprayed over with a clear sealing material, or in new storages, simply eliminated.
9. Colourbond Roofing and Access. White 'colour bond' roofing material can be selected, to reduce solar radiation. Air movement due to the daily temperature changes is therefore considerably less.
It must be emphasized that storages which are capable of controlled atmosphere disinfestation (either by modifying existing horizontal storages, or incorporating the required design criteria in new facilities) are less than desirable unless all other functions of a modern storage can be performed without breaking the seal. O'Neil (1983) has described the low cost (less than 50% of the cost per tonne of more conventional vertical storages) "total concept" 60,000 tonne capacity horizontal storage situated at Moura, Queensland, Australia. The storage has been built at a cost of $ Aus. 3 million i.e. $ Aus. 50. Tonne-1 storage capacity.
The provision of integrating the functions of fumigation unloading/outloading and aeration as a simultaneous operation is the first of its kind in horizontal storages.
The containment of modified atmospheres, such as nitrogen or carbon dioxide, appears easier than with fumigants such as phosphine, where some problems in theplacement and retreival of fumigant strips, pellets or other phosphine generating formulations is anticipated.
Although not fully commissioned, it is envisaged that by the end of 1983 most of the following difficulties will be overcome:
CO2 Application in Transit
The control of insects in containerized transport using methods involving CO2 disinfestation have been demonstrated by Banks and Sharpe (1979). Nineteen tonnes of bagged wheat loaded in a Graaf container (total internal volume of 21.2 m³) which had a leak rate of 6 x 10-3m3.sec-'of air (at 250 Pa excess pressure) was dosed with 30 kg of dry ice directly onto the wheat stack, and about 31 kg of additional dry ice was kept in an insulating box which provided a release rate of 3 kg of gaseous CO2 per day.
The concentration of CO2 after 240.7 hrs was approximately 49% measured manually by a Riken 18 interferometer and remained between 42-52% at all sampling points over a period of 10 days. The load initially had a light infestation of R. dominica and C. pusillus, and hidden infestations of C. pusilius, E. cautella, J. castanceum, but mainly R. dominica. Test cages containing multiple resistant S. oryzae were also added to this natural infestation, none of which survived the treatment after 252 hours (10.5 days) exposure. It was viewed that the insulating box of dry ice may not be required, if headspace temperature fluctuations were minimized by stowing the container below deck.
5. AIR-TIGHT OR HERMETIC SYSTEMS
The principles that apply to the control of insects in dry grain also hold for high moisture grain (i.e. grain of a moisture content over about 15%), but here it is the micro-organisms, mostly fungi, that create the oxygen-free conditions. Most moulds need oxygen for their growth, and die, or at least become inactive, in its absence. Once oxygen-free conditions have been established, however, certain other micro-organisms, known as anaerobes, can grow. These are mainly yeasts and bacteria. The latter need a very high humidity for their growth, and flurish best at grain moisture contents over 22%. Their respiration, as described earlier, involves incomplete breakdown of the carbohydrates of the grain, and results in the production of alcohol and other volatile substances. These impart a sour-sweet or "beery" taint to the grain, which is not completely removed by subsequent airing or drying. The grain is therefore normally unacceptable for human consumption, although suitable for animal feed. At high moisture contents (over 22%) or with prolonged storage, the gluten of the grain is affected, so it cannot be used for breadmaking. The germination capacity is also reduced, rendering the grain unsuitable for seed, or for malting.
Successful hermetic storage requires the creation and maintenance of low-oxygen tensions in which most moulds are unable to grow even in equilibrium relative humidities above 70% ERH (approximately 14% me). This would enable growth in open storage systems. Many storage fungi are microaerophilic but their subsequent growth is much impaired in low oxygen tensions (which is the limiting factor), while the increase in carbon dioxide concentrations may also have a slight additive effect.
If oxygen entry is restricted to a minimum, and the internal oxygen concentration remains somewhere between 0.5 -1.0% O2 (most readily obtainable O2 levels in commercial airtight stores rather than 2.0% O2) then the grain remains bright and free from visible moulding.
The field fungi are eliminated at moistures above 20%, correlated with the depletion of O2 and accumulation of CO2, while the numbers of bacteria and yeasts usually increase, especially the yeasts of the genera Hansenula and Candida at O2/CO2 levels of 1 - 2% and 15 - 40%. At higher O2 levels found in partially empty silos, mesophilic fungi such as species of Penicillium and Aspergillus, and mesophilic fungi with higher temperature optima and true thermophils such as Mucor may be found associated with developing hotspots. Other thermophilic organisms such as actinomycetes can also be present causing bridging and spoilage of the grain.
Several of these organisms are capable of causing infections, inducing allergic responses or respiratory irritation from people handling mouldy grain, and mycotoxicoses in both man and domestic animals consuming infected or mouldy grain.
Because of the more stringent requirements for gastightness with high moisture grain, many of the structures suitable for airtight storage of dry grain are therefore unsuitable. Specially constructed metal silos are sometimes galvanized or treated with epoxy-resins, or more commonly coated in a dark blue or green vitreous enamel finish to protect the steel from corrosion manifested by silage acids (lactic, acetic, propionic). To cope with pressure changes, they are fitted with pressure release valves (a few centimeters of water) in the top, while the American vitreousenamelled or "glass-lined" silos are provide with a breater bag in the beadspace to compensate for temperature induced internal pressure changes with respect to atmospheric pressure, which prevents free air interchange below the pressure setting of the safety valve.
Smaller flexible bag silos made of butyl rubber or PVC supported in metal cages of approximately 20-40 tonnes, or smaller heavy gauge polythene sacks (50 kg) are also widely used. These are easily handled and transportable, but are also easily damaged by rodents, birds, and machinery.
The Waller bin developed from European grain storage practices was simply modified by replacing the hessian or paper liner with plastic or rubber bags. The sealing becomes increasingly difficult as the diameter increases. A desirable safety feature is the inclusion of a safety door which facilitates cleaning and ground level ventillation, to reduce CO2 levels to normal atmospheric conditions. The liner cannot be hung on the mesh cylinder, so the liner is inflated by using a fan (rated at 140 m³.min-1 at 100 mm W.G. which is equivalent to 5000 c.f. m. at 4 in. W.G.) during loading. When filling operations have ceased, the silo is simply allowed to collapse onto the grain surface. This also occurs during emptying, thus reducing the headspace volume.
With undamaged liners, complete disinfestation can be rapidly achieved with an initial high population density. insect penetration of the butyl rubber sheeting has also been reported, but O'Dowd (1971) found Callosobruchus maculatus capable of penetrating polythene but not butyl rubber. Butyl rubber has limited ability to withstand U.V. radiation and high ozone concentrations, while the natural black colour and pigmented polythene is responsible for extreme temperature fluctuation in an unshaded silo resulting in moisture migration to the outer grain layers, particularly on walls facing east/west. Shading by providing a roof is necessary, since painting the walls white did not reduce the internal temperatures to acceptable levels.
Observations were carried out over a 10-month period in a flexible silo consisting of a butylrubber/EPDM welded liner inside a circular supporting weldmesh wall designed to provide an airtight seal with a capacity of 900 tonnes of wheat by Navarro (1976). Development of insect populations which become evident two months after the beginning of storage was arrested due to the airtightness, and gas concentrations unfavorable to further insect development were maintained during three stoage seasons. The insect population remained low until the end of the storage period and in the order of decreasing abundance, were Cryptolestes sp., Oryzaephilus surinamensis L., Tribolium castaneum (Herbs"), Rhyzopertha dominica F., and Sitophilus oryzae L. Grain was preserved at a satisfactory guality throughout the storage seasons. Insect accumulation occurred at the silo apex and was accentuated by migration from within the bulk. This was attributed to interrelated effects of oxygen tension within this region which permitted survival through O2 diffusion; moisture accumulation in this region attracting grain insects and providing a most favourable environment; and temperature gradients where temperatures within the centre of the bulk varied between 33-40°C, a condition which is less favourable for those insects listed and therefore promoted their outwards and upwards movement. It was concluded from these tests, that moisture accumulation at the silo apex and damage caused to the sheeting will ultimately result in some deterioration.
Both of these systems, ranging in sophistication and capacity, are suitable however to either dry grain or high moisture air-tight storage, depending on the final grain requirement (animal feed or human consumption).
Various systems for dry drain grain airtight storage hve been documented. These include traditional methods such as dried fruit cases of cucurbits, commonly known as gourds, used in the tropics for storage of small quantities of grain often intended for seed purposes, but are not sufficiently gaslight unless a surface treatment is applied and attention is given to adequately seal the stopper.
Metal bins display great potential for use as airtight containers, provided that shading or the application of external reflectant treatments offset or minimize temperature induced moisture migration by diurnal fluctuations.
The semi-underground bins developed by J. E. Waller in Cyprus and Kenya have been used for longterm storage and some were initially sealed for over 3 years with excellent results. Moisture migration was also a problem, but moulding was reduced by either placing trays on top of the grain surface below the mancover, or lining the metal covers with polystyrene sheets to prevent translocated water vapour from coming into contact with the cold metal.
Maize has been stored for three years at 12% mc and wheat for 18-24 months at 13% mc with low levels of deterioration mainly related to rise in FFA's (wheat 20-30 to about 40, maize 15-20 to about 50 based on mg KOH per Kg of grain) and loss in seed viability. After 3 years, moisture was within a 60-75% ERH range.
The airtight underground storages (Aus) in Argentina have been used for 2-3 years with loss amounting to 0.5%. Many of the 2.5 million capacity Aus storages were not built gaslight (because of the change to permanent vaulted roofs made of hollow blocks covered with reinforced concrete, rater than flexible roofs that allowed some movement) but gave better storage conditions than above ground silos through superior thermal insulation. Repeated fumigation was however still necessary to prevent insect infestation.
Grain loaded at 12% mc and at 20°C, resulted in altered atmospheres of 3-4% O2 and 10% CO2. If loaded with grain at higher temperatures, moisture migration became a problem, and construction of Aus systems have been limited to wheat producing zones where grain is harvested with a low moisture content.
The Grain Elevators Board of N.S.W., Australia, has developed a system of bunker-type storage which was originally intended as temporary storage facilities to offset lack of storage capacity during bumper harvests. It has evolved from an original C.S.I.R.O. concent of underground storage which has been practiced since time immemorial,but has since been greatly modified. Further trials by CSIRO involved the provision of an above-ground bunker storage covered by earth or sand. This involves building earthen banks and lining the base with PVC sheeting to prevent moisture migration and seepage. During the 1979/80 harvest, about 1/4 million tonnes of wheat was stored in Victoria, Australia in this manner. Bunker walls measure 13 meters at the base, rise 2.5 meters and are 6 meters wide at the top. Each bunker is about 30 meters wide and built to a length which determines its capacity. When full another earth wall was built to enclose the bunker and the surface of the wheat was covered with another polythene sheet and about 1/2 meter of earth.
Excellent results have been obtained with phosphine fumigation equivalent to a dosage rate of approximately 0.2 tablets/tonne (ca one pellet). This is due to the high level of sealing obtained combined with prolonged exposure periods.
The earth covering presented appreciable operational problems, and with the availability of high grade PVC sheeting, the earth covering has since been omitted.
One major disadvantage with this system of storage is the disruption to inloading and outloading operations caused by rain. Concrete banks (or the movable 'A' frame type of temporary storage of wooden supports covered with G.l. sheet as in Western Australia) have now replaced earthen banks which are water-proofed by a bitumen surface. A special machine has been developed which is capable of inloading at 250 tonnes. fur-' and outloading at 200 tonnes.hr-1 or better.
Problems that are associated with bunker storage, although essentially overcome, are,
Problems (1) and (2) are associated with earthen banks, while the provision of concrete banks and butimenising the storage surface completely eliminates these. Problems of hail damage still remain, but (4) has been obviated by sewing rather than using a sealant, but a suitable sealant over the stitching must be provided to ensure gas tightness. Item (5) has been minimized by the "Lobstar" machine, which can be operated in moderate winds, depending on wind direction, Item (6) is removed by supplying "man-proof" fencing.
PVC bunker-type storage of 50,000 tonne capacity costs approximately $6.00 per tonne to utilize, if used annually, but if used as an emergency storage 3 years out of 10, the costs increase to $13.20 per tonne. The equivalent costs of steel bins of 10,000 tonne capacity is $11.65 and $38.37 respectively (Druce, 1982).
Yates and Sticka (1983) claim that bunker storage has now reached a stage where
"It is quite a satisfactory form of permanent storage where only minimal capital requirements are warranted, where capital is simply not available for the construction of storages, and where a storage facility must be provided at very short notice.
The PVC-covered bulk grain storage system is shown to be capable of directly responding to the two major factors:
These storages are now routinely being constructed to 50,000 tonne capacity. A major advantage of the PVC bunkertype storage is that it can be fumigated very satisfactorily by the use of phosphine. This is a very cheap form of fumigation and experience to date indicates that it is completely effective at very low rates of application as the storages are sufficiently gas-tight."
6. SAFETY ASPECTS OF CAST
Although nitrogen is non-toxic, atmospheres containing less than 14% O2 or atmospheres containing more than 5% CO2 maybe dangerous for human life. The rapidity of onset of symptoms may vary with individuals and even at 6.5% O2, may take a few minutes. Death will ensue if an unconscious subject remains in such an atmosphere, but if removed quickly and given air, recovery is complete. There are no reliable warning symptoms concerning nitrogen asphyxia. Carbon dioxide enriched atmospheres strongly stimulate breathing, and becomes uncomfortable in atmospheres containing 3% CO2. Death occurs in 20-30 minutes on exposure to CO2, but if an affected subject is removed to fresh air even after a brief exposure to very high CO2 concentrations (e.g.70%) no irreversible physiological damage results.
No space under controlled atmosphere should be entered without protective respiration devices, and then only in emergencios. Fully self contained or compressed air breating apparatus should be permanently on hand. Gas masks provide no protection against low O2 or high CO2 atmospheres. A storage that has been under CAST should be tested for oxygen and where relevant, CO2 levels should also be checked. If CO2 levels are less than 0.5% and oxygen levels are greater than 18%, the area can be declared safe for entry.
7. ADVANTAGES OF CONTROLLED ATMOSPHERE STORAGE
CAST (or modified atmosphere or inert atmosphere storage) has some distinct advantages over practices of insect control using residual grain protectants.
Some disadvantages are that the process of disinfestation is slow (taking up to 2 weeks even under tropical conditions), sampling during CAST is difficult and the development of a suitable remote sampling method to detect infestations if it occurs is required. The commodity is immediately liable to reinfestation after outloading from CAST, so a system of transportation of the insecticide free grain from storage to its ultimate destination which does not allow the grain to become sensibly infested en route also needs to be developed.
Rather than aiming for the present 'one-shot' method of CO2 introduction (- 70% CO2 decaying to less than 35% in 10 days at temperatures greater than 25°C) it may be more efficient to introduce the intermediate CO2 system (achieving > 40% CO2 in approximately 30 days or more).
CAST systems are now at a stage in Australia where they can be regarded as one of the strategies commercially available for control of insect infestations. Their adoption is related to cost benefits derived from the system, and because of increasing costs and lowered efficacy of existing systems, CAST is already becoming competitive in many situations where there is a requirement for treatments that do not leave any chemical residues.
It also appears highly unlikely that a strain of stored products insects will evolve which is capable of completing its life cycle in micro aerophilic or oxygen free atmospheres, since they are essentially obligate aerobes. Since the possibility however remote, does exist, it is advisable to utilize controlled atmospheres as efficiently as possible to avoid undue selection of more tolerant strains.
8. APPLICATION OF THE SYSTEM IN DEVELOPING COUNTRIES
Annis (1982) has described comparative trials with bagged stacks of milled rice. With the Rice growers Cooperative Mills in Griffith, New South Wales, Australia, and Badan Urusan Logistik or BULOG, the Indonesian National Logistic Agency.
For large structures that are sufficiently sealed, a single introduction of CO2 will ensure lethal concentrations be maintained long enough to disinfest the commodity. However, with small enclosures, the same level of sealing is not practically attainable and gas loss through leakage must be replaced.
Normal disinfestation techniques with bagged commodities is to fumigate with methyl bromide or phosphine under plastic sheets secured to the floor by placing chains or sandfilled snakes (roughly 2 kg.m-2) on the sheet margins. Once the correct exposure has been achieved, the fumigation sheet is normally removed, which allows the commodity to be liable to reinfestation by resident insects within the warehouse or godown, and the requirement for repeated fumigation on a regular basis. This becomes an expensive and inefficient exercise, and in the case of methyl bromide, the possibility of objectionable levels of bromide residues being reached. It was viewed that CO2 may be an attractive and viable alternative to fumigation, especially in Jakarta in tropical conditions with a naturally high insect infestation pressure. The treatment involved three main phases.
1) Covering and sealing the stack and testing the seal.
In Jakarta, four stacks each of 180 tonnes of hagged milled rice was manually stacked on timber dunnage, and enclosed by Wavelock 41 nylon reinforced PVC sheeting built on a 0.76 mm PVC ground sheet. The outer edge of each enclosure was bonded with an urethane sealant to the ground sheet. Pressure testing was carried out after searching for obvious leaks, by connecting a small vacuum cleaner to the gas inlet port and creating a negative differential pressure of eve 1500 Pa (6 in. water gauge) with respect to atmospheric pressure. Pressure change was recorded (-ve 500 to eve 250 Pa) over time.
2) Adding the CO2
Carbon dioxide was added as snow from inverted 30 kg cylinders of food grade gas in a specially constructed frame and piped through a 17 mm copper pipe between the timber layers of dunnage out of contact with the bags. The snow sublimes and CO2 was added until the gas leaving the exit vent (200 mm x 200 mm) in the top of the stack reached 60% CO2 after which the enclosure was completely sealed. Gas concentration were measured by a Riken 18 interferometer daily for first 10 days, then subsequently by Evrite gas absorption apparatus twice a week.
3) Prolonged insect proof storage
This trial demonstrated the potential of CO2 using one 30 kg cylinder CO2 per 10 tonnes of rice, or between 2.2 - 2.9 kg.tonne-1 if using bulk gas, for up to 5 months protection in tropical conditions. The enclosures remaining in situ offered considerable protoction against reinfestation from within the warehouse. At this stage the milled rice had not deteriorated detectably, and insect control and quality maintenance were satisfactory as compared to normal operational experiences in Indonesia. A stack size of 200 tonnes can be adequately sealed to permit single CO2 introduction, while the sealing achieved with 100 tonne stacks in Greffith was only just effective (see Table 8).
A second trial was began in January, 1982, by BULOG at their National Food Technology and Research Centre at Tambun, West Java (Sukardi and Martono, 1983). It was designed to compare an initial disinfestation using phosphine where the fumigation sheet was retained to provide an insect barrier, and disinfestation using carbon dioxide.
At this trial, carbon dioxide was evaluated over a period of 16 months using 8 x 200 tonne stacks of locally produced milled rice and 8 x 200 tonne stacks of imported milled rice, where no protective insecticidal films were applied. The same system of stacking was used for the phosphine treatment, and 4 stacks of both CO2 and phosphine were opened for inspection every 4 months.
In trial 2, an application of 1.7 - 2.9 kg CO2 per tonne of rice was used to achieve a 62 - 81.8% CO2 concentration leaving the exist vent in the top of the stacks. The CO2 concentration naturally decayed to 17.4 - 22.9% after 4 months and to 1.2 6.2% after 8 months storage.
Table 8. Comparison of trials of bagged milled rice under CO2 in Australia and Jakarta.
| Griffith, N.S.W. | Tambun, W. Java | |
| Size of bagged stacks | 100 tonnes | 4 x 200 tonnes |
| Pressure decay (500- 250 Pa) | 5 mins. | 10 - 17 mins. |
| Purge | 10 cylinders | 13 - 17 cylinders |
| True CO2 application | 2.9 kg. tonne-1 | 2.2 - 2.6 kg. tonne-1 |
| Bioassay insects | Caged cultures of all developental moryzae stages of S. througtrout the stack. | Initial natural infestation of 15 live insects.kg-1, composed of R. dominica, T. castaneum, and smaller numbers of S. oryzae, C. ferrugineus, C pusillus, O. surina mensis, C. pilosellus, larvae of Ephestia sp. and Liposcelis entomo philus abundant on the floor dunnage and lower stack. |
| Results | No survival. | One stack opened on, 28, 58, 94 and 133 days after treatment. No live insect found except L. entomophilus at 58 and 1 live S. oryzae at 133 days, but may have originated externally and not survived the treatment. |
Observations at 4.8 and 12 months revealed no live insects at the surface and on the floor, while the rice was in good condition with no appreciable signs of mould. When the four phosphine treated stacks were broken down and inspected after 4 months, mould damage was apparent, and moisture condensation evident on the internal surfaces of the enclosures. To stabilize these conditions in phosphine treated stacks (equalize temperature and moisture contents) it has been necessary to adapt a system of Low Volume Exhaust Ventillation.
Several conclusions can be drawn from this interesting study.
Annis and J. van Someren Greve (1983) described trials in Lae, Papua New Guinea, which established that carbon dioxide can be successfully used for preserving the quality of green coffee beans in small stacks of 10 tonnes under plastic sheets. An average CO2 concentration of > 95% CO2 was obtained by adding 4.5 kg CO2.tonne-1. Insect control under these conditions is again comprised of .
Normally, deterioration of coffee beans stored in open stacks is rapid (3 weeks), while in the sealed enclosures treated with CO2, quality can be maintained for 6 months or more.
Calverley (1983) stated that grain quality preservation is not simply a function of storage but affected dramatically by environmental conditions operating in the region where grain is stored such that the moisture content can be cited as the "single greatest factor of grain loss throughout the world." This cannot be overemphasized where national grain agencies in the humid tropics receive a large volume of grain on procurement that has such diverse moisture content and grain quality as compared to Australian conditions where large volumes on receival are homogenous with respect to moisture ( < 12% for wheat).
Apart from extending simple airtight storage, CAST is unlikely to have any application to farm storage systems because the benefits to be gained are insufficient to justify the very considerable effort needed to establish a system requiring such a level of technology."
More relevant and low cost control measures in this form of grain storage are necessary, while at the national level, more capital intensive but cost effective control measures could be implemented.
Government storage functions cover
Storage periods are often extended beyond 12 months, with management relying on insecticides, fumigants and often to a lesser degree, good storage hygiene to effect control. Control failures are common and losses in these types of storage are oftentimes extremely high.
Through research programmes initiated by the Australian Centre for International Agricultural Research (ACIAR) in Southeast Asia, the modifications of the "Australian experience" in controlled atmospheres necessary for adaptation in this region has be or investigated. With an appropriate gas supply by generation of controlled atmospheres on site, the technology of CAST can be adapted under high moisture content regimes.
Whether we are investigating modified atmospheres using CO2 or N2 (in which there is an abundant supply in the atmosphere) or looking towards more efficacious use of phosphine or potentiating fumigation (either CH3Br of PH3 in CO2 enriched atmospheres), the need for a high standard of gastightness in whatever structure or enclosure is being used through efficient sealing methods becomes apparent.
The problems of grain quality preservation exist in those countries constrained by the environment, i.e., in the humid tropics, and not in large exporting countries. It becomes imperative that systems involving controlled atmosphere storage, and other related pest control techniques, should focus directly at problems of high moisture grain storage in developing nations.
In the humid tropics, the use of natural airtight or hermetic storage should be restricted to grain of moisture content not exceeding 13-14% m.c. Most of the grain is destined for human utilization, which therefore makes the taints that develop at high moistures unacceptable. Seed germination is also affected at warm temperatures in relatively short storage durations under sealed conditions. The deterioration of damp grain is extremely rapid in warm environments ( < 24 hours during the wet season harvest).
The degree of gastightness required to prevent mould growth is much more critical than that required to control insects in dry grain. Sufficient oxygen maybe present to allow the growht and development of the harmful organisms such as the toxin producing Aspergillus flavus oryzae group of storage fungi. Even if these are controlled by subsequent air-tight conditions, aflatoxins that they produce would remain, and invasion by moulds after outloading from hermetic storage would be rapid.
REFERENCES
ALI NIAZEE, M. T. (1971). The effect of carbon dioxide gas alone or in combinations on the mortality of Tribolium castaneum (Herbs") and T. confusum Jaq du Val. (Coleopera: Tenebrionidae). J. Stored Prod. Res. 7:243-252.
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