"Wireless sensors for monitoring O₂ and/or CO₂ concentrations, relative humidity and temperature have been developed to enable the technical personnel record the data. Such development facilitates the analysis of the MA/CA treated enclosures without the presence of the technical personnel on site. Since the exposure time of MA/CA treatments last many days and the treated structures may be located at distant sites then the location of the operator, such development offers an excellent tool that can be incorporated to the package of the MA/CA treatments."
High quality flour can be obtained only from high quality wheat. Protection of the beneficial qualities of wheat during storage depends on many factors. Among the detrimental factors that reduce the quality of wheat are insects and microflora. Wheat kernels can be stored for extended periods of time, provided there is no insect infestation and their water activity can be kept low enough to prevent microbial growth. However, in aerated storages quantitative and qualitative losses still occur. Qualitative losses, for example, may consist of changes in physical appearance, in color change, loss of flavor, in nutritional degradation due to oxidation and increase in free fatty acids, the presence of insects or their fragments, or contamination by mold or the presence of mycotoxins. If the moisture content is maintained sufficiently low, insects and quality loss remain the main concern for the quality preservation of wheat (Navarro and Donahaye, 2005). Although in modified or controlled atmospheres (MA/CA), the major emphasis is placed on the control of insect pests, for quality preservation, just maintaining the vapor pressure in the sealed structure is sufficient.
In developed countries, consumers’ preference for quality wheat, uncontaminated by insecticidal residues and that is not contaminated by molds and insects are particularly important. Whereas in developing countries, poor handling and storage methods under warm and humid climatic conditions, promote rapid deterioration of the stored foodstuffs.
Increased public concern over the adverse effects of pesticide residues in food and the environment has led to the partial substitution of use of contact pesticides (typically organophosphates and pyrethroids) and fumigants by alternative control methods. It is worth noting that of the 14 fumigants listed some 35 years ago by Bond (1984), only one remain today in regular worldwide use, namely, phosphine and methyl bromide which is used mainly for QPS (quarantine and pre-shipment) conditions. Methyl bromide kills insects relatively quickly, but because of its contribution to stratospheric ozone depletion (UNEP, 2002), it was phased out in developed countries by 2005 (UNEP, 2006). In contrast, phosphine remains popular, because it is easier to apply than methyl bromide. However, many insects have developed resistance to phosphine over the last two decades.
MA/CA offers an alternative that is safe and environmentally benign, to the use of conventional residue producing chemical fumigants for controlling insect pests attacking stored wheat grain, oilseeds, processed commodities and packaged foods. These atmospheres prevent fungal growth also and maintain product quality. An important development stimulating further work on MA, took place in the U.S. in 1980 and 1981. The Environmental Protection Agency (EPA) approved an exemption from tolerance for CO₂, N₂, and products from an “inert” gas generator when used to control insects in raw (Federal Register 45, pp. 75663 64, Nov. 1980) and processed (Federal Register 46, pp. 32865 66, June 1981) agricultural products. The development of this technology has come about mostly over public concern for the adverse effects of pesticide residues in food and the environment. Although this method has become well established for control of storage pests, its commercial use is still limited to a few countries. Investigations that are more recent, have attempted to integrate modified atmosphere application into the 21st century version of raw products and manufactured food storage and transportation (Navarro 2006).
Atmospheric manipulation for the protection of stored products such as wheat grains has been researched extensively for more than 30 years (Adler et al., 2000; Calderon and Barkai-Golan, 1990; Jay, 1984; Navarro, 2006)
MA is proposed to serve as the general term, including all cases in which the atmospheric gases composition or their partial pressures in the treatment enclosure have been modified to create in it conditions favorable for the control of insects. In a MA treatment, the atmospheric composition within the treated enclosure may change during the treatment period. In a CA treatment, atmospheric composition within the treated enclosure is controlled or maintained at a level and duration lethal to insects. The result in either case is the creation of a safe and environmentally benign process to manage food preservation (Navarro 2006).
The purpose of this work is to discuss the concepts and variations of MA and CA, their impact on pests and on the quality of wheat being treated, the structures where they may be considered for use, and their compatibility in commercial settings.
CA UNDER NORMAL ATMOSPHERIC PRESSURE
Gas supply from pressurized cylinders - CA is a modified gas composition, usually produced artificially, and maintained unchanged by adding desired gases (CO₂ or nitrogen [N²]), supplied from pressurized cylinders or otherwise. This supplementary introduction of’ gases is carried out when their concentration in the sealed container drops to below the desired level.
The objective of CA treatment is to attain a composition of atmospheric gases rich in CO₂ and low in O₂, or a combination of these two gases within the storage enclosure or treatment chamber. These set concentrations are maintained for the time necessary to control the storage pests. A widely used source for production of such atmospheric gas compositions is tanker-delivered liquefied CO₂ or N₂, when the target CA gas composition is <1% O₂ or high CO₂ concentration. For large-scale application of N₂ or CO₂, vaporizers are essential. These vaporizers consist of a suitably designed receptacle with a heating medium (electricity, steam, diesel fuel, or propane), a super-heated coil with hot-water-jacket, and forced or natural draught.
Combustible gases - For on-site generation of CAs by combustion of hydrocarbon fuel to produce a low-O₂ atmosphere containing some CO₂, commercial installations – termed exothermic gas generators or gas burners – are available. Their CA composition is designed to allow the presence of approx. 2 to 3% O₂ with CO₂ removed through scrubbers. Several adaptations are required for their use in the grain industry, i.e., tuning equipment to obtain an O₂ level of <1%; utilizing to full advantage the CO₂ generated; and removing excessive humidity from the atmosphere generated. Combustion of propane and butane yields approximately 13% and 15% CO₂, respectively. The CA generated is more toxic than a N² atmosphere deficient in O₂ due to the presence of CO₂ in the MA, causing hypercarbia, which together with hypoxia, are synergistic in their effect on insect mortality.
On-site N₂ generators - Commercial equipment, termed also “pressure-swing absorption” systems, use the process of O² adsorption from compressed air passed through a molecular sieve bed. For continuous operation, a pair of absorbers is provided that operate sequentially for O² adsorption and regeneration. Nitrogen at a purity of 99.9% can be obtained through regulation of inlet airflow; this method of N² generation is an expanding new approach in CA generation technology. Equipment is now being manufactured that is rated to supply an outlet flow of 120 m3/h at an outlet purity of 98% N₂.
EFFECTS OF CA ON INSECTS UNDER NORMAL ATMOSPHERIC PRESSURE
Effects of low oxygen levels - Insects can tolerate low levels of oxygen for prolonged periods. Using N₂ to replace O₂ must result in O₂ being below 2%, preferably 1% for rapid death. This effect is reversed below 1% O₂ in N₂ where adult rice weevils, Sitophilus oryzae (L.) (Navarro 1978) showed tolerance, increasing the lethal exposure time by apparently closing their spiracles. In particular, S. oryzae adults are killed more quickly at 1.0% O₂ rather than at 0.1 or 2% O₂ under the same conditions. Tribolium castaneum (Hbst.) in N₂ showed significant differences in adult mortality between 0.1 and 1.0% O₂ (Navarro 1978). Adults are generally most susceptible to treatment, and S. oryzae or Rhyzopertha dominica (F.) was found to be more tolerant than Tribolium spp. The lowest level of tolerance to lack of O₂ was attained around the 1% concentration level. Therefore, Annis (1987) concluded that O₂ levels of 1% are needed to kill insects in 20 days (Table 1).
Effects of high carbon dioxide levels - Elevated CO² levels cause spiracles to open resulting in insect death from water loss. Above 10% CO₂ spiracles remain permanently open. Toxic effects are entirely through the tracheae, not the hemolymph; CO₂ has direct toxic effects on the nervous system. In some cases, CO₂ can acidify the hemolymph leading to membrane failure in some tissues (Nicolas and Sillans 1989). Elevated, but sub-lethal CO₂ levels, for prolonged periods can have deleterious effects on insect development, growth, and reproduction (White et al. 1995, Nicolas and Sillans 1989). Atmospheres containing about 60% CO₂ rapidly kill stored-product insects. At 26°C, about 4 days of exposure would be sufficient to kill all stages (including eggs) of most stored-product insects (Table 1).
High carbon dioxide and low oxygen levels - Atmospheres with 60% CO₂ and 8% O₂ are very effective at killing internal seed-feeding insects, while low O₂ atmospheres are more rapid in killing external-feeding insects (Banks and Annis 1990). High CO₂ levels even with 20% O₂ rapidly kill insects because of CO₂ toxicity. CO₂ levels must be at 40% for 17 days, 60% for 11 days, 80% for 8.5 days at temperatures above 20°C or 70% declining to 35% in 15 days at 20°C (Annis 1987). Higher temperatures accelerate CO₂ toxicity as insect metabolism is elevated. Even low levels of CO₂ (7.5-19.2%) for prolonged periods sharply increase immature and adult mortality (White et al. 1995).
Effects of temperature and relative humidity on controlled atmosphere fumigation - Insect mortality increases more rapidly as temperatures rise and their metabolism speeds up. Cool temperatures slow rates of mortality while lower relative humidity (RH) hastens toxic effects, notably in high CO₂ atmospheres because of desiccation of insects (Banks and Fields 1995).
EFFECTS OF CA ON PRODUCT QUALITY
Germination of seeds - Seed below the critical moisture content is not significantly affected at high CO₂ or low O₂ atmospheres. However, with increasing grain moisture contents, carbon dioxide-rich atmospheres could reduce the physiological quality of grain by interfering with the enzymatic activity of glutamine-decarboxylase. The adverse effect of CO₂ on germination of rice, maize, and wheat, becomes more pronounced at temperatures higher than 47°C and, from observations carried out so far, this adverse effect may not be detectable at all below 30°C. Therefore, if preservation of germination is of primary importance, the use of CO₂-free low O₂ atmospheres is preferred if expected temperatures are significantly above 30°C.
Viability of corn stored under hermetic (148 days storage) and non-hermetic (120 days storage) conditions in the Philippines did not indicate significant changes between the initial and final samples (Navarro and Caliboso 1996; Navarro et al., 1998). In same trials, viability of paddy stored under hermetic conditions did not change significantly. To test viability of wheat stored under hermetic conditions in Israel, two trials were carried out with storage periods of 1,440 and 450 days only under hermetic conditions. Viability of wheat changed slightly from an initial 99% to 97% after 1,440 days, and from 97% to 91% after 450 days, respectively. In both trials, insect populations were successfully controlled and the average CO₂concentrations ranged between 10% and 15%.
Product quality preservation - Donahaye et al. (2001) reported on quality preservation of 13.4 to 31.9 tonne lots of paddy, stacked in flexible enclosures and stored outdoors for 78 to 183 days. The quality of the paddy was compared with that of three control stacks (5.3 to 5.6 tons capacity) held under tarpaulins in the open for 78 to 117 days. Percent milling recovery and levels of yellowing in the gastight stacks showed no significant change. In a study on quality preservation of stored cocoa beans by bio-generated modified atmospheres, respiration rates of fermented cocoa beans were tested at equilibrium relative humidity of 73% at 26oC in hermetically sealed containers. The O₂ concentration was reduced to <0.3%, and CO₂ concentration increased to 23% within 5.5 days. The free fatty acid (FFA) content of cocoa beans at 7.0%, 7.5%, and 8.0% moisture content under hermetic conditions of 30°C remained below or close to 1.0% after 90 and 160 days of storage (Navarro et al. 2010).
TYPES OF STRUCTURES IN WHICH CA AND MA HAVE BEEN USED
Controlled atmospheres have been used in a wide array of grain storage structures. The most important consideration is that they must be airtight for long-term storage or relatively airtight for CO₂ or N₂ fumigation. Acceptable airtightness for CO₂ fumigation is determined by negative pressure testing and should at most hold a negative pressure from 500 Pa to 250 Pa in 10 minutes (Annis and van S. Graver 1990). Attempts have been made to predict gas tightness relative to leakage areas (Mann et al. 1999; Lukasiewicz et al. 1999). Provisional guidelines based on best estimates from comparative of variable pressure tests are presented in Table 2 (Navarro 1999). The suggested times given in Table 2 were doubled for empty storages as an approximation to the intergranular airspace.
In-ground storage - Historically, in-ground storage was widely used worldwide to create hermetic storage where CO₂ was produced and O₂ consumed by respiration of grain and microflora. Its use was recorded from Spain to India and China, East Africa, and North America west of the Mississippi River (Sigaut 1988).
Bolted steel bins - Bolted steel bins are not airtight but they can be sealed for partially successful fumigation with CO₂ (Fig. 1). Alagusundaram et al. (1995) placed dry ice in insulated coolers under a CO₂ impervious plastic sheet above wheat 2.5 m deep in a 5.6 m diameter bin. CO₂ levels were 30% at 0.55 m above the floor where 90% of rusty grain beetles, Cryptolestes ferrugineus (Steph.) were killed; CO₂ levels of 15% at 2.0 m above the floor resulted in 30% mortality. A bolted, galvanized-iron silo (21.5 tons) was sealed using a polyvinyl resin formulation sprayed onto joints from the inside. The silo was loaded with wheat into which cages of insect-infested wheat were introduced, and conditions monitored with thermocouples and gas sampling lines. Oxygen levels were reduced to <1% by purging with N₂, and similar levels were then maintained by a slow N₂ bleed for 35 days, after which the silo was emptied. All adult insects were dead but, as expected, some immatures survived. This was because the maintenance period was too short to ensure complete kill at the observed grain temperatures of <15°C (Williams et al. 1980).
Sealed steel bins - Airtight, galvanized-steel bins have been manufactured in Australia for the past 30 years and are commercially available (Moylan Silos 2011). Welded-steel hopper bins can be modified for CO₂ fumigation for a few hundred dollars. Carbon dioxide from dry ice must be recirculated through the grain and a pressure relief valve installed to the bin. The top and bottom hatches must be gasket sealed. After 10 days at 20OC, 75% of applied CO₂ was retained while 99% of the caged C. ferrugineus were killed (Mann et al. 1999).
Concrete grain elevators - Carbon dioxide fumigation of grain has been successful in concrete elevators holding 209 tons of wheat. The bottom hopper was sealed and the grain purged with CO₂ for 4 hours (1 metric tonne of CO₂) and additional gas is added as needed. All caged test insects were killed (White and Jayas, 2003). A large installation for the application of CO₂-based CA was installed to treat more than 200,000 tons of rice annually in flat bins each of 5,000 tons capacity in Mianyang, China (Fig. 2).
Numerous MA and CA systems have been developed over the years to manage insect pests and microflora associated with stored products, however, their general commercial use remains somewhat limited (Adler et al. 2000). Exceptions are for organic products where use of fumigants is not possible because of residues; hermetic storage in plastic structures with application of MA is the preferred choice (Navarro 2006).
Hermetic storage - When placed in sealed airtight storage, commodities and the insects and aerobic microflora that exist within them respire, consuming O₂ and producing CO₂. This modified atmosphere technology has been utilized to a great extent for durables such as wheat grains. Hermetic grain bags (Africa, Argentina, Asia, Australia, North and South America, Middle East) and sealed bunker storage (Australia, U.S., Middle East) have been implemented into commercial application to various extents.
Bunker storage, having designed storage capacities to over 10,000 tons, is established in permanent locations with a prepared base (usually asphalt or compacted soil with a convex profile) and an airtight cover. This type of storage has been used extensively in Australia, Argentina, Israel and Cyprus (Adler et al. 2000). While low moisture content, high temperature grain supports this type of storage, condensation can remain problematic if the grain is stored with cones or ridges (Navarro et al. 1994). Sealed bunker storage has also been demonstrated as an effective means for utilizing CA or conventional fumigation where the bunker is sealed and flushed with N₂ or CO₂ .
A major challenge that South America is facing is to minimize quality and quantity losses, and improve food safety in view of the shortage of permanent storage capacity. As a result, the Silobag system for temporary storage of dry grain and oilseeds has been adopted. During the 2008 and 2010 harvest seasons, more than 33 million and 43 million tons of grain were stored, respectively, in these plastic bags in Argentina. Commodities included corn, soybean, wheat, sunflower, malting barley, canola, cotton seed, rice, lentils, sorghum, beans and even fertilizers. The Silobag technology is also being adopted in other countries such as the United States, Australia, Bolivia, Brazil, Canada, Chile, Italy, Kazakhstan, Mexico, Paraguay, Russia, South Africa, Sudan, Ukraine, and Uruguay (Bartosik 2011 personal communication). Dry grain can be stored in silo-bag for more than six months without losing quality (Bartosik 2012).
Controlled atmospheres - Nitrogen and CO₂ have been used as agents for controlled atmospheric storage for many years. Carbon dioxide has been considered to be more efficient than N² due to the concentrations necessary for control and the level of gas tightness of the structure being used. A CO₂ concentration of about 60% can provide 95% control of most stored-product insect pests at 27ºC (Jay 1971), while N² use requires interstitial O₂ levels to be reduced to 1% or less. Considerable efforts to improve bin sealing of storage bins have been made (Mann et al. 1999) which in turn facilitates ease in gas application and retention. Mann et al. (1999) demonstrated that CO₂ generated from dry ice and circulated with a vacuum pump at a concentration of 51% caused 100% mortality of C. ferrugenius after 10 days at 20ºC. Carbon dioxide can also be added to bulk stored-products as compressed gas. White and Jayas (1991) demonstrated that by circulating CO₂released from compressed cylinders, high mortality of several stored-product arthropod pests could be achieved within 14 days. They found that bin sealing was crucial to maintain efficacy especially when commodity temperature fell below 20ºC, and that utilizing pressure testing techniques (Banks and Annis 1980) is a useful means of determining a bins seal.
Nitrogen production has also changed considerably over the years. Pressure-swing absorption systems have proven successful where a 13,660 m3 bin can be purged to <1.0% O₂ in 7 days. Appropriate sealing allows for accurate calculation for additional gas application required to compensate for gas loss due to sorption and pressure cycling caused by pressure change (Cassels et al. 2000); gas concentration can be maintained for appropriate times. Liquid N₂ can be used for topping up the controlled atmosphere, but can cost twice that of other sources. Although CA treatment of grain is an old and proven technology, its applications remained limited. A recent development has been reported by Clamp and Moore (2000), in which N² supplied as a bulk liquid under pressure was used to treat 1,800 tonne bins. Since the N₂ treatment was commissioned in 1993, more than 300,000 tons were treated in the Newcastle facilities as of 2000 (Clamp and Moore, 2000).
Nitrogen can also be easily generated using molecular membrane generators. These are capable of purging vertical grain storages of 120 tons capacity within 3 hours (Timlick et al. 2002). By maintaining a slight positive bin pressure, concentrations within a sealed commercial storage could be maintained (compensation for leakage) and insect mortality was significant after 14 days at 17ºC.
In terms of efficacy and efficiency, there is not much difference between using CO₂ over traditional fumigants such as phosphine. Nitrogen has been considered unsuitable for bulk commodity treatment at export position because the length of time required for significant mortality of the pests in question is too long. However, effective management procedures can allow for N₂ use when temperatures are appropriate. All require effective sealing and monitoring and efficiency is directly correlated to temperature. While caution is necessary when utilizing any product as an atmospheric control, there are no residues of concern when utilizing MA. Aeration after treatment is of less concern, thereby allowing for outturn of product in export position minimizing concerns for worker safety. Product availability is not an issue. Flexible liners and their associated loading/unloading equipment and nitrogen generators are all available commercially and can be set up and maintained with product replenished on site.
Maintenance of sealing of hermetic storage has proven a challenge at times. Large bunkers and grain bags in Australia often have sealing breached by birds pecking holes in the liner. In Canada, deer often break the seals of hermetically stored grain in bags. Consequently, some focused research on liner integrity may be of use in these types of situations. Discovery of breached seals during CA treatments can be difficult to remedy, underlining the necessity of performing pressure testing before application.
A significant aspect of MA/CA treatment is our ability to monitor the gas concentrations in the gastight structures. Similar to fumigation, O₂ and/or CO₂ concentrations should be monitored to ensure a successful application. As a common practice gas sampling lines have been installed for monitoring using gas measurement instruments. Such monitoring is performed by technical personnel on site at predetermined regular times, particularly during gas purging and during the exposure time to MA/CA treatment. The conventional method of such monitoring is the visit of the technical personnel on regular basis to monitor the changes in gas concentrations. This method necessitates precious time and travel of the technical personnel, which makes the monitoring expensive.
Wireless sensors for monitoring O₂ and/or CO₂ concentrations, relative humidity and temperature have been developed to enable the technical personnel record the data (Centaur Analytics, 2018). Such development facilitates the analysis of the MA/CA treated enclosures without the presence of the technical personnel on site. Since the exposure time of MA/CA treatments last many days and the treated structures may be located at distant sites then the location of the operator, such development offers an excellent tool that can be incorporated to the package of the MA/CA treatments. Such wireless monitoring assists the operator in decision making on the necessity of intervention during the treatment.
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