Miller Magazine Issue 110 / February 2019

80 ARTICLE MILLER / FEBRUARY 2019 COMMERCIAL USE 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 O ₂ and producing CO ₂ . 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., Midd- le 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 pre- pared 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 (Ad- ler 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 demonst- rated as an effective means for utilizing CA or conventional fumigation where the bunker is sealed and flushed with N ₂ or CO ₂ . 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, ca- nola, cotton seed, rice, lentils, sorghum, beans and even fer- tilizers. 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 per- sonal communication). Dry grain can be stored in silo-bag for more than six months without losing quality (Bartosik 2012). Controlled atmospheres - Nitrogen and CO ₂ 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 le- vel of gas tightness of the structure being used. A CO ₂ con- centration of about 60% can provide 95% control of most stored-product insect pests at 27ºC (Jay 1971), while N² use requires interstitial O ₂ levels to be reduced to 1% or less. Con- siderable 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) demons- trated that CO ₂ generated from dry ice and circulated with a vacuum pump at a concentration of 51% caused 100% mor- tality 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 CO ₂- released 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 effi- cacy 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 succes- sful where a 13,660 m3 bin can be purged to <1.0% O ₂ 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 chan- ge (Cassels et al. 2000); gas concentration can be maintained for appropriate times. Liquid N ₂ 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 deve- lopment 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 N ₂ treatment was commis- sioned in 1993, more than 300,000 tons were treated in the Newcastle facilities as of 2000 (Clamp and Moore, 2000). Table 2 - Provisional recommended ranges for variable pressure tests carried out in Structures destined for gaseo- us treatments to control storage insects (Navarro 1999). Table 1 - Suggested provisional dosage regimes for control of all stages of the 12 most common insect species of stored grain, using modified atmospheres at temperatures between 20o and 29oC*