Miller Magazine Issue 110 / February 2019

79 ARTICLE MILLER / FEBRUARY 2019 mature and adult mortality (White et al. 1995). Effects of temperature and relative humidity on controlled atmosphere fumigation - Insect mortality increases more rapid- ly as temperatures rise and their metabolism speeds up. Cool temperatures slow rates of mortality while lower relative humi- dity (RH) hastens toxic effects, notably in high CO ₂ atmosphe- res because of desiccation of insects (Banks and Fields 1995). EFFECTS OF CA ON PRODUCT QUALITY Germination of seeds - Seed below the critical moistu- re content is not significantly affected at high CO ₂ or low O ₂ atmospheres. However, with increasing grain moisture contents, carbon dioxide-rich atmospheres could reduce the physiological quality of grain by interfering with the enzyma- tic activity of glutamine-decarboxylase. The adverse effect of CO ₂ 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 preser- vation of germination is of primary importance, the use of CO ₂ -free low O ₂ atmospheres is preferred if expected tem- peratures are significantly above 30°C. Viability of corn stored under hermetic (148 days storage) and non-hermetic (120 days storage) conditions in the Philip- pines 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 herme- tic 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 sli- ghtly 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 CO ₂- concentrations ranged between 10% and 15%. Product quality preservation - Donahaye et al. (2001) re- ported 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 un- der 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 equ- ilibrium relative humidity of 73% at 26oC in hermetically sea- led containers. The O ₂ concentration was reduced to <0.3%, and CO ₂ 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 CO ₂ or N ₂ fumigation. Acceptable airtightness for CO ₂ 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). Attemp- ts 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 vari- able 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 wi- dely used worldwide to create hermetic storage where CO ₂ was produced and O ₂ 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 CO ₂ (Fig. 1). Alagusundaram et al. (1995) placed dry ice in insulated coolers under a CO ₂ imper- vious plastic sheet above wheat 2.5 m deep in a 5.6 m diameter bin. CO ₂ levels were 30% at 0.55 m above the floor where 90% of rusty grain be- etles, Cryptolestes ferrugineus (Steph.) were killed; CO ₂ le- vels of 15% at 2.0 m above the floor resulted in 30% mortality. A bolted, galvanized-iron silo (21.5 tons) was sea- led using a polyvinyl resin formulation sprayed onto joints from the inside. The silo was loaded with wheat into which ca- ges of insect-infested wheat were introduced, and conditions monitored with thermocouples and gas sampling lines. Oxygen levels were reduced to <1% by purging with N ₂ , and similar levels were then maintained by a slow N ₂ 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 com- mercially available (Moylan Silos 2011). Welded-steel hopper bins can be modified for CO ₂ fumigation for a few hundred dollars. Carbon dioxide from dry ice must be recirculated th- rough 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 CO ₂ 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 CO ₂ for 4 hours (1 metric tonne of CO ₂ ) and additional gas is added as needed. All caged test insects were killed (White and Jayas, 2003). A large installation for the application of CO ₂ -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). Figure 2: Application of CO ₂ based CA on a 5,000 tons concrete horizontal silo bin in Mianyang, Sichuan, China.