Miller Magazine Issue 110 / February 2019

77 ARTICLE MILLER / FEBRUARY 2019 sect 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 pestici- de residues in food and the environment has led to the partial substitution of use of contact pesticides (typically organop- hosphates and pyrethroids) and fumigants by alternative cont- rol 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 environmental- ly benign, to the use of conventional residue producing che- mical 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 stimula- ting 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 CO ₂ , N ₂ , 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 pro- ducts such as wheat grains has been researched extensively for more than 30 years (Adler et al., 2000; Calderon and Bar- kai-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 par- tial 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 enclo- sure 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 vari- ations of MA and CA, their impact on pests and on the quality of wheat being treated, the structures where they may be con- sidered 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 unchan- ged by adding desired gases (CO ₂ or nitrogen [N²]), supplied from pressurized cylinders or otherwise. This supplementary introduc- tion 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 CO ₂ and low in O ₂ , or a combina- tion of these two gases within the storage enclosure or treat- ment 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 CO ₂ or N ₂ , when the target CA gas composition is <1% O ₂ or high CO₂ concentration. For large-scale application of N ₂ or CO ₂ , 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 com- bustion of hydrocarbon fuel to produce a low-O ₂ atmosphere containing some CO ₂ , commercial installations – termed exot- hermic gas generators or gas burners – are available. Their CA composition is designed to allow the presence of approx. 2 to 3% O ₂ with CO ₂ removed through scrubbers. Several adapta- tions are required for their use in the grain industry, i.e., tuning equipment to obtain an O ₂ level of <1%; utilizing to full ad- vantage the CO ₂ generated; and removing excessive humidity from the atmosphere generated. Combustion of propane and butane yields approximately 13% and 15% CO ₂ , respectively. The CA generated is more toxic than a N² atmosphere defi- cient in O ₂ due to the presence of CO ₂ in the MA, causing hypercarbia, which together with hypoxia, are synergistic in their effect on insect mortality. On-site N ₂ 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 pro- vided that operate sequentially for O² adsorption and regene- ration. Nitrogen at a purity of 99.9% can be obtained through regulation of inlet airflow; this method of N² generation is an Figure 1: Application of CO ₂ based CA in a 6,800 tons metal silo bin in Kingston, North Carolina, USA.