Miller Magazine Issue: 120 December 2019

79 ARTICLE MILLER / DECEMBER 2019 from “passive” or “natural” ventilation, which takes place in grain bins with an open manhole, or in granaries with open doors or windows, and is due to natural or convective air currents. Passive aeration takes place also in corn cribs, used traditionally in tropical and subtropical climates. Wind forces ambient air to flow through corn (maize) cribs, caus- ing slow drying of damp unshelled corn and other grains. Aeration is a widely used method for the preservation of stored grain. It is used to modify the grain bulk microclimate and to render it unfavorable for the development of damag- ing organisms in the grain, and at the same time to create favorable conditions for the sustained preservation of grain quality. Within the concept of the storage ecosystem, the role of aeration is to modify one or more of the abiotic fac- tors (temperature, humidity, atmospheric composition) and thereby to “condition” the stored grain to improve existing conditions in the grain bulk by moving air of suitable quality through the grain mass. Forced aeration is an effectively applied method in com- mercial-scale bulk storage of grain and takes advantage of two important physical properties of the grain bulk: 1- Porosity of the grain bulk: For most cereal grain, the intergranular void volume is 35–55% of the grain bulk vol- ume. The porous nature of bulk grain permits forced air to pass through and come into contact with almost all grain kernels in the bulk. 2- Thermal insulation property of the grain bulk: Due to low thermal conductivity, the grain mass is self-insulating. This enables maintenance of a “modified microclimate” long after the grain bulk has been aerated. Although the role of temperature has long been recog- nized, manipulation of this regulation by aeration tech- niques was first brought into focus in the early 1950’s in stored grain management programs in the United States. Pioneering engineering work of U.S researchers such as Foster (1953), Robinson et al. (1951), Shedd (1953), and Holman (1966) and research on technological aspects of aeration by Hukill (1953), and more recently by Cuperus et al. (1986), Arthur and Casada (2005, 2010), and Reed (2006), form the basis of modern grain aeration systems. Aeration technology is used to modify the grain bulk micro- climate to reduce or eliminate the development of harmful or damaging organisms in the grain by reducing and main- taining grain temperatures at safe levels below humidity levels which support microflora activity. Aeration helps sus- tain favorable storage conditions for the safe preservation of grain quality. Since then, several authors have reported their findings on aeration carried out in temperate climates. The knowledge accumulated over the last four decades has formed the basis for the present-day aeration technol- ogy reviewed by Navarro et al,, (2012). At present, forced aeration of grain is one of the most effective sustainable, non-chemical method in use for the control of stored grain conditions, biological activity, and grain quality losses. OBJECTIVES OF AERATION The objective of aeration is to maintain the quality of bulk grain in storage. Although aeration can improve storage conditions, aeration does not improve intrinsic quality at- tributes of grain. COOLING THE GRAIN BULK FOR PEST SUPPRESSION Cooling grain is the primary objective of grain aeration (Reed and Arthur, 2000, Reed and Harner, 1998) when discussing pest suppression. Stored grain insects are of tropical or subtropical origin and require fairly high tempera- tures, typically 24° to 32°C for development. Grain infesting insects are sensitive to low temperatures. Stored product insect development is generally stopped below 16°C; there is little insect survival above 43°C. In the southwestern and south-central U.S., temperatures of wheat, rice, and sor- ghum at harvest can range from 32° to 43°C, depending on the specific crop and location. During fall harvest in the northern U.S., grain temperatures around 10° to 18°C are typical. At temperatures below 21°C, population growth of most storage insects is significantly suppressed. Grain tempera- tures of 16° to 21°C are considered “safe” for insect man- agement, because feeding and breeding are slow. Com- plete life cycles at these temperatures take three months or more, so insect population growth remains insignificant. Insect damage caused under these low temperature con- ditions is minimal (Flinn et al., 1997; Stejskal et al., 2019). The crucial control parameter for mite pests is not only temperature, but establishing an equilibrium relative humid- ity (ERH) below about 65%. About 12.5% moisture content (MC) for wheat at 25°C suppresses mite development (Cun- nington, 1984, Navarro et al., 2002). Temperatures required to suppress mite development in damp grain 14%) to 16% MC wet-basis) are obtainable in temperate climates. Main- taining low uniform grain temperatures is too expensive at the bulk periphery when mean ambient temperatures are favorable for mite development. Although cooling moist grain is unlikely to prevent moderate mite infestation, aer- ation is expected to minimize “hot spots” and heavy mite populations associated with them. SUPPRESSION OF MICROFLORA GROWTH Low temperatures are required to prevent mold damage