Miller Magazine Issue: 120 December 2019

80 ARTICLE MILLER / DECEMBER 2019 in moist grain. Temperatures below 5°C are needed for the suppression of most mold development. For suppression of Penicillium molds, temperatures must be below 0°C. Most fungi do not grow at relative humidity below 70%, which is equivalent to roughly 13% MC for cereal grains at typical storage temperatures. The moisture content threshold is lower for oilseeds. In practice, mold growth is dependent mainly upon interstitial air humidity. Although cooling grain may not seem like an efficient method for controlling mold, at lower grain temperatures, mold damage is reduced. MAINTENANCE OF SEED AND GRAIN QUALITY Low kernel temperatures are desirable for better mainte- nance of seed and grain quality. Studies have shown that the lower the temperature (within certain limits), the longer the seeds maintain full viability. A rule of thumb (Harrington, 1973) states that a seed’s life span in storage is doubled for each 5°C decrease in temperature (within the range of 0° to 50°C) and for each 1 percent decrease in seed MC (within the range of 5% to 14%.) Seeds are commonly stored with equilibrium relative hu- midity from 30% to 40% with good results. For extended storage times of seeds, Vertucci and Roos (1990) recom- mend the best storage MC is between 19% and 27% equi- librium relative humidity. EQUALIZATION OF TEMPERATURE THROUGH-OUT THE GRAIN BULK Because of self-insulating properties, grain placed in storage during summer harvest retains initial harvest tem- peratures for a long time before cool weather arrives in the fall (except for grain near bin walls, exposed conical base, or at the surface). It is recommended that harvest heat be removed by night time suction aeration as soon as ambi- ent temperatures are 8° to 11°C below internal grain mass temperatures to prevent condensation and minimize in- sect activity at or near the grain surface. The initial cooling should be followed by additional aeration when generally lower ambient temperatures will allow cooling the entire grain mass below 21°C. PREVENTION OF MOISTURE MIGRATION IN THE GRAIN BULK As the ambient temperature drops during the cool season, the surface (and peripher- al) layers of the grain become considerably cooler than the internal grain mass. Tempera- ture gradients are established in the grain bulks that can lead to convection currents that circulate air through the inter- granular spaces. In large bulks, the cold dense air settles along the outer walls. The warmer air (which contains more moisture than cool air) moves toward the colder upper surface of the grain bulk. When the warm air reach- es the cool layers of the grain bulk, moisture condenses and creates wet layers or spots in the grain. Studies (Montross et al., 2002, Montross and Maier 2001) suggest a moisture equilibration theory for the mechanisms involved in this moisture movement in a non-aerated grain mass. Using the finite-element model they developed, additional large-scale trials to demon- strate the effect of significant temperature gradients on moisture condensation due to convection currents that carry moisture into the cool layers of the grain bulk. On the other hand, the traditional natural convection hypoth- esis suggests that the natural convection currents in the grain bulk alone are sufficient to cause large amounts of moisture to “migrate” to cooler layers or the cooler surface grain, where the air cools to “dew point” and deposits excess moisture, slowly increasing the grain moisture content in the upper parts of the grain bulk. An example of the above phenomenon is presented in Fig. 1. Particularly, the prediction algorithm developed Fig. 1. Simulation algorithm developed by Centaur, predicts airflow patterns, temperature and mois- ture content profiles leading to the development of excess moisture in the upper layers of the grain.