The 2019-20 Corn Harvest Quality Report published by the US Grains Council (USGC) states that the actual quality buyers will observe during the current marketing year will be affected by how the corn has been handled, dried, stored and blended since it was harvested in October through December of last year.
Late planting in April through June of 2019 and wet weather in October and November delayed harvest and required much more corn to be dried artificially than had been for about a decade. High harvest moisture results in more corn stored at moisture contents above the recommended safe storage level, which across the Midwestern US is 15% to 16%. Unfortunately, that was the case as it was the highest average and greatest variability since the USGC published its first report for the 2011 harvest.
Moisture content is not a grading factor and thus not part of the US Grain Standard. Thus, any US corn grade could theoretically be met with respect to test weight, heat and total damaged kernels, and broken corn and foreign material at moisture contents that may be above what is safe in the overseas locations of importers, grain handlers and feed millers. Fortunately, this is rarely the case as the aggregated moisture content data for US exports over the last six marketing years shows (See Figure 1).
Although speculative, past moisture data indicates a 1.8-percentage point decrease between harvest and export, which for the 2019-20 marketing year could mean an aggregated average of 15.7%. The recommended safe storage moisture content in tropical climates is no higher than 14% to 14.5%, especially if corn is to be stored for more than a few weeks. It is critical that importers specify a moisture content limit in their purchasing contracts.
At export terminals, corn is not dried but blended to meet contract specifications. When corn at 15% to 16% moisture content is blended with corn at 12% to 13%, a mix of corn results that has individual kernels at moisture contents that may range from 10% to 18%. Unless corn is aerated — which it is not during ocean transport — kernels will not equilibrate to the average of 14% for weeks. Those kernels at moisture contents higher than safe for storage have a substantially shorter shelf life and will allow fungi to sporulate, especially when relatively colder corn is imported and stored in warmer, more humid climates. The onset of spoilage is accelerated when corn contains large amounts of broken corn and foreign material (BCFM). Fortunately, this is not the case in most years, but the 2019 harvest BCFM data does not bode well for the 2019-20 marketing year. As was the case for moisture content, it was the highest average and greatest variability since the USGC published its first report for the 2011 harvest.
Although purely speculative, past BCFM data indicates a four-fold increase between harvest and export, which for the 2019-20 marketing year could mean an aggregated average of 4%, which would meet US grade 3 but would be one percentage point higher than US grade 2 allows. Broken kernels mean starch exposed to fungal spores that together with more dust and higher moisture kernels will accelerate the onset of spoilage and increase the potential for self-heating. Clearly, extra care needed to be taken by US farmers and grain handlers to monitor and maintain grain quality than during previous years to prevent the onset of spoilage due to mold growth during storage and transport.
Likewise, operations managers at overseas locations of importers, grain handlers, and feed millers also will need to pay greater attention to monitor and maintain stored grain quality before end use. This article focuses on best practices to do that.
Moisture and mold management
Importers of US corn, especially into tropical climates, face greater challenges to maintain quality during storage than do their counterparts in the United States. Reasons include:
The repeated handling from farm to export terminal and again from import terminal to end user produces more breakage and dust.
The ambient temperature is higher than the temperature corn was held at during the winter storage period in the United States.
The combination of ambient temperature and relative humidity in the tropics is higher than the equilibrium relative humidity of corn even if exported at 14% to 14.5% moisture content.
Relative humidity and mold development
Fungal spores are activated more by the relative humidity of air surrounding corn kernels than temperature. A relative humidity above 65% to 70% is the general activation threshold. This implies that moisture contents in equilibrium with a relative humidity of 65% to 70% at any temperature determines whether fungi will sporulate or corn will continue to store safely. For example, corn at 15°C and 15.1% moisture content will be in equilibrium with air at a relative humidity of 75% (See Table 1).
Unfortunately, at a relative humidity of 75%, fungal spores will begin to activate, which often is observed in the form of surface crusting in corn when headspace temperatures in silos and warehouses increase in warm weather. Ideally, US corn exported to the tropics should be stored closer to 13% to 13.5% to assure relative humidity surrounding grain kernels remains below 65% to 70% at temperatures above 20°C.
Condensation associated with handling and storage of imported US corn in warm climates is another challenge. It often arrives in tropical ports at a temperature at least 10°C colder than the ambient air. Water condenses on the external surfaces of grain handling equipment when cold corn is unloaded from vessels and transferred into storage silos or warehouses.
Condensed water on external metal surfaces does not affect the corn as long as it is not allowed to drip onto the corn. Ideally, corn is transferred in enclosed conveyors rather than open belt conveyors that allow warm, humid air to come into contact with the cold, dry corn. Contact between this cold grain and humid air causes water to condense on the exposed corn, which when placed in storage can result in a higher equilibrium relative humidity of the air. This in turn results in activation of fungal spores followed naturally by onset of spoilage and self-heating.
A key process technology for reducing moisture content to safe storage levels is artificial drying. Unfortunately, high capacity continuous-flow dryers are generally not found at US export terminals and overseas import terminals.
The equipment is expensive to purchase, own and operate. Thus, drying with heated air is typically not an option for managing moisture content in imported corn.
Ambient and chilled aeration
Aeration of grain involves forcing ambient or chilled air at low flow rates through a stored grain mass to lower or maintain its temperature. It requires fans and an air distribution system to push or pull air through the grain. Most silos and warehouses at overseas import terminals are not equipped for aeration. However, if needed they can be retrofitted with permanent or portable aeration systems.
When weather conditions do not allow for cooling and maintaining stored grain with ambient air, portable air-cooling units are available for chilled aeration of stored grain. This equipment is expensive to purchase, own and operate, and generally only used to preserve higher value food grains such as rice and wheat to avoid the use of protectants and fumigants.
Key to aeration is understanding the local weather conditions to determine if or when to turn on fans. For example, an analysis of five years of weather data for Tunis, Tunisia, indicates the average temperature and relative humidity of ambient air throughout the year was 19.6°C and 65.8%. Average relative humidity throughout the year ranged between 55% and 75% which implies a low potential for activating fungal spores. Thus, imported US grade 2 corn at 14% to 14.5% moisture content and a temperature of 10°C on arrival should store well in Tunisia without the need for aeration. If such corn were aerated, it would be warmed to 18.6°C and shrink to 13.2% equilibrium moisture content. This amount of shrink loss due to aeration would be costly and would not substantially improve the storability of the imported corn. An additional benefit of keeping stored grain as cool as possible is prevention of insect development, which is slowed substantially below 20°C. Exported US corn is generally free of live insects for two reasons:
- The US grain grading system limits the number of live insects that can be present without the special designation “infested” recorded on the official grade certificate.
- Most purchase contracts require fumigation aboard ship as a precautionary phytosanitary measure. Thus, insect infestation likely occurs when grain is discharged from the ship and passes through the handling and storage system of the import terminal and from there to local grain facilities and feed mills.
Mold inhibitors originally were developed for application to high-moisture corn (greater than 18%) immediately after harvest as a less expensive alternative to artificially drying corn, or to extend the holding time before corn could be dried.
Mold inhibitors never gained acceptance among US grain producers, handlers, and exporters, so they are not applied to exported US corn. They are based on propionic acid and are considered safe for human and animal consumption.
The inhibitory action is primarily by keeping fungal spores from growing rather than destroying them. In many countries they are used to extend the shelf life of animal feeds. They should not be needed for imported US corn, especially when moisture and fines are managed with best practices to prevent the on-set of grain spoilage and self-heating.
Fines and dust management
Fines in exported US corn consist mostly of starch from broken corn endosperm. Together fines and dust are good feed ingredients, but their presence can cause problems in storage and must be properly managed. When corn drops into a silo or warehouse from an overhead spout or cross conveyor, a pile shaped like a cone forms. Corn kernels, larger pieces of broken corn, and light pieces of foreign material (e.g., cob and stalk pieces) flow down the surface of the pile increasing its diameter while fines tend to accumulate in the core of the pile below the peak. In silos only one so-called spoutline forms while in warehouses spoutlines form below each conveyor drop point.
The accumulation of small pieces of broken kernels and associated exposure of starch in spoutlines is highly susceptible to the onset of spoilage, especially when the equilibrium relative humidity of the air trapped in the core is greater than 65% to 70%. Sporulating fungi respire heat, water vapor and CO2. The densely packed spoutline traps the first two, which accelerates mold growth and deterioration — neither is easily detected. Fortunately, CO2 is a gas that is not trapped and can be detected in the headspace of a silo or warehouse.
Coring and un-peaking
Spoutlines in silos can be managed by partially unloading the silo until a sufficient amount of grain is removed from the core, and the peak is inverted into the surface of the grain mass to about one-third to one-half of the diameter. This is possible because each flat-bottom silo has a center discharge gate directly under the center fill spout. The same applies to hopper-bottom silos. This best management practice of coring removes the center column of grain that contains the highest concentration of fines. An additional benefit of partially leveling the grain surface is the removal of the grain peak, which is similarly susceptible to self-heating and deterioration like the core.
Coring and un-peaking a grain mass also helps improve uniformity of airflow during aeration. Unfortunately, most flat-bottom warehouses do not have discharge gates built into floors. Instead, they are typically unloaded using front-end bucket loaders. When the amount of fines in corn to be stored in such warehouses is too high, it should be reduced to a more manageable level by pre-cleaning.
Cleaning and screening
The amount of fines — and potential problems they cause during storage — can be reduced by cleaning corn using screeners. Gravity screeners with capacities as high as 1,000 tonnes per hour can be incorporated into existing grain conveying systems. They are designed to work solely off gravity after discharge, for example, from a bucket elevator. A properly sized set of screens allows smaller particles (fines) such as broken kernels, dust and dirt to drop through, and the rest of the corn considered clean material to be discharged to storage.
It should be noted that cleaned corn does not mean it contains zero percent of fines or foreign material. Instead, a properly sized gravity screener can be adjusted and operated to reduce the amount of fines in cleaned corn so it will better preserve during storage especially in a flat-bottom warehouse (see Figure 2).
Monitoring stored grain
Monitoring stored grain is key to successfully preserving grain quality and quantity. Unfortunately, relatively few silos and even fewer warehouses of importers, grain handlers and feed millers are equipped with sensing technologies and monitoring systems that can track temperature, moisture content, and COs.
Most storage problems are caused by moisture infiltration and insect penetration into the storage structure, which cause biological respiration from germs, fungi, and insects to increase. Thus, the early detection of spoilage is essential to keep them at levels where they do not cause grain spoilage and affect economic value.
Insects and molds are aerobic organisms that respire and release CO2 into the interstitial air of a stored grain mass. Upward moving convection air currents within the grain mass transport CO2 into the headspace of a silo or warehouse even from densely packed cores of fines.
Typically, ambient air has a CO2 concentration of 400 parts per million (ppm). Past research indicates that a stable grain mass has a CO2 concentration of 450 to 600 ppm. Higher levels indicate biological activity above normal. Thus, monitoring CO2 in the headspace air of silos and warehouses, or the exhaust air stream from aeration fans with handheld or permanently installed sensors is an effective and preferred monitoring tool for stored grain managers.
Concentrations of 600 to 1,500 ppm indicate on-set of mold growth, insect presence, or moisture infiltration. Concentrations of 1,500 to 4,000 ppm and beyond clearly indicate severe mold infection or insect infestation.
Dirk Maier is a post-harvest engineer with the Iowa Grain Quality Initiative at Iowa State University. He may be reached at firstname.lastname@example.org.