Monitoring CO2 in stored grain
July 8, 2016
Millions of dollars are lost worldwide due to mold (fungi) and insect-related spoilage during grain storage resulting in substantial economic losses for farmers and stored grain managers. Maintaining stored grain quality requires a combination of multiple tools and practices to ensure that the quality and quantity of grain entering the storage facility does not deteriorate over time. Despite decades of research and the availability of advanced monitoring technology, combating pre- and post-harvest mold infection and insect infestation during storage remains a challenge throughout the world. Grain has a finite shelf-life, and these biological organisms flourish based on how grain temperature and moisture are managed.
Molds are the primary cause of spoilage in stored grain. They can cause detrimental changes in appearance, quantity and quality, thus reducing the end use value for food, feed and biofuel. More importantly, some mold species can produce toxic substances known as mycotoxins. These are secondary metabolites produced by a group of molds that belong mainly to the Aspergillus, Fusarium and Penicillium genera.
Preventing these toxins from entering the food and feed value chains is a major concern of the global grain and feed industry. Worldwide, approximately 25% of food crops are affected by mycotoxins causing millions of tonnes of food stuff lost each year. As a result, grain producers (including smallholder farmers) and grain handlers (including processors and exporters) often suffer the consequences of reduced marketability of their commodities in the form of discounts or outright rejection both domestically and internationally.
Aflatoxin and other mycotoxins (such as fumonisin) can suppress the immune system, reduce nutrient absorption and cause stunting in young children, cause cancer, and are lethal to humans and animals in high doses. Human exposure is the result of ingestion of contaminated grain foods, or indirectly from consumption of food of animal origin previously exposed to aflatoxins in feeds (e.g., dairy products and eggs). The economic impact of mycotoxins includes loss of human and animal life, increased health care and veterinary care costs, reduced livestock production, disposal of contaminated foods and feeds, post-harvest loss of crops, and higher cost for regulatory programs directed toward mycotoxins.
The early detection of spoilage due to insects and molds during grain storage is essential to keep them at levels where they do not cause grain spoilage and affect economic value. Grain sampling methods are tedious and time consuming, especially when sample analysis is involved such as mold isolation and enumeration, and insect identification. Informal surveys of farmers in the midwestern United States indicate that they check their stored grain only every four to six weeks, primarily visually and by odor. That is more than enough time for biological organisms to flourish and cause spoilage, especially when excess moisture has infiltrated the storage structure and dripped or condensed onto the grain surface. Instead of tedious sampling, more rapid methods are needed for early detection of spoilage so proactive management can prevent spoilage organisms in “real-time.”
How CO2 Sensing Works
Research conducted at Purdue University, Kansas State University and Iowa State University, as well as by INTA in Argentina, shows that CO2 sensing can be effectively used to monitor stored grain quality and detect early the onset of grain spoilage.
Insects and molds are aerobic organisms that respire and release carbon dioxide into the interstitial air of a stored grain mass. Upward moving convection air currents within the grain mass transport CO2 into the silo’s head space. Typically, ambient air has a CO2 concentration of 350-400 parts per million (ppm). Past research indicates that a stable grain mass has a CO2 concentration of 400-600 ppm. Higher levels indicate biological activity above normal.
This technology was tested and validated in 20 different sized grain storage silos in Kansas and Indiana during several storage seasons. The results clearly demonstrated that CO2 sensors can detect grain spoilage due to insects and molds usually about three to five weeks earlier than detection by traditional methods such as visual, odor, or temperature detection.
Unfortunately, temperature sensors on cables whether analog or digital will not detect heating due to mold growth and high insect density accumulation a few feet (or meters) away from the cable until the size of the spoiling grain mass is large enough to raise the temperature closer to the sensor. Given that CO2 moves with air currents much faster than heat conducts in grain, CO2 sensing overcomes the limitations of temperature cables and can give a more “real-time” indicator of the onset of grain spoilage. Once onset of spoilage is detected, the manager of a bulk storage facility, whether on or off farm, has a sufficient window of time to decide how to address the situation such as aerating, turning, or even selling the grain.
Evaluating CO2 Readings
Past research data and experience gained by early adopters of the technology clearly show that safe grain storage was observed at CO2 concentrations of up to 600 ppm. Higher concentrations indicate biological activity due to mold growth and/or insect development inside the grain mass whether stored in a silo, warehouse or ground pile.
Concentrations of 600 to 1,500 ppm indicate onset of mold growth most often triggered due to storage of grain above the safe storage moisture content, or moisture infiltration into the structure. Concentrations of 1,500 to 4,000 ppm and beyond clearly indicate severe mold infection or stored-product insect infestation. Aflatoxins and fumonisins were detected in grain samples collected from silos showing these higher CO2 concentrations. Additionally, heavy stored-product insect infestation of Cryptolestes pusillus and Sitophilus zeamais was observed in several silos with similarly high CO2 readings.
The observed data collected in silos on farms and at commercial grain facilities under real-world conditions confirmed that CO2 sensors were effective in early detection of spoilage in stored grain as well as confirming that stored grain was in stable conditions.
Placement of CO2 Sensors
Hand-held CO2 sensors can be purchased from numerous sources. Several vendors have begun selling CO2 sensors as add-ons to temperature monitoring systems. Sensors are either held in front of exhaust vents and access ports at roof level or are placed near these in the headspace of storage structures in updraft aeration systems. They are either held in front of aeration fans or are placed in aeration ducts at ground level in downdraft aeration systems.
A weekly reading during the warm season and a bi-weekly reading during the cold season suffice for a CO2 sweep. In order to take a representative reading, aeration fans need be turned on manually or automatically. A handheld sensor needs to be placed in the airstream soon after. The sensor(s) will show an increasing CO2 reading above the background level and reach a maximum value before decreasing again. Essentially the aeration fans flush out the CO2 from the stored grain mass which explains why the readings increase to a maximum before decreasing to a minimum. Depending on airflow rate and size of the storage structure, a CO2 flush and reading sweep will take several minutes. Afterwards, aeration fans can be turned off and CO2 will accumulate again until the next scheduled reading.
Key to interpreting CO2 readings is the maximum value reached during a flushing cycle, the length of time of the flushing cycle, and the change of both compared to the previous cycle. For example, if a flush results in little change above the background level of 400-600 ppm, the stored grain mass remains in stable condition. If the flush reaches a peak of 1,200 ppm and decreases to the background level within a 5-minute time interval but remained at the background level during the previous flush a week earlier, that may be a sign of the start of some biological activity. But it does not warrant any action at the time. If the following week’s flush reaches the same maximum level but the time interval increased to 10 minutes, the stored grain manager should take a look into the top of the storage structure and see whether there is any evidence of condensation on the underside of the roof or the grain surface, or of moth activity in the headspace.
If a week later the CO2 flush indicates a peak of 2,000 ppm and the time interval increased further, the stored grain manager should investigate for any evidence of grain surface crusting, odor from the flushed air, or flying and crawling insects at the grain surface. Unless a temperature cable sensor happens to be close to where the onset of spoilage is taking place, it is unlikely that an increased temperature would be detected at this point in time.
Past research indicates a delay of three to five weeks between detecting higher CO2 readings and higher temperature readings. This gives the stored grain manager sufficient time to consider options such as aeration to move a cooling front through the grain mass and determining whether temperature sensors pick up heat from spoiled grain before cooling off, or unloading some grain and determining whether additional coring of the grain mass reveals any breaking up of surface crusting or problems with flowability of grain.
An additional advantage of monitoring with CO2 sensors is with regard to deciding which storage structure to unload first. If higher CO2 readings are detected in one grain mass but the others remain in stable condition, the decision which one to unload and ship first is clear. This can save thousands of dollars in discounts or blending costs.
Carbon dioxide-based spoilage detection sensors are available and have been utilized successfully by stored grain managers for the past few years. Reducing grain spoilage lowers mycotoxin levels and the need to apply protectants and fumigants. It also minimizes chemical residues and foreign material in the food and feed supply.
Maintaining the quality and quantity of stored grain with this technology minimizes storage and handling costs. This early warning system provides more timely information for stored grain managers to make better management decisions in terms of spoilage mitigation measures such as turning, aerating, and fumigating.
The early detection of the onset of spoilage helps in moving stored grain to market in order to avoid further quality deterioration. This technology complements other stored grain management tools and practices such as sanitation, application of protectants at the time of silo filling, temperature monitoring, insect and mold detection, and last but not least, costly fumigation.