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.
It is becoming increasingly difficult to meet the food requirements of a growing global population. In spite of the need for additional food, it has been estimated that 50-60% of grain is lost after harvesting, at a cost of about $1 trillion per year.
A constant supply of grain is provided all through the year by keeping the grains in long term storage after they are harvested. It is crucial to maintain the quality of stored grain in order to prevent economic losses for Farmers and also to ensure the quality of the final food products.
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.
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.
Monitoring Stored Grain
Farmers are advised to conduct a weekly check on their stored grain in order to look out for signs of spoilage. Grains are traditionally checked both visually and by odor. Grain sampling can permit earlier detection of molds and insects, but these methods can be time-consuming and tiresome. Simple, rapid methods are required for early detection of spoilage and to prevent grain losses.2
When molds and insects grow and respire, they produce CO2, heat and moisture. Temperature sensors detect increases in temperature caused by insect infestation or mold growth, thus indicating the existence of grain spoilage. However, they are not able to detect temperature increases brought about by infestation unless the infestation is within a few meters of the sensors. CO2 sensors are capable of detecting the CO2 produced by insects and molds during respiration. As the CO2 gas moves with air currents, CO2 sensors will be able detect infestations that are located far away from the sensor than temperature sensors. CO2 measurements are thus considered to an important part of the toolkit needed to monitor stored grain quality.2
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 monitoring can be employed for monitoring the quality of stored grains, and for early detection of spoilage in stored grains.
Safe grain storage generally results in CO2concentrations below 600 ppm, while concentrations of 600-1500 ppm indicate the onset of mold growth. Severe infestations are indicated by CO2 concentrations more than 1500 ppm, and these concentrations can also represent the presence of mycotoxins.
CO2 measurements can be performed in a rapid and effortless manner and can detect infestations 3-5 weeks earlier than temperature monitoring. After spoilage has been detected, the Manager of the storage facility will address the problem by aerating, selling, or turning the grain. Additionally, CO2 measurements can help in deciding which storage structure should be unloaded first.
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.
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.