Early spoilage detection

by Arvin Donley
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It has been established in laboratory and field trials that spoiling grain produces high carbon dioxide (CO2) levels compared to good quality grain, so it is important to monitor stored grain for elevated levels of CO2 to minimize spoilage.

Why is this so important? During his presentation, “Monitoring CO2 in Grain Bins: Putting the Research to Work,” at the 2011 GEAPS Exchange in Portland, Oregon, U.S., Bob Marlow, regional operations manager of The Andersons, said it is estimated that that over $1 billion is lost worldwide each year due to grain spoilage.

“In the U.S. alone, there is well over 20 billion bushels of grain stored each year, so maintaining grain quality is essential,” said Marlow, who oversees three grain facilities in Indiana with nearly 19 million bushels of storage
capacity.

Elevated levels of carbon dioxide (CO2) in localized pockets of a grain mass are attributed to a combination of factors such as moisture content, presence of molds, insect infestation and temperature.

He said, in general, safe grain storage conditions are found at relative CO2 concentrations between 400 and 500 parts per million (ppm). Levels that are consistently around 1,000 ppm means a problem could be occurring and readings of 3,000 ppm or greater means that spoilage is certainly occurring.

“We can’t tell how many bushels are affected, but that something is going on in the way of spoilage,” he said. “Research is now being done to see if we can correlate the amount of spoilage to a certain level in parts per million.”

Marlow said detecting and monitoring CO2 levels in the headspace of a storage structure or at the aeration fan outlets can give an early warning about grain spoilage which may otherwise go undetected with conventional temperature cables. These “hot spots” may occur away from temperature cables, making early detection of spoilage difficult.

“Temperature cables have been used for years and will continue to be used for years,” Marlow said. “Temperature cables are a valuable tool in our business.”

The problem, Marlow said, is that temperature cables can detect heat, but only if they are located at relatively close range to the “hot spots.”

“Grain is an excellent insulator so it can take a long time for heat to transfer through the grain where the temperature cables can detect it,” he said.

While CO2 monitoring is not a brand new technology, its application in grain facilities is a relatively recent phenomenon.

Marlow noted that the major constraint of expensive, inaccurate and cumbersome CO2 sensors from years past has been overcome through technological advances, which have made accurate and durable CO2 sensors available at a reasonable cost.

Handheld and fixed CO2 sensors are now used in order to get an early warning about spoilage conditions in a grain mass so that stored grain managers can take corrective actions such as aerating, turning, fumigating or selling grain.

HANDHELD VERSUS FIXED SYSTEMS

If a grain facility chooses to implement CO2 monitoring as a quality control measure, it then must decide whether to use handheld or fixed monitoring systems.

The initial cost of handheld units is much lower, but they do require some training of personnel and standards should be established on how to use them. Marlow noted that the accuracy of handheld units can be affected by wide temperature swings and, in particular, extremely low temperatures.

Perhaps the biggest disadvantage of using the handheld versions is that they are more time consuming than using fixed sensors.

“One thing about using handheld sensors is it requires that you record a lot of data over a long period of time,” he said.

Fixed sensors are typically mounted in the headspace of bins and on discharge fans.

The biggest advantage in using fixed systems is their convenience, Marlow said. They are typically monitored by an outside source and the potential for human error is eliminated.

“You don’t have to worry about somebody forgetting to take the readings,” Marlow said. “They are continuously gathering data for you. The initial cost is significantly more than a handheld sensor, but they give you more flexibility.”

FIELD TRIAL

A paper entitled, “Monitoring Carbon Dioxide Levels for Early Detection of Spoilage and Pests in Stored Grain,” authored by researchers at Purdue University, outlines the results of a field trial conducted at The Andersons’ facility in Delphi, Indiana, U.S.

The trial was conducted from April through September 2005 in three of the facility’s five large steel storage tanks that contained corn (maize), and from January through May 2006 in two ground piles.

In the steel tanks, CO2 sensors were placed in the headspace and in the fan exhaust air stream of each tank. Four sensors were installed in the headspace through the four roof vents and another four sensors were installed on the grating of the exhaust air outlet. One additional sensor was installed at the eave of one of the tanks to monitor the ambient CO2 level.

For the oblong pile of grain, four sensors were installed at the fan exhaust air streams. Each pile had two sensors on two fans monitoring the exhaust air on a continuous basis. For the round pile, CO2 sensors were installed at the exhaust air outlet of four low-speed fans and four high-speed fans. One additional sensor was installed near the radio to monitor the ambient CO2 level.

The data collected in one of the storage tanks (Tank 54) showed the benefit of using CO2 sensors for early detection of spoilage. The CO2 levels in the headspace of this tank steadily climbed from 500 ppm to 5,000 ppm by the second week of June, while the headspace temperature increased to 70 degrees F. One of the sensors showed an early peak of 3,500 ppm in the first week of May while the other two sensors indicated about 1,000 ppm. This was probably due to increased spoilage activity at a localized spot below the grain surface nearer to Sensor 2 than the other two sensors.

Compared to the other two tanks, the corn in this storage unit was of better initial quality with only 3% damage compared to 17% for the other two tanks. Researchers said the increased CO2 levels in this tank compared to the other two tanks could not be satisfactorily explained.

The CO2 sensors at the air outlets at the bottom of the tank showed a peak of 2,000 ppm in the second week of May, but after aeration was started May 11, the levels dropped to the 400 to 600 ppm range for several weeks before starting to climb and reaching a peak of 4,200 ppm the second week of June. Similar spikes in CO2 were observed throughout the rest of the summer but were reduced through aeration.

Based on data collected from January to May 2006 for the round pile, CO2 levels did not increase noticeably at most locations that CO2 sensors were installed. CO2 levels of 350 to 500 ppm measured at the eight locations of the round pile were almost the same as the ambient CO2 levels from January to May.

CO2 levels measured from the oblong pile indicated high concentrations at one location (Fan 3 on the west end of the pile). CO2 levels measured monthly at Fan 3 showed a slight increase from about 1,000 ppm to over 1,200 ppm, while CO2 levels at the other three fans remained steady, between 500 to 850 ppm. Upon unloading of the grain, quality analysis confirmed higher levels of damaged grain located near Fan 3.

The trial showed that temperature cables alone might not be a reliable indicator of grain conditions in storage structures. The CO2 monitoring technology was confirmed as a viable alternative to temperature-only monitoring for “early warning” of grain spoilage, especially in outdoor ground piles where temperature monitoring is not an option.

“Our results also showed that CO2 levels correlated with growth of fungi species known to cause visible mold damage on grain kernels,” the Purdue researchers wrote. “Thus, increasing CO2 levels can give operations’ managers important information for making better decisions to protect grain quality and prevent early spoilage.”

POTENTIAL LIFE SAVER

“I think it’s really important to take a hard look at this new technology,” Marlow said. “It’s one of the best things to come along for monitoring grain conditions. Keep in mind that it won’t replace what you’ve been doing before. It doesn’t mean you should get rid of temperature cable systems, but rather you should use it to supplement those systems. It’s another thing in your tool box that you can use on the grain quality management side.”

Besides reducing grain spoilage, Marlow said CO2 monitoring technology can also keep employees from entering storage tanks, which ultimately saves lives. He recalled how years ago it was standard practice to walk on the surface of grain to try to detect spoilage problems.

Unfortunately, despite intensive educational efforts by the industry, these types of dangerous practices are still taking place. In the United States in 2010, there were 51 grain entrapments resulting in 26 deaths.

“When there’s an indication of trouble, you have the ability to react earlier than you would with traditional methods, before the temperature cable reacts,” he said. “If you do that you save a trip inside the bin. That means you or someone you could avoid becoming a statistic.”
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