World Grain - November 2022 - 60

GRAIN OPERATIONS: DETECTING SPOILAGE WITH TEMPERATURE SENSORS
Table 2
Date
1-Jan
8-Jan
15-Jan
22-Jan
29-Jan
5-Feb
Source
39.2
44.6
50.0
55.4
60.8
66.2
1 ft
39
40
42
44
46
49
2 ft
39
39
40
41
42
43
3 ft
39
39
39
40
40
41
6 ft
39
39
39
39
39
39
9 ft
39
39
39
39
39
39
Predicted temperature cable readouts for sensors located 1
foot (0.3 m), 2 feet (0.6 m), 3 feet (0.9 m), 6 feet (1.8 m), and
9 feet (2.7 m) away from the self-heating corn compared
to the temperature at the source of self-heat over a fiveweek
period in the middle of an Iowa winter. Temperatures
recorded in Fahrenheit.
Bold italicized temperatures show changes by at least 1 degree F over time.
age even after five weeks. Temperature monitoring systems
that claim to have a sensing radius in this range would therefore
give a false sense of security to stored grain managers.
Even at just 1 foot (0.3 m) of distance from the heat source,
there is considerable difference between the hot spot temperature
(solid black source line) and the temperature reading by
the sensor (orange 1 foot line).
Table 2 (left) shows the predicted temperature cable read0.2°
F (0.1° C). Most grain temperature monitoring systems
do not record temperature to the tenth of a degree and thus
grain managers would not notice a full degree temperature
change until after four weeks.
It would take at least five weeks before the grain manager
may notice that temperature has increased for two consecutive
weeks by a total of 2° F. Only then they may consider acting to
mitigate self-heating grain. During this five-week period, the
grain in the center of the hot spot would have increased by 20°
F to 25° F (11° C to14° C). Even with careful documentation
Self-heating grain is most commonly
caused by biological activity due
to molds. Preventing high ERH in a
stored grain mass is the best way to
prevent mold development.
and weekly examination, a grain manager cannot know how
close cable sensors are to the hot spot. Most importantly, this
implies that one cannot assume that a 1° F increase in a temperature
cable sensor reading is indicative of a hot spot 1° F
warmer than the surrounding grain. In this example, recall that
the hot spot was 3 feet (0.9 m) from the sensor, yet it took about
five weeks to detect it.
Figure 1 depicts the temperature change over time for sensors
on one cable at different distances from the self-heating
grain source. The distance from the sensor is a critical factor
in how quickly the hot spot can be detected as a function of
heat conduction throughout the stored grain mass.
The figure shows that at 6 feet (1.8 m) and 9 feet (2.7 m),
there is no change in temperature evidence of detecting spoil60
ings
for sensors located at the various distances from the selfheating
corn. Temperatures are rounded to what monitoring
systems would typically indicate and would form the basis for
a grain manager to make stored grain management decisions
such as turning on aeration fans to cool self-heating grain.
It is important to remember that when looking at the data
presented in Table 2, all sensors are sensing the heat from the
same hot spot. The only difference is the distance from the hot
spot to each sensor. Recall from previous installments of this
series of articles that much of the grain mass is not within 3
feet (0.9 m) for most recommended temperature cable configurations.
Thus, the ability to detect self-heating grain depends,
at least partially, on chance.
In the time it takes for a hot spot to be detected, the selfheating
grain continues to increase in temperature to 66° F (19
C) in this example. If not detected and not mitigated (such as
with aeration cooling), it will continue to increase in temperature
and volume and cause heat damage to the stored grain. In
the worst-case scenario, it could also result in a grain bin fire.
Summary
Self-heating grain most commonly is caused by biological
activity due to molds. Preventing high ERH in a stored grain
mass is the best way to prevent mold development. If mold
does develop, the heat moves slowly through the grain mass.
It is important to track warming and cooling of stored grain
by checking temperature cables weekly during warm weather
and every other week during cold weather periods. Changes in
temperature should be compared to previous weeks.
Unexplained increases in temperature may indicate spoilage.
However, reading temperature cables provides limited
protection because of the small volumetric sensing range of
temperature cables. If the source of self-heating is as little as 3
feet (0.9 m) away from a sensor, it may take four to five weeks
to detect a 1° F to 2° F change.
If grain managers do not appreciate how slowly heat moves
through a grain mass, they may have a false sense of security.
In the final installment of this series of articles, we will cover
CO2
monitoring and its effectiveness in early detection of biological
activity.
Dirk E. Maier is a post-harvest engineer with the Iowa Grain Quality Initiative at
Iowa State University. He may be reached at dmaier@iastate.edu.
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November 2022 / World Grain / World-Grain.com
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World Grain - November 2022

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