Grain Quality Fact Sheet #48

October 9, 2002

Purdue University

Cooperative Extension Service

West Lafayette, Indiana

Reconditioning Overly Dry Soybeans

Dirk E. Maier, Agricultural & Biological Engineering, Purdue University

Michael D. Montross, Biosystems & Agricultural Engineering, University of Kentucky

Using aeration technology to manage the moisture content of a stored grain mass for the purpose of raising its moisture content has long been a controversial subject. Frequently, soybeans are harvested at low moisture contents (8 to 10%), and during artificial drying, corn is frequently overdried. Crops sold at less than market moisture weigh less and thus provide less revenue than crops sold at market moisture. Any moisture added back to overdried grain increases the weight of the grain sold. Direct addition of water to any grain for the purpose of increasing its weight for marketing is considered an illegal adulteration by U.S. regulatory authorities. Incidental addition of moisture during aeration and intentional conditioning of grains and oilseeds to optimum moisture levels for processing have not been challenged.

There are significant economic incentives to recondition grain to higher moisture contents for producers and elevator managers. (Figure 1). Conditioning of low moisture grain during periods of high humidity is economically desirable but has been considered by many as technically infeasible. A temperature front moves through grain about 20-30 times faster than a drying or wetting front. Thus, a typical aeration airflow rate of 0.1 cubic feet of air per minute per bushel of grain (cfm/bu) that is adequate to complete a temperature change in one week of fan operating time may take six months to complete a desired moisture change throughout the same lot. However, research reported by Purdue University and the University of Minnesota shows that it is technically feasible to increase moisture contents in grains and oilseeds using automatically controlled aeration systems within a shorter time period.

Figure 1. Economic incentive for adding moisture to soybeans (assuming a constant test weight of 60 lb/bu).

A primary motivation for research into the conditioning of grains and oilseeds stems from the need of processors of popcorn, food corn, soybeans, and other crops to achieve moisture contents that are optimum for processing. For example in popcorn, the popping volume is maximized when kernels are uniformly conditioned to around 13.5% moisture, while soybean processors prefer an optimum moisture content around 10.5% for the flaking of beans for oil extraction. A recent Purdue University study confirmed that overly dry soybeans are undesirable for soybean crushers because they result in poor cracking and flaking performance and lower oil extraction yields.

Aeration Systems and Fan Controllers

Aeration is the forced movement of ambient air through stored grain to decrease or increase the grain temperature to the desired level. Although standard design airflow rates of 0.1 cfm/bu or less are generally too low to significantly change grain moistures by more than 0.5 percentage points, excessive aeration can shrink grain, or cause swelling of grain kernels near the air inlet.

The primary conditioning technology available to farmers and elevator managers is the use of forced ambient air from drying or aeration fans installed on grain bins, tanks, flat storages, and concrete silos. The success of a conditioning strategy to achieve a significant moisture change in a bulk of grain depends on the right combination of aeration system design, airflow rate, air and grain conditions, available time, and direction of airflow.

As grain is aerated, its moisture content gradually comes into equilibrium with the surrounding (interstitial) air relative humidity (r.h.). If air temperature increases while r.h. is constant, the grain’s equilibrium moisture content (EMC) will decrease. If r.h. increases at constant temperature, EMC will increase. Knowing the relationship between EMC and air conditions is important in properly managing aeration systems to prevent overdrying, condensation, or absorption.

Aeration based on the EMC of grain is critical for achieving the conditioning objective. A microprocessor (or computer) can be used to calculate EMC from the measured ambient temperature and relative humidity. EMC equations for corn and soybeans are available. Microprocessor- and computer-based aeration controllers are commercially available and can be programmed to achieve a specific target moisture content either by operating fans to reduce or increase the average moisture in the grain mass. The success of such a strategy depends primarily on exposing the grain to the right combination of ambient conditions (temperature and r.h.) for a sufficient length of time.

In order to accomplish a desired outcome, a microprocessor-based controller must reliably sense the air temperature and r.h. to determine the EMC, and be able to provide the right amount of fan operating time for the airflow rate of the system to produce the desired grain temperature and moisture. These sophisticated control strategies require not only reliable sensors that are regularly calibrated, but also programmable microprocessors that are well understood by the user.

A New Approach to Reconditioning

A new approach to reconditioning overly dry soybeans was evaluated as part of a Purdue University research experiment. It involved directing the airflow through the grain from the top to the bottom. This was chosen for several practical reasons. First, pulling air through the grain avoids any prewarming of the air due to fan compression, which would lower the actual air EMC. Second, during conditioning it was possible for the grain to swell. It was assumed that swelling of the grain could take place in the upper layers of the bin more readily than in the lower portions, which carry the weight of the grain above. Thirdly, any problem of spoilage or heating of the grain was expected to occur most readily in the rewetted grain. Managing such problems is easier when the rewetted layer is near the top of the bin than when it is near the bottom. Fourth, because conditioning fronts move slowly, rewetting grain from the top down is more effective because it allows for the partial unloading of the conditioned grain assuming there is a funnel flow pattern during unloading of the bin (last in - first out). If grain was conditioned from the bottom up, the benefit of rewetting would generally not become apparent until the last part of the bin was unloaded because of the relatively slow movement of a moisture front.

Site-Specific Weather Analysis

Four U.S. Corn Belt locations were investigated to determine the number of hours available to recondition soybeans. The primary concern with respect to setting certain temperature and relative humidity limits for moisture conditioning with an automatic fan controller is whether adequate fan run time is available to achieve the desired moisture content. Weather data between October and June for the years 1961 to 1990 were analyzed for the number of available hours when ambient conditions were such that the EMC for soybeans was above 13% and temperatures were between 26 and 60°F (Table 1). Indianapolis generally had the greatest amount of suitable fan run time and Des Moines had the least.

Table 1. Total run time, range, and standard deviation (hours) for four Corn Belt locations and storage periods when reconditioning soybeans (Air EMC > 13% and ambient temperature within 26 to 60°F) for 29 years of weather data (1961-1990).

10/1 - 4/1

10/1 - 6/1

11/1 - 4/1

11/1 - 6/1

Indianapolis, IN

2079

1702-2787

273

2512

2014-3199

307

1760

1302-2448

249

2019

1555-2690

276

Des Moines, IA

1614

816-2457

343

2054

1005-2949

400

1330

573-2016

304

1602

721-2309

337

Peoria, IL

1975

1249-2595

284

2438

1463-3196

336

1654

875-2195

275

1923

1020-2467

312

St. Louis, MO

1868

1249-2595

351

2234

1463-3196

424

1613

875-2195

315

1837

1020-2467

367

The limits on the temperatures were chosen to prevent air that was significantly below freezing from entering the bin in the winter and excessively warm air from entering and spoiling wetter soybeans during the spring. An ending date of April 1 was chosen because the number of available hours to run the fan decreased rapidly in the late spring/early summer (Figure 2). Extending the conditioning period beyond June resulted in limited additional run time.

Figure 2. Accumulated run time for the minimum, maximum and average years for Indianapolis, IN, when rewetting soybeans with an EMC >13% and temperatures between 26 to 60°F.

Simulated Site-Specific Conditioning

Using Indianapolis and Des Moines weather data for 29 years (1961 through 1990), the conditioning of soybeans in a typical farm bin and a typical commercial tank was investigated using a computer simulation model. Simulated conditioning started on October 1 and ended on April 1. Three unloading scenarios were investigated: a single unloading on April 1; three partial unloadings on December 15, February 1, and April 1; and six monthly partial unloadings. After each partial unloading of the top layer of grain, a level grain surface was assumed and the downward airflow rate was increased accordingly. The net economic gain was calculated as the value of the weight gain in conditioned grain quantity minus fan operating costs. During reconditioning of soybeans, the test weight was assumed to be 60 lb/bu with an initial moisture content of 10%. In all cases electricity was assumed to cost $0.07/kWh, and the price of soybeans was $7.00/bu.

Simulated Conditioning of Soybeans in a Farm Bin

The average net gain when conditioning soybeans in a farm bin was significant (Table 2). The market moisture content of 13% was not reached when the low airflow rate was used. However, the market moisture content was exceeded when the high airflow rate was used. The net economic gain increased when partial unloading was used, and the net gain was greater in the more humid Eastern Corn Belt (Indianapolis) than in the less humid Western Corn Belt (Des Moines).

Table 2. Net gain and final average moisture content when rewetting soybeans in a farm bin (30 ft diameter, 30 ft grain depth, 16965 bu) in Indianapolis and Des Moines at two airflow rates (0.13 vs 0.56 cfm/bu at 30 ft grain depth) and for three unloading schedules (results are average, range, and standard deviation over 29 years).

Low Airflow Rate

High Airflow Rate

Location

6 unloads

3 unloads

1 unload

6 unloads

3 unloads

1 unload

net gain

$/bu

0.133

0.084-0.208

0.029

0.13

0.082-0.206

0.028

0.0122

0.080-0.193

0.028

0.428

0.273-0.552

0.065

0.416

0.323-0.513

0.048

0.410

0.319-0.505

0.046

final MC,

% w.b.

11.8

11.2-12.7

0.37

11.7

11.1-12.7

0.36

11.6

11.1-12.5

0.35

15.9

14.0-17.4

0.80

15.8

14.6-17.1

0.61

15.7

14.6-17.0

0.58

net gain,

$/bu

0.096

0.013-0.170

0.035

0.081

0.020-0.173

0.031

0.074

0.019-0.164

0.029

0.356

0.158-0.500

0.073

0.334

0.147-0.480

0.068

0.329

0.163-0.478

0.066

final MC,

% w.b.

11.3

10.2-12.3

0.45

11.1

10.3-12.3

0.40

11.0

10.3-12.2

0.38

14.9

12.3-16.8

0.95

14.7

12.2-16.6

0.90

14.6

12.4-16.6

0.87

For example, in Indianapolis the average net gain was $0.13/bu, yielding an average moisture content of 11.7% over twenty-nine years with the low airflow rate and three unloads. In comparison, the average net gain was only $0.081/bu in Des Moines with an average moisture content of 11.1%. When reconditioning was done in a bin with a high airflow rate, the final average moisture content on April 1 was approximately 15.8% in Indianapolis and 14.7% in Des Moines. By using a higher airflow rate, reconditioning could have been stopped earlier when the average moisture content had reached the desired market moisture of 13%. The standard deviation (and thus variability) when reconditioning soybeans was always greater in Des Moines.

Simulated Conditioning of Soybeans in a Commercial Tank

The same general trends occurred when reconditioning soybeans in a commercial tank (Table 3). It is interesting to note that using three unloads instead of one unload at the low airflow rate led to a slightly lower net economic gain in both Des Moines and Indianapolis. This could be due to the fact that the airflow rate was too low, and as a result, it took too long to establish a moisture front. However, with one unload the moisture gradient within the bin was much greater than with three unloads. When the bin was unloaded six times, the net gain was greater than either the one unload or three unload cases. The airflow rate increased fast enough that the frequent unloading of the bin did not interfere as much with moisture fronts becoming re-established.

Table 3. Net gain and final average moisture content when rewetting soybeans in a commercial tank (60 ft diameter, 60 ft grain depth, 135700 bu) in Indianapolis and Des Moines at two airflow rates (0.11 vs 0.22 cfm/bu at 60 ft grain depth) and for three unloading schedules (results are average, range, and standard deviation over 29 years).

Low Airflow Rate

High Airflow Rate

Location

6 unloads

3 unloads

1 unload

6 unloads

3 unloads

1 unload

net gain

$/bu

0.100

0.039-0.155

0.032

0.086

0.050-0.142

0.021

0.089

0.054-0.145

0.022

0.218

0.109-0.310

0.053

0.202

0.134-0.295

0.039

0.198

0.137-0.292

0.038

final MC,

% w.b.

11.5

10.7-12.2

0.42

11.3

10.8-12.0

0.29

11.3

10.9-12.1

0.30

13.2

11.8-14.4

0.69

13.0

12.1-14.3

0.52

13.0

12.2-14.3

0.52

net gain

$/bu

0.063

0.0-0.124

0.027

0.050

0.005-0.118

0.023

0.051

0.01-0.125

0.024

0.154

0.032-0.266

0.051

0.133