“There is a misconception, very widespread, that one should not clean, dry or aerate the grain because it will lose weight. If there is one error that brings the chaos to a grain elevator it is that.”
Prof. Gustavo Sosa
Industrial Mechanical Engineer
Licensed Grain Inspector
MBA Project Management
SOSA – Engineering Consultants
First of all, I have to warn you that I am familiar only with the grain trading rules of Uruguay and Argentina. In other countries there are di erent rules and they may be applied di erently. Anyway, I think the procedure that I will describe is reasonable and can be understood by any vendor or client in case of disagreement.
There is a misconception, very widespread, that one should not clean, dry or aerate the grain because it will lose weight. If there is one error that brings the chaos to a grain elevator it is that.
All grain that enters or exits a grain elevator has some content of water and dirt. Water in grain is called moisture, and it is the sum of water on the surface of the grain, in its pores, and also that chemically bonded to other substances. The water that may be dried of grain is of the first two types. Dirt in grain is called foreign matter. It is the sum of things that shouldn’t be there. For example, bolts, sticks, bags, husks, actual soil dirt, fragments of grain, etc. Foreign matter has almost no commercial value, dificults aeration, may cause fires in dryers, may break processing and conveying equipment, and pollutes the environment. The ideal moisture content in grain depends on the specific grain and the final use. The ideal foreign matter content is zero, but it is not actually achievable, because every time you move grain it breaks a little.
Many people resist drying or cleaning grain because they think the weight they lose is tons of grain that they could have sold. They don’t realize that there are commercial standards in every country specifying acceptable contents of moisture and foreign matter. Each percentage point that you go over the limit, the buyer will charge you, making a discount on the price they pay. This discount amounts exactly to the tons of water and dirt you are trying to sell him. In this article I will teach you how those discounts are calculated.
Let’s recap the problems with moisture and foreign matter:
• A batch of grain that isn’t cleaned will fill a dryer with hulls and straws that will cause a fire in the dryer.
• A batch of grain that isn’t dried will demand continuous operation of the fans to prevent the temperature from rising.
• A batch of grain that isn’t aerated will get rotten and lost forever.
• Nobody will actually pay you for those extra tons of water and dirt.
So, ultimately, not conditioning your grain properly will mean that you lose all of it, and not just a few tons of water or garbage that you were planning to sell at grain value.
However, fear not, you wouldn’t have lost those precious tons of grain.
Imagine you handle wheat, which has a target moisture level of 14%. A farmer delivers to you 3.500 tons at 18% moisture. Now we will calculate how many tons you have to actually pay to the farmer. Of course you have to dry the grain to handle it properly, and you wouldn´t pay him for tons of water. That´s why you must apply “shrinkage” to the quantity delivered. This is, calculating how many tons of grain you will end with after drying it down to the com-mercial standard. The moisture measured by the testers (whatever the technology used) is absolute, not relative to a maximum. This is important to know because you may get confused with the terms absolute humidity and relative humidity as they apply to ambient air. No, the humidity we talk bout here is absolute. If an hypotetical shipment had 100% moisture it would mean they delivered to you tanker full of water. So, the water is the percentage measure as moisture. The rest is dry matter:
DM = W D*(100%-MD)
Where DM is the dry matter weight, WD is the weight delivered, and MD is the moisture delivered. We know the quantity of dry matter won’t change after drying, so we have this other equation:
DM = WT*(100%-MT )
Where WT is the weight at the target moisture and MT is the mois-ture set by the commercial standard, the target.
WT*(100%-MT ) = WD *(100%-MD)
will hold true in every situation. To calculate how many tons you will actually pay to the farmer, the equation is:
WT = WD * (100% - MD)/(100% - MT)
WT = 3500 * (100% - 18%)/(100% - 14%) = 3337
Now let’s say you have some 4800 tons of grain that have been stored for two years, and after so much aeration, the moisture is down to 11%. Do you think you lost money? Not so fast. Let’s apply the same equation to the grain that goes out of your facility:
WT = 4800*(100%- 11%)/(100% - 14%) = 4967
And there you find the 167 tons of water that you lost. It is true that recovering the moisture in grain is very difficult, almost impossible, because the capillars in the grain close after losing water and it makes absorption more difficult than drying. Wetting the grain with a hose will (more or less) only increase the water content in the surface and not in the grain itself. Anyway, any client who is not out to get you should be able to understand that you are actually delivering 4967 tons of wheat to him, and not 4800 tons. And if he doesn’t understand you shouldn’t be doing business with him in the first place. Just be glad that it is cheaper to find out this way than in discussions about credit terms. If, because the client is the God Emperor of Malbec or something like that, you HAVE to deliver at the target moisture, then the best practice is buying grain of higher moisture from someone else and mix it with yours. This practice is dangerous because a de cient mixing may lead to pockets of high moisture that will spoil an entire shipment, but it is better than using a hose. To find out how many tons of one and other you need, use this system of equations:
WA * (100% - MA) + WB * (100% - MB) = WC * (100% -MC)
WA + WB = WC
You fill the values of the equation with the data of the grain you can find in the market. For example, MC will be 14% in this case, because that is the moisture the client requests. WA is 4800, the quantity you have. MA is 11%. Some acquaintance has grain at 16,5% moisture. What is left to know is how many tons to buy from him and how many will you o er to your client.
4800 * (100% - 11%) + WB * (100% - 16.5 %) = WC * (100% - 14%)
4800 + WB = WC
I am assuming you know how to solve this, though, considering what so many people share on LinkedIn as “only for geniuses” perhaps I shouldn’t. It can be solved with any of different methods that you can review online at any website of high school algebra. The solution to this system is WB = 5760 and WC = 10560. It means you have to buy 5760 tons of grain from your friend in order to deliver the maximum quantity of grain (10560 tons) to the Emperor of Malbec.
The same concepts apply to foreign matter, the dirt that comes with the grain. I don’t want to bore you to death going through the same steps over and over. The only difference would be that you have to write DT (for example) for “dirt target” and DD for “dirt delivered” instead of MT and MD. The shrinkage by foreign matter should be applied first, because you are not supposed to dry or aerate dirt. Then you apply the shrinkage by moisture. First wash, then dry, like the laundry.
Just a final consideration. When you mix grains, you should do it at a special installation. Better than mediocre results can be achieved by recirculating in a hopper grain bin. The best results would be achieved using a paddle screw conveyor with metered feeding from both sources. Just throwing the two qualities of grain in the same bin does nothing, and recirculating in a flat bottom bin is pointless; you are just wasting energy.