Grain is often harvested wetter than is safe to store: to escape rain, birds and shattering, or to free the land for the next crop. But left wet, it heats, moulds and sprouts within days.
Drying removes the extra water, quickly and safely.
Done badly, it cracks kernels, kills seed, wastes fuel, or leaves wet pockets that spoil in the store.
In this video: what a moisture percentage really means, how much water you actually have to remove, why selling wet grain costs money, how dry is dry enough, the ways to dry, safe drying temperatures, and how to keep dried grain dry in store, with the sums worked out step by step. The advice is general and global; your local standards come first.
When someone says maize at fourteen per cent, they mean wet basis: the weight of water divided by the total weight of the grain. That is the number used in trade and in everyday talk.
Engineers often use dry basis instead: water divided by the dry matter only. It is convenient because the dry matter never changes during drying; only the water does.
The two numbers are not the same. Maize at twenty-five per cent wet basis is thirty-three point three per cent dry basis.
To convert, divide the wet-basis number by one hundred minus itself,
and multiply by one hundred.
Mixing the two up is one of the most common, and most costly, mistakes in grain handling.
In this video, every percentage is wet basis.
Because the dry matter stays the same, there is one simple formula.
Final weight equals the starting weight, times one hundred minus the starting moisture, divided by one hundred minus the final moisture.
Here is a worked example.
A farmer harvests one thousand kilograms of maize at eighteen per cent and must dry it to thirteen per cent. The dry matter is one thousand times eighty-two per cent: eight hundred and twenty kilograms.
Divide that by eighty-seven per cent, and the final weight is about nine hundred and forty-two and a half kilograms.
So about fifty-seven and a half kilograms of water must come out of every tonne.
Now a bigger one: ten tonnes at twenty-five per cent, dried to fourteen.
The dry matter is seven point five tonnes. Divided by eighty-six per cent, that leaves about eight point seven two tonnes. So one thousand two hundred and eighty kilograms of water come out.
And notice the trap: going from twenty-five to fourteen per cent removes not eleven per cent of the weight, but about twelve point eight.
This matters when you sell.
Suppose a buyer pays two hundred per tonne for maize at fourteen per cent. Your ten tonnes at twenty-five per cent are not worth two thousand. They are worth the dry-equivalent eight point seven two tonnes, times two hundred: one thousand seven hundred and forty-four, before any drying charge.
That is why grain is traded with moisture discounts.
The other option is to dry it yourself, and compare the cost.
Practical dryers use about four to seven megajoules of fuel energy per kilogram of water removed. At four and a half megajoules, removing one thousand two hundred and eighty kilograms of water needs about five thousand seven hundred and sixty megajoules. With diesel at about thirty-six megajoules per litre, that is roughly one hundred and sixty litres of fuel.
Put your own local prices into the same sum, and you can see which way pays.
For long-term storage in warm climates, cereals such as wheat,
maize and paddy rice are typically safe at around twelve to thirteen per cent. Oilseeds such as groundnut and sunflower must go lower, often seven to nine per cent, because their oil holds no water.
Grain a little wetter can be kept briefly, or in cool conditions.
Why those numbers?
Grain exchanges moisture with the air around it until it reaches a balance, the equilibrium moisture content.
Maize or wheat in air at sixty-five to seventy per cent relative humidity, at twenty-five degrees, settles at about thirteen to fourteen per cent. Moulds grow quickly when the air between the grains is above about seventy per cent humidity, so storage targets sit just below that. It also means well-dried grain in humid air slowly takes moisture back, unless it is sealed.
But do not overdry. It wastes fuel, it reduces the weight you sell, and it can crack kernels.
Dry to the safe level for your storage period and climate, and no further.
Sun drying on clean floors, tarpaulins or roofs uses free energy. But it depends on the weather, it takes a lot of labour, and grain can be contaminated or rewetted. Never dry on bare soil.
Solar dryers heat air in a collector and pass it through the grain.
Batch dryers, such as the flat-bed dryers common for paddy in Asia, dry one load at a time. In-bin drying blows natural or slightly heated air through grain in a storage bin over days or weeks: slow, but energy-efficient where the air is dry enough.
And continuous-flow dryers, at large farms and elevators, move the grain through while the air passes across it, along it, or in mixed flow.
Air can only dry grain if its humidity is below the grain's equilibrium humidity.
Heating the air lowers its relative humidity and lets it carry more water, which is why most dryers heat the air. In a deep bed, a drying front moves up from the bottom:
grain below it is dry, grain above it still wet. If the air flow is too low or the bed too deep, the top layers can spoil before the front reaches them.
How hot can the air be? It depends on what the grain is for. For seed, keep the air at about forty to forty-five degrees, to protect germination.
For milling grain, such as wheat for flour or paddy for rice, use moderate temperatures, often below about sixty degrees.
Dry rice gently, with resting periods between passes, called tempering. Fast drying dries the outside while the inside is still wet,
and the stress cracks the kernels, giving broken rice at the mill.
Feed grain can take higher temperatures, often eighty degrees or more in continuous dryers.
And after drying, cool the grain to near the outside temperature before it goes into the store.
Warm grain in a cool store causes condensation, which rewets part of the bulk.
The reference method is the oven: weigh a sample, dry it in an oven at a set temperature and time, and weigh it again.
Electrical moisture meters are fast and portable, but each is calibrated for particular crops against the oven. The wrong crop setting, or very wet, frozen or freshly dried grain, gives errors.
At grain receival points, near-infrared analysers measure moisture, protein and oil together.
So use the right crop setting, take samples from several places,
and check your meter against a reference now and then.
A good store keeps grain dry, cool, clean, and protected from insects,
rodents and birds, and lets you inspect it. Grain must go in clean and dry, into a store cleaned of old grain and insects.
The options range widely.
Clay and mud bins, woven baskets, cribs and bags are well adapted to local materials, but vulnerable to rodents, moisture and insects.
Improved household stores include metal bins,
drums, mud-brick bins with damp-proof bases,
and metal silos, promoted widely in Central America and East Africa, which are airtight when sealed.
Hermetic bags kill insects by using up the oxygen.
Steel and concrete silos hold grain in bulk.
And silo bags, long sealed tubes laid in the field holding about two hundred tonnes each, are widely used in Argentina for temporary storage.
Bulk grain needs aeration with cool outside air.
Otherwise temperature differences drive air currents that carry moisture from warm grain to cooler grain, where it condenses,
often near the top. To size a silo, multiply its volume by the grain's bulk density. A silo six metres across with ten metres of grain holds about two hundred and eighty-three cubic metres; at seven hundred and eighty kilograms per cubic metre for wheat, that is about two hundred and twenty tonnes.
To recap.
Moisture in trade is wet basis.
Final weight equals starting weight times one hundred minus the starting moisture, over one hundred minus the final moisture. Going from twenty-five to fourteen per cent removes about twelve point eight per cent of the weight.
Store cereals at about twelve to thirteen per cent, and oilseeds at seven to nine. Dry promptly, match the air temperature to seed, milling or feed use, cool before storing, measure with a calibrated meter,
and aerate bulk grain.
Our next video covers the moulds that make toxins in wet grain.
For the full guides and worked sums, visit laisha.ai.
And before you go: Laisha Lens number one is out, our free guide to November, for every climate zone on Earth.
North of the equator, the cool-season crops go in:
winter wheat and garlic in the temperate north,
wheat, barley and lentil on the autumn rains of the Mediterranean,
and on the plains of South Asia the whole rabi season,
wheat, mustard, chickpea and potato.
South of the equator it is spring, and the tropical south sows soybean, maize, cotton and rice on the first rains.
Lens also covers crop residue without fire, the pests worth walking the field for, and a study plan for agri students.
Open My farm's November, choose your zone and up to three crops, and it builds your field jobs, week by week, with the pests to scout for. It is free, and needs no account. Our free November Lens, with your farm's month plan, is on laisha.ai slash lens.
From all of us at Laisha: may your fields do well this season. Thank you for watching.
