Grain and post-harvest systems

Contents - Previous - Next


In the agrofood chain, harvesting is the stage between the phase of actual agricultural production and that of grain processing or, more broadly, treatment of the produce.


Whether it is done by hand or with the help of machines, the harvest should generally not take place until the produce has reached its optimal maturity.

After the harvest, it may be necessary to pre-dry the produce before the subsequent threshing or shelling operation.

The grain must be cleaned and dried, so that it can be stored or undergo further processing.

Grain can be stored in bulk or in bags, on the farms where it was produced, in collection centres, or with storage agencies.

Finally, the grain is sent from the warehouses to markets for sale to consumers, to smallscale food-processors, or to agrofood industries.

The sequence and interactions of these operations contribute to the formation of a complex system called the post-harvest system.


Harvest Manual Manual and mechanized Mechanized
Pre-drying Standing or in shocks Incribs or in shocks  
Storage in the ear In traditional In cribs granaries  
Threshing Manual Mechanized Mechanized
Pre-cleaning   Mechanized Mechanized
Drying Natural Artificial Artificial
Cleaning and sorting Winnowing in the wind Mechanized Mechanized
Storage in grains In traditional granaries In bags or in bulk In bags or in bulk
Processing Manual Mechanized Mechanized



A gramineous plant native to the tropical regions of Asia (China and Indochina), rice is the staple food of much of humanity.

Ears of rice which have reached physiological maturity are cut and left in sheaves in the field to pre-dry.

Then threshing is done manually or mechanically, to separate the paddy, that is the whole grain and its protective envelopes (hulls), from the straw.

When rice is harvested with a combine-harvester, cutting and threshing are simultaneous.

After threshing, the paddy often contains impurities (earth, stones, plant residues, etc.), and its moisture content is over 20 percent.

To preserve or hull the paddy, it must be pre-cleaned, and the moisture content reduced to about 14 percent by drying.

This can either be done naturally, by exposing the kernels to air and sun and turning them frequently; or artifically by a current of warm dry air in mechanical dryers.

Once dried, the paddy is cleaned and then stored (in bags or in bulk) in warehouses or silos.

The dried, cleaned paddy is ready to be milled, that is, to undergo the following treatments:

After sorting, the white rice may undergo other treatments - polishing or glazing (with a mixture of talc and glucose): the object is to enhance the product's commercial value and prolong its shelf-life.

The rice is then ready to be packaged and marketed.

After industrial milling, 100 kg of paddy yields about 60 kg of white rice, 10 kg of broken grains, 10 kg of bran and flour, and 20 kg of hulls.

Because rice is highly nutritious, it is mainly used for human consumption.

It is also used industrially in the production of alcohol, beer, starch, oil and other byproducts.

By-products such as broken grains and flours are often used as animal feed.

Rice-hulls are sometimes used as fuel and their ashes as fertilizer.



A cereal native to tropical zones of America, maize is one of the most widely cultivated gramineous plants in the world.

Maize can be harvested either in ears or in grains. The ears can be harvested by hand or by a mechanical corn-picker. The harvested ears are stripped of their husks and then shelled manually or mechanically.

In small-scale cultivation when the harvest takes place in the dry season, the ears (with or without husks) can be sun-dried, and then stored under cover.

In industrial cultivation, on the other hand, the maize is harvested only by machines (cornsheller or combine harvester) capable of supplying grains ready for drying or for sale.

At harvest time, particularly in the rainy season, grains of maize have too high a moisture content to keep well; therefore, before storage, the product must be dried, to lower the moisture rate to about 14 percent.

Artificial drying of the grain, by dry heated air, is done by machines in the collection or storage or in the processing units (mills, cattle-feed factories etc.).

The dried maize is cleaned, and then stored (in bags or in bulk) in warehouses or silos. The dried and cleaned maize is ready for sale or for further processing.

For human consumption, maize can be eaten as fresh ears or in the form of cakes made from dough obtained by cooking the grain. The flour or semolina from husking and grinding can also be eaten.

The processing industry also uses maize to produce oil and margarine, cattle-feed, beer, baby food, soap, glue and varnish.


Grain sorghum

Grain sorghum, also simply called sorghum (Guinea grass), is a graminaceous plant native to central and eastern Africa (Ethiopia, Sudan).

At physiological maturity, the panicles of sorghum are cut and left to dry in the sun.

The ears of grain thus obtained can be kept in traditional granaries or can directly undergo manual or mechanical threshing. Cutting and threshing can be carried out simultaneously by combine-harvesters, as with rice, but this is far less common in the tropics.

In hot climates with little rainfall, the sorghum harvest can be delayed until the grain has completely dried in the field.

If the grain has a moisture content higher than 13 percent, the product must be dried before it can be stored or treated.

Once dried, the sorghum is cleaned, then stored (in bags or in bulk) in warehouses or silos.

The dried and cleaned sorghum is ready for sale or for further processing.

Like maize, sorghum can be eaten in the form of flour or semolina from husking, then grinding.

The processing industry uses this cereal in the production of cattle-feed, beer, oils, glues or adhesives, etc.

Grain legumes


A pulse native to tropical America, the bean is very widely cultivated for its high nutritional value, owing to the high protein content.

The product can be harvested by hand, by pulling up or cutting down the plant, letting it pre-dry, and then threshing it manually or mechanically. Otherwise, a combine-harvester can be used.

After threshing, the beans often contain impurities and have a moisture content higher than 20 percent. Before they can be stored, it is necessary to pre-dry them, and then to lower their moisture content to about 14 percent by natural or artificical drying.

Traditional methods of storing grain legumes call for the use of jars, mud-walled cribs, or bottles, but the most satisfactory results are obtained by storing completely dry grain in air-tight containers (metal cans or drums, plastic bags, etc.).

Commercial storage is in bags in warehouses. Beans thus produced are ready for sale or consumption.



The groundnut, native of tropical and subtropical America, is a pulse grown chiefly because its grains are rich in oil.

The product is harvested by hand or machine, by pulling up the plant and allowing the shells to dry in the field for two or three days.

Then, when the moisture content is below 15 percent, the shells can be separated from their stems by manual or mechanical threshing.

In mechanized cultivation, these operations of pulling up and threshing can be done directly to fresh plants, with the help of special machines like grubber-threshers.

After threshing, the groundnuts are pre-cleaned, then artificially dried.

When the moisture content has fallen to 7-8 percent, unshelled groundnuts are ready to be stored in bulk in the open air (in pyramids known as "secco"), or in very dry and well-ventilated storerooms.

After drying, one can proceed directly to the shelling and then the cleaning of the groundnuts.

Once put into bags, the shelled groundnuts are stored in the open air or in warehouses.

In the processing industries (oil-mills), the shelled kernels are increasingly stored in silos. In the oil-mill, the dried grains, shelled and cleaned, are ready for oil-extraction treatments.

The oil is used for human consumption, as are the nuts themselves either whole (boiled in the fresh shell, grilled in the shell and either plain or salted, shelled and grilled or coated) or as peanut butter.

Oil-mill by-products, especially groundnut cake, make an excellent raw material for animal feed.



A native plant of North America, the sunflower is grown mainly for the richness of oil in its seeds.

The capitula (flowering heads) are harvested, by hand or machine, when the upper leaves of the plants are dry and the petals faded.

If the moisture content of the harvested product is higher than 15 percent, the heads must be pre-dried in cribs before proceeding to manual or mechanical removal of the seeds.

In mechanized cultivation, harvesting and removal of the seeds are carried out simultaneously by combine-harvesters.

When the moisture content is below 9 percent, sunflower seeds can either be stored directly or undergo oil-extraction treatments. If the moisture content is higher, the seeds must be artifically dried.

Given the excellent quality of the oil, sunflower seeds are used almost exclusively in oil-making.

In addition to its nutritional use, sunflower oil is used as a raw material in making paint colours, soap, and lamp oil.

The by-products of oil-making, especially feed cake, are an excellent livestock feed.

Contents - Previous - Next