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Grain milling

There are a number of cereals important as food sources, these include wheat, barley, maize, oats, rye and rice. Wheat and rye grain is milled into flour and is the prime ingredient used in the manufacture of products such as bread and biscuits. In some countries, rye is used for bread.

Barley is one of the oldest cultivated cereals and is mainly used for animal feed. Barley of certain quality is processed to malt and then used in industries such as brewing and distilling.

Maize is used as a source of starch but also as an ingredient in breakfast cereals and snack foods.

Oats are used for cereals, cakes, biscuits, but also for bread (Knaeckebroed).

Rice, along with wheat and maize is one of the major cereals of the world and is the basic food for much of the world’s population. Rice is pearled and sometimes precooked. The flour milling industry deals uses wheat as its main raw material.

The wheat grain has three main components, the enveloping skin (bran), the embryo (germ) and the endosperm, which makes up the majority of the grain and is used as a food source for the developing embryo. The object of the milling process is to separate the endosperm with minimal contamination from the bran and germ. The main operations associated with flour milling are: cleaning, conditioning, breaking, scalping, purification, reduction and dressing.

The incoming grain is transferred to bulk silos prior to further processing. Gas or heat treatment may be applied against insect infestation of the plant. The grain is first cleaned by passing through a series of screening, scouring, brushing and aspiration operations. These processes remove extraneous matter such as other cereals, stones, metal contaminants, chaff, loosened bran layers, seeds and dust. After cleaning, the grain is conditioned to optimise the milling process. This involves dampening the grain by the measured addition of water at a rate of between 1 and 5%; this immediately binds the kernels. Conditioning may be made by using steam and pressure. Conditioning has a number of functions, such as toughening of the bran, and thereby improves the separation of the bran from the endosperm, allowing the endosperm to be reduced more effectively in subsequent stages of the process.

The conditioned grain enters the break system of the mill, which consists of pairs of corrugated rolls which revolve in opposite directions and at different speeds. Five sets of break operations are common, with the aperture gradually decreasing and the corrugation becoming finer between subsequent sets. The grain is split by the break-rolls and the endosperm is scraped from the bran. A screening operation known as “scalping” and grading is carried out after each breakroll which separates:

• coarse particles, which contain the remains of the grain and some endosperm which is yet to be recovered
• flour
• particles of intermediate size, which contain chunks of endosperm called semolina or “middlings”, depending on their particle size and purity.
  

The semolina and middlings pass through a purification system, consisting of sieving and air classification. This removes particles of endosperm with bran adhering, for further treatment by finely fluted rolls elsewhere in the process. The semolina and middlings are then transferred onto reduction rolls, which are smooth rolls whose purpose is to crush the endosperm. As in the break section of the mill, there are a series of rolls and screening operations so that flour is screened off whilst coarser endosperm, retained on the sieves, passes to a further set of reduction rolls to be further reduced in size. At the end of the reduction process, most of the endosperm will have been converted to flour, the coarse bran has been removed and there will be a third stream of endosperm containing fine bran containing material which cannot be practically separated. There could be as many as 12 reduction stages in the process.

Flour is normally passed over a final redresser at the end of the milling process or after bulk storage to ensure the safety of the final product with respect to foreign body contamination. Flour may be dried and classified by sieves into fractions. The majority of flour is delivered in bulk; some 25% is bagged for delivery to bakers and other food manufacturers, 5% is packed for direct consumer use. Grains such as oats and barley, with a tightly adhering husk cannot simply be separated in a traditional wheat mill and are usually subjected to an abrasion process called “pearling” prior to milling.

Storage practice

See cereals storage.

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User comments

2008-03-20 09:32:22

Name: editor
A relation, established in Cotonou, Benin Republic, has received a mandate to source for a competent foreign company who can supply Flour, Wines, Canned Sardine, Canned Food, Cooking Oils, finished pharmacetical drugs, Rice, Dairy Products etc to a client who wants to buy in a large order. Please contact info@hyfoma.com

2009-01-13 11:29:41

Name: Rik
Wij vroegen ons af, wat de grootte van een mailkorrel is. Dus van een zo\'n wit dingetje die in de bekende kilozakken zit.

2009-01-16 15:05:53

Name: Editor
Beste Rik,
ervan uitgaande dat je meelpoeder bedoeld: die korreltjes hebben afmetingen tussen 0,5 en 5 µm.

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