Articles and reference material related to wheat

Wheat Conditioning

The purpose of the screenroom is to supply grain that is free from impurities, and in optimum condition for milling.

The final stage of the screenroom after cleaning, is to "condition" the wheat. To allow for the maximum extraction of flour and to ensure the quality parameters can be met, water is added to the grain. "Conditioning", "Tempering" or "Damping" are all terms used to describe this part of the process.
The addition of water toughens the bran to reduce powdering ( and subsequent darkening of the flour ) during the milling process. The bran layers are also loosened to ease the separation from the endosperm. It also helps to "mellow" the grain by softening the starchy structures of the endosperm.
  • The amount of water added at this stage will be dependant on several factors:
  • The variety of grain to be milled
  • The natural moisture content
  • If the mill is an elevator mill or pneumatic
  • The prevailing climate
  • Specification of the finished flour
Normal moisture levels in wheat vary from around 9% upto 14% dependant on variety, and these will normally need to be "conditioned" to between 15-17% prior to milling.
In cases where extremes of the above exist, the conditioning is normally done in two stages. The maximum percentage that can be added in one pass is around 5% due to the rate at which the grain can absorb the water.
After damping the grain is normally left to stand for anywhere between 6 and 24 hrs to allow the moisture to penetrate evenly through the grain (by osmosis). This time is dictated by the grain type and is shorter for soft wheats and longer for hard wheats. The theoretical rate of water addition as described above, is easy to calculate when setting up the water flow to a suitable mixer or agitator. However, in reality, the natural moisture content of grain is not uniform throughout a given parcel of wheat and these fluctuations will be carried through to the mill if a constant rate of water is added. This will lead to imbalance in the milling process causing inconsistency of quality and/or processing problems. To get round this, most modern flour mills will have automatic moisture measuring devices which control the water addition system. The nett result is a consistent moisture across the batch. These devices can be broadly split into two types - Feed Forward and Feed Back systems. Feed Forward systems measure the moisture of the dry grain and calculate the addition required and control the water flow accordingly. Feed Back systems measure the damped wheat and control the water addition accordingly.
The advantage of the Feed Back system is that it requires less instrumentation as the flow rate is irrelevant, but due to the added water being mainly on the surface of the grain immediately following damping, the accuracy is questionable. The Feed Forward system is far more accurate as it measures the dry grain both for moisture and density, combined with measuring the flow rate to calculate the required water addition rate. The disadvantage of this, apart from the cost of the system, is that if there is a malfunction and too much water is added there is nothing to check the resultant output.

Wheat Cleaning

The purpose of flour milling is to provide quality flour that is fit for purpose and free from any foreign matter or tainted in any way.

To make this happen, the miller must prepare the wheat for milling by ensuring that all foreign matter is removed prior to the milling process. Typically raw wheat can be contaminated by the following on arrival at the mill:
Diseased or shrivelled grain, straw, chaff, other cereal grains, seeds of all descriptions,string, paper, wood, stones, sand, dust, nuts, bolts, wire, glass. The larger contaminants and some dust are usually removed at intake to prevent potential damage to equipment, so the task of final clean is done in an area of the flour mill called the screenroom. The design and layout of screenrooms and machinery used in the modern flour mill can vary greatly from one country or mill to another, but one thing remains constant - they all employ the following five principles of separation to remove these impurities:
  • Separation by size
  • Separation by specific gravity
  • Separation by air resistance
  • Separation by natural peculiarity
  • Separation by shape
Separation by size
This is by far the easiest of the cleaning principles and is essentially sieving. By employing revolving, oscillating or reciprocating sieves, contaminants that are larger or smaller than the grain can be separated. A generic term for this type of machine would be a "Milling Separator" or "Drum Separator".


Separation by specific gravity
Where contaminants may be the same size as a grain of wheat (eg. a stone), separation by size is not possible. By means of a vibrating, inclined porous deck with an adjustable air flow being drawn through the deck, the denser material can be retained on the deck and will travel up the deck and fall off the back, whereas the lighter grain will float above the deck and will travel downwards and fall off the front of the deck. Various machines have been developed over the years which can make multiple separations and can therefore separate heavy grains as well as light grains from the original contaminant. This is particularly useful to split grains for separate treatments specific to their size or nature further down the process.
Typically these machines would have such names as Gravity tables, Dry stoners, Concentrators or Combinators.
Separation by air resistance
As well as the naturally lighter material such as dust, sand and chaff that comes in with the wheat, more dust and chaff is created by the mechanical transport of the wheat around the process.
By means of selective aspiration, a strong current of air is drawn through a falling curtain of wheat. This lifts the lighter contaminants out of the wheat due to its velocity. This dust laden air is then allowed to decelerate in a special expansion chamber which causes it to drop the impurities it had collected from the wheat.
Typically this machine would be called an Aspirator. Aspirators are normally installed immediately after any impact machine, as these machines generate dust, loosen dried mud from the crease of the grain as well as loosen bran layers.
Separation by Shape
Where the size and the density of a contaminant is similar to a grain of wheat, it may be possible to separate by shape. Most seeds, for example, are round and will "roll" better than a grain of wheat. There are two methods for removal of these impurities.
The first is the spiral seed separator which is built very much like a fairground spiral slide. When product is fed into the top of this inclined spiral, the round seeds will roll faster than the grain and by means of centrifugal force, will be forced to the outside edge of the slide. By having varaible flaps at the exit of the spiral it is possible to split the feed running on the inside of the spiral from that on the outside. The second method is the disc or cylinder separator which uses indented pockets in a rotating disk or a cylinder to collect or reject grain or impurities according to their shape. By selecting specific size pockets or dimples, a very fine selection or rejection of size can be controlled. Typically the selected shape is collected by the pockets as they rotate through the feed and are thrown into a collection conveyor for disposal or further treatment.
Separation by natural peculiarity
Normally this refers to metal contamination, but in recent times has also come to include colour.
There are many types of magnets used to do this from rotating drum magnets to cascade magnets, to bar magnets and even electro-magnets. The function of all of these is the same. The removal of metal, apart from the obviuos contaminant aspect, is the danger this poses to the process equipment in terms of damage or wear, and more specifically the potential to generate a spark leading to a dust explosion. With colour sorting it is now possible to identify not only different colours but also different shades of colours. By means of electronic "eyes" and pneumatic air jets it is possible to remove single grains from a stream of product that may be running through the screenroom at several tonnes an hour.

Strength & Hardness

Generally speaking, the strength of a wheat is dictated by the quality and quantity of the protein.

Strong flour produces a good elastic gluten at the dough stage and has good gas retention properties which is favoured for breadmaking as it produces loaves with good crumb structure and volume.
Weaker flour produces extensible (ie non-elastic) doughs and is preferred for biscuit and cakes.
The difference between hard and soft wheat relates to the texture of the endosperm. Hard wheats, when milled, produce gritty regular sized granules which sieve well and flow easily and starch damage is easy to achieve due to the brittle nature of the starch granules. A cleaner separation of bran and endosperm makes milling easier and produces greater extraction rates.
Soft wheats produce irregular granules which are soft in nature and tend to be squashed rather than crushed in the milling process; These characteristics lead to poor handling in the milling process, more difficult separation of bran and endosperm and lower starch damage capability.
Starch damage caused as a result of the milling process is the controlling factor in the amount of water that can be absorbed in the dough making process, and dictates the yields achieved in the bakery.
Wheat quality is therefore defined differently depending on whether you are a farmer, miller or baker. Growers are looking for high yielding wheat, millers look for hard, vitreous grain to aid high extraction, and the baker is looking for low enzyme activity flour with high starch damage (to improve absorbtion). The target for breeders is to develop wheat varieties that meet all these requirements.
So to summarise - the strength of a wheat dictates its' baking potential, and the hardness dictates its' milling potential. Both these characteristics are independently identifiable in the breeding process. The breeders therefore can develop strong/soft or weak/hard or strong/hard to meet the needs of millers and bakers at the same time.

Wheat Storage

Nearly all mills will have large scale storage facilities on site for their grain to ensure continuity of supply for running. It is not unusual to have a month or more of storage capacity on site.



This buffer stock is stored in flat storage sheds, or silos, prior to transfer to the mill. There are many ways that wheat arrives at the mill. Each country or region will have its' own methods that overcome the logistical obstacles that present themselves.

The location of the mill itself will also present its' own challenges. The merits and techniques of road, train or direct ship deliveries, or indeed whether silos should be steel, concrete or flat storage etc. are complete topics in themselves, and as such will not be dealt with here.

Whichever method of delivery is used, there will be impurities in the grain that need to be removed prior to storage. These impurities picked up during harvest, intermediate "elevator" storage or during transport, can range from rocks and boulders, vermin, chaff, straw, other grains such as oats, barley and rye to rogue varieties of milling wheats.

At point of intake it is first essential to verify the grade of wheat and its' quality data. Typically tests will be done to determine specific weight, moisture content, protein content, enzyme activity and impurity admixture. These tests are relatively quick to do, and are used to verify the product against a purchase contract. Claims may be made at this stage against the supplier if agreed specifications have not been met and a revised price negotiated. DNA testing or "electrophoresis" testing can be used to determine specific varieties or levels of contamination by other varieties if necessary, but this is a time consuming and expensive exercise and is not normal practice.

The next step is to remove any impurities that may damage equipment or pose a safety risk to the plant. A coarse sieving and some form of dust aspiration is usually employed to render the product operationally safe. Large impurities may damage transport elements or machinery, and grain dust is highly explosive if certain concentrations are exposed to an ignition source.

When stored under the correct conditions grain can be kept for years with no adverse affects on quality. However, it is important to remember that grain is a living organism, and as such needs to breathe. As part of this process the grain takes in oxygen and gives out carbon dioxide and a small amount of heat. The amount of respiration increases with the moisture content of the grain; The accepted safe limit of moisture content for storage of grain is 15% - above this level the rate of respiration and heat generation is such that germination of the grain begins, as enzymes are activated in the grain. This not only renders the grain almost useless for breadmaking, but also creates hot spots in the grain which can lead to spontaneous combustion.

Wheat Identification

The following section is taken from the DSIRCrop Research publication booklet entitled "Identification of New Zealand Wheat Cultivars"
Grain Characters
To enable accurate identification a minimum of thirty sound plump grains is required to cover variation. As grain character states are continuous in expression any attempt to identify a cultivar on a small sample or by observing only one or two states may be misleading. The examiner must build up a complete picture of the cultivar and not concentrate attention on a single grain which  may be atypical.
Where there is a range of expressions in a character state, the dominant state is listed first in the description. For example "grain shape: ovate to oval" indicates that most of the grains have an ovate shape.
Options:White /Red /Purple
Grain colour is the major key to wheat identification. This gives an immediate separation into one of three distinct groups. Within one colour, i.e red wheats, colour is not a reliable character for separation of cultivars. Most of the New Zealand commercial wheats are red, and further divisions of paleness or darkness are indicated in the grain descriptions.
Brush hair length
Options: Short/Medium/Long
Brush hair length is viewed from the top or side of the grain. Short hairs are usually barely visible to the naked eye, long hairs are at least 1mm long. Brush hair length is a consistently good identification characteristic for New Zealand wheat cultivars unless the grain has had much handling
Brush end profile
Options: pointed/medium/blunt
The profile of the brush end is viewed either from the top or the side of the grain. The character is related to the shape of the back and the base at this end of the grain, and to overall grain shape. For example a curved back and ovate shape will result in a pointed brush end.