Milling of Wheat Flour

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Milling of Wheat Flour
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST
The Cereal Kernel

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SEM Wheat kernel
Aluron Layer
The Aleurone layer is part of the
endosperm but contains almost no
starch

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Wheat Starch Granules

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Grain Cleaning
• There are two simple objectives in grain
cleaning
• Remove non-grain (e.g. non-wheat) material
• Remove grain not fit for milling

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Conditioning of grain
Conditioning
• To prepare the cleaned grain for milling, we begin
with the conditioning process.
• Defined as the application of water, steam, or heat to
the grain
Tempering
• After water is applied in the conditioning step, the
grain requires a rest period called tempering. The
tempering step allows water to penetrate the inner
endosperm.
• Tempering can last up to 24 hrs before milling.

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• Mellowing of the endosperm
– Flour extraction can be increased
– Power consumption / noise level of the Rollermills reduced and
– Flour ash content reduced
• Toughening of the bran
– Bran tends to break up less and remains in bigger pieces
– Large bran flakes can effectively be cleaned by the corrugated
rolls
– Less small bran specks in the flour
• Adjustment of the Flour moisture content
– Constant moisture level = constant milling conditions
– Constant moisture level = constant baking conditions
• Recommended tempering time, 1BK moisture:
• Hard Wheat: 24 – 36 (48) hours 16.0 – 17.0% (17.5%)
• Semi-hard Wheat: 18 – 24 hours 15.5 – 16.0%
• Semi-soft Wheat: 12 – 18 hours 15.0 – 15.5%
• Soft Wheat: 6 – 12 hours 14.5 – 15.0%
The target moisture content at 1st Break is influenced by:
• Hardness of the Wheat
– energy needed for grinding and moisture loss
• Maximum moisture content of the flour
– government or end user specifications
• Maximum moisture content of the bran
– storage problems
• Operational problems
– sifting performance / mold development

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Roller Milling
• Flour is extracted from grain in a series of steps called the gradual
reduction process
• Each step involves grinding the stock, followed by a sifting step.
•Stock is ground between two rolls of a roller mill
•Depending on which step the rolls could be corrugated or smooth
Sifting
• After the stock is ground on the roller mill we
sift the material to collect the similar sized
particles so that they can be ground again
• Every grinding operation produces flour, after
flour is removed by the sifter it is sent to a
finished product storage bin.

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Sifting principle
•“Overs” include the coarse material which does not go through
the openings in a bolting cloth.
•“Thrus or throughs” is the finer material which goes through the
openings in a bolting cloth.

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Composition of Milled products
Flour Mill By-products
• Screenings from cleaning system
• Bran from the break system
• Shorts from the residue system
• Red Dog from the low grade system
• Low grade flour from the low grade system
• Germ from the sizings system

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Bran
• Bran can contain 20-30% residual starch from milling
• May be sold in the raw form or can be pelletized at the flour mill (increase bulk
density)
• May contain a mixture of all mill by-products Bran, Shorts, Red Dog and Germ
depending on the flour mill design
• Bran buyers should know the constituents of the wheat midds or bran they are
buying
• Every flour mill will produce a unique by-product depending on the flour mill
design
Germ of Wheat
• Germ is the undeveloped embryo of the wheat kernel
• High concentration of protein and lipid
• Most often processed for human consumption
• Germ recovery rate from a commercial flour mill is ~0.5% of the
wheat milled
• Has a high value as a feed product because of its nutritive value
• High fat content leads to rancidity if not properly handled

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Shorts
• Small particle size bran pieces
• Most often sold as bran or as a mixture with bran
• Collected from the residue system
• Can have a high amount of residual starch if the flour mill is not
optimized
• Processing equipment called a shorts duster removes attached
flour particles
Red dog
• Flour rich bran particles left over from the low grade flour
system
• Consists of bran material slightly larger than flour
• Material that couldn’t pass through a flour sieve in the last
step of the milling process
• May be used as a binder in pelleting
• Can be sold for use in pet foods

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Low grade flour
• Flour sized particles from the last grinding step in the milling
process
• High ash content with little to no value in commercial baking
• Used for industrial products such as adhesives and pastes
• Used in extruded pet food such as dog biscuits and as a
binder in pelleting
Post treatment of flour
• Additions
– Oxidants to overcome the need for storage. It oxidize yellowish pigments
and increases the whiteness of flour
– Alpha-amylase: breakdown starch to dextrin, glucose and maltose.
Amylase containing flour holds less moisture and increase storage period.
– Chemical leavening agents
– Bleaching product: benzoyl peroxide (20-30 mg/kg)
– Chlorination

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Post treatment of flour
• Treatments
– Air classification
• Further size reduction of flour particles (smaller than 140 micron)
– Pin milling
– Easier with soft wheats
• Sieving not possible anymore
• Wind sifting – air classification
– Fraction 1: < 17 micron : protein rich
– Fraction 2: > 17 micron, < 35 micron : starch rich
– Fraction 3: > 35 micron : flour

Milling of Wheat Flour

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