WHEAT STARCH WET MILLING TM33-2e
WHEAT STARCH WET MILLING TM33-2e
| “Worldwide, wheat is a most important cereal crop with 600 million metric tones of grain per year - close to that of corn and rice. The high protein content of wheat grain makes it an important item of human nutrition. By-products of wheat processing are wheat germ oil, wheat gluten and wheat starch. Also the straw finds special applications as reinforcement of building materials and as bio-fuel”. |
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Starch makes up the nutritive reserves of many plants. During the growing
season, the green leaves collect energy from the sun. This energy is
transported as a sugar solution to the starch storage cells, and the sugar is
converted to starch in the form of tiny granules occupying most of the cell
interior. The conversion of sugar to starch takes place by means of enzymes.
Then, the following spring, enzymes are also responsible for the re-conversion
of starch to sugar - released from the seed as energy for the growing plant.
WHEAT VARIETIES
Wheat is a cereal plant of the genus Triticum of the family Gramineae (grass family). Modern wheat varieties are usually classified as winter wheat (fall-planted) and spring wheat - most of the wheat grown is winter wheat. Some ancient varieties of wheat like einkorn (T. monococcum), emmer (T. dicoccum) and spelt (T. spelta) are still being cultivated for specialty purposes. Triticum aestivum is by far the most important of all wheat species. Flour from hard varieties derived from bread wheat (T. aestivum) contains a high gluten content and is preferred in bakery products. The hardest-kernelled wheat is durum - macaroni wheat (T. durum); it is essential for the manufacture of pasta products.
WHEAT GLUTEN
Gluten is proteins of the wheat. Gluten forms long molecules insoluble in
water. This gives dough its characteristic texture and permits breads and
cakes to rise because the carbon dioxide released by the yeast is trapped in
the gluten superstructure. Gluten is particular important in the manufacture
of starch from wheat because gluten is a most valuable by-product
repre-senting half the turnover. In fact the starch is by some manufacturers
considered the by-product and gluten the main product. If the gluten is
extracted and gently dried in hot air at moderate temperatures it maintains
it's characteristics. If so it is designated "vital gluten". Vital gluten
may be added as a dry powder to flour otherwise low in gluten and thereby
improve the baking qualities of the flour. The Danish and Scandinavian
climate favours weak wheat of poor baking qualities. The gluten content is
low and the texture of the gluten is short. A remedy is mixing it with
French or Canadian wheat known for their better gluten quality. As an
alternative the baking characteristics may be improved by mixing it with
vital gluten powder. Commercial gluten is dried to minimum 90% dry matter
and a typical composition is:
- 70 - 80 % crude protein,
- 6 - 8 % crude lipids,
- 10 - 14 % carbohydrates,
- 0.8 - 1.4 % minerals.
Gluten in general is used as a meat extender in both food and feed. The
fermentation industry consumes large amounts of gluten and by acid
hydrolysis it is used for production of hydrolyzed vegetable protein and
glutamic acid. A gluten based meat analogue was invented by the
International Starch Group. It replaces up to one third of minced meat in
popular meat balls. Another invention combines emulsifiers and gluten into a
spray dried powder improving both baking quality of the flour and shelf life
of the bread.
A KITCHEN EXPERIMENT.
Gluten is an invisible integrated part of the wheat flour. To make it
visible and to illustrate its vital properties a small and simple experiment
may serve the purpose. Form flour into a dough with a little water. Knead by
hand a small lump of dough under a squirt of tap water. Apply water
sparingly while kneading. The white starch will run off with the water and
may be collected while the dough stays coherent. Gradually the starch is
washed out and the remaining dough is made up of pure gluten with a cohesive
chewing gum like consistence. Pulling the dough at this point will elongate
the lump until it bursts. The elongation before bursting indicates the
baking quality of the flour.
WHEAT STARCH
Wheat starch granules are divided in two groups by size, B-starch (15 - 20
%) is 2 - 15 µm diameter and the larger A-starch granules (80 - 85 %) are 20
- 35 µm. B-starch is conta-minated with pentosans, fibres, lipids and
protein to an extent requiring special treatment in the factory.
WHEAT GERM OIL
Wheat germ oil is contained at 8 - 12 % in the fresh wheat germ which is 2½
% of total grain weight. Its fatty acid composition (%) is:
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Due to its high level of linoleic acid (C18:2) wheat germ oil is used for
dietary purposes and in cosmetic preparations. Wheat germ oil is expelled or
extracted from the germ. Because the germ is removed from the endosperm
during the dry milling it is not a by-product from the industrial wet
milling of wheat.
RAW MATERIAL FOR STARCH
Wheat grain may be taken in as raw material as is the case with corn,
but typically the starch manufacturer prefers to buy flour from a flour
mill.
Composition of the wheat kernel
Bran
12½ %
Germ
2½ %
Endosperm 82 %
The number of parts by weight of flour that is produced from 100 parts of
wheat is termed the extraction rate. Flour extraction ranges from 73 to 77 %
resulting in an average mill feed production of about 25 %. It is apparent
that the mill feed contains, in addition to the bran, a significant portion
of the starchy endosperm.
Typical flour composition on dry matter basis
Moisture content: 13.5
%
Total protein content: 13 %
Fibre content: 1.0 %
Ash content: 0.75
%
The flour must be suitable for human consumption and it has to be
milled to a specific particle size distribution.
THE MARTIN PROCESS
The kitchen experiment previously described is also a demonstration of the
old Martin process still in use. The process resembles very much the
described experiment demonstrated by washing out the starch from a lump of
dough with tap water. The classic Martin process uses ordinary baking
equipment for the kneading and maturing of the dough. The combination of
kneading and time develops the gluten and makes it cohesive. Until matured
by kneading and time gluten will not allow the starch to be flushed out
without falling apart with losses of both starch and gluten.
THE SCANDINAVIAN PROCESS
Slurry processes (batter processes) are more industry friendly and make
closed continuous handling possible. Several variants have been practiced
over time, but the Scandinavian Process is by far the most elegant and
efficient. The Scandinavian process is based on wheat flour as raw material
and it is designed to process even weak (soft) Scandinavian wheat difficult
to process otherwise and nevertheless obtain vital gluten of excellent
properties. The Scandinavian process also works well with completely fresh
and unconditioned flour minimizing storage capacity requirement. In general
the Scandinavian process is very robust and of advantage to any wheat. Flour
is pneumatically conveyed from inter-mediate silos into a feeding bin
equipped with means to separate air and flour. The control system
continuously discharges flour into a stream of warm water. Water and flour
is mixed in-line and the slurry obtained is homogenized in a high-speed
in-line disintegrator. The homogenized slurry is right away separated into
the following fractions by a three-phase decanter (tricanter):
- Starch - Heavy phase
- Gluten - Middle phase
- Pentosanes - Light phase
THE STARCH FRACTION is the heavy phase containing the major part of
A-starch. It is re-slurred and refined - much in the same way as starch of
any other origin as described in "Starch Refining".
THE PENTOSANE FRACTION - the light phase from the tricanter - contains
various gums. It is preferably mixed with other by-products and used as a
wet feed. The wet feed may be dried, mixed with bran or sold as such.
THE GLUTEN FRACTION is the complex middle phase. It contains the gluten,
fibres, solubles, B-starch and some A-starch. After maturing of the gluten
these constituents are split into sub-fractions.
Gluten Maturing. Before separating the gluten fraction the
stream is carefully treated in a maturing reactor. The reactor is specially
designed for the maturing of gluten. During maturing the “gluten matrix” of
wheat flour is softened and bound starch granules are released. Glutenin and
gliadin proteins can now start to form long molecular chains i.e. gluten
formation can take place.
Gluten Agglomeration and Recovery. The gluten maturing step is
followed by a treatment in special gluten agglomerators. In the
agglomerators the matured gluten is combined into lumps formed of glutenin
and gliadi. The gluten lumps are screened off and washed on bend screens.
The wet gluten is dewatered on screw presses and dried. By gentle drying in
hot air in a ring dryer the gluten retains its vital properties. After
in-line milling and classification the product leaves the dryer ready for
packing and sale as Vital Gluten.
B-starch recovery. After gluten recovery the residual fraction
is separated on hydro-cyclones. The heavy A-starch goes in the underflow and
the lighter B-starch goes with the overflow.
A-starch recovered with the underflow is concentrated and combined
with the A-starch main stream.
B-starch is recovered from the overflow by special recovery cyclones
and dewatered on a decanter. The B-starch is dried in hot air in a ring
dryer or drum dried and used as pre-gelatinized starch.
Solubles. A clarifier removes the last bit of starch from the
overflow and only solubles and water remains. The clarified overflow leaves
the factory as an effluent to be disposed of by landspreading or biogas
digestion.
A-STARCH REFINING
Starch is refined by washing with fresh clean water. With
hydrocyclones it is feasible to reduce fibre and solubles including soluble
protein to low levels with a minimum of fresh water. To save water the wash
is done counter currently, i.e. the incoming fresh water is used on the very
last step and the overflow is reused for dilution on the previous step, and
so on. By using multi stage hydrocyclones all soluble materials and fine
cell residues are removed in a water saving process. The refined starch milk
contains an almost 100% pure starch slurred in pure water. Starch is among
the most pure of all agricultural products. Actually, purity is the most
important parameter in being competitive.
A-STARCH DEWATERING.
The purified A-starch milk is discharged to a peeler centrifuge for
dewatering. The peeler filtrate is recycled to the process. The dewatered
starch is batch-wise peeled off and discharged by gravity to the moist
starch hopper.
A-STARCH DRYING
From the moist starch hopper the A-starch is fed by a metering screw
conveyor into a flash dryer and dried in hot air. The inlet air temperature
is moderate. The dried starch is pneumatically transported to a starch silo
ready for screening and bagging. The moisture of starch after drying is
normally 12-13 %. Before delivery the starch is screened on a fine sieve in
order to remove any scale formed in screw conveyors etc.
CLEANING IN PLACE (CIP)
To secure a high standard of sanitation in the plant a cleaning system
is necessary. To minimize shutdown periods and thereby causing production
losses, all equipment is designed to minimize the need of frequent cleaning.
Cleaning and preventative maintenance must be planned once a month.
MODIFICATION
Most starch is used for industrial purposes. Starch is tailor made to meet
the requirements of the end-user giving rise to a range of specialty
products. Many and sophisticated techniques are applied. A most versatile
principle comprises a three step wet modification: By applying different
reaction conditions - temperature, pH, additives - and strict process
control specialty products with unique properties are made.
Preparation --> Reaction --> Finishing
These specialty products are named modified starches. They still
retain their original granule form and thereby resemble the native
(unmodified) starch in appearance, but the modification has introduced
improved qualities in the starch when cooked. The paste may have obtained
improved clarity, viscosity, film-forming ability etc.
STARCH SWEETENERS
Starch sweeteners are an important outlet for wheat starch and in many
plants starch is not dried at all. In stead the refined A-starch slurry is
further processed into starch syrups. For wheat starch the glucose is
particular important. Basic and typical units of operation are:
LIQUEFACTION.
The refined A-starch slurry is pH-adjusted and enzymes are added. The
prepared slurry is heated by direct steam in a steam jet. The liquefaction
is typically a two stage process. The combination of heat and enzymes
gelatinizes and thins the starch. The enzyme does the work by cutting the
long starch molecules into pieces by hydrolysis. A low DE hydrolysate is
formed and at this point the starch has been converted into a maltodextrin.
(DE= Dextrose Equivalent).
SACCHARIFICATION.
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| Saccharification |
The low DE hydrolysate is pH and temperature adjusted once again and new
enzymes added to produce glucose with a higher DE. Glucose of different
composition can be made depending on the enzymes added and the process
applied - even products close to pure dextrose.
PROTEIN FILTRATION.
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| Diatomaceous earth filtration |
New technology allows cross-flow membrane filtration of the hydro-lysate. By
dia-filtration glucose may be recovered from the filter residue leaving a
protein rich mud to be discharged as animal feed.
CARBON TREATMENT.
The glucose hydrolysate is heated and treated with activated carbon to
remove impurities and color bodies and then filtered.
ION EXCHANGE.
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| Ion Exchange |
The glucose hydrolysate is demineralized with ion exchange resins in a
"merry go round" arrangement. Cation resins remove various ions as sodium,
calcium, traces of iron and some amino acids. Anion resins remove ions like
chloride, sulphate, phosphate and most residual amino acids.
EVAPORATION.
The refined glucose syrup is concentrated by evaporation to its final
commercial dry matter content. The syrup is now ready for drumming off or
for road tanker transport.
A MULTITUDE OF SWEETENERS.
By varying the procedures a range of commercial products can be made and the
pure dextrose syrups may even form basis for further processing into High
Fructose Syrups utilizing sophisticated techniques like enzymatically
isomerising and chromatography.
APPLICATION.
Being a pure renewable natural polymer, starch has a multitude of
applications. Commercial wheat starch is used in the manufacture of
sweeteners, sizing of paper and textile and as a food thickener and
stabilizer. Nine million t per annum of starch and starch sweeteners are
manufactured in the European Union and one third is originating from wheat.
In the European Union 40% of native and modified starches is consumed by the
paper industry being the most important outlet at present. Increasing
amounts of grain, however, is supposed to be consumed by the new bio-fuel
industry. In USA this development has already started on maize as raw
material. In Europe wheat is the prime candidate.