“World corn crop is 600 million ton per annum. Nearly 10% is made into starch or
starch-derived sweeteners making corn starch the largest starch commodity in the
world.
Corn refiners see corn as a combination of starch, germ,
fibre and gluten, and wet milling is their means to separate them.”
Cleaned corn softens in hot mild acidic water. A coarse grind
releases the germ and a fine grind and screening separates fibre and starch. The
mill stream is deglutinated and refined by washing again and again in
hydroclones to remove the last trace of protein and produce high quality starch.
In small operations by-products may be combined to just one product,
but the larger the scale, the more feasible the individual by-products will be.
On a well-developed market by-products contribute substantial to the overall
economy.”
THE OCCURRENCE OF STARCH
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
reconversion of starch to sugar - released from the seed as energy for the
growing plant.
THE DIFFERENT KINDS OF CORN
Corn, Zea Mays, is grown in most countries throughout the world. It requires,
however, warmer climates than found in the temperate zones to grow to
maturity.
DENT corn, the scientific name of which is Zea Mays
Indentata, is also called "field" corn. It is a corn variety with kernels that
contain both hard and soft starch and become indented at maturity. It is a
major crop used to make food, animal feed, and industrial products. This is
the variety to be considered for cornstarch manufacturing.
FLINT
corn, known by the scientific name Zea Mays indurata, is a variety of corn
having hard, horny, rounded, or short and flat kernels with the soft and
starchy endosperm completely enclosed by a hard outer layer. It is similar to
dent and is used for the same purposes. Most of it is grown in South America.
WAXY corn is a corn variety with grains that have a waxy
appearance when cut, and that contains only branched-chain starch. It is grown
to make special starches for thickening foods.
SWEET or "green"
corn is eaten fresh, canned, or frozen. It is a type of corn that is grown in
many horticultural varieties. It is occasionally considered a distinct species
(Zea saccharata or Zea rugosa), a subspecies (Zea Mays rugosa), or a specific
mutation of dent corn. It is distinguished by kernels containing a high
percentage of sugar in the milk stage when they are suitable for table use.
POPCORN is a variety of corn, Zea Mays everta, which has small
ears and small pointed or rounded kernels with very hard corneous endosperm
that, on exposure to dry heat, are popped or everted by the expulsion of the
contained moisture, and form a white starchy mass many times the size of the
original kernel.
INDIAN corn has white, red, purple, brown, or
multicoloured kernels. It was the original corn grown by the Indians, and is
known by the scientific name Zea Mays. It is many times seen in harvest time
and Halloween decora-tions.
FLOUR corn, also called "soft" corn or
"squaw" corn, has kernels shaped like those of flint corn and is composed
almost entirely of soft starch. It is known by the scientific name Zea Mays
amylacea. The USA grows small amounts of blue flour corn to make tortillas,
chips, and baked goods. In South America this corn is grown in various colours
to make food and beer.
Dent Corn Composition (15% Moisture Basis, USA)
Number of Samples 151
Average
Range
Protein (%)
7.7
5.7-9.7
Oil (%)
3.3
2.6-4.9
Starch (%)
61.7
59.9-64.8
CLEANING
The raw material for wet milling is shelled dent corn delivered in bulk.
The corn is inspected and cob, dust, chaff and foreign material are removed.
The cleaning is normally done twice before wet processing. After cleaning the
corn is transported to the steeps.
STEEPING
A proper steeping is essential for high yields and high starch quality.
The steeping is carried out in a continuous counter-current process. The
cleaned corn is filled into a battery of large steeping tanks (steeps), where
the corn is soaked in hot water 30 to 48 hours to begin breaking the starch
and protein bonds . The gluten bonds within the corn begin to loosen and
release the starch. The steeping is actually a controlled fermentation.
Sulphur dioxide improves the fermentation by enhancing growth of favorable
micro-organisms, preferably lactobacillus, while suppressing detrimental
bacteria, molds, fungi and yeast. Solubles are extracted and the kernel
softens. The kernel swells to more than double size and increases its moisture
content from about 15% to 45%.
Steeping
STEEPWATER EVAPORATION
The steepwater containing approximately 10% dry substance is drained
from the kernels and condensed on a multi-stage evaporator. Most organic acids
formed during the fermentation are volatile and evaporate with the water. The
condensate from the first evaporator stage will therefore be discharged after
the heat is recovered by preheating the entering steepwater.
The
steepwater is condensed to an autosterile product - a valuable nutrient in the
fermentation industry - or concentrated to approximate 48% dry matter and
mixed and dried with the fibre fraction.
SO2
The sulphur dioxide may be prepared by burning sulphur and absorbing the
gas in water. Because modern processes call for stricter and narrow dosage, a
supply of sulphur dioxide gas under pressure is preferred or SO2 is replaced
by sodium hydrogen bisulphite where no local gas supply is available.
GERM SEPARATION
The soften kernels are broken up in attrition mills to loosen the hull
and break the bonds between germ and endosperm. Water is added to assist the
wet milling. A good steeping ensures that the germ will be freely released
intact from the kernel by a gentle milling operation with no free oil.
Oil constitutes half the weight of the germ at this stage and the
germ is easy to separate by centrifugal force. The lightweight germs are
separated from the ground slurry by hydrocyclones in a two step separation
with regrinding in between.
The germs are washed repeatedly
countercurrent on a three-stage screen to remove starch. Process wash water is
added at the last stage.
GERM DRYING
Surface water is removed from the germs by a tapered screw press. The
dewatered and clean germs are fed to a rotary steam tube bundle dryer and
dried to approximate 4% moisture. Low moisture content improves shelf life.
The germs are cooled and pneumatically transported to a germ silo ready for
bagging or oil extraction.
CORN OIL
Mechanical presses and/or solvent extraction are used to extract the
crude oil from the germ. The crude oil is refined and filtered. A typical
yield per ton corn is 27 kg corn oil. During refining free fatty acids and
phospholipids are removed. The finished corn oil finds uses as food and
cooking oil or as raw material for margarine. Refined corn oil has a pleasant
taste and does not develop off-flavours in cooking and frying. The high
content of polyunsaturated fats is a nutritional advantage.
The
residual extracted corn germ meal is used in animal feeds or combined with the
gluten feed.
FINE GRINDING AND SCREENING
After germ separation the mill flow is finely ground in impact or
attrition mills to release starch and gluten from the endosperm cell walls
(fibres).
The determinanter mill starch leaving the fine mill is
pumped to the first stage of a fibre washing system, where starch and gluten
is screened off. The overs, hull and larger fibres, are washed free from
adhering starch and gluten (insoluble protein) on screens in countercurrent
with process wash water added at the last stage. The last fibre washing stage
has a slightly courser screen for pre-dewatering the fibre prior to a tapered
screw press.
FIBRE DRYING
The dewatered fibres from the dewatering press may be mixed with
concentrated steepwater and cakes from the oil press and dried to approximate
12% moisture. The dried fibre are pelletized to reduce bulkiness and
pneumatically transported to a silo ready for shipping.
The fibre
fraction is a valuable constituent of animal feeds.
PRIMARY SEPARATION
Crude starch milk from the dewatering screen ahead of the fine mill and
from the first stage fibre washing is combined. The crude starch milk contains
starch, gluten and solubles.
A primary High Pressure Hydrocyclone
(HPHC) separator splits by gravity the mill stream in two fractions:
Gluten Overflow
Starch Underflow
A mill stream thickener may be applied on the separator feed.
Primary separation. Multi-Stage Hydrocyclone Unit.
GLUTEN RECOVERY
The gluten fraction from the primary separator is traditionally
concen¬trated on a nozzle type continuous centrifugal separator - a gluten
thickener.
The gluten separator splits the gluten fraction in two
streams:
Process water
Overflow
Gluten
Underflow
The underflow, which is mainly protein and a small amount of starch, is
discharged to the gluten dewatering section.
GLUTEN DEWATERING AND DRYING
The gluten slurry is dewatered on a vacuum belt filter or decanter. The
decanter removes more water, but requires strict pH-adjustment to the
iso-electrical point of the gluten.
Dewatering splits the gluten
stream in:
Process water
Gluten (moist)
The dewatered gluten is dried in a rotary steam tube bundle dryer to
approximately 10% moisture and disintegrated in a hammer mill. Drying is
facilitated by powder recycling.
The dried gluten is sold as corn
gluten meal with 60% protein. It is a valuable source of methionine. The high
xanthophyll content - typically 500 ppm - makes it an efficient pigmenting
ingredient in poultry feeds.
STARCH REFINING
Washing with fresh clean water refines the crude starch milk. 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. Starch refining
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.
With a
middling separator the overflow from the starch refining hydrocyclones may be
refined into:
Process water
Overflow
Starch
Underflow
In the strong gravitational fields of a hydrocyclones and a centrifuge, the
starch settles quickly. Refining is based on the differences in weight density
between water, fibres and starch:
Although some impurities go with
the starch in the underflow, there is - by means of a sieve - a last chance to
remove the larger particles. Impurities not removed this way are not removable
by any known technique.
CENTRIFUGES AND HYDROCYCLONES.
The centrifuge obtains its gravitational force by spinning the bowl. The
hydrocyclone has no moving parts and the separation is totally dependent on
the pressure difference over the cyclone.
The technological
development and quality demands have forced centrifuges to give way for
hydrocyclones in corn starch refining, but centrifuges still hold a defensive
position in gluten concentration.
Starch is among the most pure of
all agricultural products. Actually, purity is the most important parameter in
being competitive.
STARCH DEWATERING.
The purified starch milk is discharged to a peeler centrifuge for
dewatering. The peeler filtrate is recycled to starch refining. The dewatered
starch is batch-wise peeled off and discharged by gravity to the moist starch
hopper.
STARCH DRYING
From the moist starch hopper the 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 dried cornstarch 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.
Peeler Centrifuge
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 speciality 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.
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.
VISCOSITY
The viscosity (torque) is recorded on a Brabender instrument during
controlled heating of starch slurry and subsequent cooling. The
temperature is drawn in red.
Native corn-starch develops a characteristic set-back when cooled (blue line).
Same happens with a thin-boiling corn-starch (green line), but with this
particular product to a lesser degree. The native starch was tested in 8%
suspension and the chemical wet-modified thin-boiling corn-starch in 15% -
hence the name thin-boiling.
Thin-boiling starches are used for
paper and textile, where a high starch concentration in the size is
advantageous.
APPLICATION.
Commercial cornstarch is used in the manufacture of sweeteners, sizing
of paper and textile and as a food thickener and stabilizer. The by-products
are valuable feed ingredients.
Being a pure renewable natural
polymer, starch has a multitude of applications.
Starch finds uses
in fast food, sweets, sausages, tablets, and paper, corrugated board etc. and
plays a prominent part in our everyday life.
