I2
Wet Milling: Starch and Gluten
Purpose of wet milling
Wet milling of cereal grains differs fundament-
ally from dry milling in being a maceration
process in which physical and chemical changes
occur in the nature of the basic constituents -
starch, protein and cell wall material - in order
to bring about a complete dissociation of the
endosperm cell contents with the release of the
starch granules from the protein network in
which they are enclosed. In dry milling, the
endosperm is merely fragmented into cells or cell
fragments with no deliberate separation of starch
from protein (except in protein displacement
milling by air-classification, which is a special
extension of dry milling; cf. p. 132).
Although the grains of all cereals contain
starch, those most widely processed by wet milling
are wheat and maize. Other cereals which are less
frequently wet milled are rice, sorghum and
millet, while experimental work has been carried
out to separate starch and protein from triticale
and rye.
gluten comprise the steps of extracting the crude
starch and crude protein, purifying, concentrating,
and drying the two products. In order to obtain
gluten in a relatively pure form (in addition to
starch) it is necessary to separate the gluten from
the bran and germ. This may be done by first
milling the wheat by conventional dry processes
and using the white flour as the starting material
for the wet process.
Until the development of recent methods,
processes starting with flour were mostly variants
on three long-established methods:
Martin process: dough is kneaded under water
sprays. The gluten agglomerates, and the starch
is washed out.
Batter process: a flour-water batter is dispersed
in more water so that the gluten breaks down
into small curds. The gluten is separated from
the starch milk by screening.
Alkali process: flour is suspended in an alkaline
solution (e.g. 0.03 N sodium hydroxide) in which
the protein disperses. Starch is removed by tabling
and centrifuging, and the protein precipitated by
acidifying to pH 5.5. The protein product is in
a denatured condition, i.e. non-vital.
A number of modern processes start with flour:
Wheat
Means for separating starch from wheat by wet the Canadian process (1966) resembles the alkali
milling processes have been known from classical process, but suspends the flour in 0.2 M ammonium
times. Marcus Porcius Cat0 (234-149 B.C.) hydroxide; the Far-Mar-Co process (U.S. Pat.
described a process in which cleaned wheat was No. 3,979,375) is similar to the Martin process,
steeped for 10 days in twice its weight of water, but the dough moves through a tube in which
the water poured off, the soaked wheat slurried, it is washed and mixed; the Alfa-Laval/Raisio
enclosed in a cloth, and the starch milk pressed process (Starke, 1978, 30:8) resembles the batter
out. The residue of gluten, bran and germ would process, but uses centrifuging, decantation and
have been discarded or used as animal feed. hydrocyclones for separating the starch from the
All wet processes for manufacture of starch and gluten; the Koninklijke Scholten-Honig process
259
260 TECHNOLOGY OF CEREALS
(BP 1,596,742) also resembles the batter process
and uses hydrocyclones. A process by Walon
(U.S. Pat. No. 4,217,414) uses bacterial alpha-
amylase to solubilize the starch; the gluten is not
denatured and can be separated as ‘vital gluten’.
Some modern processes start with wheat but
differ from Cato’s method in using a steep-liquor
containing 0.03-0.7°/~ of sulphur dioxide to inhibit
development of micro-organisms . After draining
off the steep liquor, the wet grain is coarsely
milled and slurried with water. The bran and
germ are separated by screening, and the heavy
starch granules separated from the light gluten
curd by sedimentation and centrifuging.
In the Pitlsbury process (Br. Pat. No. 1,357,669)
grain is steeped in an acid medium with applica-
tion of vacuum or carbon dioxide to remove the
air pocket at the base of the crease where micro-
organisms might develop. In the Far-Mar-Co
process (U.S. Pat. No. 4,201,708) wheat is soaked
in water and flaked. The flakes are disintegrated
and the resulting bran-germ and endosperm
particles are hydrated and form a dough-like mass
which is tumbled and manipulated in water to
separate and recover vital gluten, starch and bran-
germ components.
In all processes, the starch and gluten are dried
using, in the case of gluten, methods such as
freeze-drying which do not denature the gluten.
‘Vital’ gluten, or undenatured gluten, is gluten
separated from wheat by processes which permit
the retention of the characteristics of natural
gluten, viz. the ability to absorb water and form
an extensible, elastic mass. Commercial glutens
are produced in the U.K., Europe, Australia and
Canada (McDermott, 1985). In the U.K., some
260,000-270,000 tonnes of wheat were used in
1988/89 for the manufacture of starch and vital
gluten, chiefly by the dough (Martin) or batter
processes, which start with flour. So far as is
known, processes such as the Pillsbury and Far-
Mar-Co processes that start with wheat grain are
not currently being used in the U.K.
With a yield of about 0.075 t of gluten from
each tonne of wheat, the quantity of gluten
producedin the U.K. in 1988/89 would have been
some 20,000 t. The demand for vital gluten in
the U.K. considerably exceeds this figure, and
is met by imports of gluten, chiefly from the
European continent. Imports of gluten in 1988/
89 amounted to 37,000 t, givinga totalavailability
of about 57,000 t. World production of vital
gluten in 1986 is reported as 253,000 t (Godon,
1988), of which 130,000 t were produced in
western Europe and 54,000 t in the U.S.A.,
Canada, Mexico and Argentina.
uses for vital wheat gluten
Vital gluten is used as a protein supplement:
- at levels of 0.5-3.0% to improve the texture
and raise the protein content of bread, par-
ticularly ‘slimming’ bread, crispbread and
speciality breads such as Vienna bread and
hamburger rolls;
- to fortify weak flours, and to permit the use
by millers of a wheat grist of lowered strong/
weak wheat ratio (particularly in the EC
countries) by raising the protein content of
the milled flour;
- in starch-reduced high protein breads (cf. p.
209), in which the gluten acts both as a source
of protein and as a texturizing agent;
- in high-fibre breads (cf. p. 209) now being
made in the U.S.A., to maintain texture and
volume.
Vital gluten is also used as a binder and to raise
the protein level in meat products, e.g. sausages,
breakfast foods, pet foods, dietary foods and
textured vegetable products (t.v.p.).
,,ital gluten ;,, bread
In the U.K., domestic bread consumption in
1988 averaged 30.28 oz/person/week. With a
population of about 57 million, the total domestic
bread consumption would have been about 2.5
million tonnes per year, or about 2.75 million
tonnes total consumption, allowing for about 10%
consumed non-domestically. With about 45,000
t of vital gluten used in bread in the U.K. in
1988, the average level of use would have been
about 1.6%. The usual rates of addition are 1.5%
for white bread and 4.5% for wholemeal bread.
In the U.S.A., about 70% of all vital wheat
Cleaning Steeping Evaporating
concentrates
I 1
1
1
Moisture Grinding
expelling
Moisture
expelling
*
Drying
Filtering
I I
* 011 extracting
Germ cake
Feed drying
and grinding
Bleaching
1
Oil - Deodorizing
Gluten feed
Gluten meal
262 TECHNOLOGY OF CEREALS
gluten is used in the manufacture of bread, and more slowly to 4345%. The steeping softens
rolls, buns and other yeast-raised products. The the kernel and assists separation of the hull, germ
remainder finds uses in breakfast cereals (e.g. and fibre from each other. The sulphur dioxide
Kellogg's Special K), breadings, batter mixes and in the steep may disrupt the -SS- bonds in
pasta products (Magnuson, 1985). the matrix protein (glutelin), facilitating starch/
protein separation.
After steeping, the steep water is drained off.
It contains about 6% of solids, of which 3545%
Gluten flour
Gluten flour is a blend of vital wheat gluten is protein. The protein in the steep water is
with wheat flour, standardized to 40% protein recovered by vacuum evaporation, allowed to
content in the U.S.A. settle out of the water in tanks, and dried
as 'gluten feed' for animal feeding. The water
recovered is re-used as steep water or, after
Maize
concentration, as a medium for the culture of
Maize is wet-milled to obtain starch, oil, cattle organisms from which antibiotics are obtained.
feed (gluten feed, gluten meal, germ cake) and
the hydrolysis products of starch, viz. liquid and
De-germing
solid glucose and syrup.
The maize, after steeping, is coarsely ground
in de-germing mills with the objective of freeing
the germ from the remainder of the grain without
Operations
The sequence of operations in wet milling of breaking or crushing the germ. The machine
maize is shown in Fig. 12.1. generally used for this purpose is a Fuss mill, a
For safe storage, maize must be dried because bronze-lined chamber housing two upright metal
the moisture content at harvest is generally higher plates studded with metal teeth. One plate rotates
than the desirable m.c. for storage. Drying at 900 rev/min, the other is stationary. Water and
temperature should not exceed 54°C; at higher maize are fed into the machine, which cracks
temperatures changes occur in the protein where- open the grain and releases the germ. By addition
by it swells less during steeping, and tends to of a starch-water suspension, the density of the
hold the starch more tenaciously, than in grain ground material is adjusted to 8-10.5" BC* (1.06-
not dried, or dried at lower temperatures. In 1.08 sp.gr.): at this sp.gr. the germs float while
addition, if dried at temperatures above 54°C the the grits and hulls settle.
germ becomes rubbery and tends to sink in the
ground maize slurry (whereas the process of germ
Germ separation
separation depends on the floating of the germ),
and the starch tends to retain a high oil content. The ground material flows down separating
troughs in which the hulls and grits settle, while
the germ overflows. More modern plants use
hydrocyclones which require less space and are
Steeping
The cleaned maize is steeped at a temperature less costly to maintain than flotation equipment.
of about 50°C for 2848 h in water containing Moreover, the germ separated on hydrocyclones
0.1-0.2°/~ of sulphur dioxide. Steeping is carried is cleaner than that separated by flotation.
out in a series of tanks through which the steep The germ is washed and freed of starch on
water is pumped counter-current. The moisture reels, de-watered in squeeze presses and dried on
content of the grain increases rapidly to 35-40%, rotary steam driers. The dry germ is cooked by
* Baume Scale: a hydrometer scale on which 0" represents the sp. gr. of water at 12.5"C and 10" the sp. gr. of a 10%
solution of NaCl at 12.5"C. It is also known as the Lunge Scale.
WET MILLING: STARCH AND GLUTEN 263
steam, and the oil extracted by hydraulic presses rotary or flash driers. Further fractionation to
or by solvent extraction. The germ oil is screened, obtain the alcohol-soluble zein, which comprises
filtered and stored. The extracted germ cake is about 50% of the maize gluten, by solvent extrac-
used for cattle feed. tion and precipitation may be carried out. Zein
finds a use as a water-protective coating material
for nuts and confectionery and as a binder for
pharmaceuticals.
Milling
The de-germed underflow from the germ separ- A combined dry-wet milling process for refin-
ator is strained off from the liquor and finely ing maize has been described (U.S. Pat. No.
ground on impact mills, such as an entoleter, or 4,181,748; 1980) in which the maize is dry-milled
attrition mills, such as the Bauer mill. After this to provide endosperm, germ, hull and cleaning
process, the starch and protein of the endosperm fractions. The endosperm fraction is wet-milled
are in a very finely divided state and remain in in two steps which respectively precede and
suspension. The hulls and fibre, which are not follow an impact milling step. The principal
reduced so much in particle size, can then be products are prime maize starch, corn oil and an
separated from the protein and starch on reels animal feed product.
fitted with 18-20 mesh screens. Fine fibres, which
Maize wet-milling in the U.S.A
interfere with the subsequent separation of starch
from protein, are removed on gyrating shakers
fitted with he nylon cloth. The processing of maize into wet-processed
products has increased greatly in the U.S.A. in
recent years. In 1960/61,3.94 Mt were processed
into wet-processed products, but by 1984/85 the
Separation of starch from protein
In the raw grain the starch granules are em- figure had increased to 20.7 Mt, representing
bedded in a protein network which swells during 10.6% of the entire maize harvest in 1984 of 195
the steeping stage and tends to form tiny globules Mt. Of this, 7.9 Mt were used to produce high-
of hydrated protein (Radley, 195 1-1952). Disper- fructose corn syrup (HFCS), 4.8 Mt for glucose
sion of the protein, which frees the starch, is and dextrose, 3.8 Mt for starch and 4.3 Mt for
accelerated by the sulphur dioxide in the steep alcohol (Livesay, 1985).
water. The growth of the industry in the 1970s
The effect of the sulphur dioxide, according to followed the development of a process to convert
Cox et al. (1944), is due to its reducing, not to starch into high-fructose corn syrup (HFCS). The
its acidic, property. The sulphur dioxide also has sweetness of HFCS allows it to be used as a
a sterlizing effect, preventing growth of micro- substitute for sucrose in soft drinks and other
organisms in the steep. processed foods.
The suspension of starch and protein from the
Products of wet-milling
wet screening is adjusted to a density of 6"BC
(1.04 sp.gr) by de-watering over Grinco or string
filters, and the starch separated from the protein The wet-milling of maize yields about 66% of
in continuous high-speed centrifuges such as the starch, 4% of oil, and 30% of animal feed,
Merco centrifugal separator. comprising about 24% of gluten feed of 21%
The starch is re-centrifuged in hydrocyclones protein content (made up of about 13% of fibre,
to remove residual protein and is then filtered 7% of steep water solubles and 4% of germ
and dried to 10-12O/0 m.c. in kilns or ovens, or residue), plus about 5.7% of gluten meal of 60%
in tunnel or flash driers. The moisture content is protein content. The composition of products
further reduced by vacuum drying to 5-7% m.c. from the wet-milling of maize is shown in Table
in the U.S.A., or to 1-2% m.c. in Britain. 12.1 (Wright, in Watson and Ramstad, 1987).
The separated protein is filtered and dried in Most of the starch is further processed to make
264 TECHNOLOGY OF CEREALS
TABLE 12.1
Composition of Maize Wet-Milled Products*
Fibre
Moisture Proteint Fat Crude NDF* Ash NFES Starch
(Oh) (%) (%) (Yo) (”1 (%) (Yo) (Oh)
Maize 15.5 8.0 3.6 2.5 8.0 1.2 69.2 60.6
Corn gluten feed 9.0 22.6 2.3 7.9 25.4 7.8 50.1 low
Corn meal 10.0 62.0 2.5 1.2 4.1 1.8 22.5 low
Germ meal 10.0 22.6 1.9 9.5 41.6 3.8 52.2 low
Steep liquor 50.0 23.0 0 0 0 7.3 19.2 low
* Sources: Anon. 1982; Wright, K. N., 1987.
t N X 6.25.
$ Neutral detergent fibre.
$ Nitrogen-free extract.
modified starch, sweeteners and alcohol (Long, modified starch for particular purposes, or by
1982). enzymic hydrolysis, to yield maltose, which can
Other uses for corn gluten include cork-binding be further treated to make dextrose (D-glucose),
agent, additive for printing dyes, and in pharma- regular corn syrup, high-fructose corn syrup
ceuticals. It is perhaps misleading that the protein (HFCS), and malto-dextrins.
product obtained from maize should be called In modifying starch, the objectives are to alter
‘gluten’, because maize gluten in no way resembles the physical and chemical characteristics in order
the vital gluten that is obtained from wheat (cf. to improve functional characteristics, by oxida-
p. 260) tion, esterification, etherification, hydrolysis
Ethanol can be made by yeast fermentation of or dextrinization. The methods used are acid
maize starch, and has a particular advantage thinning, bleaching or oxidation, cross-linking,
because the yeast can be re-cycled. About 85% substitution or derivatization, instantizing.
of the ethanol produced from maize starch is In acid thinning, or conversion, the glucosidic
blended with gasoline, in which it acts as an linkages joining the anhydroglucose units are
octane enhancer for unleaded fuel (May, 1987). broken, with the addition of water. The resulting
An edible film has been made from maize starch thinning reduces the viscosity of the starch paste,
amylose obtained from high-amylose maize (cf. and allows such starches to be cooked at higher
p. 99). Suggested uses for the film include the concentrations than the native starch. In a wet
packing of gravies, sauces and coffee. process of conversion, the starch is treated with
The starch obtained from the milling of waxy 1-3% of hydrochloric or sulphuric acids at about
maize (cf. p. 99), called ‘amioca’, consists largely 5OoC, then neutralized and the starch filtered off.
of amylopectin. Amioca paste is non-gelling and Acid-thinned starches are used in confectionery
has clear, fluid adhesive properties. products, particularly starch jelly candies (Moore
Heated and dried maize starch/water slurries et al. , 1984). Non-food uses include paper-sizing,
yield pregelatinized starch, known as ‘instant calendering, coating applications (Sanford and
starch’, as it thickens upon addition of cold water. Baird, 1983; Bramel, 1986). In a dry process
conversion, in which dry starch powder is roasted
with limited moisture and a trace of hydrochloric
acid, the main product is dextrins, used for
Uses for wet-milled maize products
Uses for maize starch include paper manufac- adhesives and other non-food purposes.
ture, textiles, adhesives and packaged foods, and In bleaching, or oxidation, aqueous slurries
as the starting material for further processing by of starch are treated with hydrogen peroxide,
chemical treatment, to make various kinds of peracetic acid, ammonium persulphate , sodium
WET MILLING: STARCH AND GLUTEN 265
hypochlorite, sodium chlorite, or potassium per- high viscosity, cohesiveness, and stability to
manganate and then neutralized with sodium retrogradation. They are used as emulsifiers in
bisulphite. The xanthophyll and other pigments foods and for many non-food uses (Orthoefer,
are bleached, thereby whitening the starch which 1987).
then becomes suitable for use as a fluidizing agent Dextrose is made by enzymic hydrolysis of
in, e.g. confectioners' sugar. Starch, in aqueous starch and crystallization; HFCS by partial
slurry, can be oxidized with about 5.5% (on d.wt.) enzymic isomerization of dextrose hydrolysates;
of chlorine as sodium hypochlorite. Hydroxyl regular corn syrup and malto-dextrins by partial
(-OH) groups are oxidized, forming carboxyl hydrolysis of corn starch with acid, acid +
(-C=O) or carbonyl (OH-C=O) groups, with enzyme, or enzyme only (Hebeda, 1987).
cleavage of glucosidic linkages. The bulkiness of In making dextrose, thermostable bacterial
the -C=O groups reduces the tendency of the alpha-amylase from B. subtilis or B. lichenifomis
starch to retrograde. Bleached starch finds uses is used for liquefying starch to 10-15 D.E.
in batter and breading mixes in fried foods (dextrose equivalent) followed by saccharification
(Moore et al. , 1984), to improve adhesion. Oxidized with glucoamylase from Aspergillus niger to
starch finds non-food uses in paper sizing, due 95-96% dextrose (dry basis). The glucoamylase
to its excellent film-forming and binding properties releases dextrose step-wise from the non-reducing
(Bramel, 1986). end, cleaving both alpha-1-4 and alpha-1-6
Cross-linking improves the strength of swollen bonds. The hydrolysate is clarified, refined, and
granules, preventing rupture. Granular starch is processed to crystalline dextrose, liquid dextrose,
cross-linked by treatment with adipic acid and high-dextrose corn syrup or HFCS feed.
acetic anhydride, forming distarch adipate, or Glucose and dextrose are used in beer, cider,
with phosphorus oxychloride, forming distarch soft drinks, pharmaceuticals, confectionery, baking
phosphate. The slurry is neutralized, filtered, and jams.
washed and dried. The viscosity of cross-linked The refined dextrose hydrolysate can be treated
starch is higher than that of native starch. with immobilized glucose isomerase of bacterial
Derivatization or substitution consists of the origin. This enzyme catalyzes the isomerization
introduction of substitution groups on starch by of dextrose to D-fructose. A product containing
reacting the hydroxyl groups (-OH) with mono- 90% of HFCS may be obtained by chromatographic
functional reagents such as acetate, succinate, separation.
octenyl succinate, phosphate or hydroxypropyl Dextrose and HFCS are important sweeteners:
groups. Derivatization retards the association dextrose has 65-76% of the sweetness of sucrose,
of gelatinized amylose chains, improves clarity, while HFCS is 1.8 times as sweet as sucrose and
reduces gelling and improves water-holding 2.4 times as sweet as dextrose.
(Orthoefer, 1987). Substitution may be combined The use of maize in the U.S.A. to produce
with cross-linking to yield thickeners with par- HFCS increased from 0.25 Mt in 1971/72 to 8.1
ticular processing characteristics (Moore et al., Mt in 1985/86. The explosive growth of HFCS
1984). Starch esters - acetates, phosphates, production between 1972 and 1985 was due to
octenyl succinates - find uses as thickeners in the technical breakthrough in the process for
foods, e.g. fruit pies, gravies, salad dressings, making HFCS, the high cost of sugar in the
fdled cakes, because they withstand refrigeration U.S.A., and the availability of abundant stocks
and freeze/thaw cycles well. Non-food uses include of relatively low-cost maize in the U.S.A. (May,
warp sizing of textiles, surface sizing of paper, 1987). By 1991, HFCS had become the largest
and gummed tape adhesives (Sanford and Baird, product category of the U.S. corn wet-milling
1983; Bramel, 1986). industry, accounting for more than one-third
Phosphate mono-esters of starch, made by of the industrial grind. When reporting world
roasting starch with orthophosphates at pH 5- production, however, the U.S. Dept. Agric. now
6.5 for 0.5-6 h at 12O0-16O"C, have good clarity, refers to HFSS (high-frucose starch syrup) rather
266 TECHNOLOGY OF CEREALS
than HFCS, acknowledging that alternative starch
sources, such as wheat and potatoes, can be
converted to fructose-rich syrups using enzymic
conversions, refining and separation technologies
which are currently applied in the U.S.A. to
starch separated from maize. On this basis, world
production of HFSS increased from 0.65 Mt in
1975 to 7.1 Mt in 1988. Production of HFSS in
the U.S.A. similarly increased from 0.48 Mt in
1975 to 5.3 Mt in 1988. Over this period, HFSS
production as a percentage of total world sugar
plus HFSS consumption increased from 0.9% in
1975 to 6.3% in 1988 (Meyer, 1991).
The de-germed endosperm and pericarp are
wet-screened on a nylon cloth with 70-75 ym
apertures, through which the free starch and
protein pass. The residue, mostly horny endo-
sperm, is re-ground in an entoleter, impact mill,
or other suitable mill, to release more starch, and
is again screened.
De-watering
The washed tailings of the screen are de-
watered to about 60% m.c. by continuous screw
presses. The product is known as ‘fibre’. The
fibre, blended with concentrated steep water
(containing solubles leached out during the steep-
ing) and spent germ cake, is dried in a continuous
Sorghum
sorghum closely resemble those described for
Starch/protein separation
maize (cf. p. 262), but the process is more difficult
with sorghum than with maize. The problems are
associated with the small size and spherical shape The de-fibred starch granules and protein
of the sorghum kernel, the large proportion of particles are separated in a Merco continuous
horny endosperm, and the dense high-protein centrifuge by a process of differential sedimenta-
peripheral endosperm layer. Varieties with dark- tion; the starch granules, with a density of 1.5,
coloured outer layers are not satisfactory for wet- settle out of an aqueous slurry at a faster rate
milling because some of the colour leaches out than the protein particles, density 1.1. The starch
and stains the starch. slurry is dried on a Proctor and Schwartz moving-
belt tunnel drier, or by flash drying.
Steeping
Sorghum ’gluten’
The cleaned sorghum is first steeped in water
(1.61-1.96 l/kg) for 40-50 h in a counter-current The ‘gluten’ (protein) is concentrated, filtered
process. Part of the water is charged with 0.1- and flash dried. The protein content of milo
0.16% of sulphur dioxide, which is absorbed by gluten is 65-70% on dry basis. A blend of milo
the grain and weakens the protein matrix in which
the starch granules are embedded.
TABLE 12.2
Composition of Products From Wet
Milling of Sorghum* (dry basis)
De-gerrning
Product Yieldt Protein Fat Starch
The methods used for the wet-milling of flash drier to yield ‘milo gluten feed’.
The steeped grain, in slurry form, is ground (“h) (Yo) (Yo) (OK)
in an attrition mill. The mill has knobbed plates, Germ 6.2 11.8 38.8 18.6
one static, one rotating at about 1700 rev./min. Fibre 7.4 17.6 2.4 30.6
The milling detaches the germ and liberates about Tailings 0.8 39.2 - 25.3
Gluten 10.6 46.7 5.1 42.8
half of the starch from the endosperm. The germ, Squeegee 1.2 14.0 0.6 81.6
which contains 4045% of oil, floats to the surface Starch 63.2 0.4 - 67.3
endosperm and pericarp in a continuous liquid
cyclone.
- -
of the slurry, and is removed from the heavier Solubles 6.6 43.7
* Source: Freeman and Bocan (1973).
t Percentage of dry substance in whole grain.
WET MILLING: STARCH AND GLUTEN 2 67
gluten with milo gluten feed, reducing the protein
content to 45% d.b., is known as 'milo gluten
meal'.
Products of sorghum wet milling
The yield and composition of products from
the wet milling of sorghum are shown in Table
TABLE 12.3
Composition of Products From Wet
Milling of Pearl Millet* (dry basis)
Product Yieldt Protein Fat Starch
("/.) ("1 (Yo) (Oh)
Germ 7.5 10.4 45.6 10.4
7.3 11.8 6.0 13.5 Fibre
Tailings 1.5 34.1 1.9 34.2
12.1 37.8 9.0 44.0 Gluten
12.2. Squeegee 4.1 17.7 0.8 75.5
Starch 49.3 0.7 0.1 57.5
Solubles 16.8 46.1 - -
* Source: Freeman and Bocan (1973).
t Percentage of dry substance in whole grain.
Millet
The possibility of using pearl millet as raw
material for a wet-milling process has been invest-
igated (Freeman and Bocan, 1973). The small
millet grains were much more difficult to de-germ
than were sorghum or maize, although the poten-
tial yield of oil from millet exceeded those from
the other cereals. Separation of protein from
starch was also more difficult with millet than
from sorghum or maize, and the products of
separation were somewhat less pure than those
obtained from the other cereals. The starch from
millet resembled that from sorghum and maize
in most respects, but the granule size was slightly
smaller (cf. p. 57), and the starch had a slightly
lower tendency to retrograde.
The composition of products from the wet-
milling of pearl millet is shown in Table 12.3.
Rice
residual grain, is virtually fat-free, and is thus
much more stable than that separated in the
conventional dry milling process.
In the SEM process (U.S. Pat. No. 3,261,690),
rice oil is applied to brown rice (de-hulled rough
rice) in controlled amounts, and softening of the
bran is accomplished. The bran is removed by
milling machines of modified conventional design
in the presence of an oil solvent - rice oivhexane
miscella. The miscella acts as a washing or rinsing
medium to aid in flushing bran away from the
endosperm, and as a conveying medium for
continuously transporting detached bran from
rice. The miscella lubricates the grains, prevents
rise of temperature, and reduces breakage.
The de-branned rice is screened, rinsed and
drained, and the solvent removed in two stages.
Super-heated hexane vapour is used to flush-
evaporate the bulk of the hexane remaining in
the rice, and the rice is subjected to a flow of
inert gas which removes the last traces of the
solvent.
The bradoil miscella slurry is pumped to
vessels in which the bran settles, and is then
separated centrifugally, while being rinsed with
hexane to remove the oil. The last traces of solvent
are removed from the bran by flash de-solventking,
and the bran is cooled.
The oiuhexane miscella from the bran-settling
vessels is pumped to conventional solvent re-
covery plant, where the hexane is stripped from
the oil.
SEM (X-M) plants are operated at Abbeyville,
Louisiana, U.S.A. and in many Asian countries.
Solvent extraction milling (SEM)
This is a process applied to rice in order to
obtain de-branned rice grain, and also rice bran
and rice oil as separate products. The SEM
process cannot strictly be described either as a
dry-milling or as a wet-milling process, although
it involves stages which could be included in each
of these categories.
The customary method for milling rice uses
abrasion of the rice grain in a dry condition to
remove bran from the endosperm (cf. p. 160). In
the SEM process (also called X-M) the bran layers
are first softened and then 'wet-milled' in the
presence of a rice oil solution. The separated bran
has a higher protein content than that of the
268 TECHNOLOGY OF CEREALS
Advantages of the SEM process over the MCDERMOTT, E. E. (1985) The properties of commercial
glutens. Cereals Fds Wld, 30 (2): 169-171.
MEYER, P. A. (1991) High fructose starch syrup production
and technology growth to continue. Milling and Baking
News, March 26, 70 (4): 34.
SCHANFELT, R. V. (1984) Applications of starches in foods.
In: Starch: chemistry and technology, pp. 579-592 2nd edn.
WHISTLER, R. L., BEMILLER, J. N. and PASCHALL,
E. F. (Eds). Academic Press, Orlando, FL., U.S.A.
ORTHOEFER, F. T. (1987) Corn starch modification and uses.
In: Corn: chemistry and technology. pp. 479-499 WATSON,
S. A. and RAMSTAD, P. E. (Eds). Amer. Assoc. Cereal
Chem., St Paul, MN, U.S.A.
RADLEY, J. A. (1951-2) The manufacture of maize starch.
Food Manuf. 26: 429, 488; 27: 20.
SANFORD, P. A. and BAIRD, J. (1983) Industrial utilization
of PolYsaccharides. PP. 41 1-490 in: The Pob'sacchahdes,
Vol. 2. ASPINALL, G. 0. (Ed.). Academic Press, New
York, U.S.A.
WRIGHT, K. N. (1987) Nutritional properties and feeding
value of corn and its by-products. In: Corn: Chemistry and
Technology. pp. 447478 WATSON, S. A. and RAMSTAD,
P. E. (Eds). Amer. Assoc. Cereal Chem., St Paul, MN,
U.S.A.
conventional milling process are:
1. An increase of up to 10% on head rice yield.
2. A decrease in the fat content of the rice, which
MOORE, c. o., TUSCHHOFF, J. v., HASTINGS, C. W. and
improves its storage life.
3. An increase in stability of the bran product.
4. A yield Of 2kg Of rice Oil from each lookg Of
unmilled rice (2% yield on rice wt).
The bran product has potential application in
breakfast cerea1sy baby foodsy baked goodso The
oil has edible and industrial applications, and is
a rich source of a wax with properties similar to
those ofmyricyl cerotate, or carnauba wax. Other
applications are in margarine, cosmetics, paints
(Edwards, 1967).
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BRAMEL, G. F. (1986) Modified starches for surface coatings Further Reading