9.9.7 Corn Wet Milling9.9.7.1 General1
Establishments in corn wet milling are engaged primarily in producing starch, syrup, oil, sugar,and byproducts such as gluten feed and meal, from wet milling of corn and sorghum. These facilities
may also produce starch from other vegetables and grains, such as potatoes and wheat. In 1994,
27 corn wet milling facilities were reported to be operating in the United States.
9.9.7.2 Process Description1-4
The corn wet milling industry has grown in its 150 years of existence into the most diversifiedand integrated of the grain processing industries. The corn refining industry produces hundreds ofproducts and byproducts, such as high fructose corn syrup (HFCS), corn syrup, starches, animal feed,oil, and alcohol.
In the corn wet milling process, the corn kernel (see Figure 9.9.7-1) is separated into
3 principal parts: (1) the outer skin, called the bran or hull; (2) the germ, containing most of the oil;and (3) the endosperm (gluten and starch). From an average bushel of corn weighing 25 kilograms(kg) (56 pounds [lb]), approximately 14 kg (32 lb) of starch is produced, about 6.6 kg (14.5 lb) offeed and feed products, about 0.9 kg (2 lb) of oil, and the remainder is water. The overall corn wet
milling process consists of numerous steps or stages, as shown schematically in Figure 9.9.7-2.
Shelled corn is delivered to the wet milling plant primarily by rail and truck and is unloaded
into a receiving pit. The corn is then elevated to temporary storage bins and scale hoppers forweighing and sampling. The corn then passes through mechanical cleaners designed to remove
unwanted material, such as pieces of cobs, sticks, and husks, as well as meal and stones. The cleaners
agitate the kernels over a series of perforated metal sheets through which the smaller foreign materials
drop. A blast of air blows away chaff and dust, and electromagnets remove bits of metal. Coming outof storage bins, the corn is given a second cleaning before going into "steep" tanks.
Steeping, the first step in the process, conditions the grain for subsequent milling and recoveryof corn constituents. Steeping softens the kernel for milling, helps break down the protein holding thestarch particles, and removes certain soluble constituents. Steeping takes place in a series of tanks,
usually referred to as steeps, which are operated in continuous-batch process. Steep tanks may holdfrom 70.5 to 458 cubic meters (m3) (2,000 to 13,000 bushels [bu]) of corn, which is then submerged
in a current of dilute sulfurous acid solution at a temperature of about 52°C (125°F). Total steeping
time ranges from 28 to 48 hours. Each tank in the series holds corn that has been steeping for adifferent length of time.
Corn that has steeped for the desired length of time is discharged from its tank for further
processing, and the tank is filled with fresh corn. New steeping liquid is added, along with recycledwater from other mill operations, to the tank with the "oldest" corn (in steep time). The liquid is thenpassed through a series of tanks, moving each time to the tank holding the next "oldest" batch of corn
until the liquid reaches the newest batch of corn.
Water drained from the newest corn steep is discharged to evaporators as so-called "lightsteepwater" containing about 6 percent of the original dry weight of grain. By dry-weight, the solids
1/95

Food And Agricultural Industry

9.9.7-1

Figure 9.9.7-1. Various uses of corn.
in the steepwater contain 35 to 45 percent protein and are worth recovering as feed supplements. Thesteepwater is concentrated to 30 to 55 percent solids in multiple-effect evaporators. The resulting
steeping liquor, or heavy steepwater, is usually added to the fibrous milling residue, which is sold asanimal feed. Some steepwater may also be sold for use as a nutrient in fermentation processes.
The steeped corn passes through degerminating mills, which tear the kernel apart to free both
the germ and about half of the starch and gluten. The resultant pulpy material is pumped throughliquid cyclones to extract the germ from the mixture of fiber, starch, and gluten. The germ issubsequently washed, dewatered, and dried; the oil extracted; and the spent germ sold as corn oilmeal or as part of corn gluten feed. More details on corn oil production are contained inSection 9.11.1, "Vegetable Oil Processing".
The product slurry passes through a series of washing, grinding, and screening operations toseparate the starch and gluten from the fibrous material. The hulls are discharged to the feed house,
where they are dried for use in animal feeds.
At this point, the main product stream contains starch, gluten, and soluble organic materials.
The lower density gluten is separated from the starch by centrifugation, generally in 2 stages. A
high-quality gluten, of 60 to 70 percent protein and 1.0 to 1.5 percent solids, is then centrifuged,dewatered, and dried for adding to animal feed. The centrifuge underflow containing the starch ispassed to starch washing filters to remove any residual gluten and solubles.
The pure starch slurry is now directed into 1 of 3 basic finishing operations, namely, ordinary
dry starch, modified starches, and corn syrup and sugar. In the production of ordinary dry starch,
the starch slurry is dewatered with vacuum filters or basket centrifuges. The discharged starch cakehas a moisture content of 35 to 42 percent and is further dewatered thermally in 1 of several types ofdryers. The dry starch is then packaged or shipped in bulk, or a portion may be kept for use in
making dextrin.
9.9.7-2

EMISSION FACTORS

1/95

Figure 9.9.7-2. Corn wet milling process flow diagram.1-4(Source Classification Codes in parentheses.)

1/95

Food And Agricultural Industry

9.9.7-3

Modified starches are manufactured for various food and trade industries for which unmodified
starches are not suitable. For example, large quantities of modified starches go into the manufactureof paper products as binding for the fiber. Modifying is accomplished in tanks that treat the starchslurry with selected chemicals, such as hydrochloric acid, to produce acid-modified starch; sodium
hypochlorite, to produce oxidized starch; and ethylene oxide, to produce hydroxyethyl starches. The
treated starch is then washed, dried, and packaged for distribution.
Across the corn wet milling industry, about 80 percent of starch slurry goes to corn syrup,sugar, and alcohol production. The relative amounts of starch slurry used for corn syrup, sugar, andalcohol production vary widely among plants. Syrups and sugars are formed by hydrolyzing the starch— partial hydrolysis resulting in corn syrup, and complete hydrolysis producing corn sugar. The
hydrolysis step can be accomplished using mineral acids, enzymes, or a combination of both. The
hydrolyzed product is then refined, which is the decolorization with activated carbon and the removalof inorganic salt impurities with ion exchange resins. The refined syrup is concentrated to the desired
level in evaporators and is cooled for storage and shipping.
Dextrose production is quite similar to corn syrup production, the major difference being thatthe hydrolysis process is allowed to go to completion. The hydrolyzed liquor is refined with activatedcarbon and ion exchange resins, to remove color and inorganic salts, and the product stream isconcentrated by evaporation to the 70 to 75 percent solids range. After cooling, the liquor istransferred to crystallizing vessels, where it is seeded with sugar crystals from previous batches. The
solution is held for several days while the contents are further cooled and the dextrose crystallizes.
After about 60 percent of the dextrose solids crystallize, they are removed from the liquid bycentrifuges, are dried, and are packed for shipment.
A smaller portion of the syrup refinery is devoted to the production of corn syrup solids. In
this operation, refined corn syrup is further concentrated by evaporation to a high dry substance level.
The syrup is then solidified by rapid cooling and subsequently milled to form an amorphous crystalline
product.
Ethanol is produced by the addition of enzymes to the pure starch slurry to hydrolyze thestarch to fermentable sugars. Following hydrolysis, yeast is added to initiate the fermentation process.
After about 2 days, approximately 90 percent of the starch is converted to ethanol. The fermentation
broth is transferred to a still where the ethanol (about 50 vol%) is distilled. Subsequent distillationand treatment steps produce 95 percent, absolute, or denatured ethanol. More details on this ethanol
production process, emissions, and emission factors is contained in Section 6.21, "Ethanol".
9.9.7.3 Emissions And Controls1-2,4-8
The diversity of operations in corn wet milling results in numerous and varied potentialsources of air pollution. It has been reported that the number of process emission points at a typicalplant is well over 100. The main pollutant of concern in grain storage and handling operations in corn
wet milling facilities is particulate matter (PM). Organic emissions (e. g., hexane) from certain
operations at corn oil extraction facilities may also be significant. These organic emissions (andrelated emissions from soybean processing) are discussed in Section 9.11.1, "Vegetable OilProcessing". Other possible pollutants of concern are volatile organic compounds (VOC) andcombustion products from grain drying, sulfur dioxide (SO2) from corn wet milling operations, andorganic materials from starch production. The focus here is primarily on PM sources for grain
handling operations. Sources of VOC and SO2 are identified, although no data are available toquantify emissions.

9.9.7-4

EMISSION FACTORS

1/95

Emission sources associated with grain receiving, cleaning, and storage are similar in characterto those involved in all other grain elevator operations, and other PM sources are comparable to thosefound in other grain processing plants as described in Section 9.9.1, "Grain Elevators And Processes".
However, corn wet milling operations differ from other processes in that they are also sources of SO2and VOC emissions, as described below.
The corn wet milling process uses about 1.1 to 2.0 kg of SO2 per megagram (Mg) of corn
(0.06 to 0.11 lb/bu). The SO2 is dissolved in process waters, but its pungent odor is present in theslurries, necessitating the enclosing and venting of the process equipment. Vents can be wet-scrubbed
with an alkaline solution to recover the SO2 before the exhaust gas is discharged to the atmosphere.
The most significant source of VOC emissions, and also a source of PM emissions, from corn wet
milling is the exhaust from the different drying processes. The starch modification procedures also
may be sources of acid mists and VOC emissions, but data are insufficient to characterize or toquantify these emissions.
Dryer exhausts exhibit problems with odor and blue haze (opacity). Germ dryers emit a
toasted smell that is not considered objectionable in most areas. Gluten dryer exhausts do not create
odor or visible emission problems if the drying temperature does not exceed 427°C (800°F). Higher
temperatures promote hot smoldering areas in the drying equipment, creating a burnt odor and a bluebrown haze. Feed drying, where steepwater is present, results in environmentally unacceptable odor if
the drying temperature exceeds 427°C (800°F). Blue haze formation is a concern when drying
temperatures are elevated. These exhausts contain VOC with acrid odors, such as acetic acid andacetaldehyde. Rancid odors can come from butyric and valeric acids, and fruity smells emanate frommany of the aldehydes present.
The objectionable odors indicative of VOC emissions from process dryers have been reduced
to commercially acceptable levels with ionizing wet-collectors, in which particles are charged
electrostatically with up to 30,000 volts. An alkaline wash is necessary before and after the ionizing
sections. Another approach to odor/VOC control is thermal oxidation at approximately 750°C(1382°F) for 0.5 seconds, followed by some form of heat recovery. This hot exhaust can be used as
the heat source for other dryers or for generating steam in a boiler specifically designed for this typeof operation. Incineration can be accomplished in conventional boilers by routing the dryer exhaustgases to the primary air intake. The limitations of incineration are potential fouling of the boiler air
intake system with PM and derated boiler capacity because of low oxygen content. These limitations
severely restrict this practice. At least 1 facility has attempted to use a regenerative system, in whichdampers divert the gases across ceramic fill where exhaust heats the fumes to be incinerated.
Incinerator size can be reduced 20 to 40 percent when some of the dryer exhaust is fed back into thedryer furnace. From 60 to 80 percent of the dryer exhaust may be recycled by chilling it to condense
the water before recycling.
The PM emissions generated from grain receiving, handling, and processing operations at corn
wet milling facilities can be controlled either by process modifications designed to prevent or inhibit
emissions or by application of capture collection systems.
The fugitive emissions from grain handling operations generated by mechanical energy
imparted to the dust, both by the operations themselves and by local air currents in the vicinity of theoperations, can be controlled by modifying the process or facility to limit the generation of fugitive
dust. The primary preventive measures used by facilities are construction and sealing practices that
limit the effect of air currents, and minimizing grain free fall distances and grain velocities during
handling and transfer. Some recommended construction and sealing practices that minimize emissions
are: (1) enclosing the receiving area to the extent practicable; (2) specifying dust-tight cleaning and1/95

Food And Agricultural Industry

9.9.7-5

processing equipment; (3) using lip-type shaft seals at bearings on conveyor and other equipment
housings; (4) using flanged inlets and outlets on all spouting, transitions, and miscellaneous hoppers;and (5) fully enclosing and sealing all areas in contact with products handled.
While preventive measures can reduce emissions, most facilities also require ventilation orcapture/collection systems to reduce emissions to acceptable levels. Milling operations generally areventilated, and some facilities use hood systems on all handling and transfer operations. The control
devices typically used in conjunction with capture systems for grain handling and processing
operations are cyclones (or mechanical collectors) and fabric filters. Both of these systems canachieve acceptable levels of control for many grain handling and processing sources. However, eventhough cyclone collectors can achieve acceptable performance in some scenarios, and fabric filters arehighly efficient, both devices are subject to failure if not properly operated and maintained.
Ventilation system malfunction, of course, can lead to increased emissions at the source.
Table 9.9.7-1 shows the filterable PM emission factors developed from the available data on
several source/control combinations. Table 9.9.7-2 shows potential sources of VOC and SO2, although
no data are available to characterize these emissions.

9.9.7-6

EMISSION FACTORS

1/95

Table 9.9.7-1 (Metric And English Units). PARTICULATE MATTER EMISSION FACTORSFOR CORN WET MILLING OPERATIONSa
EMISSION FACTOR RATING: E
Filterable PMbEmission Source

kg/Mg

lb/tonFabric filter

0.016

0.033

Grain handlingc (legs, belts, etc.)(SCC 3-02-007-52)

None

0.43

0.87

Grain cleaningd(SCC 3-02-007-53)

None

0.82

1.6

Grain cleaningd(SCC 3-02-007-53)

Cyclone

0.086

0.17

Starch storage bine
(SCC 3-02-014-07)

Fabric filter

0.00070.0014

Starch bulk loadoutf(SCC 3-02-014-08)

Fabric filter

0.000250.00049

Grain receivingc (trucks)

Type Of Control(SCC 3-02-007-51)

Gluten feed drying
Direct-fired rotary dryersg(SCC 3-02-007-63)

Product recovery
cyclone

0.13

0.27

Indirect-fired rotary dryersg(SCC 3-02-007-64)

Product recovery
cycloneh

0.25

0.49

Flash dryersj(SCC 3-02-014-10, -12)

Wet scrubber

0.29

0.59

Spray dryersk(SCC 3-02-014-11, -13)Fabric filter

0.080

0.16

Direct-fired rotary dryersg(SCC 3-02-007-68)

Product recovery
cyclone

0.13

0.27

Indirect-fired rotary dryersg(SCC 3-02-007-69)

Product recovery
cyclone

0.25

0.49

Starch drying

Gluten drying

Fiber drying(SCC 3-02-007-67)

ND

ND

ND

Germ drying(SCC 3-02-007-66)

ND

ND

ND

Dextrose drying(SCC 3-02-007-70)

ND

ND

ND

Degerminating mills(SCC 3-02-007-65)

ND

ND

ND

Milling(SCC 3-02-007-56)

ND

ND

ND1/95

Food And Agricultural Industry

9.9.7-7

Table 9.9.7-1 (cont.).
a For grain transfer and handling operations, factors are for an aspirated collection system of 1 ormore capture hoods ducted to a particulate collection device. Because of natural removal processes,
uncontrolled emissions may be overestimated. ND = no data. SCC = Source Classification Code.
b Emission factors based on weight of PM, regardless of size, per unit weight of corn throughput
unless noted.
c Assumed to be similar to country grain elevators (see Section 9.9.1).
d Assumed to be similar to country grain elevators (see Section 9.9.1). If 2 cleaning stages are used,emission factor should be doubled.
e Reference 9.
f Reference 9. Emission factor based on weight of PM per unit weight of starch loaded.
g Reference 10. Type of material dried not specified, but expected to be gluten meal or gluten feed.
Emission factor based on weight of PM, regardless of size, per unit weight of gluten meal or gluten
feed produced.
h Includes data for 4 (out of 9) dryers known to be vented through product recovery cyclones, andother systems are expected to have such cyclones. Emission factor based on weight of PM,regardless of size, per unit weight of gluten meal or gluten feed produced.
j References 11-13. EMISSION FACTOR RATING: D. Type of material dried is starch, butwhether the starch is modified or unmodified is not known. Emission factor based on weight ofPM, regardless of size, per unit weight of starch produced.
k Reference 14. Type of material dried is starch, but whether the starch is modified or unmodified isnot known. Emission factor based on weight of PM, regardless of size, per unit weight of starch
produced.

Table 9.9.7-2 (Metric And English Units). EMISSION FACTORS FOR CORN WET MILLINGOPERATIONSEmission Source
Steeping(SCC 3-02-007-61)
Evaporators(SCC 3-02-007-62)
Gluten feed drying(SCC 3-02-007-63, -64)
Germ drying(SCC 3-02-007-66)
Fiber drying(SCC 3-02-007-67)
Gluten drying(SCC 3-02-007-68, -69)
Starch drying(SCC 3-02-014-10, -11,-12, -13)
Dextrose drying(SCC 3-02-007-70)
Oil expelling/extraction(SCC 3-02-019-16)

VOCSO2Type Of
Control

kg/Mg

lb/tonkg/Mg

lb/ton

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND = no data. SCC = Source Classification Code.
9.9.7-8

EMISSION FACTORS

1/95

References For Section 9.9.7
1.

Written communication from M. Kosse, Corn Refiners Association, Inc., Alexandria, VA, toD. Safriet, U. S. Environmental Protection Agency, Research Triangle Park, NC, January 18,
1994.

2.

L. J. Shannon, et al., Emissions Control In The Grain And Feed Industry, Volume I:Engineering And Cost Study, EPA-450/3-73-003a, U. S. Environmental Protection Agency,
Research Triangle Park, NC, December 1973.

3.

G. F. Spraque and J. W. Dudley, Corn And Corn Improvement, Third Edition, American
Society Of Agronomy, Crop Science Society Of America, and Soil Science Society Of
America, Madison, WI, 1988.

4.

S. A. Watson and P. E. Ramstad, Corn Chemistry And Technology, American Association ofCereal Chemists, St. Paul, MN, 1987.

5.

American Feed Manufacturers Association, Arlington, VA, Feed Technology, 1985.

6.

D. Wallace, "Grain Handling And Processing", Air Pollution Engineering Manual, Van
Nostrand Reinhold, NY, 1992.

7.

H. D. Wardlaw, Jr., et al., Dust Suppression Results With Mineral Oil Applications For Corn
And Milo, Transactions Of The American Society Of Agricultural Engineers, Saint Joseph,MS, 1989.

8.

A. V. Myasnihora, et al., Handbook Of Food Products — Grain And Its Products, Israel
Program for Scientific Translations, Jerusalem, Israel, 1969.

9.

Starch Storage Bin And Loading System, Report No. 33402, prepared by Beling Consultants,Moline, IL, November 1992.

10.

Source Category Survey: Animal Feed Dryers, EPA-450/3-81-017, U. S. Environmental
Protection Agency, Research Triangle Park, NC, December 1981.

11.

Starch Flash Dryer, Report No. 33405, prepared by Beling Consultants, Moline, IL,February 1993.

12.

No. 4 Starch Flash Dryer, Report No. I-7231-1, prepared by The Almega Corporation,
Bensenville, IL, May 1993.

13.

No. 1 Starch Flash Dryer, Report No. 86-177-3, prepared by Burns & McDonnell, Kansas
City, MO, August 1986.

14.

Starch Spray Dryer, Report No. 21511, prepared by Mostardi-Platt Associates, Inc.,
Bensenville, IL, August 1992.

1/95

Food And Agricultural Industry

9.9.7-9