Corn: Classification, Dry Milling and Wet Milling

CORN: CLASSIFICATION, DRY MILLING AND WET MILLING

6.1 Introduction
Corn or Maize (Zea mays, L) is used for animal feeding, for human consumption and for the
manufacture of starch, corn syrup solids, sugar, beer, industrial spirit, etc. The products of
milling include maize grits, meal, flour, and protein and corn steep liquor. Corn is consumed as
human food in many forms. In its harvested wet form, it is consumed as vegetable. The ready-to-
eat breakfast cereal “corn flakes” is made from maize grits. Popcorn, the first snack food is
undoubtedly the oldest snack food. The majority of corn consumed as human food has
undergone milling and is consumed as a specific or modified fraction of the original cereal grain.
Like other cereal grains, corn is milled to remove hulls and germ.
6.2 Corn Classification
Maize or corn is classified commercially into four main classes as follows:
1. Dent varieties, which, when mature have a pronounced depression or dent at the top of
the kernel. These have hard patches of densely packed endosperm cells at the outer edges
of their endosperm and soft, opaque cells toward their center. Their shapes vary from
long and narrow to wide and shallow.
2. Flint varieties, which have a continuous hard layer surrounding the endosperm. When
these kernels dry, they dry evenly and therefore do not form a dent.
3. Flour or soft varieties, which are almost entirely opaque and soft. It is the soft maize
varieties that are normally used to make corn flour.
4. Waxy maize varieties that have a waxy appearance especially when broken. The starch
consists of very little amylose and is effectively 100% amylopectin (maize starch is
normally about 30% amylose and 70% amylopectin).
6.3 Corn Processing
Maize is processed by dry or wet milling. Dry milling may or may not include de-germing as a
preliminary step. Non-de-germing dry milling is carried out on a local basis in small grist mills
or in modern roller mills using sifters and purifiers. The maize is ground to
make coarse wholemeal of 85 - 95% extraction rate. This wholemeal is highly susceptible to the
rancidity as the germ is retained which has a high oil content. Wet milling and dry milling
involving de-germing are carried out in large commercial mills.
6.3.1 Dry milling
 Two different systems are used for dry milling of corn.
 The non-degerming system grinds corn into mill with hardly any separation of germ. This
corn meal has comparatively shorter shelf-life, as the germ is retained, which contains 32
- 35% oil. This oil in presence of oxygen and lipolytic enzymes is prone to oxidative and
hydrolytic rancidity.
 Hence, it is necessary to remove the germ from corn to produce corn products with much
lower fat content and greater shelf-life.
 Tempering and degerming system remove most of the germ and hull and leave the
endosperm as free of oil and fiber as possible to recover maximum yield of endosperm
and germ as large clean particles.
 Corn is cleaned to remove dirt, stones, insects, tramp iron, broken kernels and extraneous
plant materials.
 The corn is then conditioned by adding water to increase the moisture content to 20%,
and the moistened corn is allowed to equilibrate for 1 - 3 hrs. The objective of
conditioning is to loosen the germ and toughen the bran and to mellow the endosperm so
as to obtain a maximum yield of grits and a minimum yield of flour in the subsequent
milling.
 Degerming and dehulling is carried out in one of the three ways:
1. Beall de-germinator (De-germer and corn huller)
2. With roller mills and sifters
3. With impact machines such as entoleters and gravity separators
 Once the germ and hull are removed, the endosperm is reduced in size to grits with roller
mills. A complex array of additional roller mills and particle size separating equipments
is used to purify and size endosperm particles. All products must be dried prior to
packaging or bulk storage.
6.3.2 Wet milling
 Wet milling of corn is achieved by a combination of chemical and mechanical means.
Wet milling begins with steeping of cleaned corn for 30 - 48 hours with water. Sulfur
dioxide is added to the water at the rate of 0.1 - 0.2% and the solution is heated to about
50O C. This condition prevents growth of putrefying microorganisms.
 During steeping, the kernel absorb solution and swell, activating enzymes native to the
kernel to assist in breaking down the structure; the bisulfite ion reduces disulfide bonds in
the protein matrix, increasing protein solubility and diminishing interactions between
starch and protein; the lactic acid and/or exogenous enzymes produced by the lactobacilli
help soften the endosperm.
 After steeping corn is ready for grinding and fractionating in disc attrition mill. The
ground slurry is then pumped to hydroclones (liquid cyclones) to separate lighter-weight
germs. The germs are dried and processed for oil and meal. The heavier underflow from
the hydroclones is screened, and larger particles are finely reground with an impact mill
to free the starch, protein, and fiber from each other.
 Fiber is separated and washed over series of screens.
 The remaining stream of starch and protein is passed through disc nozzle type
centrifuges, where heavier starch is separated from the gluten.
 The gluten is dewatered using additional centrifuges and vacuum filters. The remaining
starch slurry is washed and passed through hydroclones. Centrifuges and/or vacuum filter
dewater the purified starch.

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