Lesson 6
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 form 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 50°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.