Print This PageDry Milling Process - Introduction
The Raw Material

Before entering into a discussion of the milling process, it is necessary to review the structure, chemical composition, physical properties, and so forth, of the corn kernel, as these characteristics play an important role in determining the success of, and to some extent the very nature of, the process used upon them.

There are six basic types of corn having different sorts of kernels.   They are:

Dent Corn Flint Corn Flour Corn
Sweet Corn Popcorn Pod Corn

Dent corn is of primary commercial interest.   The name is derived from the indentation in each kernel caused by the unequal shrinkage of different fractions of starch as the corn dries.  Considerable genetic research has been expended on the creation of hybrids from this variety of corn, and at present numerous varieties exist, each possessing a certain share of favorable characteristics.  The choice of proper hybrid for existing climactic and soil conditions is a complicated decision for the corn grower.

Kernel Diagram

The major portions of the kernel are:

Pericarp - also called the hull or bran, is the outer covering of the kernel that protects it from deterioration. It is water- and water vapor-resistant and is undesirable to insects and micro-organisms.

Endosperm - The Endosperm is about 82% of the kernel's dry weight and is the source of energy and protein (starch) for the germinating seed. There are two types of endosperm, soft (floury) and hard (horny). The endosperm near the tip of the kernel constitutes the soft portion and tends to break easily while other portions of the endosperm remain in larger segments.  Flour ground from the soft portion is called "soft flour" and that ground from the other portions is called "sharp flour", mainly from the way the particles feel when rubbed in the hand.  In the hard endosperm, starch is packed tightly together; in the soft endosperm, the starch is loose. When corn dries in the field before harvest, the moisture loss causes the soft endosperm to collapse and form a dent in the top of the kernel.

Germ - The Germ is the only living part of the corn kernel. It contains the essential genetic information, enzymes, vitamins and minerals for the kernel to grow into a corn plant. About 25% of the germ is corn oil. Corn oil is the most valuable part of the corn kernel because of its amount of linoleic acid (polyunsaturated fat) and its bland taste.

Tip Cap - The Tip Cap is the only area of the kernel not covered by the pericarp. It was the attachment point of the kernel to the cob. It is the major entry path into the kernel.

Of these portions, the bran constitutes about 5% of the kernel, the germ about 12% and the endosperm about 82%.  The average yield of endosperm products is between 65% and 70%, the residual being "shorts" (small endosperm removed from the endosperm stream), germ and bran.  After oil extraction from the germ, these three components are combined to produce hominy feed, an animal feed.

The endosperm products may be classified by size as follows:

Grits

6 to 16 United States Bureau of Standards Sieve

 

Meals

Course

16 to 24 USBS Sieve

Medium

24 to 40 USBS Sieve

Cones

40 to 70 USBS Sieve

 

Flours

Sharp

70 to 100 USBS Sieve

 

70 maximum USBS Sieve

 

Soft Corn Flour - as mentioned before, comes from the soft endosperm and is separated by sieving from the early break products (that is, soon after the process of crushing and shearing between revolving cast iron rolls is begun) because it is easily fragmented.

Sharp Corn Flour - comes from the flinty endosperm and is of similar size to the soft flour, but much more harsh to the touch.

Cones - is a product similar to sharp flour except it is coarser.  It is often employed in brewing and in other food industries where dust must be kept at a minimum.

Corn Meals - are products smaller than corn grits but much coarser than the corn flours.  They are often intended for table consumption, and often milled from white corn in that case.   They may also be used to make gelatinized corn flour or for industrial purposes quite apart from the realm of food.

Corn Grits - are most often produced in screen sizes 8, 10 and 12 and are intended for use by breweries to ferment alcoholic products.  Because of the variation in brewing method from one brewery to another, the individual brewer's specification of size must be obeyed strictly, and accuracy of sizing must be maintained.  The size of these particles makes visual inspection important, as contaminating particles are likely to be of similar size.   In the United States, cheaper yellow corn is being used more and more for this product, although formerly brewers greatly preferred white corn.

Pearl Hominy - ("number 4 grits") is used largely for production of breakfast cereals, and therefore close attention must be given in order to prevent contamination, as contaminating particles are likely to be very similar in size.  Strict visual inspection of the product is important.

 

* Some kernel information taken from ohiocorn.org.