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ISI Technical Memorandum on Corn Starch [TM]

Maize (also known as corn), common name for a cereal grass widely grown for food and livestock fodder. Maize ranks with wheat and rice as one of the worlds chief grain crops. More starch is produced from maize than any other crop.

The maize plant has an erect, solid stem, rather than the hollow one of most other grasses. It varies widely in height, some dwarf varieties being little more than 60 cm (2 ft) tall at maturity, whereas other types may reach heights of 6 m (20 ft) or more. The average is 2.4 m (8 ft). The leaves, which grow alternately, are long and narrow. The main stalk terminates in a staminate (male) inflorescence, or tassel. The tassel is made up of many small flowers termed spikelets, and each spikelet bears three small anthers, which produce the pollen grains, or male gametes. The pistillate (female) inflorescence, or ear, is a unique structure with up to 1,000 seeds borne on a hard core called the cob. The ear is enclosed in modified leaves called husks. The individual silk fibres that protrude from the tip of the ear are the elongated styles, each attached to an individual ovary. Pollen from the tassels is carried by the wind and falls onto the silk, where it germinates and grows down through the silk until it reaches the ovary. Each fertilized ovary grows and develops into a kernel.

Maize is the most important raw material for industrial starch.

THE DIFFERENT KINDS OF CORN

DENT corn, the scientific name of which is Zea mays indentata, is also called "field" corn. It is a corn variety with kernels that contain both hard and soft starch and become indented at maturity. It is a major crop used to make food, animal feed, and industrial products. This is the only variety to be considered for cornstarch manufacturing.

FLINT corn, known by the scientific name Zea mays indurata, is a variety of corn having hard, horny, rounded or short and flat kernels with the soft and starchy endosperm completely enclosed by a hard outer layer. It is similar to dent and is used for the same purposes. Most of it is grown in South America.

WAXY corn is a corn variety with grains that have a waxy appearance when cut, and that contains only branched-chain starch.  Waxy corn starch is over 99% amylopectin, whereas regular corn contains 72-76% amylopectin and 24-28% amylose. Amylopectin is a branched form of starch of high molecular weight, while amylose is a smaller unbranched or linear form of starch. Waxy corn is processed in wet milling to produce waxy cornstarch which slowly retrogrades back to the crystalline form of starch. It is grown to make special starches for thickening foods in particularly those that undergo large temperature changes in processing and preparation.

SWEET or "green" corn is eaten fresh, canned, or frozen. It is a type of corn that is grown in many horticultural varieties. It is variously considered a distinct species (Zea saccharata or Zea rugosa), a subspecies (Zea mays rugosa) or a specific mutation of dent corn. It is distinguished by kernels containing a high percentage of sugar in the milk stage when they are suitable for table use.

POPCORN is a variety of corn, Zea mays everta, which has small ears and small pointed or rounded kernels with very hard corneous endosperm that, on exposure to dry heat, are popped or everted by the expulsion of the contained moisture, and form a white starchy mass many times the size of the original kernel.

INDIAN corn has white, red, purple, brown, or multicoloured kernels. It was the original corn grown by the Indians, and is known by the scientific name Zea mays. It is many times seen in harvest time and Halloween decorations.

FLOUR corn, also called "soft" corn or "squaw" corn, has kernels shaped like those of flint corn and composed almost entirely of soft starch. It is known by the scientific name Zea mays amylacea. USA grows small amounts of blue flour corn to make tortillas, chips, and baked goods. In South America this corn is grown in various colours to make food and beer.

Corn Composition (15% Moisture Basis, USA)

Determination of dry matter is an adequate method for the purchaser to estimate the value of corn. The higher the dry matters the better the starch yield and the better the storage stability.

An even faster method for the purchaser is to measure weight density. Starch yield and other wet-milling properties are not appreciably affected unless corn test weight drops below about 61.7 kg/hl.

Only dent corn is useful for the starch process - flint corn f.e. is extremely difficult to steep

Reproductive Stages of Corn Development.

R1 Silking - begins with any silk visible outside husks. It takes 2-3 days for all silk to emerge and be pollinated.

R2 Blister - occurs 10-14 days after silking. Endosperm and inner fluid are clear. About 85% moisture.

R3 Milk - occurs 18-22 days after silking. Fluid is milky white. About 80 % moisture

R4 Dough - occurs 24-28 days after silking. About half of mature dry weight has accumulated. About 70 % moisture.

R5 Dent - occurs 35-42 days after silking. Milk line develops at the top and moves towards kernel tip during a 3-3½ weeks' time. About 55 % moisture.

R6 Physiological Maturity - occurs 55-65 days after silking. Hard starch layer advanced to cob and black (dark) abscission layer formed. 30-35 % moisture.

Drydown Period.

After reaching maturity, typical drying rates may range from 0.4% to 0.8% loss of moisture content per day. Rates of drydown vary depending on temperature and moisture levels. The optimum harvest moisture content for corn is approximately 23% to 25%. At this moisture level, kernels shell easily and stalks generally stand better, which can make harvesting more efficient. A normal harvest loss level of a timely and efficient harvest is about 1 to 2%. Delaying harvest until corn dries down to 17% to 19% moisture content can save on artificial drying costs, but there is greater potential for excess harvest losses from stalk lodging and ear drop.

Steeping

During drydown cell constituents agglomerate gradually. Steeping is required to reverse this process. The lower the moisture density the longer steeping time is required. During early harvest adjustments of steeping conditions may be required.

Image Source: Agronomy Dept., Purdue Univ.