Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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0023 One study has produced experimental magnesium

deficiency by feeding magnesium-deficient formulas

to humans recovering from surgery for oral cancer.

When subjects were switched from a magnesium-

adequate formula to a magnesium-deficient one,

urinary and fecal magnesium losses fell sharply, but

were not sufficient to prevent a steady decline in

serum magnesium. In these subjects, hypocalcemia

(low serum calcium) and hypokalemia (low serum

potassium) were also seen, even though the magne-

sium-deficient formula provided adequate calcium

and potassium. It would therefore seem that the

hypocalcemia and hypokalemia seen in magnesium-

deficient individuals are not solely due to low intakes,

as magnesium appears to have a profound effect on

potassium and calcium homeostasis.

Magnesium Excess

0024 Magnesium toxicity is rare. This condition is charac-

terized by smooth-muscle relaxation, nausea and

vomiting, mental confusion, cardiac arrhythmias,

and, ultimately, coma and cardiac arrest. Hypoten-

sion is one of the earliest signs, occurring by the time

magnesium serum levels are doubled. Death does not

usually occur until serum levels exceed 7–8 mmol l

1

0025 It is unusual for excessive dietary intakes of mag-

nesium to cause toxicity, but some cases have been

reported following excessive use of medical magne-

sium salts such as Epsom salts (magnesium sulfate,

used as a laxative) or milk of magnesia (magnesium

hydroxide, an antacid and laxative). In humans,

the most likely cause of magnesium toxicity is renal

failure and an inability to excrete magnesium in

the urine. It is possible for magnesium toxicity to

occur during therapeutic use of intravenous magne-

sium to treat preterm labor or pregnancy-induced

hypertension.

Recommended Dietary Intakes of

Magnesium

0026 Due to the lack of a reliable measurements of magne-

sium status, current recommendations for magnesium

intake are largely based on metabolic balance

studies and estimates of the magnesium required for

‘normal’ growth.

0027 The Food and Nutrition Board of the Institute of

Medicine recently published a comprehensive report

of dietary reference intake for magnesium. Its find-

ings are summarized in Table 3. A number of different

definitions were used:

0028 The estimated average requirement (EAR) is the

intake that will meet the needs of half of a given

population. At this intake, therefore, half of the

population will have an inadequate intake.

0029The recommended dietary allowance (RDA) is the

intake that will meet the needs of most (97%) of a

given population.

0030The adequate intake is given if insufficient data are

available to define the RDA. At this intake, the

needs of essentially all the population are met.

In addition, it was recommended that the EAR for

pregnant women should be 35 mg higher than for

nonpregnant women of the same age. No increase

was deemed necessary for lactating women.

Magnesium and Human Disease

0031Magnesium deficiency or low magnesium intakes

have been suggested as risk factors for a number of

important human diseases, particularly hypertension

and coronary heart disease.

Hypertension

0032Magnesium plays an important intracellular role in

decreasing the excitability of smooth muscle, leading

to a reduction in smooth-muscle tone. In addition,

intravenous magnesium therapy is a useful treatment

of hypertensive emergencies in pregnant women

(pregnancy-induced hypertension). Animal studies

have shown a consistently inverse relationship

between magnesium intake and blood pressure, and

an inverse correlation between intracellular magne-

sium concentration and blood pressure. For these

reasons, there has been considerable study of the

effect of magnesium nutrition on blood pressure.

0033The results of a number of observational studies

have suggested that low serum or urinary magnesium

tbl0003Table 3 Dietary reference intakes for magnesium

Age Gender AI EAR RDA

0–6 months M/F 30 mg

7–12 months M/F 75 mg

1–3 years M/F

65 mg 80 mg

4–8 years M/F

110 mg 130 mg

9–13 years M/F

200 mg 240 mg

14–18 years M

340 mg 410 mg

300 mg 360 mg

19–30 years M

330 mg 400 mg

255 mg 310 mg

> 31 years M

350 mg 420 mg

265 mg 320 mg

AI, adequate intake; EAR, estimated average requirement; RDA,

recommended dietary allowance.

, if insufficient data were available to

determine the RDA and EAR, an AI was recommended instead.

Data from The Standing Committee on the Scientific Evaluation of Dietary

Reference Intakes, Food and Nutrition Board, Institute of Medicine (1997)

Dietary Reference Intakes for Calcium, Phosphorus, Magnesium,Vitamin D and

Fluoride. Washington, DC: National Academy Press.

3644 MAGNESIUM

concentrations may be risk factors for hypertension.

But these studies are contradictory, and negative

results have been reported in many other studies.

Some interventional studies have evaluated the effect

of magnesium supplements on hypertensive individ-

uals. Preliminary evidence suggests that, in hyper-

tensive individuals treated with thiazide diuretics,

magnesium supplementation can lower blood pres-

sure. This is physiologically plausible, as thiazides

inhibit magnesium reabsorption in the kidney and

may lead to magnesium deficiency. However, a similar

effect has been demonstrated with potassium supple-

mentation, with no additional benefit from combined

magnesium and potassium supplementation. Further,

magnesium supplementation of hypertensive individ-

uals with normal serum magnesium levels has not

been of any benefit. Clearly, the current literature is

confusing, and further work to determine potential

interactions among hypertension, diuretic use, and

magnesium supplementation is urgently needed.

Cardiovascular Disease

0034 There is increasing evidence that populations residing

in areas supplied with hard water (which has higher

mineral content, including magnesium) have a lower

risk of cardiovascular disease than people who live in

similar regions supplied with soft water. Early studies,

however, suggested that higher levels of serum mag-

nesium can be detected in serum immediately after

myocardial infarcts. More recent studies have sug-

gested that this is a result of magnesium released

from dying heart cells, rather than an indication that

an existing high serum magnesium concentration

is a risk factor for myocardial infarct per se. Some

intervention studies have shown that intravenous

magnesium may be a useful adjuvant therapy in

acute myocardial infarcts.

Diabetes Mellitus

0035 Diabetics have increased urinary magnesium losses.

Such losses are especially high in individuals with

poorly controlled diabetes, those with high amounts

of glucosuria, and those with diabetic complications.

It is unclear whether the correlation between low

serum magnesium concentrations and diabetic

complications is a cause and effect relationship, or

whether both are independent effects of poor glyce-

mic control. However, magnesium supplementation

has been reported to improve insulin sensitivity and

glycemic control in adults with noninsulin-dependent

diabetes mellitus. In 1992, the American Diabetic

Association (www.diabetes.org) convened a consen-

sus panel to evaluate the possibility of a relationship

between magnesium deficiency and diabetes mellitus.

The panel members felt that there was evidence to

suggest that magnesium deficiency might be related to

some of the underlying causes of diabetes, such as

insulin resistance and carbohydrate intolerance.

However, they recognized the difficulty of diagnosing

magnesium deficiency, and did not recommend

universal magnesium supplementation for diabetics.

Osteoporosis

0036Thirty percent of total body magnesium is in bone.

Magnesium is important for normal bone growth and

mineralization. Some studies have suggested that

magnesium supplementation can increase bone min-

eral content in subjects on relatively low magnesium

intakes, but such benefits were not seen in women on

higher magnesium intakes. Two studies have shown

that magnesium supplementation may have beneficial

effects on bone mineral content in women with

established osteoporosis.

Magnesium as a Pharmacological Agent

0037Magnesium is unusual in that it has physiological

effects as a nutrient, and in much higher doses, has a

pharmacological effect as well.

0038One of the important biological functions of mag-

nesium is to alter the excitability of nerves and

muscles, either directly or through an interaction

with calcium. Large doses of intravenous magnesium

have been used to treat medical conditions involving

excessive excitability of nervous tissues. The classic

example is the use of intravenous magnesium to treat

pregnancy-induced hypertension. This condition,

also called preeclampsia, is characterized by protei-

nuria, edema, and hypertension. Untreated, pre-

eclampsia leads to central nervous system

excitability, convulsions and, ultimately, death. It is

the most common cause of maternal morbidity in

western countries. Intravenous magnesium has been

widely used for many years to treat this condition; it

has been shown to decrease the risk of seizures and

improve maternal and fetal outcome.

0039Magnesium is also used to delay the progression of

preterm labor, again by relaxing excitable cells, in this

case, uterine muscle cells. It is also used as a treatment

for primary pulmonary hypertension of the newborn

(or persisting fetal circulation). This latter condition

is caused by abnormal contraction of the blood

vessels entering the lung, and leads to profound hyp-

oxia and, ultimately, death. Magnesium acts to relax

these pulmonary blood vessels and improve oxygen-

ation. Unfortunately, it may also relax other blood

vessels and lead to hypotension, counteracting its

beneficial effect on pulmonary vessels.

0040There are conflicting data on the benefit of

intravenous magnesium following acute myocardial

MAGNESIUM 3645

infarcts. The LIMIT-2 study suggested that intra-

venous magnesium reduced mortality from 10.3%

to 7.8%, and also significantly decreased the risk of

congestive heart failure. The much larger ISIS-4 study

found no such benefit in a much larger sample size

(ﬃ60 000 subjects), and a possible increase in mortal-

ity in subjects with heart failure who were treated

with magnesium.

0041 Once again, magnesium offers possible benefits for

treating human disease, but the evidence (particularly

for the treatment of heart attacks) is confusing, and

further studies are needed before magnesium can be

recommended in these conditions.

Conclusions

0042 Magnesium is a vital component of the plant protein

chlorophyl, which is responsible for fixing solar

energy into chemical energy in green plants. In this re-

spect, most life on earth is dependent on magnesium.

0043 In humans, magnesium is an essential nutrient, and

is widely available in the diet. Severe magnesium defi-

ciency is very rare in otherwise healthy individuals,

but is seen in a number of disease states, particularly

renal disease, malabsorptive syndromes, and in alco-

holics. There is increasing concern about the effect of

marginal magnesium status, or low dietary intake, on

chronic diseases, including osteoporosis, myocardial

infarct, and stroke. Over the next 5–10 years, more

information is likely to accumulate to help us under-

stand the effect of dietary magnesium on these condi-

tions, and the potential use of magnesium as a therapy

for these conditions when they occur.

Acknowledgment

0044 Supported in part with federal funds from the USDA/

ARS under Cooperative agreement No. 58-6250-6-

001. This work is a publication of the US Department

of Agriculture (USDA)/Agricultural Research Service

(ARS) Children’s Nutrition Research Center, Depart-

ment of Pediatrics, Baylor College of Medicine and

Texas Children’s Hospital, Houston, Texas. Contents

of this publication do not necessarily reflect the views

or policies of the USDA, nor does mention of trade

names, commercial products, or organizations imply

endorsement by the US government.

See also: Alcohol: Alcohol Consumption; Chlorophyl;

Coronary Heart Disease: Etiology and Risk Factor;

Diabetes Mellitus: Etiology; Hypertension:

Hypertension and Diet; Osteoporosis

Further Reading

Aikawa JK (1981) Magnesium: Its Biological Significance.

Boca Raton: CRC Press.

American Diabetic Association(1992) Magnesium supple-

mentation in the treatment of diabetes. Diabetes Care

15(8): 1065–1067.

Committee on Diet and Health, Food and Nutrition Board,

Commission on Life Sciences, National Research Coun-

cil (1989) Diet and Health, Implications for Reducing

Chronic Disease Risk. Washington, DC: National

Academy Press.

Czajka-Narins DM (1996) Minerals. In: Mahan LK and

Escott-Stump S (eds) Krause’s Food, Nutrition and

Diet Therapy, 9th edn. Philadelphia: WB Saunders,

pp. 109–140.

Elmadfa I and Koenig JS (1993) Magnesium. In: Macrae R,

Robinson RK and Sadler MJ (eds) Encyclopedia of

Food Science, Food Technology and Nutrition. London:

Academic Press, pp. 2822–2824.

Ensminger AH, Ensminger ME, Konlande JE and Robson

JRK (eds) (1994) Magnesium. In: Food and Nutrition

Encyclopedia, 2nd edn. Boca Raton: CRC Press, pp.

1338–1341.

Goulding A and Robinson M (1998) Calcium and magne-

sium. In: Mann J and Truswell AS (eds) Essentials of

Human Nutrition. Oxford: Oxford University Press, pp.

123–136.

Sauberlich HE (1999) Magnesium. In: Laboratory Tests for

the Assessment of Nutritional Status. Boca Raton: CRC

Press, pp. 331–342.

Shils ME (1969) Experimental production of magnesium

deficiency. Annals of the New York Academy of Science

162: 847–855.

Shils ME (1996) Magnesium. In: Ziegler EE and Filer Jr. LJ

(eds) Present Knowledge in Nutrition, 7th edn. Wash-

ington, DC: ILSI Press, pp. 256–264.

Shils ME (1999) Magnesium. In: Shils ME, Olson JA, Shike

M and Ross AC (eds) Modern Nutrition in Health and

Disease, 9th edn. Baltimore: Williams & Wilkins, pp.

169–192.

The Standing Committee on the Scientific Evaluation of

Dietary Reference Intakes, Food and Nutrition Board,

Institute of Medicine (1997) Magnesium. In: Dietary

Reference Intakes for Calcium, Phosphorus, Magne-

sium, Vitamin D and Fluoride. Washington, DC:

National Academy Press, pp. 190–249.

Widdowson E and Dickerson JWT (1964) Chemical com-

position of the body. In: Comar CL and Bronner F (eds)

Mineral Metabolism. Vol. II: The Elements. New York:

Academic Press, pp. 2–207.

Maillard Reaction See Browning: Nonenzymatic; Enzymatic – Biochemical Aspects; Toxicology of

Nonenzymatic Browning; Enzymatic – Technical Aspects and Assays

3646 MAGNESIUM

MAIZE

S R Eckhoff and M R Paulsen, University of Illinois,

Urbana, IL, USA

S C Yang, Providence University, Taiwan, Republic of

China

Introduction

0001 Maize, Zea mays L. (corn), is the second most abun-

dantly produced cereal in the world, exceeded only

by rice. Maize is grown in every continent except

Antarctica. North America grows over 43% of the

world’s maize crop, with 90% of the US total grown

in the Corn Belt consisting of 12 midwestern and

north central states. Other world leaders in maize

production include China (18%), the EU (7%), Brazil

(6%), the Balkan area (3%), and Mexico (where

maize is generally agreed to have originated) (3%),

Argentina, India, and South Africa (2% each). The

ease with which maize can be dried, stored, and

transported has made it a major source of energy for

animal food and a stable raw material for the produc-

tion of starch and starch-based industrial products.

Maize is also a primary food source in many areas

of the world, including South America, Central

America, and Africa, where it is converted directly

into food products via grinding, alkali processing,

boiling/cooking, or fermentation.

0002 This article reviews the structure and composition

of maize, the genetic diversity of maize, the issues

related to genetically modified maize, the drying,

storage, handling, and grading of maize, and the

major fractionation/processing methods.

Morphology/Anatomy

0003 The maize kernel is the reproductive seed of the maize

plant. As such, its structure and composition exist for

the purposes of reproduction rather than processing.

Figure 1 shows that the kernel is composed of four

main parts expressed as a dry weight percentage of

the whole kernel: germ, 12%, endosperm (horny and

floury), 82%, pericarp, 5.2%, and tip cap, 0.8%.

Each of these sections has distinct compositional

characteristics that are important for utilization of

the maize kernel (Table 1).

0004 The germ, the primary focus of maize kernel from a

reproductive standpoint, contains all the essential

enzymes, nutrients, and genetic material to produce

a maize plant. If the germ is removed from the

kernel, planted, and supplied with the necessary

chemicals for growth, it will produce a new plant.

The remainder of the kernel’s purpose is to protect the

germ and to provide the energy and structural com-

ponents necessary for the early growth of the plant.

0005The germ contains approximately 34% oil, 19%

protein, and 28% solubles (sugars, water soluble pro-

teins, mineral, and vitamins), and the balance insol-

uble materials. Although germ is relatively high in

protein, the main interest to processors is the oil it

contains. Maize oil is a highly desired food vegetable

oil due to its relatively high level of linolenic fatty acid

(polyunsaturated oil) and excellent flavor. The germ

contains over 65% of the kernel’s albumins (water-

soluble proteins) and globulins (salt soluble proteins),

25% of the total maize protein, and a high percentage

of the kernel’s soluble sugars, vitamins, and minerals.

0006The tip cap is the point of the kernel attaching to the

cob and the passageway for the movement of nutri-

ents to the developing kernel. Near the end of matur-

ation of the kernel, a black layer (hilar layer) forms

across the tip cap region in order to serve as a diffu-

sional barrier and to provide some protection for the

kernel from invading insects and microorganisms.

This black layer can be seen in Figure 1.

0007Enveloping the entire kernel (except the tip cap

region) is the pericarp, which is comprised entirely

of dead, empty cells. These cells are high in cellulose

and hemicellulose and are arranged in several distinct

layers. The epidermis, the outer one or two cell layers,

is thick walled and covered with cutin, a waxy

substance that restricts the entry of water, water

vapor, and many other gases and liquids. Below the

Floury

endosperm

Horny

endosperm

Pericarp

Germ

Tip cap

fig0001Figure 1 (see color plate 102) Maize kernel.

MAIZE 3647

epidermis is the mesocarp, which is composed of 17–

25 layers of elongated cells, up to 1 mm long, con-

nected by openings through which liquids and gases

can diffuse. The next four to six layers of cells are

large open cells, called the cross and tube cells, which

allow for a high flux diffusion of gases or liquids.

Through these cross and tube cells, water, which

enters the tip cap region, can be rapidly transported

around the kernel to allow for diffusion into the germ

and endosperm.

0008 Below the pericarp is a semipermeable membrane

called the seed coat, which resists the movement of

macromolecules into and out of the kernel. Just inter-

ior to the seed coat is the outermost endosperm layer,

the aleurone, composed of large, thick-walled cells of

high protein content. This aleurone layer may also

impart semipermeable characteristics due to the com-

pact arrangement of the cells in this layer. Damage to

the pericarp and/or the seed coat increases the rate of

water or chemical absorption.

0009 The endosperm comprises hard (translucent,

horny, and vitreous) and soft (opaque, floury, and

corneous) endosperm. Both comprise a protein

matrix, which encapsulates granules of starch. The

translucent hard endosperm is composed of densely

packed starch granules surrounded by a thick protein

matrix, while the less dense soft endosperm has some-

what larger round starch granules surrounded by a

thin protein matrix. As moisture escapes from the ear

during the in-field drying of the dent type maize, the

thin protein matrix ruptures, and the loosely packed

soft endosperm structure collapses, leaving many

microfissures which interrupt the structural continu-

ity and give the kernel an opaque appearance. The

external result of the soft endosperm structural col-

lapse is that the crown region is pulled inward giving

a dented appearance. During drying, the thicker pro-

tein matrix in the hard endosperm area shrinks

but does not rupture. The pressure forces the starch

granules into angular shapes, and the density in-

creases. During processing, the hard endosperm

structure resists disruption or modification, while

the soft endosperm is easily disrupted.

0010The protein matrix in the endosperm is composed

primarily of glutelin, a protein soluble in concen-

trated alkali, and zein, an ethanol-soluble protein.

The glutelin is highly cross-linked with disulfide

bonds and provides continuity and the structural

characteristics of the endosperm. Zein does not add

significantly to the endosperm structure because it

primarily exists in spherical balls embedded in the

protein matrix.

0011Starch granules are approximately 99.8% starch.

Each granule in dent and flint maize starch is a mixture

of two polysaccharides, amylose and amylopectin.

Amylose is a linear polymer of glucose (several

thousand anhydroglucose units per molecule), and

amylopectin is a large multibranched polymer

tbl0001 Table 1 Maize kernel composition by parts for nine midwestern US dent maize hybrids compared to flint and flour hybrids (on a

percentage dry-weight basis)

Proportion of whole kernel Starch Protein Oil Ash Sugar

Kernel

Mean of dents 100 71.5 10.3 4.8 1.4 2.0

Range of dents 67.8–74.0 8.1–11.5 3.9–5.8 1.3–1.5 1.6–2.2

Flint 67.8 13.6 5.6 1.5 2.0

Flour 66.8 13.2 5.6 1.5 2.0

Endosperm

Mean of dents 81.9 86.4 9.4 0.8 0.3 0.6

Range of dents 80.3–83.5 83.9–88.9 6.7–11.1 0.7–1.11 0.2–0.5 0.5–0.8

Flint 80.6 84.7 12.8 0.8 0.4 0.7

Flour 79.7 84.7 12.8 0.8 0.3 0.8

Germ

Mean of dents 11.9 8.2 18.8 34.5 10.1 10.8

Range of dents 10.5–13.1 5.1–10.0 17.3–20.0 31–38.9 9.4–11.3 10–12.5

Flint 13.5 8.0 20.2 35.3 9.5 10.0

Flour 14.1 8.0 19.9 35.0 9.2 9.9

Pericarp

Mean of dents 5.3 7.3 3.7 1.0 0.8 0.3

Range of dents 4.4–6.2 3.5–10.4 2.9–3.9 0.7–1.2 0.3–1.0 0.2–0.5

Fling 5.1 7.0 4.6 1.0 1.2 0.3

Flour 5.5 7.4 4.6 0.9 2.0 0.3

Tip cap

Means of all samples 0.8 5.3 9.7 3.8 1.6 1.6

Range of all samples 0.8–1.1 9.1–10.7 3.7–3.8 1.4–2.0

Adapted from Earle FR, Curtis JJ and Hubbard JE (1946) Composition of the component parts of the corn kernel. Cereal Chemistry 23(9): 504–511.

3648 MAIZE

(50 000–200 000 anhydroglucose units per module).

The spherical starch granule consists of a radial

arrangement of the amylose with the amylopectin inter-

dispersed. Dent maize starch contains * 25% amylose

and * 75% amylopectin, while waxy maize starch

contains * 100% amylopectin, and high amylose

maize starch contains 65–85% amylose. (See Starch:

Structure, Properties, and Determination.)

Genetic Diversity

0012 Zea mays is a member of the grass family and is one

of the most diverse species of plants. Maize plants

contain both male and female reproductive structures

and reproduce by both cross-pollination and self-

pollination. In most commercially viable maize geno-

types, the female structure (the ear) projects outward

from a central stalk, while the male structure (the

tassel) projects out the top of the stalk. Pollen from

the tassel is carried by the wind to other maize plants,

where fertilization of the individual kernels on the

ear occurs. The ears of maize may range in size from

about 2.5 to over 45 cm long. The kernel size, shape,

and color also vary widely.

0013 Although there are hundreds of ‘races’ of maize,

most of the commercially grown hybrids emanated

from only a few major races. For the purposes of

discussing the commercial importance of maize,

maize types can be subdivided into four categories

not related to race.

0014 Dent maize is the primary type of maize grown in

the US Corn Belt as well as in Europe, South Africa,

and China. Dent varieties have been adapted through

hybridization and selection to provide a wide range of

agronomic and kernel characteristics. Special dent

varieties have been produced with unique starch char-

acteristics. High-amylose (linear starch) and waxy

(branched starch) maize genotypes have been grown

commercially for many years. Other unique geno-

types such as high-oil maize and high-lysine maize

are also being produced.

0015 Flint maize is genetically different in ancestry from

dent maize and is characterized by hard, round

kernels. Flint maize endosperms consist predomin-

antly of hard or vitreous endosperm. The hard endo-

sperm allows flint maize to withstand greater impact

forces before being damaged, which has advantages

in commerical merchandizing. The agronomic char-

acteristics of flint maize differ from dent maize, pri-

marily due to the unique needs of the historical

growing regions.

0016 Popcorn is a flint-type maize that has been genetic-

ally selected for its ability to expand or ‘pop’ when

heated. Popping occurs when the kernels are rapidly

heated to *240



C. The dense endosperm restricts

water vapor diffusion that causes pressure to build

within the kernel until it explodes. It is the starch

granules that explode and in the process stretch the

protein matrix. As the protein matrix cools, it be-

comes rigid. The white fluffy part of popped popcorn

is gelatinized starch splattered on the surface of the

expanded cell protein matrix.

0017Sweetcorn is dent-type maize that is harvested

while still immature for canning, freezing, and direct

consumption as a vegetable. Sweetcorn hybrids con-

tain a gene that retards the conversion of glucose to

starch in the endosperm. There are generally three

to four times more short-chain polysaccharides accu-

mulated in sweetcorn than in other maize varieties.

Genetically Modified Maize

0018Biotechnology has made it possible to transfer genetic

material within a species as well as between species.

As a result of this development, the first genetically

modified (GM) maize seed to resist pests, known as Bt

(Bacillus thuringiensis) maize, was released in 1997.

Bt maize has saved farmers millions of dollars annu-

ally due to the reduction of pesticides needed to

reduce corn-borer infestations. It has been estimated

that 25% of the maize area planted in the USA in

2000 was GM.

0019While biotechnology offers many opportunities to

enhance maize production yields, functionality and

millability, genetic engineering of crops has been

met with resistance. Scientific data support the safety

of genetically engineered crops. However, there are

still a variety of nontechnical sociological issues sur-

rounding the acceptance of these crops. Concerns

vary from a general distrust of new technology to

ethical concerns. Resistance to GM crops has been

exacerbated by inflammatory press releases and

action by some who view the issue as a means of

passive tariffs. It appears that the controversy will

not be settled in the near future. In the meantime,

marketing systems for keeping GM grain separate

from conventional grain are developing to provide

customers with a choice.

Drying, Storing, Handling, and Grading

0020Maize is generally mechanically harvested at mois-

ture contents ranging from 18 to 30% by weight and

dried to a safe storage moisture. If the maize is har-

vested on the ear, it is harvested at moisture contents

not exceeding 21% and dried in cribs or wire bins by

using natural ventilation. Once dried, the ears are

shelled by a stationary sheller. Nearly all maize in

the major maize-growing regions is now harvested

using mechanical picker-shellers, which separate the

MAIZE 3649

kernels from the cob while in the field. The shelled

maize can then be dried using one of a variety of

drying procedures including high-temperature con-

tinuous driers, batch-in-bin high-temperature driers

and low-temperature in-bin driers. The selection of

a drying procedure depends upon the availability

of equipment, energy cost, harvesting rate, and har-

vesting/drying weather.

0021 Drying kernels at temperatures above 55



C can

decrease product yields and cause processing prob-

lems, as will be discussed in the following sections.

Rapid drying causes internal stress fractures that

reduce the mechanical strength of the kernel and

results in more broken kernels during handling. An

increase in the amount of broken maize during hand-

ling is the primary factor leading to grade reduction.

0022 Most grades and standards for the merchandizing

of maize contain the similar criteria of test weight

(bulk density), broken maize, mold-damaged kernels,

heat-damaged kernels, and foreign material (non-

maize material). In any grading standard, specific

levels of the grade factors are assigned for any desig-

nated grade. Nongrade determining factors such as

moisture content, mycotoxin levels, etc. can be added

to the grade certificate depending upon contract

specifications.

0023 For long-term safe storage of maize, a moisture

content of 13% is required; however, for storage up

to one year in the temperate climates of the US Corn

Belt, maize can be stored at 14% moisture content.

The actual maximum moisture allowed for safe stor-

age varies with the temperature and relative humidity

of the air in the interstitial spaces surrounding the

kernels. Fluctuations in temperature and atmospheric

moisture can result in moisture migration within a

mass of maize, which can result in quality reduction

due to growth of fungi and insect infestation.

0024 Maize may be handled as many as 20 times be-

tween harvesting in the field and final use. Each

handling provides a series of impacts that can cause

an increase in breakage. Breakage is measured in the

USA as the percentage of the maize mass that passes

through a 12/64 inch (4.76 mm) rounded hole sieve.

This value may be 0.5–1.0% at the farm gate, 2.0–

3.0% at in-land grain terminals, 2.5–4.0% at export

terminals, and 5–15% at the ultimate international

user. Breakage accumulates at each handling and

depends upon the breakage susceptibility of the

maize kernels and the impact experienced by the

kernel during handling. The drying temperature and

rate of drying have the greatest effect upon breakage

susceptibility. Variety differences are significant, with

hard endosperm maize having a lower breakage

during handling as compared to softer dent hybrids.

Breakage also increases with decreasing grain

temperature. To reduce breakage during maize

movement, gentle handling is needed. Long spout

drops should be reduced so that the kernel velocity

does not exceed 12 m

1

(less than a 12-m free-fall

drop). Velocity reduction equipment and proper facil-

ity design can greatly help reduce maize damage.

Processing and Food Uses

0025Although the major portion (*80%) of maize grown

worldwide is used directly as animal food, processing

maize for human food or chemicals is growing rap-

idly. In the USA, the amount of maize used for these

purposes has grown from 7.2 10

t in 1960 to over

4.98 10

t in 2000. Worldwide growth in maize

processing is expected to continue as the demand for

processed food increases.

0026There are three major processes utilized in the pro-

duction of maize for human food usage: dry milling,

wet milling, and alkali processing. All three processes

have developed specific products and markets.

Dry Milling

0027Dry milling is the simplest method of producing

maize products for human consumption. Grinding

whole kernel corn in a grind stone or roller mill to

produce flour or meal is a simple method used world-

wide when the ground products are to be consumed

shortly after processing. The storability of such prod-

ucts is limited due to the presence of crushed germ in

the flour and/or meal. Oil from broken germ cells is

easily oxidized, producing a rancid odor and flavor.

Whole ground maize can also be used as the sub-

strate for beverages or fuel-ethanol production. (See

Milling: Principles of Milling.)

0028To overcome any storage problems, most larger dry

milling facilities remove germ as the first step in pro-

cessing. Maize is tempered with water to a moisture

level of 18–24% to toughen the germ and induce

differential swelling of the germ, endosperm, and

pericarp for ease of separation. A degerminator

mechanically impacts the kernel to break loose the

germ and pericarp from the endosperm. The resulting

material is sieved, aspirated, gravity-separated, and

roller-milled to separate the germ, pericarp, and

endosperm pieces of different sizes.

0029The large grits (flaking grits) are used in the pro-

duction of flaked breakfast cereals. Smaller grits are

also used in some breakfast cereals, extruded maize

snacks, and alcoholic beverage brewing. Meal frac-

tions are used in extruded snack foods, food mixes,

and brewing. Flour is used in food mixes, food coat-

ings, and breadings. The low cost of maize flour

relative to wheat flour makes it a good choice for

many food applications where the strength of wheat

3650 MAIZE

gluten is not important. Maize dry-milled products

also have a number of nonfood uses, including the

manufacture of gypsum board, plywood, enzymes,

chemical binders, and biodegradable plastic fillers

and in intermediate chemical and biochemical

fermentations.

0030 Dry-milled germ can be pressed or solvent-

extracted to recover the valuable oil. The defatted

germ meal is most often combined with the pericarp

fraction to produce an animal food product know as

hominy food. Because the mechanical separation pro-

cesses used in dry milling are imprecise, the germ and

pericarp fractions often have significant amounts of

attached endosperm material. The maize hybrid,

drying temperature, tempering conditions, and deger-

minator adjustment affect the amount of attached

endosperm in the pericarp and germ fraction, and the

amount of broken germ in the endosperm products.

0031 Table 2 lists representative dry milling yields of

degerminated maize products. When properly deger-

minated, the larger grits will be low (0.45–0.55%) in

oil. As the degerminated product size decreases, the

amount of oil in the fraction increases to the content

of 1.5–2.5% oil in the flour. Low-fat maize flour can be

derived by the roller milling of larger low-fat grits or

meal. Dry milling provides a variety of ingredients for

use in food products, but the heterogeneous nature of

the dry-milled products makes it difficult to find new

markets. Maize dry milling in the USA only grew at an

annual rate of

sim 1.5% per year during the last several

decades, from 2.9 10

tin1960to3.510

tin

1998. The major advantages of maize dry milling are

the lower use of energy in fractionation and lower

capital costs as compared to wet milling.

Wet Milling

0032 In the USA, the growth area in the utilization of maize

has been wet milling, which has grown from

3.9 10

t in 1960 to over 43.2 10

t in 1998 due

to the increasing demand for high-fructose corn syrup

and ethanol. The relatively clean separation of maize

into its functional components offers wet milling

many opportunities for future growth. Maize yields

several large molecules (starch, protein, and oil),

which can be used as feed stocks for conversion to a

wide variety of new food and industrial chemicals,

including intermediate chemicals, amino acids, citric

acid and other organic acids, synthetic fibers, plastic

and styrofoam filler, and low-calorie fat. Environ-

mental issues and regulations concerning petrochem-

ical and crude oil transportation and utilization will

likely allow these and other maize-based products

to become more economically competitive in the

future.

0033Wet milling is the use of biochemically and chem-

ically induced changes in the maize kernel to facilitate

mechanical separation of components. The kernel is

first steeped for 20–48 h in a counter-current manner,

which results in microaerophilic lactic acid fermenta-

tion during hydration of the kernel, followed by ex-

posure to increasing levels of sulfurous acid (sulfur

dioxide in water, 1000–2000 p.p.m.). Steeping

toughens the germ, removes solubles from maize,

which increases the oil content of the germ, loosens

the interfacial forces between the endosperm germ

and pericarp, and disrupts the protein matrix, in

which starch granules are embedded by reducing di-

sulfide bonding and enzymatic hydrolysis of the

matrix protein, and hydrates the kernels to 45–50%

(by weight) moisture. Concentrated steepwater, con-

taining * 7% of the total solids, 3–5% protein

and most of the soluble sugars, vitamins, and min-

erals, is often used as nutrient source for biochemical

fermentations.

0034The kernels are then lightly milled to cause the

germ to separate from the pericarp and endosperm.

Germ recovery is accomplished in cyclones utilizing

the density difference between the light germ and the

heavier endosperm. The germ containing 45–50% oil

is dry-pressed or solvent-extracted to recover the oil.

Extracted germ meal is sold as a 20% protein animal

food or added to the gluten feed fraction.

0035Nongerm material is further milled to mechanically

disrupt the protein and starch in the endosperm.

This fine grinding allows for the separation of cellular

fiber from the starch and protein. The pericarp

and cellular fiber are recovered using a series of bent

pressure fed screens (4–7 units). The fiber is de-

watered in a press, dried, and combined with concen-

trated steepwater to produce corn gluten feed (21%)

protein. The underflow from the screens is thickened

and centrifuged, resulting in a primary starch–protein

separation.

tbl0002 Table 2 Comparison of product yield for laboratory dry-milled

medium-hard endosperm and medium soft endosperm dent

maize dried at two temperatures

Drying

temperature (



Product yield (percentage dry-weight basis)

Medium-hard

endosperm

Medium-soft

endosperm

30 105 30 105

Grits

3.5, þ5 30.3 11.0 20.0 5.1

5, þ7 19.5 19.4 21.1 18.7

7, þ10 4.7 14.5 5.7 15.8

To t a l gri t s 54.5 45.8 46.7 39.6

Meal and flour 15.6 20.9 26.5 31.5

Germ fraction 20.5 23.1 26.5 31.5

Standard US mesh.

MAIZE 3651

0036 The lighter protein fraction is further centrifuged

to concentrate and purify. Belt filters are used to

dewater the protein to 35–45% (by weight) moisture

and then dried to produce corn gluten meal. The

underflow of the primary centrifuge is starch con-

taining * 1% protein. This starch slurry is pumped

through a series of small cyclones (10–14 units) and

washed with fresh water. The resulting starch, with a

protein content of 0.25–0.50%, is dried in a flash

drier. The starch can be chemically modified to

enhance its functional characteristics for food or in-

dustrial use, chemically or enzymatically hydrolyzed

for use in the production of fructose syrups, dextrose

syrups, dextrins, or acid- or enzyme-thinned starch,

or left unmodified for use in a variety of food and

industrial applications.

0037 Wet milling of specialty maize hybrids (high-

amylose, waxy, high-oil, and high-lysine), produces

starch or other components with enhanced character-

istics. The steeping parameters of these specialty

hybrids are often different from those of ordinary

yellow dent maize. Currently, there is considerable

interest in the USA in the development of specific

maize hybrids which will have unique starch charac-

teristics or which could increase wet milling profit-

ability by enhancing millibility.

0038 Flint and flint-type maize is less desirable than

yellow dent maize for wet milling. In general, the

flint maize takes longer to steep (50–60 h) and yields

slightly less starch. Softer endosperm maize is more

desirable for wet milling. Recent tests with all soft

endosperm maize has shown that steep times can be

reduced to 8–12 h, with no loss in starch yield and

good starch quality.

0039 The most deleterious factor affecting wet milling is

the temperature at which the maize was dried when

harvested (Table 3). When the drying temperature is

about 50



C, there is potential for starch gelatiniza-

tion, protein denaturation, and endogenous protease

inactivation. Processing of high-temperature dried

maize results in a reduced starch yield, difficulty in

protein (gluten) dewatering, increased starch in the

fiber, and lower hydration of the maize kernels during

steeping. Most US millers purchase US No. 3 or better

maize because of the volume of maize purchased,

although there has long been interest in being able

to delineate high-temperature-dried maize from low-

temperature-dried maize in the marketplace. Inter-

national millers have observed major differences in

the way in which US No. 3 maize mills is compared

to natural-dried South African or Chinese maize.

Natural air-dried or low-temperature-dried maize,

as in South Africa or China, steeps easier, is less

variable, has lower amounts of broken maize, and

creates less dust. During wet milling, the fiber de-

waters easier, gluten filters easier, and the starch

yield is greater.

Alkali Processing

0040Alkali processing is the traditional lime cooking

method employed by the Native Latin American

Indians and Mexicans to produce tortillas. Tortillas

are the major source of energy and nutrition in many

Central American countries. In the USA, alkali-

processed ethnic food and snack food is increasing

at a faster rate than most other food uses of maize.

0041In alkali processing of maize, the maize is mixed

with water and lime in a ratio of 1:1.2–3.0:0.0005

(maize:water:lime) and cooked at 94



C for 50 min.

The cooked maize is then steeped for 14 h in the lime

solution before being washed with fresh water to

remove loosened pericarp and residual alkali from

the maize. The washed maize is milled to a gritty

textured product, called masa. This masa is rolled

into flat cakes and baked in an oven for 1–2 min to

produce the traditional tortilla. Masa can also be

deep-fried to produce tortilla chips or maize chips,

or dried and finely milled to produce masa flour. The

specific conditions and procedures for alkali process-

ing maize vary with each processor.

tbl0003 Table 3 Wet milling yields (percentage dry-weight basis) for several types of maize and the effect of drying temperatures on wet

milling yields

Laboratorymilling Commercial

LTD HTD LT D LTD HTD

Waxy High amylose High amylose Medium-soft dent Medium-soft dent Dent

Starch 59.0 29.8 45.0 65.2 62.5 67.5

Gluten 14.9 37.3 26.6 10.2 12.6 5.8

Germ 7.2 7.4 7.4 7.0 7.0 7.5

Fiber 10.2 18.1 11.3 9.3 10.8 11.5

Steepwater 5.0 4.5 5.3 4.4 3.9 7.5

Filtrate 3.2 2.5 4.0 2.6 2.5

HT, high-temperature-dried; LT ¼ low-temperature-dried.

From Anderson RA and Watson SA (1982) The corn milling industry. In: Wolff IA (ed.) CRC Handbook of Processing and Utilization in Agriculture,Vol. II: Part

1 Plant Products Boca Raton, FL: CRC Press.

3652 MAIZE

004 2 The quality of maize used in the alkali process is

very important. The largest US processor desires hard

endosperm yellow dent maize in order to provide a

uniform endosperm texture for cooking. Mechanical

damage to the kernel results in increased rates of

water absorption and thus increased rates of cooking.

Damaged or stress-cracked kernels tend to be over-

cooked, and they leach out more starch into the

nejayote (steep water). Drying temperature is also

important as high-temperature drying increases

stress-crack damage and damages the starch prior to

cooking. The use of hard-endosperm maize is not

required for masa production, and quality masa prod-

ucts can be produced from high-quality soft endo-

sperm or average dent maize with appropriate

changes in processing conditions.

004 3 High-lysine hard-endosperm maize, called quality

protein maize (QPM), is being developed to increase

the nutritive value of alkali-cooked products. Maize

protein as a total protein source is deficient in the

amino acids tryptophan and lysine. The QPM is

*20–35% higher in these amino acids, and alkali

processing of QPM results in masa products with

improved levels of both amino acids. (See Amino

Acids: Properties and Occurrence.)

Seealso: Ethnic Foods; Milling:PrinciplesofMilling;

Types of Mill and Their Uses; Characteristics of Milled

Products