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Waxy corn

Waxy corn

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Waxy corn was found in China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

 in 1909. As this plant showed many peculiar traits, the American
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 breeders long used it as a genetic marker
Genetic marker
A genetic marker is a gene or DNA sequence with a known location on a chromosome that can be used to identify cells, individuals or species. It can be described as a variation that can be observed...

 to tag the existence of hidden genes in other maize breeding programs. In 1922 a researcher found that the endosperm of waxy maize contained only amylopectin
Amylopectin
Amylopectin is a soluble polysaccharide and highly branched polymer of glucose found in plants. It is one of the two components of starch, the other being amylose.Glucose units are linked in a linear way with α glycosidic bonds...

  and no amylose
Amylose
Amylose is a linear polymer made up of D-glucose units.This polysaccharide is one of the two components of starch, making up approximately 2-30% of the structure...

 starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...

 molecule in opposition to normal dent maize varieties that contain both. Until World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, the main source of starch in the USA was tapioca
Tapioca
Tapioca is a starch extracted Manihot esculenta. This species, native to the Amazon, Brazil, Colombia, Venezuela, Cuba, Puerto Rico, Haiti, the Dominican Republic, Honduras, and most of the West Indies, is now cultivated worldwide and has many names, including cassava, manioc, aipim,...

 but when Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

 severed the supply lines of the States, they forced processors to turn to waxy maize. Amylopectin or waxy starch is now used mainly in food products, but also in the textile, adhesive, corrugating and paper industry.

When feeding trials later on showed that waxy maize could produces more efficient feed gains than normal dent maize, interest in waxy maize suddenly mushroomed. Geneticists could show that waxy maize has a defect in metabolism precluding the synthesis of amylose in the endosperm
Endosperm
Endosperm is the tissue produced inside the seeds of most flowering plants around the time of fertilization. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This makes endosperm an important source of nutrition in human diet...

. It is coded by a single recessive gene (wx). Waxy maize yield about 3.5% less than their normal dent counterparts and has to be isolated from any nearby normal maize fields by at least 200 meters.

History


The exact history of waxy maize is unknown. The first mentions of it were found in the archives of the U.S. Department of Agriculture (USDA). In 1908, the Rev. J. M. W. Farnham, a Presbyterian missionary in Shanghai
Shanghai
Shanghai is the largest city by population in China and the largest city proper in the world. It is one of the four province-level municipalities in the People's Republic of China, with a total population of over 23 million as of 2010...

, sent a sample of seeds to the U.S. Office of Foreign Seed and Plant Introduction. A note with the seeds called it: “A peculiar kind of corn. There are several colours, but they are said to be all the same variety. The corn is much more glutinous than the other varieties, so far as I know, and may be found to be of some use, perhaps as porridge.” These seeds were planted on May 9, 1908, near Washington, D.C.
Washington, D.C.
Washington, D.C., formally the District of Columbia and commonly referred to as Washington, "the District", or simply D.C., is the capital of the United States. On July 16, 1790, the United States Congress approved the creation of a permanent national capital as permitted by the U.S. Constitution....

, by a botanist named G.N. Collins. He was able to grow 53 plants to maturity and made a thorough characterisation of these plants, including photographs, which were published in a USDA bulletin issued in December 1909.

In 1915, the plant was rediscovered in Upper Burma
Upper Burma
Upper Burma refers to a geographic region of Burma , traditionally encompassing Mandalay and its periphery , or more broadly speaking, Kachin and Shan States....

 and in 1920 in the Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...

. Kuleshov, when screening the distribution of maize in Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

, found it in many other places.

The discovery in China of a distinct type of maize bears the historical question whether maize was known in the Orient
Orient
The Orient means "the East." It is a traditional designation for anything that belongs to the Eastern world or the Far East, in relation to Europe. In English it is a metonym that means various parts of Asia.- Derivation :...

 before the discovery of America. The question was closed at the end of the 19th century by De Candolle who stated: ”Maize is of American origin, and has only been introduced into the old world since the discovery of the new. I consider these two assertions as positive, in spite of the contrary opinion of some authors.“

But the finding of this unique variety of maize suggested a re-examination of the question. He also states that Portuguese
Portuguese people
The Portuguese are a nation and ethnic group native to the country of Portugal, in the west of the Iberian peninsula of south-west Europe. Their language is Portuguese, and Roman Catholicism is the predominant religion....

 arrived in China in 1516, simultaneously introducing maize. Collins supposed that waxy maize has arisen by a way of mutation in Upper Burma. For some scholars it was difficult to conceive that from 1516 on the American plant had had time to penetrate into a wild country inaccessible to foreigners, to produce a mutation, and as such a mutant to spread from the Philippines to Northern Manchuria and the Primorsky region within three to four hundred years.

Nowadays we are able to counterpart both of these arguments. At first we know that the waxy mutation is quite common (see #Genetics). Secondly, the fact that maize, if introduced into Asia in Post-Columbian times, must have been rapidly accepted merely indicates that, like the potato in Ireland, it met an acute and pressing need.

Goodrich states that there are now in China some 6000 local histories called gazetteers written from A.D. 347 on. Maize was first accurately described in one of them, published in the sixteenth century.
Ho, an eminent Chinese historian, stated: “Summing up the introduction of maize into China, we may say that maize was introduced into China two or three decades before 1550 . . . ” It might be, as various students concluded, that maize reached Asia before 1492, but currently we are not aware of a single plant fragment, artifact, illustration, or written record to prove it. Therefore, any undocumented statement about its occurrence there in earlier times is to be regarded with scepticism until substantiated. Thus, the two assertions of De Candolle are still valid.

In his publication, Collins characterised the new plants as possessing a number of unique characters. No indications of these characters in any recorded form of Zea mays had thus far been found. Several of the unique features combine to enable the plant to resist the drying out of the silks by dry, hot winds at the time of flowering. Although the plants produced such small ears that they could find no place in direct competition with the improved varieties, the possession of this adaptation gave the new type an economic interest, particularly in some parts of the semiarid Southwest. Consequently, the effort has been made to combine by hybridising the desirable characters of this small variety with those of larger and more productive types.

And when Collins found such a distinct difference in the appearance of normal and waxy maize endosperm, he suspected a difference in chemical composition, but the analysis did not yield any unusual results. The percentages of starch, oil, and protein were all within the normal range. Yet, he was intrigued by the physical nature of the starch, and wrote: “In view of the recent development of specialised maize products as human food, the unique type of starch may be of some economic importance." So actually, for many years the main use of waxy maize was a genetic marker for other maize breeding programs. Breeders were able to use some of the traits to “tag” the existence of hidden genes and follow them through breeding programs. It is possible that waxy maize would have become extinct again in the USA without this special application in breeding.

In 1922, another researcher, P. Weatherwax of Indiana University in Bloomington, reported that the starch in waxy maize was entirely of a “rare” form called “erythrodextrin
Dextrin
Dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch or glycogen. Dextrins are mixtures of polymers of D-glucose units linked by α- or α- glycosidic bonds....

”, known today as amylopectin. He found that this rare starch stained red with iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

, in contrast to normal starch which stained blue. Bates, French et al. and Sprague, Brimhall, et al. confirmed that endosperm starch of waxy maize consists nearly exclusively of amylopectin. The presence of amylopectin in rice had been demonstrated previously by Parnell.

In 1937, just before World War II, G.F. Sprague and other plant breeders at what was then called Iowa State College had begun a crossbreeding program to attempt to introduce the waxy trait into a regular high-yielding hybrid maize. By this time, the waxy plant no longer had the peculiar structural traits noted by Collins, probably due to years of crossing into various genetic stocks. Only the unique endosperm had been retained. At this time, waxy maize was not so important because the main source of pure amylopectin still was the cassava plant, a tropical shrub with a large underground tuber.

During World War II, when the Japanese severed the supply lines of the States, processors were forced to turn to waxy maize. Waxy maize appeared to be especially suitable for this purpose because it could be milled with the same equipment already extensively used for ordinary maize. H. H. Schopmeyer has advised that the production of waxy maize in Iowa for industrial use amounted to approximately 356 metric tons in 1942 and 2540 tons in 1943. In 1944, there were only 5 varieties of waxy maize available for waxy starch production. In 1943, to cover all the special requirements for amylopectin, approximately 81650 tons of grains were produced. From World War II until 1971, all the waxy maize produced in the U.S. was grown under contract for food or industrial processors. In fact, most of the maize was grown in only a few counties in Iowa
Iowa
Iowa is a state located in the Midwestern United States, an area often referred to as the "American Heartland". It derives its name from the Ioway people, one of the many American Indian tribes that occupied the state at the time of European exploration. Iowa was a part of the French colony of New...

, Illinois
Illinois
Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...

, and Indiana
Indiana
Indiana is a US state, admitted to the United States as the 19th on December 11, 1816. It is located in the Midwestern United States and Great Lakes Region. With 6,483,802 residents, the state is ranked 15th in population and 16th in population density. Indiana is ranked 38th in land area and is...

.

But in 1970, as most maize growers remember well, the Southern corn leaf blight epidemic (Helminthosporium maydis Nisik. and Miyake) swept the U.S. corn belt. At the same time, at least 80% of the maize being grown in the U.S. was susceptible to the blight because this maize contained the “Texas type” male-sterile cytoplasm, which allowed production of hybrid seed without mechanical or hand detasseling. So quite naturally, there was a mad scramble in 1971 to find any kind of maize that had normal cytoplasm – cytoplasm that would resist the blight. Consequently, some seed of waxy maize worked its way into the market. Through backcrossing, also has been used extensively to transfer individual genes such as wx (waxy), o2 (opaque 2) and the Htl gene for resistance to the leaf blight was transferred to regular dent corn.

Some farmers who fed this waxy grain to their beef cattle observed that animals thrived on it. Feeding trials were set up which suggested that the waxy maize produced more efficient weight gains than normal dent. Interest in waxy maize suddenly mushroomed, and this maize type abandoned the status of botanical curiosity and speciality product to become the subject of major research importance.

In 2002, an estimated 1,200,000 to 1,300,000 tonnes of waxy maize was produced in the United States on about 2,000 km², representing only 0,5% of the total maize production.

Chinese maize


Collins noted, among others, these unusual traits of the Chinese maize:
  • Several unique structural features that enabled the plants to resist the drying out of the silks by wind at the time of flowering
  • Unusual growth behaviour in that the top four or five leaves all appeared on the same side of the main stem of the plant. Extremely erect leaves of the upper nodes, while the lower leaves were more spread and drooping
  • One of the main things he noted was the composition of the endosperm
    Endosperm
    Endosperm is the tissue produced inside the seeds of most flowering plants around the time of fertilization. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This makes endosperm an important source of nutrition in human diet...

     of the maize kernels
    Seed
    A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...

    . He wrote: “The texture of the endosperm is one of the unique features of this maize. Cut in any direction it separates with a sort of cleavage, exposing a dull, smooth surface. The texture suggests that of the hardest waxes, though it is still harder and more crystalline. From this optical resemblance to wax the term cereous or waxy endosperm is suggested.” The moisture content of the kernel must be 16% or lower before the waxy trait can be recognised visually.


The starch of normal dent maize is characterised by a content of about 25% amylose with the remainder being amylopectin and the intermediate fraction (see 3.5 Biochemistry). But these percentages vary among cultivars and with kernel development. For example, amylose percentage ranged from 20 to 36% for 399 cultivars of normal maize. There are maize germplasm
Germplasm
A germplasm is a collection of genetic resources for an organism. For plants, the germplasm may be stored as a seed collection or, for trees, in a nursery.-See also:*Germ plasm, the germ cell determining zone...

 collected that range from less than 20 to 100% complement of amylopectin. And waxy maize contains 100% amylopectin..

Waxy starch is of main interest because fractionation of normal starch to obtain pure amylose or amylopectin is very costly.

Waxy endosperm is inherently a defect in metabolism, and its low frequency in most maize populations in the face of recurring mutations indicates that it is acted against by natural selection.

Genetic drift


Experiments by Sprague have shown that ten to twenty plants are required for adequate representation of genetic diversity in an open-pollinated maize variety. Since the number of ears saved for seed by ancient Asian maize cultivators with only small plots of land at their disposal was often smaller than this and, indeed, since new maize populations are sometimes established by growing the progeny of a single ear, it follows that there must often have been genetic drift
Genetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...

 – changes in gene frequencies resulting from the creation of small breeding populations.

A striking example of genetic drift in maize is the occurrence in parts of Asia of varieties with waxy endosperm. In maize races of America such a variety is unknown, but the waxy character itself has been discovered in non-waxy varieties: in a New England
New England
New England is a region in the northeastern corner of the United States consisting of the six states of Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut...

 flint maize and in a South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

n variety.

The fact that waxy maize occurs so commonly in a part of the world that also possesses waxy varieties of waxy rice, sorghum
Sorghum
Sorghum is a genus of numerous species of grasses, one of which is raised for grain and many of which are used as fodder plants either cultivated or as part of pasture. The plants are cultivated in warmer climates worldwide. Species are native to tropical and subtropical regions of all continents...

, and millet
Millet
The millets are a group of small-seeded species of cereal crops or grains, widely grown around the world for food and fodder. They do not form a taxonomic group, but rather a functional or agronomic one. Their essential similarities are that they are small-seeded grasses grown in difficult...

 can be attributed to artificial selection. The people of Asia being familiar with waxy varieties of these cereals and accustomed to using them for special purposes recognised the waxy character in maize after it was introduced into Asia following the discovery of America and purposely isolated varieties purely for waxy endosperm. But the fact that waxy endosperm came to their attention in the first place is probably due to genetic drift. The gene for waxy endosperm, which has a low frequency in American maize, apparently attained a high frequency in certain samples of Asian maize.

Indeed, the practice reported by Stonor and Anderson of growing maize as single plants among other cereals would result in some degree of self-pollination and, in any stock in which the waxy gene was present, would inevitably lead in a very short time to the establishment of pure waxy varieties with special properties that people accustomed to the waxy character in other cereals could hardly fail to recognise.

Genetics


Genetic research of this genetic drift started first with describing (phenotyping
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

) the mutant kernel appearance of maize and other mutant changes in the maize. Lateon these description were coupled to mutant
Mutant
In biology and especially genetics, a mutant is an individual, organism, or new genetic character, arising or resulting from an instance of mutation, which is a base-pair sequence change within the DNA of a gene or chromosome of an organism resulting in the creation of a new character or trait not...

 genes genotypes. More than 40 mutant allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...

s are known for the waxy locus
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...

, making up the finest collection of mutations found among higher plants.

Some of these waxy mutants are very stable whereas others are very unstable. The genotype of the stable mutants remains unchanged whereas the one of unstable mutants changes because of the insertion of transposable elements (5-8). For a listing of all these mutations, the excellent book of Neuffer, Coe et al. is greatly recommended.

Because the waxy mutation is expressed in an easy identifiable nonlethal phenotype, it has been the subject of major research during the 20th century. Nelson made a fine structure genetic map of most of these mutations
  • For waxy maize, a single recessive gene
    Gene
    A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

     (wx) was located on the short arm of chromosome 9, codes for the waxy endosperm of the kernel (Wx codes for endosperm with normal starch). This was first shown by Collins and Kempton.
  • The structure of the wildtype waxy (wx+) locus has been determined through DNA sequencing
    DNA sequencing
    DNA sequencing includes several methods and technologies that are used for determining the order of the nucleotide bases—adenine, guanine, cytosine, and thymine—in a molecule of DNA....

    . The gene has 3718 bp (14 exons and 13 introns).
  • Waxy endosperm is the counterpart in maize of the “glutinous” character in rice.
  • There is a wide range of species also presenting the waxy mutation, including rice, sorghum, millet, barley and wheat, which were characterised by starch granules staining red with iodine.
  • In crosses between heterozygous plants for the waxy character, a small but significant deviation from an expected Mendelian ratio in self pollination is produced. Bear obtained from 71 segregated ears on the F1 generation 23,77% of waxy kernels and 76,23% of non-waxy kernels. This is evidenced by the two heterozygous types, Wx Wx wx and wx wx Wx.


The waxy gene is epistatic
Epistasis
In genetics, epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is called epistatic, while the phenotype altered or suppressed is called hypostatic...

 for all known other amylose and amylopectine forming mutants genes like dull (du), sugary-1 (su1 ) and sugary-2 (su2),. wx gene for example increases sugars and water-soluble polysaccharides (WSP) in a su1 background and it causes dramatic increases in sugars and reduction in starch with ae or aedu mutated genes.
  • The mutation from Wx to wx is not uncommon in Corn Belt varieties, Bear having found three separate mutations to waxy in three consecutive years in a total population of some 100,000 selfed ears.
  • Mangelsdorf found also many mutants on his trial fields.
  • Argentine waxy (wx-a) corn, an allele at the waxy locus first reported by Andrés and Bascialli, is known to produce small amount of amylose (< 5%) and gives an intermediate staining reaction with iodine.
  • Other mutant alleles at the waxy locus have been reported which possess similar starch properties to those observed with wx.

Genotype and characterisation with iodine


The wx locus is expressed in the endosperm, in the male gametophyte
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....

 (pollen) as well as in the female gametophyte
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....

 (embryo sac). Amylose and amylopectin have different iodine binding-properties
Iodine test
The Iodine test is used to test for the presence of starch. Iodine solution — iodine dissolved in an aqueous solution of potassium iodide — reacts with the starch producing a purple black color. The colour can be detected visually with concentrations of iodine as low as 0.00002M at 20°C...

, with maize amylose and amylopectin giving iodine affinity (IA) values of about 19 to 20 and 1%, respectively, depending upon the source. Weatherwax discovered this process in 1922.

The amount of apparent amylose can be determined either by measuring the absorbency of the starch-iodine complex (blue-value) and relating this value to that of pure amylose and amylopectin standards or by measuring the amount of iodine (mg) bound per 100 mg of starch in a potentiometric titration and relating the value to the amount bound by an amylose standard.

Values used on the iodine binding, however, are only estimates of amylose content because of differences in the binding abilities (and structure) of amylose and amylopectin among starch types. For example, amylopectin molecules with long external branches bind more iodine than those with short branches do, resulting in a small measure of apparent amylose.

Chromatographic profiles of wx-containing starches, however, reveal no amylose peak. The wave-length at which a starch-iodine complex has maximum absorbency is referred to as the lambda max.

Plants which are heterozygous on the waxy gene (Wx:wx) can be characterised by staining the pollen with iodine. Half of the pollen will be blue and half brown whereas the kernels will stay blue (very helpful in backcrossing program). If the plant is homozygous recessive (wx:wx) the whole pollen will be brown and the kernel too. Being homozygously dominant (Wx:Wx) the iodine will appear only blue.

Biochemistry


Normal dent maize has two different pathways for starch formation one leading to branched chain (amylopectin) and the other to straight-chain polysaccharides (amylose).
  • The amylopectin consists of chain of α-D-(1-4) and α-D-(1-6)-glucosidic linkages that form a branched molecule.
  • Amylose is primarily linear with α-D-(1-4)-linked glucose residues.


The locus Wx codes for a specific starch granule-bound enzyme, NDP-glucose-starch glucosyltransferase
NDP-glucose-starch glucosyltransferase
In enzymology, a NDP-glucose-starch glucosyltransferase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are NDP-glucose and n, whereas its two products are NDP and n+1....

. This specific starch synthase
Starch synthase
In enzymology, a starch synthase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are ADP-glucose and a chain of D-glucose residues joined by 1,4-alpha-glycosidic bonds, whereas its two products are ADP and an elongated chain of glucose residues...

 enzyme is responsible for amylose biosynthesis. The Wx gen catalyses the 1–4 linkage from glucose residues to amylose synthesis in the developing endosperm. This enzyme is located in the amyloplasts and is the major component of the starchbound protein in maize. Nelson showed that starch granules from wx wx wx endosperm had very low starch granule-bound glucosyltransferase activity.

When measuring if the activity of the transferase was a function of the Wx dosage in diploid and tetraploid maize, Akatsuka noticed a linear proportionality between a preparation of Wx Wx Wx and Wx Wx Wx Wx Wx Wx . Nevertheless the amylose content was the same in both types suggesting that activity of the transferase is not directly linked to the amylose content.

In maize and some other plants, there is evidence of a starch molecule that is intermediate in size to amylose and amylopectin. The intermediate fraction contains chains of (1–4)-linked alpha-D-anhydroglucose residues, but the average length of these chains and the number of chains per molecule are different from those in either amylopectin or amylose. Several researchers demonstrated the presence in normal maize starch of about 5 to 7% intermediate polysaccharides, basing their conclusions on indirect evidence from IA.

As early as in 1956, it was stated that amylopectin contained three different types of chains. In each macromolecule
Macromolecule
A macromolecule is a very large molecule commonly created by some form of polymerization. In biochemistry, the term is applied to the four conventional biopolymers , as well as non-polymeric molecules with large molecular mass such as macrocycles...

 there is one C-chain, which carries the only reducing group. The B-chains are linked to the macromolecules linked by their potential reducing group, and may contain one or more A-chains that are similarly linked. The ratio of A-B chains (1:1 to 1,5:1) is a measure of the degree of multiple branching and is an important property describing amylopectin. Nevertheless the exact arrangement of chains within the amylopectin molecule is still not clear.

Combining the recessive mutant (wx) maize variant with other mutant as for example amylose extender (ae)maize and dull (du) maize has an effect on the amylose and amylopectine structure of the starch.
  • The amylose extender waxy (aewx) starch contain 21% apparent amylose and has a lambda max. of 580 for the iodine-starch complex. The aewx outer chains are longer than those of wx mutant and fewer in number per weight of starch. In general, the aewx starch had a unique structure that is similar to the anomalous amylopectin (intermediate fraction) reported in ae starch.
  • Increased dosage at the ae locus, regardless of the genotype at the wx locus, resulted in amylopectin with increased linearity.
  • Short-chained amylose (approximately 100 glucose units) was observed in all ae genotypes in a homozygous Wx background.
  • Amylopectin of the aewx mutants had an increased proportion of long B-chains and a decreased proportion of short B-chains compared with wx amylopectin, whereas amylopectin of the dull waxy (duwx) mutant had a decreased proportion of long B-chains and an increased proportion of short B-chains, thus confirming the novel nature of aewx and duwx amylopectin.

Agronomic features


Producing waxy maize starch on an industrial scale requires extra measures compared to standard dent maize.

New varieties with the waxy locus are relatively easy to breed through back-crossing breeding with dent maize varieties, but their productivity is approximately 3 to 10% less than that of dent maize.

Due to the waxy gene being recessive, waxy maize has to be isolated from any nearby dent maize fields by at least 200 meters to prevent cross-pollination. Volunteer dent maize plants sprouting from the previous year's debris are also a problem. A few dent maize volunteers in a waxy field will be enough to contaminate the whole field, resulting in dent grains instead of waxy grains with amylopectin starch.

Almost all waxy grain is produced under contract for starch (wet milling) companies. A premium is paid as compensation for the extra costs incurred from the lower yield and the extra handling, such as quality control procedures to ensure starch the grain is not contaminated.

Utilization


Amylopectin
Amylopectin
Amylopectin is a soluble polysaccharide and highly branched polymer of glucose found in plants. It is one of the two components of starch, the other being amylose.Glucose units are linked in a linear way with α glycosidic bonds...

 or waxy cornstarch
Cornstarch
Corn starch, cornstarch, cornflour or maize starch is the starch of the corn grain obtained from the endosperm of the corn kernel.-History:...

 is relatively easy to gelatinise
Starch gelatinization
Starch gelatinization is a process that breaks down the intermolecular bonds of starch molecules in the presence of water and heat, allowing the hydrogen bonding sites to engage more water. This irreversibly dissolves the starch granule...

, produces a clear viscous paste with a sticky or tacky surface. The paste rheology
Food rheology
Food rheology is the study of the rheological properties of food, that is, the consistency and flow of food under tightly specified conditions. The consistency, degree of fluidity, and other mechanical properties are important in understanding how long food can be stored, how stable it will...

 resembles pastes of root or tuber starches, such as potato starch
Potato starch
Potato starch is starch extracted from potatoes. The cells of the root tubers of the potato plant contain starch grains . To extract the starch, the potatoes are crushed; the starch grains are released from the destroyed cells...

 or tapioca starch
Tapioca
Tapioca is a starch extracted Manihot esculenta. This species, native to the Amazon, Brazil, Colombia, Venezuela, Cuba, Puerto Rico, Haiti, the Dominican Republic, Honduras, and most of the West Indies, is now cultivated worldwide and has many names, including cassava, manioc, aipim,...

 (made form cassave). Amylopectine starch have also a lower tendency to retrogradate
Retrogradation
Retrogradation is the term for the landward change in position of the front of a river delta with time. This occurs when the mass balance of sediment into the delta is such that the volume of incoming sediment is less than the volume of the delta that is lost through subsidence, sea-level rise,...

 and are thus more viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

 stable. These different properties compared to normal dent corn starch, containing also amylose
Amylose
Amylose is a linear polymer made up of D-glucose units.This polysaccharide is one of the two components of starch, making up approximately 2-30% of the structure...

, are utilized mainly in following different applications.

Food products


Modified waxy maize starches
Modified starch
Modified starch, also called starch derivatives, are prepared by physically, enzymatically, or chemically treating native starch, thereby changing the properties of the starch. Modified starches are used in practically all starch applications, such as in food products as a thickening agent,...

 are used to improvement of uniformity, stability, and texture in various food products. The clarity and visco stability of amylopectin starch make it especially suitable for thickening fruit pies. It improves smoothness and creaminess of canned food and dairy products as well as freeze-thaw stability of frozen foods. It gives a more desirable texture and appearance to dry foods and mixes [24]. Waxy maize starch is also the preferred starting material for the production of maltodextrin
Maltodextrin
Maltodextrin is a polysaccharide that is used as a food additive. It is produced from starch by partial hydrolysis and is usually found as a creamy-white hygroscopic spraydried powder. Maltodextrin is easily digestible, being absorbed as rapidly as glucose, and might be either moderately sweet or...

s because of improved water solubility after drying and greater solution stability and clarity.

Adhesive industry


Starch from waxy maize differs from regular maize starch in both molecular structure and pasting characteristics. Pastes made from waxy starch are long and cohesive; whereas, pastes made from regular maize starch are short and heavy bodied. Waxy maize starch is a major starch component in adhesives used for making bottle labels. This waxy starch based adhesive imparts resolubilizing resistance to the labels which prevents their soaking off the bottle if immersed in water or being subjected to very high humidity conditions. Moreover, waxy maize starches are commonly used in the USA for the manufacture of gummed tapes and envelope adhesives.mklm/

Livestock, dairy and poultry feeding research


The research in feeding of waxy maize began in the 1940s. Beginning with a research report in 1944, waxy maize seemed to have the potential to increase feed conversion efficiencies compared to dent maize. Many other feeding trials were started and generally indicated an slight to clear positive advantage for feeding waxy grain. Increases of both milk production and butterfat content for lactating dairy cattle, increase in daily weight gains in fattening lambs and when fed to finishing beef cattle.

Still the extensive (mushroomed) agro-research did not lead to any large scale use in the feed industry due to analytical research analysing the pancreatic digestibility of starches of several genotypes. Waxy starch of the genome type ae, as also the genome type du and su2, for starches with a high amylose content, show an excellent digestibility. Thus, amylopectin, waxy starches alone, cannot be correlated to good digestibility. Sandsted suggests that digestibility could lie more in the structure of starch granule, in differences in bonding of the starch molecules and in possible anomalous linkages between the molecules.