EFFECTS OF CHEMICALS USED IN STORING MAIZE ON IT S ZEIN CONTENT OVER A DURATION OF TIME

Abstract

Maize is the largest and most important agricultural commodity in the world.  Zein is the storage protein in maize known as prolamine.  It is water insoluble  and forms odorless,  tasteless,  clear edible films.  The zein content of maize is 25% of the dry weight of maize seeds, it is resistant to hydrolysis by  very  dilute acids and  contains the  following amino  acids:  glutamic acid  and glutamine,  leucine,  proline,  alanine,  phenylanlanine,  isoleucine,  serine, tyrosine and asparagine but lacks lysine and trytophan.  Stored maize is attacked by three major groups of pests namely fungi, insects and rodents.  Methods such as the use of natural materials like plants, minerals and oil are still in effective use, chemicals such as dichlorovous and calcium hydroxide are used for storage over a long period.  This study was aimed at investigating the effects of these chemicals used on zein content. Maize was stored for ninety days with dichlorovos and calcium hydroxide. After 2 weeks interval,  zein was extracted and tested for malondialdhyde level,  total protein content and composition of amino acids present. MDA levels increased  over the storage period from 2.629mg/100g±  0.186 to 4.685mg/100g  ± 0.003.  The mean protein content of crude zein extract showed  a significant decrease from 31.215% ± 0.002  to 26.264%±  0.004,  the mean moisture content showed a decrease from 7.160% ± 0.003 to 2.645% ± 0.003, mean ash content Increased from 3.343% ± 0.004 to 6.655% ± 0.003, mean fat content increased  from 3.373% ± 0.003  to 3.385% ± 0.003, mean fiber content increased from 1.3097%  ± 0.002 to  1.432%  ± 0.003, mean carbohydrate content increased  from 53.605% ± 0.011  to 59.41 % ±0.005.  Amino acid content of the zein samples decreased over the duration of storage.  The chemicals used in storage increased the MDA levels, reduced the amino acid content and had no negative impact on the moisture, ash, fiber, fat and carbohydrate contents of the zein samples.

CHAPTER ONE

1.1  OVERVIEW OF MAIZE

1.2 ORIGIN

Maize  is known  in many English  speaking  countries  as Com.  It  is a grain domesticated  by indigenous peoples in Mesoamerica in prehistoric times (Green,1999).  The leafy stalk produces ears which contain  seeds called kernels, though technically  a grain.  The Olmee and Mayons cultivated it in numerous varieties throughout central southern Mexico between  1700 and 1750, the crop spread through America (Boyd et al.,2006). The region developed a trade network based on surplus and varieties of maize crops. After the European contact with the America in the late

15″ and early 16 centuries, explorers and traders carried maize back to Europe and introduced it to other countries. Maize spread to the rest of the world due to its  ability to grow in diverse climates  (Pipemo  et al.,  2009).The term “Maize”  was derived  from the Spanish form of the indigenous Taino word for the plant,  Maiz.  It is known by other names around the world.  The term  “Maize”  was used  in the United  Kingdom  and Ireland,  where  it is now usually  called “sweetcorn”, a common crop known to people there (Cudderford,1995). Com was originally the English term for any cereal crop. In North America, its meaning has been restricted since the 19″ century  to maize  as it was  shortened  from “Indian corn.  The term  Indian  com  now refers specifically to multi-coloured  “field com”  (flint com) cultivars (Herman and Larkins,1999). In scientific and formal usage, “maize”  is normally used in a global content. In English speaking countries, the word com is often used in culinary contexts, particularly in naming products such as popcorn,  cornflakes  and baby com.  In Southern Africa,  maize is commonly referred to as mielle  or mealie,  from the portugese  millo.  It is called Masara  in Hausa,  Oka.  In Igbo,  and Agbado in Yoruba.  Maize is used in agricultural and scientific references  (Femandez-Armisto,

2001). Maize is the most widely grown grain in the world with 800 million metric tons produced annually.  Maize is domesticated variant of teosinte.  The two plants have dissimilar appearance, maize having a single tall stalk with multiple leaves and teosinte being a short, bushy plant. The difference between the two is largely controlled by differences in just two genes.  Several theories had been proposed about the specific origin of maize in Mesoamerica (Ranere et al., 2009)

Fig 1    :   Freshly harvested maize.

1)   It is a direct domestication of a Mexican annual teosinte Zea mays,  spp. parviglumis,  native to the Balsas River valley in South-Eastern Mexico,  with up to 12%  of its genetic material

obtained from Zea mays spp.  mexicana through introgression (Dunn, 2005).

2)  It  has  been  derived  from hybridization  between  a  small  domesticated  maize  (a  slightly changed form of a wild maize) and a teosinte of section Luxuriantes,  either Z. luxurians or

Z.diploperennis  (Sluyter et al., 2006)

3)  It has undergone two or more domestications either of a wild maize or of a teosinte.  (The term “teosinte  describes all species and subspecies in the genus Zea,  excluding Zea mays

ssp.  mays.)

4)  It has evolved from a hybridization of Z. diploperennis  by   Tripsacum  dactyloides  (Kriz,

2009).

In the late 1930s,  Paul Manglesdorf suggested that domesticated maize was the result of a

hybridization event between an unknown wild maize and a species of Tripsacum,  a related genius.  This theory about  the origin of maize has been refuted by modem genetic testing, which refutes Mangelsdorf’ s model and the fourth theory listed above (Wilkes and Garrison,

2004). The teosinte origin theory was proposed by the Russian botanist Nikolai Ivanovich Vavilov in  1931  and later the American Nobel Prize winner, George Beadle  in  1932. It is supported experimentally and by recent studies of the plant genomes (MacNeish et al.,2000). Teosinte and maize are able to cross breed and produce fertile offspring.  The domestication of maize is of particular interest to researchers, archaeologists, geneticists, ethnobotanists, geographers, etc. The process is thought by some to have started 7,500 to 12,000 years ago. Research from the 1950s to 1970s originally focused on the hypothesis that maize domestication occurred in the highlands between Oaxaca and Jalisco,  because the oldest archaeological remains of maize known at the time were found there.  Genetic  studies  led by John Doehley identified Zea mays spp.  parviglumis,  native to the Balsas River valley and also known as Balsas teosinte,  as being the crop wild relative teosinte genetically most similar to modem maize (Benz, 2006). However, archaeobotanical studies published in 2009 now point to the lowlands of the Balsas River valley,  where stone milling tools with maize residue have been found in an 8,700 years old layer of deposits.  Some of the earliest pollen remains from Latin America have been found in lake sediments from tropics of Southern Mexico and Upper Central America, up to Laguna Martinez and have been radiocarbon dated

to 4,700 years ago (Lieberei, 2007).

Archaeological   remains of early maize ears,  found at Guila Naquitz Cave in the Oaxaca

valley, date back roughly 6,250 years; the oldest ears from caves near Tehuacan, Puebla, date ca.2750 B.C.  Little  change occurred in  ear form until  ca.1100 B.C when great changes appeared in ears from Mexican caves;  maize  diversity  rapidly increased and archaeological

teosinte was first deposited (Felipe, 2011).

As  maize  began  to  spread  rapidly,  it  was  introduced  to  new  cultures,  new  uses  were developed and new varieties selected to serve those uses (Arai,  et al.,2001).  Maize was the staple food,  or a major staple (along with squash, Andean region potato,  quinoa,  beans and amaranth),  of most pre-Columbian North American,  Mesoamerican,  South American and Caribbean cultures (Baden et al.,  2001).The  Mesoamerican civilization was strengthened upon the field crop of maize; through harvesting it, it’s religious and spiritual importance and how it impacted on their diet. Maize formed the Mesoamerican people’s identity.  During the first millennium AD,  maize cultivation spread from Mexico into the U.S.  Southwest and during  the  following  millennium  into   the  U.S.   Northeast  and  Southeastern  Canada, transforming the landscape as Native Americans cleared large forest and grassland areas for the new crop.  It is unknown what precipitated it’s domestication, because the edible portion of the wild variety is too small and hard to obtain to be eaten directly,  as each kernel is enclosed in a very hard bivalve shell.  However, George Beadle demonstrated that the kernels of teosinte are readily “popped”  for human consumption,  like modem  popcorn  (Barlow,

2002).  Some have argued that it would have taken too many generations of selective breeding to produce large, compressed ears for efficient cultivation.  However, studies of hybrids made by  intercrossing  teosinte  and  modem  maize  suggest  this  objection  is not  well  founded (Buckler et al., 2009).

Fig 2:  Centeotl, the Aztec deity of maize.      (Femandez-Armisto, 2011)

1.1.2:  Scientific Classification.

Maize is classified normally in this order

Kingdom                Plantae

Sub kingdom            Tracheobionta Super division          Spermatophyta Division                   Magnoliophyta Class                          Liliopsida

Sub class                 Commelinidae

Order                      Cyperales

Family                     Poaceae-Grass family

Genus                       Zea com

Species                     Zea mays

Binomial Name        Zea  mays

There are  six general varieties of maize differentiated by the characteristics of the kernel. They are

1)   Dent Com:  is the leading type of com  grown mostly  in Africa,  the sides of the kernel

consists of hard,  horny starch and the crown contains soft  starch,  as the grain matures this soft starch shrinks, forming the characteristics dent.

2)  Flint Com: the horny starch extends over the top of the kernel, so there is no denting.

3)  Pop Com:   a light, highly popular snack, is a variant of flint com with small kernels of great hardness.

4)  Flour Com:  contains a preponderance of soft or less densely packed starch,  and it is readily

ground into meal.

5)  Sweet Com:  is the type commonly  grown in United  States for consumption.  The sugar

produced by the sweet com plant is not converted to starch during growth.

6)  Pod Com:  seldom used as food but is often grown as a decorative plant,  each kernel is enclosed in its own set of diminutive husks (Goodman, 2005).

1.3  :   STRUCTURE AND PHYSIOLOGY OF MAIZE.

Maize stems superficially resemble bamboo canes and the intemodes can commonly be 7 inches.  Maize has a distinct growth form; the lower leaves being like broad flags, generally 50-100 centimeters long and 5-10 centimeters wide; the stems are erect conventionally 2-3 meters in height,  with many nodes,  casting off flag leaves at every node under these leaves and close to the stem grow the ears. Unlike major grain crops, the maize plant has separate male and female flowering parts.  The ears are female inflorescences, tightly covered by several layers of leaves and so closed  in by them to the stem.  The silks are elongated stigmas that look like tufts  of hair,  at first green and later red or yellow.  The apex of stem ends  in  the  tassel,  an  inflorescence  of male  flowers.  When  the  tassel  is  mature  and conditions are suitably warm and dry, anthers on the tassel dehisce and release pollen. Maize pollen is anemophilous (dispersed by wind) and become pollinated to produce one kernel of maize.  Young ears can be consumed raw or with the cob and silk, but as the plant matures, the cob become tougher and the silk dries to inedibility.  By the end of the growing season, the kernels dry out and become difficult to chew without cooking them tender first in boiling water (Ritchie et al,  1993). Maize, as well as sorghum and sugarcane belong to C4 plant (plants that use C4 photosynthesis essentially eliminate the oxygenase activity of rubisco via anatomical, biochemical and ultrastructural modifications in leaves). It is a facultative long night plant and flowers in a certain number of growing days in the environment to which it is adopted. The magnitude of the influence that long nights have on the number of days that must pass before maize flowers is genetically prescribed and regulated by the phytochrome system (Bortiri and Hake, 2007). Photoperiodicity can be eccentric in tropical cultivars such that the long day characteristics of higher latitudes allow the plants to grow so tall that they do not have enough time to produce seed before being destroyed by frost or cold, these attributes may prove useful in tropical maize for biofuels (Duvick and Cassman,  1999).

1.2.1:   Genetics

Maize has ten chromosomes.  The combined length of the chromosomes is 1500  cm.  Some of the maize chromosomes have what are known as “chromosomal knobs” highly repetitive heterochromatic domains that stain darkly (Cudderford, 1995). Individual knobs are polymorphic among strains  of both maize and teosinte.  Barbara McClintock   used these knob markers to validate her transposon  theory of “jumping genes”,  for which she won the

1983  Nobel Prize in Physiology or Medicine (Schwartz.,1979).  Maize is still an important

model  organism  for  genetics  and  developmental  biology  today.   The  Maize  Genetics

( Recombination rate (cM/Mb)

( Mu insertions (sites/Mb)                                109

0 MF Enrichment (%reads)       ‘.d o’

0 Repeat coverage(%)                                     100

0 Gene density (genes/Mb)                               74

( Rice chromosomes                                                s

1              2        3       4        5      6        7        8        9       10    11      1

I E  [I I

3       4        1              6       9      10           7       2                 5       8

(9 Sorghum chromosomes

Fig 3: Genetic sequence of maize.       

1.2.2:   PRODUCTION

Maize is often planted in a two crop rotation with a nitrogen fixing  crop such as fluted pumpkin in  cooler climates  and  soy  beans  in regions with higher  temperatures.  Many  of the  maize varieties  grown are hybrids,  the genetically modified hybrids  tolerate glyphosate or provide protection against natural pests. Immature maize shoots accumulate a powerful antibiotic substance,  2, 4-dihydroxyl -7- methyl, 4-benzoxazin -3-one (DIMBOA).  DIMBOA is a member of a group of hydroxamine acids (also known as benzoxazinoids) that serve as a natural defense against a wide range of pests,  including insects, pathogenic fungi and bacteria.  DIMBOA is also found in  related grasses,  particularly wheat. A maize mutant  (bx)  lacking DIMBOA is also responsible for the relative resistance of immature maize to the   European com borer (family Crambidae). As maize matures, DIMBOA levels and resistance to the com borer decline.  Maize is susceptible to droughts,  intolerant   to nutrient deficient soils and prone to be uprooted by severe winds because of it’s shallow roots. Inorganic fertilizer and herbicides are used for the cultivation and weed control respectively.

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