Understanding Prolamins: The Unique Proteins of Cereal Grains

Prolamins stand as distinctive proteins found exclusively in grasses, particularly cereals, encompassing a varied array of molecules such as glutelin, glutenin, and gliadin. These proteins, with subunits ranging from 30 to 90 kDa, serve as the primary nitrogen reservoir within the endosperm of cereal seeds. This role is facilitated by extensive gene duplication in endoreduplicated cells, leading to massive accumulation levels.

Comprising notable quantities of proline and glutamine, prolamin proteins derive their name from these amino acids. However, they contain relatively low levels of arginine, lysine, and histidine. Structurally, prolamins feature α-helix globular domains, enriched with 6-8 cysteine residues and 3-4 disulfide bonds, rendering them highly soluble in ethanol (40-70%).

Prolamins play a pivotal role in determining the hard texture of mature cereal seeds, a critical trait for grains like maize. Nonetheless, deficiencies in essential amino acids within prolamins contribute to subpar grain protein quality.

Across various cereal species, prolamins exhibit diversity. In wheat, barley, and rye, prolamins are categorized into sulfur-rich (α-, β-, γ-gliadins), sulfur-poor (ω-gliadins), and high-molecular-weight proteins. Furthermore, these proteins are named based on their cereal source, such as secalin (rye), hordein (barley), and avenin (oats), among others.

However, the significance of prolamins extends beyond their nutritional attributes. In wheat and related grains, these proteins are implicated in celiac disease, a condition sometimes termed as gluten-sensitive enteropathy due to its association with the wheat prolamin fraction known as gluten.

Understanding prolamins not only sheds light on the intricate biology of cereal grains but also underscores their implications for human health. From their pivotal role in nitrogen storage to their influence on grain texture and protein quality, prolamins stand as a testament to the complexity and significance of plant proteins in agriculture and nutrition. Moreover, their involvement in conditions like celiac disease underscores the importance of continued research into these unique proteins and their impacts on human health.
Understanding Prolamins: The Unique Proteins of Cereal Grains

Cereal of rye

Cereal rye grows rapidly in cool weather and forms a dense, tall stand with a more extensive root system than other cereals. Among the cereal, it is the most cold tolerant and is one of the best species to use if planting is delayed until late fall or winter.

Rye is a cereal grass that is second only to wheat in world popularity for bread baking. It has strong hearty flavor in bread and as a cereal.  Crisp bread also made from rye flour, often whole meal rye, possibly with some wheat flour added, with or without yeast leavening.

Pumpernickel is also produced from 100% rye meal, using the sour-dough process. It is distinctive due to the very long baking time involved - between 18 and 36 hours.

Rye flours are used occasionally in snack type crackers. A very light rye has been recommended for use in soda crackers and some cookies as partial replacement for wheat flour.

Rye flours are also used in some pancake and waffle mixes to produce a special flavor. In the United States, one of the main uses for rye is rye whisky.

Rye flours are used as fillers for sauces and soups. Rye is also flaked to make a hot breakfast cereal.

Rye contains B-complex vitamin, protein, calcium, iron, magnesium, phosphorus, and potassium.
Cereal of rye

Glutenin in flour

Flour itself does not contain gluten. One of gluten’s main protein fractions in the flour is gliadin which a complex mixture.

At least forty different components may be identified in a single wheat variety. Gluten’s other main protein fraction is glutenin.

Glutenin are protein polymers consisting of high and low molecular weight glutenin subunits HMW-GS and LMW-GS linked though disulphide bonds.

The polymeric glutenin protein have molecular sizes ranging up into the tens of millions of Daltons. The great length of these chains is presumed to confer in wheat its unique dough forming properties.

Glutenin is thought to provide most of the strength also called tenacity, to gluten, while gliadin provides its stretchiness or extensibility. Glutenin also provides elasticity to gluten; that is its ability to bounce back one it is stretched or pressed.

The network of glutenin and gliadin is formed when the long glutenin molecules cross link with hooks on the globular gliadin molecules.

This network is very stretchy and liable, and it appears that the combination of gliadin and glutenin responsible for gas retention in wheat four dough.

The stretchiness lets the dough structure hold on the gas of carbon dioxide, which is produced by yeast or baking powder and form the crumb or texture of the bread.

In cases of gluten intolerance, the gliadin fraction is the cause of the symptoms; the glutenin fraction is not.
Glutenin in flour

Cereal nutrition and diet

Cereals, the most important plant foods in the human diet are derived from seeds of domesticated members of the Graminease, the grasses.

Long before people learned to cultivate the grasses that are today’s cereal grain, they relied upon such grains as a source of nutrients.

It is thus only natural that breakfast cereals made from the cereal grains through modern processing techniques have become primarily contributors of nutrients to our diets.

Now, cereals remain the staple food in most does and in many parts of rural Africa and Asia provide more than 70% of the energy intake.

The nutrients provided by this first meal of the day include those that are indigenous to the cereal grains as well as some that are added in the manufacture of the cereal.

The nutrient contribution of cereal applies to all age groups. Processed cereals are usually the first solid food fed to infants and the cereal feeding frequently is the first of the day.

Carbohydrate is the main substance in cereals. Cereals have crude fiber and starch,. Though the nutritive value of fiber is not much it performs over other valuable functions which enable the motility of intestine.

No one can doubts on the popularity of breakfast cereals among children; all one has to do is look on store shelves at the large number of breakfast cereals designed to appeal to children.

According to one study, among children aged 5-12, those who ate ready to eat cereal three or more times a week consumed significantly less fat and cholesterol and more fiber, B-vitamins and vitamins A and D than those who ate no ready to eat cereal at breakfast.

Cereal also present an important source of protein. Cereal protein is especially valuable in diets when we consider complementation of amino acids between various plant sources.

The Nutrition Facts panel on a box of breakfast cereal shows an abundance of vitamin and minerals, many of which have been added through fortification.

Cereal made contributions to adult as well. One study shows that breakfast consumption patterns of adult aged 50 and over and concluded that for all age and sex classes, consumption of ready to eat cereal at breakfast increased the average daily intake of all vitamins and minerals particularly those identified as under consumed by elderly individuals.

Adding milk or yoghurt makes cereal a great vehicle for delivering calcium and other nutrients in milk.

For this reason, cereal labels often give the Nutrition Facts for cereal only and for cereal with added milk.

With consumer awareness of the importance of whole grains, granolas have become popular, These typically contain vegetable oil to make them tastier. Thus a cup of commercial prepared granola can easily provide 600 calories or more.
Cereal nutrition and diet

Chemical Composition of Cereals

Chemical Composition of Cereals
The chemical composition of the cereals varies widely and depends on the environmental conditions, soil, variety and fertilizer.

Wheat has a higher protein content than other cereals: The protein content varies from 7 to 22% depending on the variety. However, because of low availability of some essential amino acids in wheat, its biological value requires addition or supplementation with other amino acids.

Several research efforts have focused in producing different wheat varieties with higher protein and essential fatty acids content.

Carbohydrates are the major chemical composition of the corn. However, the maize corn kernel is more than a rich source of carbohydrate, it is a source of enzymes for the study of biosynthesis and genetic markers for genetic, biochemical, and genetic engineering studies.

The starch granule is formed inside an amyloplast and arranged in an insoluble granule. Starch is the major carbohydrate in the kernel and comprises close to 72% of its dry weight.

Starch also is found in the embryo, bran, and tip cap. Amylose makes up 25-30% of the starch whereas amylopectin composes 70-75% of the starch.

Monosaccharides , such as fructose and glucose are found in equal proportions in the endosperm. Among the disaccharides sucrose is the major sugar in kernels that comprise only 4-8% of kernel dry weight: maltose is also found at less than 0.4% of the kernel dry weight.

The corn bran consists of 70% hemicellulose, 23% cellulose and 0.1% lignin on a dry weight basis.
The protein content of the corn shows that it is poor in essential amino acids such as tryptophan, lysine and threonine, valine and sulfur amino acids.

The corn has only 4.4% oil (dry basis), but the amount of corn oil production is enormous, even though it is not considered as an oil seeds crop.

Triglycerides are the major composition (98.8%) of the refined commercial corn oil.

Corn oil is very stable compared with other seed oils owing to its flow level of linolenic acid and the presence of natural antioxidant.

The composition of rice and its fraction depends on the cultivars, environmental conditions and processing. The rice components distributes differently in aleurone, embryo and other parts of the grain.

The average brown rice protein content ranges from 4.3 to 18.3% with a mean value of 9.2%.

Protein is the second most important rice component after carbohydrates.

The ouster tissue of the rice grain are rich in water soluble proteins (albumin) and also salt soluble proteins (globulin), but the endosperm is rich in glutelin.

The milling fraction of the rice grain has a limited prolamin (alcohol free proteins), and the non-protein nitrogen (NPN) of the rice is about 2 -4%.

Rice starch is composed of linear fraction - amylose and branched fraction -amylopectin – that is a major factor in the eating and cooking quality of the rice.
Chemical Composition of Cereals

Cereal in general

Cereal in general
Cereals are monocotyledonous plants that belong to the grass family. Based on botanists’ approximation, there are about 350,000 plant species, of which about 195,000 species are economically important flowering plants.

Nearly 50 species are cultivated worldwide and as few as 17 species provide 90% of human food supply and occupy about 75% of the total tilled land on earth.

They consist of wheat, rice, corn, potato, barley, sweet potato, cassava, soybean, oat, sorghum, millet, rye, peanut, field bean, pea, banana and coconut.

The cereal grains such as wheat, rice, corn, barley, oat, rye, sorghum and millet provide 50% of the food energy and 50% of the protein consumed on earth.

Wheat, rice and corn together make up three-fourths of the world's grain production.

In general, cereal grain have been considered as the source of carbohydrates to supply food energy to the diet. Cereal grains, especially rice and wheat, provide the bulk of energy consumed on earth.

The cereal crops that are grown for their edible fruit are generally called grain, but botanically referred to as caryopsis. The cereal seed consist of two major components, the endosperm and embryo or germ.

The endosperm encompass the bulk of the seed and is the energy source of stored food.

An outer wall called the pericarp that develops from the ovary wall encases the endosperm. A semi-permeable layer under the epicarp, which is called testa, surrounds the embryo and is derived from the inner ovary wall.

The testa is permeable to water but not to dissolved salts an is important for germination. The third layer, which is called aleurone, contains thick walled cells that are free of starch. The pericarp, testa and aleurone layer are collectively called the bran.
Cereal in general

Fibers

Food Science
Dietery fibers include the nondigestible carbohydrates. These may be either water soluble or water insoluble. Both have nutritional significance. The water insoluble group that includes wheat products and wheat bran is believed to reduce chances of colon cancer by increasing bulk and diluting the effect of secondary bile acids.

The soluble fibers such as those found in the brans of some cereals (e.g. oats and rice) and in pectin are believed to lower the levels of serum cholesterol by binding bile acids and causing removal of cholesterol in the feces.

While the claims may be made for cholesterol level lowering properties of brans from different grains (e.g., the bran from oats and the bran from psyllium seed, which have much higher amounts of bran than that of other grains), the more productive course for trying to control cholesterol level is limit the consumption of foods that are high in cholesterol and high in fats.
Food Science