Endosperm Composition: Nutrients and Structure

Whole grain kernels are made up of three components: the bran, germ, and endosperm, each of which contains beneficial nutrients.

The endosperm consists of two distinct cell types: aleurone cells, forming the outermost layer with thick walls, and central starchy endosperm cells that are rich in starch and gluten proteins.

The aleurone layer, a single layer of living cells, acts as a barrier between the starchy endosperm and the germ and is abundant in essential nutrients such as minerals, vitamin B, and protein.

The starchy endosperm is the main and largest part of the grain, serving as a storage compartment to provide energy for the germinating plant. Within the endosperm cells, proteins and starch are stored, resulting in a unique and highly dense structure.

The composition of wheat endosperm varies depending on its source, typically containing around 68 to 76% starch and 6 to 18% protein. It holds the highest proportion of protein, carbohydrates, iron, and B-vitamins like riboflavin and niacin, making it a valuable source of nutrients and soluble fiber.

Constituting approximately 83% of the kernel weight, the endosperm is the primary source of white endosperm flour. In white flour, proteins play a crucial role as binding agents, ensuring the cohesion of starch granules within the endosperm.
Endosperm Composition: Nutrients and Structure

Wheat kernel

Wheat is the primary grain used in U.S. grain products — approximately three-quarters of all U.S. grain products are made from wheat flour. The kernel or wheat berry, is the seed from which the wheat plant grows. The kernel’s size generally ranges from 5 to 9 mm long and weighs between 35 to 50 mg.

Wheat kernels are loaded with nutrients because they are whole grains. A cup of cooked berries has about 300 calories and is packed with vitamins, fiber, protein, and iron.

Each tiny seed contains three distinct parts that are separated during the milling process to produce flour —the endosperm, bran and germ

Bran
The bran is the multi-layered outer skin of the edible kernel and makes up around 14.5 percent of its weight. The bran contains a small amount of protein, trace minerals, and dietary fiber – primarily insoluble. The bran is an excellent source of insoluble fiber, which makes its inclusion in whole wheat flour valuable to improving digestive health for the end consumer.

Endosperm
Endosperm is about 83% of the kernel weight and the source of white endosperm flour. It has a high portion of protein, along with carbohydrates and iron. The endosperm is also a source of soluble fiber.

Germ
The germ is the embryo which has the potential to sprout into a new plant. It is the center of the kernel and only 2.5 percent of its weight. The germ contains minimal quantities of high-quality protein and a greater share of B-complex vitamins and trace minerals. Germ is often separated from flour during milling because the germ’s high fat content can limit the shelf life of flour.
Wheat kernel

Aleurone cells of cereals

Cereal endosperm is composed of two distinct tissues: the starchy endosperm that is primarily a storage organ filled with starch and protein reserves and the surrounding aleurone layer.

Botanically, the aleurone layer (which represents 5–8% of the wheat grain) is the outer part of the starchy endosperm.

However, as it stays attached to the hyaline layer during milling and is therefore removed from the endosperm with the grain outer layers, the miller considers the aleurone as a part of the bran.

The aleurone cells are cuboid in shape when viewed in cross section and polygonal when viewed on its top surface. Aleurone cell size varies according to cereal; cell size, for example, being smaller in maize than in wheat.

In cereals, the aleurone layer has important functions in the accumulation of storage compounds during seed development, and in the mobilization of storage compounds during germination.

During the phase of storage product accumulation, the aleurone cells accumulate protein, lipids, and minerals. Polyploidy in barley aleurone has been reported. The cytoplasm of mature aleurone cells appears granular, as it contains numerous protein storage vacuoles, which are also called aleurone grains. They are about 1–5 μm in diameter and consist of a matrix of mainly 7S globulins.

The aleurone layer is particularly rich in nutrients. Indeed, the intracellular medium of aleurone cells is characterized by high amounts of protein, minerals, phytates, B vitamins such as niacin and folates, and lipidic compounds such as plant sterols.

The storage function of aleurone cells in cereals involves the accumulation of high levels of phytic acid, which chelates several minerals. In barley grains, the aleurone is the major storage site for phosphate, magnesium, potassium, and calcium, accumulating over 70% (97% for magnesium) of the endosperm stores of these minerals.

Aleurone provides ∼15% of the total wheat protein but also ∼30% of the total lysine (which is the first limiting essential amino acids in wheat). At least 80% of total niacin in wheat is found in the aleurone layer and a considerable amount of other B vitamins.

Aleurone has reported dietary and health benefits; aleurone flour dramatically decreased the incidence of colon adenomas in rats, while raising red blood cell folate levels and decreasing plasma homocysteine levels in humans.
Aleurone cells of cereals

Starch content in barley

Barley has starch content of 62.5%, a protein content of 7.5% and a low fat content of 1.2%.

Barley seeds were loaded with starch, that provides energy for seeds to grow into sprouts. It consists of hundred of sugar molecules linked together to form long chains. Starch is a polysaccharides, alpha-glucan and can be divided into straight-chain amylase and branched chain amylopectin.

These starch granules contain traces of lipids, minerals, protein and nucleotide.

The “diastatic” enzymes responsible for degrading the starch in germination are phosphorylase, alpha glucosidase, alpha-amylase, beta-amylase, debranching enzymes, and transglucosylase.

These starches in the endosperm are 95% deposited in 11-28 days after ear emergence and the ratio of amylopectin increases to the final ratio. Starch granules of the starchy endosperm are embedded in the protein matrix. This association of starch granules and protein in the starchy endosperm helps to give the endosperm its structure.

The starch granules in the barley endosperm are laid down within amyloplasts and fall into two size groups: 1.7-2.5 um and 22.5 – 47.5 um. About 90% of the starches granules are small and about 10% are large.

Large starch granules are composed of 25% amylose and 75% amylopectin. Small starch granules are reported to have similar contents of amylose and amylopectin but the very small starch granules (less than 4 um) can contain as much as 40% amylose.

In normal barleys the ration of amylase to amylopectin is about 1:3, in high amylase Glacier about 1:1 and waxy barley is 97-100% amylopectin.
Starch content in barley