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

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