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
