The common flax, Linum usitatissimum L., is the only representative of the flax species belonging to the Linaceae that is cultivated on a significant scale. The fine-leaved plant, which grows to a height of up to 120 centimeters, is predominantly annual. Its stem with its useful bast fibers bears numerous short and narrow-lanceolate stalkless leaves. In summer, the five-petaled flowers, over 2 centimeters tall, with mostly light blue to pink or white petals, are in panicle-like inflorescences.
Flax is predominantly self-pollinator. A membranous globose fruit capsule with five seeds in separated compartments develops from the ovary. Flax seeds are flat, shiny, brown or yellow depending on the variety, about 5 millimeters long and about half as wide.
Flax has been domesticated for over 6000 years and is found almost worldwide. Its original origin has not been definitively clarified, but it certainly descends from the biennial flax from the Mediterranean. Cultivated flax exists in four varieties of which the combination flax, the tall-growing fiber supplier flax, closing or fiber flax and the large-seeded oil flax have the greatest commercial relevance.
The soil requirements of flax are modest, except that it does not tolerate wetness. Flax does not thrive well near the equator and dry heat severely reduces the oil content of the seeds. As a long-day plant, flax is sown in mid-latitudes as early as spring. Flowering starts here from June until August, induced by at least 14 to 16 hours of daylight. After 110 to 120 days of vegetation, linseed is harvested by combine threshing, yielding between 2 and 3 tons per hectare. Harvesting for further processing into linseed fiber is much more complex.
Today's main linseed growing regions include Russia, Canada, China, USA, India Egypt and Ethiopia. In 2019, according to the FAO, the world flaxseed harvest was over 3 million tons, with 1 million coming from Kazakhstan and 660,000 from Russia.
There are two common oil extraction processes: the cold pressing process below 40 degrees Celsius and the extraction process with solvents.
The cold pressing process is used in the cosmetic, food and nutrition industries. The seeds are first crushed and the resulting pulp is then pressed using screws or screw presses. The oil is then filtered and packaged. Mechanical pressing leaves a higher proportion of oil in the press cake: The oil yield from pressing flaxseed is about 20 to 30 percent, and the mucilage from the outer testa layer is subsequently removed by a bleaching earth filter. The cold-pressed oil has a golden-yellow color. The press cake can be processed into animal feed.
If the oil is used in industry as much oil as possible should be extracted from the seed. For this purpose the seeds are crushed and pressed with heat. The thermal energy used in this process leads to increased oil yield. The remaining press cake can then be further extracted with solvents to remove residual oil (residual oil content is then <1% in the press cake). Since this method also increasingly extracts unwanted substances, such as mucilage, the oil is finally refined. Hot-pressed oil has a brownish-yellow color which changes to yellow after refining.
Pressed linseed oil has a wide range of applications for technical, pharmaceutical-medical, cosmetic and nutritive purposes. Industrial applications resort to hot pressing, extraction and subsequent refining, while linseed oil for other uses is always cold pressed in screw presses up to a maximum of 40 °C.
Linseed oil is particularly rich in the essential fatty acids linolenic acid and linoleic acid, which do not owe their names to it for nothing. The main ingredient, alpha-linolenic acid (ALA), can account for a content of between 45 and 71 percent, depending on its origin, which is why linseed oil is the richest vegetable source of omega-3 fatty acids. 12 to 24 percent of the oil represents the omega-6 fatty acid linoleic acid, 10 to 24 percent oleic acid (omega-9 fatty acid). The high vitamin E content of (440 to 590 milligrams per kilogram), predominantly in the form of gamma-tocopherol, is also nutritionally significant. Nevertheless, the fats in linseed oil oxidize very easily. Because of its sensitivity to oxidation, linseed oil must therefore be stored and processed with special care: as cool as possible, protected from air and light.
Tthe fatty acid pattern of linseed oil is considered outstanding. Humans can convert a certain proportion (maximum 10 percent) of ALA into the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) - otherwise found in fatty sea fish or algae oil. The omega-6 fatty acids, which are a hindrance in this process, are favorably proportionally in the background: linseed oil can therefore partially replace fish oils. According to clinical studies, the intake of EPA and DHA, now also in the form of linseed oil, is attributed positive effects on thrombosis risks, arteriosclerosis, hypertension and hyperlipidemia, as well as on allergic, chronic inflammatory and malignant diseases (in estrogen-sensitive breast cancer tumors). Antibacterial effects against multi-resistant germs have also been reported. In addition, during brain development, the EPA-DHA requirement is increased.
In terms of taste, freshly cold-pressed flaxseed oil has a hay- to grass-cut-like note with a nuttiness. Despite proper storage, a pungent, bitter and fishy taste begins to assert itself after a very short time. Nevertheless, linseed oil is popular in cold cuisine, and regional dishes still bear witness to the former importance of this very old foodstuff.
As a feed additive, linseed oil can support skin function, improve coat luster, aid digestion and, given separately, serve as a laxative.
In pharmaceutical applications, linseed oil is an ointment base, the basis of thick liniment preparations or an additive for powder preparations. Cosmetically, the omega-3 fatty acids are said to have a beneficial effect on skin aging, and linseed oil soaps are also on the market.
Industrially, linseed oil is used in various degrees of processing as a versatile primer, binder and impregnating agent. It is a raw material for oil paints and coatings, varnishes, putty and linoleum, among other things. This is because linseed oil is one of the hardening vegetable oils: when exposed to oxygen and light, its unsaturated fatty acids slowly polymerize into linoxin and form a weather-resistant, elastic skin.
Besides the oil flax (Linum usitatissimum convar, mediterranum), two other types of flax are known: the fiber flax (Linum usitatissimum convar, usitatissimum) and the jumping flax (Linum usitatissimum convar, crepitans). The flax was cultivated in Central Europe until the 20th century to produce textiles.
Objects soaked in linseed oil or textiles with linseed oil residues may pose a risk of spontaneous combustion. Brushes, rags and clothing catch fire on their own when exposed to atmospheric oxygen. Since linseed oil is one of the hardening vegetable oils, heat is generated during hardening, which can lead to a smoldering fire. Storage should therefore be under water, hermetically sealed or laid out flat for curing. Washing out the rags/brushes can also reduce this risk.
With the use of linseed in food increasing for about 100 years, the incidence of atopic allergies, also to the oil, is unfortunately increasing.