Cyanogenic glycosides are chemical compounds that occur naturally in plant foods such as apricot or apricot kernels, bitter almonds and flaxseed. Since prussic acid (hydrogen cyanide) is released from these compounds during chewing and digestion, excessive consumption can lead to severe poisoning (prussic acid poisoning), which can even be fatal.

Bitter almonds, bitter apricot or apricot kernels, but also cassava (manioc) and sorghum (especially in livestock farming) have a high content of cyanogenic glycosides. The cyanogenic glycoside amygdalin contains prussic acid in a bound form (cyanide) and serves some plants as a protective substance against natural enemies. It is mainly found in bitter almonds and apricot kernels.

Chewing the raw, peeled kernels releases the prussic acid from the amygdalin. The more and longer one chews the hulled kernels, the higher amounts of cyanogenic acid are released.

Flaxseed also contains cyanogenic glycosides. Crushing or grinding can release prussic acid, which can be absorbed by the body. In whole, unground flaxseed, prussic acid is hardly ever released.

Flavoring

Traditionally, bitter apricot or apricot kernels, or bitter almonds, are used for their flavoring properties in marzipan, persipan and related products, although there is a regulation in the form of the flavoring ordinance that limits the prussic acid content of these products to safe levels. Bitter almonds, which are comparable to bitter apricot kernels in their properties and possible applications, are also used for flavoring in baked goods (e.g. Christmas stollen). In these baked goods, prussic acid is usually not a problem because it largely volatilizes when the kernels are heated.

Hydrocyanic acid causes poisoning. When apricot or apricot kernels are chewed and digested, prussic acid is formed from the cyanide released by amygdalin. The more and longer one chews the peeled kernels, the higher amounts of prussic acid are released. Prussic acid can also be absorbed by the body when eating ground flaxseed.

The human body is capable of breaking down certain amounts of prussic acid. However, if too much prussic acid is absorbed, various symptoms of poisoning can occur. The symptoms of acute poisoning range from headaches, shortness of breath, dizziness and convulsions to cyanosis, coma and death. In children, very small amounts are sufficient to cause severe poisoning. In older or sick people, too, it must be expected that the body's own detoxification system does not work sufficiently. These poisonings can even be fatal.

Apricot kernels or "apricot kernels bitter" are increasingly sold in stores and via the Internet for direct consumption as a "snack". In some cases, they are also advertised as a cheap and effective anti-cancer agent. According to the manufacturers, the alleged effect is based on the vitamin B17 ("laetrile", amygdalin) contained in the seeds or on the toxic hydrocyanic acid, which is supposed to kill cancer cells. However, these healing effects have not been scientifically proven.

Routine examination of apricot or apricot kernels

Apricot and apricot kernels are regularly tested for compliance with EU maximum levels as part of official food inspections. The maximum level of prussic acid in unprocessed whole, grated, ground, cracked or chopped apricot or apricot kernels placed on the market for consumers is 20 milligrams(mg) per kilogram of food and is regulated in Regulation (EU) 2017/1237. This maximum level applies to both bitter and sweet apricot or apricot kernels.

Monitoring of flaxseed

An EU maximum level for prussic acid in ground flaxseed is set in Regulation EU 2022/1364.

In 2020, a monitoring was carried out in Austria. The aim of the monitoring was to obtain an overview of the prussic acid content of ground linseed. Furthermore, the labeling should be checked for the application of the warning according to action values for certain contaminants in food (published by the Austrian Food Book, ÖLMB).

Affixing of the warning according to the Austrian Food Codex

From the EFSA assessment, updated in 2019, as well as from our risk assessment, it appears that even taking into account the reduced bioavailability, a consumption restriction is necessary and the focus should be especially on children. For an adult person, from a toxicological perspective, the amount consumed at one meal is of greater importance than the total flaxseed consumption spread over a day.

In the action values for certain contaminants in foodstuffs of the Austrian Food Codex(ÖLMB), it was stipulated that flaxseed products should be labeled with an appropriate warning, including consumption restrictions, so that the consumer is made aware of the problem by a corresponding statement:

Flaxseed contains substances that release hydrogen cyanide when crushed(e.g., ground). When crushed linseed is consumed, prussic acid can be absorbed by the body. In any case, the following consumption restrictions should be indicated:

A consumption of not more than one tablespoon of ground flaxseed per meal by adult persons shall not be exceeded.

A consumption of not more than one teaspoon of coarsely ground flaxseed per day may not be exceeded by children.

The ground product is not suitable for children under 4 years of age.

For whole flaxseed, a similar advice is also recommended.

• Do not eat apricot kernels or bitter apricot kernels as snacks.
• Refrain from consuming "vitamin B17" ("laetrile", amygdalin)
• Adults should consume a maximum of one tablespoon of ground flaxseed per meal
• Children over 4 years of age should consume a maximum of one teaspoon of ground flaxseed per day.
• Children under 4 years of age should generally not consume ground flaxseed

Emergency contact

If symptoms of poisoning occur, contact a health care professional. If in doubt, contact the Poisoning Information Center: 01/406 43 43 (around the clock).

Plants contain hydrocyanic acid in the form of cyanogenic glycosides. Cyanogenic glycosides are chemical compounds consisting of a sugar and the prussic acid in its bound form (cyanide). In this case, the cyanide and the sugar are bound together. The actual toxic substance is the cyanide (salt of prussic acid), which can be cleaved by the plant's own enzyme β-glucosidase. In this process, the enzyme cleaves the cyanogenic glycoside into the cyanide and a sugar.

Amygdalin is the dominant cyanogenic glycoside in bitter almonds and bitter apricot kernels, and linamarin is the dominant cyanogenic glycoside in cassava.

In flaxseed, the dominant cyanogenic glycosides are linustatin, neolinustatin, and linamarin. When flaxseed is crushed or ground, prussic acid is enzymatically released from the bound form (cyanide) by β-glucosidase, making it bioavailable to the body and absorbed by the body after consumption. However, compared to bitter almonds, bitter apricot or apricot kernels, or cassava, flaxseed contains low levels of β-glucosidase, resulting in lower bioavailability of cyanide and lower absorption of hydrogen cyanide. In whole, unground flaxseed, hydrocyanic acid from glycosides can hardly be absorbed by the body.

EFSA updated its 2016 risk assessment on bitter apricot kernels and confirmed the acute reference dose (ARfD) of 20 micrograms (µg) of cyanide per kilogram of body weight, regardless of whether the cyanide is absorbed from bitter almonds, bitter apricot kernels, or ground flaxseeds. This safe dose is equivalent to approximately three small apricot kernels per day for adults (EFSA 2019).

According to EFSA, setting different ARfDs for different foods does not seem appropriate. However, to account for differences in bioavailability, different factors are applied in the exposure assessment. For cassava and bitter almonds this is 1, for linseed 3 and for marzipan/persipan 12 (EFSA 2019).