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 "bitter apricot kernels" are increasingly being sold in shops and on the internet for direct consumption as a "snack". They are also sometimes advertised as an inexpensive 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 as part of official food inspections to ensure compliance with EU maximum levels. The maximum level of hydrocyanic acid in unprocessed whole, grated, ground, cracked or chopped apricot kernels placed on the market for consumers is 20 milligrams(mg) per kilogramme of food and is regulated in Commission Regulation (EU) No. 2023/915. This maximum level applies to both bitter and sweet apricot and apricot kernels.

Monitoring of linseed

An EU maximum level for hydrocyanic acid in crushed linseed is also laid down in Commission Regulation (EU) No. 2023/915.

Monitoring was carried out in Austria in 2020. The aim of the monitoring was to obtain an overview of the hydrocyanic acid content of crushed linseed. Furthermore, the labelling was to be checked for the application of the warning label in accordance with the action values for certain contaminants in food (published by the Austrian Food Codex, ÖLMB).

Labelling of the warning according to the Austrian Food Codex

The EFSA assessment, which was updated in 2019, and our risk assessment indicate that, even taking into account the reduced bioavailability, a restriction on consumption is necessary and the focus should be placed in particular on children. For an adult, from a toxicological point of view, the amount consumed in one meal is of greater importance than the total daily consumption of flaxseed.

The national action values for certain contaminants in foodstuffs of the Austrian Food Codex(ÖLMB) stipulate that linseed products must be labelled with an appropriate warning including consumption restrictions so that the consumer is made aware of the problem:

Linseed contains substances that release hydrocyanic acid when crushed(e.g. ground). When crushed linseed is consumed, hydrocyanic acid can be absorbed by the body.

In any case, the following consumption restrictions must be observed:

Consumption of a maximum of one tablespoon of ground linseed per meal by adults must not be exceeded.

Children must not consume more than one teaspoon of ground linseed per day.

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

A similar advice is also recommended for whole linseed.

• 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).