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All plants contain at least tiny amounts of [https://en.wikipedia.org/wiki/Hydrogen_cyanide hydrogen cyanide] ('''HCN'''), however some plants also release high amounts of HCN from a class of glycosides called "cyanogenic glycosides", also called "cyanoglycosides". [https://en.wikipedia.org/wiki/Amygdalin Amygdalin] and [https://en.wikipedia.org/wiki/Linamarin linamarin] are common examples of cyanogenic glycosides <ref name="Gleadow_2014"></ref>. HCN is released from cyanogenic glycosides just like other types of glycosides: beta-glucosidase enzyme or exposure to low pH breaks the bond between a glucose molecule and an unstable compound called "cyanohydrin" (or "alpha-hydroxynnitrile"), which then disassociates into a ketone or benzaldehyde and an HCN molecule. In cyanogenic glycosides, this reaction is called "cyanogenesis". Cyanogenesis is stimulated by maceration, and by bacteria in the human gut <ref name="Speijers">[http://www.inchem.org/documents/jecfa/jecmono/v30je18.htm "Cyanogenic Glycosides", First Draft. Dr G. Speijers. National Institute of Public Health and Environmental Protection Laboratory for Toxicology, Bilthoven, The Netherlands. Retrieved 08/25/2016.]</ref>. Although the optimum pH of cyanogenesis (at least for amygdalin) is 5.0 - 5.8, cyongenesis can occur at a wide range of pH values, and can occur in the presence of acid <ref>[http://www.sciencedirect.com/science/article/pii/S0308814601003132 Total cyanide determination of plants and foods using the picrate and acid hydrolysis methods. M Rezaul Haque, J Howard Bradbury. 2002.]</ref>. If seeds containing cyanogenic glycosides are ground up, the coarseness to which they are ground effects how quickly cyanogenesis occurs. Finely ground seeds extract HCN within an hour, where as coarsely ground seeds extract within 24 hours <refname="tuncel">[http://www.sciencedirect.com/science/article/pii/030881469599841M The effects of grinding, soaking and cooking on the degradation of amygdalin of bitter apricot seeds. G Tunçel, M.J.R Nout, L Brimer. 1995.]</ref>. HCN boils at a relatively low temperature (25.6°C / 78.1°F) <ref name="Gleadow_2014"></ref>. In some cases, soaking, cooking, and/or sometimes fermenting foods with certain bacteria or yeast (this has not been fully documented with ''Saccharomyces'' or ''Brettanomyces'') that contain cyanogenic glycosides allows the HCN to be released, and then subsequent cooking afterwards will boil off the cyanide <ref>[http://www.sciencedirect.com/science/article/pii/016816059400115M International Journal of Food Microbiology. M.J.R. Nout, G. Tunçe, L. Brimer. 1995.]</ref><ref name="Chaouali"></ref>.
After being released from cyanogenic glycosides, HCN is highly toxic to animals. The human body is used to breaking down trace amounts of cyanide into the less toxic substance thiocyanate with an enzyme called rhodanese, which then leaves the body via urination <ref name="Gleadow_2014">[http://www.annualreviews.org/doi/full/10.1146/annurev-arplant-050213-040027 Cyanogenic Glycosides: Synthesis, Physiology, and Phenotypic Plasticity. Roslyn M. Gleadow and Birger Lindberg Møller. 2014.]</ref>. Although there are more than 3,000 plant species that are cyanogenic (a number of them cultivated by farmers perhaps because their cyanogenic properties deter animals from eating them), only a few parts of plants that are considered foods contain enough HCN from cyanogenic glycosides to be considered dangerous (generally, other forms of cyanide are considered more dangerous, such as from exposure to air or water that is polluted with cyanide) <ref name="CDC1">[http://www.atsdr.cdc.gov/toxprofiles/tp8.pdf Toxicology Profile for Cyanide. Agency for Toxic Substances & Disease Registry. July 2006. Retrieved 08/25/2016.]</ref>. The location of the cyanogenic glycosides and the enzymes that release them are often each located in different (or all) parts of plants, and those locations are diverse across species. In some plants, the cyanogenic glycosides are concentrated in the stems or leaves of the plant and not the seeds (e.g. sorghum, barley, and lima beans). In fruits sometimes the seeds contain concentrated amounts (e.g. black cherry pits), and other times in the fruit itself (e.g. ''Passiflora edulis''). In rosaceous stone fruits, cyanogenic glycosides are located in the seeds, but the beta-glucosidase enzyme that the plant uses to release HCN is located in the roots of the plant. The concentration of cyanogenic glycosides is generally higher in seedling plants compared to mature plants, however there are a few exceptions where this is the opposite (e.g. some ''Eucalyptus'' species, and lima beans).
Upon learning about cyanogenic glycosides, brewers often question the toxicity of cherry pits or apricot kernels in beer. Cherry pits have traditionally been used in [[lambic]] kriek beers in Belgium. However, the dilution of HCN from cherry pits in beer results in benign levels. Assuming full breakdown of these glycosides, and that none of the HCN boils off (25.6°C boiling temperature), levels of HCN introduced from cherry pits are too low to cause harm to adult humans. The EU regulates that alcoholic beverages cannot exceed 1 mg of HCN per ABV percentage (v/v%) per liter <ref>[http://ec.europa.eu/food/fs/sfp/addit_flavor/flav09_en.pdf COUNCIL DIRECTIVE of 22 June 1988 on the approximation of the laws of the Member States relating to flavourings for use in foodstuffs and to source materials for their production (88/388/EEC). The European Food Commission, Food Safety. Retrieved 08/26/2016</ref>. Luk Daenen, a glycoside researcher, calculated that for a 4% ABV alcohol beer, 4 mg of HCN per liter is allowed. With 200 grams of cherries per liter, and the pits being 10 - 14 grams of that weight, there is 22 - 30.8 mg amygdalin per liter of beer. Around 6% of the weight of amygdalin is converted into HCN. Assuming maximum extraction of HCN from the amygdalin glycoside, which is unlikely because the pits are not ground up when used in beer, this equates to 1.3 - 1.82 mg of HCN per liter of beer, which is less than the 4 mg of HCN per liter that the EU regulation states. Considering that ~42 mg of HCN is required to kill a person that weighs 70 kilograms (154 pounds), that person would need to drink around 23 liters of beer <ref name="daenen">[https://www.uclouvain.be/cps/ucl/doc/inbr/documents/presentation-luk-daenen.pdf "Use of beta-glucosidase activity for flavour enhancement in specialty beers," slideshow by Luk Daenen. 2012. Retrieved 08/26/2016.]</ref>. Considering that 350 mL of pure alcohol would kill a 70 kilogram adult <ref>[http://www.alcohol.org.nz/alcohol-its-effects/health-effects/alcohol-poisoning "Alcohol Poisoning". NZ Health Promotion Agency. Retrieved 08/26/2016.]</ref>, the amount of 4% ABV beer required to kill a 70 kg adult from alcohol poisoning is around 8.75 liters. Alcohol would kill such a person far before cyanide poisoning would become a concern. In general, the potential cyanide in most plants will become too dilute to have any health problems when added to beer in normal amounts, however there might still be plants that are extremely high in HCN content and should be avoided in beer (see the table below).
Some cyanogenic foods can have their cyanogenic glycosides reduced by cooking them at 230°C for 15 minutes (flaxseed, for example) <ref name="Chaouali"></ref><ref name="flax"></ref>, however some amygdalin based cyanogenic plants may have their amygdalin content reduced to about 25% by cooking alone (apricot seeds, for example; only the cooking temperature of 100°C was tested <ref name="tuncel"></ref>). Fermentation by certain species of microbes can have a greater effect on reducing amygdalin to HCN than cooking alone. Microbes that have been shown to break down amygdalin include some species of lactic acid bacteria including ''Lactobacillus plantarum'', and fungi such as ''Endomyces fibuliger'', ''Pichia etchellsii'', and ''Hanseniaspora valbyensis''. Some strains of ''Brettanomyces'' that have high beta-glucosidase activity might be able to break down amygdalin by around 64%, and some strains of ''S. cerevisiae'' might be able to break down up to around 10% of amygdalin, but this needs to be verified by science. Once the amygdalin is broken down into HCN, the HCN can then be volatilized off by cooking in a ventilated space <ref>[http://link.springer.com/article/10.1007/BF00151870 Simple screening procedure for microorganisms to degrade amygdalin. L. Brimer, G. Tunçel, M. J. R. Nout. 1993.]</ref><ref>[http://www.ncbi.nlm.nih.gov/pubmed/7710917 Microbial degradation of amygdalin of bitter apricot seeds (Prunus armeniaca). Nout MJ, Tunçel G, Brimer L. 1995.]</ref><ref name="Daenen1"></ref>. In normal brewing procedures, however, the beer is not cooked nor ventilated, so any HCN that is produced by the breakdown of cyanogenic glycosides should be presumed to remain in the beer.
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