13,703
edits
Changes
no edit summary
'''(In progress)'''
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 <ref>[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 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 this is there are a few exceptions where this is the opposite (e.g. some ''Eucalyptus'' species, and lima beans).