Changes

Monoterpenes generally exist as aromatic and flavorful alcohols that tend to smell floral with low odor thresholds (100-400 ppb), and are present in plant material. These alcohols mostly consist of linalool(a major contributor to hop aroma <ref>[http://www.mbaa.com/brewresources/Pages/Linalool-in-Hops.aspx Linalool in Hops. MBAA. Stefan Hanke, University Weihenstephan, Germany. 2009.]</ref>), geraniol, nerol, and linalool oxides, but also includes other monoterpenes such as citronellol, alpha-terpineol, hotrienol, nerol oxide, myrcenol, the ocimenols, and other oxides, aldehydes and hydrocarbons. In wine (and probably sour beer), these alcohols bind with acids to create aromatic monoterpene ethyl esters and acetate esters, and can also be transformed into other types of monoterpenes by yeast metabolism <ref>[http://www.sciencedirect.com/science/article/pii/S0168160510006653 Monoterpene alcohols release and bioconversion by Saccharomyces species and hybrids. A. Gamero, P. Manzanares, A. Querol, C. Belloch. 2011.]</ref><ref name="Daenen2">[http://onlinelibrary.wiley.com/doi/10.1111/j.1567-1364.2008.00421.x/pdf Evaluation of the glycoside hydrolase activity of aBrettanomyces strain on glycosides from sour cherry (Prunus cerasus L.) used in the production of special fruit beers. Luk Daenen, Femke Sterckx, Freddy R. Delvaux, Hubert Verachtert & Guy Derdelinckx. 2007.]</ref>. Monoterpenes can also be odorless polyols, some of which can break down easily to produce pleasant aromas. For example, diendiol can break down into hotrienol (tropical, floral, fennel, ginger aroma <ref>[http://www.thegoodscentscompany.com/data/rw1374491.html "Hotrienol." The Good Scents Company. Retrieved 05/11/2016.]</ref>) and nerol oxide (green, vegetative and floral with a minty undernote <ref>[http://www.thegoodscentscompany.com/data/rw1001891.html "Nerol Oxide." The Good Scents Company. Retrieved 05/11/2016.]</ref>). While some monoterpenes are free within the plant material, around 2-8 times that amount are bound up in flavorless glycosides <ref name="Maicas">[http://link.springer.com/article/10.1007/s00253-004-1806-0/fulltext.html "Hydrolysis of terpenyl glycosides in grape juice and other fruit juices: a review." Sergi Maicas, José Juan Mateo. May 2005.]</ref>. It is thought that some wine strains of ''Saccharomyces cerevisiae'' might be able to produce monoterpenes such as geraniol during fermentation in ways not related to beta-glucosidase activity <ref>[http://femsle.oxfordjournals.org/content/243/1/107.abstract De novo synthesis of monoterpenes by Saccharomyces cerevisiae wine yeasts. Francisco M. Carrau, Karina Medina, Eduardo Boido, Laura Farina, Carina Gaggero, Eduardo Dellacassa, Giuseppe Versini, Paul A. Henschke. 2005.]</ref>.