(Updated 10/15/16: fixed broken links, removed two photos. 12/10/16: update on candy cap cheesecake)
 
Some time ago I heard about a mushroom that smells like maple syrup, and given my interest in fenugreek’s maple aroma (Fenugreek’s Flavors, When Fenugreek Leads to a False Diagnosis), I had to find out more.

A quick search in the UC Berkeley libraries led me to papers about two fungi that are in the Lactarius genus that have maple syrup aromas or flavors. One is Lactarius helvus, which is native to Europe, and the other is Lactarius fragilis var. rubidus, which also known as “candy cap” and grows in northern California. (Full citations for the papers are at the bottom of this post)

Fungus 1: Lactarius helvus

Interestingly enough, the Encyclopedia of Life page for L. helvus gives “Fenugreek milkcap” as the common name, so there’s a long-standing connection between fenugreek and the mushroom. The mushroom is considered to be poisonous.

In the research article, the team describe a search for volatile components using solvent extraction and gas chromatography/mass spectrometry (GC/MS). Dried, ground mushroom was rehydrated and treated with diethyl ether to stop enzymatic activity. A concentrate of the organic extract was fed into the GC/MS analyzer, whereupon thirty-eight volatile compounds were found. For a dry L. helvus sample that was described as smelling like fenugreek, the machines found sotolon, the chemical that is prominent in fenugreek and also used as an artificial maple syrup flavor. After a bunch of chemistry talk, the authors note that a previous article said that “the characteristic smell of fenugreek occurred during the dehydration process of [fenugreek] seeds. Thus, the drying process produced the fenugreek odor in both L. helvus (Basidiomycota) and T[rigonella] foenum graecum (Fabaceae).” The study might have practical implications, as this was the first identification of sotolon in Basidiomycota fungi, which could lead to using fungi to produce “natural” flavor compounds (i.e., perhaps someone can grow the fungus and extract the sotolon).

Fungus 2: Lactarius fragilis var rubidus (Candy Cap)

The candy cap mushroom* has a sweet aroma that is reminiscent of maple syrup, butterscotch or burnt sugar, and consequently is used to flavor desserts, while also adding some novelty (“Wow! This ice cream is really made with mushrooms?”).  As with L. helvus, candy cap mushrooms do not have much odor when fresh, and drying is required to make their characteristic aromas appear.
For their analysis of L. fragilis var rubidus, the research team gathered the “headspace volatiles” from dried mushrooms*. The team used GC/MS to identify the compounds in the headspace, and did not find sotolon. They found other chemicals that have maple syrup odors, however, including quabalactone III. Of note is that the quabalactone III compound can form sotolon when it reacts with water, as would happen when candy cap mushrooms are cooked or when the volatiles meet moisture in the nasal cavity.

Unfortunately, the authors aren’t terribly specific about the practical or esoteric motivations behind the study, so this one was simply about expanding the range of knowledge.

Candy Cap Mushrooms in Restaurants

To give some examples of how candy cap mushrooms are used in restaurants, I searched a few of my favorite food sites, and here are some of the results (in no particular order):

• The Tablehopper’s 707 Scout from February 8, 2013 has a report on a candy cap ice cream in a blue corn crepe at Mateo’s Cocina Latina in Healdsburg, California (the 707 area code covers this area, hence the name).
• Tablehopper has a photo of a candy cap mushroom ice cream sandwich at San Francisco’s Americano, with the recipe for said sandwich available at 7×7 magazine.
• A description of a cocktail that uses candy cap mushrooms from the Tablehopper in 2008: “Or how about the Mushroom Hunter: Old Overholt Rye infused with locally foraged candy cap mushrooms, plus Cossart Gordon Rainwater Madeira, Aperol, orange bitters, thyme tincture, and flamed orange for garnish. Again, dude. The earthy note of the mushrooms was so funky-cool, reminded me of a truffle grappa I had in Venice when I was 20. (Yes, a while ago.) Talk about an intersection of culinary and cocktail! ”
• An update by San Francisco Chronicle food editor and chief restaurant critic Michael Bauer at the Inside Scoop SF blog:  “At Fifth Floor I was impressed with the persimmon pain perdu ($12) with  kiwi sorbet (When was the last time you saw this fruit? Maybe it’s time for a comeback) and a cheesecake puree. Pastry chef Francis Ang always includes a savory ingredient in his desserts; this time it was the earthy candy cap mushrooms.” (Update: Fifth Floor is now Dirty Habit)
  
• A 2010 review of P-30 in Sebastapol, California by Carey Sweet in the San Francisco Chronicle starts with this: “They had me at candy cap mushroom ice cream. The fanciful fungus, plentiful across California’s north coast, has a distinctive fragrance of maple syrup and is delicious in desserts, but is rarely seen on menus.”
• In the San Francisco Chronicle, Alissa Merksamer wrote about the San Francisco Cheesecake Company in Who needs New York cheesecake when you have a San Francisco one?  One of the company’s distinctive flavors is candy cap.  Merksamer writes, ” In true Bay Area style, these cheesecakes are crammed with local ingredients: chocolate from Guittard (Burlingame), dairy from Challenge (Dublin), spreads from Inna’s Jams (Emeryville) and even candy cap mushrooms from Far West Fungi in the Ferry Building. That’s right — mushroom cheesecake. After all, candy cap mushrooms taste remarkably like maple; besides, it wouldn’t be a San Francisco product without a little innovation. “

Notes
* The name “candy cap” is also used for several other species, including L. camphoratus and L. fragilis var. fragilis.
** Headspace volatiles are the gases that are emitted above from sample, like the aromas of a hot bowl of soup or fragrant bowl of strawberries. They are often collected by flavor chemists, so that they can isolate the key chemicals and synthesize them in the lab for use in their products. An interesting 2009 article in the New Yorker explained how they do it (subscription required).

References
“The Fenugreek Odor of Lactarius helvus”, by Sylvie Rapior, Françoise Fons and Jean-Marie Bessière, Mycologia, Vol. 92, No. 2 (Mar. – Apr., 2000), pp. 305-308.  Stable URL: http://www.jstor.org/stable/3761565

“The maple syrup odour of the ‘candy cap’ mushroom, Lactarius fragilis var. rubidus”, by William F. Wood, Jay A. Brandes, Brian D. Foy, Christopher G. Morgan, Thierry D. Manna, Darvin A. DeShazer, Biochemical Systematics and Ecology 43 (2012) 51–53. DOI: 10.1016/j.bse.2012.02.027.

Image Credits
Drawing of Lactarius helvus from Wikimedia Commons, extracted from “Handbook of British Fungi,” by Mordecai Cubitt Cooke; published 1881, in public domain.