By Ernesto Salcedo, PhD
I was recently at a beer tasting event at the Denver
Museum of Nature & Science, whereupon someone remarked that a
particular ale had an aroma that smelled distinctly of melon. Now
I've heard, especially in wine, how particular varietals can give
off odors of pepper, leather, graphite, or barnyard animal. Am I to
believe that a Northern Rhone syrah and a billy goat share a
similar chemical makeup? What's the deal?
Great question! Why is there a barnyard smell in my $20
glass of wine? Well, as I mentioned in my last post (for Earl and Wim), odors are
typically made up of hundreds of different volatile molecules
called odorants. Each of these molecules has its own distinctive
scent, but when blended together in specific combinations, they
make up the scent of a more complex odor.
In the nose, each of these different molecules activate
different sets of olfactory receptors cells and the collective
activation of these different cells are thought to be converted
into a code that the brain can use to identify and discriminate
between different smells.
So why does your wine smell like barn? A given wine
accumulates its distinctive aroma from several different sources
during the wine making process. These sources are often organized
into three major categories:
- Primary Aromas arise from the fruit used to
make the wine and have a fruity or floral smell.
- Secondary Aromas come from the fermentation
process and have a yeasty, bakery, milky, buttery, or yogurt-type
- Tertiary Aromas arise from the aging process,
both from the barrels and in the bottles and can have a range of
smells including vanillin, coconut, menthols, etc.
Brettanomyces bruxellensis is a well-know type of yeast
(called 'Brett' in some circles) that can release compounds into
wine that contribute certain sensory characters to the wine's
aroma. One of these sensory compounds is 4-ethylphenol, which among
other things, smells like a barnyard. A little 4-ethylphenol and
you have an arguably complex wine character evocative of simple
winemaking, while too much 4-ethylphenol and you have a sloppily
made wine that smells like goat.
Now, why a billy goat smells like a chemical compound in yeast
remains a question for another day.
~Dr. Salcedo, resident Olfaction Expert of the Blue Tongue
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