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 smell.
- 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 Blog