Daily Archives: November 18, 2010

Taste vs. Flavor: A retronasal excursion

So, when we talk about what a food or drink “tastes like”, it’s pretty common to get confused about the terminology.  You may hear someone say that something tastes “fruity” or “rancid” or whatever.  What they are actually discussing is how the food smells.  As we discussed already, “taste” applies only to the basic tastes. Everything else, apart from the various tactile sensations, is aroma. Combine the taste, the aroma, and the mouthfeel and you’re now talking about the overall flavor of the substance.

Here’s an exercise for you to try to drive home this point:

Eat or drink something while your nose is plugged.  What I like to use is a piece of gum or a mint candy or something. It might be tricky to swallow like this, but it’s possible.  So while your nose is plugged, what are you sensing?  Sweetness, sourness, saltiness, bitterness, temperature, texture, etc.  It’s all just taste and mouthfeel, right?  No aroma.    Now, unplug your nose and breath out through it.  NOW we’re in flavor country, eh? This aroma that you smell while food or drink is in your mouth is called “retronasal aroma” (backwards nose). It is distinct from “orthonasal aroma” (straight nose) which is sensed when you put your face over the food and smell normally through the front of the nose.

So apparently, the majority of food and beverage flavor is perceived by your nose.  What’s happening is, as the substance is in your mouth, it’s warming up.  This warming action allows the volatile aroma compounds to leave the food and enter the air in your mouth and sinus.  Also, the surface area of the food is increasing as you chew it and spread it around your mouth, which also allows more volatilization of flavors.  Other things might be happening as well, like bursting carbonation bubbles carrying even more flavors out of the beverage.  All these things are causing aromas to become “airborne” which allows them to be carried into your sinus (via the back of the mouth/throat). This retronasal method leads to distinct differences in the aroma of your food compared to the orthonasal method, since with the orthonasal method the warming and the agitation of the sample are considerably less. It’s not uncommon for the flavor profile of a food or beverage smelled retronasally to be quite unique from the same product smelled orthonasally, as certain compounds may not be volatile enough in the glass to be detected; they may need to be warmed and agitated to be detected at above-threshold levels. This demonstrates the importance of smelling AND tasting the product before you try to describe it.

Here is a diagram showing a cross-section of the human head, where you can see how the back of the throat is connected to the back of the sinus cavity.  This is where the retronasal aromas access the olfactory bulb at the top of the sinus (essentially the bottom of the brain) which houses the various receptors responsible for detecting aromas.

Cross section of mouth and sinus, showing how retronasal aromas access olfactory bulb.

So now that we understand how taste, aroma, and flavor are all related, we can use the correct terminology when we discuss our sensations and assess our beers with proper diligence.

Diacetyl – “Who put butter in my beer?”

Diacetyl (dye-assa-TEEL, or dye-ASS-itle) is probably one of the most well-known flavors related to brewing.  It’s buttery aroma is easily recognized at levels above threshold but, as much as is known and recognized about this compound, I’m constantly amazed and disappointed by how much “butter-beer” is still being produced. This post will briefly explore the various ways that diacetyl arises in beer.


Diacetyl (2,3-butanedione) is one of a class of compounds called “vicinal diketones” (VDK). In chemistry-speak, “vicinal” essentially means “adjacent”, and “diketone” means that there are two ketone functional groups (a ketone is an oxygen double-bonded to a carbon in the middle of a carbon chain). If you look at the molecular structure in the link above, you’ll see that it is a 4-carbon chain (hence the “butane” root in the name), and the two ketones are on the vicinal positions of the #2 and #3 carbons (ergo, 2,3-butanedione: two ketones on #2/3 carbons of a 4-carbon chain). The other main VDK in beer is 2,3-pentanedione (pentane – can you guess what this one looks like?), but it is usually found in beer at levels below that of diacetyl.

Diacetyl, as mentioned already, has a buttery or butterscotch-like aroma. Open a bag of microwave buttered popcorn and you’re hit in the face with diacetyl. At very high levels it can even start to affect the mouthfeel of the beer, causing a slick or oily mouthfeel.  The detection threshold of diacetyl in beer is typically between 10 and 40ppb, although I have determined the personal thresholds for 11 of my panelists and they range from 7-190ppb, with an average of about 60ppb. Personally, my threshold is about 20ppb and beers with higher levels are so offensive to me that any beer with detectable diacetyl usually goes down the drain. Many times you may be served a beer which appears to have no hint of diacetyl, but as you work your way down to the bottom of the glass you begin to detect it. This is because the beer is warming up, which allows more diacetyl to volatilize into the headspace of the glass (and your nose). Too many times have I started in on a good and cold beer only to dump the second half because the “Big D” had begun to show its stinky face.

Common levels for diacetyl in beer range from 30ppb to over 1ppm [1]. Until recently, I would have had a hard time believing that there was packaged beer out there with 1ppm diacetyl. That is, until my panel tasted a beer from a local micro-brewery which was so high in diacetyl that we just had to run it through our gas chromatograph to find out how much was actually in there: almost 900ppb. Nearly all of our beers that we make here at The Company are below 30ppb or so, with most falling easily below 20ppb. I would hazard a guess that the big American Lagers/Pilsners are about 10ppb or less, but I don’t know for sure.

There are a few different sources of diacetyl in beer, only two of which are widely discussed.  The first is during fermentation, where it’s created by the brewer’s yeast.   The other well-known source is from bacterial infection.  The third source is probably the least known:  beer aging.  Below, we’ll examine each of these sources.

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