I have shared how various products rely on the talents of chemical engineering.  Coffee (a course being taught at Berkeley made this clear), winemaking, soda pop, etc. all demonstrate the need for great chemical engineering talents to maximize the value of these products.

Today, we’ll discuss how the bourbon industry has improved its products- and its profitability- by harnessing the talents of chemical engineers.  (Keep in mind that 19/20 [95%] of this liquor [160+ proof or 80% alcohol, as produced; 80+ proof as sold] is produced in Kentucky, primarily from corn!)

So, it starts from ground corn (along with rye, wheat, barley, etc.) – this is called the grist-  dumped into water and heated (the process is called mashing) so the yeast fermentation can proceed.

Since yeast only grows and converts sugars, the mashing process serves to convert the starch to glucose; that’s why enzymes (i.e., high temperature alpha amylase) are now added to the mix, to enhance the conversion. Once the starch chains are freed from the grist, saccharification (the chemical bonds of the polymerized starch are enzymatically degraded to simple sugars) can proceed.

Now that the sugars are present, the mixture is moved to the fermenter (a biological reactor).  This step depends on whether the mash was “sweet” or “sour”.  A sweet mash is derived from grains, water, and yeast.  Sour mashes also include stillage (the liquid portion from a previous production batch, minus the alcohol); it’s called sour because there are copious levels of organic acids, meaning the solution has a lower pH.  (Distillers continually argue which mash is best.  It’s up to the consumer to decide who is right.)

Note that this operation is not heated- the fermentation manages to raise the temperature to the desired level (~31 C).  Moreover, this operation needs to be cooled to ensure that the fermentation proceeds properly.  (Yes, that much heat is generated.)

Not surprisingly, the sugars are converted to ethanol (alcohol) and carbon dioxide- the CO₂ bubbles that form further mix the contents. Enough bubble mixing is effected that no agitators (mixers) are needed for the process.  (So much carbon is  produced that it also has to be vented to ensure safe unit operations.)

There are side products (typically esters, aldehydes, and ketones, carboxylic acids, higher alcohols) produced in the “distiller’s beer” over the course of the next 3 or 4 days.  The esters (organic ‘salts’), aldehydes, and ketones (if their boiling temperatures match those of alcohol- more about why later) all enhance the flavors of the bourbon.

(This might be a good time to explain the difference between what happens in the fermentation of beer, Scotch and Irish whiskies,  and that of bourbon.   It’s the solids content of the broth from this phase.  To produce beer or whiskey, the solids are removed- leaving a clear wort. Bourbons are “grain on”- the solids are kept in the fermenter, yielding a whole mash.)

Once the fermentation is done, the distiller’s beer is sent to a distillation column.  (You know, the “still”, as most of you have heard mentioned when bootleggers are involved.)  The stills can be continuous (as would be done for most petroleum processes) or batch (as in the pot stills of bootlegging fame).  Many of the pot stills are made from copper- because of its antimicrobial capabilities and because copper sulphate is formed (i.e., this means sulphur is removed from the distillate, greatly improving the flavor).

Lower boiling compounds (like ethanol- the alcohol for the bourbon) are stripped and rise with the steam.  Since the temperature decreases as one rises up the column, compounds are stripped out,  based upon their boiling temperatures.  (Some of the distillate- 125 proof “low wine”- is sent to a second distillation system to purify it to 140 proof.)

The stillage (the left over, non-distilled material) is high in proteins and nutrients- and water.  This is sold (or given away) as livestock feed.  (Seasonality- wintertime means high demand since there is no grass for the livestock; summertime means the stillage has to compete with the grasses, which are also free.)  Some brewers process the stillage via ultrafiltration to reclaim the water and to reduce the volume of stillage that needs to be transported.

Now comes the critical step.  Aging.  Which may mean that the distillate (which is clear and colorless) needs to be diluted- since 160 proof won’t work.  The charred oak barrel process is best when the distillate is below 125 proof. The aging process (four years or more) – and that charred oak barrel- yields the distinctive colors, flavors, and mouth-feel of the bourbon we buy.

Note that bourbon must be aged in new (virgin) barrels.  Scotch and Irish whiskeys have no such restriction.  As a matter of fact, many of the other distillers purchase the used barrels from the bourbon producers.

Which brings up the trade secrets of the distillers. The raw wood.  The amount of seasoning (drying, over a few years) of the wood, toasting and charring (which also breaks down the wood components) are all factors to produce the desired bourbon flavor.

Since the aging process (in the rackhouses) is not subject to climate control, the bourbon flavor is influenced by the temperature changes.  In the summer, the whiskey expands- forcing itself into the wood structure of the barrels.   In the winter, the whiskey contracts, and the liquid- with enhanced flavor (wood celluloses and hemicellulose) from the wooden barrels- is extracted from the wood.  The celluloses [polysaccharides] degrade to various sugars during the aging process; lignins are occasionally converted to vanillin and other compounds; while the tannins are simply extracted. Plenty of other compounds are extracted from the wood and char layers- lactones, furfurals, etc.  All of which are distinctive to the bourbon flavor and finish.

Another factor- the aging process allows esterification to occur.  The organic acids extracted from the wood react with the mash alcohol to produce esters.  (Alcohol is basic- so when it combines with the organic acid, it forms those esters or “organic salts”.)  The problem is that not all esterification is desirable- some of those esters are deleterious to the bourbon flavor.  Especially if they incorporate the sulphur that wasn’t removed in the copper stills.

Now, comes the bottling process.  The products from multiple barrels are combined to yield the consistent flavor profile desired.  Water may also be added to mix to obtain the alcohol level desired (80 to 110 proof).

We should all recognize that our choice of drinking- neat, with water, or on the rocks- also affect the taste.  Warm water brings out the more volatile components, enhancing the aroma.  (Rolling the bourbon glass in our hands and/or retaining the sip in our mouths also raise the temperature.)

Maker’s Mark, anyone?