However for a whiskey to be known as a bourbon, it has to adhere to very particular guidelines. For one, it must be made within the U.S. or a U.S. territory – though nearly all is made in Kentucky. The opposite guidelines have extra to do with the steps to make it – how a lot corn is within the grain combination, the growing older course of and the alcohol proof.
I’m a bourbon researcher and chemistry professor who teaches lessons on fermentation, and I’m a bourbon connoisseur myself. The advanced science behind this fragrant beverage reveals why there are such a lot of distinct bourbons, regardless of the strict guidelines round its manufacture.
The mash invoice
All whiskeys have what’s known as a mash invoice. The mash invoice refers back to the recipe of grains that makes up the spirit’s taste basis. To be labeled as bourbon, a spirit’s mash invoice should have at the very least 51% corn – the corn provides it that attribute sweetness.
Virtually all bourbons even have malted barley, which lends a nutty, smoky taste and offers enzymes that flip starches into sugars later within the manufacturing course of.
Many distillers additionally use rye and wheat to taste their bourbons. Rye makes the bourbon spicy, whereas wheat produces a softer, sweeter taste. Others may use grains like rice or quinoa – however every grain chosen, and the quantity of every, impacts the flavour down the road.
The chemistry of yeast
As soon as distillers grind the grains from the mash invoice and blend them with heated water, they add yeast to the mash. This course of is named “pitching the yeast.” The yeast consumes sugars and produces ethyl alcohol and carbon dioxide as byproducts through the course of known as fermentation – that’s how the bourbon turns into alcoholic.
The fermented mash is now known as “beer.” Whereas related in construction and style to the beer you may purchase in a six-pack, this product nonetheless has a technique to go earlier than it reaches its last type.
Yeast fermentation yields different byproducts in addition to alcohol and carbon dioxide, together with taste compounds known as congeners. Congeners may be esters, which produce a fruity or floral taste, or advanced alcohols, which might style robust and fragrant.
The longer the fermentation interval, the longer the yeast has to create extra flavorful byproducts, which reinforces the complexity of the spirit’s last style. And totally different yeasts produce totally different quantities of congeners.
Separating the fermentation merchandise
Throughout distillation, distillers separate the alcohol and congeners from the fermented mash of grains, leading to a liquid spirit. To do that, they use pot or column stills, that are massive kettles or columns, respectively, usually made at the very least partially of copper. These stills warmth the beer and any congeners which have a boiling level of lower than 350 levels Fahrenheit (176 levels Celsius) to type a vapor.
The kind of nonetheless will affect the drinks’ last taste, as a result of pot stills usually don’t separate the congeners as exactly as column stills do. Pot stills end in a spirit that usually comprises a extra advanced combination of congeners.
The specified vapors that exit the nonetheless are condensed again to liquid type, and this product is known as the distillate.
Totally different chemical compounds have totally different boiling factors, so distillers can separate the totally different chemical compounds by accumulating the distillate at totally different temperatures. So within the case of the pot nonetheless, because the kettle is heated, chemical compounds which have decrease boiling factors are collected first. Because the kettle heats additional, chemical compounds with greater boiling factors vaporize after which are condensed and picked up.
By the tip of the distillation course of with a pot nonetheless, the distillate has been divided into just a few fractions. Certainly one of these fractions is known as the “hearts,” containing largely ethanol and water, but in addition small quantities of congeners, which play an enormous function within the last taste of the product.
The alchemy of time and wooden
After distillation, the “hearts” fraction (which is evident and resembles water) is positioned in a charred oak barrel for the growing older course of. Right here, the bourbon interacts with chemical compounds within the barrel’s wooden, and about 70% of the bourbon’s last taste is decided by this step. The bourbon will get all its amber shade through the growing older course of.
Bourbon could relaxation within the barrel for a number of years. In the course of the summer season, when the temperature is sizzling, the distillate can cross by the internal charred layer of the barrel. The charred wooden acts like a filter and strains out a few of the chemical compounds earlier than the distillate seeps into the wooden. These chemical compounds bind to the charred layer and don’t launch, type of like a water filter.
Underneath the charred layer of the barrel is a “purple line,” a layer the place the oak was toasted through the charring course of of constructing the barrel. The toasting course of breaks down starch and different polymers, known as lignins and tannins, within the oak.
When the distillate seeps to the red-line layer, it dissolves the sugars within the barrel, in addition to lignin byproducts and tannins.
In the course of the chilly winter months, the distillate retreats again into the barrel, but it surely takes with it these sugars, tannins and lignin byproducts from the wooden, which improve the flavors. When you disassemble a barrel after it has aged bourbon, you may see a “solvent line,” which reveals how far into the wooden the distillate penetrated. The kind of oak barrel can have a profound impact on the ultimate style, together with the barrel’s dimension and the way charred it’s.
For many distilleries, barrels are saved in massive buildings known as rickhouses. Ethyl alcohol and water within the distillate evaporate out of the barrel, and the humidity in that a part of the rickhouse performs an enormous function.
Decrease humidity usually results in higher-proof bourbon, as extra water than ethanol leaves the barrel. As well as, air enters the barrel, and oxygen from the air reacts with a few of the chemical compounds within the bourbon, creating new taste chemical compounds. These reactions are inclined to soften the style of the ultimate product.
There are 1000’s of bourbons in the marketplace, and they are often distinguished by their distinctive flavors and aromas. The number of manufacturers displays the numerous decisions that distillers make on the mash invoice, fermentation and distillation situations, and growing older course of. No two bourbons are fairly the identical.
Michael W. Crowder, professor of chemistry and biochemistry, Miami College
