The Science Behind the Taste of Boubon

What follows is a guest post by none other than our very own Cheddarman.  If you've ever wondered how bourbon is made... and what makes it taste the way it does...  well... here's is your PhD level dissertation on the matter.  Fair warning...  if you hate chemistry this is going to make your head explode.  If you like chemistry... you'll love it.

Now... if I can only get him to explain, in scientific terms, why scotch sucks...

Bourbon flavor and its development: some artisanal and industrial considerations

In honor of Nate and his friends and blog followers, Rebels, Copperheads and Yankees, and even any anonymous Ohio State University sports fans or dedicated pink Hello Kitty Glock users that hang out at the ATF blog, I have put together a little “review” of bourbon flavor formation. Specifically, I will cover a number of the factors involved in the development of bourbon flavor, beginning with the raw ingredients, and following the process through to the end. I have not been able to find any published scientific articles on the science and craft of making Bourbon. However, I have a 20+ year career as a practitioner of flavor chemistry, and can draw on my experience in that area that includes extensive reading of the scientific literature as well as working on a number of flavor related projects in industrial and academic settings. Finally, for what it is worth, I could be considered a world class expert in the flavor of chemistry of ripened cheeses, and this gives me some unique insight into the development of bourbon flavor. All these credentials and 50$ will get me a good bottle of bourbon (it never got me anything else, nudge nudge, wink wink). There are also a number of excellent sites on the world wide web devoted to making bourbon from an artisanal approach that were helpful in putting this review together. I hope you enjoy it. If you want to better understand Bourbon, here is your chance to dig deep.

“Artisanal” versus “Industrial” Bourbon

As one looks at the types of bourbon available on the market, there appears to be 2 separate schools of thought in how to make it, clever marketing tactics aside. One is the artisanal school (I use that term because these distillers are practicing an art), where bourbon is made the old fashioned way, in small batches, using pre-industrial techniques. Whereas artisanal producers specialize in making more expensive, more intensely flavored, or more “interesting” bourbons, the “industrial” bourbon makeruses modern methods of production to make a more affordable product that is generally more consistent from batch to batch, and has a wider over all appeal due to its relatively milder flavor profile. We will cover aspects of both approaches below.

Ingredients

Water

One way to make bad bourbon is to start with low quality ingredients. High quality water is very important to bourbon. Traditionally, water from limestone springs is used to make the malted barley and the mash. Some writes have said that the calcium dissolved in the water somehow contributes to high quality bourbon flavor. I doubt that is true, for a couple of reasons: 1) Calcium carbonate from limestone filtered water has little if any flavor. 2) The fermentation micro-organisms need some calcium for growth, but they can obtain it from the grains. There a number of reasons why the use of limestone filtered water was a common historical practice. This water is filtered by slow passage through limestone, and is generally free of off-flavors, chlorine and iron. This is very important, as iron or chlorine in the water can cause the formation of off some very potent undesirable flavors. Iron will help oxidize oils present in the grains, and causes a variety of “green” off flavors, like including freshly cut grass, metallic flavors (imagine a penny on your tongue), drying oil based paint, and fishy. Chlorine can react with phenols that are produced during barley malting, the sour mash fermentation or charring of barrels. These chloro-phenols have very potent “saltwater fishy” aromas. They can ruin otherwise good bourbon. Enough said. The industrial producer uses a number of steps to clean up their brewing water. These may include ion exchange resins to remove iron and other mineral,, as well as sand filters, activated carbon, UV light and Ozone to remove all traces of taste or aroma from the water. As a result, the industrial distiller can have water of even higher quality than that coming out of a natural limestone formation.

Grains, Malt and Enzymes

By U.S. law, Bourbon must contain at least 51% corn as well as barley, rye and wheat. The primary purpose of the grain is to provide glucose for the yeasts to ferment into alcohol. The grains should be high in starch, and low in vegetable oils. The oil can be a source of “oxidized fat” off flavors. The grains are ground to improve the conversion of starch into glucose and other sugars. If the grains get too hot during milling, it will damage the starch and protein, and they can’t be broken down by enzymes. Also, high amounts of heat building up in the grains during milling can oxidize the vegetable oil.

The artisanal distiller generally uses malted barley. In the malting process, dry barley is soaked in water to activate enzymes in the grain. Wild yeasts and lactic acid bacteria grow on the surface of the grains during the malting process. The slow drying of the malt “hardens” these micro-organisms, so they can survive the mash boiling step and ferment the mash. These “wild” micro-organisms can be a source of the batch to batch flavor variation seen with artisanal distilled bourbon.

The enzymes activated by the malting process are needed to break down starches and proteins in the other grains. Without these enzymes being active in the mash, the fermentation process would be slow and difficult to control. Glucose and other sugars are produced from the breakdown of starches by a set of enzymes called amylases; proteins are broken down into amino acids by protease enzymes. The activity of these enzymes on starch and protein helps set the stage for rapid growth of yeast and lactic acid bacteria during the sour mash fermentation. Glucose and amino acids are also a converted into flavor compounds by the yeast and lactic acid bacteria.

The industrial producer may choose not to use malted barley, and instead use a commercial mixture of “microbial” amylase and protease enzymes produced by food grade bacteria or fungi. It is faster and cheaper than using malted barley, and can help keep “wild” yeast and lactic acid bacteria out of your mash fermentation.

By varying different amounts of the 4 grains, one can change some of the flavor profile of the bourbon. Each grain has some unique flavor components that can be released during the fermentation. Wheat, as an example, is supposed to have a mellowing effect upon the bourbon. Consider the differences in flavor between a sour dough wheat, rye barley, or corn bread; these differences in flavors can be partly attributed to the interaction of the yeasts and bacteria with the individual grains. The yeasts and lactic acid bacteria have special enzymes that will release specific classes of flavor compounds from the grains. The different grains will have varying levels of these flavor compounds. These flavor compounds are often attached to a sugar molecule or other non-volatile molecule; by the cutting the chemical bond, the glucose can be harvested, and the flavor molecule is released into the mash.

Mashing

The mixture of grains and water is called mash. The mash is heated to help dissolve the starch and protein in the grains. This also has the effect of killing off some of the wild yeast and other bacteria that could be present on the grains. If we use microbial enzymes instead of malted barley, we can cook the mash at higher temperatures for a longer time. This will, again, kill off more wild microorganisms naturally present on the grains, and help us have a more consistent product over time. The microbial enzymes are more heat stable that the enzymes from malted barley and are active at relatively high temperatures, and can be added when the boiled mash is still hot.

Fermentation/ or making Sour mash

If you mom, sister, girl friend or wife has ever made sour dough bread, she has come pretty close to making a sour mash and is a stones’ throw from hooch, if I am using that term correctly. But that is the difference between us and them. They will walk up to the line and not cross it (No offense, Miss Susan, I know you are reading this). We will, because BOURBON, DAMMIT!!!

Fermentation is any process where micro-organisms are grown. In a sour mash fermentation, Lactic Acid Bacteria and Yeasts grow up together for their and our mutual benefit. The Lactic acid bacteria grow faster than the yeast, and quickly produce lactic acid. They also use up the oxygen. This is important, as yeast makes carbon dioxide and water if it has oxygen. Nate is not pleased if good grain sugar for a bourbon sour mash is being converted to CO₂! Without oxygen, the yeasts make ethanol, and Nate is happy! (Note to self: unhappy Alphas are not much fun, and it is probably dangerous to laugh at them) This cooperation between the two inhibits their competitors, and allows the Lactic Acid Bacteria and yeast to dominate the process, and keep the potential spoilage bacteria from growing. You could perhaps explain this cooperation to the simple minded as something like tag team professional wrestling, but on a very small scale with a whole lot of tag teams.

The lactic acid bacteria can and do make a number of other flavor compounds, in addition to producing lactic acid. They actually produce two types of lactic acid, the “d” (Latin abbreviation for right) and “l” (Latin for left) forms. The lactic acid can and does react with the ethanol, to produce esters. The “d” and “l” esters have different aromas. You probably already know this, but Lactic acid bacteria also ripen cheese and other fermented dairy products, transforming relatively bland milk into pure Cheese awesomeness. (Sorry my Southron friends, but no one makes a great Cheddar Cheese like New England Yankees, and I say that as an Ohioan who believes the South was right and who did 1 year hard time in Vermont workin’ at a cheese plant). The take home message is that the yeast and Lactic acid bacteria can contribute a lot more to the Bourbon flavor that acid and alcohol.

Europeans and Lactic acid bacteria have a few things in common. For instance, they have a peculiar habit of trying to kill each other off every so often. Some species of Lactic acid bacteria make chemicals called bacteriocins that can inhibit the growth of closely related Lactic Acid Bacteria species. This would be like my Scotts-Irish cousins many generations removed, developing a superweapon to completely dominate my Irish Catholic cousins, many generations removed. The lactic acid bacteria can also be attacked by viruses called “bacteriophage.” Bacteriophage can wipe out a specific strain of lactic acid bacteria that dominates fermentation, and allow another strain to grow up and dominate. Some strains of lactic acid bacteria actually have bacteriophage hidden in their DNA just waiting for the right moment to pop out of hiding and take out a competing strain.

Yeasts also have ways of competing with each other, although I am not familiar with these mechanisms of competition. A dominant yeast strain can also be killed off by a virus. The take home point in all this is that if you have a favorite artisanal bourbon, and the flavor changes over time, it could be due to a formerly dominant strain of Lactic Acid bacteria or yeast being wiped out by a virus or other form of competition, and replaced by a competitor.

Distillers normally save some of the previous batch of sour mash, and use it to introduce yeast and lactic acid bacteria into a mash after it has been boiled, and cooled. There are a number of simple techniques an artisanal distiller can use to preserve their mash, such as using good manufacturing practice borrowed from the dairy industry for starter culture preservation, freezing and storing portions of the mash, and having different mashes that they use on a rotating schedule.

An industrial distiller would have a number of practices in place to protect the integrity of their sour mash. As an example, they may have what is known as a mother culture. The mother culture is used to grow up a “starter culture” in a small closed tank, from there it can be pumped it into the mash fermentation vat. External sources of contamination like airborne dust containing dehydrated wild yeast of lactic acid bacteria, are controlled through careful plant design, good manufacturing practices and air filtration.

A home brewer could make their own sour mash from scratch, using a process similar to making a sour dough starter culture.

We want the sour mash fermentation to stop before the yeasts and LABs run out of glucose. If they run out of glucose and the ethanol concentration is not high enough to make them go dormant, the micro-organisms will start breaking down amino acids or fibers from the grains for energy. This is not good, and can lead to off-flavor formation (Think cowy, cow barn, piggy, fecal, rotten egg, skunk, tomcat urine).

Distillation process:

Another advantage to stopping the fermentation before the glucose is exhausted it that free glucose will react with amino acids and other substances produced by the fermentations when we start heating the Bourbon beer for the distillation step. These reactions produce a number of “sweet” (think of yourself at a county fair and walking past the cotton candy stand) and “brown” (soy sauce-like) flavor notes. A similar reaction is used industrially to make cocoa and a multitude of manufactured flavors that go into processed foods.

We now have a soup that is rich in flavor compounds, most likely smells similar to beer, and has a lot of chemicals that can react with each other upon distillation heating to create other flavors. The distillers sometimes refer to it as the Bourbon beer. At this point, the distiller uses a selective process called distillation, where the bourbon beer is carefully heated to remove the ethanol. The distillation process changes the flavor profile into something that starts to resemble bourbon. (If at this point, for some unknown reason, Nate told the distiller to make some Vodka-like distilled spirits for his fellow Alpha buddy Vladimir Putin, the distiller would install an series of activated filters in the still to remove most of the flavor chemicals, we would indeed have a Vodka-like product)

A lot of things happen during distillation: 1) we reduce the concentration of the low boiling flavor compounds that are responsible for the “top notes” of the beer flavor. The heat of distillation can also cause some of these chemicals to form other, more complex aromas 2) We concentrate the alcohol 3) We reduce the concentration of the high boiling compounds from the Bourbon beer.

The actual concentration of the high boiling and low boiling compounds in the distilled ethanol depends on the design of the still. Generally, a higher distillation column will be more selective than a shorter column; it will more effectively separate the low boiling and high boiling chemicals from the ethanol. The industrial distiller might perform a “quick” distillation to strip of the alcohol, and then do a second, slower distillation to remove more of the low boiling and high boiling flavors. Obviously, by controlling the relative amounts of the low boiling and high boiling flavor compounds in the distilled ethanol, we control some of the flavor that is present in the final product.

The distillation process does one more thing that never gets any press; it introduces copper into the distilled ethanol and remaining flavors. Acids and sulfur chemicals from the beer distillation will attack the copper in the distillation column, and leach some copper into the distilled ethanol. More on what this does in the next section.

Barrel Charring and Aging

Most of you already know that bourbon is aged in charred oak barrels. Here is a little more information on how this affects the flavor. There are three natural polymers present in wood: cellulose, hemicellulose and lignin. The barrel heating and charring affects each one differently, in terms of the bourbon flavor.

Heating lignin releases the chemical vanillin (Think artificial vanilla flavor, cake batter vanilla flavor, or Ben and Jerries’ chocolate chip cookie dough vanilla flavor) and smoky flavors. It also produces most of the color. (Vanillin is a little different in odor than the vanilla flavor extracted from fermented vanilla beans). The reaction can also produce phenol, which sort of dulls the tongue, and eugenol, a chemical in black pepper and other spices, and several other potential flavor chemicals.

Heating the cellulose and hemicellulose causes the sugar molecules in them to break down. This can form another class of compounds including and related to the cotton candy flavor we talked about earlier. These chemicals include sotolone and furaneol. These chemicals also occur in aged cheeses with dry rinds, and give them a “sweet” flavor.

The heating process also does something else that never gets any press, it converts the inside of the oak barrel into a large catalytic surface. The carbon surface inside the barrel, along with the copper from the distillation column or still, most likely catalyze a number of chemical reactions that inactivate sulfur compounds, and catalyze the formation of some other flavor compounds. (As an example, methyl glyoxal, and hydroxy acetone, two chemicals formed from cellulose and hemicellulose heating, can react to form the furaneol cotton candy flavor chemical) Some individuals claim that the aging process allows a lot of the low boiling off flavors to evaporate slowly through the wood. I think that much of the mellowing is due to the process I just described.

Obviously, an artisanal distiller may have a significant variation in charring for the barrels used to age their bourbon, and this will affect the flavor. The industrial distiller will probably use barrels for aging that are made with relatively tight quality control practices, leading to less flavor variability, and fewer surprises.

Tasting Wheel

I would like for Nate to write a section on Bourbon tasting. He is the expert on that. I would, however, suggest that if you really want to train yourself to pick out some of the complexities of Bourbon flavor, Google “bourbon flavor wheel” or “whiskey flavor wheel”, and print it out or save it to your phone of wheel. Look at the wheel when you enjoy your next glass of bourbon, and see what you can pick out.

God Bless ya’

Cheddarman