The Five  Sources of Flavor for Bourbon: 

Grain:  All whiskey is made from grain.  There is no sugar in grain, only starch. But malting the grain (soaking it in water) produces enzymes that convert the starch into sugar and that’s what is fermented.  The neutral spirits are distilled from wheat, rye or maize (corn), but there is no distinctive flavor (that’s why they’re called neutral spirits), so the only flavor element is the barley. The requirement is for one – usually either corn or rye – to make up over half of the grain in the mix with the others adding flavor. Neither of those grains malts well, so the third grain to be added is nearly always malted barley. Corn adds sweetness and lends a grainy edge to the whiskey. Rye adds spiciness. Barley malt not only contributes sweetness; it also helps convert the starch in the corn and rye into sugar.  Wheat, could be a fourth grain, added mellowness.  By law, bourbon must consist of at least 51% corn, but in reality the percentages are always much higher; 70% - 78% is pretty common.  The grain is then inspected and milled at the distillery before being added to the vat of water.  In the colonial days whiskey was primarily rye based because corn did not grow well in the New England areas.  It was only until settlers in Kentucky used corn for distilling since it was so readily available and abundant, grew well and tasted good.

Water:   Kentucky is unique in that one of it's greatest resources is it's limestone filtered water, which is water that has trickled through vast bed of limestone rock which filter out iron and other bad tasting minerals, like sulfur.  The water distilleries use typically comes right from a protected spring, very natural and pure.  The water is pumped directly into a vat where it is heated to 150 degrees and then the milled grain is added to it - a process called "slicking".  Water is also used in the maturation and bottling stages to adjust the  proof but this is the same spring water but it has been de-ionized and is totally  characterless. 

Fermentation: Yeasts are single-celled,  micro-organisms that turn sugar into alcohol by feeding on sugar and other nutrients. They excrete carbon dioxide and alcohol - hence the large amount of bubbling in a vat.  Sour mash is actually the equivalent of cooked yeast mash.  The reason that only a small amount of yeast is needed is that they multiply drastically. In three to five days most of the sugars and nutrients the yeast eat will be gone, and the level of alcohol will have reached about 8%, which is too high for the yeast to live in. So they stop reproducing and die. There are thousands of yeast varieties (strains).   One way the distiller maintains this control is by not letting nature decide which of the thousands of yeast strains will become the dominant one. Every distillery maintains it's own, carefully tended strain of yeast. Keeping the company yeast healthy and fresh and ready to go t o work one of the most important tasks the distiller does. Right from the start, a fairly large dose of this yeast is introduced into the mash and allowed to gain a good, solid foothold. By the time a wild strain discovers the new mash and begins to move in the established strain will have already had a chance to gain strength and fortitude, and thus drive other strain the intruders away or at least keep their numbers down.   

Distillation:  The fermented mash is pumped into the still house where it is distilled.  This is where heat converts the "beer" into spirits.  Scotch and Irish whiskies are made in copper pot stills, as was bourbon at first (steel these days), and with these whiskies only the liquid "wash" is sent to the pot stills. One important way that bourbon differs from those whiskeys is that it derives a significant part of its flavor from cooking the grain solids in the mash during the distilling process. The first still is the beer still which produces low wine.  The liquid is directed to the second still, called the wine still which produces high wine.  With the exception of Labrot & Graham, all bourbon is distilled in two "continuous" or "column" stills invented in the mid 1800’s.  At Labrot & Graham, Brown-Forman wanted to revive the old methods of using copper pot stills, but they needed a way to include cooking the mash in the copper still as part of the process so they built a third still - the sprit still - which  is larger and slightly different than the other two and produces the new make whiskey.  Copper supposedly reduces the effects of sulfur.  Most of what vaporizes from the mash is ethyl alcohol (ethanol), there are dozens of other alcohols that will evaporate below the boiling point of water. These include all the elements that give new whiskey its flavor (since ethanol has no flavor). The flavor in distilled spirit comes from other chemicals that vaporize with the alcohol called congeners. Alcohol distilled at lower proof has more impurities than high proof alcohol.  160 proof is generally thought to be the highest you can go and still maintain an acceptable level of whiskey flavor. There is no lower limit, but whiskey distilled at a lower proof will carry more of the grain and fermentation elements into the barrel, and the lower alcohol level will tend to extract less of the flavors available in the oak barrel itself as opposed to a higher alcohol level spirit which is more capable of extracting larger amounts of tannin, sugars, and vanilla, resulting in a finished whiskey very different in character. 

Maturation:  New, charred, white American Oak barrels is what bourbon is stored in and matured.  The practice of charring new oak barrels is discussed in the history of bourbon whiskey, but from a practical matter it was used to rid the wood of insects, organic material and other impurities.  White oak is the perfect wood for whiskey barrels. It will stay watertight for many years. It will take a char consistently, and it contains the right balance of tannins and sugars, not to mention American White Oak is tough. Barrels can only be used once and are usually  sold in Scotland and Japan. Unlike wine barrels, whiskey barrels tend to be moved and rolled around a lot. Wine likes to sleep in its casks, with as little movement and variation in temperature as possible. Whiskey’s maturation process is active. It wants to expand into the wood during the hot summer days and be sucked back out as it contracts during the cold winters. In some distilleries, bourbon wants to be stored in the 135º (summer)- 20º (winter) upper floors of the warehouse a couple years, then be brought down to the constant 55º lower middle floors for the next year or two.  How many years, or how many cycles, makes a big difference in the character of the finished product.  The only bad news about maturation is that 3-5% of a barrel's content annually evaporates - this is known as the "angel share".  As volume goes down due to the loss of water via evaporation, the new make whiskey that went into the barrel at 110 proof will come out some years later around 120-125 proof.  If there is no age claim to a bourbon, it was likely aged four years.