If bourbon contains no carbohydrates (and therefore no sugar) what gives it a sweet flavour? What are the chemicals or compounds?
Many chemical compounds, like saccharin for example, can taste sweet. There are hundreds of chemical compounds in bourbon and a number of them or combinations of them can lead to that sense of sweetness. Ethanol itself has a sweet flavor. The sugar from the corn itself however, will not make it through the distillation process. If we consider for a moment corn based vodka, you'll notice that it doesn't have the same sweet flavor.
I'd say that what sets bourbon apart is the number of chemical compounds present, collectively adding to the sweetness. An important contributor to these compounds are the flavors that come from the barrel, most noticeably vanillin. Vanillin tastes like, well..... vanilla. Another is 3-Methyl-4-octanolide, also known as whisky lactone, which adds more vanilla flavor as well as coconut. The other major contributor are esters. Esters are one of the congeners, things other than ethanol that make it through distillation. Esters add more fruit flavors. One such ester is ethyl hexanoate, which can provide like a pineapple or slightly green banana flavor.
You can actually get totally different compounds in the bottle while using the exact same production materials too. For example in the barrel, a lower alcohol content won't penetrate the wood as deeply as a higher alcohol content would. The sweeter flavors are pretty shallow in the wood so if I wanted to make something sweet, I'd dilute the alcohol a little before aging. You won't be able to tell this by looking at the alcohol content on the bottle though, because the higher proof stuff going into the barrel will be diluted after aging. It's all part of the process.
I did not see this mentioned...wood is made up of fibers of cellulose and hemicellulose. Both of these substances are polymers of sugar, just like starch is a polymer of sugar. When the barrel is charred, it will create caramel compounds, just like charring sugar. When bourbon is stored in a charred oak barrel, these caramel compounds will be extracted into the spirit, adding sweetness.
Similarly, the cellulose and hemicellulose are bound together by a compound called lignin. When lignin is broken down, phenylpropenes and methoxyphenols are created. Examples of these compounds are vanilla and clove flavors
So why is bourbon so sweet?
It’s all about the corn mash. The stuff those geniuses ferment.
Bourbon’s mashbill is made of at least 51% corn, often far more (closer to 70% as an average). Four Roses has two mashbills, one with 60% corn and one with 75% corn. There are various high-rye, low-rye, wheat and whatever else mashbills but all share one characteristic – at least 51% corn.
Scotch, on the other hand, uses mostly or all barley in its mash and the glucose content of corn is much higher. That glucose is what gives bourbon its easily recognizable sweetness.
51%+ corn in the mash is why bourbon is so sweet.