Methodology – Bourbon Classification

I have spent some time here explaining how – for malt whiskies – you can take a validated set of extended flavour categories, scored for intensity by expert reviewers, and reduce the complexity down to a scientifically-valid, two-dimensional flavour map. This requires extensive validation of subjective flavour perceptions, building on well-characterized  norms, as well as modern statistical rigour and analysis (something most maps out there have not done, sadly).

I frequently get asked whether something similar could be done for bourbons and other American whiskies. In principle, the answer is yes – although in practice, it is complicated by the narrower set of flavours most reviewers report in bourbon and related whiskies. But this actually opens the door to a much simpler way to classify bourbons.

Unlike most jurisdictions, American rules around bourbon production actually narrow the range of options where flavour diversity can come from. Specifically, the most relevant rules are:

  • Made from a grain mixture that is at least 51% corn
  • Aged in new, charred oak containers (i.e., virgin American oak wood)
  • Distilled to no more than 160 proof (80% ABV)
  • Entered into the barrel for aging at no more than 125 proof (62.5% ABV)

There is more to it than just the above, and I recommend you check out the Wikipedia bourbon page for more info.

But the point is that bourbon makers don’t have a lot of flexibility in how they introduce flavour.  You are limited in your starting materials, and have to maintain core flavour characteristics of the grain (due to relatively low distillation limits). Design of stills and their application are also a lot more consistent than elsewhere. Use of virgin American oak wood barrels also introduces consistent – and heavy – wood-sourced elements. And you cannot rely on the mixing of additional additives through custom barrel finishes, etc. As a matter of common practice, most bourbon makers blend barrels from across their entire warehouses, in order to even-out temperature/humidity differences during aging – further reducing variability. It is true that other non-bourbon American whiskies have more flexibility – but even here, a lot of common whisky production methods persist (e.g., use of virgin wood, etc).

To summarize, American whisky producers typically ensure consistency and quality through a different means than most jurisdictions, by standardizing processes in every area but one – the custom mashbill of grains used for fermentation. In most other jurisdictions, they typically rely on blending varying amounts of different components, distilled and aged separately, under different conditions, through batch-matching processes at the end. See my Source of Whisky Flavours for more info on the background sources of flavour, or my Single Malts vs Blends page for more info on the classic Scottish method.

All of this is by way of introducing the true source of bourbon’s distinctiveness: the custom mashbill. While every bourbon has to be at least 51% corn, there is a lot of flexibility in that remaining 49%. Common elements used for adding distinctive flavours are wheat and rye grain, among others.

As a Canadian whisky drinker, I am very familiar with rye flavouring in my whisky. I am also aware of how little rye is needed to produce noticeable levels of the classic “spiciness” this grain imparts. When I first started trying different bourbons, the overwhelming virgin wood-derived characteristics were typically what I first noticed (“tastes like bourbon”). But then I quickly started characterizing them along a gradient by how much rye grain I could detect.

I subsequently discovered I was not alone here – a lot of bourbon drinkers tend to classify this beverage on the basis of relative rye content. If you search online, you will quickly see people referring to “low rye” and “high rye” bourbons, based largely on subjective flavour impressions. While there are no consistent definitions for those terms, this general concept seems fairly ubiquitous. See for the example the generally excellent Beginner’s and Intermediate Guide to Bourbon on Reddit.

Since the production methods of bourbon are so well codified, a lot of that subjective rye flavouring impression can be traced back to actual rye content in the mashbill. Note the qualification above: I am not saying that this is due solely to rye content, as actual distillation methods matter too. But once I started to compare various subjective classification systems, I realized these actually track very closely to reported rye content in the mashbill. As an aside, many bourbon makers reveal at least the general ranges of grains in their mashbills, facilitating these comparisons.

I had originally thought to make a three-category system for bourbons based on subjective rye levels (i.e., no rye, low rye, and high rye), with a fourth category for “rye whisky” (which, for American whisky, refers to a mashbill >51% rye grain). I posted a discussion item on the /r/Bourbon forum of Reddit, and got a lot of good feedback (you can see the full thread here).

In the end, after further review of published mashbills, I’ve decided to stick with a rye grain content system for classifying all American whiskies. While the category labels below are arbitrary, I am using specific mashbill percentages (known or speculated) for the classification.

Here’s my system, showing the classification label (R0-R4), actual rye content, and rough descriptor:

  • R0 – (r=0%) – “No Rye” whisky with 0 rye gain (i.e., no rye in the mashbill or in the resulting taste, includes pure corn whiskies and “wheaters”)
  • R1 – (r<=10%) – “Low Rye” whisky of 10% or less rye grain (i.e., lower rye content and flavour than typical)
  • R2 – (10%<r<=15%) – “Standard Rye” whisky of 10-15% rye grain (i.e., classic bourbon recipe)
  • R3 – (15%<r<51%) – “High Rye” whisky of more 15% rye (i.e., more rye content or flavour than typical, but not enough to classify as a rye whisky)
  • R4 – (r>=51%) – “Rye” whisky of more than 51% rye (aka “straight rye”, although that brings with it other requirements for aging)

Of course, not all mashbills are made public. But there is sufficient speculation online – and enough consistency among subjective descriptors of those with extensive tasting experience – that I am able to classify all American whiskies in my database fairly easily into this five-category system.

I hope you find it useful as an additional guide for American whiskies!