Rum Aging Science American Oak Barrels
Rum Aging Science - Rum Central
Introduction
In our first 12-part series, published from January through December 2020, we explored the transformation of rum, while aging in an ex-whiskey barrel. We selected an ex-whiskey barrel for that initial series because the majority of the rum aged around the world employs this type of barrel. The reason for the popularity of this choice is the fact that whiskey producers are required to age their spirit in new oak barrels and, once emptied, they cannot re-fill the barrels, thus creating a surplus of barrels that many other spirits are happy to use in their aging programs.
During the course of the first series, we received a considerable number of inquiries, asking how the results presented would differ if we were using new barrels. This new 12-part series is devoted specifically to address this topic: we will be conducting the same type of research as we did previously, examining the month-to-month changes to the rum while it ages in a new barrel, but also presenting side-by-side comparisons to the corresponding results from the used barrels.
There are many different levels of heat treatment that can be applied to a brand new barrel. This series focuses exclusively on American Oak with a Char #1 (staves and heads), from Independent Stave Company/Missouri Cooperage. Future series will explore different Char and toast level combinations, as well as, barrels constructed using French Oak.
Understanding Barrel Char Levels:
Char Barrel Levels
One of the last steps during barrel making is toasting or charring the inner surface. The intensity of the heat/flames results in different levels of toast or char. On average, a Char #1 means 15 seconds of direct ignition, Char #2 is 30 seconds, Char #3 is 35 seconds and Char #4 (Alligator Char) is 55 seconds.
Experimental Design and Equipment
Environmental Parameters
As part of this study, we will be monitoring daily temperature and relative humidity inside the cellar, near the location of the selected barrel.
Physical Parameters
We will also be monitoring physical changes of the rum inside the barrel, including: pH, ABV, Color and Composition.
Equipment
The environmental parameters will be recorded using Rum Central’s Amprobe Temperature and Relative Humidity Data Logger, which is configured to log readings hourly.
Once per month we will withdraw a sample from the barrel we selected for this study. The sample will then be analyzed for physical changes using the following equipment:
- pH will be measured using a Hanna Instruments Edge pH Meter.
- ABV will be measured using an Anton Paar Snap 50 Alcohol Meter.
- Color and composition will be recorded using a Thermo Scientific Spectronic 200 Spectrophotometer, with a wavelength range from 340nm to 1000nm.
Expectations
- Acidity should increase (pH should decrease).
- Total volume should decrease (due to evaporation).
- ABV should change (due to esterification and differences in evaporation rates).
- Tannic and wood extractable content should increase.
- The concentration of aldehydes, acids and esters should also increase over time (due to the interaction of the different alcohols with oxygen inside the barrel).
How fast will these transformations take place and how will they differ from the results obtained from the rum aged in the ex-whiskey barrel?
The Rum
The white rum used to fill the barrel is column-distilled, from fermented High Test Molasses (“miel virgen”). The entry ABV into the barrel is 63.43% ABV.
Rum Aging Science Oak barrels
The Barrel
The barrel was made by Independent Stave Company, using White American Oak. The heat finish of the barrel is a #1 Char and it has no previous use: the rum used in this study will be its first fill. Rum Central received the barrel shortly after it was produced and the barrel was filled quickly afterward.
Since American Whiskey and Bourbon are aged using new barrels like this one, we expect the type and concentration level of wood extractives in our rum to be similar to those in the aforementioned spirits. These extractives include:
Cellulose - is the most abundant natural polymer on earth. It consists of linear chains of glucose units and remains relatively intact even after wood curing and toasting.
Hemicellulose - also known as a “wood sugar” is a two-dimensional polymer comprised of many simple sugars, including: Glucose, Xylose, Mannose, Arabinose, Galactose and Rhamnose.
Lignin - despite the fact that it is also one of the most abundant nature-produced materials on earth, lignin remains one of the least understood. Oak ligning consists of two building blocks: guaiacyl and syringyl. The former is responsible for producing coniferaldehyde, vanillin and vanillic acid, which -especially the vanillin- are easily recognized in cask-condition spirits.
Oak tannins - these plant polyphenols derive their name from the Latin word tannum, which means “crushed oak bark,” since in early times oak trees served as a major source of tannin for the leather-tannin industry. Tannins improve aged rum’s character by increasing the perception of balance, complexity and roundness.
Join us next month, as we share our observations from the first month of aging at the cellar!