Decanting in this context would be to eliminate the headspace in an open bottle of whisky by pouring the remaining amount into a smaller bottle. (Headspace is the space in the bottle filled with air between the top of the whisky and the cap.) While pouring the rest of the whisky into a single bottle just large enough to hold the remaining amount left seems optimal, after further consideration it is not. This is because every time you take a pour to drink from the decanted bottle, you would need to decant (yet again) what was left to a smaller bottle. Each time slightly degrading the whisky. The optimal route would be to decant to bottles small enough to finish at one sitting, but large enough it would not degrade the whisky. We choose 4 oz amber glass round boston bottles with a polycone caps, filled to the top and then vacuum sealed. We also tried a 2 oz version that was also filled to the top and vacuum sealed to see if there was a difference between 2 oz and 4 oz bottles. All bottles were stored upright in a dark temperature controlled environment to eliminate all other variables.
NOTE: When we speak about the differences in taste or score, it is the difference between a specimen and the control, and another specimen and the control. Not the direct difference between the two specimens.
A Year Later
Our initial hypothesis as to the best way to store an open bottle of whisky was this method: to decant to smaller bottles to remove headspace, vacuum seal and then store it in a cool dark place. We were proved correct. However, because the results of vacuum sealing were all over the place we do not suggest taking that taking step.
Decanting to full and keeping the bottles in a cool dark place can make a huge difference in the taste of a whisky. If you did nothing and just left the whisky on a shelf you would be left with a whisky that had a “noticeable difference, but passable.” However, if you decanted you would have whisky that only had a “vague hint of difference” even over the course of a full year. While heat and light are the top two factors that will affect the taste of whisky, headspace is the third.
The results also show that there was virtually no difference between 4 oz or 2 oz bottles as long as they were filled all the way up. I would suggest using the 2 oz bottles as it gives you the most flexibility in the long term to minimize the number of times you would need to decant or open a bottle.
The average scores of each set of specimens. (Lower is better.)
2 oz decanted bottle, 2 oz fill, cool dark place, vacuum sealed
4 oz decanted bottle, 4 oz fill, cool dark place, vacuum sealed
750ml bottle, 150ml fill, exposed to indirect sunlight