After the three weeks the next thing you need to do is lower the ABV to 35-40% which is the average for bitters. To do this – strain the solids from the alcohol and put the solids to the side (you’re going to use them again). Measure the volume of alcohol left. You’ll need to work out what volume of your chosen water-based solution (with an ABV of 0%) you need to add to the spirit to come up with an ABV of 35-40%. This could be plain old water or it could be something infused such as foraged dog-rose water. You’ll need to know the ABV of your spirit and the total amount of strained spirit you’re working with. For example, adding 500ml of water to 500 ml of alcohol at 75%ABV, would result in an ABV of 37.5% which would be perfect.
But don’t add the water to the alcohol just yet! First add the solids you drained from the alcohol to the water and heat, simmering for about 15 minutes. Then strain the solids again and discard. At this stage you can add a little sugar to the liquid to balance the bitterness, but since the spirit used at Bruichladdich had a touch of sweetness to it we didn’t add any. For 500 ml. of decoction, Ellen suggested using about 50-100 ml. sugar. Whisk it into the hot decoction to dissolve.
Now combine the botanical infused alcohol with the water based solution and you have bitters.
With thanks to Ellen Zachos