of H, which is a measure of total ions per unit volume rather than mass per unit-volume. For each one-unit change in pH, the hydrogen ion concentration changes ten-fold. In other words, the amount of acid required to change the pH of water from pH 7 to pH 6 is one-tenth of the amount required to take it from pH 6 to pH 5. Why is this important? Because each day the rum is inside the barrel, the amount of ethanoic (acetic) acid increases, which then leads to formation of esters, particularly of ethyl acetate.
WHY IS THE ABV DECREASING EVERY MONTH? As acids form inside the barrel, the acid molecules combine with the alcohol molecules to create esters. What this means is that, as the quantity of esters increases, the amount of alcohol molecules decreases. If you want to learn more about this fascinating topic, do a search online for Fischer esterification (or Fischer– Speier esterification).
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WHAT’S NEXT? Average temperatures will continue to increase over the next three months. The constant expansion and contraction of the rum inside the barrel will allow it to extract more elements from the wood, and the increased evaporation loss will result in higher acidification and esterification of the rum (decreased pH and ABV). While some of these changes may seem like losses, they are all part of the price we pay in exchange for the maturation of the rum.
Luis Ayala is an international rum consultant and broker of specialty aged rums. He is founder of The Rum University, Rum Central and Got Rum? magazine. For more info visit www.gotrum.com or email luis@gotrum.com.
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