Evaporation Rate, the proportion of the total volume of liquid in the wort kettle that is evaporated as steam expressed as a percentage of the original volume of wort. Brewers strive to achieve a target evaporation rate in boiling their wort. If the evaporation rate is too low, the necessary changes to the wort achieved by boiling may not occur. If it is too high, then energy is being wasted. Historically the target was 10%–15% evaporation over 90 min of boiling, but modern brewers tend to boil for a little over 1 h; as a result, evaporation of 6%–8% of the total liquid volume is now more usual.