アク
Meanings
Noun
1. lye (Usually kana)
2. harsh taste; bitter taste; alkaline taste; astringency (Usually kana)
3. scum (on a soup, broth etc.) (Usually kana)
4. (excessive) self-assertiveness; strong individuality; strong idiosyncrasy (Usually kana, also written incorrectly as 悪)
Other readings
Examples2
４．
4. Just before bringing it to the boil, set to a low flame and remove the scum again.
アク
When the scum stops appearing pour the syrup back into a clean bottle and keep it in the fridge.