A slightly acidic environment is enjoyed by yeast, and also inhibits the development of
bacterial contaminants. The pH of the brew should be adjusted to between 4.0 and 4.5
prior to fermentation, using citric or lactic acids.
You can also use lemon juice rather than citric acid -
it works great in distilling, but is bad in winemaking. Just use it on an
equal volume basis- 1TBSP of acid blend = 1TBSP of lemon juice.
This calculation seems on the low side practically - it must be that the citric acid
sold in supermarkets / brewshops isn't 100% pure. Always double-check the pH using
pH papers or some other test.