Expert: Spencer W Thomas - 6/17/2006

Question
Remind me again what exactly gypsum does for the brewing water?  Some older recipes say to add gypsum to every batch, but is this necessary or wise?  Thanks, john white.

Answer
The main purpose of gypsum is to add calcium to the brew water.  When mashing, calcium is needed to bring the mash pH (acidity) into the proper range.  A secondary effect of gypsum is to add sulfate ions to the brew water.  A proper level of sulfate will accentuate hop bitterness.

Gypsum should never be added blindly to your brew water, though.  Unless you're starting with distilled water, your water will already have some level of calcium and sulfate.  If you add gypsum, you may increase one or both to unpleasant levels, resulting in a beer with a strong "mineral" flavor, and/or harsh bitterness.  

My recommendation: if you are not mashing, don't add any gypsum.  If you are making a Pale Ale, IPA, or similarly hoppy beer, you may want to add some gypsum to accentuate the bitterness.  I suggest making two batches of the same beer, (or perhaps splitting a batch of beer into two fermentors), one with gypsum and one without.  If you can't taste any difference, then don't bother with the gypsum.  If you prefer one over the other, then there's your answer.

If you are mashing, you should know the mineral content of your water, and only add minerals as needed to (a) get sufficient levels of calcium for the mash, and/or (b) adjust the mineral content as appropriate for the style you are brewing.  If you have municipal water, you should be able to get a report of the average mineral levels from your water authority.  If you're on a well, you can buy test kits from many homebrew or aquarium supply shops that will give you a rough measure of your mineral content.  

Hope this helps.