You're right - you need minerals! Different minerals in the brewing water perform a number of roles througout the brewing process:
- mashing: during the mash, minerals are used to adjust the pH - around 5.2 is considered a comprimise between the pH ranges favored by alpha and beta amylase. Chalk (Calcium Carbonate) and Baking soda (Calcium Hydrogen Carbonate/Calcium bicarbonate) increase pH, while Gypsum (Calcium Sulphate), Calcium Chloride and Epsom Salts (Magnesium Sulphate) reduce pH.
- yeast nutrients: various metals, such as calcium, zinc, manganese, phosphorus, copper and iron are used by the yeast.
- flavour: Sulphates accentuate hop bitterness, while Chlorides accentuate the maltiness, while too much of both gives a harsh bitterness.
- stability: calcium adds stability to the beer, helping produce clearer beers.
The references give the information in much more detail, including recommended amounts in parts per million (ppm.)
When brewing from extract or steeping grains, there is no mash, so of course mash pH is not a concern. The primary concern then is ensuring required minerals are available to the yeast. Wyeast make a nutrient blend that contains all of these minterals, so you could put that in the last 5 mins of the boil to make up for any deficiencies in the RO water. (In this case, avoid ammonium phopsphate, since that contributes few of the minerals required.) The extract brewer whose got all the variables nailed down and looking for more refinement can look to secondary concerns such as adding chlorides and sulphates to adjust the balance between maltiness and hoppiness. This also adds calcium - good for the yeast and good for colloidal stability.
The partial mash/all grain brewer also needs to feed the yeast and strike a flavour balance, so he will use all of the above, but will also add minerals to control mash pH. When controlling the mash and flavour, each salt performs multiple functions. For eample, Calcium Chloride reduces mash pH and accentuates the malt, so it's quite an art to finding the correct proportion of all the salts to give the desired effect. Brewing softwawre makes this process much simpler.
The quantities of salts added to a 5 or 10 gallon brew are measured in grams. A digital jewellers scale that measures 0-100g with a precision of 0.01g typically costs less than $10 and is perfect for weighing these small amounts.
To get started, you might pick up some yeast nutrient and these salts from your LHBS:
- Calcium Sulfate (Gypsum)
- Calcium Carbonate (Chalk)
- Magnesium Sulfate (Epsom Salts)
- Calcium Chloride
And from your kitchen
- Sodium Chloride (Table salt)
- Calcium Bicarbonate (Baking soda)