[...]2- sure you can do a 2.5 gallon boil and then top off with filtered or boiled and cooled water I would not use distilled water though as the mineral content in spring water is beneficial to the flavor and the yeast health. Just a FYI don't ever use reverse osmosis water though as it can kill the yeast as the osmotic pressure of RO water is higher than the cell wall of the yeast.
Are you sure about this? RO water is simply water that has been purified by being forced through a semi permeable barrier. Distilled water has less minerals than RO as it was evaporated and recondensed.
Regardless in an extract batch all those minerals are already in the extract and there is no need for more.
Yes I'm sure about the RO water we learned that in school as I went to school for brewing..... it is moreso for yeast starters or even in high amounts in fermentation. What will happen to RO water when all and I mean all of the minerals, toxins, VOC, etc are pulled through a thin film cellulose it actually changes the composition of the water and creates what is called osmotic pressure. There have been numerous brewing case studies as well as microbiology studies showing the impact of the Osmotic pressure of H2o on cell walls causing them to implode not explode but implode as the pressure of the water has increased to a level higher than a cell can tolerate. Now distilled water is fine except for the fact that any of the waters character has been removed I love distilled water when I build a profile to a specific region. for extract though you are correct all the monerals and nutrient are already present in the LME or DME.
Not trying to be nit picky but you have it a little mixed up.http://en.wikipedia.org/wiki/Reverse_osmosis
The solvent (water) is passed through the membrane, the solute (minerals, etc.) stay on the input side and are flushed away.
Perhaps some of the water scientist can clarify this a bit for us but my understanding is that the less 'stuff' in the water the lower the osmotic pressure on the cell would be thus too many minerals would leave the cell wall. I can see how that would be problem in a starter or rehydration solution.
My confusion with your argument is coming from the fact that you are saying that distilled is fine. RO water is far closer in chemical makeup to tap water than distilled is to tap water. Why would moving from a to b on a continuum be worse than moving from a all the way to c?