Headspace can cause oxidation, especially in secondary fermenters.
Anything that is done to eliminate air helps minimize oxidation. And
adding de-aerated water can reduce the odds of oxidation, but the
process deserves more than a simple answer. So read on Adam!
In the parlance of commercial brewing this method is typically referred
to as high gravity brewing, or simply HGB. The idea with HGB is simple; a
brewery can expand the production capacity of a fermenting vessel by
brewing high gravity wort in the brewhouse and then diluting the beer
post fermentation. The primary motivation to use HGB is monetary since
fermentation vessels represent a major expense in terms of capital
costs, floor space and labor associated with cleaning, filling and
racking. It is fairly common to “extend” a batch by around 25% between
the fermenter and package with HBG. There are some breweries who have
invested a lot of serious talent into very high gravity brewing (VHGB)
and routinely brew lagers with an original gravity in-line with a
doppelbock, but the finished color, aroma and flavor of a pale lager.
The challenge with HGB, and particularly with VHGB, is that yeast are
temperamental little critters and generally don’t like it when wort
gravity gets too high. In a brewery brewing a large volume of normal
gravity beer and a smaller volume of high gravity specialties it is
common to pitch yeast from normal brews into specials and then to retire
the yeast from the special at the end of fermentation. But when you
brew everything from high gravity wort much greater focus is placed on
yeast because of the harsher conditions.
Furthermore, the biochemical pathways used for metabolism are affected
by wort gravity and this has a direct influence on finished beer aroma.
Most brewers utilizing HGB methods want to brew beers that taste very
similar to beers brewed at gravity, meaning no dilution, and much of the
research conducted by these brewers relates to this particular “magic
Now onto your question about water . . . boiling water does indeed kill
microorganisms that may be living in it and it also decreases the oxygen
content. But the oxygen content does not get nearly as low as some may
believe because even when water is boiling it is still being exposed to
atmospheric pressure and the two variables affecting oxygen solubility
in water are temperature and pressure. This means that there is still
about 5 ppm of oxygen dissolved in boiling water, and this is far, far
greater than the targets of commercial brewers. Most breweries these
days have some method for making and storing de-aerated, carbonated
water (DCW). At Springfield Brewing Company we make DCW and use it for
filter and line presses when we move beer. High gravity brewers use DCW
to blend with beer and the oxygen targets in DCW are typically less than
20 ppb . . . or 250 times lower than the oxygen content of boiled water. This
means that topping your fermenter up with boiled water is actually not a
good method of controlling oxidation.
Most systems used to produce DCW use gas flushing under vacuum or
membrane diffusion. It is possible to strip oxygen from water by
bubbling nitrogen into it. This is what we do at Springfield Brewing
Company and empirically I know that it works, but since we currently do
not own a dissolved oxygen meter I do not know the oxygen content of our
homemade DCW. I do know that when we began making DCW that oxidized
notes in our beer following filtration (our filter runs begin and end
with water presses) disappeared.
To sum it up, this method you propose using does work, but you need to
use water that is de-aerated by a better means than boiling. It also can
be used to stretch your volumetric yield but you need to plan for this
and brew beer that is concentrated and intended for later dilution.