Does adding pre-boiled water to the carboy after I rack my beer help against oxidation (less headspace) or hurt it (risk of splashing)?
Adam Woods
Belding, Michigan

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 trick.”
   
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.