Excluding oxygen by gassing headspaces and purging containers is one of the most important winemaking jobs we have. Oxygen exposure during aging can create all sorts of problems from premature oxidation and loss of aroma to spoilage microbe growth. The tough part is just what you mention — how do we know, with our own unique bottles, carboys, kegs, and barrels (not to mention the PSI and size of our gassing setup) that a barrel or carboy is “gassed” enough for the job? Sadly, there is no easy answer because as I’ve intimated there are so many unique factors involved. I can, however, provide a solid set of guidelines to help you make the best decisions for any situation you may encounter while racking. We “gas the headspace” using inert gases heavier than air, typically argon and carbon dioxide gas. Nitrogen is sometimes used in the commercial winemaking process to push hose lines, etc., but since nitrogen has about the same gas density as air (nitrogen has 1.25 g/L to air’s 1.29 g/L) it’s not terribly practical for blanketing headspace.
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