Filtering on the fly

This works: 8HD_3187

It often happens when it would be nice to bring some beer to an event – which usually involves at the very least racking a keg to another, or more often for me, filtering out a recent batch so there is a full keg to bring instead of the 1/2s or 1/3rds of kegs I have sitting in my keg fridge.

The trouble with this is, if the keg isn’t all consumed in one hit, the leftover filtered beer doesn’t have the keeping properties that it would otherwise have – or if I’ve filtered out a very fresh batch, oxidation, or off flavours develop that the yeast would have otherwise consumed.

So doing it on the fly has some benefits:

  • What beer is left remains unmolested
  • Less beer wastage in transfers
  • The filter itself need not be sanitised as thoroughly as it would otherwise be, which saves a lot of effort
  • No process of ultra chilling the beer, cleaning, sanitising and purging the receiving keg, transferring, using up a bunch of CO2, etc etc.

There are some conditions:

  • The flow resistance (long length of tube/stretched tube) must be after the filter, not before.  Otherwise the contents of the filter will empty out into the glass as useless foam.  It helps if this resistance is more than usual too.
  • Once the keg is carbonated, it is good to run top pressure on the keg more than usual – like 20 psi.  This helps to keep the CO2 in solution at all times.  Since this is only for one session the extra pressure won’t overcarb the beer.  The longer tube length will keep the dispense rate appropriate.
  • The filter must be kept upright (so the gas in the filter is at the top), and cold (to keep it from foaming up the beer).

For a transportable keg, I usually put the keg in a empty grain sack, throw in a bag of ice, and put the lot in a flexible laundry basket for easy carrying, and to avoid leaking melted water everywhere.  So I just jammed the filter in the side of the bag along with the ice.

Here is the before and after
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