Topping off the brew kettle


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Feb 11, 2013
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Morning: This is my first day on this sight so if this has been addressed before I appologize.
I have noticed some extract receipes say start your boil of 3 gal, then when finished top off the
wort with cooled water to the desired volume. What is the reason for this, what are the advantages,
and are there risk of contamination doing this? Thanks for your help.

A smaller boil avoids having to buy a huge kettle. A full wort boil is really closer to 6.5 or 7 gallons because the boil causes evaporation. A typical kitchen stove has a hard time getting more than 3 gallons up to a rolling boil as well. So, kettle size and capacity of burner are the main reasons extract brewers opt for a smaller boil. It also speeds up the process, since the smaller volume can be boiled and subsequently chilled quicker, and there is less to clean up.

Most people top off the primary fermentor to the desired volume. This also helps cool the wort down to pitching temp faster.

Yes, there are risks to contamination by topping off with water that has not been boiled. Another problem with that is, at least with most city tap water, a chlorine taint can shine through in the finished beer.

To get around this, some brewers buy bottled spring water. Another option is to boil the water the night before, which drives off the chlorine and other bad stuff.

The advantages of a full wort boil - higher hop utilization. In practice this amounts to maybe $0.50 of bittering hops per batch. Owning big kettle feels good too. Mine is 15 gallons so I can do 10 gallon batches. If/when you do step up, go for the biggest kettle you can afford - buy it once, instead of upgrading to a 7, then an 11, then a 15...
A word of caution when topping off a partial boil. Do not add your hot wort/semi-cooled wort directly to the fermentor. If your recipe is for 5 gallons and you boiled 3 gallons, first add 2 gallons of cold water to fermentor. Add your wort and then top off to 5 gallons to account for boil off. Adding warm wort directly to the fermentor, especially glass, can cause it to fracture. A better bottle can handle somewhat warmer wort.