When doing the post mash hop boil during a brew, is it possible to do this with a 5L sub-sample of the wort, instead of the full 23L of wort? In practise it would mean taking 5L from the post mash wort and boiling it separately in a stock pot, then pouring it back into the main wort in the fermenter post-boil.
The idea behind this would be to save energy bills by not having the boil the full 23L of wort for an hour. there may also be an advantage in being able to cool it quicker..
Conventional brewlore recommends a 60 minute boil to drive off DMS. However, this is starting to be questioned (so proceed at your own risk): http://brulosophy.com/2015/03/11/the-impact-of-boil-length-ale-exbeeriment-results/
That said, all of the wort must be boiled for at least 15 minutes. In addition to killing off the bugs present in the malt, this will ensure a good a hot break to denature the mash proteins and help clarify your beer.
You also will get less extraction from the hops by boiling them in a smaller amount of liquid.
Most of the info here is personal anecdote, but it's possible. For extract + steeped grain brewing you can make good beer without boiling the wort at all. Do the hops boil on their own in water, say 5L or so, I've used even less and had good results. Hops utilisation goes down as gravity goes up, so boiling in water at SG 1.000 in theory should give good hops utilisation in lower volumes of water than normal. In practice I've found it works fine.
DME or Liquid Extract has typically already been boiled (see also Molot's answer) in the process of creating it. So DMS precursors have already been driven off, and the hot break has occurred already.
You have to accept some additional risk of contamination as the wort is not reaching sanitation temp and there is a small chance the extract and/or grain is contaminated. But personally I've brewed probably 10 batches like this when I had limited time and space and never had an infection.
See also a related question.
No. But not because of the hop.
You need to boil ALL the wort for 60-90 minutes to reduce DMS (cooked corn flavor) especially in all grain brewing. If any of the wort is unboiled, you will have lacto and other critters still alive, and unbroken proteins.
But as for extracting the bittering, flavor, aroma in a partial boil. It does work. I'm sure there are subtle differences on how the hops react to the limited boil, but it is a common practice is brewing. For example boiling a high gravity wort 1.080 then blend it with water for a 1.040 beer. (Known as partial boil)
Key is that all the wort is boiled. If you want to save energy with a lower boil volume, increase gravity of the wort and add sanitary water to the wort post boil.
I agree with the idea that there is no overwhelming need to boil all the wort. And no need at all if making beer from malt extract. If isomerisation of hop bittering compounds is required it can be done efficiently using water alone to boil the hop and then the hop tea can be strained into the wort. It also seems to be true that hop conversion is more efficient in pure water and less efficient in wort.
There is an emerging debate on whether wort made by an all grain process (not extract) needs to be boiled as holding the wort at 65 degrees will be an effective method of pasteurisation.
The wild yeast in the grain don't seem to hold their alcohol very well, a strong yeast starter will work well almost every time IME. I learned to brew 35 yrs ago, we never boiled beer wort then. These days I take every precaution, which includes boiling 100% of the wort! I just cant afford to dump a large batch, and also believe every little nuance will effect the finished product!
