Question about hop schedule and BIAB

Question

I've done three brews so far, one with a partial mash, one with only extract and one BIAB. I'm restricted to working with a 10L kettle (a little more than 2.5 gallons). For obvious reasons, this yields quite small batches. I've been reading quite a bit different BIAB techniques and have a question about something similar. More specifically, I'm curious about the difference between two ways of brewing.

In the first scenario, let's say we have 12.5L of wort to boil for an hour. The boil-off is 2.5L so that at the end of the boil we're left with 10L.

In the other scenario, we start with 10L of wort in our main kettle. But this was obtained using BIAB, and we re-use the bag in a second (smaller) kettle. So when the boil starts we are boiling 10L in one kettle and around 4-5L in a second kettle. As wort boils off in the main kettle, we add wort from the second kettle. Let's say we do this every 15 minutes.

The two scenarios obviously differ, but at the end of the boil they should have the same amount of wort left: 10L. A simple figure may illustrate this.

My questions are:

1. Is the second technique bad?
2. Would the hop amounts be different in the two techniques (provided nothing is added to the second kettle)?

Answer 1

Moving the wort between kettles complicates things, since you're dealing with both hopped and unhopped extract. While you'll be able to make beer this way, it complicates hitting IBUs.

Since you're boiling both kettles anyway, why not add hops in proportion to their volumes, e.g. 2/3 in the 10l kettle and 1/3 in the 5l kettle. There is then no need to transfer wort between the kettles which makes your day a bit easier.

Answer 2

1. Not necessarily a bad method, but it's a little complicated, and it's difficult to predict the outcome in terms of OG and hop utilization. (The hop utilization depends strongly on the SG of the extract you are boiling in, see

   http://www.howtobrew.com/section1/chapter5-5.html

2. Yes, the resulting IBUs will differ, because the the hop utilization depends on the SG of the extract you are boiling. You will not have very good control of the SG of the wort you boil, so you will not have good control of the hop utilization. On the other hand, the numbers may not differ much, maybe a couple of IBUs. Moreover, the human palate has a resolution of about 5 IBUs so it won't matter that much. At the same time, there are many other unknowns affecting hop utilization in homebrewing, such as boil vigor, so it is difficult to predict IBUs anyway!