There isn't a way to measure efficiency before you start lautering, right? In my head, I'm imagining that pulling wort off the top of the grain bed is going to be different than the wort under the grain, but maybe I'm not thinking clearly in the morning. Would it be a good idea to dip enough wort out of the mash to take a gravity reading before I start lautering?
Yes there is a way to measure the efficiency before you start lautering. But not in the conventional way since you don't know the volume of the wort in the mash. But one can determine the amount of sugar that has been dissolved based on the volume of water that was used and the current gravity of the wort. That is the idea behind the mash gravity test. But as you pointed out, the mash needs to be well mixed for that. Just like the kettle wort has to be well mixed before you can take a pre boil gravity measurement.
Using Kai's chart and this gravity (assuming I let it cool before reading it) would eliminate the need for an iodine test.
Do people do this?
I do this all the time. More than the iodine test. If I expect 80+% efficiency from my batch sparging process I know that my conversion efficiency has to be in the high 90's. If it is not I have a few options:
- let it mash for longer (I commonly do this)
- raise the temp to boost a-amylase (I also do this when needed or as part of the mash schedule)
- add more malt (I would only do this if I suspect low enzymatic strength to be the culprit. In that case I'd add pilsner malt)
- decide to go two batch sparges instead of one (this compensates for low conversion efficiency by boosting lauter efficiency, but that may have a negative impact on beer flavor)
- don't do anything
You don't have these options if the only place you test for efficiency is the boil kettle.
The iodine test checks for starch in the wort. If your conversion efficiency is close to 100% I would be surprised if the iodine test is positive. But if you have conversion issues you may have low conversion efficiency which you could compensate for by adding more grain. But even if you do that you need to make sure that the wort that is boiled is iodine negative unless the beer needs starches and long dextrins for character (e.g. Wit and other Belgians).
As I said before, I do the iodine test on the wort that I run into the kettle when I brew Doppelbocks and Dunkels just to make sure that the boiled wort will be starch free.