Why is my post-boil gravity consistently lower than pre?

brazilhead · Jul 17, 2016

Hello fellow homebrewers!

I've been brewing all-grain for 10 years now and lately I've noticed something that I really can't figure out. I'll try and explain the relevent points of my process. I always keep notes on my brew sessions and use these notes for water temps and quantities and gravity readings among similar recipes. I batch sparge and take refractometer readings of my first, second, and third runnings and preboil density. These readings are always quite consistent between recipes. Then my postboil density (OG) is consistently lower than the preboil density, which of course makes no sense. I take this last reading after chilling from the small amount of liquid left over in the kettle, Could that somehow explain it? Today, for example, I brewed an English IPA with a preboil density of 14.0 Brix. Then the post-boil reading was 13.0. I have a hunch that my apparently crappy efficiency of around 71% may be due to faulty OG readings. Any ideas?

I really appreciate any help on this!

liquidcourage · Jul 17, 2016

Slow leak in your chiller adding water to the pot as you chill?

addled_mind · Jul 17, 2016

Suger does not evaporate during the boil so if your post boil volume is less than your pre boil volume there is no way it can't be an measurement error.

If either your pre or post boil measurements are wrong it could easily explain the discrepancy. I would bet the refractometer is your culprit and would suggest using a hydrometer at least in parallel for a couple batches - that way you will be sure of your readings. Obviously make sure both devices are calibrated correctly beforehand.

jddevinn · Jul 17, 2016

What type of refractometer are you using? My digital one does not like cloudy samples (like what would be left behind post-boil).

stella_tigre · Jul 17, 2016

We've just discussed on another thread, hot wort skews your refractometer reading more than you think it will, it appears higher than it should be. Make sure the preboil sample is cooled to 80 deg f or so. Or calibrate your refractometer with hot water...

Qhrumphf · Jul 17, 2016

As you've correctly identified, it's not possible to boil off water and have the gravity go down. What are your pre and post-boil volumes?

Leaking into the kettle is one possibility, sure. As far as the full batch, for your gravity to go down, not up, you'd by definition have to leak in as much water as you boiled off during your boil, and I have a hard time seeing that happen for that long without you noticing (I'd sure as hell notice it the very first time, and you probably would too). But if you're reading from the kettle dregs at the end, especially if it's only a few drops, just a little added water could easily throw your reading off. I also wonder, if your ground water is cold enough to leave condensation on the chiller, if that couldn't throw it off. Not sure how much your "small amount" left over is, but I'm skeptical about that last one.

It's also possible you're observing some form of stratification (think the mixing problem between high gravity extract wort and top off water, just on a less extreme scale) on some or all of your readings. It's easily identifiable in pre-boil wort though (unless thoroughly stirred so you're getting a representative sample), as the gravities of your various runnings are quite different. If your preboil reading was more first runnings than consistent overall, it would make your pre-boil reading seem artificially inflated. Post-boil it's far less likely to have noticeable stratification since the boil itself will mix it all. An easy check since you're already checking the gravity of individual runnings is to check gravity points (ie abstract representation of total sugars present instead of density) at any given point in the process once all your wort has been collected, as the gravity points shouldn't change without actually adding additional sugars. Multiply the gravity of each runnings by the actual volume collected and add them together (let's say 2 gallons of 20° first, 2.5 gallons of 8° second, and 2.5 gallons of 3° third, that'd be 2*20+2.5*8+2.5*3=67.5). Then multiply the total preboil volume by the preboil gravity. It should be the same number. Same should be the case for your post-boil volume multiplied by post boil gravity (before any losses to chiller/trub/etc). If it's not, then at least one of your readings is wrong. I suspect if you accurately get the volumes of your various individual runnings and total those, you'll find it mathematically lines up with your post-boil actual gravity, and due to stratified pre-boil wort it's your preboil reading that's the erroneous one.

It could be an equipment problem too. How sure are you that your refractometer is accurate? Are you correcting your readings with an appropriate WRI? Or that the ATC function on it isn't failing (if it has one)? I would obviously start by calibrating it to 1.000 in distilled water at reference temperature. And then I would make a few tiny sample "worts" out of DME, 10 grams DME per 100ml of wort (as in add water to DME to bring to combined volume, not add 10 grams to 100ml of water) should be about 1.036/9°P (and functionally 9° Brix too), and you could adjust the ratios to adjust the gravity, so that you're dealing with a known number. If you could double check with an accurate hydrometer even better just in case your weights are off a bit. And then, I'd check your readings at various temperatures and see if they change.