Pre-boil gravity HIGHER than post?

[brewjack]

So here's one that doesn't make any sense to me:

Last few batches I've noticed that when I check the gravity of my wort after runoff, it's higher than after the 60-90 minute boil. Yesterday I brewed up a batch of Hefe' and decided to check it out carefully; I figured that since I use a refractometer while collecting/sparging/boiling and a hygrometer post-cooling, that the instruments simply weren't calibrated to one another. So, I checked my pre and post boil wort with both instruments, and both agree that the wort is becoming weaker as it boils.

Obviously, it shouldn't work like that. So what's going on?

Jack

 

[iijakii]

Are you correcting for temperature?

 

[brewjack]

Are you correcting for temperature?

yup

 

[HollisBT]

Are you thoroughly stirring your wort before taking a sample? Sounds like you have stratification between each runoff, and if you are taking the sample via a ball valve and pick up at the bottom of the kettle, you are only capturing your first runnings.

Stir the entire boil volume thoroughly, and make sure that you are mixing the runnings well so that you get an accurate reading

 

[GilaMinumBeer]

Use a hydrometer instead to keep your measurements relative.

 

[brewjack]

I'm not collecting samples through a ball valve, I have a large quart sized ladle that I plunge deep into the wort a couple times to stir it before pulling the sample.

Use a hydrometer instead to keep your measurements relative.

I used both tools both times, comparing refractometer to refractometer and hygrometer to hygrometer. Both tools agreed that the gravity dropped after the boil.

 

[HollisBT]

Well unless you are adding water post boil, I maintain that the wort is not thoroughly mixed before collecting the sample...




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[Shooter]

hygrometer to hygrometer

Aha, there's your problem, you're using the wrong instrument!!!

 

[GilaMinumBeer]

I used both tools both times, comparing refractometer to refractometer and hygrometer to hygrometer. Both tools agreed that the gravity dropped after the boil.

A hygrometer measures relative humidity.

A hydrometer measure specific gravity.

 

[GilaMinumBeer]

Aha, there's your problem, you're using the wrong instrument!!!

"You dropped a Baume on me ... baby."

 

[Hopper5000]

Even with calculators I have noticed that at least my hydrometer reading are more accurate when I am within 10ish degrees of calibration. For my pre-boil reading I mix very thoroughly and get wort in the sampling container and refrigerate it for about a half an hour and take a reading after making sure the temp is down to room tempish.

Something isn't calibrated right if you aren't getting accurate readings and you are doing full boils with no top off. If you are topping off in the fermenter then you have to mix that thoroughly.

I am assuming you are doing all grain batches, no? If it's extract then you might not be mixing the extract in well enough.

 

[brewkinger]

Well unless you are adding water post boil, I maintain that the wort is not thoroughly mixed before collecting the sample...

THIS ^^

And I further present the theory and maintain that there is NO way that this can be realistically happening.

The boil drives off water in the form of vapor, ALL the sugars stay behind and the wort is MORE concentrated (higher gravity) after the boil than it is before the boil.

Possibilities:

1) both your refractometer AND hydrometer are not calibrated correctly.

2) you are not correctly converting for temp differences

3) you are getting a non-homogeneous sample when you are taking it.

4) Any combination (or all) of the above.

 

[Qhrumphf]

As has been said, there's no physical way for this to happen, unless there's this several week lag in between your mash and boil and things start fermenting, but I'd think you'd have disclosed that part. No other way for the gravity to go down when water is removed.

Now, if we're wrong, and what you've described is actually happening, I suggest you quarantine off the room you're doing this in, leaving everything exactly as is, and contact your local university's science department immediately, as I'm assuming they'd like to examine it before whatever force in there stops breaking the laws of physics.

 

[brewjack]

ok ok. hydrometer. You win.

It's possible it's as simple as not being thoroughly mixed. But I've noticed the same trend in the last three batches. Each time I look at my pre-boil gravity and think "wow, I must have gotten really good efficiency! I guess I'll have to water down at the end" but then by the end of the boil, the gravity has dropped.

Could the hot break have anything to do with it? The first measure was taken before the break when all those proteins would be in suspension, later I could see large "rafts" in the boil, then when took the last sample (post boil) most of those proteins had fallen to the bottom of the kettle. Seems like a stretch... what do you think?

 

[HollisBT]

Could the hot break have anything to do with it? The first measure was taken before the break when all those proteins would be in suspension, later I could see large "rafts" in the boil, then when took the last sample (post boil) most of those proteins had fallen to the bottom of the kettle. Seems like a stretch... what do you think?

Nope, absolutely not. The only two reasonable explanations are:

1) Not mixing thoroughly
2) You have broken physics

 

[kh54s10]

I doubt you got enough of anything into your preboil reading that would make the gravity higher than after the boil. As said it has to be a measuring error. When you boil off water the sugars become more concentrated, raising the gravity. It is impossible for the gravity to be lower after the boil. Unless somehow you are ending up with more water in the wort after boiling compared to before.?!?!?!?

 

[j1n]

ok ok. hydrometer. You win.

It's possible it's as simple as not being thoroughly mixed. But I've noticed the same trend in the last three batches. Each time I look at my pre-boil gravity and think "wow, I must have gotten really good efficiency! I guess I'll have to water down at the end" but then by the end of the boil, the gravity has dropped.

Could the hot break have anything to do with it? The first measure was taken before the break when all those proteins would be in suspension, later I could see large "rafts" in the boil, then when took the last sample (post boil) most of those proteins had fallen to the bottom of the kettle. Seems like a stretch... what do you think?

LOL cmon

It is literally impossible for the pre boil gravity to be higher than post.

Obvious human error is obvious.

 

[Yooper]

ok ok. hydrometer. You win.

It's possible it's as simple as not being thoroughly mixed. But I've noticed the same trend in the last three batches. Each time I look at my pre-boil gravity and think "wow, I must have gotten really good efficiency! I guess I'll have to water down at the end" but then by the end of the boil, the gravity has dropped.

Could the hot break have anything to do with it? The first measure was taken before the break when all those proteins would be in suspension, later I could see large "rafts" in the boil, then when took the last sample (post boil) most of those proteins had fallen to the bottom of the kettle. Seems like a stretch... what do you think?

What I didn't see mentioned is temperature.

Any preboil reading should cool quickly, as a drop cools quickly, but I'd try making sure that the refractometer is at room temperature. Also, check the refractometer against the hydrometer, to make sure they are calibrated against each other and apply the correction factor for your instruments.

If you take a hydrometer reading, make sure it is cooled to under 100 degrees and then take the reading and apply the correction. A reading over 100 degrees is so inaccurate as to be useless (possibly what happened here?)

 

[brewjack]

hmmm... I donno. Maybe a mix of things, I guess I'll just stir better, be a little more careful with temp, and see if I get the same results next time.

 

[Hermit]

If you take a hydrometer reading, make sure it is cooled to under 100 degrees and then take the reading and apply the correction. A reading over 100 degrees is so inaccurate as to be useless (possibly what happened here?)

I didn't realize there was a 100 degree cutoff. I gave up trying to use correction factors at all since they never seemed to work for me. I float my hydrometer and tube in a 55 gallon drum of water that I use to capture the drain water from my RO system. That seems to cool it off quick enough.

 

[DSorenson]

I think everyone else is wrong.

It is likely gnomes are pilfering your tasty wort sugars when you are not looking.

Watch the pot more closely.

 

[DSorenson]

I think everyone else is wrong.

It is likely gnomes are pilfering your tasty wort sugars when you are not looking.

Watch the pot more closely.