Sugars lost during the boil

[eddiewould]

Let's say I had 45L of 1.047 wort before I start boiling = 45*47=2115pts.

I then boil it down to 27 litres. If there was no sugar lost then I should end up with starting gravity of 1.078. But I end up with 1.074 (1998 pts).

Does that mean my measurements are off, or that some sugar is lost in the boil (it was quite a long boil - 2 hours). Or maybe both?

 

[Hex23]

Let's say I had 45L of 1.047 wort before I start boiling = 45*47=2115pts.

I then boil it down to 27 litres. If there was no sugar lost then I should end up with starting gravity of 1.078. But I end up with 1.074 (1998 pts).

Does that mean my measurements are off, or that some sugar is lost in the boil (it was quite a long boil - 2 hours). Or maybe both?

Could be many things. Most likely its measurement error in volume or SG. How did you measure?

You should not lose any solids in the boil, but it seems possible that some ongoing transformations could affect SG.

 

[lumpher]

it's also possible the boil could redistribute/even out the sugars, and afterward it's more evenly distributed, so you're getting a better reading

 

[Cerevisaphile]

Did you measure both volumes at the same temperature? The volume will decrease as the temperature decreases. I have read the volume is around 4% greater at boiling than at pitching temperature.

 

[passedpawn]

Did you measure both volumes at the same temperature? The volume will decrease as the temperature decreases. I have read the volume is around 4% greater at boiling than at pitching temperature.

The 4% number is true, but that means that the wort would increase in density when it cooled (i.e., the specific gravity should go up, now down). That's assuming the preboil measurement was at a high temp and the post boil was when cooled.

I'm guessing there is some error at the time you measure the gravity, perhaps due to temperature-dependency of the tool or evaporation of the sample.

 

[eddiewould]

Hex23: Measured my marking on the side of pot to get volume. So that's not gonna be very accurate

Cerevisaphile: Both were done at the same temperature (but with a refractometer with ATC).

 

[drainbamage]

If anything, your SG would go up from a long boil, as the water would boil off and leave the sugars behind. Wouldn't it? (Same principle as distilling)

More likely is that it wasn't mixed up enough when you took a reading.

 

[Hex23]

Hex23: Measured my marking on the side of pot to get volume. So that's not gonna be very accurate

Cerevisaphile: Both were done at the same temperature (but with a refractometer with ATC).

Depending on the markings on the pot, it may not be very accurate.

I suspect the other source of error is in the refractometer. There are some hints in this thread that the WCF might change between pre-boil and post boil: https://www.homebrewtalk.com/f36/refractometer-question-362437/index9.html (see post 89 conclusions). Although it seems the data there may be the opposite direction to explain your situation. The point is that refractometers need careful calibration to produce accurate results.

 

[Cyclman]

Sugar doesn't boil off. Could be imprecise measurements, brewing tools aren't that accurate, or- wort naturally stratifies, so measurements could be affected by measuring a non-homogenous wort.