Pre- and Post-Boil Mash Efficiency Calcs Different

[slurms]

I calculate my mash efficiency at both pre-boil and post-boil. Given that gravity*volume is constant during the boil, the mash efficiency should theoretically be the same between the two. I usually brew with a 15gal kettle and the differences between the two calculations were maybe 2 percentage points (i.e., 78% vs 76%). Thought no big deal and chalked it up to some sloppy measurements. Today I brewed my first batch on an anvil foundry and found the difference much different (77% vs 72%).

I measure my volumes using a ruler, and I correlated gallons to centimeters. Pre-boil volume is measured at mash temperatures (obviously), and adjust it based on water expansion at that temp (usually around 3% or so). It was also mixed pretty well with the recirc pump so there shouldn't have been any gravity/temperature stratification for the gravity reading. Post-boil is after everything is cooled.

I find it odd that the foundry has a bigger difference than my 15gal kettle. With the narrower diameter foundry, the wort expansion would result in a much larger change in the ruler reading vs the wider kettle, which maybe leads to some kinda factor based on that which I'm overlooking. But, I don't see why the diameter of the boil kettle should play a part in it. Just measure the volume at whatever temperature and adjust accordingly. Or, maybe there is something wrong with my methodology for pre-boil? I think I would trust the post-boil efficiency more so than pre, just because there isn't any temperature effects there.

Anyone have any insight on this? Much appreciated in advanced.

 

[brewfun]

I suspect that most of the error is from how you're calculating expansion. Water is at its most dense at about 4c, a temperature your beer is almost never at. The expansion from there is about 4.25% at 100c. For purposes of brewing, we can easily round to 4%, max. Although you didn't say how cold you chill, if you're near 20c, there is still about 0.5% expansion from max density. The expansion at mash temps is less than 3%. It's about 2.3% at 70c. From those two points, your actual expansion differential is 1.8%. There may be a false accuracy in being so absolute, but it'll get you into the ballpark. If you're measuring at the same point in the process, at the same temperature (as you've pointed out, you already do) then the inaccuracies are more about equipment than technique, as you've observed.

There will still be differences between preboil and post boil efficiency because you've added some items (hops) that both displace and absorb some wort. This is where the concept of brewhouse efficiency (BHE) becomes more consistent in determining mash efficiency.

BHE is the amount of wort you get into the fermenter, instead of what's in your kettle. The trub loss and other unrecoverable wort becomes accounted for. BHE is the %of post boil wort in the fermenter * actual mash efficiency. There is still plenty of wiggle room in BHE, but it's more helpful for figuring overall yield and scaling recipes between systems.

 

[CascadesBrewer]

Anyone have any insight on this? Much appreciated in advanced.

Are you calculating efficiency yourself or are you using software to help with this? What instrument are you using to measure gravity?

 

[doug293cz]

Exactly when in the process are you taking your pre-boil gravity measurement? The malt pipe in the Anvil creates three different zones - inside the malt pipe, below the malt pipe, and between the malt pipe and vessel wall. While recirculating during the mash the wort inside and underneath should be fairly well homogenized. However, the wort between the pipe and wall is at a much lower SG, since it is isolated from the malt, and recirc does not flow thru that zone. Unless you mix aggressively after lifting the malt pipe, your wort will not be homogeneous w.r.t. SG. There is a good chance that your pre-boil SG measurement sample was pulled from a higher than average SG portion of the wort.

Brew on

 

[slurms]

Are you calculating efficiency yourself or are you using software to help with this? What instrument are you using to measure gravity?

I'm calculating it on my own, but they are similar to any online calculator. I'm using a refractometer to measure. Think there is just an inherent uncertainty with this for reading the blurred blue line, so I will probably use both hydrometer and refractometer next time to compare the two.

Exactly when in the process are you taking your pre-boil gravity measurement?

With the Foundry, I pulled the grain out and recirculated for a bit, trying to get the small grain particles out of the wort (different problem...). With my cooler mash tun and boil kettle, I always measured after transferring from MT to BK, so it should have been well mixed. I understand what you're getting at with the stratified gravity, but I don't think that's the problem here (definitely was earlier on in my brewing).

I ended up tossing in 6 gallons of water into the Foundry at 70F and heated up to 160F. The volume expansion coefficient was slightly off from what I originally was using (can't remember where I got that data from), but it is in line with another set of data I found (based on US Geological Survey agency), which should be more reputable. That definitely helped (as in make preboil more in line with post boil). And with the margin of error of using a refractometer for the reading, I think that is most likely where my issue is coming from. And for what I'm aiming for, I think this is good enough for a predictable efficiency. Thanks all!

 

[cactusgarrett]

Unless you mix aggressively after lifting the malt pipe, your wort will not be homogeneous w.r.t. SG. There is a good chance that your pre-boil SG measurement sample was pulled from a higher than average SG portion of the wort.

I absolutely just saw this on a high OG maibock i brewed last week. Second brew on my Brewzilla 65L: pulled the malt pipe and took a quick sample right from the top for the refractometer. Came out 1.063 (vs BeerSmith predicted 1.065), so I thought I was good. Post boil turned out to be 1.072, vs target of 1.090, so the only thing I could surmise was that I took the sample too early and should have waited for a better homogenization of the pre-boil volume.

 

[SonomaBrewer]

FWIW I've also noticed it's important to give wort a good stir before taking samples to measure gravity. No matter what the temperature, there always seems to be some stratification. I also filter with a small screen as any suspended break material seems to make my digital refractometer less accurate.