Boil & Evaporation Rates

[hafmpty]

I have a question about boil off & evaporation rates.

In the 01/26/10 episode of Brew Strong ( http://thebrewingnetwork.com/shows/600 ), Palmer mentions George Fix's advice that an evaporation rate greater than 15% should be avoided. I found the place in An Analysis of Brewing Techniques where Fix says this.

When I brewed 10gal batches my evaporation rate was around 13%. Because of back problems, I'm moving to brewing 2.5gal batches. With a 2.5gal batch, I don't think there is any way I can get a good rolling boil and come in under the 15% mark.

From my reading, it seems most "small batch brewers" are boiling off more than 15%. My estimates are that I will have ≈4.5gal at the start of the boil. I'm assuming the amount I boil off using the same burner will be about the same. But even if I were to boil off 1gal per hour, that puts me at ≈22% for the evaporation rate.

Is this a problem? Is there anything I can do to make sure my beers don't suffer from the negative flavor & stability effects of too high of an evaporation rate?

Thanks for your help!

[BDawg]

Percentages are horrible ways to measure boil off. They cannot remain constant, as you would need to reduce the heat constantly as the liquid level decreased.

What you want to strive for is consistency. You want a slow, rolling boil that looks the same batch to batch so that you can accurately hit your desired final volume from batch to batch. This takes practice so that you can recognize the level of boil activity that 'looks right' for your system - indicating that you aren't boiling off more or less than you desire.

On my system, I try to boil off about a gallon and a half over the 75 minute boil, leaving me with 5.75 gallons in the boil kettle, and about 5.25 gallons in the fermenter. You should be targeting a boiloff volume of roughly 3/4 of a gallon plus or minus. The geometry of your kettle and the heat source will drive the exact number. It is up to you to zero in on what works best for your setup.

This may require adjusting your recipes up or down, and playing with the exact finishing wort volume you want to hit, until you are satisfied with the final results.

I know that this is somewhat vague, but there really isn't a better way. Most will agree that starting with about a 3/4 to 1 gallon boil off volume for your first attempt at a 2 1/2 gallon batch will at least get you in the ballpark. Then, adjust your amounts and keep repeating until you can recognize what the right boil off rate looks like. You'll then be able to hit it every time.

HTH-

[mabrungard]

Bdawg, you are confusing total loss with loss rate. While I agree that using percentage is really cumbersome in the brewery and using gal/hr (or something similar) is much easier to use in practice. But what I found is that we do need to understand how much of the original volume we end up boiling off from each batch of beer. That is where percent is worthwhile and that is what I understand hafmpty to have meant.

All you have to do is look at these examples:
Starting vol = 7 gal, Ending vol = 6 gal after 1 hr boil, Loss rate = 1 gal/hr, % loss about 15%
Starting vol = 3 gal, Ending vol = 2 gal after 1 hr boil, Loss rate = 1 gal/hr, % loss about 33%

The examples show the same loss rate, but the amount of boil off expressed as a percentage of the original volume is very different. Needless to say, the wort concentration and the mineral concentration for each of those examples will be quite different.

Bdawg, I used to be firmly in your camp with saying that % was a stupid thing for brewing. But with more brewers doing small batches, it became clear that we do need to evaluate the total volume reduction since it can profoundly effect the resulting wort.

Regarding the control of boil-off rate, most set ups can allow for throttling the heat input. The other thing we can adjust is the amount of exchange between the headspace over the boiling wort and the atmosphere. Partially covering the boil can be used to reduce boil-off rate while keeping a strong boil. Most pro-brewer's kettles are covered and have a small vent pipe. Boiling in an uncovered kettle invites excessive boil-off. I don't buy the argument that DMS and other off-flavor producers will affect the beer since the pro's have covered kettles and most of them don't have that problem.

[hafmpty]

...what I found is that we do need to understand how much of the original volume we end up boiling off from each batch of beer. That is where percent is worthwhile...

I can see both your points. I need to know my system for sure. Part of that is knowing how much I normally boil off (volume) to get the proper O.G. But at the same time, Fix is talking in percentages which is a ratio of starting to ending volume...what Martin is talking about and what I'm trying to understand as it relates to flavor development...especially OFF-FLAVOR development.

I posted this question on HBT and received some helpful input. One thing that became clear upon closer reading is that Fix is talking about EXCESSIVE THERMAL LOADING which he defines as long boil times or high-temperature/high-pressure boils. The evaporation rate isn't the issue. He's using the evaporation rate as an INDICATOR of excessive thermal loading. I can hear him saying, "15%? If you're boiling off that much...you've got a really hot boil or your boiling too long." But that's without him knowing where MY volumes started or even without knowing what my boil looked like.

Also, at another place in Fix he says, "On the other hand, excessive thermal loading (e.g. uncontrolled high-temperature [...] boils) can transform the simple melanoidins into less desireable heterocyclics...Nevertheless it is excesive heat treatement - not inadeqate boiling - that is responsible for their formation."

It seems to me that the HIGH HEAT applied during a boil is the cause, not the concentration...to a certain extent. The "extract twang" that extract brewers get is related to the formation of maillard reactions during the extract production process and the continued darkening of the extract that occurs over time. So when extract brewers boil very, VERY high gravity worts for 60+ minutes, it will occur more.

In my case, I'm not doing that. I'm boiling a lower gravity wort down to an "average strength" wort, not boiling a high gravity wort and then diluting it down to an average strength wort.

Martin I have a question for you. I'm using v.3.4 of the Supporter Edition of your spreadsheet. How would you recommend I use it to calculate mineral additions? Since I'm using BIAB and full volume mashing, should I just keep the sparge water at 0 or is there something else you would recommend? Thanks!

[BDawg]

Martin-

No, I wasn't confused. The example you gave is exactly what I had in mind when I said that percentage is a horrible way to express boiloff rates. I don't know about yours, but my boil off volume does not shrink the longer I boil. It seems to boil off approximately the same volume per unit time, all other things remaining constant. Stating that it boils off some constant percentage per unit time is still, IMHO, a useless and misleading statement. A 90 minute boil with a 15% per hour boiloff rate simply will not yield a final volume 77.5% of the original volume. Why should we use that as the measurement rate then?

[mabrungard]

Bdawg, I agree that the percentage rate wouldn't be reasonable if the calculation was integrating the interim losses in its calculation. My boil-off observations are also relatively consistent during the boil.

But reading my Promash instructions, it does say that the percent loss rate is based on the 'total' volume. I'm assuming that total is referring to the original kettle volume, but I'm not totally sure.

Hafmpty, for those of us with relatively modest total boil-off in the 10 to 15 % range don't really have to worry too much about the concentrating effect of the boil. But those with greater total boil-off would have a dilemma. I guess we just identified another feature that needs to be added to the program. It should be easy enough. Now if I just didn't have a real job that gets in the way!

[brewinhard]

[quote="mabrungard" I guess we just identified another feature that needs to be added to the program. It should be easy enough. Now if I just didn't have a real job that gets in the way![/quote]

Wait a minute...You have a real job too?

[Ozwald]

Just popping in to +1 BD's posts.