Author Topic: Temperature and pH  (Read 6321 times)

Offline tygo

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Temperature and pH
« on: February 27, 2011, 06:51:08 PM »
I was going to add this as an aside to Kai's pH tool thread but figured it might be better as a separate topic.  I've looked around on the web before and haven't been able to find a good conversion for pH by temperature.  We say "mash temp" and "room temp" but is there a good reference for what that means and how temp affects pH?

I'm sure that for home brewing this is one of those topics where close enough is good enough but I was curious.
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Offline nateo

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Re: Temperature and pH
« Reply #1 on: February 27, 2011, 08:32:54 PM »
IIRC, pH at "mash temp" is about -0.3 lower than at "room temp." Mash temp is generally 149-160, I'd say. But I've checked pH at the protein rest step, which is 120-132*-ish and gotten closer values to "room temp" something more like -0.15. There is a relation between temp and pH, but I don't think it's linear.

Just checked myself, and to quote Kai (quoting Briggs):  "Briggs quotes a difference of 0.35 in pH between a mash pH measured at mash temps and the same liquid measured at room temp. The pH measured at mash temp is lower "
« Last Edit: February 27, 2011, 09:16:01 PM by nateo »
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Offline sharg54

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Re: Temperature and pH
« Reply #2 on: February 27, 2011, 10:45:28 PM »
I don't know much about ph but I did run a short experament one time with my water to see what heat wouuld do to the ph levels and the hotter I got it the higher the ph would go and it only took a matter of seconds at boiling to blow it off the scale. In a mash however it acts totaly different depending on what malts you are using. With the use of say 10% crystal malts you can actually lower your ph by .03 to .05 depending on your grain bill and the use of acid malts can change it as well. I don't think there is actually a dependable standard chart you can get as there are to many varibles and the best thing you can do is monitor your ph with a good set of test strips and make adjustments as you go if need be. You can make a good base line from your water profile and grain bill but after that you need to teat and correct if need be on the fly.
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Offline mainebrewer

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Re: Temperature and pH
« Reply #3 on: February 28, 2011, 05:52:14 AM »
From what I've read, the target mash pH is around 5.3.
Is that a room temp or a mash temp measurement?
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Offline tygo

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Re: Temperature and pH
« Reply #4 on: February 28, 2011, 06:01:42 AM »
That's the mash temp pH target (or thereabouts).
Clint
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Offline mainebrewer

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Re: Temperature and pH
« Reply #5 on: March 01, 2011, 06:58:46 AM »
Thanks!
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Offline Kaiser

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Re: Temperature and pH
« Reply #6 on: March 01, 2011, 07:46:13 AM »
This comes up once in a while and I just had an e-mail discussion with a fellow home brewer on the same subject.

Fact is that the pH of a solution changes with temperature. It is caused by a change of the dissociation constants of the various acids/bases that are in the solution. Even water is considered an acid since it can donate hydrogen ions although in most cases it is not dominating pH at all. The extent of the change depends on the substance. Even the pH optimum of enzymes may shift with temperature. I believe that the0.35 correction factor for mash temp (65 C) vs. room temp (25 C) pH contains both the aspect that the actual pH in the mash is lower at 65 C that it is at 25 C and that the pH optimum of the amylase enzymes shifts a bit from the value that can be observed by room temperature mashing.

But none of this matters since by convention pH values in brewing are reported as the pH of a room temperature sample. This arises from the laboratory practice of cooling pH samples before pH is tested. This also means that reported pH optima and pH ranges are for room temperature samples even though the actual reaction happens at higher temperatures. A.J. deLange mentioned to me the “by convention” aspect. Another thing we express “by convention” is SG. By convention we always correct SG for temperature since we all assume that the reported SG applies to a 68 F sample. The same is and should be done for pH measurements. To be exact you’ll have to cool hot samples and warm cold (e.g. beer) samples.

It’s also helpful to take into account how we arrived at these pH optima/ranges. They are determined by conducting a series of mashes (at correct mash temp for that enzyme) with differing pH. The pH is tested in a room temp sample. The amount of product produced during these reactions (sugar, for example) is then plotted over this room temperature pH.

The same is true with boil pH recommendations.

One problem is that hardly any author is explicit about this. I assume that most of them see it as a given that they talk about pH from room temperature samples. Briggs was the only one I found that made a distinction. This lack of explicitness, if this is a word, seems to cause a lot of confusion with home brewers.

Kai

Offline johnf

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Re: Temperature and pH
« Reply #7 on: March 01, 2011, 08:25:23 AM »
That's the mash temp pH target (or thereabouts).

Optimal mash pH ranges are varying expressed as anywhere between 5.2 and 5.7, at room temperature. So 5.3 at either temperature would fall within that range.

The important point being that any professional or academic source quoting a pH or range would be talking about room temperature. Only homebrewers talk about pH at mash temperature because they mostly do not use pH meters and do not know that it is impractical to measure at mash temperature.

Offline glastctbrew

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Re: Temperature and pH
« Reply #8 on: March 01, 2011, 08:49:47 AM »
Ok, just want to make sure I've got this straight since I'm an accountant and not a chemist.  If I use a ph meter with temperature compensation, it is giving me a reading  at "room temp" as long as my sample is within the temperature range the meter is rated for, right?  For example.  I have a Milwaukee ph56.  The effective temp range is 23 to 140 F (-5 to 60 C).  As long as I have my sample is cooled to at least 140F the reading I get will be as if the sample was at 68F.
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Offline Kaiser

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Re: Temperature and pH
« Reply #9 on: March 01, 2011, 08:55:47 AM »
I think John Palmer started some of this confusion.

While he may have corrected this in subsequent editions the following is from the widely referenced on-line edition of How-To-Brew (http://www.howtobrew.com/section3/chapter15-2.html):

“When you mash 100% base malt grist with distilled water, you will usually get a mash pH between 5.7-5.8. (Remember, the target is 5.1-5.5 pH.)”

In this sentence he mixes room temp and mash temp pH values. The 5.7-5.8 base malt pH is correct when seen as the pH of a room temperature mash sample while the 5.1-5.5 pH target is only correct when seen as a mash temp pH target with a conversion factor of 0.35. With this the room temp sample pH target range is 5.45 – 5.85, which is more correct.

The pH optima that John cites for various enzymes seem to be mash temp pH values. He doesn’t quote a source but the only source that I found which lists mash temp pH values is Briggs’s Brewing Practice and Science book. In this he also gives room temp pH numbers.

Kai



Offline johnf

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Re: Temperature and pH
« Reply #10 on: March 01, 2011, 09:26:25 AM »
Ok, just want to make sure I've got this straight since I'm an accountant and not a chemist.  If I use a ph meter with temperature compensation, it is giving me a reading  at "room temp" as long as my sample is within the temperature range the meter is rated for, right?  For example.  I have a Milwaukee ph56.  The effective temp range is 23 to 140 F (-5 to 60 C).  As long as I have my sample is cooled to at least 140F the reading I get will be as if the sample was at 68F.

A pH meter without ATC would read different at two different temperatures for two reasons.

1. The way the probe works is that temperature in and of itself affects the reading regardless of actual pH.
2. The actual pH is different at different temperatures.

The ATC attempts to correct for the first. I say attempts because the ATC assumes the probe is in perfect condition and the sub $100 probes most homebrewers use probably weren't all that perfect out of the box and certainly aren't after months of use. ATC does nothing to correct for the second. Even if your probe is perfect, if you read at 140 and then again at 75 you will get two different readings because of the second factor.

Furthermore if you measure at 140F you will place a great deal of stress on the probe and dramatically shorten its life. Very inexpensive probes are around $40 these days so my choice though I am not a very cheap homebrewer is to not going sticking them in hot stuff when I can just leave the sample in a small steel bowl in the freezer for a few minutes.

In terms of having consistent readings that you can compare from batch to batch, you should try to always measure at the same temperature. If you want readings that you can compare to literature or other brewers and you don't want to be buying probes every couple of months 25C is the best temperature to do the readings at.

My advice is measure at room temp and forget you ever heard about mash temp pH.

Offline Kaiser

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Re: Temperature and pH
« Reply #11 on: March 01, 2011, 10:01:33 AM »
My advice is measure at room temp and forget you ever heard about mash temp pH.

I can only agree with what John is saying and a pH meter being labeled as “ATC” may lure some into thinking that it corrects for that ominous 0.35 pH shift that is commonly cited.

I’m even going so far to say that there is no use for the ATC feature in brewing. pH samples should be measured at room temp (I chose 25C since this is the temp at which the calibration buffers are correct) and thus no ATC is needed.

I have has varying methods of cooling pH samples. My current method uses a 12ml glass vial on a stiff copper wire that I use to take a sample of mostly wort. Then I dip this in ice water and stir with the thermometer until the reading falls below 28 C. After that I immediately transfer this into a small medicine measuring cup where I then test its pH. Most of my recent pH experiments also used this method.

Kai


Offline glastctbrew

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Re: Temperature and pH
« Reply #12 on: March 01, 2011, 10:17:30 AM »
Huh?  ATC?  Never heard of it ;D

This sounds like good advice.  I'll come up with a consistent sampling method that will work in my environment and never look back.

Thanks for the feedback.
Scott
Still Hill Brewery

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Re: Temperature and pH
« Reply #13 on: March 01, 2011, 10:36:59 AM »
Yep, no ATC is the way to go.  There are too many variables and an ATC pH meter's abilities do not cover all the changes in mash pH that occur with temperature change.  All pH measurement must be performed at room temperature.
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Re: Temperature and pH
« Reply #14 on: March 01, 2011, 10:46:58 AM »
And just so it's explicitly stated in this thread, room temp mash pH should be in what range?  5.2-5.6?
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