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Cider Digest #0846
Subject: Cider Digest #846, 29 January 2000
From: cider-request@talisman.com
Cider Digest #846 29 January 2000
Forum for Discussion of Cider Issues
Dick Dunn, Digest Janitor
Contents:
Cider Digest #844, 13 January 2000 (Dave Burley)
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Subject: Cider Digest #844, 13 January 2000
From: Dave Burley <Dave_Burley@compuserve.com>
Date: Thu, 13 Jan 2000 15:16:00 -0500
James Elphick's explanation of how two ciders could have the same pH but
taste like they had a different acidity is pretty close to correct for
most of his explanation.
I discount his idea that pH meters can vary a lot from day to day as an
explanation. If that's what he was saying. However, the things Jim says
about degrading buffers (most are preserved OK) and electrodes degrading
(keep yours in buffer solution not water) and such can be accounted for
properly. So pH meters can be reliable instruments if properly cared for.
The taste of acidity is in fact due largely to the undissociated acid -
since there may be more of it, but also relates to the free acid since it
is also titrable. (more later) and not due to the pH which as Jim said is
related to the dissociated acid. From freshman chemistry we learned that pH
of an acid solution is directly related to acid content in *unbuffered*
solutions, but not so tightly in buffered solutions. Buffered solutions
are those that contain the acid and salts of the acid, for example. This is
what we have in cider, wine and fruit meads. So a pH reading has little
resemblance to titrable acidity and taste in many cases. The higher the
extract (more acid and acid salts), the less the resemblance, I guess..
Historically, the titrable acidity of wines was considered to be directly
related to the acid taste. Most recent work in the area of wines and acid
taste relates that acid taste is related to both titrable acidity and pH.
Thus:
X = TA (in g/l) - pH
Wines (and I assume in the realm of ciders and meads) with the same X
value will taste the same as far as acidity goes. Or so goes the theory.
TA in wines range from about 5 (low) to 11 (very high) and typically in
the range of 6 to 7. pH is typically in the range of 4 (high) to 3 (low).
So X values for normal wine are in the range of X = 6.5 - 3.5 = 3. In any
event, we see that even though pH is brought into this equation, in
buffered solutions, like wines and ciders and fruit meads, the pH range is
small (why they are called "buffers") and the main influence is from the
much larger ranging titrtable acidity. This connects the history of TA and
acid taste with the present interpretation. Perhas as we get into things
like non-fruit meads and such with low buffer content, the pH will be more
important.
Something is wiggling around in my bean about the reason for the negative
sign on the pH but I can't recall it. I don't think it made sense to me
when I read it. I know pH = -log(Hion), but that doesn't do it for me,
since this relates to the exponential sign and not the value of the pH.
Anyway, TA is a good start. What I don't know is if this TA is that of the
acid or is using sulfuric acid as the base. I assume it is tartaric acid in
wines as the standard, as this is a modern concept. The only thing that
puzzles me is that there was a note that said "this is 10 times the normal
acidity numbers". Which would be true if TA was expressed as a %. Which it
used to be when we used sulfuric acid as the standard. The concept should
be the same, except the TA will be different by the MW factors (really the
normality) of sulfuric acid/tartaric acid. Always give the standard used
when expresssing TA.
Dave Burley
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End of Cider Digest #846
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