A lot of discussion here lately has revolved around acidity in wines. It appears to me that there are two valid but by no means equivalent ways of measuring acidity: Total Acidity (TA) and pH. I know that pH goes up as acidity goes down (or as alkalinity goes up, but this is not an issue with wines). It would also seem obvious that acidity would go up as TA goes up.
From a practical point of view, how are these two measures related? Perhaps a better question would be: What is their relative importance with regard to the sensory evaluations of different wines?
Vino To reply, add "x" between letters and numbers of e-mail address.
le/on Sat, 17 Apr 2004 04:42:12 GMT, tu disais/you said:-
Not quite as obvious as that.
This is somewhat technical, Vino, but given that Vincent seemed to be treating them as if they were wildly different, I think it might be worth explaining a bit.
Total acidity is (as I understand it) the sum of the weights of the various acids (malic, lactic, citric, tartaric (especially)) present in a litre of wine. I'm not sure how it's calculated, (whether by titration, for example, or by estimation from pH) or how it's expressed - I warned you this was technical, but one MAY take the view that it is most useful to express total acidity as IF all the acids were Tartaric, because that's the predominant acid present so you titrate the acids and then work out what that would give in grams/litre of Tartaric acid.
As for pH, this is a measure - quite simply - of the proportion of Hydrogen ions in the solution. Some acids dissociate more than others, so they taste more acid and have a lower pH than others. In winemaking a common example is of malic and lactic acids. Malic acid is a "stronger" acid than Lactic, as it is more dissociated, it tastes sourer and has (in equal gram molecular strengths) a lower pH. However if one were measuring JUST the acidity in grams, you might have MORE lactic acid, therefore a higher TA, but end up with a wine that was less sour, due to the fact that lactic acid is a weaker acid. OK?
Well, when wine undergoes malo-lactic "fermentation", the malic acid is converted to lactic (I've absolutely NO idea of their valency or chemical formulae, btw) and the pH drops, as you know. However, there's no reason to expect the total acidity to change much, IF this is a real measure of the weight acids present.
So pH is a measure of how highly acid the wine behaves, and sour it tastes (which CAN be masked by lots of sugar, obviously). Total acidity is a matter of simple weights, but doesn't necessarily relate to the freshness of the wine. It would, if all the acid was always Tartaric, of course.
More insults Ian? "wildly different"? What the heck are you talking about. anything to make me look wrong while you said just about the same thing I did with more words. You are slime.
Hi, Ian - I can elaborate a little on this topic. TA (titrable acidity) is measured by titrating a known volume of a degassed (to remove error from dissolved CO2) sample of wine against a standard NaOH solution, using either phenolphthalein for an indicator or while observing the pH with a meter. The result is expressed as though it were all tartaric acid in the sample. IIRC, the Europeans express TA as sulfuric - which gives a number that differs by a scalar.
Right concept, but ML causes the pH to _rise_; the TA drops.
Malic acid is diprotic, meaning it has two active hydrogens. The conversion to lactic acid (which is a monoprotic acid) is a decarboxylation of malic acid by leuconostoc oenos (or similar strain) of bacteria, releasing CO2 gas in the process and losing an active hydrogen:
COOH-H2OC-H2C-COOH ==>
CH3-H2OC-COOH + CO2
However, there's no reason to
Because malic acid is generally a minor component of the TA there will usually be only a slight decrease in the TA upon completion of ML.
le/on Sat, 17 Apr 2004 18:41:03 GMT, tu disais/you said:-
Aha!! Thanks.
Blast, yes of course, stoopid mistook.
Given that I misunderstood TA to mean total acidity. not titrable acidity.... Sigh. And I'm sure I've seen references to TA meaning Total Acidity. Never mind.
I hope therefore that between us, vino has a better idea.
The concept of pH is quite general. It is related to the concentration of H+ hydrogen ions in a solution on a log scale so that the numbers cover a convenient range rather than having to write many leading and trailing zeros. Actually we must consider the concept of activity rather than just simple hydrogen ion concentration, but this is meaningless to those who have not studied physical chemistry or chemical thermodymaics, and concentration is close enough for most practical uses. The reference standard for pH involves use of standard hydrogen electrodes using a reference solution and a standard solution, and measuring the electrical emf generated. I did this once in a physical chemistry course lab, and it is anything but user friendly. The so-called glass electrode made pH meters possible for laboratory use without so much bother. With modern electronics, pocket pH meters are possible, and one good enough for most practical industrial and home purposes can be had for under $US 100 these days. To be accurate, they must be tested against a standard buffer solution from time to time and adjusted to give the corect pH for the buffer. In addition, there are pH paper strips to be dipped into a liquid. They give different colors for different pH ranges. There are wide range strips to get the approximate pH and narrow range strips to more accurately approximate the pH. The strips are good enough for many industrial applications.
Liquids that have a pH below 7 are called acids. The lower the number, the stronger the acid. Liquids with a pH above 7 are called bases. The higher the number, the stronger the base, with 14 being at the top.
Here are a few approximate pH values for several common substances to give you a feel for the range. These are taken from the 64th ed. of Handbook of Chemistry and Physics.
The pH values for acids to follow are of the same concentration(0.1 N) unless otherwise noted .
Hydrochloric -1.1; Tartaric,Malic,Citric - 2.2; Acetic 2.9; Carbonic(saturated) - 3.8; Boric - 5.2. Thus hydrochloric acid is very strong and boric acid is quite weak.
The pH values for some 0.1 N bases follow.
Sodium hydroxide(lye) - 13; Ammonia 11.1; Sodium Bicarbonate(baking soda) - 8.4. Thus sodium hydroxide is a very strong base and sodium bicarbonate is a rather weak one.
Typical pH ranges for body fluids follow.
Human blood - 7.3 to 7.5; Human saliva - 6.5 to 7.5; Human gastric contents - 1.0 to 3.0. The stomach produces hydrochloric acid to digest food which makes the stomach contents extremely acid. Other body fluids are introduced and decrease the extreme acidity as the food goes down lower in the digestive system.
Typical pH ranges for a few foods follow
Limes - 1.8 to 2.0; Lemons - 2.2 to 2.4; Oranges - 3.0 to 4.0; grapes -
3.5 to 4.5; Drinking water - 6.5 to 8.0; Wines - 2.8 to 3.8; Tomatos -
4.0 to 4.4; Apples - 2.9 to 3.3.
Keep in mind these are just typical values for foods. You often can find a few exceptions both at the lower and upper end of the range.
I was going to ask that, as the context of Ian's message suggests pH would rise, lactic being the weaker acid. Just a "slip of the tongue", I'm sure. And thanks Ian, for a good explanation.
Yes, but the wine has a buffer capacity which means that the relationship between pH and TA is not linear. Thus, one may need to add lots of acid to get a minor change in pH until the buffer capacity is overcome, in which case linearity occurs for a certain interval. The buffer capacity is caused by salts, such as potassium tartrate and potassium acid tartrate in solution. So it's fair to say that titratable acidity is really a measurement of the absolute concentration of the acid anions, whereas the pH is a measurement of the acidic portion of the cation concentration (H+).
Already answered.
Perceived acidity is a complex function of both, but there is no clear relationship such that a sincle perceived acidity can be related to different values of TA and pH. There tend to be other factors as well. Lower pH wines tend to taste "fresher" than higher pH wines at the same degree of perceived acidity.
A left-handed compliment, to be sure. I like it! :^D
Hey, wait a minute. It hasn't been established that VV is an American, so please don't be so quick to blame him on US! He's more likely a Spaniard or Argentinian who was kicked out of his native country for being so obnoxious. I infer that he is not a native of the USA from his grammar and syntax, which strike me as foreign-born.
le/on Sun, 18 Apr 2004 08:00:09 GMT, tu disais/you said:-
Absolutely right, and I was fumbling around with this concept, trying to work out how to bring it up. Thanks for doing so with admirable clarity.
Excellent, ad I'm sure you or Mark can explain this last part if anyone doesn't understand.
Well put. I have read of a recent tasting of rieslings. Very interesting, though I doubt that I ought to say more without express permission by email from the person concerned. In this, TA varied from 6.6 g/l to 9 g/l, though I don't know if these were ALL expressed as Tartaric acid, as some were from Europe. At the same time, the pH varied from 2.9 to 3.32, with, not surprisingly the wine containing 9/l having the lowest (got it right this time, dumbo that I am) pH. BUT might surprise, and which I found interesting is that as you suggest, there was no simple correlation between pH and TA.
Note that domaine aaaa had quite a bit higher TA, but almost identical pH. The alcohol contents were the same, but yet the sugars were wildly different. While it's logical to guess that the wine with a higher acidity would taste fresher, in the case of these two, I take leave to doubt it. In fact I think the difference in perceived acidity would come from the widely different sugar contents, rather than the acidity, particularly in view of their VERY close pH.
From the figures here, the trick question is, "which of these two wines came from further north?".
That said, there is a certain "je ne sais quoi" about his (happily) inimitable phraseology, which makes me suspect that he might not be a native anglophone. If I had to plunge for a country, I'd say "France". In fact, at first I wondered if he wasn't a well known troll from one of the french language news groups, currently posing as a resident of Corsica.
Call me a fool if you like,, but I understand your resentment against Americans being that you are a lying, French bastard. I have nothing in common with you, nor do I want to. I take a Frenchman's disagreement with me as a compliment. Your live in a nation of pigs and scoundrels yet you call yourself superior. I don't like your kind and I don't like your nation. If it wasn't for the USA you would be drinking German beer and burning Jews. I understand your jealousy,, but attacking me wont make up for what you and your country lack.
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