Testing for Acid content

[lockwood1956]

From Winemaker mag
http://www.winemakermag.com
Subscription is $45 per year (delivered) For UK


An acid testing kit is one of most important items in your lab. Acids, as most people know, are chemicals which, in high concentrations, are caustic and damaging. Their chemical opposites are known as bases or alkalis (such as lye). These are also, paradoxically, caustic.

When acids are dissolved in water, their molecules dissociate - break apart - with hydrogen ions being released into the liquid (some acids release more hydrogen ions than others, and thus are more "active" than others; food acids are typically very weak). Chemically, hydrogen ions have the symbol H+. Bases, on the other hand, dissociate by releasing hydroxyl ions, with the chemical symbol OH-. If the two encounter each other, they neutralize each other, combining to form plain water, while releasing energy in the form of heat.
Virtually all fruit contains acid, but fruit acids are mild, and are found in relatively small concentrations.In juices acid gives a sharpness or zest to the flavor; pineapple juice, for example, is higher in acid - and sharper in flavor - than the juice of ripe apples. In a wine, too much acid makes the taste unpleasantly sharp, but too little leaves it bland and uninteresting.
We can measure acid in two ways; in titratable acidity, which measures the total amount of fixed acid present, regardless of its strength; and in pH, which measures the activity or strength of an acid, regardless of its concentration. Although our taste is more correlated with acid activity than with its concentration, the latter is easier to measure, and - if we are dealing with the normal acids one experiences in food and drink - reasonably accurate.

Your home lab should therefore contain an acid titration kit. This consists of one or two Titrets - a syringe without needle, but calibrated in cubic centimetres (CCs), also known as milliliters (mL) - plus a plastic or glass testing jar and two containers of reagent (that's chem-speak for chemical solutions). One of these is sodium hydroxide, at a known strength of 1/5 normal (0.2N), and the other is phenolphthalein solution, which has the interesting property of being colorless in an acidic solution, but which turns red in an alkaline solution.

The process is simply to add a measured quantity of the sodium hydroxide to a measured quantity of the wine, which is acidic, and noting exactly when the color change occurs to indicate that all of the acid has been neutralized. This is a lot simpler than it seems and the process is as follows:

Using 15 mL of wine (to which 3 drops of phenolphthalein solution has been added) one adds, a small quantity at a time, a 0.2N sodium hydroxide solution (trust me; it works). The color change from white to pink indicates the neutralization of the acid. The magic formula is that the acid content of the wine, in grams per liter (or parts per thousand), exactly equals the number of mL of sodium hydroxide used; thus, 7.5 mL equals an acid content of 7.5 grams per liter, measured "as tartaric". Acid titration kits are available virtually wherever winemaking supplies and ingredients are sold.

Do be sure, though, that you are measuring apples with apples; British books use "as sulfuric" by their standards, and the values obtained are only 2/3 our measure of "as tartaric" equivalent. Also, as a rule of thumb, the sodium hydroxide should be replaced every six to nine months, but the phenolphthalein will last a couple of years.



See here for more info
If you use a US acid test kit it will express acid as Tartaric acid (left column on the sheet below))
UK kits express acid as Sulphuric H2SO4 (right column on the sheet below)
http://www.winesathome.co.uk/downloads/TAtestsheet.pdf

[lockwood1956]

You can also test for acid using a PH meter

free download here

http://www.winesathome.co.uk/downloads/PHmetertotestTA.pdf

Using a pH Meter in Titration to Measure TA.
Please read these instructions before using this equipment.

1.) A pH meter is a sensitive analog tool. The electrode is filled with electrolyte that must be rejuvenated before use to
assure an accurate reading. Always calibrate your pH meter before using.
Before using your pH meter it is necessary to soak the electrode for a minimum of 2 hours in pH Storage Solution
followed by a rinse (water) and minimum soak of 30 minutes with it’s switch “ON” in buffer solution 7.01 pH. Then
calibrate at the high end of your range for this test (7.01 pH is recommended). pH meters are most accurate at or near
their calibration points. Rinse before and after measuring your sample. Shake or blow on your pH meter to dry the
electrode and remove all excess water.
Always store your electrode in storage solution by filling the cap with a few drops and securing it over the electrode.
Rinse electrode after every measurement, shake or blow on electrode to remove excess water. “DO NOT
TOUCH” electrode!
If your pH meter has been idle for an extended period repeat the rejuvenating step listed above.

2.) Fill your Wash Bottle with distilled or DI water. You will use this to rinse your burette, syringe, beaker, everything
so it’s best to start with it full.

3.) Set up your Titration stand by bolting the steel rod to white base.

4.) Attach your dual burette holder to the titration stand, and insert your burette in the holder by squeezing open the
holder. Adjust the burette height so that is just above your 250 flask (sample).

5.) Close the valve (stopcock) of your burette before filling, the valve should be horizontal to close.

6.) Use your pipette to draw 15cc sample of wine (must) and place in 250 ml Flask or 50 ml Beaker. To draw a sample
into the pipette, apply a vacuum (you may gently suck on the red end) to the end of the pipette when inserted in your
sample. “Do Not Use This Pipette to sample the .2N Sodium Hydroxide”.

7.) Draw 20 cc of water into your pipette and add it your sample.

8.) Fill the syringe with 15-20 cc of .2N Sodium Hydroxide and gently squeeze the solution into the top of your
“Closed” burette. “Caution Sodium Hydroxide is Hazardous”. Avoid all skin contact, if contact occurs wash with
water. For eye contact wash with cool water for 15 minutes and seek medical attention. If swallowed contact your
poison control center immediately.

9.) Place an empty container under the burette (not your wine sample) and slowly open the burette valve (stopcock) and
allow 1-2 ml’s of solution to escape and fill the burette spout. Close the valve.

10.) Record the amount of solution in your burette. You will subtract the amount used from this number to determine
your TA.

11.) Measure the pH of your sample with your pH meter and place your wine sample (flask) under the burette.

12.) With one hand on your wine sample and the other on the burette valve, gently swirl the sample while “SLOWLY”
adding sodium hydroxide.

13.) Stop frequently and measure your pH with your meter. With practice you will be able to hold your pH meter in
your sample while swirling and adding sodium hydroxide.

14.) Titrate until your pH reads 8.2. Slow down as your pH approaches 8. It is easy to over shoot your target.

15.) Record the ending solution level in your burette once you’ve achieved a pH of 8.2 in your sample.

16.) Subtract the recorded starting solution level from the ending solution level to determine your TA. Your TA equals
.1 times the amount of .2N Sodium Hydroxide use to reach a pH of 8.2 in your sample. If you used 5 ml of solution
to reach a pH of 8.2 your TA is .5g/L.

17.) After testing, place your sodium hydroxide container under the burette and pour the balance (if any) back into the
container for later use.
Clean up – place an empty container under the burette and fill the burette with water from your wash
bottle. Then open the valve and repeat until clean. Be certain to rinse your syringe and all containers as
well. Best Wishes on a Successful Project!

[lockwood1956]

Further info

http://www.wineperspective.com/the_acidity_of_wine.htm

THE ACIDITY OF WINE
By Alexander J. Pandell, Ph.D.
Ó 1999 by Alexander J. Pandell, All Rights Reserved

Acids are very important structural components of wine. If a wine is too low in acid, it tastes flat and dull. If a wine is too high in acid, it tastes too tart and sour. Usually, the winemaker can easily manipulate the acidity.
What does it mean when a wine label states the total acidity is 0.60 % (0.60 grams acid per 100 mL) and the pH is 3.5? What follows is a primer on the role of acids in wine and an explanation of concepts such as total acidity (TA) and pH.
THE PRINCIPAL ACIDS FOUND IN WINE
The principal acids found in grapes, and therefore wine, are tartaric acid, potassium hydrogen tartrate (cream of tartar), malic acid and potassium hydrogen malate. Tartaric acid and potassium hydrogen tartrate predominant in wine. Since potassium hydrogen tartrate and potassium hydrogen malate are derivatives of tartaric and malic acids, respectively, only tartaric and malic acids will be discussed with the understanding that their derivatives are also present in wine. The relative amounts of tartaric and malic acids vary depending on the grape variety and on where the grapes are grown. For example, in Burgundy, the Chardonnay has a lower concentration of malic acid than the Chardonnay grown in the Napa Valley of California. We will come back to that later.
VOLATILE ACIDITY
Both tartaric and malic acids are nonvolatile which means that they do not evaporate or boil off when the wine is heated. This is to be distinguished from volatile acidity (VA) in wine that represents acetic acid (vinegar). Acetic acid does boil off when heated, and high VA is undesirable in a wine. A VA of 0.03-0.06% is produced during fermentation and is considered a normal level.
CLIMATE: ACID vs. SUGAR
Tartaric and malic acids are produced by the grape as it develops. In warm climates, these acids are lost through the biochemical process of respiration. Therefore, grapes grown in warmer climates have lower acidity than grapes grown in cooler climates. For example, Chablis (France) produces grapes with high acid because the climate is very cool, while Napa Valley produces grapes with lower acidity because the climate is warmer.
Sugar production is the complete opposite of acid production. The warmer the climate the higher the sugar content of the grapes. Sugar content of grape juice is expressed in percent (%) or ° Brix (e.g., 24 % sugar is equal to 24° Brix).
In summary, warmer climates result in high sugar and low acid whereas cooler climates result in low sugar and high acid. The Chablis region of France is a very cool region and normally produces grapes with low sugar and high acid. The big concern in Chablis is getting enough sunlight and warmth to get reasonable sugar levels. In low sugar years, they are allowed to add sugar to the grape juice. The process is called chaptalization.
The addition of sugar in winemaking is not allowed in California. However, the addition of tartaric acid (and others acids) is allowed to increase the acidity of the wine.
ROLE OF THE MALOLACTIC FERMENTATION
The malolactic fermentation (MLF) is an important natural process for adjusting acidity. The MLF lowers the acidity by converting malic acid to lactic acid and carbon dioxide. Many white wines are encouraged by the winemaker to undergo MLF and almost all red wines "automatically" undergo MLF. Although it is usually difficult to stop in red wines, many winemakers inoculate to control the timing of this important secondary fermentation. The acid is so high that Chablis requires a malolactic fermentation (MLF) to lower the acidity. Since some wines have less malic acid in them than others, the MLF is not as significant in shaping the wines as in those with a higher malic acid content. For example, a White Burgundy typically contains less malic acid than a Napa Valley Chardonnay. Therefore, when a white burgundy undergoes MLF, very little acidity is lost and the character of the wine is preserved. On the other hand, a California Chardonnay contains more malic acid so when it changes to lactic acid the acidity can change appreciably.
The problem in cool climates is too much acid whereas the problem in warm climates is too little acid.
TOTAL ACIDITY
In the U.S., the total acidity (TA) of a wine is measured assuming all the acid is tartaric. This allows one to determine a value for total acidity that is consistent. A high TA is 1.0%. Most people would find this level of acidity too tart and too sour for consumption. A low TA, say 0.4%, results in flat tasting wine that is more susceptible to infection and spoilage by microorganisms. Most red table wines are about 0.6% total acid. White wines are usually a little higher.
pH
I will not provide you with the complicated mathematical definition, but I will say that pH is a measure of a solutions acidity and is analogous to the Richter scale used to measure the intensity of earthquakes, since both scales are logarithmic. For example, wine with a pH of 3 is 10 times more acidic than a wine with a pH of 4. The thing to remember about pH is that the higher the pH, the lower the acidity, and the lower the pH, the higher the acidity. So a wine with a pH of 4.0 is LESS acidic that one with a pH of 3.6. Although total acid and pH are related, they represent different ways of measuring acidity of wine. The pH can be measured with a pH meter, an instrument that determines pH quickly and easily. It represents the active acidity of the wine. If the pH of a wine is too high, say 4.0 or above, the wine becomes unstable with respect to microorganisms. Low pH inhibits microorganism growth. Tartaric acid is sometimes added to fermenting grape juice in California to insure that an acceptable final pH can be realized, since some acid is lost during fermentation thus reducing the total acidity and raising the pH.
TYPICAL VALUES FOR pH AND TA
A typical premium California Chardonnay has a total acidity of 0.58 grams per 100 mL (0.58%) and a pH of 3.4. It is interesting to compare these values with a total acidity of 1.10 grams per 100 mL (1.10%) and a pH of 2.91 found in a late harvest Johannisberg Riesling with 21% residual sugar. Generally speaking, sweet wines require a higher acidity than table wines to balance the high sugar. This is true for Sauternes, Alsatian SGN and German TBA wines.
SUMMARY OF IMPORTANT POINTS
· The principal acids of wine are tartaric and malic.
· Volatile acidity (undesirable) is due to acetic acid (vinegar).
· Cool climate grapes have high acid and low sugar.
· Warm climate grapes have low acid and high sugar.
· The malolactic fermentation can be used to lower acidity of wine.
· Total acidity is reported as grams of tartaric acid per 100 mL of wine.
· Table wines generally have a total acidity of 0.6 to 0.7%.
· Sweet white dessert wines generally have a total acidity above 1% to balance the sugar.
· pH is a measure of "active" acidity.
· The lower the pH, the higher the acidity; the higher the pH, the lower the acidity.
· Table wines generally have a pH between 3.3 and 3.7.

[lockwood1956]

How to use Acid testing kits

Testing the Acidity of your must and adjusting if neccesary, is a very important aspect of your winemaking if you wish to produce quality wines.

I purchased a very expensive acid test kit for this purpose, and I am very happy with it for white wine testing, but for red wines the colour change is not so easy to see.

Test kits are slightly different depending on what tye they are, but they all work on the same principle, you take some of your must, add a colour changing agent and add the test solution until the must sample changes colour. some express the titrateable acidity in ppt (parts per thousand) as sulphuric acid (most UK and French test kits) others express the acidity as g/l (grammes per litre..which is the same as parts per 1000 as i litre weighs 1 kilo) as long as you know which type of test kit it is, you are good to go (it awill tell you in the instructions.

It's all very easy once you get over the initial uncertainty.

I purchased a Ritchie's acid test kit in order to use it and do a photo tutorial, much to my surprise I found it very easy to use and also much better than my expensive test kit when it came to detecting colour changes in red wine must samples. although my test kit is more accurate this one is plenty accurate enough and will give good results to 1/4 ppt, so I urge you all to try it out and see how you get on with it, it WILL help you make better wines.


here is a chart explaining how it works, and it will also help you to convert from ppt Tartaric to ppt Sulphuric, if you need to adapt a recipe that has measurements in the "other" form of acid

http://www.winesathome.co.uk/downloads/TAtestsheet.pdf

as you can see I bought the test kit from hop and grape and it cost me £5.20

[lockwood1956]

Using the syringe provided put 5ml of must in the test tube (sorry for the dodgy pic)

here is a white must sample, note if the musyt has started ferenting you will need to drive off the CO2 it will contain by either vigorous shaking or boiling the sample ( blast it for 20 secs in a microwave on high..allow it to cool before you test it though )

[lockwood1956]

Add to it some distilled water, to about 2 inches in the tube, the amount isn't too important as it wont affect the test results

[lockwood1956]

Add 3 to 4 drops of the indicator solution, and shake

[lockwood1956]

Note... Kitchen roll wrapped around the neck of the test tube to prevent sodium hydroxide getting on your hands (ouch)

Add the sodium hydroxide slowly, it will turn pink when you put it in the test tube, but will dissapear if you shake it, once it doesnt dissappear when shaken, but remains pink, (see picture) record the ammount of mls of sodium hydroxide you have used and this is your ppt figure, for example if you used 4ml of it then your ppt figure is 4

discard the test solution...do not put it back into your must.

ideally you want your must to be in the3.0-4.5 range.
if it isnt then you need to adjust, depending on what kind of fruit you have, for example you would adjust the acidity of grape musts using only tartaric acid, so you may for example wish to add tartaric and Citric to plums, as they are high in malic acid anyhow, so you would use an acid that would compliment the original fruit.

here is a chart detailing the acid content of common fruits

http://www.winesathome.co.uk/downloads/sugarandacid.pdf

[lockwood1956]

And for red must tests

5ml red must with distilled water added

[lockwood1956]

And colour change looks like this, obviously its not as easy to see as in the white must test, but it's pretty clear when the change happens.

Hope all of this helps
regards
Bob

[Hippie]

Doesn't the distilled water dilute the test sample, and what makes up for that?

[lockwood1956]

well yes and no

the distilled water makes the test solution less acidic from a % point of view, but, even though the test sample is less acididic, there is still the same ammount of acid in the sample, and therefore it takes the same ammount of reagent to change the colour, I kno it doesnt necessarily seem logical, but ive tested it several times using just must and must + distilled water, and the results are always the same. (i tested soooo many times as it didnt make sense to me either.


let me express it this way, for arguments sake you have a 5ml sample that contains 4 g/l of acid, (which for arguments sake is "x" ammount of acid) you dilute it with distilled water, the sample is now 25 ml, but it still contains the same ammount of acid ("X") that needs to be neutralised with the reagent, no matter how much the sample is diluted, it still takes the same ammount of reagent to turn the sample pink.

does that make sense to everyone?

it took a while beofre it made sense to me.

[Hippie]

Yep. I reckon that explains it well. The ph would likely go up with the water dilution, but still the same amount of acid in it.

Thanks Bob.

[lockwood1956]

yay!