Friday, May 2, 2008

Care and Feeding of a Baby Wine: The Acid Test

Acidity and pH go hand in hand.  The last winemaking post discussed testing and adjustment possibilities regarding the pH of your wine.  This one will explain how to test the acidity of juice or wine.

There are kits and instructions on doing an acidity test that involve color change.  The problem is that, while this is easy with white juice, it is very difficult with red.  The best way to test total acidity is with a pH meter.  Place 5 ml of juice in a container that will hold about 150 ml (about 5 ounces) of liquid.  Using a 10cc syringe (like for hypodermic needles but without the needle, of course) works great for doing these measurements (1 cc = 1 ml).  Add enough distilled water to cover your pH probe.  

A .10N Sodium Hydroxide solution is used for this test.  The "N" stands for "normal solution" which is a complicated chemistry term that you don't have to worry too much about.  Suffice to say that Sodium Hydroxide solutions are sold in varying strengths represented as a percentage of N.

These solutions can be made in the lab from stronger ones or bought at the correct strength at your local wine supply store (recommended for amateurs).  Draw 10 ml of .10N Sodium Hydroxide into your syringe.  Immerse the pH probe in the wine/water solution.  Add drops of the sodium hydroxide to the wine/water, mixing each time.  When the pH meter reads 8.2, you have reached your end-point.

The calculation to determine your total acidity is as follows:  The amount of sodium hydroxide you added (10 ml minus the amount of sodium hydroxide you had left over) X .10 (the strength of your sodium hydroxide) X 1.5.  By using this calculation, you can actually use any strength sodium hydroxide within reason.  However, if it is too strong you will get to your endpoint too quickly, and use so little, that there will be little accuracy in your calculations.

In order to be certain of the strength of your sodium hydroxide, you can get a chemical called Potassium Acid Phthalate, available from Presque Isle Wine Cellars, among other places, to test the strength of your product.  Their directions are simple and the stuff is inexpensive and lasts a very long time.  You never again have to worry if your sodium hydroxide has weakened with age.  Whatever strength this test indicates for your sodium hydroxide, simply use that number in place of the .10 in the above equation. 

So here is an example of the calculation.  You started with 10ml of sodium hydroxide and have 4.3ml left, meaning that you added 5.7ml.  5.7ml X .10 X 1.5 = .86 total acidity.  If you tested your sodium hydroxide and it is weaker, say .095, your calculation would be 5.7ml X .095 X 1.5 = .81 total acidity. 

Depending on the style of wine, you may want to raise your total acidity.  A general guideline is that you want a somewhat higher acidity if you plan to sweeten the wine later so that there is nice balance and not what they call a flabby or cloying wine.  Also, some acidity is lost during the fermentation and cold stabilization (to be discussed later) so you need to keep that in mind.  I usually like my white juice to be between .70 and .80 to begin with.  Reds, I like somewhat lower, at .60 to .70.  It is important to not have a too high acid finish on a dry red wine as they are more desirable with a soft, full mouthfeel.

Next week, I’ll cover ways to adjust the total acidity of your juice.

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