“pH and Acid Titration”

W.C. PAETZ - January 21, 2004 Michigan State University

Abstract:

During our lab exercise we followed the protocol for discerning accurate measures for pH and titratable acidity for six different wines. As a prelude to this analysis the calibration of the equipment and staging of samples was carried out to facilitate consistent and expedient laboratory results.

INTRODUCTION:
A comprehensive program for grape and wine analysis includes several critical metrics. Being able to discern the pH and titratable acidity are essential skills. These factors come in to play at every stage of wine production from harvest, as juice or must, after primary fermentation, during problem solving, prior to and during aging and finishing and especially prior to bottling.

For the enologist to safely and to effectively produce accurate results, they must be familiar with the equipment and the tests.

With regard to pH, this factor must be known to control wine quality, to avoid spoilage and to strategically use chemicals and acids to protect the final product.

With regard to titratable acidity, “(1.) Fruit, including grapes, contain different types of acids. Too much acid renders a wine undrinkable. On the other hand, if wine does not have enough acidity, the result is flat or insipid tasting wine. That’s why it is important to check the acid levels and adjust if necessary.”

“(2.) There are two ways to measure an acid. They are by titration for total titratable acid and by measuring pH. One of the hardest things to understand is that they do not measure the same thing and do not correlate with one another. In my mind if they both measure acid, one should be able to convert one to the other but it just doesn’t work that way.”

“(2.)Acids are molecules and they must be made up of at least two atoms. At least one of those atoms must be hydrogen. This hydrogen ion is the magic part of acid, has a positive charge and is written [H+].” Titration measures the total [H+] ion concentration, the ones floating around in solution, plus the ones still holding on to their constituents. The pH measures only the ones that are dissociated and floating around in the solution. Titration measures how many could react. The pH measures how many are ready to react.”

“(3.) Like a lot of other things in life, pH is what you make of it. It is easy to learn to use pH, and you don’t really need to understand it. Most of us don’t understand the cars we drive or the TV sets we so blithely manipulate. Start by recognizing pH as a very important number that can help you make better wines.”
Total titratable acidity is not a measure of total acid, but of the acid that is available to react with the NaOH solution with which we titrate, pH is a measure of the acid strength in the must or wine. It is the pH, rather than the total acidity, which indicates the ability of the must to resist oxidation and invasion by bacteria, and which determines how much SO2 is needed.

These are some of the reasons that support this study.

Materials and Methods

Titratable Acidity:
Equipment:
1.) 250 ml Erlenmeyer flasks
2.) 10 ml Volumetric A Pipet
3.) Titration assembly (burette set up)
4.) 0.1N Sodium Hydroxide
5.) pH meter
6.) Wine Samples
7.) magnetic stirring bar 3/8
8.) distilled water

Method:
1.) Standardize pH meter.
2.) Fill burette with 0.1 N NaOH.
3.) Add distilled water to ensure pH meter covered.
4.) Rinse pH electrode with a few ml of distilled water.
5.) Place in wine sample
6.) Position burette so outlet is above beaker.
7.) Place and activate stir bar.
8.) Tartrate the pH of sample to 8.2
9.) Read measure of NaOH and record this volume.
10.) Multiply by .75 Titre value.

“pH Measurement of Wine Sample”
Equipment:
1.) Corning Scholar 425 pH meter
2.) Beakers with various samples
3.) Distilled water
4.) Wipes to dry electrode probe.
5.) 3/8 magnetic stir bar

Method:
1.) Standardize the pH meter.
2.) Place stir bar.
3.) Immerse the electrode into wine sample beaker.
4.) Slowly activate stir bar in sample beaker
5.) Take reading of stabilized pH from meter and record.
6.) Rinse and wipe electrode and place in storage solution while preparing next sample for analysis.

Results
(Figure 1)
Cultivar pH TA
Carbonic Maceration Foch 2.95 9.6
Delaware 3.02 7.6
N.W. Vignoles 3.36 12.4
Sparkling Foch 2.91 12.4
Late Harvest Vignoles 3.12 14.9
Frontenac Juice 3.03 18.5

Discussion
There is a need to exercise caution when using a pH meter as pH is exponential and quickly moves from pH 7 to pH 8. The use of this equipment when analyzing red wines is critical as the ability to discern the end point when using Phenolphthalein or another color indicator is quite problematic.
According to Jackson, (5.) “A pH range of 3.1 – 3.4 is the goal for white wines and 3.3 to 3.6 for most red wines.”

Conclusion
This study was not only worth while but extremely important. A person can not consistently make desirable wines with out a profound understanding of these analytical tests and ensuing results. One needs to be able to confidently follow the test procedure and know what the results mean in terms of the status of the wine and what desirable metrics should be aspired to as well as what action needs to be taken to correct and protect the wine.

Appendix

Refer to figure 1 in text.

References:

1.) Martin Jordan, Host of Video, 1995 “Making Wine”, The comprehensive how to video for making wine in your home. Bacchus Productions Inc.Toronto, Ontario
2.) Acid Titration, G. Goodwine , http://www.geocities.com/medievalbrewers/acid.html
3.) J.E. Underhill, pH Without Pain, The Grapevine a news letter for winemakers in British Columbia.
http://www.bcawa.ca/winemaking/ph.htm
4.) R.S. Jackson, Wine Science Principles Practice Perception, Cool Climate Oenology and Viticulture Institute, Ontario Canada. (2000), Page 240

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