Measuring acidity in fruit and juice samples
Dr. Beth Chang and Dr. Joy Ting
In order to make good decisions about additions to juice and wine, you need to have a good measure of what you are starting with. Much of the information you need can be obtained in the winery lab with minimal equipment, time, and expense.
pH
The most important measurement is the pH of the wine. To ensure you have an accurate measure of pH, you must have a good working pH meter that is properly calibrated. Once you get your pH meter up and running, you will also use it for acid trials and measurement. Though each pH meter is different, there are a few principles to keep in mind when calibrating and measuring pH of grape juice and wine.
pH Protocol and Best Practices
Acid trials
There is a complex relationship between the concentration of acids in grape juice and wine and their effect on the pH of the solution. Grape juice and wine are buffered solutions, meaning there are a number of components in the juice that will resist changes in pH when acid is added (known as the buffering capacity)(6). The best way to determine how a juice or wine will respond to addition of acid is to measure it yourself on the lab bench. This process is very simple as long as you have a well calibrated pH meter (link again), and a micropipette.
Titratable acidity
Technically, titratable acidity is the sum of all of the free (dissociated) and bound (undissociated) protons in a solution. This measurement correlates to the total amount of acid molecules in wine including all of the tartaric, malic, citric, lactic, acetic and succinic acids as well as which form they are in (6,7). The TA of a juice or wine can give you a good indication of the sensory perception of acidity. Though there is a little bit of initial set-up, TA can also be measured easily in the winery lab with a well calibrated pH meter, a burette and a stir plate.
Sending out samples to service labs
If you want to verify your in-house readings, or if you want additional tests such as malic acid and YAN, you may consider sending samples for analysis at a service lab. Be sure to follow any instructions given by that lab in terms of sample size, hours/days of delivery, and sample preparation. Without intervention (use of preservative, freezing or boiling), grape juice samples will likely begin fermentation during shipping, altering several chemical parameters. Due to potential of tartrate precipitation, juice and wine samples that have been frozen may have some alteration of pH and TA (malic acid and YAN will remain the same with freezing). To determine the best method of shipping for your samples and desired analysis, it is recommended to contact the lab and ask before sending samples. Lab personnel are generally very friendly and informative and can be a huge help when determining the best course of action.
