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An Inexpensive Diagnosis

for High pH/High TA

 

 

Here's a simple formula that works - 

if you remember your chemistry

by Clark Smith

Every winery's nightmare: Juice in the tank has a pH of 3.9 and a TA of 10 g/L. What in the world's going on? And what can be done? The dilemma can be agony: how to keep spoilage in check without ending up with a wine too tart to drink?

Who is at fault? Is this something that should be cured in the vineyard, or can adjustments be made at the winery? Should the grower concentrate on "farming for numbers?" Is there a flavor trade-off, or is this condition linked to poor flavor development?

The search for answers begins with the realization that there are really two different high pH/high TA conditions, with different causes and different treatments.

AHigh potassium@ is a common condition, often associated with high ripeness and stress, which is much more easily dealt with at the winery, and is sometimes a reasonable price the winery pays for rich flavor.

AHigh malate,@ or more properly Alow tartrate:malate ratio,@ requires more drastic steps such as double-salt treatment, and is frequently linked to overcropping, dense canopy, and low flavor and color.

The best winegrowing teams will proceed with an experimental program to address these questions. A problem that arises immediately is the cost and tedium of the analysis involved. Economical measurement of berry composition presents a daunting barrier to commercial vineyard research. On-site analysis involves substantial expense. A wet chemical method for tartrate does exist, but it requires a solvent hood and noxious materials which present disposal problems. Preservation of juice for analysis by private labs isn=t practical since freezing or fermentation alters potassium and tartrate content through precipitation.

The expense of specialized equipment for potassium determination and the inconvenience of tartrate analysis may be avoided if it is recognized that minimum precision will suit our purposes and that these values are not independent of other quantities we are measuring.

At the 1983 ASEV meeting, James Vahl observed that in tartrate-malate buffers such as grape juice, the following relationship can be applied:

 

                 -a x TA

pH = ___________________ + b

               total anion

 

                                    a x TA

so: total anion = __________________

                                    b - pH

 

where TA and anion are expressed as equivalents and a and b are weak functions of the tartrate:malate ratio as shown on the attached figure, which Jim derived by computer. It may be further assumed for juice that (if all quantities are expressed as equivalents per liter):

1. total anion = total cation

2. total anion = tartrate + malate (only for juice!)

    so: tartrate = total anion - malate

3. total cation = K+ + H+

    so: K+ = total cation - H+

Using equation 1's substitution, we get:

4. K+ = total anion - H+

Total H+ is measured by titratable acidity (TA), but to express it as equivalents per liter, we divide the normal expression by 75 gm/eq. Then:

5. K+ = total anion - TA

An iterative procedure (repeated stepwise refinement from an initial guess) makes use of these derivations. It actually works because a and b (see figure) are not strong functions of the tartrate:malate ratio, so approximate values yield a good preliminary estimate of total anion from a given pH and TA.

So much for theory. Here=s what you do.

1. Get good numbers for pH, TA and malate. These must be done on juice. TA and pH (using a reliable temperature-compensating instrument) are readily determined on fresh juice, and malate analysis may be done later on small samples preserved by freezing, by enzymatic analysis or farmed out by HPLC.

2. Make a guess about the Ta:Ma ratio (50:50 is as good as any), and consult the chart. From the ratio on the bottom line, read values for Aa@ (top) and Ab@ (bottom). Plug the values for a and b into the Vahl Equation. Solve for total anion.

3. Subtracting malate from total anion (Equation 2) gives a first approximation for tartrate.

4. Use the new Ta:Ma ratio. Go back to the chart for a new, refined estimate of a and b. Plug these back into the Vahl Equation for a more exact total anion estimate.

5. Continued iteration yields stable values for tartrate and total anion. Keep repeating steps 3 and 4 until the Ta:Ma ratio stabilizes. This usually takes only two or three iterations - maybe five minutes, once you get the hang of it.

6. Potassium can now be calculated from Equation 5.

In our experience, the method is usually good to within 5%, and provides a powerful quality management tool at practically zero cost. In these days of kilobuck instrumentation and gigabyte computation, this five minute solution with a hand calculator really beats workin=.

 

{Clark Smith's article "An Inexpensive Diagnosis for High pH/High TA" was originally published in the January/February 1996 issue of Vineyard & Winery Management magazine}

 

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