Principals and Practices of Bench Trials

Presented by Dr. Sandy Kirk            2019-09-10 

Ideally we do a perfect job of adjusting juice or must and finish up with a gold medal wine that requires no adjustment prior to bottling.  More realistically an assessment of the wine may indicate that it is out of balance with respect to acid/sugar balance or too low or high in tannin. So the question arises of how best to adjust it prior to bottling.

Ted Underhill in his book “Making Better Wines” has a discussion of how to assess various wines and then to improve the product by blending. This is often an excellent way to deal with a wine that is not ideal.  However assuming that one does not have an appropriate number of wines for blending one can use bench test to determine how to adjust the wine for sugar, acid or tannin.

General Procedure.

There are two common but equivalent approaches that differ only in the amount of wine taken for each test and the calculation of the result. I will focus on acid adjustment of a flabby wine as this is the easiest change to make without running into complications. (E.G adjusting by adding sugar risks re-fermentation unless sorbate is used and many winemakers don’t want to go that route for various reasons)

Being a Scot I prefer to make up a 5.0% solution of either sugar, acid* or a finishing tannin and make tests using 50 mL ( that is 0.05 L or 1/20 of a litre) samples of wine. (Others make a 10% solution. e.g BCAWA site on acid adjustment, and use 100 mL test samples). As I said these are equivalent except for the result calculation.

A typical test for acid adjustment;

1. Take say six 50 mL samples of the wine in six wine glasses and label them 1 through  6 or A through F.

2. Add small increasing volumes of the 5% acid solution to five of the glasses in random order keeping a list of the amount of acid added to each glass and leaving one unchanged as a control. A sequence such as 0.5, 0, 0.1, 0.9, 0.3, 0.7 mL for example.

3. Now find a third party (spouse, friend, judge, etc) to taste these samples and rate them.

4. Say the sample with 0.3 mL is the best. The 0.3 mL of 5% solution weighs 0.3 g and contains 5 % acid by weight so you have added 0.3×5/100 = 0.015 g of acid to the 0.05 L volume of wine.

5. Result: To adjust 1 L of wine you need to add twenty times as much (just the ratio of volumes) or 0.015 g x 20 = 0.3 g acid. i.e. 0.3 g/L.

Put simply, the g/L of acid to be added is the same as the volume 5% solution used on the test sample. (If you use the 10% procedure you get the same result.)

What to use?

Sugar; Sucrose (table sugar), glucose, glucose solids or fructose. All are fermentable!

*Acid: Tartaric (may form wine diamonds), citric. Malic and citric mandate no ML

Tannin; Various finishing tannins. This is a whole separate presentation or workshop. Marg Best and

Gerry Resch hosted an excellent one at Rick Homer’s some years ago.

Other Sources of information. BCAWA site. Winemaker magazine.