Testing Times August 18
Low pH red wine
At our laboratories we service a wide array of winemaking clients, from large wine companies through to backyard hobbyists. We also see a wide variety of wines, from finished products through to works very much ‘in progress’.
Recently we encountered a red wine from a very small producer with an unusually low pH. With some knowledge of what was typical of the variety, region and vintage from which the grapes had been sourced, a discussion was initiated with the client to determine the reasons behind, and impacts of, such an unexpectedly low result.
Analysis of the wine early in the winemaking process had shown high pH and low titratable acidity values. In an effort to remedy these figures to more standard levels, the winemaker had added tartaric acid – but unfortunately a lot more was added than was really needed. While the actions taken to decrease the pH and increase the acidity were correct, the calculation made to determine the amount of tartaric acid to add was not. The winemaker had ended up with a pH of around 3.0 and a titratable acidity of 9 g/L. Malolactic fermentation (MLF) had not occurred as yet, and with such a low pH it was very unlikely to proceed to completion, if at all. And of course the wine was not well balanced in taste.
Potential remediation measures discussed included the addition of calcium or potassium carbonates to deacidify the wine. Deacidification is not a simple process and requires some careful considerations, for example potassium carbonate or bicarbonate can cause a big shift in pH relative to the TA drop. Calcium carbonate has a more moderate pH shift but can cause calcium instability and associated problems. An attempt at cold stabilisation to drop out some potassium hydrogen tartrate was also discussed.
A partial improvement by one of these steps to achieve a higher pH would lead to a more favourable environment for malolactic fermentation to occur. This in turn would also lead to further favourable movements in the pH and acidity figures.
This is not the sort of scenario that would be commonly encountered in a winery, however in this example, results from laboratory testing were able to identify a problem, and by considering the processes that went into the winemaking, help to determine where the problem occurred, in this case a calculation error. Using this information, the winemaker is then able to make decisions regarding what actions they wish to make to improve their wine.
Titratable Acidity in cider is not the same as in wine!
Titratable Acidity (TA) is a commonly requested test for a wine laboratory to perform. A determination of TA measures the concentration of all the available hydrogen ions present in the sample of juice or wine.1 Typically the concentration is determined by titrating a known volume of wine with a strong base, such as sodium hydroxide, to an equivalence point which may be determined by pH meter or by using a pH indicator. The TA result is most commonly calculated to an equivalence point of pH 8.2 in Australia (but to pH 7.0 in Europe) and expressed in g/L as if all the hydrogen ions present were associated with one acid. In Australia, the TA of a wine is typically expressed as if all the acid present was tartaric acid, which makes sense as tartaric acid is the main (but not the only) acid contributing to acidity in a wine. It is worth noting that some other countries don’t quote TAs in the same tartaric acid units, for example the French use sulfuric acid units.
Recently some of our customers in the cider industry have advised a preference to have their TA measurements expressed in g/L as if all the acid present was malic acid. This makes sense as in apples the main organic acid present is malic acid.
To do this the same test method is done; it is only in the calculation of the final result where a change is needed. This difference relates to the molecular weight of the acid, be it tartaric or malic. A TA result expressed as malic acid will give a figure lower than the equivalent tartaric acid result as malic acid has a lower molecular weight than tartaric acid. For example, a TA result of 6.2 g/L (as tartaric acid) will be equivalent to a result of 5.5 g/L (as malic acid).
As in wine, TA is an important parameter in cider making and the use of a standard unit of measurement is most useful.
Bentonite fining/Protein stability/Heat stability
We were recently asked by a client why and how to do bentonite fining. This has prompted us to prepare the simple refresher item as it might be worthwhile to some winemakers, particularly at this time of the season.
All wines contain proteins, but problems arise if the content is too high. White wines can produce a haze from having excess protein. There are several ways to remove proteins but the simplest and most common way is to add a particular clay mineral (bentonite) that causes some of the proteins to be removed from the wine.
The correct amount of bentonite varies with each individual wine and depends upon several factors, including the grape variety it has been made from and the type of bentonite used. To be able to work out how much bentonite to add, simple lab bench trials are done with the same bentonite that is to be used in the fining process. These bentonite fining trials determine the amount of bentonite to add.
To see if a wine is protein stable a standardised test (the Heat Stability test) is done on a bentonite-treated wine sample. The treated, filtered wine is placed in a sample tube that is heated in an 80oC water bath for 6 hours and once cooled is then viewed to see if there is any haze. If there is no haze, the wine is classified as protein stable. If there is a haze, a higher level of bentonite needs to be added and the Heat Stability test repeated until a level of bentonite addition is found that produces a protein stable wine. Common textbooks on winemaking have more detailed sections on this topic.
1. Iland, P. Bruer, N. Edwards, G. Caloghiris, S. and Wilkes, E. Chemical analysis of grapes and wine: techniques and concepts 2nd Edition, 2014. P39