Predicting acidity and pH changes during the winemaking process
Van Rensburg, P
University of Stellenbosch. Faculty of AgriSciences. Institute for Wine Biotechnology
Lambrechts, M G
Rohwer, E A
Botes, M P
The aim of this project is to develop a database regarding the pH, total and titratable acidity and organic acids of different cultivars, from the grape until the end product. This information will be used to test the ability of the ACID computer program to accurately predict pH and acid changes during the winemaking process. This will enable the winemaker to do acid adjustments before the alcoholic fermentation, which is the optimum time considering wine quality.
The crystallisation of tartrate salts is a major source of instability in wine. Both potassium hydrogen tartrate (KHT) and calcium tartrate (CaT) may crystallise, the most common being KHT. Tartrate salt crystallisation occurs spontaneously during both alcoholic fermentation and wine storage. Several factors influence this instability of which pH, alcohol content, buffer capacity and potassium concentration are the most important. Furthermore, during the malolactic fermentation malic acid is converted to lactic acid resulting in a pH shift and lowering of the titratable acidity. Therefore, adding acid to musts and wines is common practice in warm wine growing regions. Yet, attempts to quantify and predict changes that occur in a wine during an acid adjustment, have been difficult in the past due to laboratories not being able to accurately and cheaply perform the necessary analyses and the difficult mathematical analysis required to process the data. Individual wine makers base their decisions on what they have seen from specific vineyards in past seasons, but the effectiveness of this approach is limited by vintage variation. This has resulted in several wines being spoilt due to excessive acid additions etc. As a result wines may receive acid additions well into their life rather than in the fermentor as wine makers would prefer, thus negatively affecting the quality of the wine. Therefore, winemakers have a growing need to be able to accurately add acid at the beginning of the winemaking process. This situation could be improved if there were a tool for predicting how individual wines will behave during acid additions and malolactic fermentations.
Innovative Winery Solutions sells a computer programme (ACID) with which they claim to be able to predict pH and acid changes accurately and to be able to routinely evaluate and compute different acid adjustment scenarios – tartaric vs. malic vs. a mixture – with or without malolactic fermentation. The ACID programme uses the following information for the computation: pH, ethanol concentration, buffer capacity, tartaric acid concentration, malic acid concentration and potassium concentration.
Thus the aims of this project are:
- To develop a method to accurately determine potassium content of grape skins, pips and flesh.
- To determine the extent of exchange of potassium between pips, skins and the grape juice taking into account the different winemaking practices.
- To predict acidity and pH changes of wines using the ACID computer programme.
- To determine the effect of acid additions at various stages (after crushing, after alcoholic fermentation, after malolactic fermentation) in the winemaking process on the quality (e.g., organoleptic quality, tannin structure, etc.) and microbial stability of wines.
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Goussard, P G and Vivier, M A. 1998. The induction and maintenance of embryogenic cell-lines of Chardonnay and Merlot noir for use in transformation experiments. Paper presented at the 23rd National Congress of the South African Society for Enology and Viticulture. 12-13 November, Somerset West, South Africa.
Vivier, M A and Pretorius, I S. 1998. Grapevine transformations: A new beginning for an old genotype. Paper presented at the 2nd Congress of the South African Society of Biochemistry and Molecular Biology. Potchefstroom, South Africa.