Cool-Climate White Wine Oenology

CHAPTER 2

PRE-FERMENTATION STRATEGIES

Pre-fermentation operations performed between the grapes' crushing and the start of fermentation have a widely underestimated impact on wine quality and its sensory stability during storage. A skin contact period of up to one day is frequently used to enhance aromatics by their extraction from the skins, provided that the grapes are perfectly ripe. More important than the technical modalities of pressing is the issue of the generally recommended addition of sulphur dioxide to must. The oxidation and browning of must resulting from omitting it is not related to the oxidation of wine, and even mitigates it by lowering detrimental phenols, thus improving the wine's shelf life and reducing its astringency. Aroma losses frequently attributed to it only occur in a few specific grape varieties. Protein stabilisation by bentonite fining and any acidity corrections deemed necessary are already useful at this stage. Another important measure to achieve flawless wines with pristine aroma is juice clarification. Choice of clarification procedure is not decisive, but rather the level of clarification obtained, evaluated as residual turbidity. The use of pectolytic enzymes is strongly recommended for this purpose.

Closed-cage membrane press awaiting its next load of grapes.

2.1 Must Acidification and the Issue of Safe pH

From a historical perspective, until the end of the twentieth century, must acidification had never played a major role in cool-climate growing areas. The acidity was usually high enough and often too high, so that deacidification was more important. High acidity was also accompanied by low pH, although this inverse correlation is weak. As is generally known in the wine industry, a low pH contributes to microbial safety. Hence, no thought had ever been given to microbial hazards caused by high pH levels. However, this situation has changed in the meantime, and the pH has become a hotly debated topic of conversation even in cool-climate areas. There are two reasons for that.

The first reason can be found in the development of the New World wine industry in the second half of the twentieth century. Most of their wine growing areas sprouted in hot regions that yielded low acidity and high pH figures. Therefore, acidification became a necessity to achieve a balanced taste and, at the same time, a decrease in pH to improve microbial safety. Since absolute safety was considered paramount, much importance was and still is attached to the lowering of pH to values considered safe through the addition of tartaric acid. As these countries quickly became opinion-leading in the global wine industry, the fear of supposedly too high pH levels spread to Old World wine-producing countries as well.

The second reason is global climate change. It has led to the fact that even in cool-climate growing areas, hot and dry vintages occur more often, providing musts with low acidity and actually high pH figures. That is why most of the countries concerned have now legalised must and wine acidification. This raises the question of how far the pH should be lowered and the acidity increased.

Interpreting pH Correctly

With a few exceptions, pH in musts ranges from 3.0 to 4.0. Some very conservative schools of thought continue advocating lowering pH to 3.5 for safety reasons and to add as much tartaric acid as necessary to achieve this goal regardless of the sensory outcome. As a consequence, many winemakers are terrified by higher pH figures, because they mean more risk of adverse microbial activity since SO2 is less effective in the higher pH wine. This is absolutely true. The molecular gaseous fraction of free SO2, which alone is responsible for microbial protection, decreases logarithmically with increasing pH (Figure 5.1).

Other winemakers have learnt to handle high pH musts and wines by using modern techniques such as cooling, filtration and sterile bottling that were not available some decades ago. These winemakers feel quite comfortable vinifying and bottling particularly high-end wines with pH in the range from 3.5 to 3.8, stating that wines acidified according to pH taste thin, harsh and tough. This is also true. Many of the great wines of the world would be considered undrinkable by contemporary quality standards if their pH was lowered to 3.5. Unpleasant wines that have been distorted by overacidification just for pH concerns are easy to find.

Factors Affecting pH

It is frequently believed that low titratable acidity (TA) automatically leads to high pH and vice versa. However, this relationship is not that clear, as shown in Figure 1.1. Indeed, pH is also affected by potassium. It is the most common alkali mineral cation in wine and neutralises acids. Just as acids drive pH down, potassium drives it up (Figure 4.4). Thus, the pH actually measured is the result of an interaction between acids and potassium. Musts with high TA from bad rainy years do not necessarily display low pH figures, because they also tend to have high potassium levels. However, the reverse can also become true when, in the course of global climate change, hot and dry growing conditions yield musts with little potassium (section 4.2).

Figure 1.1: Relationship between titratable acidity (TA) and pH. Data in the oval circle represent white wines.

When Lowering pH Through Acidification is Really Necessary

Generally, a TA in the range of 7-9g/L and a pH of 3.2-3.6 is preferred in white musts. Somewhat higher acidity levels can be useful for the production of sparkling base wines. After the precipitation of potassium bitartrate during and after fermentation, wines generally display a lower TA and also a lower pH than the musts they have been obtained from (section 4.2). This counterintuitive behaviour, that despite less TA the pH is also lower, is partially explained by the decrease of potassium that accompanies this process. The pH decrease always occurs unless the initial must pH is above 4.1, which is practically never the case. Older theories, according to which the initial pH only decreases when it is below 3.65 and increases when it is above, are based on aqueous solutions that do not take into account the impact of alcohol content and ionic strength of wines (Boulton et al. 1996).

The pH drop due to potassium bitartrate precipitation relativises the significance of high pH figures in must. Only when must is actually high in pH (>3.8) or very low in acidity (for example, less than 6g/L TA), should these numbers be corrected by acidification at this early stage. For that purpose, tartaric acid is used, usually in amounts of 1 or 2g/L. It is the organic acid that gives the greatest reduction in pH. An addition of 1.0g/L tartaric acid leads to a reduction of pH by 0.15-0.20, depending on how much of it remains in solution and on the buffering capacity of the individual must. It is important to note in this context that about 80 per cent of sensorially perceived sourness is determined by TA, whilst pH only contributes approximately 20 per cent. Therefore, TA must be given at least the same importance as pH (Schneider and Troxell 2022).

Special Features of Acidification with Tartaric Acid

Tartaric acid differs from the other acids contained in wine in that it can precipitate as insoluble salts. This results in the following peculiarities of its application:

The addition of 1.0g/L tartaric acid only temporarily increases TA by 1.0g/L, because a significant part of it precipitates together with potassium as a potassium bitartrate after some days or weeks.

If the added tartaric acid were to precipitate completely as potassium bitartrate, an addition of 1.0g/L would only result in a permanent increase in TA of 0.5g/L. In practice, this value is usually around 0.6g/L.

Potassium in wine contributes to sensory sensations that are described as volume, weight and body on the palate. Its inevitable decrease by addition of tartaric acid makes the wine thinner. Details are outlined in section 4.2.

These properties indicate that any acidification of must, if really necessary, should be approached with caution. Under cool-climate conditions, questions about acidity and pH management arise primarily in connection with deacidification. Whether this is performed on the must or in the wine depends on numerous factors. This issue will be discussed in detail in Chapter 4. Furthermore, it is not admissible to state in a simplified way, as so often happens in the lay press, that low pH or high TA improve the shelf life of wines. On the contrary, shelf life and aroma stability are primarily determined by variables such a phenolic composition and must treatment (section 2.2), oxygen uptake post-fermentation (Chapter 7) and storage temperature.

2.2 Reductive vs Oxidative Grape Processing

When white grapes are crushed without previous SO2 additions, the juice they release undergoes rapid browning and displays a smell reminiscent of fresh bread. Many winemakers are startled by this appearance and believe it will remain in their later wine. Therefore, they immediately add SO2 to the grapes or, at the latest, to the juice.

Grapes and wine contain polyphenolic compounds that easily oxidise to brown pigments when the juice picks up atmospheric oxygen. This oxidation is an enzymatic one caused by grape-derived enzymes called polyphenoloxidase...