For the last two decades market trends have seen an increased demand for more concentrated wines – wines with more colour, more flavour, more tannin, more texture. This is leading to longer hang times and harvesting of riper fruit as well as prolonged pre- and post fermentation macerations. Global warming and more extremes in precipitation are changing ripening patterns of grapes. Both winemaking and climate variations are bringing higher pH wines and raising issues for winemakers regarding the effect of pH on wine stability and quality. This article will first explain what pH is before taking a look its very important role in the microbial and chemical/physical stability of wine post fermentation. The significant role of pH for malolactic fermentation (MLF) and maturation in barrel will then be discussed. Finally, the role of pH in determining a wine’s quality (colour, flavour and mouthfeel, and longevity) will be addressed.
pH is the negative of the log to base 10 of the concentration of hydrogen ions in a solution. It is a scale of measurement of the concentration of the effective acidity of a wine, the relationship between its acids and natural salts such as potassium. The buffering effect of potassium raises pH without altering total acidity (TA). As grapes ripen, potassium content increases and acidity decreases which results in increasing pH. Potassium is also higher in vintages with abundant rain during the growing season. A good demonstration is the 2009 vintage in Germany, a warm vintage with abundant rain. Acidity was so low that Germany received legal permission from the EU to acidify, yet pH was higher than average. Wine pH is usually between 2.8 and 4.2 on a scale of 14. In this essay low pH is determined as <3.2 and high pH >3.6. pH is an important parameter because it is a controlling factor in how wine looks, tastes and lasts.
pH plays a vital role in maintaining the microbial stability of wine because the hydrogen ion concentration controls the effectiveness of SO2. Because the hydrogen ions combine with bisulphite ions, in low pH wines more SO2 is kept in the effective molecular form that protects wine from oxidation and spoilage organisms. With the effectiveness of SO2 diminished, high pH wines are more susceptible to oxidation and offer an ambient solution for various bacteria and yeasts. This is particularly true if there is any residual sugar and the reason why sterile filtration with a 0.45?m membrane is standard procedure for off-dry commercial wines like white Zinfandel. Contrarily, the high pH environment is essential for a flourishing population of flor yeast that determines the style and aroma of fino sherry. Tolerant of 15% abv, flor yeast ousts competition from other microbes and a surface carpet of flor inhibits oxidation. Looking at a completely different style, László Mészáros of Disznókö states that while Tokaji aszú typically has a pH of 3.6-3.8, TA is usually over 9 g/l. To prevent microbial spoilage and excess oxidation during the long maturation in cask, Mészáros adds a high dose of around 150 mg/l SO2 and filters before racking into clean, hygienic barrels. A high pH wine style which has become highly fashionable is big, ripe, concentrated dry red wines. Due to their high pH and possible trace residual sugar after fermenting to over 14.5% abv, these wines are particularly susceptible to Brettanomyces (Brett) infection. Studies at the AWRI of 129 dry red wines showed that over half had low level Brett aromas with a high correspondence between phenolic content and 4-ethylguiacol. Cabernet Sauvignon and Shiraz were particularly vulnerable. Low threshold Brett is often interpreted as a positive attribute of complexity in tannic red wines, but at higher levels it is certainly an unpalatable taint. Additions of up to 200 mg/l DMDC are permitted to help eradicate Brett. To summarize, low pH enables the winemaker to protect the wine with lower additions of SO2 and less intervention.
High pH diminishes the ability of bentonite to bind with proteins in the fining process of white wines. Winemaker Rudi Strasser of Napa Valley has an experimental plot of Grüner Veltliner, which by nature is a high protein variety. The first harvest in 2007 came in with a pH of 3.6 and Strasser underestimated the amount of bentonite needed to achieve protein stability. A few cases were sold, but had to be recalled due to protein haze. Heinz Frischengruber of Domaine Wachau advocates addition of 1.5 to 2 g/l of bentonite to Grüner Veltliner must before fermentation. Maturation on lees post fermentation will often render the wine protein stabile, but if not, Frischengruber recommends further adjustment with the sodium bentonite GranuBent PORE-TEC because the sodium makes it more effective in high pH wines and less bentonite is needed. Frischengruber states the advantage of applying bentonite on must is that it strips less flavour, protecting those created during fermentation.
Potassium bitartrate (KHT) and/or calcium tartrate (TCa) can precipitate from wine as crystals and although harmless, most consumers object to them. Potassium, calcium, tartaric acid, alcohol and pH play a role in tartrate stability of wine. Tartrate stability can be improved by either removing things responsible for the formation of crystals or by adding substances that inhibit their formation. High pH wines pose a special problem for tartrate stabilization due to their content of TCa. Only KHT is removed in the process of cold and contact stabilization. Electrodialysis is an alternative with advantages being much lower energy costs, no loss of wine volume, reliable removal of both KHT and TCa, no additives, and the ability to tailor treatment to each wine. The technology is expensive and viable for purchase only by large wineries or hired as a mobile service.
For many styles of wines, malolactic fermentation (MLF) and/or maturation in oak barrels is employed. MLF generally requires a minimum of 3.1 pH. High acid wines with pH levels below this where MLF is desired can be adjusted with the addition of calcium carbonate or potassium carbonate. An ambient temperature of around 20°C is necessary for MLF to occur. A warm, unprotected high pH red wine provides the ideal environment for both desirable lactic bacteria and undesirable bacteria. Likewise any fermentable residual sugar also leaves the wine exposed to fermentive organisms such as Brettanomyces (see above). An efficient MLF can be promoted either through inoculation with a lactic bacteria culture or by putting the wine into clean, hygienic barrels where successful MLF has already occurred. A dose of 10 – 30 mg/l free SO2 can help mitigate spoilage in endangered high pH wines without hindering MLF. A new gene modified yeast developed by food biotechnologist Dr. Hennie van Vuuren at the University of British Columbia makes it possible inoculate both alcoholic and malolactic fermentation to occur simultaneously. This yeast has just been approved in North America and is now on the market. Because it has the added benefit of producing wines that are free of allergenic bioamines, this may become an attractive option not only for producers of high pH wines.
In low pH wines, anthocyanins are present in bright red coloured forms, while in increasingly high pH wines the colour is dull purple ultimately even slightly greyish or taupe. SO2 additions may seem to bleach a young wine with high pH, but this actually stabilizes red colour by protecting some monomeric pigment which is restored as pigments polymerize. Robert Parker generally seems to praise the deep dark colour of very ripe high pH wines and many consumers also find this appealing. Low pH in young white wine brings pale greenish hues which are often seen in high acid varieties like Riesling or Sauvignon Blanc. The consumer often associates this pale green-yellow with positive refreshing qualities.
Abundantly praised, in particular by American wine critics, is the rich finish of full-bodied, concentrated red wines. According to Clark Smith of Vinovation in California, this is not entirely attributable to high alcohol and phenol content; potassium and sulphate both contribute to the back palate mouthfeel and are natural by-products of high pH winemaking. Not only New World producers of dry red wines conscientiously make the decision to make high pH wines, these textural goals are also seen in the past 20 years in Bordeaux, particularly on the right bank. The quality attributes of sparkling wine are also affected by pH. Arnaud Weyrich, winemaker at Roederer Estate in California has also worked at the parent company in France and can make a good comparison between typical pH values in Anderson Valley and Champagne. Weyrich states that the pH in Champagne is significantly lower and as a result the second fermentation takes much longer resulting in a more delicate brioche and floral autolytic character compared to more forward bread-like aromas from the higher pH sparkling wines of Alexander Valley. With similar acidity and alcohol levels, the Alexander Valley wine is also somewhat broader mid-palate.
Longevity of a wine after bottling is dependent on pH, TA, alcohol content, and dry extract. While high alcohol and high extract content connected to high physiological ripeness of dry red wines contribute positively to a wine’s longevity, low acidity and high pH can be detrimental. Like many other right bank properties in Bordeaux, Château Pavie has seen an extreme shift in style towards bigger, riper, high pH wines. After purchasing the property, Gerard Perse followed the advice of Michel Rolland for harvesting later and employing cold maceration. The rich voluptuous wines were highly praised by Parker and prices achieved increased dramatically. Sceptics criticized the style as unbalanced and not worthy of aging. Gerard Perse counters that these wines are now 15 years old, drinking splendidly, and still improving. Leo Alzinger, vintner in the Wachau, Austria is more critical of the trend for big bodied, high alcohol white wines. “I make small quantities of a dry late harvest ‘reserve’ Grüner Veltliner and a dry late harvest ‘reserve’ Riesling because this style gets the highest accolades from critics who taste the wines in their youth. The more mineral, more longevous wines are the single-vineyard ‘Smaragd’ wines. In my opinion, higher pH is does not improve balance and it does not support expression of terroir.”
Praise from certain critics as well as high accolades in wine competitions for rich, concentrated wines have inspired many wine producers to strive for higher physiological ripeness. This, together with global climate change is bringing higher pH values. High pH reduces the effectiveness of SO2 and renders wine prone to microbial spoilage and oxidation. More intervention is required by the winemaker to lend these wines balance and keep them stable. Acidification, high SO2 additions, addition of DMDC and sterile filtration are just some of the methods employed. While these measures can possibly diminish the quality of wine, potential advantages for aroma, flavour and texture make it an interesting and viable option for some styles. A scrutinizing eye must be kept on the concept of physiological ripeness to keep it from compromising balance, longevity and expression of terroir in high quality premium wines.
