One of the challenges with our cold-hardy hybrid grapes is that they tend to have high acid and low pH. This can make some winemaking processes more difficult, as well as resulting in an overly tart wine for consumers. For these reasons, there are a range of tools we can use to lower acidity and raise pH.
However, you may have noticed that the pH of your wines naturally increases during fermentation, even without putting any of those strategies to use.
What causes this pH increase?
There can be a few different factors at play, but let’s look at just a couple of them.
Maceration on skins (in red wines)
Grapes contain a large amount of potassium, an alkali mineral, and K+ ions are readily leached out of the skins and dissolved into the juice. The longer the juice stays on the skins, the more these potassium cations will cause the pH to rise (Rakonczas et al., 2015; Australian Wine Research Institute). This means over the course of fermentation for red wines, the pH will increase as a result of the time on skins, even without any other manipulation.
Malic acid degradation by yeast
While commercially available wine yeasts don’t convert malic acid to lactic acid like ML bacteria do, they do take up small amounts of malic acid and use it in other metabolic pathways (Saayman and Viljoen-Bloom, 2006; Casal et al., 2008). This means that as some amount of malic acid is consumed, the overall acid level of the must decreases and the pH goes up. However, some types of yeast are more efficient at taking up malic acid than others. Saccharomyces cerevisiae, the dominant fermentative wine yeast, doesn’t consume malic acid as quickly as other yeast species like Schizosaccharomyces pombe. This is because S. cerevisiae lacks an active transporter to efficiently bring malic acid into the yeast cell and has different enzyme activity for malic acid breakdown. The presence of non-Saccharomyces yeasts like S. pombe in the early stages of fermentation, plus S. cerevisiae’s low rates of malic acid consumption during the more active stages of fermentation, combine to result in lowered overall acidity and increased pH.
Co-inoculation (or contamination) with ML bacteria
Many winemakers intentionally co-inoculate their musts with the malolactic bacteria Oenococcus oeni so it can start working during fermentation, but there’s also the possibility of ML bacteria from the environment getting into the musts without being intentionally added. In either case, ML bacteria convert malic acid to lactic acid. This conversion raises the pH of the must because lactic acid (pKa = 3.86) is a weaker acid than malic acid (pKa1 = 3.40). However, it’s difficult to predict the actual change in pH due to ML conversion, because the amount of increase depends on the buffering capacity of the must (Boulton et al., 1999).
The bottom line on rising pH
It’s normal to see your pH increase during fermentation! The cause could be a combination of any of the above factors. But if these aren’t enough on their own, check out this guide to managing acidity by chemical deacidification.
Boulton, R., et al. Practices and Principles of Winemaking. Springer Science+Business Media, Inc., 1999.
Casal, M., et al., 2008. Transport of carboxylic acids in yeasts. FEMS microbiology reviews. DOI: 10.1111/j.1574-6976.2008.00128.x
Rakonczas, N., Andrási, D., and Murányi, Z., 2015. Maceration affects mineral composition and pH of wines. International Journal of Horticultural Science. DOI: 10.31421/IJHS/21/3-4./1163.
Saayman, M. and Viljoen-Bloom, M., 2006. The Biochemistry of Malic Acid Metabolism by Wine Yeasts – A Review. South African Journal of Enology and Viticulture. DOI: 10.21548/27-2-1612.