High pH winemaking - risks and solutions

The effects of climate on grape and wine composition

In many wine-producing regions worldwide, the phenological development of grapevines is being accelerated by climate change. Consequently, grape ripening increasingly occurs during the warmest periods of summer. It is estimated that grape harvests have advanced by at least two to three weeks over the past four decades.¹ Changes in ripening conditions, including temperature, water availability and sun exposure, affect multiple compositional parameters of grapes at harvest, such as increased sugar concentrations, reduced anthocyanin concentrations, elevated pH and decreased titratable acidity (TA), mainly due to lower malic acid concentrations.^2,3 The resulting wines from these grapes have higher alcohol content and altered aroma composition and sensorial properties.

Two of these compositional changes in must and wine, increased pH and decreased titratable acidity, directly affect microbiological control, sensory balance, chemical stability and colour expression, and therefore necessitate adaptation strategies.⁴

This article examines the implications of elevated pH and reduced acidity in must and wine, and presents LAFFORT^® solutions to address these challenges.

Mechanisms driving higher grape and wine pH

During berry ripening, sugar concentration increases, while titratable acidity gradually declines.³ Malic acid is temperature-sensitive and is actively respired by berries as ripening progresses. Higher temperatures accelerate this degradation, resulting in lower malic acid concentrations at harvest. Tartaric acid, by contrast, is relatively stable during ripening. However, its contribution to overall acidity can be affected by interactions with cations, especially potassium.

Potassium accumulation significantly contributes to elevated pH in must and wine.² Heat stress and changes in vine water relations can increase potassium concentrations in berries. Potassium’s association with organic acids reduces free acid concentrations. This neutralisation effect raises the pH, even when titratable acidity appears to remain relatively unchanged.

The combined effect of malic acid loss, potassium accumulation and altered acid equilibria is an increase in must pH. This shift can continue to influence the wine composition after fermentation, particularly following malolactic fermentation and potassium bitartrate precipitation.

Reduced antimicrobial effectiveness of SO₂

The challenge

The antimicrobial (antiseptic) efficacy of sulphur dioxide is strongly influenced by pH. In juice and wine, free SO₂ exists in several forms, including molecular SO₂, bisulphite and sulphite. At the typical pH of wine, bisulphite predominates, with only a minor proportion as molecular SO₂.⁵ However, molecular SO₂ is the only component with antimicrobial activity, and its concentration depends on the medium’s pH. As pH increases, the proportion of free SO₂ as molecular SO₂ decreases. Consequently, two wines with identical free SO₂ concentrations may show different levels of antimicrobial protection, depending on their pH.

This has practical implications for higher-pH musts and wines. At elevated pH, higher free SO₂ concentrations are required to achieve the same molecular SO₂ concentration. If SO₂ management is based solely on free SO₂ without considering pH, antimicrobial protection may be insufficient.

The risk is particularly relevant before and during the early stages of alcoholic fermentation. At this stage, ethanol concentration is still low, and the selective pressure against spoilage microorganisms is limited. A higher must pH can therefore increase the risk of unwanted microbial growth before the inoculated yeast population becomes dominant.

The solutions

One way to mitigate this risk is through bioprotection using non-Saccharomyces yeasts.^6,7 Specific strains of Torulaspora delbrueckii and Metschnikowia pulcherrima, such as ZYMAFLORE™ ÉGIDE^TDMP and ZYMAFLORE™ KHIO^MP, can protect the must during the pre-fermentative phase by colonising the medium and/or scavenging oxygen, thereby preventing the proliferation of spoilage microbes.

During fermentation and ageing, the use of a biocontrol product containing chitosan, such as FLORACONTROL™, is recommended.^8,9 With its chitosan- and inactivated yeast-based formulation, FLORACONTROL™ is effective in reducing the overall microbial load and controlling the growth of Brettanomyces species (including many SO₂-resistant strains) and most lactic acid bacteria.

Microbial management of wine acidity

The challenge

Acidity contributes to wine freshness, balance, appearance, stability and longevity.³ Freshness has long been associated with white, rosé and sparkling wines, but it is increasingly important for red wines as well, reflecting changing consumer behaviour. As titratable acidity decreases, maintaining freshness becomes more challenging.

The solutions

Several chemical and biological acidification methods exist, with bioacidification, which uses selected acidifying yeasts, gaining popularity.

Non-Saccharomyces Lachancea thermotolerans strains, such as ZYMAFLORE™ OMEGA^LT, can produce lactic acid from grape sugars alongside ethanol.¹⁰ The amount of lactic acid produced depends on the specific strain and the inoculation regime with the Saccharomyces cerevisiae starter culture. The effects on TA and pH can be significant, depending on the amount of lactic acid produced.

Saccharomyces cerevisiae strains can also modify wine acidity by consuming, preserving, or producing malic acid.^4,11 Most S. cerevisiae strains consume malic acid, but novel strains, such as ZYMAFLORE™ KLIMA, can produce it, thereby increasing wine acidity and freshness. Depending on the must composition, ZYMAFLORE™ KLIMA can also slightly lower the pH by 0.1 - 0.2 units. The amount of malic acid produced depends on the strain, the initial malic acid concentration, and the pH of the must.

Citric acid is another organic acid that can contribute to the perception of freshness in wine.¹² Malolactic fermentation starter cultures of Oenococcus oeni vary in their capacity to metabolise citric acid. Strains characterised by slower citric acid degradation, such as LACTOENOS™ BERRY Direct, help preserve acidity after MLF. In addition, reduced citric acid metabolism can influence the wine’s sensory profile by limiting diacetyl formation, a compound associated with buttery aroma notes.

These microbial strategies may be applied individually or in combination, depending on the desired acidity level and sensory expression of freshness.

Calcium tartrate instability

The challenge

Higher pH is also associated with an increased risk of calcium tartrate (CaT) instability.¹³ As pH rises, the dissociation state of tartaric acid changes, increasing the availability of tartrate ions (T^2-) that can bind with calcium. This can favour the precipitation of calcium tartrate under suitable conditions. This risk of CaT crystal formation is distinct from potassium bitartrate (KHT) instability and cannot be controlled by conventional cold stabilisation practices, which are primarily aimed at KHT precipitation. Adding exogenous tartaric acid to lower TA may further increase the risk of CaT instability.

Other organic acids can compete with tartaric acid for calcium ions, thereby preventing the formation of crystalline nuclei.¹⁴ Malic acid has been reported to reduce the risk of CaT crystal formation, but the exact concentration and conditions required for complete inhibition remain unknown. In warm vintages, malic acid respiration in berries increases, resulting in low residual malic acid in the final wines. This reduces the inhibitory effect on CaT crystal formation.

The solutions

There are several options to reduce the risk of CaT crystal formation. The most common method is the selective removal of calcium using racemic tartaric acid (CA²⁺STAB™) or micronised calcium tartrate (CAFINISH™). Avoiding chemical acidification and opting for bioacidification instead are other ways to reduce the risk.

Effects on red wine colour chemistry

The challenge

The colour of red wine is strongly influenced by pH.² Anthocyanins exist in various forms in wine. At low pH values, the anthocyanin equilibrium favours the red flavylium cation, resulting in stronger red colour intensity. As pH increases, the proportion of the colourless carbinol pseudo bases increases, reducing the bright red colour. At pH values above approximately 3.7, colourless anthocyanin species become more prevalent, thereby significantly influencing red wine colour.

Higher pH can also reduce the efficiency of copigmentation. Copigmentation occurs when anthocyanins interact with other phenolic compounds, intensifying colour and contributing to blue-purple hues in young red wines. When pH increases, these interactions may become less favourable, limiting early colour expression and reducing the conditions required for stable colour development. For red wines, elevated pH may therefore affect both immediate colour intensity and long-term colour stability.

The solutions

In addition to lowering the pH through acidification, a well-targeted split addition of oenological tannins can help preserve the wine’s colour potential. To this end, LAFFORT^® combines the complementary action of two of its tannins:

• TANIN VR SUPRA™ (at tank-filling) acts from the start to protect the grape’s natural tannins through its sacrificial effect on grape proteins. It also provides greater protection through its antioxidant and anti-laccase actions.
• TANIN VR COLOR™ (at one-third of alcoholic fermentation) balances the tannin/anthocyanin ratio. As a highly reactive tannin, it promotes co-pigmentation and condensation, two essential mechanisms for lasting colour stabilisation.¹⁵

Conclusion

Higher pH and lower acidity are not isolated analytical changes. They affect several interconnected aspects of wine production and stability. Higher pH reduces the antimicrobial effectiveness of SO₂, diminishes perceived freshness, shifts anthocyanin equilibria, and can increase the likelihood of calcium tartrate instability. For this reason, pH should be treated as a central technical parameter from harvest through to bottling. Managing pH and TA early can help protect fermentation security, sensory balance and stability.

References

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6.   Windholtz S, Nioi C, Coulon J, Masneuf-Pomarede I. Bioprotection by non-Saccharomyces yeasts in oenology: Evaluation of O₂ consumption and impact on acetic acid bacteria. Int J Food Microbiol. 2023 Nov 16;405. doi:10.1016/J.IJFOODMICRO.2023.110338 PubMed PMID: 37506548.

7.   Windholtz S, Nioi C, Thibon C, Bécquet S, Vinsonneau E, Coulon J, et al. Bioprotection as an alternative to SO₂ in the pre-fermentation phase: Original language of the article: English. IVES Technical Reviews, vine and wine. 2023 Sep 21. doi:10.20870/IVES-TR.2023.7763.

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10. Hranilovic A, Gambetta JM, Schmidtke L, Boss PK, Grbin PR, Masneuf-Pomarede I, et al. Oenological traits of Lachancea thermotolerans show signs of domestication and allopatric differentiation. Sci Rep. 2018 Dec 1;8(1):1–13. doi:10.1038/S41598-018-33105-7;SUBJMETA=1495,171,193,252,2522,2538,326,631; KWRD=APPLIED+MICROBIOLOGY,FOOD+MICROBIOLOGY,FUNGAL+BIOLOGY PubMed PMID: 30287912.

11. Hranilovic A, Capitanio J, Mansour C, Bernard M, Muro M, Marullo P, et al. Novel yeast strains for fresher wine profiles. In: American Society of Enology and Viticulture National Conference, Portland, USA. 2024.

12. Eicher C, Coulon J, Favier M, Alexandre H, Reguant C, Grandvalet C. Citrate metabolism in lactic acid bacteria: is there a beneficial effect for Oenococcus oeni in wine? Front Microbiol. 2023 Jan 4;14:1283220. doi:10.3389/FMICB.2023.1283220/TEXT.

13. Cataldo E, Fei M, Eichmeier A, Mattii GB, Domizio P. Understanding calcium tartrate precipitation in wines – A comprehensive study from soil and grapevine to stabilisation strategies before bottling. OENO One. 2026 Apr 28;60(2). doi:10.20870/OENO-ONE.2026.60.2.9613.

14. McKinnon AJ, Scollary GR, Solomon DH, Williams PJ. The Influence of Wine Components on the Spontaneous Precipitation of Calcium L(+)-Tartrate in a Model Wine Solution. Am J Enol Vitic. 1995 Jan 1;46(4):509–17. doi:10.5344/AJEV.1995.46.4.509.

15. García-Estévez I, Alcalde-Eon C, Puente V, Escribano-Bailón MT. Enological Tannin Effect on Red Wine Color and Pigment Composition and Relevance of the Yeast Fermentation Products. Molecules 2017, Vol 22, Page 2046. 2017 Nov 23;22(12):2046. doi:10.3390/MOLECULES22122046 PubMed PMID: 29168752.

For more information, contact Morné Kemp at [email protected].