In this article we take a closer look at acids — which have become crucial commercially — the mechanisms of ripening, climatic influences, and the options available for improvement in the vineyard.
Acid dynamics during ripening
During berry ripening, sugars rise while acids fall. Tartaric and malic acid make up around 90% of the acids in the berry, while the remaining organic acids (primarily citric) are present in significantly smaller amounts. Both acids are synthesized in the early, “green” phase of berry development — the period of cell division and growth — when they also serve as the main osmotic agents drawing water into the berry.
The curve depicting acid behavior throughout the season was first published by Dr. Bryan Coombe, the Australian researcher who laid the foundations of modern grape berry development and ripening science. His double sigmoid growth curve model, developed with colleagues during the 1970s and 1980s, remains the standard framework for describing grape ripening physiology today. It is also a great example of how scientific breakthroughs enable practical success — the Australian wine industry, relying precisely on this measurable approach to ripening, grew dramatically during the 1980s and 1990s.
The curve shows that malic and tartaric acid behave differently during ripening. From berry formation onward, tartaric acid declines, while malic acid first rises and only later declines.
The diagram shows their concentrations in mg/g. This unit is close to the commonly used g/L found on regular must and wine analyses. The diagram can therefore be read proportionally in g/L as well.
Tartaric acid
From berry formation, tartaric acid declines only very slightly per berry, and its total amount per berry remains largely stable throughout ripening. What visually appears as a “drop” on the diagram is largely the result of dilution, since the berry simultaneously grows and increases its juice volume. Tartaric acid is not consumed as an energy substrate.
Malic acid
Malic acid behaves differently: it first rises during the green phase, then begins to decline sharply from véraison onward, precisely because the vine actively “consumes” it as a cellular energy source (a process known as malic acid respiration). This difference in the metabolic role of the two acids explains why malic acid responds far more sensitively to temperature than tartaric acid.
Beyond this natural characteristic of the ripening process itself, acid levels are determined by two further factors: the vineyard’s climatic characteristics and the variety grown — and these two factors are not entirely independent, since certain varieties prefer a particular type of climate.
pH, Potassium, and SO₂ — why the numbers aren’t everything
Acid concentration is almost always used alongside the pH value, and potassium ions complicate matters further. Although potassium is essential for the vine, excess potassium displaces hydrogen ions from acids, forming salt and raising pH — put simply, “potassium can neutralize acids while they’re still in the berry.”
pH, in this context, is not just a number on an analysis — it directly determines how effective sulfur protection will be. SO₂ in wine exists in equilibrium across several forms, and only the so-called molecular (free) SO₂ acts as an antimicrobial and antioxidant. The share of molecular SO₂ within total free SO₂ falls as pH rises — above pH 3.65, this equilibrium shifts so much that free SO₂ rapidly loses effectiveness, directly threatening the wine’s microbiological stability.
This is why high pH combined with seemingly adequate total acidity is not a harmless combination — and it is precisely the most common scenario in warm years, when the berry simultaneously loses acidity and accumulates potassium.
Climate Influence
Climate exerts its greatest influence by modulating the mechanism known as malic acid respiration. The vine uses malic acid as one of its cellular energy sources — it is synthesized before véraison, as an energy reserve, and from véraison onward its consumption increases, resulting in a drop in concentration.
In climates with higher growing season temperature (GST), in warmer growing seasons, malic acid levels typically decrease because vines consume more of it throughout the day and night. As average temperatures rise across the Balkans, this exact trend is being observed. Cool nights can moderate this effect: at lower nighttime temperatures the vine slows its physiological activity, requiring less energy, which in turn slows the consumption of malic acid. This is also why vineyard regions with a pronounced day-night temperature amplitude (e.g., higher altitude, proximity to large bodies of water) tend to preserve better acid structure even at higher daytime temperatures.
In warmer climates, tartaric acid levels are also lower. It is not, however, an energy source and is not consumed in the same way, but in warm climates it binds more with potassium, a process further supported by increased potassium uptake through the roots at higher soil temperatures.
It has also been shown, as in this study, that the key temperature effect comes specifically from the temperature of the cluster and berry itself, not solely air temperature — the higher it is, the lower the acids. This directly links cluster temperature and direct sun exposure to the protective measures applied in the vineyard.
Varietal characteristics
White Varieties
White varieties, since they generally prefer cooler climates, tend to have higher acidity. Their typical tartaric-to-malic acid ratio at harvest ranges from 1:1 to 4:1. This ratio also holds for white varieties from warmer climates — for example, Pošip and Grk from Dalmatia, where ratios of 2.5:1 to 3.5:1 were recorded for the 2009, 2010, and 2011 harvests.
More on varietal variability using the example of a wine-growing region in Bosnia and Herzegovina.
Red varieties
Red varieties typically come from warmer climates, and their typical tartaric-to-malic acid ratios range from 2:1 to 5:1. Corvina and Pinot Noir are exceptions: Corvina naturally has high malic acid, often at a 1:1 ratio, which allows it to withstand drying during the appassimento process (characteristic of wines like Amarone) and retain acidity despite water loss. Pinot Noir is grown almost exclusively in cool sites, giving it an acid ratio of 1.5:1 to 2:1.
What can be done in the vineyard
Depending on the stage the planting is in, several concrete measures are available:
- Variety and planting site selection — climate profile, growing-season temperature analysis, and nighttime temperatures are especially important here, since these parameters are difficult to correct later.
- Plant health, photosynthesis, and a well-ventilated canopy — these measures allow greater acid accumulation in the berries before ripening and véraison begin, while better air circulation further lowers temperature in the cluster zone.
- Shading the cluster zone with nets or leaf canopy, from véraison to harvest — this measure lowers cluster temperature and the berry surface exposed to direct sunlight, thereby slowing malic acid respiration precisely during the period when it is most intense.
- Irrigation strategies that reduce nighttime plant activity, where irrigation is applied — the goal is to avoid stimulating vine metabolism during the period when it would otherwise be slowing its consumption of energy reserves.
- Controlling potassium input during fertilization, both in the soil and through the vine, to limit subsequent acid binding and pH increase.
In closing
Winessense is a reliable tool for tracking all the climatic effects that directly influence acids in the vineyard. Data, however, is only the foundation — it is the work carried out in the vineyard that delivers results. This year’s acid levels will largely depend on how dry and hot August turns out to be. Still, with careful vineyard management, the outcome can be excellent.
