Over recent decades, the alcohol concentration of wines has increased due to the higher sugar concentration of grapes when harvested. This also led to an increase in pH and a decrease in the total acid concentrations of wines.
Tartaric and malic acids are the two major fixed acids in grape berries, with tartaric acid being the strongest. Tartaric acid increases during the early stages of berry growth but decreases after further development, especially after véraison. Tartaric acid, like malic acid, is not usually used in respiration metabolism, whose concentration decreases with increasing metabolic respiration. Rising temperatures increase respiration. The decreasing concentration of both acids leads to unfavourable acid-sugar and acid-aroma balances. This trend can be caused by climate warming.
The role of potassium in total acid concentration
Potassium in the form of its ion (K+) decreases the total acid concentration due to the formation of potassium bitartrate, which influences the free tartaric acid concentration. The potassium bitartrate precipitation may require more acid adjustments, adding to the production costs of the wine. K+ ions are about 80% of all cations in grapes and mainly accumulate during ripening.
Different ways of decreasing the K+ concentration in grapes
Decreasing these ions may be a logical solution for addressing the total acid problem, but it is not so simple. K+ ions play a critical role in cell membrane function. The potassium accumulation stops about the same time as cells begin to die in the berry. Seeing that K+ deficiency should be avoided, other ways should be addressed to prevent access to it. Many soils are high in potassium, and rootstocks with limited uptake should be selected. Slowing the ripening process can also limit the uptake. This can be obtained by adapting the reproductive and vigour balance by increasing the yield or the number of bunches and leaf removal.
By limiting the direct exposure of berries to solar heating, the temperature of the berries will be lower, causing less respiratory loss of malic acid. Evaporative demand and wind influence transpiration and the transport of nutrients like potassium. Opening the canopy in different ways will limit K+ accumulation, but in warmer regions, this benefit is cancelled by the negative impact of overexposure to solar radiation. By decreasing irrigation, the K+ uptake can also be reduced. However, it is risky because other essential nutrients may also be limited, and stress conditions of the vine can cause more respiration, leading to lower malic acid and total acid concentrations.
Cation antagonism is where competition exists between different nutrients for root uptake. Mg++, Ca++ or Na+ cations, if available in high concentrations, may substitute the K+ uptake by the roots. However, research regarding this to lower the K+ uptake by vine roots has been inconsistent.
Different vineyard surveys
Different vineyard surveys have been implemented to investigate the role of K+ concentration on the pH and total acid concentration of grapes. The vineyard blocks varied in cultivar, soil types, altitude and vine age to cover a range of growing conditions. The main results can be summarised as follows:
- The juice pH was positively correlated with the K+ concentrations of Chardonnay, Sémillon, Pinot gris and Shiraz, but the trend was not so strong with Cabernet Sauvignon.
- The juice total acid concentration was negatively correlated with the same cultivars.
- There was a positive correlation between berry and juice K+ concentration in some of the cultivars, indicating differences between the berry compositions of different cultivars.
- Petiole K+ concentrations were inversely correlated with petiole Mg+2 concentrations, indicating the possibility of cation antagonism.
- A high Mg/K ratio is associated with lower pH and higher total acid concentration in the case of several cultivars, but it depends on the cultivar, region and season.
Although further research is required, the following actions can be recommended to correct berry pH and total acid concentration:
- Assess soil nutrient analysis, especially the cation exchange capacity (CEC), K, Mg, Ca and Na.
- Reduce K fertilisation if not required and apply Mg fertilisation if needed.
- Tissue analysis should be applied if the pH and total acid concentration are not within the required range. If necessary, adjustments should be made to improve it.
- Monitor the soil and plant water status to avoid over-irrigation.
- The monitoring of the K/Mg ratio should become part of soil nutrient management with an emphasis not only on growth and yield but also on berry composition.
Rogiers, S.Y., Baby, T., Xiao, Z., Holzapfel, B.P., Schmidtke, L.M. & Walker, R.R., 2023. Australian and New Zealand Grapegrower and Winemaker 709: 34 – 42.