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A Technical Guide for Wine Producers
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RECENT ARTICLES | WYNBOER HOME
Vine Balance: Its Importance To Successful Cultivation
Edited by Prof. E Archer, Stellenbosch University
INTRODUCTION
In the wine world everybody agrees that the quality of a wine is directly related to optimal ripeness. The question to be asked at this point is: what is optimal ripeness? In short, optimal ripeness means that the grapes, when pressed, must have the right sugar, good acid (high tartaric acid and low malic acid), low pH and good colour (in the case of red grapes) and flavour in accordance with a specific wine goal.
The most important factors that influence optimal ripeness in the vine are: terroir (physical and chemical characteristics of the soil and climate), the growth pattern of the vineyard and the microclimate inside the canopy. The above factors will therefore also play a big role in the formation of tartaric acid and malic acid in the vines.
In order to manage grape composition for optimal acid development, it is therefore necessary to manage the above factors. Seeing that soil and climate in South Africa are a given, the focus will fall on that which is manageable, namely the growth pattern and the canopy microclimate of the vineyard.
TOTAL ACID
The three most important organic acids that occur in the grape berry, are tartaric acid (TA), malic acid (MA) and citric acid. Tartaric acid and malic acid constitute 90 % and more of the total acid concentration, while citric acid only occurs in very limited quantities in the grape berry.
Both the leaves and the berries are able to form tartaric acid and malic acid. The young leaves in particular play a role in the synthesis of tartaric acid and malic acid. Photosynthesis of the green berry may contribute up to approximately 50 % of the synthesis of tartaric acid and malic acid. Tartaric acid is mostly synthesised from sugars and malic acid mostly from pyrovates or phosphoenolpyrovates (Fig. 1).
The distribution of tartaric acid and malic acid in the berry differs during berry development. In the developing berry tartaric acid usually occurs towards the outside of the berry and malic acid towards the inside, with the highest concentrations of malic acid in the pulp of the berry. In the ripe berry the acid concentration shifts, with the highest concentration of total acids occurring near the pips and the lowest concentration of total acid occurring near the skin (Fig. 2).
Tartaric acid already starts to accumulate during Phase 1 of berry development and reaches a maximum concentration shortly before véraison. As soon as véraison occurs, as well as during Phase 3, there is a sharp decline in tartaric acid in all parts of the berry. Malic acid, on the other hand, starts to accumulate before véraison and reaches a peak even before the onset of véraison. During véraison malic acid sometimes occurs in higher concentrations than tartaric acid in the berry (Fig. 3).
Following véraison and during ripening there is a decline in total acid which may be ascribed mainly to the following:
A very important component of the grape is phytochrome. It is a light receptor that intercepts photones at two specific wavelengths (660 nm red light and 730 nm farred light) in order to provide energy for certain enzyme reactions to be able to take place. This phytochrome occurs in an active and an inactive form. In dense canopies the ratio between red and farred light is more in favour of red light and the phytochrome is mostly inactive. In less dense canopies this ratio is inverse and most of the phytochrome is active. The energy that is thus generated is used to precipitate certain enzyme reactions in the plant. The most important enzymes that play a particular role with regard to organic acids and pH are:
Dense canopies are consequently detrimental to the function of these enzymes and the joint result is that the acid concentration drops and the pH increases.
pH
pH may be defined as the negative logarithm of the hydrogen ion concentration in a medium, in other words the free hydrogen ions in solution. The higher the concentration of free hydrogen ions, the lower the pH. The most important sources of hydrogen ions in the grape berry are tartaric acid and malic acid which are only partially ionised. Only the free part of the acids' hydrogen ions determines the pH.
During ripening the pH of the grapes changes as a result of the change in acid composition, acid concentration, dissociation of acids and formation of salts. Initially the pH drops from just after berry set, whereafter it remains constant for a while and then begins to increase rapidly as dilution of the acid takes place and/or the acids are neutralised.
Factors that play a role in the change in pH are climate, soil, water status, cultivar, plant material and cultivation practices. Potassium also plays an important role in the pH balance of grapes and wine. If high potassium levels are present in the berry, the hydrogen ions of the acids are displaced by the potassium ions. Acid salts (tartrates and malates) are formed, which means that the pH increases, as a result of lower concentrations of total acid. Cultivation practices such as nutrition and canopy management, for example, also play a significant role with regard to the pH of grapes.
pH plays a very important role in the colour, taste and stability of wine, however. In red cultivars high pHs reduce the colour stability of the wine as a result of the tannins occurring in an insoluble form in the wine. These tannins sink down and consequently reduce the colour of the wine. Furthermore anthocyanins occur in an uncoloured form in high pHs, which contributes even more to lower colour stability.
The freshness and complexity of wines are also lower when the pHs are high and the wines tend to be "flabby". High pH wines oxidise more readily, do not mature as well and are microbiologically unstable as a result of the free SO2 which is less effective when the pH is high. The effectiveness of bentonite is also reduced by high pH. When the pH is lower, proteins precipitate more readily and potassium bitartrate occurs in the undissociated form, which means that the wine is more protein and cold stable.
THE EFFECT OF GROWTH PATTERN ON THE ACID COMPOSITION AND pH
When vigorous growth takes place, excessive shadowing of the leaves occurs and the photosynthetic activity is therefore reduced. This results in the accumulation of potassium ions, which are translocated to the berry, which in turn causes the neutralisation of the organic acids in that the hydrogen ions are displaced by the potassium ions. The acid concentration is therefore reduced. Seeing that the sunlight in the canopy is not sufficient, nitrate reductase does not function optimally and potassium is not used effectively in the berries, which means that the hydrogen ions of the acids are displaced by the potassium ions. As already mentioned, overshadowing also causes the suppression of phospho-enolpyrovate carboxylase and tartrate synthetase and consequently sufficient synthesis of malic acid and tartaric acid is not possible. The reduction in organic acids then obviously results in a higher pH.
In vineyards with poor vigour the tempo of photosynthesis is normally high, there are few yellow leaves and little potassium is translocated to the bunches, with the result that conditions are favourable for a high acid and a low pH. This is not the case, however, since the leaf to fruit ratio is too small. This causes the berries to begin respirating as a result of insufficient active leaf surface and high berry temperatures of grapes hanging in the sun. Photorespiration therefore causes the nett product of photosythesis to be considerably lower, with a resulting reduction in acid content and therefore an increase in pH. During respiration it is usually the organic acids that are first used as substrate to form the sugars. The simplified comparisons for photosynthesis and respiration are as follows:
Shoot length also plays a significant role in the acid concentration and the pH of grapes. In the case of short, weak shoots the organic acid content was found to be low and the pH therefore high. In the case of excessively long shoots it was also found that the composition of the grapes was not ideal, with high total titratable acid and high pH.
For optimal acids and pH the ideal would therefore be to obtain an average canopy with the above conditions. This means that moderate vigour should be envisaged in order to obtain optimal use of sunlight. The most important factors influencing moderate vigour are: soil, rootstock & scion combination, trellis system, vine spacing, pruning policy, canopy management practices, water housekeeping and availability of nutrients.
MANAGEMENT ACTIONS FOR OPTIMAL ACID AND pH
South Africa is a warm winemaking country and one should accept that the same acid concentrations and pHs as in cold winemaking countries cannot possibly be obtained. To achieve optimal acids and pHs in the light of this fact, the goal must be optimal canopies. This means that an attempt should be made to obtain sufficient exposed leaves, protected grapes and moderate vigour. Already in the planning stages of a vineyard the above requirements should be borne in mind.
Ensure that the right cultivar stands on the right soil and that the combination of rootstock and scion induce moderate growth. The plant width and trellis system must be such that it will be able to handle the growth in order to achieve optimal leaf exposure. Yellow leaves as a result of excessive shadow or stress conditions must be avoided at all times, seeing that the yellow leaves are the source of potassium in the bunches. During vine development care must also be taken to develop well-balanced vines, in other words vines with straight trunks so that the cordon arms are equally strong and even in length (Fig. 4). The trellis system must keep the shoots straight and prevent toppling over with the subsequent grape exposure (Fig. 5).
Canopy management for leaf exposure must be applied in such a way as to obtain the biggest possible effective leaf surface for photosynthesis to occur optimally. This process starts with pruning and already then the correct bearer spacing (± 12cm) and the number of bearers per vine or per metre cordon (approximately 8 bearers per metre) must be implemented. Stringent suckering should take place in spring, leaving sufficient shoots per bearer for good sunlight penetration and aeration (approximately 16-20 shoots per metre cordon - Figs. 6 & 7).
Shoot growth and shoot lengths may be controlled through dedicated tipping actions during the season. Stringent topping must be avoided at all costs since insufficient shoot lengths are obtained and a lot of young leaves removed. As mentioned before, the young leaves play a very important role in the production of acids and as many young leaves as possible (including lateral shoot leaves) must be retained during the ripening process. Shoots must also be tucked in and positioned regularly and in good time in order to obtain optimal leaf exposure.
To achieve optimal acid and pH, all stress conditions must be avoided. Stress conditions may include the following:
It is clear that there are a number of factors influencing the acid composition and pH. These factors form a complex network of management actions that may be executed in the vineyard so as to manage the grape composition and pH. In a nutshell, the right long term decisions, together with good and healthy canopy management practices, play a very significant role in the acid composition and pH of grapes and an attempt should therefore be made to execute and plan the above practices and decisions with the ideal constantly in mind.
REFERENCES
ARCHER, E. & VAN WYK, J., 2000. Die Bestuur van Druifsamestelling. Lecture delivered at Pinotage Seminar and written by J. van Wyk. |
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Willem Botha (pictured), Viticultural consultant: Klein Karoo, VinPro Viticultural Consultation Service, Robertson
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