The pH of wine grapes, in addition to flavour and colour, is one of the most important and controversial quality parameters for making wine. The pH of grapes is a manageable component in red and white cultivars, especially in irrigation areas.
Influence on wine quality
In grapes or wine a high pH could have the following negative effects on quality:
- Juice with a higher pH oxidises more easily.
- The taste, as well as the freshness and complexity of red wine in particular, are affected negatively by a pH higher than 3,5-3,6, thus producing flabby wines.
- Wine made from grapes with a high pH do not age well and develop “off” flavours more easily.
- High pH juice is microbiologically unstable.
- Free SO2 is more active when the pH is low.
- Colour stability is poor when the pH is high.
- Protein precipitates more easily when the pH is lower and consequently wines that ferment at a lower pH, contain less protein (are more protein stable).
- Tannins are insoluble at a high pH. This explains why the colour of such wines has no staying power.
- At a low pH, potassium bitartrate occurs in an undissociated form. It therefore remains in solution and is consequently cold stable.
- The effectiveness of bentonite is reduced by high pH in wine.
- At a high pH, the addition of much more acid is required, which has cost implications.
- Juice with high pH (eventually resulting in wine with high pH) can be rectified in the cellar by one of two methods, namely:
- By the addition of extra acid (e.g. tartaric acid). This technique has limited value, however (Personal communication, C.J. van Wyk, U.S. 2001), since the addition of tartaric acid to wine with a pH of 3,6 and more, seldom results in the desired reduction in pH, and the pH may even increase as a result of the precipitation of potassium bitartrate (Butzke, C.E., & Boulton, R.B.) Moreover, tartaric acid is relatively expensive: R45-R50/kg. Therefore it will cost 12 cents/litre to make an acid adjustment of for example 2,5 grams/litre using tartaric acid to adjust the acid in the must (at the start of fermentation) to 7,5 grams/litre. As mentioned, the addition of acid will not necessarily reduce the pH in a linear manner, due to the chemical buffer characteristics of wine.
- By blending wines that have a high pH with wines that have lower pH values, an acceptable pH may be obtained. However, this option is not desirable, since wines of various quality levels have to be blended to adjust the pH, which has a negative impact on the overall quality.
It is clear that pH plays a very important role in wine quality, and that pH adjustments in the cellar offer a limited solution, but are by no means ideal.
pH of grapes
What is pH pH is simply a figure that indicates the negative logarithm of the H+ ion concentration (the “acid”) in a solution. A pH of 3 means there are 10-3 H+ ions in solution and a pH of 4 means there are 10-4 H+ ions in solution. These H+ ions obviously derive from the organic acids occurring in the berry.
The most important organic acids in grapes are D-tartaric acid (Ta.A) and L-malic acid (MA) which together are responsible for 90% or more of the total acids (TA). Acid in ripe grapes is determined largely by relative concentrations of Ta.A and MA – especially by the potassium salts (Winkler et al., 1974.) The ratio of tartrates (salts of Ta.A) to malates (salts of MA) differs considerably and depends mainly on the environment, and is to a lesser extent cultivar dependent. Cultivars with inherently higher malate concentrations or tartrate concentrations do occur, but so far experimental values have shown that there is little significant difference among the top six cultivars (Merlot, Cabernet Sauvignon, Shiraz, Chardonnay, Pinotage, Sauvignon blanc) provided the management practices are correct. The ratio of Ta.A to MA can range from 0,6 to 3,4, but since Ta.A is a stronger acid than MA, it makes more sense, with regard to low pH, to have Ta.A in the composition of a grape (Winkler et al., 1974).
Formation/synthesis of acids
According to Winkler et al. (1974) photosynthesis in the green berry is responsible for approximately 50% of the accumulating acids (to before veraison). Accumulation of the acids occurs at the beginning of berry development and has already progressed quite a bit by the onset of ripening. The young leaves in a vineyard’s foliage are then responsible for producing acid precursors, conveying them to the bunch, and by so doing maintaining the amounts of acid in the bunch, while the original acid concentration is reduced by respiration, metabolisation, K+-H+ exchange, etc. (as discussed below)
During ripening of grapes the pH changes constantly from about 2.8 to values of 3.5 and more, depending on the cultivar, the growing season and the macroclimate (Amerine et al., 1970.) This increase in pH occurs simultaneously with a sugar accumulation, and a reduction in tritatable acid.
The breakdown of acids
The reduction in the concentration of Ta.A and MA during the ripening process may be ascribed to the following factors:
- A reduction in the translocation of acids from the leaves to the berry (Amerine, 1956; Hardy, 1969);
- The metabolisation of organic acids to sugars (Drawert & Steffan, 1966; Ribereau-Gayon, 1968);
- Dilution of the acids due to an increase in berry volume (Winkler et al., 1974);
- Increased accumulation of potassium which combines with Ta.A and MA in the berry to form salts (Saito & Kasai, 1968; Kliewer, 1971);
- A reduction in the ability of the berry to synthesise organic acids during ripening (Hardy, 1969);
- An increase in membrane permeability, causing the acids to accumulate in the cell vacuoles and mainly allowing malic acid to be respirated (Hardy, 1968; Kliewer, 1971). Respiration of malic acid usually takes place more rapidly in warm areas, especially where grapes are hanging in the ‘open’, and are therefore exposed to direct sunlight which heats the berries (Butzke & Boulton, 1997).
pH values are influenced mainly by the ratio of Ta.A:MA, as well as by the extent to which the H+ of these acids has been translocated by potassium. Tartaric acid is a much more stable acid than malic acid. Writers from various origins have also found tartaric acid to be more stable with regard to increases in temperature (Ruffner, 1982).
Reasons for high pH in grapes
Since no enzymes that are able to break down Ta.A in grapes could be identified, the reduction in TA as a result of respiration can be ascribed mainly to the breakdown of MA via the Krebs cycle (Winkler et al., 1974). In cooler seasons or cultivation areas, less malate is respirated, thus contributing to high titratable acid concentrations and the accompanying low pH that commonly occur under such conditions (Beelman, 1984). The reverse is also true in warm seasons or in warm cultivation areas.
According to Boulton (1980) a significant exchange between K+ and H+ ions occurs in berries during ripening, causing an increase in pH as a result of the nett loss in H+ ions. He determined, through experimentation, that it was mainly the measure of exchange between K+ and H+ ions, and not only the amount of K+ ions in the juice, that caused the pH increase. He also proved that, in any degree of exchange, a lower tartrate to malate ratio in the juice would cause a bigger increase in pH. This exchange between K+ and H+ ions is helped along by dense foliage and/or excessive vigour in vineyards.
In many areas with a cool climate and long daylight hours in the growing season, the grapes develop not only high sugar concentrations, but also high TA and pH. The cool conditions are probably also conducive to the preservation of high malate concentrations. Furthermore it is also possible for higher than normal exchange of K+ with H+ to occur in the berries as a result of the competition between sugar accumulation and K uptake from the leaves. The longer growing season and period in which the grapes are exposed to potassium uptake from the leaves, can therefore have a more negative effect on pH than in a short growing season (Butzke & Boulton, 1997). In this instance, therefore, respiration of MA is not the problem, but exchange between K+ and H+ ions.
Zoeklein (1982) found that any activity that may influence carbohydrate production and translocation, will also affect potassium levels in the grape, and consequently also the pH of grapes. He demonstrated that the reduction of excessive leaf surface/vine, the removal of yellow leaves in compacted foliage and the limitation of excessive vigour in the vineyard, would bring about drastic reductions in potassium translocation to the bunches, and therefore stabilise the grapes at excessively high increases in pH.
According to Smart (2001) and various other writers, the growth point of the shoot displays the biggest demand for carbohydrates in the vine. This demand is especially important during flowering and veraison. The second largest point of demand is the grape itself and the third biggest, the building of reserves in the trunk and roots. When a shortage of photosynthetic product is created as a result of insufficient leaves/bunch, the growth point will therefore benefit to the detriment of the ripening bunch, thus delaying ripening. Sugars therefore accumulate slowly, colour is poor and the pH is high, because malate, and therefore malic acid, must be metabolised to form sugars.
Archer (2001) compared the sugar, acid and pH of Cabernet Sauvignon, Merlot and Sauvignon blanc experimentally on three different shoot lengths. Shoots of + 60 cm, +120 cm and >2 m were compared. Conclusions to be drawn from the results of this experiment, are that shoots that are either too short (too little effective leaf/bunch) or too long (excessively vigorous) produce grapes with significantly lower sugar content, poor flavour components and high pH. It is interesting to note that excessively vigorous shoots produce grapes with a high total acid content, while still causing the pH to be high (>3.7). This can be blamed on excessive shade levels in the foliage. The same phenomenon was observed in 2001 in vineyards along the Olifants River, by comparing the analyses of grapes on shoots displaying excessively vigorous growth (> 300 cm/shoot), moderate growth (120 cm/shoot) and poor growth (30 cm/shoot). Excessively vigorous vines and vines with poor growth from the same block produced significantly higher pH values than vines with moderate vigour. (Table 1.)
Potassium uptake through grape roots occurs through membrane bound ATPase in the cell-walls of roots. Potassium accumulation in the grape vine is therefore enzyme controlled, and may be inhibited by unfavourable conditions for enzyme working (e.g. cold weather). Freeman & Kliewer (1983) proved that between set and ripeness, shoots and leaves are nett exporters of potassium to the rest of the vine. Accumulation of potassium occurs mainly in bunches due to the availability of ATP in the bunches. In warmer cultivation areas, where higher pH values in grapes are a common occurrence, the enzyme activity is higher as a result of favourable temperature, and the TA levels are relatively low due to the respiration of malate. The tempo of respiration usually doubles with each 10C increase in temperature (Smart & Robinson, 1992). High crop load causes delayed potassium accumulation per berry, as well as TA in grapes as a result of a dilatory effect, but it also delays ripening, and in warm areas, together with malic respiration, it will result in low TA levels. (Boulton, 1980)
The tempo of potassium uptake through the roots appears to be independent of the amount of potassium available in the soil. When potassium concentration in soils is too low to saturate the (H+/K+)-ATPase, uptake will be linear to the soil’s potassium status. Should the cultivar and/or rootstock grow vigorously, the effect of the potassium status of the soil could play a more significant role in the grape’s pH (Butzke & Boulton, 1997). This is probably due to the larger amounts of potassium required by vigorous vineyards for the bigger mass of exposed leaves that photosynthesise and transpire (Esteman et al. 1999). Potassium content of soils in the Vredendal vicinity is generally high (pH = 7.5 – 8), especially in the traditional karoo and silty soils. With irrigation, potassium will be lyed out of the soil, especially the deep, red sandy soils, but shortages seldom occur (Personal communication, F.Ellis, U.S. 2001). Consequently factors that inhibit excessive vigour (rootstock, scion, irrigation management, soil potential, etc.) will also limit potassium uptake through the roots, with the indirect result that pH in grapes will be lower (Boulton, 1980). Summary High pH values in grapes can therefore be ascribed to one or more of the following reasons:
- Warm climate/season – bigger malate respiration and K+ion exchange
- Compacted foliage – high translocation of K+ to bunches
- Excessive vigour – photosynthetic products are channelled to growth points only
- Too few effective young leaves per bunch to produce, inter alia, sugars and acids
Below follows a discussion of vineyard management to improve grape pH.
The management of a vineyard to improve grape pH
Together with flavour and colour, the pH of wine grapes is one of the most important, and controversial quality parameters for making wine. The pH of grapes is a manageable component in red and white cultivars, especially in irrigation areas.
Golden rule I: Balanced vines with moderate vigour
(Shoots with 15 – 25 nodes, to obtain a shoot length of + 120 cm (enough effective leaves to ripen grapes, with the shoot growth arrested naturally at/after veraison), especially in cultivars tending to a higher pH.
- Too few effective leaves per bunch will delay ripening, with the above consequences. (The ideal for white cultivars is 16 to 20 leaves, the ideal for red cultivars is 20 to 26 leaves)
- Avoid excessively vigorous vines that display active growth points after veraison, mainly by adjusting irrigation management. Excessively vigorous vines translocate photosynthetic products from the leaves to the apical growth point in particular, and the grape is forced to metabolise MA to form sugars for ripening.
- The trellis system that has been selected must be able to accommodate the vigour of the vine, so that the producer is not forced to top too much. With excessive topping, the capacity of the vine for photosynthesis, with regard to young leaves, is destroyed, or the vine is forced to form lateral shoots in the bunch zone, thus causing compaction.
Golden rule II: Exposed leaves, protected grapes
- Compacted foliage causes ineffective exposure to light (with short wavelengths) on the shaded leaves. Enzyme working (pep-carboxylase and tartrate synthetase), as far as the synthesis of organic acids is concerned, is repressed when there is a shortage of direct sunlight. Compaction of foliage must therefore be limited (Personal communication, E.Archer, U.S. 2001).
- Compaction of the foliage, in the bunch zone especially, causes yellowing of leaves, which, as a result of the working of nitrate reductase with limited direct sunlight, serves as a “potassium pump”, so to speak, to the bunch. This potassium therefore exchanges with the H+ of organic acids in the bunch, which in turn causes the pH to increase. Yellow leaves in the bunch zone must therefore be avoided (Personal communication, E.Archer, U.S. 2000).
- What is more, the vineyard can also be tipped regularly from a shoot length of 60-100 cm (depending on cultivar!), so that lateral shoots form above the bunch zone (in the top 2/3 of the foliage), which also doubles or triples the amount of young leaves that are exposed to direct sunlight. The production points for organic acids (Ta.A in particular) are therefore multiplied. The production point for organic acids (Ta.A and MA) during ripening is young leaves, especially young leaves on lateral shoots.
- Grapes should preferably not be totally exposed to direct sunlight, as the temperature inside the red grape berry could rise to 12C above the ambient temperature, thus causing the MA to respire, which in turn causes the TA to decrease and the pH to increase.
- A row direction of East-West to Northwest-Southeast in warmer cultivation areas is conducive to lower pH because the sun moves all day long over the top of the foliage, and consequently the bunches are less exposed to direct sunlight. Young leaves at the canopy of the foliage therefore also get maximum exposure to direct light.
- To a lesser degree pH in grapes may decrease if, at a late stage, the production of MA and Ta.A is faster than possible potassium uptake, e.g. with irrigation after veraison (Butzke & Boulton, 1997).
Avoid excessive potassium fertilisation, which may have a significant effect on pH, especially with regard to profusely growing cultivars. Unnecessary applications of nitrogen to a vineyard that is already vigorous, may cause further growth and compaction, with catastrophic results for general grape quality (pH, flavour, colour).
Regulated Deficit Irrigation and Partial Rootzone Drying are two irrigation tecniques, inter alia, to manipulate the berry size of grapes. Both Dry et al. (2001) and Esteban et al. (1999) have found that the reduction of the berry size through irrigation management results in higher TA, lower pH, higher anthocyanin and phenol levels, with no shrivelling towards the end of ripening in Shiraz grapes. The most likely reasons for this is the lower juice/skin ratio of the smaller berries, the higher concentration of acids and solids in the berry, as well as the less vigorous/compact foliage (and therefore improved microclimate) obtained by the moderate influence on vigour. Deficit irrigation at the wrong time of the season may be undesirable, however, e.g. drought stress shortly before ripening may cause the stomae of the leaves to close. This will prevent continued photosynthesis during this critical period of sugar accumulation, and the vine will metabolise organic acids in the berry to sugars. On the other hand excessive irrigation, especially during berry set, can result in ‘swollen’, large berries, which will have an accompanying dilatory effect on the juice. The dilatory effect on Ta.A in particular will cause the pH of the grape to increase. The tempo of potassium uptake by the roots is significantly higher in irrigated vineyards than in dryland vines. This is probably a result of the bigger stream of transpiration through the plant, since irrigated vines usually have a larger exposed leaf surface for transpiration, as well as easily obtainable water for the roots. (Esteman et al. 1999). An irrigation programme should therefore prevent the vineyard from experiencing excessive drought stress after veraison (after veraison, irrigation no longer has an effect on berry enlargement), or be too extravagantly irrigated during berry set (effect on berry size).
Unfortunately pH measurement at the weighing bridge during the harvest could be a misleading parameter of quality. When pH (as well as sugar) is measured, an average vallue is allocated. In practice this means that when a truck load of grapes with a mixed degree of ripeness is measured at the weighing bridge, the AVERAGE of the load could be right. This average could consist, however, of green grapes with sugars of + 19 balling and a pH of 3.1, as well as overripe grapes of 26.5 balling and a pH of 3.8. Furthermore the pH reading at the weighing bridge is also an optimistically low value of what the pH would be after crushing and during fermentation, seeing that the potassium from the skins is not taken into account when measuring at the weighing bridge. In the cellar the pH increases even more when the entire content of the bunch is taken into account. Furthermore the malic acid is also removed from the wine’s composition during malolactic fermentation. These problems will be eliminated to a large extent if homogeneous shoot lengths can be obtained in vineyards.
The measurement of pH on a scale is a very important, but limited measure of quality, and in future, with improved technology, the focus should also fall on the quantification of colour and the flavour intensity of grapes. Healthy and correct long and short term vineyard management practices remain the only way to ensure the improvement of quality, especially with a view to the impact this has on obtaining vineyards with homogeneous foliage.
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