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Investigation into the role of potassium and nitrogen in must stability, with special reference to organic acids

by | Oct 28, 2020 | Uncategorized

Project Number
WW 08-13

Project title
Investigation into the role of potassium and nitrogen in must stability, with special reference to organic acids

Project leader
Hunter, J J

Team members
Volschenk, C G
Conradie, W J

Project description
The effect of different nitrogen and potassium fertilisation combinations 0:0, 50:50, 100:100, 50:0, 100:0, 0:50, 0:100 kg per hectare, respectively) on must stability of Sauvignon blanc was investigated at Nietvoorbij. The potassium and inorganic contents of the soil varied according to the fertilisation level. Vegetative growth was increased by all fertilisation treatments, particularly the nitrogen:potassium combinations – that resulted in a poorer canopy microclimate and bunch exposure, while tendencies of water stress occurred. Canopy development in the early summer period was, however, stimulated and the source capacity of the leaves and thus contribution to the bunches accelerated. For the potassium fertilisation treatments and the 50:0 nitrogen:potassium treatment generally less vegetative growth in comparison to the rest of the fertilisation treatments was found. Both nitrogen and potassium fertilisation (separated and in combination) increased yields, mainly via a stimulation in berry set. However, as a result of fertilisation, a large percentage of berries were shed after berry set, probably because of competition with vegetative growth. The fast development of the canopy in summer therefore also had negative implications. Noticeably more bunch rot also occurred for the nitrogen:potassium treatments. The high nitrogen and nitrogen:potassium fertilisation treatments increased the free amino nitrogen content and arginine concentration of the must. For the potassium treatments, addition of nitrogen to the must would be necessary.

At ripeness, the nitrogen content of the leaves and berries was only increased with nitrogen fertilisation. Potassium fertilisation had no effect on the nitrogen content and even decreased the nitrogen content in the bunches at ripeness. The potassium content of the berries was increased to a larger extent by nitrogen:potassium combinations than by potassium fertilisation alone; no differences occurred in leaves. At veriason, maximum and higher potassium levels were induced in the leaves by nitrogen:potassium and potassium fertilisation. The accumulation of high levels of both nitrogen and potassium in the soil must be avoided in order to restrict the build-up of potassium in the bunches. Potassium in particular actively binds to tartaric acid to form tartaric acid salts and excessive translocation of this mineral to the bunch must therefore be avoided. The nitrogen:potassium fertilisation stimulated the accumulation of malic acid in the bunches. In spite of the increase in lateral shoot growth and potential source of tartaric acid resulting from that, a high accumulation of tartaric acid in the bunches of the fertilisation treatments were restricted by the simultaneous increase in canopy shade and translocation of potassium to the bunches. Vigorous growth and shade conditions favour the accumulation of malic, but inhibit the accumulation of tartaric acid in the bunches. The nitrogen and potassium fertilisation therefore have direct as well as indirect effects on tartaric acid recovery. Lateral shoots nevertheless had generally higher tartaric concentrations than main shoot leaves (malic acid concentrations were slightly less). Except for potassium, lateral shoots also had a higher mineral content, particularly nitrogen, than the main shoot leaves. That may favour the accumulation of tartaric acid in the bunches and facilitate fermentation and flavour development. Vigorous growth must therefore be efficiently accommodated and the removal of lateral shoots avoided.

Tartaric acid losses already occurred in the intact berry and were generally increased by fertilisation, particularly the nitrogen:potassium combinations. Before pressing, the skins contained 50% tartaric and malic acids, while the rest was mainly located in the pulp at 50% tartaric and 45% malic acid. After pressing, a tartaric acid distribution of 47:2:41:10 was found in the skin, pulp, seeds, settled juice, and lees fractions. Malic acid differed with respect to the settled juice and lees. Approximately 58% tartaric acid and 47% malic acid were lost during pressing. Here, salt formation in the skins and pulp played a big role. Winemaking procedures which would increase the recovery of organic acids during grape pressing must be investigated. The highest losses as a result of salt formation occurred with 100:100-, 50:0- and 0:50 treatments. The tartaric acid concentration in the settled juice was effectively decreased by fertilisation. That has negative implications regarding must stability as well as must and wine quality. Improved canopy microclimate and bunch metabolism can contribute to the utilisation of tartaric acid, which is available in the leaves as a result of growth stimulation. Fertilisation increased the aroma of the wine. The best wine quality was found for the 50:50 nitrogen:potassium treatment. Despite the increase in aroma composition, the rest of the treatments either had no effect or decreased wine quality.

The effect of canopy management on the utilisation of the potential positive impact of a more vigorous canopy on grape, must and wine quality as well as must stability was also investigated. Only the 100:100, 50:0 and 0:50 nitrogen:potassium treatments and a non-fertilised control were used. Canopy management treatments comprised suckering and topping (control), and suckering, topping and leaf removal, respectively. Canopy management increased yields and total soluble solids in the must. In most cases titratable acidity was also slightly increased by canopy management, whereas the pH of the must was unaffected or decreased. Higher tartaric acid concentrations in both berries and settled juice were induced by canopy management, particularly when nitrogen fertilisation was applied.

The effect of different nitrogen and potassium fertilisation combinations (0:0, 100:100, 100:0, 0:50, 0:100 kg/ha, respectively) on must stability of Merlot was also investigated. The potassium fertilisation resulted in marked tartaric acid losses in the settled juice because of salt formation. In spite of the lower pH, skin colour was decreased by 100:100, 100:0 and 0:50 treatments. For the 0:100 treatment, skin colour was increased; improved light conditions probably played a big role in this. The data indicated that as for Sauvignon blanc, high applications of nitrogen and potassium, in combination and individually, must be avoided in order to prevent a decrease in skin colour and ensure must stability.


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