The aim of any grower is a high yielding, top quality wine grape that satisfies the wine-maker. However, many vineyard managers and wineries believe that a high yielding grape crop cannot produce quality wine. We believe this apparent conflict has not been rigorously tested. There are many examples around the world of vineyards producing top quality wine from high yielding grapes that have been provided with a balanced nutritional programme.
Wine grape quality requirements vary between countries, as well as between individual wineries. Many have been established as a result of local preference, taste and tradition and it is important to meet the specifications demanded by the buyer. The major wine grape quality parameters include total soluble solids (TSS), acidity, pH of the juice/must, anthocyanin content, and arginine concentration.
Determination of maturity is very important and close monitoring of the stage of development of the crop, using local experience and a knowledge of the variety, allows growers to plan harvest dates well in advance. As harvest approaches, the key checks needed to confirm maturity include measurement of the sugar content and acidity of a sample must.
There are a large number of factors that affect top quality wine production. Many are within the control of the grower, under given climatic and soil conditions.
Crop Nutrition and Wine Grape Quality
Nitrogen has a direct effect on wine grape berry size. Strong growing vines as a result of too much nitrogen have been shown to have higher acidity levels and can create strong canopy growth, increasing shading and pH. High nitrogen levels can create vigorous growth and shading, increasing wine grape berry size and thereby reduce the ratio of grape skin to pulp. Total nitrogen content as well as Nitrogen-composition, influences the speed and quality fermentation, as well as the alcohol production process.
Phosphorus affects yield directly, because it is the central component of energy required during metabolism and transportation of photosynthetic products. Application of phosphorus fertilizer increases wine grape berry setting and crop yield.
Potassium has also a marked effect on the total acidity, helping to neutralize acids and controlling the acidity of the grape juice. There is a strong relationship between potassium and pH. Too much potassium results in a high wine pH and a poorer quality wine. Excessive potassium can increase grape pH, which influences the anthocyanins, reducing the intensity of the red coloration
Calcium is a key component of cell walls maintaining membrane structure. It also directly influences the regulation of enzyme systems, phyto-hormone activities, and nutrient uptake. Insufficient supply of Ca limits crop growth and yield. Low calcium concentration in fruit lead to earlier maturity and softer skin.
Sulphur is a component of enzymes and proteins. It is required for chlorophyll formation. Sulphur deficiency is rarely observed in vine orchard as a result of high rates of S containing fungicide (80 – 95% elemental S) is sprayed seasonally.
Boron influences wine grape berry size and has a positive influence on wine grape TSS. The nutrient has also a positive effect on wine grape berry colour.
Level of zinc supply affects growth and yield of crop, because zinc is involved in development and function of growth regulators that influence internode elongation. Application of Zn fertilizer resulted in increase of fruit setting.
Other Crop Management Practices Influencing Wine Grape Quality
- The best wine grapes need a long, hot, dry season to ensure good, early maturity and the right mix of sugars and acidity, to provide a good wine.
- Adapting canopy structure to bring on or delay berry maturity will also have a direct effect on TSS and acidity as well as wine colour.
- The manipulation of moisture availability is important in ensuring the right berry size and the desired concentrations of sugar and phenolic concentrations of sugar and phenolic compounds in the wine.