The chemical composition of the grape is generally much more important than physical characteristics. Nonetheless, poor handling or incorrect viticulture that leads to split grapes or disease problems at maturity will influence the quality of the wine. Split grapes can lead to oxidation of the must and reduced ‘freshness’ of the wine. In addition, the solid matter – stems, skin, and pips – may begin to disintegrate, releasing bitter tasting substances into the wine.
Grape market 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.
It is also important to ensure cleanliness at harvest so that contamination from dust, soil, or from oils or lubricants on mechanical harvesters is minimized and rejections by the wineries avoided.
Berry size influences the concentration of a range of compounds in the wine making process. Smaller berries will have a greater proportion of seeds and skin to the pulp. This affects the levels of phenolic compounds, particularly anthocyanin, which is the main colorant in red wine, as well as tannins and other flavor-giving compounds.
An even berry size across the vine is thus highly desirable, to ensure an even grape maturity and to maintain consistency of wine production.
Moulds such as Botrytis and bird or pest damage can adversely affect the winemaking process and result in poor wine quality. High levels of disease or pest damage can lead to rejection or reduced prices.
Grapes stored at high temperature and those found to be fermenting upon arrival at the winery, may be penalized with reduced prices, or be returned to the grower.
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. In regions where acidity levels tend to diminish rapidly during maturation, e.g. under high temperatures, it is sometimes necessary to harvest the grapes before they are completely ripe.
In colder growing regions where sugar levels are insufficient, delayed harvesting of over-ripe berries is the norm.
pH of Grape Juice/Must
The pH of the juice/must affects the taste, sugar/acid balance and stability of wines. In red wine the colour is also effected. It is determined by the balance of the major anions (malate and tartrate) and presence or absence of major cations (mainly K).
Thus, it is important to strike the right balance of potassium in the grape. In France, for example, accepted levels of potassium in the must of French wines are between 1-2.5g/l, resulting in 0.7- 1.6g/l in the final wine.
High pH values can also alter the colour of anthocyanin solutions – responsible for red wine colour - by changing the structure of the anthocyanin molecule, which becomes bluer and therefore less desirable.
A high pH must, results in wines that have a flat taste and red wines with a brownish colour. Generally, a pH above 3.6 is undesirable as it can have a negative effect on a range of wine quality characteristics.
Nitrogen Compounds in the Must
The total nitrogen content of wine grapes as well as the relative distribution of nitrogen compounds influences the production of yeast biomass and fermentation rate. It can also affect the different types of alcohol present, as a result of yeast metabolism, in the resultant wine. Free amino nitrogen (FAN) is often used as a measure of the suitability of nitrogen types available for yeast growth and fermentation.
Arginine, one of the amino acids present in high concentrations, can be used as an indicator of the nitrogen richness or quality of the must. On the other hand, levels of proline, another amino acid present at high levels in the must, bare little relation to the total nitrogen content. For complete, good quality fermentation, a must containing a minimum of 140 mg assimilable nitrogen/l is required. Without these levels of nitrogen in the must, there is a reduced capacity to produce alcohol resulting in a sluggish or stuck fermentation.
Total nitrogen content, as well as arginine concentration, is directly influenced by application timing of nitrogen fertilizers to the vine. As with potassium, the correct balance of nitrogen for vine growth and grape development is important.
Phenolic compounds directly affect the colour of the wine and help preserve its flavour and aroma. They are located in the grape skin cells and consist mainly of anthocyanins and tannins.
Berry size has a direct effect on the colour of the wine. The smaller the berry, the greater the proportion of skin used to make the wine and the greater the concentration of phenolic, colour-conferring compounds in the must. A high anthocyanin content provides a strong red, higher quality wine. Colour can be assessed visually or by measuring anthocyanin levels. Anthocyanin levels in wines generally range from 0.1 - 8g/l.
The final quality of the wine is mainly determined by sensoric or organoleptic tests ("look, smell, taste"), and is expressed by a rating or ranking from 0- 100 points.
Sensory analysis is often viewed as a skilled art rather than a science. Different countries have a completely different range of organoleptic ratings to determine the quality ratings of any wine.