Wine grapes and Vine plant characteristics
The wine grape comes from the plants of the botanical genus Vitis, and mostly from one important commercial species, vinifera, of the vine plant. This genus belongs to the family, Vitaceae, characterized by a climbing habit, and includes other genera such as Parthenocissus quinquefolia, otherwise known as Virginia creeper. The genus Vitis includes also other species, for example V. labrusca, which has some interest for wine-making. The Concorde grape is thought to be a cross between V. lambrusca and V. vinifera, with its characteristic content of the ester, methyl methranilate that gives Concorde its specific flavour. Other species produce grapes more suitable for eating fresh.
Varieties (and cultivars) of V. vinifera selected for specific flavour and agronomic purposes abound. An obvious division is made between grapes with light green skins for making white wines, and those with black skins, generally for red wines, though not exclusively so. Some of the 10 000 known varieties are described as ‘noble’, largely grapes of French origin. The prevalence of French ‘noble’ grapes is, to an extent, due to tradition and to the history of France. ‘Noble’ grapes are those that give wines that are appreciated by wine connoisseurs and wine-drinkers in general for their sensory properties. Table 1 shows eight noble ‘whites’ and seven ‘reds’ – although this selection is not without some controversy. There are many other viniferous grapes of quality, which are largely of local rather than international significance, such as the Tempranillo grape of Spain and the Nebbiolo grape of Italy. These and possibly others may well become popular in the future, since wine-makers are ‘seeking’ to test their skills on new varieties, while consumers like to drink new wines.
The main controversy probably arises around two of these ‘noble’ grape varieties, Müller-Thurgau and Gamay. All these grapes and their vines originate in France and Germany (except Zinfandel); most have international repute, having been successfully transported to the newer wine-producing countries of the world, such as the USA, New Zealand, South Africa and Australia, and to other longer established European wine producing areas.
|1 ‘Noble’ varieties of the wine grape.
A differentiating characteristic between grape varieties is their ‘aromatic’ property, some grape varieties being quite ‘neutral’, others being quite ‘aromatic’. Most of the odour/flavour components in wine, originate during fermentation and ageing, the so-called secondary and tertiary aromas. Some of the ‘noble’ varieties above have varying contents of terpenoid compounds, with the Muscat varieties having the highest. These compounds confer a ‘grapey’ or sometimes ‘floral’ flavour, which can give distinctive wines made from those varieties with sufficiently high terpene content, so that terpenes from the grapes constitute the main part of the so-called primary aroma in the wine. However, although many of these so-called ‘noble’ grape varieties tend to give wines with a recognizable sensory aroma, in most cases unique impact compounds have not (yet) been identified. The typical chemical composition of a grape variety supplies the ‘raw materials’ for fermentation and ageing processes, and even grape varieties with not very distinctive varietal aromas give wines with recognizable flavour characteristics, which in wine appear highly dependent on the grape variety.
These noble ‘quality’ varieties do not constitute necessarily the largest acreage of vines cultivated in any given country. Some quality grapes are widely planted, but there are other varieties that also fill that position (Table 2), which produce viniferous grapes destined for lower quality denominations such as Vin-de-France quality or they are used in blending wines. Some wines are specifically produced to be distilled into brandy.
There are several other grape varieties with quality connotation, which are currently mostly of local significance. The currently widely planted noble grapes were, to an extent, fairly haphazardly selected and in future we may well see other quality grapes gain in popularity with both the wine industry and the consumer. Viognier is an example of a grape traditionally grown in the Rhône region but now becoming more widely planted, and giving white wines with a distinctive varietal aroma.
|2 Grape varieties with the largest area of vine plantation.
|Table 3 Some grape varieties with a poor quality connotation.
There are also some grape varieties that have a poor quality connotation (Table 3), although they are widely planted (Table 2). Presumably their popularity is due to other factors, such as good yield. The quality connotation may not always be well deserved, for example, Carignan can give good wine, especially from old, low yielding bush vines.
Vine plant characteristics
The vine plant (Vitis vinifera) is indigenous to the Northern Hemisphere and grows in temperate regions, though with restrictions based upon soil and actual meso-climate (climate of the vineyard) generally. It is a shallow rooting plant, whilst the aerial parts have some particular botanical characteristics, apart from a climbing habit already noted, requiring training of cultivated species on poles. Information on botanical and genetic aspects can be found elsewhere. Very importantly, the cultivated plant is primarily a self-pollinating plant. The plant needs to reach some maturity before cropping fruit, since a young vine does not bear flowers until it is in its second or third season. The duration of full sunlight is important to achieve fruit bud development and flowering occurs within eight weeks of bud formation.
The fruits of the vine are, strictly speaking, berries. Maturity or ripeness of the grapes is expected to occur between 12 and 22 weeks after flowering, dependent on climatic conditions and grape variety.
After fruits set, the grapes develop, for 40 to 60 days, but remain approximately half their final size, hard, high in acid and green. Dependent on the climate, the next stage is veraison, the commencement of ripening period, taking between 30 days in hot regions and 70 days in cool regions. Some varieties are early maturing, others late maturing, needing less or more time to reach maturity when grown under the same circumstances. During veraison the grapes gain their red or yellow colour, gain sugar, lose acid, soften and increase in size. Other changes include the development of flavour (actual or precursor) compounds typical for the grape variety, often referred to as varietal aroma. Samples are normally taken throughout the growing season for chemical analysis of acidity, and sugar content, etc. and a tentative harvest date is set. However, in cool regions complete grape maturity at harvest may not be achieved, so that adjustments may have to be made at the vinification stage. Harvesting is usually accomplished in September or October in France and Germany. A special case is Beaujolais Nouveau, where vinification, bottling and distribution are accomplished by the official release date set on the third Thursday in November each year. In warmer vine growing regions, such as Southern Spain, the harvesting is carried out in August or September to avoid the grapes becoming over-mature.
The leaves of vine plants photosynthesize, so that carbon dioxide taken from the air is converted into carbohydrates and oxygen is expired. Photosynthesis takes place via the so-called Calvin biochemical pathway using UV light as an energy source. The leaves also absorb water as available. The roots also absorb water, but also take up many other essential nutrients, some in trace quantities. The roots absorb nitrogenous components, mainly the nitrate ion (NO32-), from the soil, regardless whether the soil is enhanced by organic manure or artificial fertilizers in aqueous solution. They also absorb phosphorus compounds (phosphate ions) and metallic cations, mainly potassium (K+) but also a wide range of other compounds, some in trace quantities. An adequate input of water through the roots is essential for the effective and healthy growth of the plant.
A recent review by Conde et al.(2007) states that wine quality largely depends on the vineyard and the wine grower. Sugars and acids are produced by the leaves, whilst acids and phenolics are produced in the berry. Some molecules related to aroma and taste, the so-called primary or varietal aromas, are formed during fruit development and are typical for the grape varieties used for wine-making. The characteristics of the grape when picked at commercial maturity for wine-making are crucial for the resulting wine quality.