Soil, climate and ripeness for Grape
The quality of the wine is thought to depend to a great extent on the vineyard and the skill of the viticulturalist being able to produce good quality grapes. Hence it is not surprising that the nature of the soil in which the vine is grown has always been a part of the mystique of desirable wine flavour and this is usually referred to as the effect of terroir. In the review of Conde et al. (2007) the definition is much broader defined, the term terroir is used by viticultur- alists to define rather widely the geographical and environmental conditions of the vineyard in which the grapes were grown, with many factors contributing, such as composition of soil, climate, topography, even including strains of micro-organisms on the berry skin contributing the fermentation process. A terroir offering good growing conditions to a particular grape cultivar helps the plant to produce good quality grapes, forming a good start for a good vintage.
An important aspect of soil quality is water retention and drainage. This capacity of the soil is related to both chemical and physical characteristics. The vine plant does not like waterlogging, hence excessively ‘clayey’ soils are undesirable. Gravel (defined as mineral particles, >2 mm in diameter), in contrast to clay (particle size around 0.2 mm or less) is necessary for free water draining properties. However, clay has the advantage of attracting and holding humus, containing nitrogen and other nutrients. The vine plant is thought not to need an especially fertile soil; in any event, the plant will not pick up nutrients in excess of its need, even though they may be present. Suitable soils for growing quality grapes are not particularly fertile or deep (see Conde et al., 2007). A recent review on geology and wine suggests that the importance of geology in wine-making may have been over rated (Huggett, 2006). Most of the nourishment vines take from the top 0.6 m, although the plants can take up water from as far down as 2 m, even deeper in drought conditions. In most areas, even where the soil is thin, geological influences on vines will be minimal. The quality of the grapes is only influenced indirectly by factors such as soil composition, geomorphology and water retention. The direct effect of soil on resultant wine flavour is therefore questionable and no scientific proof currently exists. Indirect effects along with the meso-climate (conditions of the vineyard, as for example influenced by the slope and orientation towards the midday sun) will be important. Very light soil will reflect the sunlight, warming the area round the vine. Currently the Old World tends to emphasize the effect of terroir (generally the nature of the soil rather than the wider definition), while the New World tends to emphasize climate as having the greatest influence on wine quality and flavour. Further studies are needed to show the true relevance of these two factors. This difference is possibly also reflected in the aims of the winemakers, Old World wines tend to be made reflecting the style typical for the area, whereas New World wines tend to be made to reflect the best the fruit can give.
Climate conditions above the soil have an important influence on growth of micro-organisms on grapes, which in some cases cause the grapes to rot. Botrytized grapes (resulting from Noble Rot) have a separate commercial importance, since they can be prized for the production of quality sweet wines, with a very distinctive flavour. Noble Rot is associated with particular grape varieties, the type of grape cluster and meso-climate (i.e. cyclical conditions of fluctuating humidity, humid mornings followed by dry afternoons). Heavy rains and protracted damp periods are more likely to lead to conventional rotting, such as bunch rot, which essentially makes the grapes unfit for wine-making.
Different varieties of grapes will have documented agronomic characteristics, which mean they are favoured for use in particular growing areas and the associated climates. Specific botanical factors can be also important, such as the type of cluster formation of the grapes on the vine, and resistance to various forms of fungal rot and diseases, which, to an extent, depend upon such factors as the thickness of grape skins. Pruning of vines involves removing growth, usually in winter, although lighter summer pruning can also be carried out. It ensures that the otherwise rampant vine is kept in a manageable shape, thus also avoiding so-called over-cropping. The excess production of grapes (over-cropping) of a vine leads to inferior quality grapes and wines, and it is thought to compromise the life span of the vine. Hence, controlling the yield is an important issue in vineyard management, although there does not seem to be firm scientific evidence to indicate the optimum yield for vines. There are a number of different pruning techniques which influence the character of the grape and the resulting wine, even within one varietal type. For example, insufficient pruning in cooler climates will give grapes that take too long to mature; hence a heavier pruning technique would be more appropriate to ensure a ripe grape crop. Summer pruning is often associated with canopy management and helps to ensure a desirable canopy micro-climate, for example, sufficient sunlight on the bunches of grapes.
A most important characteristic of the vine is the rate at which the grapes can reach maturity, i.e., whether the vine is early or later ripening. Late maturing varieties need a warmer climate with a sufficiently long warm autumn and they do not thrive in cool climates. As already mentioned, many grape varieties are currently only of local regional interest but a relatively small number of others, with well known names, are internationally grown and are suitable for vinification in widely different countries. However, often much research is carried out regarding climate, meso-climate and even microclimate to be created around the vines, in addition to the suitability of the soil. All this ensures that, once the preferred variety is selected and planted, there will be healthy vines yielding healthy good quality grapes.
In Tables 1 and 2 the second column indicates some of these agronomic characteristics, which can be studied in greater detail in other texts. The third column indicates some botanical features of the grapes, including some general comments on the chemical composition of the grape must, in particular, relation to tannin content and acidity which are both modified during wine-making. Some grape varieties are used on their own in wine production, giving the so-called varietal wines, particularly popular in the New World. Many others are used for blending purposes, and despite the negative connotation of the word, it forms an essential part of good wine-making practice. For example, blending gives the opportunity to add complementary characteristics, adjust sweetness, enhance acidity, or reduce the ‘oaky’ character of a heavily oaked wine from ageing in barrels.
Chemical composition both of the must and of the resultant wine is the key to a full study of vinification practices and wine flavour. Notably, internationally grown varieties such as Cabernet Sauvignon and Chardonnay can show some marked differences in wine flavour according to the region of vinification (e.g. France compared with Australia or California); these differences are explored in more detail later.
An important characterizing feature of chemical composition is the so- called terpene content of particular grapes, which is relatively unaffected by subsequent vinification. These terpenes are volatile compounds that confer a very distinctive grapey flavour, often referred to as ‘aromatic fruity’, in contrast to ‘ester fruity’ flavour aroma which can be generated during fermentation. Terpene content varies with different varieties, and is highest in the Muscat variety, but it is also significant in certain ‘noble’ varieties, notably Riesling and Gewurztraminer, and hence in the wines produced by these grapes. Varieties with very low or zero concentrations of terpenes tend to be more neutral in character but often develop a more generally ‘fruity’ aroma. These distinctions are also reflected in comments in the third column of the tables. Terpene content can be used for distinguishing purposes between some varieties. During ageing, changes occur both in the type and amount of terpenes, which has a noticeable influence on the aroma bouquet of the wine (discussed in detail below).
|Table 1 Some grape variety characteristics for red wines.
|Black grapes (red wines)|
|Nebbiolo (Italy)||Late ripening.||Wine made is markedly high|
|Susceptible to powdery||in tannin and acidity.|
|Grapes need to be ripe (sugar|
|Grown largely in Piedmont.||content) to counterbalance. Requires long ageing.|
|Sangiovese||Late ripening.||High acid.|
|(Italy)||Susceptible to bunch rot. Most planted type in Italy. The ‘Chianti’ grape.|
|Tempranillo||Early ripening.||High in tannins and acidity.|
|(Spain) (Other||Subject to both powdery||Mid-sized/thick skinned.|
|Spanish regional||and downy mildews.||Usually aged.|
|names e.g.||Important in Rioja and||Not necessarily high in|
|Cencibel)||Spain’s best quality grape.||alcohol content.|
|Zinfandel (USA)||Uneven ripening.||Gives red, white and rosé|
|(= Primitivo of||Needs warm climate and|
|S Italy)||long growing season. Most planted variety in California.|
|Grenache||Flourishes in barren soils.||Thin skins.|
|(France)||Needs careful pruning to||High alcohol producer.|
|(Garnacha (tinto)||keep down the yield.||Generally used for blending|
|Spain)||World’s most widely planted red wine grape.
The ‘Southern grape’ (France).
|purposes, e.g. S Rhone.|
|Malbec (France)||SW France grape.||Used in blending or as|
|Fragile in cooler climates.||varietal wines (Argentina).|
|Very popular in Argentina.|
|Petit Verdot||Very late ripening limits|
|(France)||possible growing regions. Small quantities in Bordeaux. Some interest for Chile, Australia and California.|
|Mourvedre||Needs very warm climate to||Wines with high alcohol.|
|(= Monestrell)||Grape planted in SE France|
|(Spain)||and SE Spain.
Old fashioned grape coming into fashion again.
|Pinotage (South||Early ripening. High yielding||South African variety by|
|Africa)||vine, hence requires pruning||crossing of Pinot Noir and|
|to maintain quality and yield.|
|_____________||South Africa.||_____________________________ ‘|
|Table 2 Some grape variety characteristics for white wines.
|Variety (country of||Botanical/composition/|
|Muscat Ottonel||Grown in cooler climates,||Lower terpene content than|
|Morio-Muscat||Needs a good site for growing||German hybrid of Silvaner and|
|(Germany)||and rots easily.||Weissburgunder (Pinot Blanc) High terpene content.|
|Silvaner (Germany) =||Ripens earlier than Riesling,|
|Sylvaner (France)||but later than Müller-Thurgau. Central Europe planting.|
|Müller-Thurgau||Ripens early, and suitable||Crossing, Riesling and|
|(Germany)||for cool climates.||Chasselas (1882).|
|Subject to both powdery/||High yielding mid-sized grapes.|
|downy mildew and bunch rot.||Used in cheaper German|
|Grown in Germany and UK.||wines (QbA).|
|Traminer (Germany),||Planted in cooler regions.||Modest clusters of small|
|Savagnin Rose||Subject to powdery mildew|
|(France)||and bunch rot. N.B. Sauvignin||Tough skins.|
|grape of Vin Jeune (Jura).||Terpene content moderate.|
|Gewürztraminer||Planted particularly in Alsace.||Higher terpene content than|
|(Germany)||Produces wines of high alcohol content, but may be low acidity.|
|Aligoté (France)||Burgundy’s second wine grape; also grown in Romania and elsewhere.||Acid wine.|
|Muscadelle (France)||Planted in Bordeaux and SW||Largely used for sweet white|
|Related to Muscat.|
|Viognier (France)||Northern Rhone, but also||Fashionable variety. Wines|
|S France, Australia and||best drunk young: good for|
|California. Can be poor|
|yielding in cool climates.|
|Pinot Gris (France)||Quality grape in Alsace, also||Colour mutation of Pinot|
|(= Pinot Grigio, Italy)||grown in Germany (Baden/|
|(= Ruländer, or||Pfalz).||Pink skinned.|
|NB. EU insists on|
|Melon de Bourgogne||Grown for Muscadet; and||Dry wine, neither very acid|
|(France)||little known elsewhere. Pays||nor strongly flavoured. Often|
|Nontais area (R Loire).||‘Sur lie’. Alleged highly|
|Withstands cold well.||sulfured wines.
Harvesting the grapes at optimum ripeness is now thought even more important than in the past and research has focused on ensuring that grapes are picked at the level of maturity that gives the most desirable wine flavour characteristics. Traditional physiological ripeness is reached when grapes have accumulated sufficiently high sugar levels, without having lost too much acidity. However, compounds contributing to the flavour of the grape and the resulting wine also are taken into account. Studies on changes of grape composition, with particular interest on compounds thought to influence wine flavour, have been comprehensively reviewed by Conde et al.(2007). The development and production of compounds during grape maturation contributing to the sensory properties of the grape and wine are still not well understood. During ripening, the grapes become larger, sweeter and less acid, mostly due to malic acid being metabolized, resulting in a change of the frequently used acid/sugar balance. On a per berry basis tannins and aromatic compounds formed during the early part of berry development decline.
Concurrent there are also important changes in volatile compounds characteristic for the grape variety and contributing to the aroma and regional character of the wine, often referred to as forming part of the wine quality. Best studied are the monoterpenes, C13-norisoprenoids, methoxypyrazines and sulfur compounds. Conde et al.(2007) summarize the monoterpene concentrations in three categories: (1) up to 6 mg L-1 in intensely flavoured grapes (Muscat), (2) between 1 – 4 mg L-1 in some aromatic varieties (Tramines, Huxel and Riesling) and (3) very low concentrations, generally below perception threshold and thought to contribute only minimally to varietal flavour (Cabernet Sauvignon, Sauignon Blanc, Merlot, Shiraz and Chardonnay). The levels of methoxypyrazines, contributing to the herbaceous aroma of, for example, Cabernet Sauvignon, diminish during maturation. Carotenoids break down during ripening; the process is enhanced by sunlight. Excessive
hydrocarbon smells, due to 1,1,6-trimethyl-1,2-dihydronaphtalene, can develop in Riesling wines and have been attributed to extremely high temperatures during grape maturation. This illustrates that although hot climates favour the accumulation of sugars in grapes, it may not always bring out the best in terms of flavour quality.
The increased attention given to the production of wines with the varietal characteristics in terms of optimized flavour has increased the level of ripeness at which the grapes are picked. This means the sugar contents are higher and the fermentation leads to wines with higher concentrations of ethanol (even up to 15% v/v) than in the past. There is research ongoing in selecting yeast strains less efficient in the conversion of sugar into ethanol, enabling the production of wines with lower ethanol concentrations from very sweet and ripe grapes.