106
M. B. Johnson and M. Mehrvar
2 Overview of the Wine-Making Process
The process of wine-making can be divided into two main stages: viticulture, the
agricultural phase yielding grapes as the final product; and vinification, turning the
grapes into wine. Although other fruits can be used in the wine-making process, such
as apples, pears, plums, peaches and a variety of berries [38], this chapter focuses
on the traditional form of wine-making that is based on grapes alone.
The vineyard is the heart of the viticultural phase of wine-making. Total global
agricultural area dedicated to grape growing for wine, table grapes and raisins is
estimated at a steady 7.4 million ha [29]. The grapevine species Vitis vinifera is the
source of almost all of the world’s wine-producing grapes. To address the worldwide
introduction of a damaging pest, Phylloxera vastatrix, the Vitis vinifera vine is grafted
onto the rootstock of pest-resistant American vine species that produce lower quality
fruit. Through grafting, it is possible to choose rootstock best suited to the local
climate and soil conditions of the vineyard while maintaining fruit quality. The
lifespan of a vine can be up to 100 years, although most are replaced sooner [25].
While older vines are considered to be the source of better quality fruit, the yield
decreases as vines age [58].
Vine density varies significantly from vineyard to vineyard, ranging from 1,100 to
10,000 vines per hectare [33]. Factors that influence spacing and density include soil
quality, harvesting methods (with mechanical harvesting requiring more spacing
between vines than manual harvesting), and, often, tradition. Most New World
wineries tend to have lower density vineyards, while more traditional regions, such
as Burgundy and Bordeaux, have some of the highest densities [25, 33].
The grape berry is comprised of a number of components. The stalks, rich in
tannins, may or may not be completely removed prior to vinification. The skins are
rich in other tannins, such as anthocyanins, that add colour, aroma and flavour to
wines, and are particularly important in the production of red wines and roses [44].
Wild yeasts and other bacteria are also found on the outer waxy layer of the skin,
called the bloom. While wild yeasts are sometimes used in the fermentation process,
they generally cannot tolerate ethanol concentrations greater than approximately 4%
v/v; therefore, wine yeasts that can tolerate concentrations of up to 15% v/v are
added during the fermentation stage of vinification [25]. The pulp of the grape berry
contains water, sugars (glucose and fructose), fruit acids (with tartaric acid the most
predominant when ripe), proteins and minerals (with potassium the most abundant
at up to 2,500 mg/L) [25]. The pips, or seeds, are high in tannins, and can impart a
bitterness to the wine if they are split during vinification.
In the northern hemisphere, the grape harvesting is typically between August and
October, while spanning the period February to April in the southern hemisphere.
The production of ice wine requires sustained temperatures of −8 °C or lower prior to
harvest, and the northern hemisphere wineries that produce this sweet wine typically
harvest between December and January, with the exact date dictated by local weather
conditions [63]. Because the vine growth stops at temperatures less than 10 °C, the
M. B. Johnson and M. Mehrvar
2 Overview of the Wine-Making Process
The process of wine-making can be divided into two main stages: viticulture, the
agricultural phase yielding grapes as the final product; and vinification, turning the
grapes into wine. Although other fruits can be used in the wine-making process, such
as apples, pears, plums, peaches and a variety of berries [38], this chapter focuses
on the traditional form of wine-making that is based on grapes alone.
The vineyard is the heart of the viticultural phase of wine-making. Total global
agricultural area dedicated to grape growing for wine, table grapes and raisins is
estimated at a steady 7.4 million ha [29]. The grapevine species Vitis vinifera is the
source of almost all of the world’s wine-producing grapes. To address the worldwide
introduction of a damaging pest, Phylloxera vastatrix, the Vitis vinifera vine is grafted
onto the rootstock of pest-resistant American vine species that produce lower quality
fruit. Through grafting, it is possible to choose rootstock best suited to the local
climate and soil conditions of the vineyard while maintaining fruit quality. The
lifespan of a vine can be up to 100 years, although most are replaced sooner [25].
While older vines are considered to be the source of better quality fruit, the yield
decreases as vines age [58].
Vine density varies significantly from vineyard to vineyard, ranging from 1,100 to
10,000 vines per hectare [33]. Factors that influence spacing and density include soil
quality, harvesting methods (with mechanical harvesting requiring more spacing
between vines than manual harvesting), and, often, tradition. Most New World
wineries tend to have lower density vineyards, while more traditional regions, such
as Burgundy and Bordeaux, have some of the highest densities [25, 33].
The grape berry is comprised of a number of components. The stalks, rich in
tannins, may or may not be completely removed prior to vinification. The skins are
rich in other tannins, such as anthocyanins, that add colour, aroma and flavour to
wines, and are particularly important in the production of red wines and roses [44].
Wild yeasts and other bacteria are also found on the outer waxy layer of the skin,
called the bloom. While wild yeasts are sometimes used in the fermentation process,
they generally cannot tolerate ethanol concentrations greater than approximately 4%
v/v; therefore, wine yeasts that can tolerate concentrations of up to 15% v/v are
added during the fermentation stage of vinification [25]. The pulp of the grape berry
contains water, sugars (glucose and fructose), fruit acids (with tartaric acid the most
predominant when ripe), proteins and minerals (with potassium the most abundant
at up to 2,500 mg/L) [25]. The pips, or seeds, are high in tannins, and can impart a
bitterness to the wine if they are split during vinification.
In the northern hemisphere, the grape harvesting is typically between August and
October, while spanning the period February to April in the southern hemisphere.
The production of ice wine requires sustained temperatures of −8 °C or lower prior to
harvest, and the northern hemisphere wineries that produce this sweet wine typically
harvest between December and January, with the exact date dictated by local weather
conditions [63]. Because the vine growth stops at temperatures less than 10 °C, the