Enological Chemistry

By Juan Moreno and Rafael Peinado

Enological Chemistry is written for the professional enologist tasked with finding the right balance of compounds to create or improve wine products. Related titles lack the appropriate focus for this audience, according to reviewers, failing either to be as comprehensive on the topic of chemistry, to include chemistry as part of the broader science of wine, or targeting a less scientific audience and including social and historical information not directly pertinent to the understanding of the role of chemistry in successful wine production.

The topics in the book have been sequenced identically with the steps of the winemaking process. Thus, the book describes the most salient compounds involved in each vinification process, their properties and their balance; also, theoretical knowledge is matched with its practical application. The primary aim is to enable the reader to identify the specific compounds behind enological properties and processes, their chemical balance and their influence on the analytical and sensory quality of wine, as well as the physical, chemical and microbiological factors that affect their evolution during the winemaking process.

• Organized according to the winemaking process, guiding reader clearly to application of knowledge
• Describes the most salient compounds involved in each step enabling readers to identify the specific compounds behind properties and processes and effectively work with them
• Provides both theoretical knowledge and practical application providing a strong starting point for further research and development

• Language: English
• Publisher: Academic Press
• Release date: May 30, 2012
• ISBN: 9780123884398

Book preview

Index

Chapter 1

The Vine

Outline

1. Biological Cycles of the Vine

1.1. The Growth Cycle

1.2. The Reproductive Cycle

2. Morphology of the Grape Clusters

2.1. The Stem or Stalk

2.2. The Grape Berry

3. Chemical Composition of the Fruit

3.1. Composition of the Stalk

3.2. Composition of the Seeds

3.3. Composition of the Skin

3.4. Composition of the Pulp

3.5. Composition of the Must

1 Biological Cycles of the Vine

Vines are herbaceous or sarmentose shrubs. Their leaves are simple and more or less palmate or lobulate in shape, and they have tendrils that grow in the opposite direction. Their flowers have five petals joined at the apex which are grouped in narrow panicles. Commonly, the flowers are unisexual. The fruit is an oligospermic berry with a spherical or ovoid shape that contains pear-shaped seeds and a soft pulp.

The vine belongs to the Vitaceae family, which includes a dozen genera. Among these are Ampelopsis and Parthenocissus, which include wild vines, and Vitis, which is responsible for all table and wine grape varieties. The genus Vitis contains around 40 species, the most important being Vitis vinifera or the European species, which is used in the production of high-quality wines, and the American species Vitis rupestris, riparia, berlandieri, labrusca, etc., which have been used as rootstocks and direct-producing hybrids. Within each species, there are varieties that only conserve their characteristics by vegetative propagation, and can therefore be considered as clones. There are 6800 known varieties of V. vinifera alone, although no more than 100 are used to produce the most recognizable wines worldwide.

FIGURE 1.1 Growth and reproductive cycle of the vine .

As vines are perennial plants, they undergo characteristic morphological changes with the seasons. Throughout each yearly cycle, the vine undergoes:

• The growth and development of the vegetative organs (shoots, leaves, tendrils, and roots), its survival through the accumulation of reserves (withering), and the acquisition of latency by the buds. This is the growth cycle.

• The growth and development of reproductive organs (inflorescences, flowers, and berries) and their maturation. This is the reproductive cycle.

The morphological cycle of the vine has particular characteristics known as phenological events. In chronological order these are weeping, bud burst, flowering, fruit set, ripening, and leaf fall.

There are two clearly distinguishable periods in the yearly cycle of the vine, namely winter dormancy, during which no morphological changes occur and which extends from leaf fall until bud burst, and active growth, which begins with bud burst and ends with leaf fall. During the period of active growth, the organs are constructed, the seeds and berries form, and the materials necessary for survival accumulate in the living parts of the plant. The period of active life contains two very different phases: growth, which occurs from bud burst until fruit set, and ripening, which occurs from veraison until leaf fall.

1.1 The Growth Cycle

Prior to the onset of vegetative growth at the end of the winter, liquid oozes from the wounds created by pruning. This is known as weeping and can last for a number of days. It is caused by the activation of the root system as a result of the increasing temperature of the soil, and is halted by the development of bacteria that form a viscous mass within the liquid, that ultimately obstructs the xylem vessels.

Bud burst is the process by which the protective scabs covering the buds break open. Not all buds break open and the process is not simultaneous for all buds on the vine, a phenomenon known as acrotonic budding. Buds that burst in the spring do so because a latent bud will have formed during the previous growth cycle.

Foliation involves the appearance and development of the leaves. This phenomenon cannot be separated from the growth of the shoots.

Withering begins at the end of the veraison and continues until leaf fall. The most important event that occurs during this process is the accumulation of reserves, particularly of starch, in the trunk and shoots.

At the end of the period of active life, the leaves lose their green color, photosynthesis ceases, and the leaves fall. At this point the plant can be considered to have entered a dormant phase, although the translocation of reserve substances continues for a few days after leaf fall.

1.2 The Reproductive Cycle

The reproductive cycle begins during the period of growth and continues during part of the withering period. It comprises two successive cycles, since the flower cluster exists in embryonic form in the fertile buds formed during the previous growth cycle.

Flowering involves the opening of the corolla of the flower and is linked to fertilization. It is difficult to separate these two processes in time since the same vine will carry flowers that have yet to open and others that are already fertilized. After flowering, the inflorescence is termed a raceme or cluster. It is made up of a principal axis, together with secondary axes formed by the stalks or stems that support the fruit or berries. The structure of the raceme and the number and volume of berries are determined by the inflorescence; the cluster can be loose, intermediate, or compact.

The berries begin to develop at fertilization. The development process can be divided into four periods: herbaceous growth, veraison, ripening, and over-ripening.

In varieties containing seeds, the fruit begins to develop after fertilization of the ovary. At this stage the fruit is said to be set. Under favorable conditions for production, this type of growth and formation of the fruit generates berries of the maximum size for each variety.

The herbaceous period extends from fruit set until veraison. Normally not all of the flowers are fertilized and therefore do not form berries. The grape cluster behaves as a green organ during the herbaceous period, and as it contains chlorophyll, it contributes to photosynthesis. The stalks also reach their final size during this period and the berries increase in volume but remain hard and green. Their sugar content is low, but acids begin to accumulate and reach their maximum concentration when the grapes are close to veraison.

Veraison is characterized by a change in the color of the grapes, leading to the development of the color typical of the variety. Not all grapes change color at the same time and the process takes approximately two weeks.

During veraison, the berries become softer and more elastic due to changes in the cell walls. They lose chlorophyll and change color due to the formation of pigments; white grapes become translucent and some develop a yellowish color, whereas red grapes begin to develop their characteristic color in a series of increasingly strong red tones. By the end of veraison, the seeds or pips of the grapes are perfectly formed and able to reproduce the plant; they have thus achieved physiological maturity much earlier than the fruit.

During veraison, the pulp rapidly begins to accumulate sugars while the acidity is considerably reduced. At this point, the grape berries enter the next stage of their development, namely ripening. During this stage, the composition of the berry is modified extensively by the accumulation of substances derived from other organs and by the transformation of those that are already present. Once ripeness has been achieved, the grapes enter a phase of over-ripening. During this phase, external physical factors have a greater influence than the function of the plant, and the grapes become increasingly fragile. The grape receives almost no contribution from the plant and there is a partial evaporation of water from the pulp that leads to concentration of the sugars. In parallel, respiratory combustion of some acids continues and the grapes begin to change their consistency. In other words, over-ripening reduces the yield of fruit juice but increases the richness of the juice in terms of sugars and reduces its acidity. Over-ripening is essential to obtain wines with high alcohol content, since the concentration of alcohol in the final product is proportional to the sugar content of the grapes from which it was produced.

2 Morphology of the Grape Clusters

The grape clusters comprise two elements, namely the stalk and the actual fruit. The different elements of the inflorescence progressively achieve their final dimensions while the ovaries of the flowers are transformed into fruits and the ovules into seeds. All these elements together form the raceme or cluster.

2.1 The Stem or Stalk

The stalk of the grape clusters comprises a basal stem or peduncle, which is the part that joins the shoot, and all of its ramifications. The longest arm forms the main axis or backbone of the cluster. The finest branches, or pedicels, end in a swelling or receptacle into which the grape berry is inserted. This swelling is where the vascular bundles that transport nutrients to the inside of the berry travel. One part of these bundles, the brush, remains connected to the receptacle when the ripe berry is removed.

The stalk reaches its final dimensions during veraison, and during ripening of the grapes the peduncle becomes woody while the rest of the stalk remains herbaceous.

The texture of the cluster depends upon the length of the pedicels. If these are long and thin, the berries remain separate and the clusters are loose. If, on the other hand, the pedicels are short, the clusters will be compact and the berries are pressed against each other. Whereas loose clusters are desirable in table grapes, most varieties used in winemaking require tightly packed clusters.

Under normal conditions in which no accidents such as fruit shatter or failure to seed have occurred, the stalk accounts for between 3% and 7% of the weight of a ripe grape cluster. The proportion of stalk in the clusters is determined by the grape variety and the type of cluster (simple, branched, conical, winged), and even clusters belonging to the same grape variety can vary according to the type of care they receive, the weather conditions, diseases, etc. Consequently, it is difficult to establish an accurate average. In varieties with very tight clusters and fine stalks, and in climates and years with a wet summer, the stalk may account for only 2% of the total weight of the cluster. In contrast, it can account for 7% in fruits from plots with clusters that are spread out, either naturally or due to fruit shattering, in years with dry or very dry summers, and in poor soils with little additional management during the growing season. In general, an acceptable percentage is 4% for most grape varieties grown in Spain and 2% for Pedro Ximénez grapes grown in the Montilla-Moriles region.

2.2 The Grape Berry

Grapes are fleshy berries. Their shape can vary substantially between varieties but is consistent within the same variety. They can be lobular, elongated, flattened, ellipsoid, ovoid, etc. Prior to veraison, they are green, contain chlorophyll, and can perform photosynthesis. Following veraison, the berries of white grape varieties acquire a yellowish color, whereas berries from red varieties acquire a reddish-violet color.

The grapes are always very hard until veraison and after that their consistency depends upon the variety. The size of the berries depends on a number of factors, principally the soil, cultivation method, development of the seeds, and also the number in the cluster. Morphologically, the grapes comprise skin, pulp, xylem and phloem vessels, and seeds or pips.

The skin has a heterogeneous structure comprising cuticle, epidermis, and hypodermis. On the surface of the skin at the other end from the pedicel there is a small, darker spot, the navel, which is clearly visible in white grapes and corresponds to the remnant of the stigma. The cuticle is very thin in varieties of V. vinifera and is covered by the bloom, a layer with a waxy appearance that comes off to the touch. This layer is important as it captures microorganisms present in the air. Most notable among these microorganisms are the yeasts, which are responsible for spontaneous fermentation of must. The bloom also acts as an impermeable layer that blocks the evaporation of water from inside the berry.

The epidermis and hypodermis, which lie below the cuticle, contain layers of cells of different sizes that contain pigments, aromatic substances, and tannins. While these substances are only weakly soluble in cold water, they are soluble in alcohol, and consequently diffuse during fermentation when the sugary juice becomes enriched in alcohol. The odorant substances mainly comprise monoterpene alcohols and heterosides, and they endow the wine with floral and fruity notes corresponding to the varietal aroma. Tannins are more abundant in red than white grapes.

The proportion of the ripe grape made up by the skin depends mainly on the variety and the climate (due to its influence on transpiration). In varieties grown in Spain, the proportion is 7% to 8%, in France it is 15% to 20%, and in California, 5% to 12%.

The pulp is made up of large cells containing vacuoles, the structures which contain the fluid that will form the must. The grape contains 25 to 30 layers of cells from the epidermis to the endocarp. Since the same number is present in the ovary, it is apparent that the swelling of the grape berry is caused by an increase in the volume of the cells rather than by cell division.

The vascular system of the berry, which carries sugars from the leaves and minerals from the roots, comprises 10 to 12 bundles that are left attached to the receptacle when the berry is removed and form the so-called brush. The bundles branch off within the pulp to form a network.

The proportion of pulp in the mature berry varies between different varieties of V. vinifera, but the differences are not substantial and the pulp accounts for 85% of the mass on average. In the case of Pedro Ximénez grapes, the percentage is between 90% and 92%.

The seeds acquire definitive characteristics at the end of veraison, when they achieve physiological maturity. They therefore remain relatively unaffected by the chemical changes that occur in the berry between veraison and full ripeness.

Although the berry should normally contain four seeds, derived from the four ovules in the ovary, there are nearly always fewer due to the absence or abortion of one or more of these ovules. If fertilization is defective, the berries contain no seeds, or the seeds are smaller than normal and hollow. When this occurs, the berries will remain very small, although they may ripen and even develop high concentrations of sugars. The mass of the berry, its sugar content, and often its acid content are related to the number of seeds. An absence of seeds may also be typical of the vine, which is desirable when producing table grapes or raisins.

The seeds contain two enveloping woody layers in the form of a skin. These are known as the testa and tegmen (rich in tannins), and they surround the endosperm (rich in fatty acids). Inside the endosperm, towards the tip of the seed, the germ or embryo of the new plant is found.

The proportion of the mass formed by the seeds depends upon the variety and, in particular, the number of seeds within the berry. The average found in varieties grown in Spain is 4%. In grapes belonging to the Pedro Ximénez variety, the seeds account for between 3% and 4% of the mass.

The grape berry increases steadily in volume and mass from the time the fruit is set until it is ripe. In addition to the effect of the variety and the number of seeds, the weather conditions in a given year also affect the mass of the grapes.

From mid-veraison to ripeness, the mass of the berries increases by 50%. The volume and mass of the ripe berry depend mainly on the rainfall after veraison and the water reserves in the soil. The volume and mass of the berry can be affected by various diseases. The maximum weight is achieved a few days before the harvest, and a slight loss of mass can be observed in the week before harvesting due to loss of water from the berries. This can reach up to 10% of the total mass.

3 Chemical Composition of the Fruit

3.1 Composition of the Stalk

The chemical composition of the stalk is similar to that of the leaves and tendrils, although it is particularly rich in polyphenols. It has a low sugar content and intermediate concentrations of acid salts due to the abundance of minerals, and its cell content has a high pH (>4). Contrary to popular belief, maceration of the stalks during vinification of red grapes without destemming leads to a reduction rather than an increase in acidity, with a slight increase in pH.

The ash from the stalks accounts for 5% to 6% of the dry weight and comprises approximately 50% potassium salts. After potassium, the most abundant cations are calcium and magnesium, followed by sodium, iron, copper, manganese, and zinc in much lower proportions.

The stalks are rich in phenolic compounds (particularly in red grape varieties), and the concentrations of these compounds in wine is therefore increased when vinification is carried out with the stalks remaining present. The polyphenols present in the stalks have a bitter flavor, however, and therefore reduce the quality of the wine.

TABLE 1.1. Chemical Composition of the Stalk (Milliequivalents per kg of Stalk)

Although the stalks account for only around 4.5% of the weight of the cluster, they contribute around 20% of the total phenolic compounds, 15% of the tannins, 26% of the leucoanthocyans (constituents of condensed tannins and therefore linked to astringency), 15% of the catechins, 16% of the gallic acid, and 9% of the total caffeic acid.

3.2 Composition of the Seeds

The outer layers (woody parts) of the seeds are rich in tannins, containing, depending on the crop, between 22% and 56% of the total polyphenols of the grape. These include the procyanidins (67% to 86%) and a substantial proportion of the total gallic and caffeic acid. The woody part (testa and tegmen) is surrounded by a thin film that is also rich in tannins.

The endosperm contains a lipid fraction that comprises on average 50% linoleic acid, 30% oleic acid, 10% saturated fatty acids, and 1% unsaponifiable residue. This oil is commonly extracted from the flour obtained upon pressing the grape seeds using an appropriate solvent, and up to half a liter of oil can be extracted per hectoliter of wine.

Whereas the substances contained in the seed coat can be beneficial (phenolic compounds, nitrogenated substances, and phosphates that are partially dissolved during the production of red wines), those present on the inside of the seeds would have a negative effect on the quality of the wine if they were to dissolve, hence rupture of the seeds during pressing should be avoided.

When the seed reaches physiological maturity, it begins to lose up to a fifth of its nitrogen content in the form of ammonium cations. Nevertheless, the seeds remain richer in nitrogen than the remaining solid parts of the grape cluster.

The minerals contained in the seeds account for 4% to 5% of their weight and the distribution of cations differs from that of the other parts of the cluster, since calcium tends to be the most abundant (particularly in chalky soils) followed by potassium, magnesium, and sodium, and then much lower levels of iron, manganese, zinc, and copper, in that order.

TABLE 1.2. Chemical Composition of the Seeds (Percentage of the Total Mass)

3.3 Composition of the Skin

The skin of the grapes has an important role to play in winemaking, since the type of wine (white or red) is defined by the way in which the different parts of the grapes are used in vinification. The skins contain most of the substances responsible for the color and aroma of the grapes and make a substantial contribution to the color, aroma, and flavor of musts and wines.

The bloom is made up of two thirds oleanoic acid and the remainder comprises hundreds of different compounds such as alcohols, esters, fatty acids, and aldehydes. Grape skins contain appreciable quantities of malic acid, but its concentration declines during ripening, and the skins of ripe grapes contain mainly tartaric, malic, and citric acids, in that order. The most characteristic substances in the skins of ripe grapes are yellow and red pigments and aromatic substances. The typical color of the grape variety begins to appear at the veraison and peaks when the grape is ripe.

TABLE 1.3. Chemical Composition of the Skin (Milliequivalents per 100 g of Skin)

The amounts of phenolic compounds in the skins are highly variable, and depend mainly on the grape variety. The skin contains between 12% and 61% of the total polyphenol content of the fruit, between 14% and 50% of the tannins, 17% to 47% of the procyanidins, and almost all of the anthocyans in red grape varieties. They are rich in cellulose, insoluble pectins, and proteins. Chlorophyll, xanthophyll, and carotenoids are present in appreciable quantities when the grapes are green, but their concentrations are lower in the ripe grape.

The minerals in the skins have an almost identical distribution to that in the stalks, with potassium accounting for more than 30% of the total mineral content. In decreasing order of concentration, potassium is followed by calcium and magnesium, and then at much lower concentrations, by sodium, iron, copper, manganese, and zinc.

3.4 Composition of the Pulp

The pulp contains those components that predominate in the grape juice or must. The solid part of the pulp is made up of cell walls and vascular bundles, and accounts for no more than 0.5% of its mass. It is this that forms the sediment or deposit that remains in the tanks after the must is decanted.

The sugars in the pulp are mainly glucose and fructose. During veraison, the glucose content is twice that of fructose, whereas in ripe grapes the two sugars are present in almost equal proportions. Sucrose is only present in wine grapes in trace amounts, since, although it is the main sugar synthesized in the leaves, it is hydrolyzed during translocation to the fruit. In addition to glucose, fructose, and sucrose, other sugars such as arabinose, xylose, rhamnose, maltose, and raffinose have been identified in grapes.

Sugars are not uniformly distributed in the grape berry, and the part at the opposite side to the pedicel is richer in sugars than that closest to it. Similarly, if the pulp is divided into three parts, one closest to the skin, one surrounding the seeds, and one in the region in between, it is this last intermediate region that is richest in sugars. This distribution has consequences for the winemaking techniques used, particularly for the production of white wines, since free-run juice is richer in sugars than subsequent press fractions.

TABLE 1.4. Chemical Composition of the Pulp (Milliequivalents per kg of Pulp)

TABLE 1.5. Chemical Composition of Grape Must (g/L)

TABLE 1.6. Physical and Chemical Composition of Different Fractions of Grape Berries (Percentage Fresh Weight)

Other major components of the pulp are organic acids, mainly tartaric and malic acid. Citric acid is also present, although at lower levels.

3.5 Composition of the Must

The liquid component of the pulp, which is obtained when the grapes are crushed, makes up what is commonly referred to as must. The must is characteristically made up of sugars, acids, and other substances in proportions that are very similar to those of the pulp from which it is derived, and its composition depends upon various factors, such as the grape variety, the location of the winery, the composition of the soil, and the weather conditions during the growth and ripening of the grapes. Diseases of the vine and the processes used to obtain the must and begin fermentation can also influence its composition.

Logically, the compounds that are present at the highest concentrations are important because they form the main raw materials for the transformation of must into wine. On the other hand, a number of minor components are necessary for the growth and survival of yeast, and others contribute sensory qualities. These last components include polyphenols, which give color to the wine, and monoterpene alcohols, which are important due to their delicate aromas, responsible for the varietal aroma of the wine. In the following chapters, we will therefore explore each of these groups of compounds in detail before considering how they change during the ripening of the grapes.

Chapter 2

Composition of Grape Must

Outline

1. Grape Must

2. Chemical Families Present in Must

2.1. Sugars

Disaccharides

2.2. Organic Acids

2.3. Nitrogen Compounds

2.4. Minerals

2.5. Polyphenols

2.6. Vitamins

2.7. Aromatic Compounds

Terpenes

Carotenoids

Pyrazines

Alcohols and Aldehydes

1 Grape Must

Grape must is the liquid obtained by the gentle crushing or pressing of grapes. Pressing takes place once the grapes (either destemmed or still in clusters) have been gently crushed. Even within the same winemaking region, must composition varies according to several factors, including:

• The type and variety of grapes used,

• The ripeness and health of the grapes (ripeness depends on a range of factors, such as the climate during the growing season, the type of soil, and the fertilizers used),

• The pressure exerted on the grapes.

Musts are classified as free-run must (or juice), which is obtained by the simple crushing of grapes, or press-fraction must, which is obtained by subjecting the grapes to increasing levels of pressure. There are therefore many types of must.

TABLE 2.1. Composition of Musts Obtained at Increasing Press Pressures

Adapted from De Rosa, 1988.

Quality white wines are made only from free-run must (known in Spain as mosto flor or mosto yema) or first-press fractions. Subsequent fractions can be used to make other products, such as the more intense, deeper-colored press wines.

As the press fractions, and logically, the pressure exerted on the grapes increase to improve the yield, the resulting juice becomes increasingly rich in substances derived from the solid parts of the grape, such as the stems (when the grapes are crushed in clusters), the skins, and the