Enological Chemistry

Juan Moreno, Rafael Peinado

• Publisher: Academic Press
• Year: 2012
• ISBN:
  9780123884398
• Topic: Tecnología e ingeniería
• Subtopic: Ciencia de los alimentos

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

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 pedi...