MONDAY IN THE VINEYARD SERIES
Exploring the Anatomy of Grape Berries throughout the Growing Season
Grape Berries Anatomy
1. Grape Berries Anatomy: Early Grape Development
During this stage, grapes are hard and green and they rapidly increase in size.
Acids: grapes accumulate tartaric and malic acids.
Sugars: Sugar levels remain low.
Tannins: Tannins accumulate thanks to sunlight exposure and are very bitter at this stage.
Aroma compounds: Some aroma compounds and aroma precursors start to develop.
Water: flow into the grapes is high, mainly facilitated by the xylem.
Some factors that accelerate or prologue this stage:
Excessive water and nitrogen can prolong this stage, reducing the time available for ripening and risking cooler weather interfering with harvest.
Mild water stress can expedite this stage, resulting in smaller grapes with a higher skin-to-pulp ratio, potentially enhancing quality.
What are aroma precursors?
Aroma precursors are compounds that do not have a specific aroma at this stage but will develop one during fermentation. Methoxypyrazines, for instance, serve as an example of an aroma precursor responsible for herbaceous aromas in wines such as Sauvignon Blanc, Cabernet Sauvignon, and Cabernet Franc.
What is the xylem?
The Xylem is a vascular tissue in plants that conducts water and dissolved nutrients upwards from the root and also helps to form the woody element in the stem.
2. Grape Berries Anatomy: Véraison
Véraison is the onset of the ripening of the grapes. The term comes from French and indicates the change of color of the grape berries. It represents the transition from berry growth to berry ripening.
Lag Phase: is the designation of the few days, before véraison, during which grape growth slows down.
Skin: become more supple and changes color. Green-colored chlorophyll in skin cells is broken down and in white cultivars, carotenoids are formed, while in red cultivars, anthocyanins (if you want to know more about anthocyanins please read this article) and xanthophylls are formed.
3. Grape Berries Anatomy: Ripening
In this stage, grape cells undergo rapid expansion, leading to the accumulation of sugar and water while acid levels decrease. Tannins, color, aroma precursors, and aroma compounds also develop. Typically, this stage concludes with the harvest, which generally takes place between August and September in the Northern Hemisphere or February and April in the Southern Hemisphere. The length of the ripening stage is a complex interplay between numerous factors:
Grape variety: some ripen earlier, some later, and some unevenly.
Climate: Sunlight, temperatures, and water availability all influence the ripening process.
Vineyard management: yields and canopy management have great influence.
Timing of harvest: depends on weather conditions, capacities at the winery, wine styles.
Let's have a look at how the grape's mains components evolve during this stage:
ACIDS
The overall quantity of tartaric acid present in grapes typically remains constant, but its concentration decreases as the grape ripens due to the increase in sugar and water inside the grape berries.
The concentration of malic acid decreases even more compared to tartaric acid. This is due to dilution caused by the accumulation of sugar and water in the grape and additionally, to the fact that malic acid is metabolized (=consumed) through respiration during the ripening process.
What factors affect the acid levels of grape berries?
The acidity level of grape berries is primarily influenced by temperature, as it directly affects the speed of respiration. Respiration is slower (and therefore malic acid consumption is slower) in cooler temperatures and faster at warmer ones. As a consequence, cooler climates or climates with cool night-time temperatures generally produce wines with higher acidity than warmer climates.
During the last months of ripening, temperature is particularly important:
If the average temperatures drop below 15°C (59°F), the reduction in acid loss can reach a level where the acidity levels in the must become excessively high.
If the average temperatures exceed 21°C (70°F) acidity can diminish excessively.
What is the respiration of the vine?
Vines undergo respiration to obtain energy. In cellular respiration, organic molecules, such as sugars, other carbohydrates, and also malic acid, are broken down in the presence of oxygen (which is collected via the stomata) to release energy which is used for various processes such as growth, maintenance of cellular structures, reproduction, and other metabolic activities. Carbon dioxide is a byproduct of respiration and is released from the cells into the surrounding environment.
SUGARS
The accumulation of sugar undergoes a rapid increase during the initial phase of ripening, gradually decelerating as it approaches the final stages.
How is sugar produced?
The vine's leaves carry out the production of sugar through photosynthesis.
Photosynthesis is the process by which green plants use sunlight, carbon dioxide (CO2), and water (H2O) to produce glucose and release oxygen (O2) as a by-product. This process occurs in specialized cell structures called chloroplasts, primarily found in the leaves, where chlorophyll pigments capture sunlight and initiate the chemical reactions involved in photosynthesis.
This metabolic process reaches its peak efficiency when temperatures range from 18 to 33°C (64 to 91°F) and sunlight levels exceed one-third of full sunshine.
How is sugar transported from the leaves to the grape berries?
The sugar is transported in a sugar solution (a mixture of mainly sugar and water) from the leaves to the grapes by the phloem in a process known as translocation.
What is the phloem?
The phloem is a complex tissue responsible for the transport of organic nutrients, such as sugars and amino acids, throughout the plant. It is one of the two main vascular tissues, alongside the xylem. The phloem consists of specialized cells called sieve tube elements, which are arranged end-to-end to form long, interconnected tubes. These sieve tube elements are living cells that lack a nucleus and other organelles, allowing for efficient nutrient transport.
The phloem also transports other organic compounds, such as hormones and signaling molecules, which are essential for coordinating various physiological processes within the plant.
What factors affect sugar accumulation in grape berries?
Photosynthesis rate: slow photosynthesis reduces sugar accumulation and can be caused by too cold weather, constant cloud, or extreme water stress in very hot, dry conditions.
Transpiration rate: The movement of the sugary solution from the leaves into the grape is directly correlated to the rate of grape transpiration. Grape transpiration occurs at a faster rate in warm, dry conditions compared to cool, humid conditions. In warm conditions, the rapid accumulation of sugar can outpace the full development of aromas, flavors, and tannins in the grapes.
TANNINS
Tannins decrease slightly throughout the ripening process. They also change their structure. As ripening proceeds, tannins undergo polymerization, leading to the formation of larger molecular structures. This polymerization process contributes to the softening of tannins and the refinement of their astringency. The maturation of tannins is influenced by factors such as grape variety, climate, and vineyard practices. Sunshine plays a very important role in tannin development. Before véraison, exposure of grapes to sunlight promotes the accumulation of tannins, while after véraison, it encourages increased polymerization of tannins.
AROMA COMPOUNDS
The diversity of aroma compounds and their precursors in grapes is vast, and understanding their relationship with grape variety and natural factors is highly intricate, making it challenging to establish clear cause-and-effect relationships.
In general:
Aroma compounds increase: during the ripening stage, under favorable weather conditions, most aroma compounds and their precursors experience an increase (except for methoxypyrazine, which is responsible for herbaceous aromas and typically decreases, and some types of terpenes - such as linalool and geraniol - and thiols)
Climate influences aromas. Very generally, sunnier climates tend to produce grapes and wines with higher to perceived ripeness in their aromas compared cooler and less sunny climates.
3. Grape Berries Anatomy: Extra-Ripening
Once the grapes are ripe, if they are left on the vine, they will go into extra-ripening showing following evolutions:
- Shrivelling.
- Concentration: Grapes will continue transpiration and lose water content as no more water is imported by the phloem at this stage. As a consequence means that sugars and aromas are concentrated.
- Aromas: Extra-ripe aromas will develop.
Extra-ripening time on the vine is not always possible as highly dependent on weather conditions (ideally hot and sunny)
I hope you found this article interesting and insightful.
Next Monday we will talk about propagation.
Happy week and happy studying,
Nicky