Assess the role of sunlight in determining grape quality. (2023, Q6) Flashcards
How does sunlight affect sugar levels in grapes?
Sunlight drives photosynthesis, increasing sugar production in leaves and translocation to berries. More sunlight = more sugar accumulation = higher potential alcohol.
How does shading (lack of sunlight) affect potassium levels in grapes?
Shaded grapes tend to have higher potassium concentrations. This is often linked to delayed ripening and altered berry metabolism, which allows more K⁺ to accumulate.
What is the impact of high potassium on wine acidity and pH?
High potassium can raise juice pH by neutralizing acids (mainly tartaric), even if total acidity remains high. This leads to:
• Lower acidity perception
• Higher pH, which can compromise color stability and microbial safety
Why is managing sunlight important for potassium control in the vineyard?
Excessive shading (e.g., dense canopies) can lead to high K⁺ levels and unstable musts with high pH. Canopy management practices like leaf removal improve light penetration and help maintain better acid-pH balance.
Why can high potassium in shaded grapes lead to high pH despite high acidity?
Potassium neutralizes acids, forming salts that reduce free hydrogen ions, increasing pH even when total acidity remains high.
Which phenolic compounds are influenced by sunlight?
Anthocyanins (color), tannins (structure), and flavonols (co-pigmentation and UV protection).
What effect does shading have on phenolic development?
Shaded grapes have lower anthocyanin and tannin content, as shown in studies on Shiraz and Bordeaux varieties.
What is the risk of too much sunlight on phenolics?
Excessive sun can overheat berries (>35°C), degrading anthocyanins, causing color loss and “cooked” flavors. Excess flavonols may also add bitterness.
What timing of sunlight is ideal for norisoprenoid development?
Moderate exposure around veraison. Late exposure or excessive heat may degrade aroma precursors.
What is the difference between free and bound monoterpenes?
Free monoterpenes are immediately aromatic; bound ones are glycosidically linked and released during fermentation or aging.
How does sunlight affect monoterpenes in aromatic varieties?
Moderate sun (e.g. 50% defoliation) enhances monoterpene levels. Excess sun can cause volatilization or degradation of aroma compounds.
Why is east-side leaf removal preferred in many regions?
East-facing fruit receives gentle morning sun, enhancing aroma development while avoiding heat damage from afternoon (west) sun.
How does sunlight affect thiol concentration in grapes?
Moderate sunlight increases thiol precursors, often through thickening of skins. Early leaf removal improves thiol potential in Sauvignon Blanc and Petit Manseng.
What risk does excessive sun pose for thiols?
It may cause sunburn and reduce freshness or shift flavor balance toward overripe/tropical, away from desired citrus/gooseberry notes.
How does sunlight affect methoxypyrazine levels?
Sunlight degrades methoxypyrazines (e.g. IBMP), reducing green/herbaceous flavors in grapes like Cabernet and Sauvignon Blanc.
When and how is leaf removal used to reduce pyrazines?
East-side defoliation at veraison is common in cool climates (e.g. Bordeaux) to encourage degradation of IBMP while avoiding heat stress.
What are the sensory risks of excessive sunlight
Sunburned berries may develop bitter, oxidative, or “cooked” flavors, reducing grape quality and fermentation potential.
How does sunlight reduce fungal disease in vineyards?
UV rays lower humidity, improve airflow, and directly inhibit spore growth (e.g. Botrytis, powdery mildew). Open canopies reduce disease incidence.
Which regions use sunlight to manage rot risk or promote noble rot?
Bordeaux and Tokaj use leaf removal and canopy openness to prevent grey rot or to time Botrytis development under controlled conditions.
How does sunlight influence ripening consistency?
Even light exposure helps synchronize veraison and maturation across clusters, allowing uniform harvest timing and better grape quality.
How does light exposure affect next year’s yield?
Buds in shaded positions may not develop inflorescences, reducing fruitfulness. Light exposure also promotes cane lignification and winter hardiness.
How does sunlight help maintain vine balance?
It prevents excessive vegetative growth by ensuring interior leaves remain photosynthetically active, supporting fruit development over foliage.
Where are most aroma precursors located in the grape berry?
Mostly in the skins, though some are also in the pulp. This is why skin contact and careful vineyard management are important for aroma potential.
How does sunlight influence aroma compounds in general?
Sunlight promotes the synthesis of aroma precursors (like thiols, terpenes, norisoprenoids), and accelerates the degradation of undesirable compounds (like methoxypyrazines). Moderate exposure increases aroma complexity; excessive sun may degrade sensitive compounds.
What are norisoprenoids and why are they important?
Norisoprenoids are aroma compounds derived from the breakdown of carotenoids in grape skins. They contribute floral, fruity, or aged aromas like petrol (TDN) in Riesling, or tea/rose in Syrah.
How does sunlight affect norisoprenoid levels in grapes?
Sunlight (especially UV) helps break down carotenoids into norisoprenoid precursors. Moderate exposure around veraison increases their concentration, improving aging potential.
What are monoterpenes and what do they smell like?
Monoterpenes are aromatic compounds giving floral and citrus aromas, like rose, orange blossom, or bergamot. Found in Muscat, Gewürztraminer, Riesling, and to a lesser extent, Sauvignon Blanc.
What’s the difference between free and bound monoterpenes?
Free monoterpenes are immediately aromatic.
Bound monoterpenes are attached to sugar molecules (glycosides) and become aromatic only after fermentation or aging releases them.
How does sunlight affect monoterpene levels?
Moderate sunlight increases both free and bound monoterpenes. However, excessive sun can volatilize or degrade free monoterpenes, reducing aromatic intensity.
Which viticultural practice enhances monoterpenes without damaging them?
Early-season leaf removal on the east side of the canopy allows gentle morning sun to reach clusters, enhancing monoterpenes while avoiding heat damage from afternoon sun.
What are varietal thiols and what aromas do they contribute?
Thiols are sulfur-containing aroma compounds responsible for tropical and citrus notes like grapefruit, passionfruit, and gooseberry. They’re especially important in Sauvignon Blanc.
Are thiols present in grapes as aromas?
No — they exist in non-aromatic precursor forms (conjugated to amino acids or sugars) and are released by yeast during fermentation.
How does sunlight affect thiol precursors in grapes?
Sunlight increases thiol precursor accumulation by:
• Promoting berry skin thickening
• Possibly shifting amino acid metabolism
• Slightly stressing the vine to boost sulfur-based pathways
What canopy management technique boosts thiol expression?
Early leaf removal around flowering or fruit set improves thiol precursors, especially on the east side of the canopy. This is common in Marlborough Sauvignon Blanc production.
What do methoxypyrazines smell like, and in which grapes are they found?
They smell like green bell pepper, cut grass, or asparagus, common in Sauvignon Blanc, Cabernet Sauvignon, Merlot, and Carménère.
When are methoxypyrazines formed and how do they degrade?
They’re formed early in berry development (pre-veraison) and begin to degrade after veraison, especially with light and heat exposure.
How does vineyard management reduce methoxypyrazines?
Leaf removal around veraison (especially on the east side) increases sun exposure and accelerates methoxypyrazine breakdown.
Why are some methoxypyrazines desirable?
In small amounts, they contribute freshness or a varietal signature (e.g., subtle grassy notes in Sauvignon Blanc). Excess, however, is considered a flaw in most styles.
How does excessive sun affect aroma?
Too much sunlight or heat can:
• Volatilize or degrade delicate aroma compounds (especially monoterpenes and thiols)
• Sunburn grape skins, leading to oxidative aromas, bitterness, and reduced fermentability
What is 3MH and what aroma does it contribute?
3MH (3-mercaptohexanol) is a volatile thiol that gives grapefruit, passionfruit, and guava aromas, especially in Sauvignon Blanc and Petit Manseng.
What is 4MMP and what aroma does it contribute?
4MMP (4-mercapto-4-methylpentan-2-one) is a thiol associated with boxwood, cat’s pee, or blackcurrant bud aromas. It’s very potent and found in Sauvignon Blanc and sometimes Syrah.
How does sunlight affect the concentration of 3MH and 4MMP in grapes?
Moderate sunlight increases the precursors of both 3MH and 4MMP. This is achieved through berry skin thickening and possibly enhanced sulfur-containing amino acid metabolism. Early leaf removal has been shown to raise precursor levels.
Where are thiol precursors located in the grape?
Primarily in the skin. This is why exposure to light (which thickens the skin) and machine harvesting (which breaks the skin) can both increase final thiol concentration in wine.
Why are 3MH and 4MMP considered important for wine quality?
They are extremely aromatic at low concentrations and define the tropical/citrus style of wines like Marlborough Sauvignon Blanc. Without them, the wine can seem flat or neutral.
What is IBMP, and what aroma does it give?
IBMP (isobutyl methoxypyrazine) gives green bell pepper, asparagus, or cut grass aromas, mainly in Bordeaux varieties like Cabernet Sauvignon, Merlot, and Sauvignon Blanc.
How does sunlight reduce IBMP levels in grapes?
IBMP is synthesized early and degrades post-veraison. Sunlight accelerates this degradation, especially when clusters are exposed via leaf removal. UC Davis trials showed significantly lower IBMP in exposed vs. shaded fruit.
Why is IBMP considered a flaw in many wine styles?
In high concentrations, it dominates the aroma with unpleasant “green” notes. However, small amounts can be desirable in cool-climate Sauvignon Blanc or for stylistic freshness.
Which climate and viticultural strategies help manage IBMP?
• Cool climates: Careful leaf removal around veraison (esp. east side) to expose clusters
• Hot climates: Sometimes retain some foliage to avoid losing all varietal identity
What is the compound responsible for meaty/savoury aromas in Syrah?
The compound is 4-methylthio-2-butanone (4-MTB), also known as “meaty” thiol. It can give bacon, smoked meat, or umami notes.
How might sunlight influence meaty/savoury aromas in Syrah?
Although research is still developing, moderate sunlight exposure seems to enhance the reductive sulfur pathways that generate meaty notes (possibly linked to amino acid precursors like methionine). Open canopies can increase precursor concentration, especially in cooler climates like the northern Rhône.
Are meaty notes in Syrah considered positive?
Yes — when balanced, they are seen as a signature of high-quality Syrah, especially in traditional northern Rhône wines (e.g., Côte-Rôtie). Excess can be off-putting, though.
What is linalool, and what aroma does it give in wine?
Linalool is a monoterpene that contributes orange blossom, citrus, and floral aromas. It is one of the most important aroma compounds in aromatic varieties like Muscat, Gewürztraminer, and Riesling.
What is geraniol, and what aroma does it give in wine?
Geraniol is another monoterpene responsible for rose and geranium notes. It adds floral intensity and is found in Muscat, Gewürztraminer, and to a lesser extent in other aromatic varieties.
Are linalool and geraniol present as free aromas in grapes?
They exist in two forms:
• Free form (volatile and aromatic)
• Bound form (glycosidically linked, non-aromatic until released during fermentation or aging)
How does sunlight affect linalool and geraniol concentrations in grapes?
• Moderate sunlight exposure (e.g. early east-side leaf removal) increases both free and bound monoterpenes.
• Excessive exposure or heat can volatilize or degrade them, reducing floral intensity.
In which grape varieties are linalool and geraniol especially important?
• Muscat Blanc à Petits Grains
• Gewürztraminer
• Riesling
• Found in small amounts in Sauvignon Blanc and Albariño
What viticultural practice optimizes linalool and geraniol expression?
Early leaf removal (around fruit set or early veraison), especially on the east side of the canopy, to expose clusters to gentle morning sun while avoiding degradation from afternoon heat.
What is the impact of high potassium on wine acidity and pH?
High potassium can raise juice pH by neutralizing acids (mainly tartaric), even if total acidity remains high. This leads to:
• Lower acidity perception
• Higher pH, which can compromise color stability and microbial safety
How does sunlight exposure influence potassium and pH in grapes?
Grapes exposed to moderate sunlight tend to have:
• Lower potassium
• More favorable pH levels
This is due to faster, more balanced ripening and improved acid retention.
Why is managing sunlight important for potassium control in the vineyard?
Excessive shading (e.g., dense canopies) can lead to high K⁺ levels and unstable musts with high pH. Canopy management practices like leaf removal improve light penetration and help maintain better acid-pH balance.
How are norisoprenoids like TDN formed, and what role does sunlight play?
They are formed from carotenoid breakdown under UV/light exposure. Mid-season sun increases their precursors, contributing to aged aromas like petrol in Riesling.
