3.4 Concentrating Grape Sugars

Most ripe grapes already contain enough sugar for dry table wine. Concentrating sugars beyond ordinary ripening is how many of the world’s great sweet wines are made. WSET Level 3 expects you to distinguish four pathways, the conditions each needs, and the resulting wine style implications. Yields drop sharply whenever water is removed from berries — concentration is expensive by nature.

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Why Concentrate?

As water leaves the berry (via fungal desiccation, shrivel, or ice separation), sugars, acids, and flavour compounds become more concentrated in the remaining juice. Winemakers can then ferment to a sweet balance (stopping fermentation or using yeast limits) with richer texture and intensity. Risks include volatile acidity, unwanted grey rot, labour for selective picking, and vintage inconsistency.

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Noble Rot (Botrytis cinerea in Noble Form)

Grey rot is usually destructive. Under a narrow climatic window, the same fungus can produce noble rot: skins become permeable, water evaporates, and berries shrivel into intensely sweet, honeyed, saffron-tinged fruit with botrytis character.

The Weather Pattern (Critical Exam Trap)

Noble rot needs:

1. Humid, misty mornings — moisture that lets Botrytis establish on ripe, healthy berries (often near rivers, lakes, or autumn fog).
2. Dry, preferably breezy afternoons — evaporation concentrates juice and keeps rot “noble” rather than sliding into wet, spoilage-dominated grey rot.

Trap: Students often say noble rot “needs rain” or “needs humidity all day.” Continuous wetness favours destructive rot, not the noble cycle. Ideal sites combine a moisture source in the morning with drying wind or sun later.

Growers pick selectively in multiple tries (tries), taking only sufficiently rotted berries — extremely labour-intensive. Not every humid vineyard can produce noble rot wines; grape variety (thin skins help), bunch architecture, and vintage weather must align.

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Drying on the Vine

Drying on the vine (sometimes called passerillage when intentional shrivel is sought) leaves bunches hanging after physiological ripeness so berries dehydrate naturally. Sugar concentration rises as water is lost. Unlike noble rot, this pathway does not require Botrytis — though botrytis may appear unwanted if rains return.

Conditions favouring clean on-vine drying include dry autumns, healthy fruit, and varieties that hold without rotting. Wines can show dried-fruit characters without the full fungal spice of noble rot. Risk: desiccation that goes too far, sunburn, or rain-induced rot before harvest.

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Drying After Picking

Here grapes are harvested and then dried in controlled or semi-controlled environments — lofts, drying rooms, mats, or racks — for weeks or months (appassimento is the well-known Italian framing). Airflow and humidity are managed to shrivel berries while limiting unwanted mould.

Consequences:

• Higher sugar and potential alcohol; intense dried-fruit flavours.
• Significant weight loss and low yield of must.
• Style fork: ferment to dryness with high alcohol (for example Amarone logic) or leave residual sugar (Recioto logic) — both start from dried grapes.

This method decouples concentration from autumn vineyard weather more than noble rot does, but it still needs healthy starting fruit and careful drying hygiene.

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Freezing on the Vine (Eiswein / Icewine)

Ice wine production waits until grapes freeze naturally on the vine. At press, frozen water remains as ice crystals and is largely excluded; the liquid fraction is extremely high in sugar (and acid). Legal definitions typically require harvest below a set temperature and prohibit artificial freezing of fruit for the designation.

Key points:

• Fruit should be healthy going into winter — frozen rotten grapes do not make fine Eiswein.
• Harvest often occurs at night/dawn in deep cold; logistics are demanding.
• Climates must be cold enough to freeze reliably after ripeness (Germany, Austria, Canada’s Niagara, etc.).
• Flavours emphasise pure concentrated fruit and high acidity rather than botrytis spice.

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Comparing Outcomes for Exam Answers

When an SAQ asks how a producer achieves high sugar for a sweet wine, pick the method that matches the region’s climate story:

• River fog + dry afternoons → noble rot.
• Dry late autumn hang → on-vine drying.
• Warm region with drying lofts → post-harvest drying.
• Reliable hard freeze after ripeness → Eiswein/Icewine.

Mention trade-offs: lower yields, higher cost, vintage risk, and (for noble rot) the humid-morning/dry-afternoon requirement. Do not confuse late harvest (riper fruit, possibly some shrivel) with true ice wine (frozen at press) or with fortification (alcohol added — a winery method, not vineyard concentration).

Key Takeaways

• Four vineyard concentration routes: noble rot, dry on vine, dry after pick, freeze on vine.
• Noble rot = humid mornings and dry afternoons — the highest-frequency trap.
• All routes concentrate sugar by removing water; none replaces the need for ripe, suitable fruit.