Must Concentration — Reverse Osmosis, Vacuum Evaporation, and Partial Concentration of Grape Must

Must concentration refers to mechanical or thermal processes that remove water from grape must before fermentation, thereby increasing the concentration of sugars, acids, phenolics, and aromatic compounds. The two principal methods are reverse osmosis (RO) and vacuum evaporation (also called high-vacuum evaporation, or HVE). Both have their origins in food and beverage processing technology and were progressively adopted by European winemakers from the 1980s onward as legally sanctioned alternatives to chaptalization, especially in cool or wet vintages where rain dilutes harvested fruit. Unlike traditional grape juice concentrate, which is produced at high temperatures and loses much of its aromatic character, modern must concentration operates under conditions designed to preserve volatile compounds.

• Reverse osmosis and vacuum evaporation are the two primary must concentration methods used in premium winemaking
• Adoption in Bordeaux accelerated during the 1980s and 1990s, with use spreading across classified estates facing difficult vintages
• Distinct from industrial grape juice concentrate, which is produced at high temperatures and used mainly in bulk wine production
• Appassimento (drying harvested grapes) can also be understood as a traditional form of must concentration used in styles such as Amarone

Reverse osmosis forces grape must under high pressure (typically up to 60 bar) past a semi-permeable membrane. Water molecules, along with small molecules such as ethanol and acetic acid, pass through the membrane, while larger molecules responsible for color, flavor, aroma, and structure remain on the pressurised side as a concentrated retentate. The permeate (water removed) is then discarded, leaving a more concentrated must. Because the process occurs at low temperatures, heat-sensitive aromatics are largely preserved. Vacuum evaporation heats must under reduced atmospheric pressure, which lowers the boiling point of water significantly and allows evaporation at temperatures well below 100°C, also protecting delicate volatile compounds. Both methods proportionally increase sugars, total acidity, and phenolics. Research confirms that RO raises both tartaric and malic acid content alongside sugars, an important distinction from chaptalization which affects only sugar.

• RO uses high pressure (up to 60 bar) to force water through a membrane, retaining all larger flavor and phenolic molecules on the concentrate side
• Vacuum evaporation removes water under reduced pressure and low temperature, preserving volatile aromatic compounds better than conventional thermal processing
• Both techniques increase sugars, acids, and phenolics proportionally; RO has been shown to increase must total acidity by approximately 25-28% alongside a 4-degree Brix increase
• EU rules cap total must concentration at 20% of juice volume per treatment

Must concentration is governed in the European Union by Regulation (EU) No 1308/2013 (Annex VIII), which lists partial concentration including reverse osmosis as an authorised enrichment method for grape must. Under standard rules, enrichment by concentration may not increase the natural alcoholic strength by more than 2% vol. In years of exceptionally unfavourable climatic conditions, the Commission may issue derogations permitting higher limits: for the 2013 vintage, for example, limits were raised to 3.5% vol in Zone A, 2.5% vol in Zone B, and 2.0% vol in Zones C I and C II for affected regions. Must concentration treatments must be declared and carried out under the responsibility of an oenologist or qualified technician. Concentration and alcohol removal may not be combined on the same lot of wine. EU rules also permit the partial concentration of wine (not just must) under specific conditions.

• Governed by EU Regulation No 1308/2013, Annex VIII, which explicitly names reverse osmosis as a permitted partial concentration technique
• Standard limit: no more than 2% vol increase in natural alcoholic strength; derogations in difficult vintages can raise this (e.g., up to 3.5% vol in Zone A for the 2013 harvest)
• Total volume of must concentrated may not exceed 20% of total juice volume
• Treatments must be declared and conducted under the responsibility of a qualified oenologist or technician; concentration and dealcoholisation may not be combined on the same wine lot

The primary practical motivation for must concentration is dilution caused by rainfall before or during harvest, a recurring challenge in cool Atlantic-influenced regions such as Bordeaux, Burgundy, and parts of Germany. By removing excess water, winemakers can restore the concentration of sugars, acids, and phenolics to levels consistent with fully ripe fruit, improving the structure, color depth, and aging potential of the resulting wine. Importantly, unlike chaptalization, RO and vacuum evaporation concentrate all dissolved must components together, including acids and phenolics, not just sugars. Critics note, however, that must concentration cannot improve phenolic or physiological ripeness if grapes were simply unripe at harvest: the technique addresses dilution, not the absence of genuine maturity. As climate variability increases pressure on harvest timing decisions, must concentration remains a relevant tool for precision correction of weather-related dilution.

• Primarily used to address dilution from rain at or before harvest, restoring concentration of sugars, acids, and phenolics proportionally
• Unlike chaptalization, concentration raises acidity alongside sugar, which can benefit structure and aging potential
• Cannot improve phenolic ripeness if grapes were genuinely unripe; research confirms RO should not be used to compensate for lack of fruit maturity
• Increasingly relevant as climate variability creates more unpredictable harvest conditions in traditionally cool wine regions

Reverse osmosis machines became widespread among classified Bordeaux châteaux from the late 1980s through the 1990s, with estates openly acknowledging their use during estate visits of that period. A 2001 Decanter report noted that approximately 60 Entropie vacuum concentration machines were operating in Bordeaux alongside a comparable number of RO units, with the Saint-Emilion cooperative having purchased machines that extended the technology beyond the crus classés. In Burgundy, use is less common given the smaller scale of most producers, though some négociants and growers have employed RO in difficult years. In Oregon, producers such as Ken Wright at Ken Wright Cellars have used vacuum concentration to manage dilution in rainy vintages. Several Bordeaux châteaux, including Pontet-Canet, reportedly moved away from RO in subsequent years as viticulture and vintage conditions improved.

• Bordeaux: widespread adoption among classified estates from the late 1980s-1990s; both RO machines and Entropie vacuum units in use, with some châteaux later discontinuing the practice
• Burgundy: use exists but is less common given small-scale production; some growers and négociants have used RO in wet vintages
• Oregon: producers such as Ken Wright Cellars have used vacuum concentration to address dilution from heavy autumn rainfall
• Mobile RO and vacuum concentration services are also available on a contract basis, making the technology accessible to estates without dedicated equipment

Must concentration is frequently compared with chaptalization, saignée, and the addition of commercial grape concentrate. Chaptalization adds external sucrose or rectified concentrated grape must to raise sugar levels, increasing potential alcohol without addressing dilution of acids, phenolics, or flavor compounds. Must concentration, by contrast, raises all dissolved must constituents together. Saignée involves bleeding off a portion of juice from red must after crushing to increase the skin-to-juice ratio, concentrating phenols through a different mechanism and typically yielding 10 to 20% juice removal; the bled juice is commonly vinified separately as rosé. Unlike must concentration, saignée does not remove water from the remaining must but instead adjusts the ratio of solids to liquid. Commercial grape concentrate addition involves products processed at high temperatures, which diminishes aromatic freshness and is more associated with bulk wine production. Must concentration at low temperatures aims to preserve the volatile aromatic profile of the original fruit.

• Chaptalization: adds only sugar externally, with no effect on acidity or phenolics; prohibited in many warm southern European appellations
• Saignée: bleeds off 10-20% of juice to raise skin-to-juice ratio in red winemaking; concentrates via a different mechanism and produces rosé as a byproduct
• Commercial concentrate addition: high-temperature processing reduces volatile aromatics; associated with bulk production rather than quality winemaking
• Must concentration: removes water at low temperature to raise all dissolved constituents proportionally, preserving aromatic profile better than high-temperature alternatives