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Various methods of concentrating grape must or wine by removing certain substances and/or reducing the pure water content. The oldest form is the drying of grapes, which was already used in ancient times. Today, this is achieved by late harvesting and/or drying on straw mats/reeds and hanging the grapes on racks for longer periods (see Trockenbeerenauslese and Recioto). Osmosis (reverse osmosis, osmotic distillation) utilises different molecular sizes of liquids or the substances they contain.

Vacuum concentration

Other possibilities for concentration are the utilisation of different boiling or freezing points. Low-pressure and low-temperature evaporators are used for this. This concentrates the sugar, but also removes volatile flavours. An improved technique is vacuum concentration, as water evaporates at low air pressure at just 30 °C (and below), which is not much higher than a conventional fermentation temperature. This means that far fewer flavours are lost. A relatively new process is the spinning cone column. These techniques are used in the production of RCGM (rectified grape must concentrate), among other things.

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