Various processes to concentrate grape must or wine by removing certain substances. This is usually done by reducing the pure water content. The oldest form is the drying of grapes, which was already used in antiquity. Today, this is achieved by a correspondingly late harvest and/or drying on straw mats/reeds as well as hanging the grapes on racks for a longer period of time (see also Trockenbeerenauslese and Recioto). In osmosis (reverse osmosis, osmotic distillation), different molecular sizes of liquids or of substances contained in them are exploited.
Further possibilities of concentration are the exploitation 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, because water evaporates at lower air pressure at already 30 °C (and less), this is not much higher than a conventional fermentation temperature. As a result, much less flavour is lost. A relatively new process is spinning cone column. These techniques are used in the production of RCGM, among others.
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Diplom-Sommelier, Weinakademiker und Weinberater, Volders (Österreich)