Process used in winemaking; see at RCGM.
RCGM is the most commonly used abbreviation for "rectified grape must concentrate". Rectification is a thermal separation process for separating a homogeneous solution of two or more substances by repeated distillation or countercurrent distillation. This results in a concentration of all soluble substances. The result is a product without the caramel flavour caused by heating in other processes. It is produced by partially removing water using various methods. This is also the difference to concentrated grape must.
It is usually produced by vacuum evaporation of grape must at very low temperatures to prevent the undesirable caramel flavour (which is why heat is obsolete)....
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Markus J. Eser
Weinakademiker und Herausgeber „Der Weinkalender“