Real Degree Of Fermentation (Rdf) measures the degree to which sugar in wort has been fermented into alcohol in beer, defined as “attenuation.” A sweet beer has more residual sugar and lower attenuation. The RDF expresses the percentage of extract that was fermented. RDFs in the 50s represent full-bodied beers with over 40% of their original extract left unfermented, whereas RDFs in the 80s represent highly attenuated beers with less than 20% of their original extract unfermented. Mouthfeel is largely determined by RDF percentage; the higher the RDF percentage, the lighter and drier the beer. Conversely, a beer with a lower RDF percentage may have a round and even syrupy mouthfeel. The term “real” in this instance separates this measurement from those that are “apparent.” Because alcohol is lighter than water, a sample of beer may “appear,” when a hydrometer is used, to have lower residual sugar than it actually does.
