Abstract
The determination of the alcoholic strength in spirits and liqueurs is required to control the labelling of alcoholic beverages. The reference methodology prescribes a distillation step followed by densimetric measurement. The classic distillation using a Vigreux rectifying column and a West condenser is time consuming and error-prone, especially for liqueurs that may have problems with entrainment and charring. For this reason, this methodology suggests the use of an automated steam distillation device as alternative. The novel instrument comprises an increased steam power, a redesigned geometry of the condenser and a larger cooling coil with controllable flow, compared to previously available devices. Method optimization applying D-optimal and central composite designs showed significant influence of sample volume, distillation time and coolant flow, while other investigated parameters such as steam power, receiver volume, or the use of pipettes or flasks for sample measurement did not significantly influence the results. The method validation was conducted using the following settings: steam power 70 %, sample volume 25 mL transferred using pipettes, receiver volume 50 mL, coolant flow 7 L/min, and distillation time as long as possible just below the calibration mark. For four different liqueurs covering the typical range of these products between 15 and 35 % vol, the method showed an adequate precision, with relative standard deviations below 0.4 % (intraday) and below 0.6 % (interday). The absolute standard deviations were between 0.06 % vol and 0.08 % vol (intraday) and between 0.07 % vol and 0.10 % vol (interday). The improved automatic steam distillation devices offer an excellent alternative for sample cleanup of volatiles from complex matrices. A major advantage are the low costs for consumables per analysis (only distilled water is needed). For alcoholic strength determination, the method has become more rugged than before, and there are only few influences that would lead to incomplete distillation. Our validation parameters have shown that the performance of the method corresponds to the data presented for the reference method and we believe that automated steam distillation, can be used for the purpose of labelling control of alcoholic beverages.
Highlights
The distillation step before the densimetric measurement is required to remove sugars and other solutes that would otherwise lead to false results, as the tables for converting density to alcoholic strength are based on pure water-alcohol mixtures (European Commission 2000; OIV 2009)
The EU reference method states some specifications for the distillation, which include that the apparatus must be leak-tight, the regularisation of the distillation rate must be possible, a rapid and complete condensation of the alcohol vapours must occur, and the first distillation fractions must be collected in an aqueous medium
Improved automatic steam distillation devices offer an excellent alternative for sample cleanup of volatiles from complex matrices
Summary
The distillation step before the densimetric measurement is required to remove sugars and other solutes that would otherwise lead to false results, as the tables for converting density to alcoholic strength are based on pure water-alcohol mixtures (European Commission 2000; OIV 2009). The classical distillation is comparably time-consuming, difficult to automate and problematic for some samples (such as liqueurs) that may have problems with entrainment and charring. For these reasons, our group has introduced the use of automatic steam distillation for the purpose in 2003 (Lachenmeier et al 2003) [English summary version published in 2004 (Lachenmeier 2004)]. In follow-up studies, we have shown the applicability for complex matrices such as egg liqueurs (Lachenmeier et al 2005), alcohol powders (Bauer-Christoph and Lachenmeier 2005) or cherryspirit containing cakes (Lachenmeier et al 2007)
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