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 absolute standard deviations were between 0.
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.
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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. In the regulatory control of alcoholic beverages, the determination of the alcoholic strength is considered as pivotal parameter Lachenmeier et al.
Sample preparation by distillation followed by some form of density measurement i. For density measurement, electronic densimetry based on electromagnetically-induced oscillation of a U-shaped glass tube, is superior in performance and accuracy compared to pycnometry, hydrostatic balance or hydrometry Strunk et al. 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 ; OIV Only for some very clean spirits such as vodka a direct densimetric measurement may be feasible.
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.
As example, the EU reference method provides a classical distillation apparatus consisting of a cm Vigreux rectifying column, a cm straight-rimmed West condenser a variant of a Liebig condenser , and a cm cooling coil European Commission However, other suitable distillation devices may be applied, e. 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 Lachenmeier et al. In follow-up studies, we have shown the applicability for complex matrices such as egg liqueurs Lachenmeier et al.
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Following the initial experiments with steam distillation, instruments with increased steam power became available showing a larger optimal range independent of specific settings such as distillation time, alcoholic strength of sample, sample volume or receiver volume Lachenmeier et al. In this paper, we introduce the use of a completely new steam distillation device.
The device not only comprises an increased steam power but also a redesigned geometry of the condenser, as well as a larger cooling coil with controllable flow. Based on a method development and validation study, this paper describes the optimized procedure for alcoholic strength determination using steam distillation. The automated steam distillations were facilitated with the Gerhardt Vapodest new device, Fig.
Both devices were equipped with an alcohol extension set, which consists of a blind cap for the NaOH inlet which is unnecessary for alcohol distillation and may lead to losses and an outlet tube with reduced diameter to facilitate the use of a measuring flask as receiver. Both instruments were obtained from C.
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The steam generator water supplies were coupled to tanks filled with distilled water. The instrument was adjusted with air and water according to the manufacturer. The adjustment was checked daily using water standards.
The alcoholic strength was calculated automatically from the measured density using the stored official alcohol table data. Between measurements, the connecting tubes were purged with air.
Before shutdown of the system, the hoses were cleaned with distilled water and purged with acetone and air until they were dry. Automated steam distillation device with setup for the determination of alcoholic strength in spirits. The sample preparation was conducted as previously described Lachenmeier et al. Rests of the sample sticking to the edge of the tube were rinsed down with distilled water. Subsequently the tube was clamped in the distillation device.
After termination, the receiver and the tube were replaced and the hoses were rinsed with distilled water to be ready for the next sample. The residues in the flask then need to be carefully rinsed 3 times with distilled water into the Kjeldatherm digestion tube. The influence of all basic operation parameters of the method including the steam power of the distillation device, the time of distillation, sample volume, receiver volume, sample preparation pipetting or measuring flask , and cooling water flow through the cooling coil were examined.
As we have previously shown that some of these settings show interactions, and a partly non-linear behaviour Lachenmeier et al.
The designs and calculations were conducted using the Design Expert V7. Optimal working settings for steam power, sample volume and distillation time were initially investigated.
The same central composite design as described previously Lachenmeier et al. To research the influence of coolant flow setting, and additionally again the steam power, a central composite design was used.
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To research the influence of sample preparation, a d -optimal response surface design was used. Finally as third factor, volumetric glass pipettes or graduated flasks were used to transfer the sample volume into the Kjeldatherm tube. As first step of the validation, the requirement of the EU reference method was verified European Commission The absolute loss of the distillation step was then calculated to compare with the requirement of a loss of alcohol of not more than 0.
The alcoholic strength of the dilutions was determined before and after distillation using density measurement. Linear regression analysis was used to compare both measurement series. The root mean squared error RMSE was calculated to estimate bias. All calculations were conducted with Origin Pro v7.
Statistical significance was assumed at below the 0.
The basic operating parameters of the steam distillation device were assessed using three experimental designs. By means of response surface analysis, the regression coefficients of the models are determined and the statistical analysis of variance ANOVA approach calculates the individual significance of each coefficient. Regression coefficients in coded values for the optimisation of steam distillation on different instruments. The first experiment investigated steam power, sample volume and distillation time.
The factors are given in coded values, which make the models directly comparable between each other and offer the opportunity to find the importance of each regression term in the model. Significant differences between the two steam distillation devices were found.
The old instrument Vapodest 30 showed a significant influence of steam power and distillation time, an additional quadratic influence of distillation time, as well as an interaction between steam power and distillation time. The new instrument Vapodest only showed a significant influence of distillation time as well as an additional quadratic influence of distillation time. In the second experiment, the influence of the new possibility to regulate the coolant flow was investigated which was not possible with the old instrument.
The second experiment also confirmed the lack of influence of the steam power for the new instrument. The highest influence was found for the sample volume. Small differences were found for receiver volume double volume is better and the use of pipettes or flasks for sample transfer pipette use is better. Some interactions were found between sample volume and receiver volume at lower volumes, the use of smaller receiver volumes is overproportionally worse and between sample volume and use of pipettes or flasks at lower volumes, the use of flasks is overproprotionally worse than the use of pipettes.
For all analysed products, the method shows an adequate precision, with relative standard deviations below 0. Validation results for alcoholic strength measured with steam distillation and oscillation-type densimetry. Using the old device, we had always recommended to fill an amount of water in the receiver to avoid evaporation at the beginning of the distillation, when the liquid was comparably warm with high concentrations of ethanol.
With the new device, we have upheld this protocol to fill water into the receiver, so that the distillation outlet tube is immersed in the water. Experiments data not shown have found that the water in the receiver is not absolutely necessary due to the improved condenser, but we have decided to still follow the old protocol to be on the safest possible side and to allow the switching between devices without changes in the standard operating procedure that may lead to operator errors.
The significant influence of distillation time including a quadratic influence also corroborates our previous findings Lachenmeier et al. It is clearly deleterious to remove the receiver too early during the distillation, when the ethanol transfer is not complete. For the same reason, it appears to be advantageous to shake the receiver during the distillation process, which may reduce the vapour pressure of ethanol due to immediate dilution, especially at the start of the distillation, and also reduce the volume due to the volume contraction effect of ethanol—water mixtures.
The influence of sample volume may derive from a larger volume error at smaller volumes, rather than problems in the distillation itself. For the same reason, we suggest to use a larger receiver volume than the original sample volume.
The error increase due to dilution appears to be compensated by the more complete distillation. Due to the instrumental improvements, the method has become more rugged than before, and there are only few influences that would lead to incomplete distillation.
The major parameters are monitored by the software during operation and in case of deviations e. 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, while not strictly mentioned in the reference procedure, can be used for the purpose.
The excellent reproducibility of the procedure can be explained by the full automation of the distillation as well as the density measurement. A manual density measurement either by pycnometry or by manual oscillation-type densimetry was previously found as leading to higher errors Lachenmeier et al.
The validation data with repeatability limits r in the range of 0. Our previous validation data using steam distillation old instrument were also very similar [r range 0. Improved automatic steam distillation devices offer an excellent alternative for sample cleanup of volatiles from complex matrices.
The major advantages are the low costs for consumables per analysis only distilled water is needed compared to techniques such as liquid—liquid extraction which requires solvents or solid-phase extraction which requires extraction tubes.
Besides for alcohol distillation, we expect that the new devices may also improve other challenging steam distillation applications such as the determination of formaldehyde Jendral et al.
DWL conceived of the study, designed the experiments, coordinated the work, supervised the analyses, and drafted the manuscript. MK, BK and AK conceived the development of the steam distillation device, and contributed to the experimental design.
Vapodest 30S Programmable Distillation Systems Gerhardt
All authors read and approved the final manuscript. Hannelore Heger is thanked for her excellent technical assistance. No funding was specific to the production of this manuscript. The salaries for authors were provided by the affiliated organizations.
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KG, the manufacturer of the instrument described in this article. Dirk W. Lachenmeier, Email: ed.