Eur Food Res Technol (2006) 223: 261–266 DOI 10.1007/s00217-005-0193-9
ORIGINAL PAPER
Dirk W. Lachenmeier · Stephan G. Walch · Waltraud Kessler
Using experimental design to optimise precision of steam distillation for determining alcoholic strength in spirits
Received: 23 August 2005 / Revised: 24 October 2005 / Accepted: 26 October 2005 / Published online: 23 December 2005 C Springer-Verlag 2005
Abstract An experimental design, in combination with oscillation-type densimetry is introduced as a novel procedure to optimise steam distillation for the determination of alcoholic strength in different types of alcoholic beverages. A central composite design was chosen to study the effects of variation in levels of receiver volume (25 and 50 ml), steam power (30–100%), distillation time (20– 140 s) and sample volume (5–50 ml). Three commercial spirit drinks with different alcoholic strengths (4.9, 35.3, and 54.7% vol.) were compared on two automated steam distillation devices of different manufacturers by separately completing the experimental design for each. The models fitted for the prediction of the alcoholic strength, as indicated by an r2 value of more than 0.93. Various surface plots were generated to describe the relationship between operating variables and predicted alcoholic strength. Optimum conditions were 50 ml receiver volume, 70% steam power, 100 s distillation time and 25 ml sample volume. Slight deviations from the optimum did not result in substantial decrease of alcoholic strength. Validation experiments carried out under predicted conditions showed excellent correspondence to the reference procedure (R=0.999). D. W. Lachenmeier () Chemisches und Veterin¨aruntersuchungsamt (CVUA) Karlsruhe, Weißenburger Str. 3, D-78187 Karlsruhe, Germany e-mail:
[email protected] Tel.: +0721-926-5434 Fax: +0721-926-5539 S. G. Walch Zentrales Institut des Sanit¨atsdienstes der Bundeswehr Koblenz, Andernacher Str. 100, D-56070 Koblenz, Germany W. Kessler Hochschule Reutlingen, Fakult¨at Angewandte Chemie, Alteburgstr. 150, D-72762 Reutlingen, Germany
Keywords Ethanol . Alcoholic strength . Steam distillation . Oscillation-type densimetry . Experimental design . Optimisation . Response surface methodology Introduction Within the work of official food control, alcoholic strength by volume is one of the most important parameters in spirit drink analysis. In comparison to other countries, the European Union allows only a marginal tolerance of ±0.3% vol. concerning the indication of the actual alcoholic strength by volume in the labelling [1]. In 2004, more than 15% of all samples of liqueurs and fruit spirits analysed in the German federal state of Baden-W¨urttemberg had to be excluded due to faulty labelling of the alcoholic strength [2]. Exceeding the tolerated limits can have grave consequences for the manufacturers such as fines, high costs of recall and relabelling of a production lot. If, on the other hand, the alcoholic strength of the products is adjusted too high, an economic loss will be the consequence. Until recently, distillation with subsequent pycnometric determination of the density has been the reference method for the determination of alcoholic strength in spirit drinks. Pycnometers were generally considered to provide the greatest accuracy, but the method is time-consuming and requires special training of personnel if reproducible results are to be obtained because of a relatively greater opportunity to cause experimental errors in making the weight measurements necessary for pycnometry [3]. Automated methods such as gas chromatography or liquid chromatography could not replace pycnometry as a reference method because they are instrumentally complex and therefore more expensive. Furthermore, handling required trained personnel, and some methods did not have the required accuracy [4–8]. Secondary analytical technique that use near-infrared spectroscopy or Fourier transform infrared spectroscopy in combination with chemometric techniques may be used for a fast screening of alcoholic strength but also do not qualify as reference procedures [6, 7, 9–11].
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In the Eighties, electronic densimetry, which is based on electromagnetically-induced oscillation of a U-shaped glass tube, was introduced into the analysis of alcoholic strength, showing similar performance in terms of accuracy and precision in comparison to established methods like pycnometry, hydrostatic balance or hydrometry [3, 12–14]. This is nowadays the state-of-the-art method, because it is time-saving and cost-saving as well as simple to perform [3, 15]. In consequence, oscillation-type densimetry has been introduced into the European community as reference method for the analysis of spirit drinks [16]. The remaining limiting factor is the conventional distillation step commanded in the reference methods. This distillation is time-consuming and requires high personnel expenditures. Steam distillation was recently introduced as a promising alternative for determining alcoholic strength in all kinds of spirit drinks [17, 18]. With the carrier vapour, the distillable part of a mixture is separated from the nonvolatile residue. Simultaneously a boiling point depression occurs and a thermally mild treatment is achieved. By passing steam into the spirit sample, the alcohol is expelled and significantly shorter times of distillation are achieved in comparison to conventional distillation. In combination with oscillation-type densimetry, the procedure takes less than 8 min per sample. In this study, the influence of all basic operation parameters of the steam distillation device such as steam power of the steam generator, time of distillation, as well as sample volume and receiver volume on the outcome of the alcoholic strength was examined. As these settings influence each other, show interactions and, additionally, a partly nonlinear behaviour, the possibility to give one setting for all feasible alcoholic strengths and the transferability between steam distillation devices of different manufacturers was investigated. Materials and methods Instrumentation The automated steam distillation was accomplished with the Gerhardt Vapodest 30 (C. Gerhardt, Fabrik und Lager chemischer Apparate, Bonn) as well as the B¨uchi K-355 (B¨uchi, Flawil, Switzerland). The devices were coupled to tanks filled with distilled water. Before every start up the steam generators were preheated with a water sample at full steam power (according to the manufacturers’ instruction). For the tempering of the sample the heating circulator bath DC10-W26 (Haake, Karlsruhe, Germany) was used. The determination of density was accomplished with the density meter DE51 with sample pump ASU-DE by Mettler-Toledo (Giessen, Germany). The instrument was adjusted with air and water according to the manufacturer. The adjustment was checked daily using certified water standards (ρ=0.99820 at 20 ◦ C). The sample temperature in all measurements was adjusted to 20 ◦ C. 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 and cleaned with distilled water. Before shutdown of the system, the hoses were purged with acetone and air until they were dry. The reference method for determination of the alcoholic strength in spirit drinks was applied without modification as prescribed in the Commission Regulation (EC) No. 2870/2000 laying down Community reference methods for the analysis of spirit drinks [16]. The pycnometers and the distillation apparatus were purchased at Paris, Technische Glasbl¨aserei (Karlsruhe, Germany) after the reference method’s specifications. Sample preparation and measurement The sample was temperated in a water bath at 20 ◦ C. Following this, the sample was pipetted into a 250 ml Kjeldatherm digestion tube. Rest 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 placing a graduated flask filled with 3 ml of distilled water under the distillate outlet tubing, the programme was started, and the distillation was automatically performed. 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 graduated flask with the distillate was temperated and filled up to the calibration mark. The alcoholic strength was determined with the oscillation-type density meter. Optimisation using experimental design The following operation parameters of steam distillation were optimised with regard to maximum alcoholic strength: receiver volume (25 and 50 ml), steam power (30–100%), distillation time (20–140 s) and sample volume (5–50 ml). To cover the usual operation range, measurements with samples of different alcoholic strengths were carried out. A herbal liqueur with reference alcoholic strength of 35.28% vol., an alcopop drink with a reference value of 4.88% vol. and an absinthe with a high alcoholic strength of 54.69% vol. were tested. All test series were performed on both the Gerhardt and the B¨uchi device. In order to find the optimal working settings with minimum amount of experiments, the information contained in each experiment and furthermore the relation between the experiments has to be fully exploited by discovering interactions and nonlinear dependence. The important step was to set up an orthogonal experimental design space. This was done with a central composite design [19, 20]. The chosen levels of the variables are shown in Table 1. Studying three factors at five levels in a complete design would require 53 or 125 samples, whereas using a central composite design requires only 20 samples and still totally covers the experimental space and allows to calculate all interactions and nonlinearities. The calculations were done using the Software Package Design Expert V6 (Stat-Ease Inc., Minneapolis, USA).
263 Table 1
Levels of variables used in the central composite design
Variable
Name
Levels
A B C
Steam power Distillation time Sample volume
30, 44, 65, 86, 100 20, 44, 80, 116, 140 5.0, 14.1, 27.5, 40.9, 50
To further check the applicability and trueness, 54 samples from the study stack of the CVUA Karlsruhe (alcoholic strength range 2.0–55.5% vol.) were measured with both densimetry following steam distillation and the pycnometric reference method under within-laboratory reproducibility conditions (different operators on different days).
Results and discussion Optimisation of receiver volume Graduated flasks with volumes of 25 and 50 ml were available as receivers for the distillate. As a change in outlet volume obviously leads to a complete deviation of the other parameters, the first experiments were aimed to find the optimal receiver volume before the optimisation of the other parameters. As can be seen in Fig. 1, the experiments with the 25 ml receiver led to higher deviations especially at low distillation times, which can be attributed to higher pipetting and dilution errors. In contrast, the 50 ml receiver had a higher operating range with reproducible results above 80 s of distillation time. This confirms previous empirically determined results that a 50 ml measuring flask is best [17, 18]. All further experiments were, therefore, conducted using 50 ml receivers.
Fig. 1 Comparison between 25 and 50 ml graduated flasks as receiver for the alcoholic distillate (B¨uchi instrument). To reach comparability, the experiments for the 25 ml receiver were conducted with half the sample volume and half the distillation time compared to those used for the 50 ml receiver. At longer distillation times than 70 or 140 s, the receiver is filled above the calibration mark
Optimisation of steam power, sample volume and distillation time The experiments of the central composite design were evaluated using Analysis of Variance (ANOVA) to find the significance of the linear, quadratic and interaction terms in the response surface models. A very important step was to check the significant models for consistency by looking at the lack of fit and possible outliers. In all six designs the extreme settings with a very low distillation time and those with a very high sample volume showed to be clear outliers and had to be left out of the analysis. For both instrumental setups (Gerhardt and B¨uchi) it is possible to find the optimal working range. By means of response surface analysis the regression coefficients of the model are determined and the statistical ANOVA approach calculates the individual significance of each coefficient. Table 2 lists the regression coefficients. The factors are given in coded values, which make the six 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 B¨uchi instrument is independent of the variation in steam power (A), whereas at the Gerhardt instrument, the variation in steam power only slightly influences the distillation of alcopop, highly influences that of absinthe and has no influence on herbal liqueur. This difference can easily be explained by the fact that the B¨uchi instrument has a significantly higher steam power (2200 W) than the Gerhardt instrument (1600 W). The changes in distillation time (B) and sample volume (C) are very decisive in all the cases for measuring the alcoholic strength. The distillation time has a quadratic influence (B2 ) for all instruments. The Gerhardt instrument is again different to the B¨uchi instrument as steam power shows an additional quadratic influence (A2 ). The interaction between distillation time and sample volume (BC) has a lower influence on the Gerhardt instrument than for the B¨uchi one. With these models, it is possible to define an optimum working area for each instrument for determining the alcoholic strength of unknown spirits. In Figs. 2 and 3, the overlay response surface plots for the optimal working area for the two different steam distillation devices is shown for three samples with very different alcoholic strengths, respectively. For easy visualization, the region fitting the pycnometric reference value of alcoholic strength is marked white in the response contour. The response surface plots show clearly that the optimum working area is not much influenced by the steam power, that a sample volume between 15 and 30 ml and a distillation time between 90 and 110 s give best results for both instruments and spirits of different alcoholic strengths. The results are summarized in Table 3. Altogether, the method proved to be very rugged. The optimal settings for either instrument are independent of the varying influences of the parameters including the alcoholic strength of the sample and the type of steam distillation device in a wide range. The steam distillation device
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Fig. 2 Response surface plots showing the optimal working area (white) of steam power, distillation time and sample volume for different alcoholic strengths determined using the B¨uchi instrument (lines show reference values for herbal liquor, alcopop and absinthe sample)
Alcopop 4.9% vol. Gerhardt B¨uchi
Herbal Liqueur 35.3% vol. Gerhardt B¨uchi
Absinthe 54.7% vol. Gerhardt B¨uchi
Intercept A B C A2 B2 C2 AB AC BC r2 Standard deviation
5.22 0.15∗∗∗ 0.34∗∗∗ −0.21∗∗∗ 0.09∗ −0.17∗∗∗ no effect −0.13∗ no effect 0.13∗ 0.918 0.12
34.53 no effect 2.66∗∗∗ −1.23∗∗ −1.20∗∗ −1.20∗ 1.53∗∗∗ 0.33∗ 0.45∗∗ 0.42∗ 0.998 0.17
54.04 4.29∗∗∗ 4.12∗∗∗ −3.26∗∗∗ −2.59∗∗∗ −1.85∗∗∗ no effect −3.01∗∗∗ 1.21∗∗∗ 1.14∗∗∗ 0.991 0.67
4.89 no effect 0.27∗∗∗ −0.21∗∗ no effect −0.12∗ −0.22∗∗ no effect no effect 0.15∗ 0.930 0.13
110
50
95
40
35.06 no effect 1.57∗∗∗ −1.50∗∗∗ no effect −0.93∗∗∗ −0.83∗∗∗ no effect no effect 1.01∗∗∗ 0.994 0.17
54.58 no effect 3.75∗∗∗ −4.20∗∗∗ no effect −1.65∗∗∗ −1.93∗∗∗ no effect no effect 3.17∗∗∗ 0.997 0.41
A: steam power = 76%
C: sample volume [ml]
A steam power, B distillation time, C sample volume ∗ P≤0.05; ∗∗ P≤0.01; ∗∗∗ P≤0.001
Regression coefficient
B: distillation time [s]
Table 2 Regression coefficients in coded values for the optimisation of steam distillation with different alcoholic beverages on different instruments
Herbal liqueur: 35.28 80
Alcopop: 4.88
30
Absinthe: 54.69
Absinthe: 54.69 20
65
Alcopop: 4.88 Herbal liqueur: 35.28
C: sample volume = 25 ml
50 40
55
10
70
85
50
100
65
95
110
95
110
50
110
95
Herbal liqueur: 35.28 80
C: sample volume [ml]
A: steam power = 76%
B: distillation time [s]
Fig. 3 Response surface plots showing the optimal working area (white) for steam power, distillation time and sample volume for different alcoholic strengths determined using the Gerhardt instrument (lines show reference values for herbal liquor, alcopop and absinthe samples)
80
B: distillation time [s]
A: steam power [%]
Alcopop: 4.88
40
Absinthe: 54.69 30
Herbal liqueur: 35.28
20
65
Alcopop: 4.88 Absinthe: 54.69
50
C: sample volume = 25 ml 40
55
10
70
85
100
50
65
Table 3 Results of method optimisation for Gerhardt and B¨uchi instruments
80
B: distillation time [s]
A: steam power [%]
Parameter
Admissible range Gerhardt
B¨uchi
Optimised setting Overall
Reveicer volume (ml) Steam power (%) Distillation time (s) Sample volume (ml)
50 55–80 90–115 15–30
50 40–100 70–110 15–35
50 70 100 25
265 Table 4 Linear correlation between analysis results using the reference method (pycnometry with standard distillation) and oscillationtype densimetry with steam distillation for the determination of alcoholic strength in spirit drinks under within-laboratory reproducibility conditions (B¨uchi instrument) Parameter
Result (n=54)
Slope (LCI, UCI) Intercept (LCI, UCI) Correlation coefficient Probability Mean bias (%vol.)
0.998 (0.992, 1.004) 0.011 (−0.189, 0.210) 0.99979
