Abstract
The purpose of this work was to evaluate the applicability of two different carbon dioxide sensors with respect to the wine and bottling production fields. The instruments considered use gas-phase carbon dioxide detection through absorption spectroscopy. The applicability of this method was examined to extract measurements of dissolved carbon dioxide in the liquid phase and the sample pressure. Limitations of these measurements are discussed.
Carbon dioxide content is crucial in wine production because it is produced by yeasts and bacteria during the alcoholic and malolactic fermentation processes, as well as during the second alcoholic fermentation (“prise de mousse”) in closed bottles of sparkling wines and Champagnes (Jackson 2014). Because most processing and treatment of wine is accomplished in a low carbon dioxide environment, a fraction of the carbon dioxide produced is lost to the atmosphere before bottling. Carbon dioxide is largely responsible for the effervescence in champagne and sparkling wines; however, its influence on the organoleptic properties is important even for non-sparkling wines where its concentration is below the perception threshold (500 mg/L). Dissolved carbon dioxide concentrations range from 300 mg/L for red wines, 800 to 1800 mg/L for white and rosé wines, and up to 10 g/L for Champagne (Vidal et al. 2012, Liger-Belair 2012).
Techniques currently used for measurement of carbon dioxide in wine include MicroGC (gas chromatography), CarboQC (double headspace expansion, based on the higher solubility of carbon dioxide versus nitrogen and oxygen), Orbisphere probe (thermal conductivity), and Carbodoseur (headspace expansion). For further description of these techniques and for a comparison of tunable diode laser absorption spectroscopy-based measurements and other established methods, refer to OIV validation protocols (Vidal et al. 2012).
If wine samples are contained in filled transparent vessels and equilibrium between gas and liquid is ensured before measurement, gas-phase absorption spectroscopy (Werle et al. 2002, Lackner 2007) is a method suitable for wine analysis (Mulier et al. 2009, Vidal et al. 2012) even though the dissolved carbon dioxide concentration is measured indirectly. In a closed container at thermodynamic equilibrium, the relation between the partial pressure of carbon dioxide in the gas phase and its concentration in the liquid phase is given by Henry’s law. This equilibrium can be influenced by external perturbations (such as temperature variations and changing the headspace composition, e.g., by opening the container), and shaking the vessel will accelerate carbon dioxide exchange between the phases and bring the solution close to Henry’s equilibrium (Vreme et al. 2015). Samples in wine bottles can be measured noninvasively with this technique after shaking, and the process can be repeated as many times as desired to ensure equilibrium is achieved. Dissolved carbon dioxide concentrations are calculated using formulas available from the literature (OIV 2008).
The instruments examined herein are two commercially available models that use laser spectroscopy. Both instruments use a gas-phase carbon dioxide sensor designed for the bottling industry. Although gas-phase analysis is the primary measurement obtained using this technique, both of the instruments provide an estimate of dissolved carbon dioxide content.
Materials and Methods
The two instruments examined were: (1) LPRO srl (Camisano Vicentino, Italy) L.sensor.CO2 (http://www.lsensor.net/?portfolio=l-sensor-co2-2) and (2) ACM GmbH (Vienna, Austria) LAB.CO (http://www.acm.co.at/en/produkte/laborbereich/lab-co). Both instruments use tunable diode laser absorption spectroscopy where the working principle involves the transmission of light from a tunable, monochromatic light source through the gas sample (e.g., the partially transparent sample bottle) and sensing the light transmitted with a photodetector while the source wavelength is scanned. The profile recorded contains a spectral feature unique to gas-phase carbon dioxide, which is independent of external factors such as other gases and broadband attenuation from the container. The spectral profile acquired is analyzed to detect both partial carbon dioxide pressure (proportional to integral absorption line intensity) and total pressure (related to the absorption line width because of collisional broadening effects) (Mulier et al. 2009).
The first step in the evaluation of the two instruments was performed using off-the-shelf sample wine bottles to ensure proper operation of the instruments under nominal conditions. Next, a laboratory calibration cell was used to obtain a large number of samples by flushing and pressurizing its optical path with certified sample mixtures (100, 75, 50, 25, and 15% of carbon dioxide diluted in nitrogen, specified to be within ± 0.1% accuracy) at different pressures (evaluated with a Druck DPI 705 digital pressure indicator (General Electric, Boston, MA) specified within a total accuracy of ± 7 mbar). The cell was a 20-mm thick gas reservoir with two poly(methyl methacrylate) windows and flushing connections for the sample gas. The headspace of an arbitrary sample was simulated by filling the cell with the required carbon dioxide mixture at the desired pressure. This was repeated for the different mixtures at the various pressures (Figures 1–5).
The concentrations and pressures of the sample gases considered were chosen to test both of the instruments on the widest operational range encountered in the wine and beverage bottling industry. Higher pressures up to 7 bar (relative) were tested only with high carbon dioxide concentrations (up to an equivalent of 12 g/L, dissolved in the liquid phase) because they are typical of sparkling wines; lower carbon dioxide concentrations (up to an equivalent of 1.5 g/L, dissolved in the liquid phase) were tested only at lower pressures, which are more commonly used for still wines.
Validation of the displayed total pressure (and carbon dioxide partial pressure, which was a measurement directly provided only by the LPRO instrument) was then direct from the sample composition and manometer readout. The validation of the dissolved carbon dioxide concentration measurement was not as straightforward because, for practical reasons, we avoided any liquid phase in the test cell, which would require an additional liquid phase carbon dioxide measurement. Such a measurement would require a third instrument that uses a different technique and could introduce another source of error because of potential deviations from liquid-gas equilibrium.
We used a theoretical model given by Henry’s law, which states that at a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. To account for the temperature dependence, we used as Henry’s coefficient the polynomial expansion to obtain the following equation:
Values generally acknowledged in the literature (Sander 1999) for the parameters in the given formula are: with T temperature in Celsius, PpCO2 carbon dioxide partial pressure in bar, Cd dissolved carbon dioxide concentration in g/L. The temperature at the time of the measurements was 26°C.
The assumptions used are fair because the instruments measure independently different wines and beverages. Any further refinement in the model (available as additional features of the LPRO instrument, but not used for those measurements), such as the correction for the effect of liquid composition on Henry’s law, was not considered in this work.
Results and Discussion
The validation tests were performed using a range of concentrations and pressures readily encountered in the bottling of wine, beer, and soft drinks. Regarding operations at low relative pressures, the ACM instrument did not return a measurement below 1 bar (relative), while the LPRO instrument operated equally well at low pressure and atmospheric pressure (e.g., a wine sample with a defective seal). At high relative pressures, the maximum detectable pressure for the ACM and LPRO instruments were 6 bar and 8 bar (relative), respectively. Pressures above 6 bar (relative) were not measurable with the ACM instrument.
Both instruments detected total pressure and partial carbon dioxide pressure, the primary measurements accessible with the technique. In addition, total pressure was directly displayed from both instruments. The other parameter available from both user interfaces was dissolved carbon dioxide concentration, meaning that some post processing is performed by the instruments to apply Henry’s law.
Figures 1 to 5 summarize the data collected. For each figure, the left panel shows the displayed total pressure as a function of reference manometer total pressure, and the right panel shows the displayed dissolved carbon dioxide concentration as a function of its calculated theoretical value. Each point in a graph originates from the average of three measurements. Deviations of the points from the black line (y = x) show pressure errors from the manometer value (graphs, left panel) and concentration errors from the results of Henry’s law with given pressure and concentration (graphs, right panel). Accuracy is shown by the agreement between the values obtained by either LPRO or ACM instruments versus the true values from a reference manometer and the calibrated gas mixture specifications. Tables 1 and 2 show the standard deviations of the data series (in the graphs) for dissolved carbon dioxide and total pressure measurements, respectively.
Conclusions
The primary difference between the two instruments examined lies in the calculation of dissolved carbon dioxide. The ACM instrument does not show any statistically significant dependence of the displayed dissolved carbon dioxide content versus partial carbon dioxide pressure in the sample headspace. This behavior is not consistent with Henry’s law, and limits the applicability of the ACM instrument, but only for those samples with 100% carbon dioxide concentration in the headspace. The LPRO instrument shows applicability in the total pressure measurement with reasonable scaling of the dissolved carbon dioxide concentration according to Henry’s law. Furthermore, the LPRO instrument allowed manual override of the auto-detected temperature and provided custom correction parameters for carbon dioxide solubility to account for any peculiar measurement condition. An important feature of the LPRO instrument was its ability to take measurements at lower total pressures, which is a condition frequently encountered in wine bottling. This situation arises on leaky or noncompliant soft drink bottles and in polyethylene terephthalate bottles after aging because of the permeability of the material. Results at pressures below 1 bar (relative) and above 6 bar (relative) were not obtained with the ACM instrument. In conclusion, we compared the results obtained using two commercially available instruments that use laser spectroscopy to measure the content of carbon dioxide in the headspace of bottles to measure dissolved carbon dioxide in the liquid phase and the sample pressure. Advantages and limitations of the two measurement techniques were examined.
- Received March 2015.
- Revision received April 2015.
- Revision received June 2015.
- Revision received August 2015.
- Accepted September 2015.
- Published online December 1969
- ©2016 by the American Society for Enology and Viticulture
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