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Ultrasound based measurements of sugar and ethanol concentrations in hydroalcoholic solutions
Food Control 19 (2008) 31–35 www.elsevier.com/locate/foodcont
Ultrasound based measurements of sugar and ethanol concentrations in hydroalcoholic solutions Michael Van Sint Jan a, Marcelo Guarini a,*, Andre´s Guesalaga a, J. Ricardo Pe´rez-Correa b, Yolanda Vargas c b
a Departamento de Ingenierı´a Ele´ctrica, P.O. Box 306, Santiago, Chile Departamento de Ingenierı´a Quı´mica y Bioprocesos, Pontificia Universidad Cato´lica de Chile, P.O. Box 306, Santiago, Chile c Facultad de Fı´sica, Universidad de Santiago de Chile, Santiago, Chile
Received 27 August 2005; received in revised form 28 November 2006; accepted 28 November 2006
Abstract Industrial automation is useful to reduce production costs and improve product quality. The wine industry has been increasingly adopting industrial automation seeking these objectives. In Chile, wineries have been lagging behind in embracing new technologies to control the fermentation process. In most cases, enologists decide control actions (cooling, heating, pumping over) based on off line periodic measurements. In this work we explore the applicability of ultrasound to measure the sugar and alcohol concentrations of hydroalcoholic solutions mimicking fermenting musts. Additionally, implementation issues such as the attenuation effect of bubbles and tank curvature are analyzed. We show that working at two sufficiently distinct frequencies, we can measure sugar and alcohol content simultaneously. Measurement resolution achieved for the wave time of travel translates to a better than 0.02% for both concentrations, depending on the container size. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Wine fermentation; Process monitoring; Online measurements; Frequency dispersion
1. Introduction A good control of the state of wine fermentation is essential to assure its quality. Therefore a reliable system to measure the must concentrations on line would be extremely useful for the enologist to assess the state of the fermentation and then apply the corrective actions on time, namely: cooling, heating or pumping over. Ultrasound waves appear as an attractive alternative for this purpose, since it has been shown that provides reliable, accurate, affordable, hygienic and non-invasive measurements (Contreras, Fairley, McClements, & Povey, 1992). Today, refractometry is the most widely used method to measure sugar content in fermenting musts. The technique
*
Corresponding author. Tel.: +56 2 3544287; fax: +56 2 5522563. E-mail address: [email protected] (M. Guarini).
0956-7135/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodcont.2006.11.009
yields a precise reading at a relatively low cost. In turn, alcohol content is usually derived from density measurements. In several Chilean wineries, these variables are measured only two or three times a day, resulting many times in poor control. Ultrasound measuring devices are based on the analysis of a received signal, which is initially emitted by a transducer towards the fluid of interest. The emitted signal, typically a pulse or a wave packet, can be captured as a reflection from the body under test (echo) or by another transducer, after it has traveled through the body. The use of ultrasound wave propagation speed has already been applied to measure the average concentration of a single compound in a complex mixture (Contreras et al., 1992; Kress-Rogers & Brimelow, 2001). For example, sugar composition has been measured, using ultrasound speed, in fruit juices and drinks with an accuracy of 0.2% w/v when the temperature of the sample was controlled to ±0.1 °C.
32
M. Van Sint Jan et al. / Food Control 19 (2008) 31–35
However, the method described by these authors is not directly applicable to measuring the concentrations of several compounds in a complex mixture simultaneously. In the case of fermenting musts, dissolved sugar increases the density and decreases the compressibility of the fluid. These changes have opposing effects on the speed of sound waves. However, the latter effect is dominant, causing a significant rise in the speed of ultrasound. Additionally, increasing amounts of dissolved ethanol in fermenting musts, up to about 15% w/w, cause an increase in the speed of sound waves. At higher concentrations, the effect is reversed and sound speed decreases (Dispersion Technology, 2007). Therefore, it is interesting to explore the applicability of ultrasound waves to measure sugar and ethanol concentrations during wine fermentations. Although sugar and alcohol slightly modify the ultrasound speed dependence on temperature, one can obtain a rough idea of this variation by looking at ultrasound speed in water at temperatures found in fermentation. These temperature shifts produce variations on the order of 10 m/s (Del Grosso & Mader, 1972). The temperature effect over the speed of sound in solutions of sugar in liquids, depends on the concentration level and is stronger at lower concentrations for temperatures in the range of 0–30 °C (Smith & Winder, 1983). In this work we developed an ultrasound based method that may be applied to measuring sugar and alcohol content simultaneously during wine fermentations. Here the method using hydroalcoholic sugar solutions within the concentration range usually found in fermenting musts is tested (Boulton & Singleton, 1996). Additionally, implementation issues like attenuation effects of CO2 bubbling and tank curvature are analyzed. 2. Materials and methods The time between emission and detection (time of flight, TOF) of an ultrasound wave pulse was measured. In the case of wine, the TOF is of particular interest since it is simple to obtain and varies significantly with sugar and alcohol contents. All TOF measurements have an accuracy of 0.1 ls and all solutions were prepared with distilled water, sucrose (table sugar) and disinfectant alcohol (96% v/v ethanol and 4% v/v methanol). Meriakri and Chigrai (2004) found that sugar and alcohol concentrations could be determined using frequency dispersion (i.e., different frequencies propagate at different velocities) of electromagnetic waves in the microwave region of the spectrum. This is the key point in the method proposed here, as ultrasound speed in sugar hydroalcoholic solutions was measured at three different frequencies simultaneously, namely 54 kHz, 500 kHz and 1 MHz. Two transducers (emitter and receiver) for each frequency were used. The 54 kHz measurements were made with a PUNDIT ultrasonic pulse velocity meter, which consist of two stainless steel cased cylindrical transducers 5 cm in diameter and 6 cm long. The 500 kHz transducer is a Panametrics-NDT VIDEOSCAN Transducer, part number
V318-SU, cylindrical in shape with a diameter of 2.5 cm and 3.2 cm long. The 1 MHz transducers were prototypes manufactured by the Physics Department of the Universidad de Santiago, Chile. These are made of PXE5 ceramic and have a 200 kHz bandwidth (3 dB), these are also of cylindrical shape with 3.5 cm in diameter and 6.2 cm long. The speed of ultrasound was established for sugar hydroalcoholic solutions at 21 °C in the range 0–25% w/w sugar and 0–16% w/w alcohol at 54 kHz. On the other hand, frequency dispersion measurements were made measuring the temperature but not controlling it. Measurements at 54 kHz were carried out in an acrylic tank 2 m long, 15 cm wide and 20 cm high. To reduce acoustic coupling lateral walls were covered with expanded polystyrene sheets of 4 cm thickness. In addition, walls located at both ends of the tank were acoustically isolated from lateral walls by a thick layer of (elastic) silicon rubber. Here, transducers were not submersible, and were located in the outside of the walls at both ends of the tank. To achieve good ultrasound transmission, transducers were firmly attached to these walls using ultrasonic coupling gel. For measurements at 500 kHz and 1 MHz a smaller glass container 40 cm (L), 10 cm (H), 8 cm (W) with 3 mm thick glass was built. In this case we used submersible transducers, and there was no need to isolate the walls. Transducers were immersed in the solution and fixed to a piece of wood that held them 30 cm apart. Wood was used since it is a good and inexpensive ultrasound insulator. Installing an ultrasound device in a fermentation tank to measure on-line must concentration is not straightforward. Some practical issues must be addressed, namely: attenuation due to suspended material in the must and transmission quality through the must-steel interface at the tank walls. Due to their significant acoustic impedance difference with respect to water, suspended bubbles in the must scatter ultrasound, attenuating the received signal. In the fermentation process CO2 is liberated in the form of bubbles, which grow as they ascend in the tank. Their diameters expand at a rate of 0.05 mm per centimeter. Bubbles found at 1 m from the bottom of the tank can reach diameters up to 5 mm. To measure attenuation, air bubbles ranging approximately from 0.5 mm to 5 mm were injected using aquarium air pumps and different diffusers between the emitter and receiver transducers. Air was injected at a rate similar to that normally found in a fermentation tank during a CO2 production peak (7 l/min/m3). The signal intensity was measured and compared with the intensity without bubbles. 3. Results and discussion 3.1. Sugar concentration Fig. 1 summarizes the outputs from ultrasound speed measurements in sucrose solutions, displaying a clear linear trend (R2 = 0.9995). The achieved sample error in the measurement of TOF is less than 0.3 m/s, which is quite
M. Van Sint Jan et al. / Food Control 19 (2008) 31–35
33
Fig. 1. Ultrasound wave speed vs. sucrose concentration. Fig. 3. Ultrasound wave speed in a mimic fermentation.
low, corresponding to less than 0.02% in the sucrose concentration. The linear fit is given by Eq. (1), where v is the sound speed in (m/s) and s is the sucrose concentration in (% w/w) v ¼ 3:14s þ 1485:
ð1Þ
nol, it is expected that the net change in sound speed is fairly small through the fermentation process, as verified in Fig. 3. Therefore, measuring speed of sound alone is not enough to determine concentration levels accurately even if the temperature is known.
3.2. Alcohol concentration
3.4. Frequency dispersion
Wines rarely exceed 15% w/w of alcohol concentration. As shown by experimental results displayed in Fig. 2, the relationship between sound speed and ethanol concentration in the range below 16% w/w is also linear (R2 = 0.9972) and therefore can be represented by:
Results revealed appreciable dispersion, which was more evident at the extreme frequencies (54 kHz and 1 MHz). The data plotted in Fig. 4a and 4b indicates that the dispersion is stronger in ethanol solutions than in sucrose solutions, where the variations lie within the measurement error. Due to enthalpy changes caused by the addition of solute, the temperature in each measurement, and at each concentration, was different. However, it was the same for the three frequency measurements. Reported values for speed of sound and ultrasound in pure water are identical regardless of the frequency used, e.g., 1482 m/s at 20 °C (Del Grosso & Mader, 1972; Yost et al., 2005). Therefore, pure water has no appreciable frequency dispersion and thus the observed dispersion cannot be attributed to temperature and thus it is consequence of the solute
v ¼ 7:48e þ 1487;
ð2Þ
where v is the ultrasound speed in (m/s) and e is the ethanol concentration in (% w/w). 3.3. Hydroalcoholic solutions Since the conversion rate of sugar into alcohol is roughly 0.5 and the effect on v is twice as much as in etha-
Fig. 2. Ultrasound wave speed vs. ethanol concentration.
Fig. 4a. Ultrasound frequency dispersion in ethanol solutions.
34
M. Van Sint Jan et al. / Food Control 19 (2008) 31–35
Fig. 4b. Ultrasound frequency dispersion in sucrose solutions.
concentration. Additionally the rightmost three sets of points were measured at almost the same temperature and they exhibit very different values. Results show that measuring TOF at two sufficiently separate frequencies, allows sugar and alcohol concentration estimation, provided that the temperature is known. At the beginning of the fermentation process, when the solution is mostly sugar, the dispersion between ultrasound waves at both, 54 kHz and 1 MHz, is 4 m/s. As sugar is transformed into ethanol, the dispersion raises to about 20 m/s for a dry wine (2% w/w sugar and over 12% w/w ethanol). Practical reasons prevent us to measure dispersion in intermediate mixtures of water–ethanol–sugar so we only know the end points of this curve. It can be observed that concentration is closely related to dispersion whereas temperature can be measured in a classical manner using contact sensors. Therefore, it should be easy to obtain a precise value for sugar and alcohol content. We can speculate that another measurement at a sufficiently separated frequency can provide information to derive temperature without the need of contact sensors. Moreover, Contreras et al. (1992) have found that ultrasound speed was sensitive to the sugar species; therefore, measuring at an additional frequency can provide independent estimation of glucose and fructose concentrations in the fermenting must, thus obtaining temperature, glucose, fructose and alcohol measurements with a completely non-invasive method. 3.5. Effect of CO2 bubbles The results of the attenuation measurements due to CO2 bubbles described in the last paragraph of Section 2 are summarized in Table 1.
Fig. 5. Transmitted power of a sound wave passing through a water-steel interface.
This experiment shows that although attenuation due to bubbles is quite strong, the signal to noise ratio (SNR) remains high enough to make sufficiently accurate TOF measurements. In fact, the lowest SNR measured was 22 dB (with the 54 kHz transducers), providing a large and safe margin. The high SNR is mainly due to the almost complete absence of ultra-acoustical noise at the used frequencies: 1 MHz, 500 kHz and 54 kHz. Since the proposed method is based in TOF measurements, attenuation poses no serious threats to obtaining accurate results. 3.6. Transmission through the tank walls If a non-invasive probe is used, ultrasound transmission through the tank walls is a concern. Industrial fermentation tanks are made of stainless steel, which has high acoustic impedance compared to water. In the best case, an acoustic signal passing at a 0° angle with respect to the normal line through a steel-water interface suffers an attenuation of about 24 dB. This attenuation increases rapidly with the angle of incidence and it reflects completely at a certain critical angle as it is shown in Fig. 5. Additionally, since mechanical waves travel more than three times faster in steel than in water or wine, the ultrasound pulse traveling through the tank wall will reach the receiver prior than the one traveling through must, causing a false TOF reading, unless the reading hardware can handle this kind of problem. 4. Conclusions
Table 1 Attenuation due to bubbles at the three frequencies used Frequency
Attenuation (dB)
54 kHz 500 kHz 1 MHz
30 21 29
TOF measurements at a single frequency are inadequate to determine sugar and alcohol concentrations in wine fermentation. However, measuring TOF at two or more sufficiently separated frequencies (e.g., 54 kHz and 1 MHz) provide enough information to calculate these parameters.
M. Van Sint Jan et al. / Food Control 19 (2008) 31–35
CO2 bubbles are inevitable but they do not pose a serious limitation since the SNR remains high enough to carry out TOF measurements, despite producing a strong attenuation in the received signal. Ultrasound traveling through the stainless steel tank will cause false TOF readings in most commercial ultrasonic TOF equipment. Additionally the poor transmission of ultrasound through the tank walls, especially at oblique angles, poses a limitation to the measurements. Nonetheless, both problems can be solved by the proper design and placement of acoustic adaptors to refract the sound in a suitable direction. Both limitations can be solved attaching ‘‘acoustic adaptors’’ inside the tanks at the points where the transducers are located. These adaptors should be made of a material of appropriate impedance and must be shaped in a particular form as to refract sound waves in the direction of the other transducers. Another, but invasive, solution is to submerge the transducers and align the emitter with the receiver. The method can easily be extended to measure the spatial distribution of the aforementioned variables by taking TOF readings in various angles and positions throughout the tank. In this case, for the method to remain non invasive, it is essential the use of acoustical adaptors to direct the sound from one transducer to another. This work aimed at establishing the applicability and limitations of ultrasound measurements in order to obtain
35
relevant wine fermentation variables. Implementation of these methods requires measuring dispersion curves in real fermentation tanks at various temperatures and concentrations to establish accurate calibration curves. References Boulton, R., & Singleton, V. (1996). Principles and practices of winemaking. New York: Chapman Hall. Contreras, N. I., Fairley, P., McClements, D. J., & Povey, M. J. W. (1992). Analysis of the sugar content of fruit juices and drinks using ultrasound velocity measurements. International Journal of Food Science and Technology, 27, 515–529. Del Grosso, V. A., & Mader, C. W. (1972). Speed of sound in pure water. The Journal of the Acoustic Society of America, 52(5 (part 2)), 1442–1446. Dispersion Technology, Inc. (2007). Influence of Chemical Composition on the Acoustic Properties of Homogenous Liquids (March 2001). (Accessed 28-03-2007). Kress-Rogers, E., & Brimelow, C. J. B. (2001). Instrumentation and sensors for the food industry (2nd ed.). Woodhead Publishing Limited. Meriakri, V. V., & Chigrai, E. E. (2004). Determination of alcohol and sugar content in water solutions by means of microwave. In MSMW 2004 Symposium Proceedings. Smith, D. E., & Winder, W. C. (1983). Effects of temperature, concentration and solute structure on the acoustic properties of monosaccharide solutions. Journal of Food Science, 48, 1822–1825. Yost, W. T. et al. (2005). System for determination of ultrasonic wave speeds and their temperature dependence in liquids and in vitro tissues. The Journal of the Acoustic Society of America, 117(2), 646–652.
Ultrasound based measurements of sugar and ethanol concentrations in hydroalcoholic solutions Michael Van Sint Jan a, Marcelo Guarini a,*, Andre´s Guesalaga a, J. Ricardo Pe´rez-Correa b, Yolanda Vargas c b
a Departamento de Ingenierı´a Ele´ctrica, P.O. Box 306, Santiago, Chile Departamento de Ingenierı´a Quı´mica y Bioprocesos, Pontificia Universidad Cato´lica de Chile, P.O. Box 306, Santiago, Chile c Facultad de Fı´sica, Universidad de Santiago de Chile, Santiago, Chile
Received 27 August 2005; received in revised form 28 November 2006; accepted 28 November 2006
Abstract Industrial automation is useful to reduce production costs and improve product quality. The wine industry has been increasingly adopting industrial automation seeking these objectives. In Chile, wineries have been lagging behind in embracing new technologies to control the fermentation process. In most cases, enologists decide control actions (cooling, heating, pumping over) based on off line periodic measurements. In this work we explore the applicability of ultrasound to measure the sugar and alcohol concentrations of hydroalcoholic solutions mimicking fermenting musts. Additionally, implementation issues such as the attenuation effect of bubbles and tank curvature are analyzed. We show that working at two sufficiently distinct frequencies, we can measure sugar and alcohol content simultaneously. Measurement resolution achieved for the wave time of travel translates to a better than 0.02% for both concentrations, depending on the container size. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Wine fermentation; Process monitoring; Online measurements; Frequency dispersion
1. Introduction A good control of the state of wine fermentation is essential to assure its quality. Therefore a reliable system to measure the must concentrations on line would be extremely useful for the enologist to assess the state of the fermentation and then apply the corrective actions on time, namely: cooling, heating or pumping over. Ultrasound waves appear as an attractive alternative for this purpose, since it has been shown that provides reliable, accurate, affordable, hygienic and non-invasive measurements (Contreras, Fairley, McClements, & Povey, 1992). Today, refractometry is the most widely used method to measure sugar content in fermenting musts. The technique
*
Corresponding author. Tel.: +56 2 3544287; fax: +56 2 5522563. E-mail address: [email protected] (M. Guarini).
0956-7135/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodcont.2006.11.009
yields a precise reading at a relatively low cost. In turn, alcohol content is usually derived from density measurements. In several Chilean wineries, these variables are measured only two or three times a day, resulting many times in poor control. Ultrasound measuring devices are based on the analysis of a received signal, which is initially emitted by a transducer towards the fluid of interest. The emitted signal, typically a pulse or a wave packet, can be captured as a reflection from the body under test (echo) or by another transducer, after it has traveled through the body. The use of ultrasound wave propagation speed has already been applied to measure the average concentration of a single compound in a complex mixture (Contreras et al., 1992; Kress-Rogers & Brimelow, 2001). For example, sugar composition has been measured, using ultrasound speed, in fruit juices and drinks with an accuracy of 0.2% w/v when the temperature of the sample was controlled to ±0.1 °C.
32
M. Van Sint Jan et al. / Food Control 19 (2008) 31–35
However, the method described by these authors is not directly applicable to measuring the concentrations of several compounds in a complex mixture simultaneously. In the case of fermenting musts, dissolved sugar increases the density and decreases the compressibility of the fluid. These changes have opposing effects on the speed of sound waves. However, the latter effect is dominant, causing a significant rise in the speed of ultrasound. Additionally, increasing amounts of dissolved ethanol in fermenting musts, up to about 15% w/w, cause an increase in the speed of sound waves. At higher concentrations, the effect is reversed and sound speed decreases (Dispersion Technology, 2007). Therefore, it is interesting to explore the applicability of ultrasound waves to measure sugar and ethanol concentrations during wine fermentations. Although sugar and alcohol slightly modify the ultrasound speed dependence on temperature, one can obtain a rough idea of this variation by looking at ultrasound speed in water at temperatures found in fermentation. These temperature shifts produce variations on the order of 10 m/s (Del Grosso & Mader, 1972). The temperature effect over the speed of sound in solutions of sugar in liquids, depends on the concentration level and is stronger at lower concentrations for temperatures in the range of 0–30 °C (Smith & Winder, 1983). In this work we developed an ultrasound based method that may be applied to measuring sugar and alcohol content simultaneously during wine fermentations. Here the method using hydroalcoholic sugar solutions within the concentration range usually found in fermenting musts is tested (Boulton & Singleton, 1996). Additionally, implementation issues like attenuation effects of CO2 bubbling and tank curvature are analyzed. 2. Materials and methods The time between emission and detection (time of flight, TOF) of an ultrasound wave pulse was measured. In the case of wine, the TOF is of particular interest since it is simple to obtain and varies significantly with sugar and alcohol contents. All TOF measurements have an accuracy of 0.1 ls and all solutions were prepared with distilled water, sucrose (table sugar) and disinfectant alcohol (96% v/v ethanol and 4% v/v methanol). Meriakri and Chigrai (2004) found that sugar and alcohol concentrations could be determined using frequency dispersion (i.e., different frequencies propagate at different velocities) of electromagnetic waves in the microwave region of the spectrum. This is the key point in the method proposed here, as ultrasound speed in sugar hydroalcoholic solutions was measured at three different frequencies simultaneously, namely 54 kHz, 500 kHz and 1 MHz. Two transducers (emitter and receiver) for each frequency were used. The 54 kHz measurements were made with a PUNDIT ultrasonic pulse velocity meter, which consist of two stainless steel cased cylindrical transducers 5 cm in diameter and 6 cm long. The 500 kHz transducer is a Panametrics-NDT VIDEOSCAN Transducer, part number
V318-SU, cylindrical in shape with a diameter of 2.5 cm and 3.2 cm long. The 1 MHz transducers were prototypes manufactured by the Physics Department of the Universidad de Santiago, Chile. These are made of PXE5 ceramic and have a 200 kHz bandwidth (3 dB), these are also of cylindrical shape with 3.5 cm in diameter and 6.2 cm long. The speed of ultrasound was established for sugar hydroalcoholic solutions at 21 °C in the range 0–25% w/w sugar and 0–16% w/w alcohol at 54 kHz. On the other hand, frequency dispersion measurements were made measuring the temperature but not controlling it. Measurements at 54 kHz were carried out in an acrylic tank 2 m long, 15 cm wide and 20 cm high. To reduce acoustic coupling lateral walls were covered with expanded polystyrene sheets of 4 cm thickness. In addition, walls located at both ends of the tank were acoustically isolated from lateral walls by a thick layer of (elastic) silicon rubber. Here, transducers were not submersible, and were located in the outside of the walls at both ends of the tank. To achieve good ultrasound transmission, transducers were firmly attached to these walls using ultrasonic coupling gel. For measurements at 500 kHz and 1 MHz a smaller glass container 40 cm (L), 10 cm (H), 8 cm (W) with 3 mm thick glass was built. In this case we used submersible transducers, and there was no need to isolate the walls. Transducers were immersed in the solution and fixed to a piece of wood that held them 30 cm apart. Wood was used since it is a good and inexpensive ultrasound insulator. Installing an ultrasound device in a fermentation tank to measure on-line must concentration is not straightforward. Some practical issues must be addressed, namely: attenuation due to suspended material in the must and transmission quality through the must-steel interface at the tank walls. Due to their significant acoustic impedance difference with respect to water, suspended bubbles in the must scatter ultrasound, attenuating the received signal. In the fermentation process CO2 is liberated in the form of bubbles, which grow as they ascend in the tank. Their diameters expand at a rate of 0.05 mm per centimeter. Bubbles found at 1 m from the bottom of the tank can reach diameters up to 5 mm. To measure attenuation, air bubbles ranging approximately from 0.5 mm to 5 mm were injected using aquarium air pumps and different diffusers between the emitter and receiver transducers. Air was injected at a rate similar to that normally found in a fermentation tank during a CO2 production peak (7 l/min/m3). The signal intensity was measured and compared with the intensity without bubbles. 3. Results and discussion 3.1. Sugar concentration Fig. 1 summarizes the outputs from ultrasound speed measurements in sucrose solutions, displaying a clear linear trend (R2 = 0.9995). The achieved sample error in the measurement of TOF is less than 0.3 m/s, which is quite
M. Van Sint Jan et al. / Food Control 19 (2008) 31–35
33
Fig. 1. Ultrasound wave speed vs. sucrose concentration. Fig. 3. Ultrasound wave speed in a mimic fermentation.
low, corresponding to less than 0.02% in the sucrose concentration. The linear fit is given by Eq. (1), where v is the sound speed in (m/s) and s is the sucrose concentration in (% w/w) v ¼ 3:14s þ 1485:
ð1Þ
nol, it is expected that the net change in sound speed is fairly small through the fermentation process, as verified in Fig. 3. Therefore, measuring speed of sound alone is not enough to determine concentration levels accurately even if the temperature is known.
3.2. Alcohol concentration
3.4. Frequency dispersion
Wines rarely exceed 15% w/w of alcohol concentration. As shown by experimental results displayed in Fig. 2, the relationship between sound speed and ethanol concentration in the range below 16% w/w is also linear (R2 = 0.9972) and therefore can be represented by:
Results revealed appreciable dispersion, which was more evident at the extreme frequencies (54 kHz and 1 MHz). The data plotted in Fig. 4a and 4b indicates that the dispersion is stronger in ethanol solutions than in sucrose solutions, where the variations lie within the measurement error. Due to enthalpy changes caused by the addition of solute, the temperature in each measurement, and at each concentration, was different. However, it was the same for the three frequency measurements. Reported values for speed of sound and ultrasound in pure water are identical regardless of the frequency used, e.g., 1482 m/s at 20 °C (Del Grosso & Mader, 1972; Yost et al., 2005). Therefore, pure water has no appreciable frequency dispersion and thus the observed dispersion cannot be attributed to temperature and thus it is consequence of the solute
v ¼ 7:48e þ 1487;
ð2Þ
where v is the ultrasound speed in (m/s) and e is the ethanol concentration in (% w/w). 3.3. Hydroalcoholic solutions Since the conversion rate of sugar into alcohol is roughly 0.5 and the effect on v is twice as much as in etha-
Fig. 2. Ultrasound wave speed vs. ethanol concentration.
Fig. 4a. Ultrasound frequency dispersion in ethanol solutions.
34
M. Van Sint Jan et al. / Food Control 19 (2008) 31–35
Fig. 4b. Ultrasound frequency dispersion in sucrose solutions.
concentration. Additionally the rightmost three sets of points were measured at almost the same temperature and they exhibit very different values. Results show that measuring TOF at two sufficiently separate frequencies, allows sugar and alcohol concentration estimation, provided that the temperature is known. At the beginning of the fermentation process, when the solution is mostly sugar, the dispersion between ultrasound waves at both, 54 kHz and 1 MHz, is 4 m/s. As sugar is transformed into ethanol, the dispersion raises to about 20 m/s for a dry wine (2% w/w sugar and over 12% w/w ethanol). Practical reasons prevent us to measure dispersion in intermediate mixtures of water–ethanol–sugar so we only know the end points of this curve. It can be observed that concentration is closely related to dispersion whereas temperature can be measured in a classical manner using contact sensors. Therefore, it should be easy to obtain a precise value for sugar and alcohol content. We can speculate that another measurement at a sufficiently separated frequency can provide information to derive temperature without the need of contact sensors. Moreover, Contreras et al. (1992) have found that ultrasound speed was sensitive to the sugar species; therefore, measuring at an additional frequency can provide independent estimation of glucose and fructose concentrations in the fermenting must, thus obtaining temperature, glucose, fructose and alcohol measurements with a completely non-invasive method. 3.5. Effect of CO2 bubbles The results of the attenuation measurements due to CO2 bubbles described in the last paragraph of Section 2 are summarized in Table 1.
Fig. 5. Transmitted power of a sound wave passing through a water-steel interface.
This experiment shows that although attenuation due to bubbles is quite strong, the signal to noise ratio (SNR) remains high enough to make sufficiently accurate TOF measurements. In fact, the lowest SNR measured was 22 dB (with the 54 kHz transducers), providing a large and safe margin. The high SNR is mainly due to the almost complete absence of ultra-acoustical noise at the used frequencies: 1 MHz, 500 kHz and 54 kHz. Since the proposed method is based in TOF measurements, attenuation poses no serious threats to obtaining accurate results. 3.6. Transmission through the tank walls If a non-invasive probe is used, ultrasound transmission through the tank walls is a concern. Industrial fermentation tanks are made of stainless steel, which has high acoustic impedance compared to water. In the best case, an acoustic signal passing at a 0° angle with respect to the normal line through a steel-water interface suffers an attenuation of about 24 dB. This attenuation increases rapidly with the angle of incidence and it reflects completely at a certain critical angle as it is shown in Fig. 5. Additionally, since mechanical waves travel more than three times faster in steel than in water or wine, the ultrasound pulse traveling through the tank wall will reach the receiver prior than the one traveling through must, causing a false TOF reading, unless the reading hardware can handle this kind of problem. 4. Conclusions
Table 1 Attenuation due to bubbles at the three frequencies used Frequency
Attenuation (dB)
54 kHz 500 kHz 1 MHz
30 21 29
TOF measurements at a single frequency are inadequate to determine sugar and alcohol concentrations in wine fermentation. However, measuring TOF at two or more sufficiently separated frequencies (e.g., 54 kHz and 1 MHz) provide enough information to calculate these parameters.
M. Van Sint Jan et al. / Food Control 19 (2008) 31–35
CO2 bubbles are inevitable but they do not pose a serious limitation since the SNR remains high enough to carry out TOF measurements, despite producing a strong attenuation in the received signal. Ultrasound traveling through the stainless steel tank will cause false TOF readings in most commercial ultrasonic TOF equipment. Additionally the poor transmission of ultrasound through the tank walls, especially at oblique angles, poses a limitation to the measurements. Nonetheless, both problems can be solved by the proper design and placement of acoustic adaptors to refract the sound in a suitable direction. Both limitations can be solved attaching ‘‘acoustic adaptors’’ inside the tanks at the points where the transducers are located. These adaptors should be made of a material of appropriate impedance and must be shaped in a particular form as to refract sound waves in the direction of the other transducers. Another, but invasive, solution is to submerge the transducers and align the emitter with the receiver. The method can easily be extended to measure the spatial distribution of the aforementioned variables by taking TOF readings in various angles and positions throughout the tank. In this case, for the method to remain non invasive, it is essential the use of acoustical adaptors to direct the sound from one transducer to another. This work aimed at establishing the applicability and limitations of ultrasound measurements in order to obtain
35
relevant wine fermentation variables. Implementation of these methods requires measuring dispersion curves in real fermentation tanks at various temperatures and concentrations to establish accurate calibration curves. References Boulton, R., & Singleton, V. (1996). Principles and practices of winemaking. New York: Chapman Hall. Contreras, N. I., Fairley, P., McClements, D. J., & Povey, M. J. W. (1992). Analysis of the sugar content of fruit juices and drinks using ultrasound velocity measurements. International Journal of Food Science and Technology, 27, 515–529. Del Grosso, V. A., & Mader, C. W. (1972). Speed of sound in pure water. The Journal of the Acoustic Society of America, 52(5 (part 2)), 1442–1446. Dispersion Technology, Inc. (2007). Influence of Chemical Composition on the Acoustic Properties of Homogenous Liquids (March 2001).