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Evaluation of ultrasonic propagation to measure sugar content and viscosity of reconstituted orange juice
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Journal of Food Engineering 86 (2008) 84–90 www.elsevier.com/locate/jfoodeng
Evaluation of ultrasonic propagation to measure sugar content and viscosity of reconstituted orange juice Feng-Jui Kuo a, Chung-Teh Sheng b, Ching-Hua Ting a,* a
Department of Biomechatronic Engineering, National Chiayi University, 300 University Road, Chiayi 600, Taiwan b Department of Bio-Industrial Mechatronics Engineering, National Chung Hsing University, Taichung, Taiwan Received 1 December 2006; received in revised form 13 July 2007; accepted 12 September 2007 Available online 22 September 2007
Abstract This paper describes the application of a non-contact ultrasonic system to measure the sugar content and viscosity of reconstituted orange juice. The system, which operates in either pulse-echo (PE) or transmission-through (TT) mode, detects the above two factors using responsive velocity of ultrasound and power attenuation. Experimental results of both modes show that the power attenuation is ineffective for such a purpose, and that the velocity of ultrasound tends to respond linearly to sugar content and exponentially to viscosity. The velocity exhibits a high linear correlation with Brix in orange juice, r = 0.994 in PE mode. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Orange juice; Sugar content; Viscosity; Ultrasonic velocity; Power attenuation
1. Introduction The juice production industry plays an important role in the harvest season as the availability of fresh fruits is usually in excess of demand. Fresh juice may be stored as a concentrated solution and later delivered to the market in a reconstituted form. Reconstituted juices may be blended with additives, diluted with water, or have added sugar to arrive at a specific, consumer-favoured flavour. The sweetness and viscosity are two controllable physical factors in reconstituting. The sweetness is a taste sensation that is difficult to measure scientifically; therefore, the sugar content is measured as an alternative (Harker et al., 2002). Practical methods currently used for measuring the sugar contents in a fluid include Brix gauge probing and chemical analysis. Brix gauge probing is traditional, accurate and effective, but can only be operated manually (Lynnworth, 1989) which is unacceptable in automated food processing. To measure *
Corresponding author. Tel.: +886 5 2717642; fax: +886 5 2717647. E-mail address: [email protected] (C.H. Ting).
0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.09.016
the viscosity, a manual spindle viscometer that measures the shear force of the fluid may be used. Hygiene and efficiency are an essential requirement in automated food processing. The best way to avoid contamination in quality measurement of food processing is to use non-contact detectors. Low-power ultrasound and infrared which are emitted from equipment isolated from the target material are a well-studied candidate in food processing, as they do not introduce extraneous contamination nor alter the properties of the substance (Cartwright, 1998; Gan, Pallav, & Hutchins, 2006; Rodriguez-Saona, Fry, McLaughlin, & Calveyb, 2001). Ultrasonics was first studied in the early 1920 and nowadays it is successfully and safely used in daily clinic diagnoses (Nyborg, 2002). It is cheap and has been used by the authors for various inspection of food products. Thus, this paper extends the applications of the ultrasonic techniques to measure the sugar content and viscosity in orange juice. It has been well understood that the propagation of ultrasound in a fluid is affected by the density, compressibility, temperature and composition of the fluid (Harker et al., 2002). Thus, ultrasonics has been utilised for more
F.J. Kuo et al. / Journal of Food Engineering 86 (2008) 84–90
85
List of symbols r
correlation coefficient between two measurements v velocity of ultrasound propagation j adiabatic compressibility q, qs, qw, qc specific gravities (densities) of fluid, sugar, water, and aqueous sugar solution Ws, Ww weights of sugar and water Sc concentration of aqueous sugar solution Zm near field distance at expected acoustic frequency and propagating velocity
than half a century as an effective tool in measurements of suspensions in milk (Hueter, Morgan, & Cohen, 1953) and density of fluid (Pryor & Roscoe, 1954). The Brix index of fluid is directly linked with the concentration of the dissolved sugar (Boulton, Singleton, Bisson, & Kunkee, 1996; McClements, 1995). Hence, we may expect a correlation between the Brix index of fluid and the behaviour of the ultrasound travelling in the fluid. A similar correlation can also be expected on viscosity since a fluid with more sugar becomes more sticky and hence has a higher viscosity (Greenwood, Adamson, & Bond, 2006). Both pulse-echo (PE) and transmission-through (TT) ultrasonic techniques are employed in this study to investigate how the sugar content in orange juice affects the nature of ultrasound propagation. The investigation, carried out by the authors, was initiated on aqueous solutions with various concentrations of sugar. The responsive velocity and power attenuation of ultrasound propagation were analysed before adopting either as the instrumental indicator. The preliminary investigation on aqueous solutions provides a macroscopic point of view of ultrasonic velocity in response to various sugar concentrations. Based on the preliminary results, the approach was extended to screened orange juice blended with various amounts of sugar solutions. 2. The ultrasonic measurement system 2.1. Ultrasound for measuring sugar content Accurate density measurement combined with ultrasonic velocity measurement is a widely used and indispensable method for determining the adiabatic compressibility of fluids (No¨lting, 1995) and a wide range of thermodynamic parameters (Ewing, 1993). The velocity of sound in aqueous sugar solution appears to correlate with the temperature, sugar concentration, and solution density (McClements, 1995). Different sugar compositions exhibit different characteristics in acoustic velocity. An inter-composition investigation on the composition of sugar types – glucose, fructose, and sucrose – has been well established (Contreras Montes de Oca, Fairley, McClements, & Povey,
R f x ta, tb, td
radius of ultrasound probe frequency of ultrasound propagation travelling distance of an ultrasonic waveform occurrences of waveforms ‘‘a”, ‘b”, and ‘‘d” (relative to Va, Vb, Vd) a power attenuation of ultrasound Va, Vb, Vd amplitudes of waveforms ‘‘a”, ‘b”, and ‘‘d”
1992). However, this study treats the sugar used as a mixture. The velocity of sound and the density of solution is linked with the so-called Wood equation (Wood, 1964): 1 v ¼ pffiffiffiffiffiffi jq
ð1Þ
where j is the adiabatic compressibility and q is the density of solution. This equation indicates that a denser fluid (with a bigger q) or a more compressible fluid (with a bigger j) reduces the velocity of sound propagating in the fluid. However, the propagation velocity of sound travelling in a fluid is a competing factor between density q and compressibility j. The relationship between the two variables is described with the concept of ‘‘concentration increments”, denoted as (Sarvazyan, 1991): ½j ¼ 2½v ½q
ð2Þ
The notation in bracket denotes the specific concentration of the enclosed variable. In organic solution the compressibility (j) decreases to a greater degree if its density (q) increases (Sarvazyan, 1991). The combined effect of a considerably decreased compressibility subject to an increased density will increase the velocity of ultrasound (v) in solution. This fact of physics elucidates the use of ultrasonic velocity in determining the density and compressibility of a solution (McClements, 1995). Ultrasonics can hence be used as an indirect measurement of the sugar content in solutions. The sugar in solution is assumed to be completely soluble and the resultant density qc is: qc ¼
WwþWs 1 W w q1 w þ W s qs
ð3Þ
where Ww is the weight of water, Ws the weight of sugar, qw the specific gravity of water, and qs the specific gravity of sugar. The sugar content of a solution provides an alternative, indirect measurement of sweetness sensation supposing the solution is free of artificial sweetener (Harker et al., 2002). Additional sugar dissolved in a solution alters the sensation of sweetness. The sugar content (Sc) in water (Ww) is a fraction of the dissolved sugar (Ws):
86
Sc ¼
F.J. Kuo et al. / Journal of Food Engineering 86 (2008) 84–90
Ws WwþWs
ð4Þ
The food industry uses ‘‘Brix” to measure Sc. For fruit juices: one degree Brix is about 1–2% sugar by weight. This usually correlates well with perceived sweetness. Eq. (3) provides a correlation between the Brix of Eq. (4) and ultrasonic velocity of Eq. (2). The velocity of ultrasound could be therefore applicable in measurement of the sugar content in fluids. 2.2. The apparatus Figs. 1a and 2a show the two schemes of ultrasonic measurement systems developed in our laboratory. The sample chamber accommodates 400 ml of fluid being detected and is immersed in water kept at a constant temperature to mitigate temperature effects on acoustic propagation. The ultrasonic pulser and receiver (1 MHz, Western NDE, Canada) are mounted onto the sample chamber. The transducer (£20 mm) was excited to emit ultrasound at a maximum power of 800 mW/m2 and a duty duration of 60 ls. This low-power acoustic stimulation in a very short period is unlikely to introduce effective heat or sonochemical effects onto the juice, as normally, only ultrasound with a power intensity much bigger than 1 kW/m2 is used in heating up or altering the micro-organisms of a bio-material (Knorr, Zenker, Heinz, & Lee, 2004; Valero et al., 2007). A computer-controlled ultrasonic system (WT-UT-001A, Western NDE, Canada) signals the transducer to emit ultrasound into the fluid and the received ultrasonic waves
Fig. 2. The TT ultrasonic measurement technique.
are amplified at a gain of 60 dB. Ultrasonic responses and inferred data are displayed on-line for human intervention and recorded for subsequent off-line analysis. The transducer was calibrated using reverse osmosis (RO) water as the reference solution. The sample chambers are designed with a dimension within the ultrasonic near field. The near (Fresnel) field is the ideal region for measurement of highest accuracy because of a maximum beam power. Ultrasonic waves diverge while propagating beyond the designated near-field distance. The near field, Zm, is defined as the distance within: R2 f ð5Þ v where R is the radius of the probe, f the ultrasonic frequency, and v the velocity of ultrasound. In this study, the expected ultrasonic velocity ranges from 1.465 103 m/s in water to 1.714 103 m/s in dense aqueous sugar solution (Contreras Montes de Oca et al., 1992). Thus, the maximum ultrasound near-field distance should be less than 58 mm. To assure less resistant transmission, the inner space of the sample chamber is chosen to be 33 mm.
Zm ¼
Fig. 1. The PE ultrasonic measurement technique.
2.2.1. Pulse-echo measurement The pulse-echo system, as shown in Fig. 1b, emits an epoch of ultrasound (wave ‘‘a”) through the solution until the acoustic waves impinge on the steel wall of the container. Part of the acoustic energy is reflected (wave ‘‘b”) depending on the size of the interface and the relative impedance difference (Lynnworth, 1989). The reflection
F.J. Kuo et al. / Journal of Food Engineering 86 (2008) 84–90
process, at the same frequency, continues until the ultrasound vanishes in the fluid. The velocity of sound in a medium depends only on the property of the medium (McClements, 1995; Wood, 1964). Hence, all the above waves travel at the same speed regardless of respective power intensities. The two reflection waveforms, waves b and d in Fig. 1b, acquired by the probe, show conceptually that the magnitudes are attenuated after reflection. The attenuation is understandable since part of the ultrasonic energy is scattered in fluid (scattering effect) and also not all ultrasonic waves are reflected by the steel wall (adsorption effect). The velocity of ultrasound is measured by counting the time elapsed between two consecutive peak amplitudes, as
3. Preliminary experiments on aqueous sugar solutions Preliminary experiments were conducted using the PE scheme. The results provide a reference for further experimental design for studies on orange juices.
ð6Þ
This equation states that the propagation route x is a significant factor in waveform attenuation. A bigger coefficient denotes a more attenuated propagation. 2.2.2. Through-transmission measurement The through-transmission (TT) scheme of Fig. 2 has an ultrasonic transmitter that emits ultrasound through the probed fluid, and a receiver opposite that collects the transmitted ultrasound. The ultrasound that travels through the fluid is hindered depending on the properties of the fluid, i.e. mainly the adiabatic compressibility and the density. The TT technique has the transducer arrangement as shown in Fig. 2b. Ultrasonic waves are emitted from the transmitter and then propagate through the fluid to the receiver. The velocity of ultrasound is reckoned by counting the time elapsed between the emission and the receiving events, as x v¼ ð8Þ tb ta where ta denotes the occurrence of emission and tb the occurrence of receiving. And the coefficient of power attenuation, a, is defined as 1 Va a ¼ log10 ð9Þ x Vb where Va is the perceived magnitude of the emission and Vb is the magnitude of the received waveform.
3.1. Preparation of aqueous sugar solutions Different quantities of white refined cane sugar were dissolved in RO water to obtain 25 sample solutions with concentrations ranging from 2% to 50% at incremental steps of 2%. The sample chamber of Fig. 1a was filled with sample solution and then covered with a lid to avoid environmental influence on acoustic transmission within the chamber. The ultrasonic velocity, attenuation, Brix, and viscosity of each sample solution were measured and recorded. Fig. 3 records Brix and density measurements as functions of sugar contents Sc of Eq. (4). The Brix index is an alternative measurement of solution density qc of Eq. (3). The relation between Sc and qc can hence be considered as an ideally linear function. This correlation allows one to change the Brix of a solution by regulating the amount of sugar additive. Also, the correlation with qc allows one to investigate the sugar content using the Wood equation of Eq. (1) by means of the velocity of ultrasound. 3.2. Ultrasonic velocity and Brix As developed in Section 2.1, Eqs. (1) and (2) state that an increased ultrasonic velocity in response to an increased density can be expected. Rewriting Eq. (2) in the form:
60 50 Brix: y =1.8427 + 0.99815x r = 0.999
40 30 20
1200
1100 Density: z =986.8124 + 4.5316x r = 0.999
10 0
2.3. Measurement of physical characteristics
1300 3
where td and tb denote the occurrence times of peaks Vd and Vb and x = 33 mm is the inner space of the sample chamber. The power attenuation, a, is defined as (dB/m): 1 Vb a ¼ log10 ð7Þ 2x Vd
Solution Density (kg/m )
2x td tb
ultrasonic propagation. The sugar content was quantitatively measured using a Brix meter (N-50E, Atago, Tokyo, Japan) with a reference temperature of 20 °C. The actual Brix indices were calibrated to fit the ambient (bath water) temperature at 30 °C. The viscosity of solution was measured using a spindle viscometer (LVDV-II+, Brookfield, MA, USA). A complete measurement was carried out within 1 min to avoid possible degrading effects on the juice.
Brix
v¼
87
0
10
20
30
40
1000 50
Sugar Concentration (%)
The bath water was kept at 30 ± 0.5 °C to alleviate possible temperature influence on fluid properties and also
Fig. 3. The Brix and density of an aqueous sugar solution is linearly proportional to the amount of added sugar.
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F.J. Kuo et al. / Journal of Food Engineering 86 (2008) 84–90
½j ½q 2
A positive [q] is associated with a more negative [j] thus giving a positive [v], i.e. the velocity increases. Experimental results in Fig. 4 show there is a positive linear correlation between ultrasonic velocity and Brix. The graphic representation does not show a perfect correlation between the two variables. However, the modelling discrepancy is considered trivial, as it has a standard deviation of 68 m/s which is far smaller than the average velocity of 1550 m/s. This preliminary result on aqueous sugar solutions demonstrates the possibility of ultrasonic velocity as an effective indicator of Brix index and density in fluid. 3.3. Ultrasonic velocity and viscosity The viscosity of a solution is a function of the sugar content in the solution (Harker et al., 2002). Fig. 5 shows a non-linear correlation existing between viscosity and Brix. The viscosity is linearly proportional to Brix at values less than 35 and is exponential beyond 35. This behaviour shows that a solution containing more sugar is much more sticky than a light solution. The relationship between the ultrasonic velocity and the viscosity is nearly exponential, as shown in Fig. 6, though there are insufficient samples
Ultrasonic Velocity (m/s)
1700 y =1434.328 + 5.0741x r = 0.983
1600 1550 1500 1450
0
10
20
30
40
50
60
Fig. 4. A linear correlation between PE ultrasonic velocity and Brix in prepared aqueous sugar solutions.
12 10
Viscosity (mPa.s)
1650 1600 1550 1500 1450
2
3
4
5
6
7
8
9
10
11
Viscosity (mPa.s)
Fig. 6. PE ultrasonic velocity as a function of viscosity in prepared aqueous sugar solution.
to reveal this trend properly. There exists a nearly linear correlation for viscosity less than 3 MPa s. The ultrasonic velocity almost saturates for viscosity values beyond 3 MPa s. This is understandable as a sticky solution exhibits a larger shear resistance to the employed rotary viscometer in fluid mechanics. A solution with a viscosity value of 3 MPa s is equivalent to 30 Brix, as illustrated in Fig. 5, which is considered too sweet to be acceptable in the juice industry. 3.4. Ultrasound attenuation and sugar content
3.5. Summary from preliminary experiments Brix
8 6 4 2 0
1700
Attenuation in ultrasound is a complicated function of wave propagation in a fluid. Factors that may affect ultrasound attenuation include the viscosity, compressibility, wall material, and scattering and adsorption effects (Povey, 1997). No meaningful correlation was found between the attenuation coefficient and Brix and viscosity in the experimental results. Thus, ultrasound attenuation is presented in subsequent sections.
1750
1650
1750
ð10Þ Ultrasonic Velocity (m/s)
½v ¼
10
20
30
40
50
60
Brix
Fig. 5. Exponential correlation between viscosity and Brix in prepared aqueous sugar solutions.
The ultrasonic velocity in solution is an effective, nondestructive indicator of Brix and viscosity, as shown in Figs. 4 and 6. In theory, ultrasound propagating in a fluid is attenuated and hence distinguishable attenuation coefficients could be expected (McClements, 1995). However, the laboratory study revealed a non-applicable attenuation coefficient profile. This means that the velocity of ultrasound is a more reliable indicator for instrumentation in identification of the sugar content and viscosity in fluids. The characteristics of the inspected solution will not be altered by the ultrasound as low-power ultrasound was used. Though the preliminary study was carried out on water dissolved with various amounts of sugar, the result may be used as a guide for studies relating to juice production. Encouraged by the good correlation between ultrasonic indices and physical properties, the experimental study was extended to detect the Brix index and viscosity of orange juice blended with sugar solutions of various concentrations.
F.J. Kuo et al. / Journal of Food Engineering 86 (2008) 84–90
4. Application to orange juice
A juice sample consists of 300 ml of fresh orange juice and 600 g of sugar solution. Orange juice was obtained from fresh oranges with flesh suspensions being screened out using a stainless steel mesh filter with an aperture size of 0.061 mm2. The screening mitigates possible scattering effects and alleviates acoustic impedance because of the solid suspensions. Sugar solutions were prepared in concentrations ranging from 0% to 26% at incremental steps of 2%. This studied Brix range is large enough since reconstituted juices normally have a Brix index between 10.6 and 11.6 (USDA, 1995). The sample chambers of Figs. 1a and 2a were filled with the blend and were covered tightly with lids. The laboratory work was the same as that used in the preliminary study on aqueous sugar solutions. 4.2. Results and discussion Experimental results from the TT scheme behave almost identically to these from the PE scheme and hence only PE results are presented. Fig. 7 shows a comparison between juice blend and aqueous solution measured using the PE scheme. The ultrasonic responses shows a good linear correlation between the ultrasonic velocity and the Brix index in orange juice. This is quite promising since the Brix indices of both aqueous solution and orange juice can be effectively indicated by the velocity of ultrasound. The aqueous sugar solution expresses a slower ultrasonic velocity than orange juice probed using either the pulse-echo or the throughtransmission scheme. It is worth noting that the orange juice contains many more chemical compositions than the aqueous sugar solution. This, in turn, contributes to the density of the solution and hence the responsive ultrasonic velocity. The compositions in the solution are a significant factor that alters the behaviour of ultrasound in the solution (Contreras Montes de Oca et al., 1992). They diminish
Ultrasonic Velocity (m/s)
1580 * — — pure juice ο — — pulse—echo y =1465.8993 + 3.9261x r = 0.994
1540 1520
+ aqueous sugar solution y =1465.6246 + 3.1886x r = 0.997
1500 1480
5
10
15
20
25
Ultrasonic Velocity (m/s)
1570
4.1. Preparation of sample solutions
1560
89
1560 * — — pure juice ο — — pulse—echo + — — aqeous sugar solution
1550 1540 1530 1520 1510 1500 1490 2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
Viscosity (mPa.s)
Fig. 8. A comparison of PE ultrasonic velocity as function of viscosity in orange juice and prepared aqueous solutions.
the compressibility j and hence increase the ultrasonic velocity v, as indicated with Eqs. (1) and (2). The solid suspensions in juice samples will introduce scattering effects that incur severe power attenuation in ultrasonic propagation. Hence, the velocity of ultrasound is a more robust and reliable measurement than the coefficient of power attenuation. This behaviour is implied in Eq. (1), where the formula accounts for propagation velocity depending only on the occurrences of reflective (PE scheme) or arriving (TT scheme) waveforms. The velocity is only a function of adiabatic compressibility and density. These two thermodynamic properties of fluid are not affected by any attenuation-related variables. This explains why the ultrasonic velocity is more robust and reliable than the attenuation coefficient. The parallel contours in Fig. 7 denote that the velocity of ultrasound is an effective indicator in identification of the sugar content in a fluid. Though a solution with Brix above 20 is considered impractical, the result alludes to the possibility that a solution can be blended to any specific Brix by controlling sugar additives through the responsive velocity of ultrasound. This satisfies a parsimonious demand for production of reconstituted juice, whereby concentrated juice is diluted with water to arrive at a specific sweetness. As shown in Fig. 8, in comparison with Fig. 6, the ultrasonic velocity tends to have an exponential response to the viscosity. There is a nearly linear relationship for viscosity below 2.7 MPa s, i.e. a sugar content less than 20 Brix. The orange juice is considered too sticky and sweet to drink for a viscosity beyond 2.7 MPa s or a Brix index bigger than 20. When the sugar content in solution is increased, the viscosity increases proportionally in two different rates and is exponential. This behaviour comes from the fact that the viscosity of fluid depends on the shear force (Greenwood et al., 2006). 5. Conclusions
30
Brix
Fig. 7. A comparison of PE ultrasonic velocity as function of Brix in orange juice and prepared aqueous sugar solutions.
An ultrasonic measurement system has been developed for determination of sugar content and viscosity in filtered orange juice. The ultrasonic variable most suitable for the determination is the propagation velocity. It shows a good
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linear correlation with sugar contents in solution denoted by Brix. The performance is independent of either pulse-echo or through-transmission ultrasonics. Ultrasonics may be used to measure the viscosity of solution through suitable mathematical calibration. The use of a power attenuation coefficient is discarded in this study as its relationships with Brix and viscosity are not linear but have big dispersions (results not shown). This study is valuable if we can predict the amount of sugar and additives required to reconstitute juice to a designated sugar content and viscosity. The experimental results, as illustrated in Figs. 7 and 8, provide such prediction models for an automatic control system. The measurement system was operated in a static manner and the sample solution was confined in an isolated chamber. For the purpose of industrial applications, the measurement should be capable of dealing with flowing fluids and other juices should be investigated. This problem is challenging as flowing fluids will introduce scattering effects on the impinging ultrasound. Dynamic measuring would allow us to build an automated inspection system that will lead to an automatic juice quality control system in juice production. This is the sequel now being studied. References Boulton, R. B., Singleton, V. L., Bisson, L. F., & Kunkee, R. E. (1996). Principles and practices of winemaking. New York: Chapman & Hall. Cartwright, D. (1998). Off-the-shelf ultrasound instrumentation for the food industry. In M. J. W. Povey & T. J. Mason (Eds.), Ultrasound in food processing. London: Blackie, chap. 2. Contreras Montes de Oca, N. I., Fairley, P., McClements, D. J., & Povey, M. J. W. (1992). Analysis of the sugar content of fruit juices and drinks using ultrasonic velocity measurements. International Journal of Food Science and Technology, 27, 515–529. Ewing, M. B. (1993). Thermophysical properties of fluids from acoustic measurements. Pure and Applied Chemistry, 65, 907–912.
Gan, T. H., Pallav, P., & Hutchins, D. A. (2006). Non-contact ultrasonic quality measurements of food products. Journal of Food Engineering, 77, 239–247. Greenwood, M. S., Adamson, J. D., & Bond, L. J. (2006). Measurement of the viscosity-density product using multiple reflections of ultrasonic shear horizontal waves. Ultrasonics, 44(Suppl.), e1031–e1036. Harker, F. R., Marsh, K. B., Young, H., Murray, S. H., Gunson, F., & Walker, S. (2002). Sensory interpretation of instrumental measurements 2: sweet and acid taste of apple fruit. Postharvest Biology and Technology, 24(3), 241–250. Hueter, T. F., Morgan, H., & Cohen, M. S. (1953). Ultrasonic attenuation in biological suspensions. Journal of Acoustic Society of America, 25(6), 1200–1201. Knorr, D., Zenker, M., Heinz, V., & Lee, D. (2004). Applications and potential of ultrasonics in food processing. Trends in Food Science and Technology, 15, 261–266. Lynnworth, L. C. (1989). Ultrasonic measurements for process control techniques: Applications. New York: Academic Press. McClements, D. J. (1995). Advances in the application of ultrasound in flood analysis and processing. Trends in Food Science and Technology, 6, 293–299. No¨lting, B. (1995). Relation between adiabatic and pseudoadiabatic compressibility in ultrasonic velocimetry. Journal of Theoretical Biology, 175, 191–196. Nyborg, W. L. (2002). Safety of medical diagnostic ultrasound. Seminars in Ultrasound, CT, and MRI, 23(5), 377–386. Povey, M. J. W. (1997). Ultrasonic techniques for fluids characterization. London: Academic Press. Pryor, A. W., & Roscoe, R. (1954). The velocity and absorption of sound in aqueous sugar solutions. Proceedings of the Physical Society. Section B, 67(1), 70–81. Rodriguez-Saona, L. E., Fry, F. S., McLaughlin, M. A., & Calveyb, E. M. (2001). Rapid analysis of sugars in fruit juices by FT-NIR spectroscopy. Carbohydrate Research, 336, 6374. Sarvazyan, A. P. (1991). Ultrasonic velocimetry of biological compounds. Annual Review of Biophysics and Biophysical Chemistry, 20, 321–342. USDA. (1995). United States standards for grades of concentrated tangerine juice for manufacturing (Tech. Rep. No. 20 FR 7281). Washington DC, USA: US Department of Agriculture. Valero, M., Recrosio, N., Saura, D., Mun˜oz, N., Marti, N., & Lizama, V. (2007). Effects of ultrasonic treatments in orange juice processing. Journal of Food Engineering, 80, 509–516. Wood, A. B. (1964). A textbook of sound (3rd ed.). London: Bell.
Journal of Food Engineering 86 (2008) 84–90 www.elsevier.com/locate/jfoodeng
Evaluation of ultrasonic propagation to measure sugar content and viscosity of reconstituted orange juice Feng-Jui Kuo a, Chung-Teh Sheng b, Ching-Hua Ting a,* a
Department of Biomechatronic Engineering, National Chiayi University, 300 University Road, Chiayi 600, Taiwan b Department of Bio-Industrial Mechatronics Engineering, National Chung Hsing University, Taichung, Taiwan Received 1 December 2006; received in revised form 13 July 2007; accepted 12 September 2007 Available online 22 September 2007
Abstract This paper describes the application of a non-contact ultrasonic system to measure the sugar content and viscosity of reconstituted orange juice. The system, which operates in either pulse-echo (PE) or transmission-through (TT) mode, detects the above two factors using responsive velocity of ultrasound and power attenuation. Experimental results of both modes show that the power attenuation is ineffective for such a purpose, and that the velocity of ultrasound tends to respond linearly to sugar content and exponentially to viscosity. The velocity exhibits a high linear correlation with Brix in orange juice, r = 0.994 in PE mode. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Orange juice; Sugar content; Viscosity; Ultrasonic velocity; Power attenuation
1. Introduction The juice production industry plays an important role in the harvest season as the availability of fresh fruits is usually in excess of demand. Fresh juice may be stored as a concentrated solution and later delivered to the market in a reconstituted form. Reconstituted juices may be blended with additives, diluted with water, or have added sugar to arrive at a specific, consumer-favoured flavour. The sweetness and viscosity are two controllable physical factors in reconstituting. The sweetness is a taste sensation that is difficult to measure scientifically; therefore, the sugar content is measured as an alternative (Harker et al., 2002). Practical methods currently used for measuring the sugar contents in a fluid include Brix gauge probing and chemical analysis. Brix gauge probing is traditional, accurate and effective, but can only be operated manually (Lynnworth, 1989) which is unacceptable in automated food processing. To measure *
Corresponding author. Tel.: +886 5 2717642; fax: +886 5 2717647. E-mail address: [email protected] (C.H. Ting).
0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.09.016
the viscosity, a manual spindle viscometer that measures the shear force of the fluid may be used. Hygiene and efficiency are an essential requirement in automated food processing. The best way to avoid contamination in quality measurement of food processing is to use non-contact detectors. Low-power ultrasound and infrared which are emitted from equipment isolated from the target material are a well-studied candidate in food processing, as they do not introduce extraneous contamination nor alter the properties of the substance (Cartwright, 1998; Gan, Pallav, & Hutchins, 2006; Rodriguez-Saona, Fry, McLaughlin, & Calveyb, 2001). Ultrasonics was first studied in the early 1920 and nowadays it is successfully and safely used in daily clinic diagnoses (Nyborg, 2002). It is cheap and has been used by the authors for various inspection of food products. Thus, this paper extends the applications of the ultrasonic techniques to measure the sugar content and viscosity in orange juice. It has been well understood that the propagation of ultrasound in a fluid is affected by the density, compressibility, temperature and composition of the fluid (Harker et al., 2002). Thus, ultrasonics has been utilised for more
F.J. Kuo et al. / Journal of Food Engineering 86 (2008) 84–90
85
List of symbols r
correlation coefficient between two measurements v velocity of ultrasound propagation j adiabatic compressibility q, qs, qw, qc specific gravities (densities) of fluid, sugar, water, and aqueous sugar solution Ws, Ww weights of sugar and water Sc concentration of aqueous sugar solution Zm near field distance at expected acoustic frequency and propagating velocity
than half a century as an effective tool in measurements of suspensions in milk (Hueter, Morgan, & Cohen, 1953) and density of fluid (Pryor & Roscoe, 1954). The Brix index of fluid is directly linked with the concentration of the dissolved sugar (Boulton, Singleton, Bisson, & Kunkee, 1996; McClements, 1995). Hence, we may expect a correlation between the Brix index of fluid and the behaviour of the ultrasound travelling in the fluid. A similar correlation can also be expected on viscosity since a fluid with more sugar becomes more sticky and hence has a higher viscosity (Greenwood, Adamson, & Bond, 2006). Both pulse-echo (PE) and transmission-through (TT) ultrasonic techniques are employed in this study to investigate how the sugar content in orange juice affects the nature of ultrasound propagation. The investigation, carried out by the authors, was initiated on aqueous solutions with various concentrations of sugar. The responsive velocity and power attenuation of ultrasound propagation were analysed before adopting either as the instrumental indicator. The preliminary investigation on aqueous solutions provides a macroscopic point of view of ultrasonic velocity in response to various sugar concentrations. Based on the preliminary results, the approach was extended to screened orange juice blended with various amounts of sugar solutions. 2. The ultrasonic measurement system 2.1. Ultrasound for measuring sugar content Accurate density measurement combined with ultrasonic velocity measurement is a widely used and indispensable method for determining the adiabatic compressibility of fluids (No¨lting, 1995) and a wide range of thermodynamic parameters (Ewing, 1993). The velocity of sound in aqueous sugar solution appears to correlate with the temperature, sugar concentration, and solution density (McClements, 1995). Different sugar compositions exhibit different characteristics in acoustic velocity. An inter-composition investigation on the composition of sugar types – glucose, fructose, and sucrose – has been well established (Contreras Montes de Oca, Fairley, McClements, & Povey,
R f x ta, tb, td
radius of ultrasound probe frequency of ultrasound propagation travelling distance of an ultrasonic waveform occurrences of waveforms ‘‘a”, ‘b”, and ‘‘d” (relative to Va, Vb, Vd) a power attenuation of ultrasound Va, Vb, Vd amplitudes of waveforms ‘‘a”, ‘b”, and ‘‘d”
1992). However, this study treats the sugar used as a mixture. The velocity of sound and the density of solution is linked with the so-called Wood equation (Wood, 1964): 1 v ¼ pffiffiffiffiffiffi jq
ð1Þ
where j is the adiabatic compressibility and q is the density of solution. This equation indicates that a denser fluid (with a bigger q) or a more compressible fluid (with a bigger j) reduces the velocity of sound propagating in the fluid. However, the propagation velocity of sound travelling in a fluid is a competing factor between density q and compressibility j. The relationship between the two variables is described with the concept of ‘‘concentration increments”, denoted as (Sarvazyan, 1991): ½j ¼ 2½v ½q
ð2Þ
The notation in bracket denotes the specific concentration of the enclosed variable. In organic solution the compressibility (j) decreases to a greater degree if its density (q) increases (Sarvazyan, 1991). The combined effect of a considerably decreased compressibility subject to an increased density will increase the velocity of ultrasound (v) in solution. This fact of physics elucidates the use of ultrasonic velocity in determining the density and compressibility of a solution (McClements, 1995). Ultrasonics can hence be used as an indirect measurement of the sugar content in solutions. The sugar in solution is assumed to be completely soluble and the resultant density qc is: qc ¼
WwþWs 1 W w q1 w þ W s qs
ð3Þ
where Ww is the weight of water, Ws the weight of sugar, qw the specific gravity of water, and qs the specific gravity of sugar. The sugar content of a solution provides an alternative, indirect measurement of sweetness sensation supposing the solution is free of artificial sweetener (Harker et al., 2002). Additional sugar dissolved in a solution alters the sensation of sweetness. The sugar content (Sc) in water (Ww) is a fraction of the dissolved sugar (Ws):
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Sc ¼
F.J. Kuo et al. / Journal of Food Engineering 86 (2008) 84–90
Ws WwþWs
ð4Þ
The food industry uses ‘‘Brix” to measure Sc. For fruit juices: one degree Brix is about 1–2% sugar by weight. This usually correlates well with perceived sweetness. Eq. (3) provides a correlation between the Brix of Eq. (4) and ultrasonic velocity of Eq. (2). The velocity of ultrasound could be therefore applicable in measurement of the sugar content in fluids. 2.2. The apparatus Figs. 1a and 2a show the two schemes of ultrasonic measurement systems developed in our laboratory. The sample chamber accommodates 400 ml of fluid being detected and is immersed in water kept at a constant temperature to mitigate temperature effects on acoustic propagation. The ultrasonic pulser and receiver (1 MHz, Western NDE, Canada) are mounted onto the sample chamber. The transducer (£20 mm) was excited to emit ultrasound at a maximum power of 800 mW/m2 and a duty duration of 60 ls. This low-power acoustic stimulation in a very short period is unlikely to introduce effective heat or sonochemical effects onto the juice, as normally, only ultrasound with a power intensity much bigger than 1 kW/m2 is used in heating up or altering the micro-organisms of a bio-material (Knorr, Zenker, Heinz, & Lee, 2004; Valero et al., 2007). A computer-controlled ultrasonic system (WT-UT-001A, Western NDE, Canada) signals the transducer to emit ultrasound into the fluid and the received ultrasonic waves
Fig. 2. The TT ultrasonic measurement technique.
are amplified at a gain of 60 dB. Ultrasonic responses and inferred data are displayed on-line for human intervention and recorded for subsequent off-line analysis. The transducer was calibrated using reverse osmosis (RO) water as the reference solution. The sample chambers are designed with a dimension within the ultrasonic near field. The near (Fresnel) field is the ideal region for measurement of highest accuracy because of a maximum beam power. Ultrasonic waves diverge while propagating beyond the designated near-field distance. The near field, Zm, is defined as the distance within: R2 f ð5Þ v where R is the radius of the probe, f the ultrasonic frequency, and v the velocity of ultrasound. In this study, the expected ultrasonic velocity ranges from 1.465 103 m/s in water to 1.714 103 m/s in dense aqueous sugar solution (Contreras Montes de Oca et al., 1992). Thus, the maximum ultrasound near-field distance should be less than 58 mm. To assure less resistant transmission, the inner space of the sample chamber is chosen to be 33 mm.
Zm ¼
Fig. 1. The PE ultrasonic measurement technique.
2.2.1. Pulse-echo measurement The pulse-echo system, as shown in Fig. 1b, emits an epoch of ultrasound (wave ‘‘a”) through the solution until the acoustic waves impinge on the steel wall of the container. Part of the acoustic energy is reflected (wave ‘‘b”) depending on the size of the interface and the relative impedance difference (Lynnworth, 1989). The reflection
F.J. Kuo et al. / Journal of Food Engineering 86 (2008) 84–90
process, at the same frequency, continues until the ultrasound vanishes in the fluid. The velocity of sound in a medium depends only on the property of the medium (McClements, 1995; Wood, 1964). Hence, all the above waves travel at the same speed regardless of respective power intensities. The two reflection waveforms, waves b and d in Fig. 1b, acquired by the probe, show conceptually that the magnitudes are attenuated after reflection. The attenuation is understandable since part of the ultrasonic energy is scattered in fluid (scattering effect) and also not all ultrasonic waves are reflected by the steel wall (adsorption effect). The velocity of ultrasound is measured by counting the time elapsed between two consecutive peak amplitudes, as
3. Preliminary experiments on aqueous sugar solutions Preliminary experiments were conducted using the PE scheme. The results provide a reference for further experimental design for studies on orange juices.
ð6Þ
This equation states that the propagation route x is a significant factor in waveform attenuation. A bigger coefficient denotes a more attenuated propagation. 2.2.2. Through-transmission measurement The through-transmission (TT) scheme of Fig. 2 has an ultrasonic transmitter that emits ultrasound through the probed fluid, and a receiver opposite that collects the transmitted ultrasound. The ultrasound that travels through the fluid is hindered depending on the properties of the fluid, i.e. mainly the adiabatic compressibility and the density. The TT technique has the transducer arrangement as shown in Fig. 2b. Ultrasonic waves are emitted from the transmitter and then propagate through the fluid to the receiver. The velocity of ultrasound is reckoned by counting the time elapsed between the emission and the receiving events, as x v¼ ð8Þ tb ta where ta denotes the occurrence of emission and tb the occurrence of receiving. And the coefficient of power attenuation, a, is defined as 1 Va a ¼ log10 ð9Þ x Vb where Va is the perceived magnitude of the emission and Vb is the magnitude of the received waveform.
3.1. Preparation of aqueous sugar solutions Different quantities of white refined cane sugar were dissolved in RO water to obtain 25 sample solutions with concentrations ranging from 2% to 50% at incremental steps of 2%. The sample chamber of Fig. 1a was filled with sample solution and then covered with a lid to avoid environmental influence on acoustic transmission within the chamber. The ultrasonic velocity, attenuation, Brix, and viscosity of each sample solution were measured and recorded. Fig. 3 records Brix and density measurements as functions of sugar contents Sc of Eq. (4). The Brix index is an alternative measurement of solution density qc of Eq. (3). The relation between Sc and qc can hence be considered as an ideally linear function. This correlation allows one to change the Brix of a solution by regulating the amount of sugar additive. Also, the correlation with qc allows one to investigate the sugar content using the Wood equation of Eq. (1) by means of the velocity of ultrasound. 3.2. Ultrasonic velocity and Brix As developed in Section 2.1, Eqs. (1) and (2) state that an increased ultrasonic velocity in response to an increased density can be expected. Rewriting Eq. (2) in the form:
60 50 Brix: y =1.8427 + 0.99815x r = 0.999
40 30 20
1200
1100 Density: z =986.8124 + 4.5316x r = 0.999
10 0
2.3. Measurement of physical characteristics
1300 3
where td and tb denote the occurrence times of peaks Vd and Vb and x = 33 mm is the inner space of the sample chamber. The power attenuation, a, is defined as (dB/m): 1 Vb a ¼ log10 ð7Þ 2x Vd
Solution Density (kg/m )
2x td tb
ultrasonic propagation. The sugar content was quantitatively measured using a Brix meter (N-50E, Atago, Tokyo, Japan) with a reference temperature of 20 °C. The actual Brix indices were calibrated to fit the ambient (bath water) temperature at 30 °C. The viscosity of solution was measured using a spindle viscometer (LVDV-II+, Brookfield, MA, USA). A complete measurement was carried out within 1 min to avoid possible degrading effects on the juice.
Brix
v¼
87
0
10
20
30
40
1000 50
Sugar Concentration (%)
The bath water was kept at 30 ± 0.5 °C to alleviate possible temperature influence on fluid properties and also
Fig. 3. The Brix and density of an aqueous sugar solution is linearly proportional to the amount of added sugar.
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F.J. Kuo et al. / Journal of Food Engineering 86 (2008) 84–90
½j ½q 2
A positive [q] is associated with a more negative [j] thus giving a positive [v], i.e. the velocity increases. Experimental results in Fig. 4 show there is a positive linear correlation between ultrasonic velocity and Brix. The graphic representation does not show a perfect correlation between the two variables. However, the modelling discrepancy is considered trivial, as it has a standard deviation of 68 m/s which is far smaller than the average velocity of 1550 m/s. This preliminary result on aqueous sugar solutions demonstrates the possibility of ultrasonic velocity as an effective indicator of Brix index and density in fluid. 3.3. Ultrasonic velocity and viscosity The viscosity of a solution is a function of the sugar content in the solution (Harker et al., 2002). Fig. 5 shows a non-linear correlation existing between viscosity and Brix. The viscosity is linearly proportional to Brix at values less than 35 and is exponential beyond 35. This behaviour shows that a solution containing more sugar is much more sticky than a light solution. The relationship between the ultrasonic velocity and the viscosity is nearly exponential, as shown in Fig. 6, though there are insufficient samples
Ultrasonic Velocity (m/s)
1700 y =1434.328 + 5.0741x r = 0.983
1600 1550 1500 1450
0
10
20
30
40
50
60
Fig. 4. A linear correlation between PE ultrasonic velocity and Brix in prepared aqueous sugar solutions.
12 10
Viscosity (mPa.s)
1650 1600 1550 1500 1450
2
3
4
5
6
7
8
9
10
11
Viscosity (mPa.s)
Fig. 6. PE ultrasonic velocity as a function of viscosity in prepared aqueous sugar solution.
to reveal this trend properly. There exists a nearly linear correlation for viscosity less than 3 MPa s. The ultrasonic velocity almost saturates for viscosity values beyond 3 MPa s. This is understandable as a sticky solution exhibits a larger shear resistance to the employed rotary viscometer in fluid mechanics. A solution with a viscosity value of 3 MPa s is equivalent to 30 Brix, as illustrated in Fig. 5, which is considered too sweet to be acceptable in the juice industry. 3.4. Ultrasound attenuation and sugar content
3.5. Summary from preliminary experiments Brix
8 6 4 2 0
1700
Attenuation in ultrasound is a complicated function of wave propagation in a fluid. Factors that may affect ultrasound attenuation include the viscosity, compressibility, wall material, and scattering and adsorption effects (Povey, 1997). No meaningful correlation was found between the attenuation coefficient and Brix and viscosity in the experimental results. Thus, ultrasound attenuation is presented in subsequent sections.
1750
1650
1750
ð10Þ Ultrasonic Velocity (m/s)
½v ¼
10
20
30
40
50
60
Brix
Fig. 5. Exponential correlation between viscosity and Brix in prepared aqueous sugar solutions.
The ultrasonic velocity in solution is an effective, nondestructive indicator of Brix and viscosity, as shown in Figs. 4 and 6. In theory, ultrasound propagating in a fluid is attenuated and hence distinguishable attenuation coefficients could be expected (McClements, 1995). However, the laboratory study revealed a non-applicable attenuation coefficient profile. This means that the velocity of ultrasound is a more reliable indicator for instrumentation in identification of the sugar content and viscosity in fluids. The characteristics of the inspected solution will not be altered by the ultrasound as low-power ultrasound was used. Though the preliminary study was carried out on water dissolved with various amounts of sugar, the result may be used as a guide for studies relating to juice production. Encouraged by the good correlation between ultrasonic indices and physical properties, the experimental study was extended to detect the Brix index and viscosity of orange juice blended with sugar solutions of various concentrations.
F.J. Kuo et al. / Journal of Food Engineering 86 (2008) 84–90
4. Application to orange juice
A juice sample consists of 300 ml of fresh orange juice and 600 g of sugar solution. Orange juice was obtained from fresh oranges with flesh suspensions being screened out using a stainless steel mesh filter with an aperture size of 0.061 mm2. The screening mitigates possible scattering effects and alleviates acoustic impedance because of the solid suspensions. Sugar solutions were prepared in concentrations ranging from 0% to 26% at incremental steps of 2%. This studied Brix range is large enough since reconstituted juices normally have a Brix index between 10.6 and 11.6 (USDA, 1995). The sample chambers of Figs. 1a and 2a were filled with the blend and were covered tightly with lids. The laboratory work was the same as that used in the preliminary study on aqueous sugar solutions. 4.2. Results and discussion Experimental results from the TT scheme behave almost identically to these from the PE scheme and hence only PE results are presented. Fig. 7 shows a comparison between juice blend and aqueous solution measured using the PE scheme. The ultrasonic responses shows a good linear correlation between the ultrasonic velocity and the Brix index in orange juice. This is quite promising since the Brix indices of both aqueous solution and orange juice can be effectively indicated by the velocity of ultrasound. The aqueous sugar solution expresses a slower ultrasonic velocity than orange juice probed using either the pulse-echo or the throughtransmission scheme. It is worth noting that the orange juice contains many more chemical compositions than the aqueous sugar solution. This, in turn, contributes to the density of the solution and hence the responsive ultrasonic velocity. The compositions in the solution are a significant factor that alters the behaviour of ultrasound in the solution (Contreras Montes de Oca et al., 1992). They diminish
Ultrasonic Velocity (m/s)
1580 * — — pure juice ο — — pulse—echo y =1465.8993 + 3.9261x r = 0.994
1540 1520
+ aqueous sugar solution y =1465.6246 + 3.1886x r = 0.997
1500 1480
5
10
15
20
25
Ultrasonic Velocity (m/s)
1570
4.1. Preparation of sample solutions
1560
89
1560 * — — pure juice ο — — pulse—echo + — — aqeous sugar solution
1550 1540 1530 1520 1510 1500 1490 2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
Viscosity (mPa.s)
Fig. 8. A comparison of PE ultrasonic velocity as function of viscosity in orange juice and prepared aqueous solutions.
the compressibility j and hence increase the ultrasonic velocity v, as indicated with Eqs. (1) and (2). The solid suspensions in juice samples will introduce scattering effects that incur severe power attenuation in ultrasonic propagation. Hence, the velocity of ultrasound is a more robust and reliable measurement than the coefficient of power attenuation. This behaviour is implied in Eq. (1), where the formula accounts for propagation velocity depending only on the occurrences of reflective (PE scheme) or arriving (TT scheme) waveforms. The velocity is only a function of adiabatic compressibility and density. These two thermodynamic properties of fluid are not affected by any attenuation-related variables. This explains why the ultrasonic velocity is more robust and reliable than the attenuation coefficient. The parallel contours in Fig. 7 denote that the velocity of ultrasound is an effective indicator in identification of the sugar content in a fluid. Though a solution with Brix above 20 is considered impractical, the result alludes to the possibility that a solution can be blended to any specific Brix by controlling sugar additives through the responsive velocity of ultrasound. This satisfies a parsimonious demand for production of reconstituted juice, whereby concentrated juice is diluted with water to arrive at a specific sweetness. As shown in Fig. 8, in comparison with Fig. 6, the ultrasonic velocity tends to have an exponential response to the viscosity. There is a nearly linear relationship for viscosity below 2.7 MPa s, i.e. a sugar content less than 20 Brix. The orange juice is considered too sticky and sweet to drink for a viscosity beyond 2.7 MPa s or a Brix index bigger than 20. When the sugar content in solution is increased, the viscosity increases proportionally in two different rates and is exponential. This behaviour comes from the fact that the viscosity of fluid depends on the shear force (Greenwood et al., 2006). 5. Conclusions
30
Brix
Fig. 7. A comparison of PE ultrasonic velocity as function of Brix in orange juice and prepared aqueous sugar solutions.
An ultrasonic measurement system has been developed for determination of sugar content and viscosity in filtered orange juice. The ultrasonic variable most suitable for the determination is the propagation velocity. It shows a good
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linear correlation with sugar contents in solution denoted by Brix. The performance is independent of either pulse-echo or through-transmission ultrasonics. Ultrasonics may be used to measure the viscosity of solution through suitable mathematical calibration. The use of a power attenuation coefficient is discarded in this study as its relationships with Brix and viscosity are not linear but have big dispersions (results not shown). This study is valuable if we can predict the amount of sugar and additives required to reconstitute juice to a designated sugar content and viscosity. The experimental results, as illustrated in Figs. 7 and 8, provide such prediction models for an automatic control system. The measurement system was operated in a static manner and the sample solution was confined in an isolated chamber. For the purpose of industrial applications, the measurement should be capable of dealing with flowing fluids and other juices should be investigated. This problem is challenging as flowing fluids will introduce scattering effects on the impinging ultrasound. Dynamic measuring would allow us to build an automated inspection system that will lead to an automatic juice quality control system in juice production. This is the sequel now being studied. References Boulton, R. B., Singleton, V. L., Bisson, L. F., & Kunkee, R. E. (1996). Principles and practices of winemaking. New York: Chapman & Hall. Cartwright, D. (1998). Off-the-shelf ultrasound instrumentation for the food industry. In M. J. W. Povey & T. J. Mason (Eds.), Ultrasound in food processing. London: Blackie, chap. 2. Contreras Montes de Oca, N. I., Fairley, P., McClements, D. J., & Povey, M. J. W. (1992). Analysis of the sugar content of fruit juices and drinks using ultrasonic velocity measurements. International Journal of Food Science and Technology, 27, 515–529. Ewing, M. B. (1993). Thermophysical properties of fluids from acoustic measurements. Pure and Applied Chemistry, 65, 907–912.
Gan, T. H., Pallav, P., & Hutchins, D. A. (2006). Non-contact ultrasonic quality measurements of food products. Journal of Food Engineering, 77, 239–247. Greenwood, M. S., Adamson, J. D., & Bond, L. J. (2006). Measurement of the viscosity-density product using multiple reflections of ultrasonic shear horizontal waves. Ultrasonics, 44(Suppl.), e1031–e1036. Harker, F. R., Marsh, K. B., Young, H., Murray, S. H., Gunson, F., & Walker, S. (2002). Sensory interpretation of instrumental measurements 2: sweet and acid taste of apple fruit. Postharvest Biology and Technology, 24(3), 241–250. Hueter, T. F., Morgan, H., & Cohen, M. S. (1953). Ultrasonic attenuation in biological suspensions. Journal of Acoustic Society of America, 25(6), 1200–1201. Knorr, D., Zenker, M., Heinz, V., & Lee, D. (2004). Applications and potential of ultrasonics in food processing. Trends in Food Science and Technology, 15, 261–266. Lynnworth, L. C. (1989). Ultrasonic measurements for process control techniques: Applications. New York: Academic Press. McClements, D. J. (1995). Advances in the application of ultrasound in flood analysis and processing. Trends in Food Science and Technology, 6, 293–299. No¨lting, B. (1995). Relation between adiabatic and pseudoadiabatic compressibility in ultrasonic velocimetry. Journal of Theoretical Biology, 175, 191–196. Nyborg, W. L. (2002). Safety of medical diagnostic ultrasound. Seminars in Ultrasound, CT, and MRI, 23(5), 377–386. Povey, M. J. W. (1997). Ultrasonic techniques for fluids characterization. London: Academic Press. Pryor, A. W., & Roscoe, R. (1954). The velocity and absorption of sound in aqueous sugar solutions. Proceedings of the Physical Society. Section B, 67(1), 70–81. Rodriguez-Saona, L. E., Fry, F. S., McLaughlin, M. A., & Calveyb, E. M. (2001). Rapid analysis of sugars in fruit juices by FT-NIR spectroscopy. Carbohydrate Research, 336, 6374. Sarvazyan, A. P. (1991). Ultrasonic velocimetry of biological compounds. Annual Review of Biophysics and Biophysical Chemistry, 20, 321–342. USDA. (1995). United States standards for grades of concentrated tangerine juice for manufacturing (Tech. Rep. No. 20 FR 7281). Washington DC, USA: US Department of Agriculture. Valero, M., Recrosio, N., Saura, D., Mun˜oz, N., Marti, N., & Lizama, V. (2007). Effects of ultrasonic treatments in orange juice processing. Journal of Food Engineering, 80, 509–516. Wood, A. B. (1964). A textbook of sound (3rd ed.). London: Bell.