Journal of Food Engineering 126 (2014) 62–64

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Journal of Food Engineering

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Research Note

Ultrasonic evaluation of coconut water shear viscosity

0260-8774/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jfoodeng.2013.11.007

⇑ Corresponding author. Tel.: +33 (4) 67 14 34 30; fax: + 33 (4) 67 52 15 84.E-mail address: didier.laux@univ-montp2.fr (D. Laux).

Didier Laux a,⇑, Olivier Gibert b, Jean-Yves Ferrandis a, Marc Valente b, Alexia Prades b

a Institut d’Electronique du Sud, UMR 5214, Université Montpellier II/CNRS, Place E. Bataillon CC 082, 34095 Montpellier, Franceb CIRAD, UMR Qualisud, TA B-95/16, 73 Rue JF Breton, 34398 Montpellier, France

a r t i c l e i n f o

Article history:Received 11 July 2013Received in revised form 20 September 2013Accepted 12 November 2013Available online 19 November 2013

Keywords:Coconut waterRheologyViscosityUltrasonic wavesNewtonian fluids

a b s t r a c t

Longitudinal viscosity of coconut water with soluble solids content (SSC) between 7 and 60 �Brix has beenassessed using a high frequency ultrasonic method which consists in measuring the ultrasonic velocityand attenuation of longitudinal waves. These measurements have been linearly correlated with thoseobtained with flow tests performed with a rheometer. Thanks to their non destructive character, andto the fact that ultrasound can propagate through tubes and pipes, this ultrasonic approach and the linearrelation proposed are recommended for in line measurements during coconut water processing.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Recently the popularity of coconut water (CW) on the interna-tional market has been continually increasing. This refreshingliquid extracted from the coconut fruit has been described as a‘‘rehydrating beverage’’ and has drawn the attention of somemanufacturers as a natural functional drink (Prades et al., 2012).Consequently, an increasing number of investigations are carriedout to process and store this fragile resource. In order to reachthe large volume expected by the international markets, one ofthe key processes seems to be ‘‘coconut water concentration’’.However, producing high quality concentrated CW remains a chal-lenge. Obviously, the rheological behavior is a critical factor duringconcentration, where the viscosity trait, in relation to the chemicalcomposition and the sugar content, is very important. Viscosityassessment can be easily achieved with usual devices such asrheometers, rotational or capillary viscosimeters. However, suchmeasurements cannot be easily carried out in line during an indus-trial process. Moreover, performing such measurements on largebatches is extremely time-consuming. It is well-known that shearviscosity can also be assessed by shear ultrasonic waves usingultrasonic shear wave reflectometry. Such a method, successfullyapplied by several authors generally leads to poor accuracy andestimation at low viscosities (Kulmyrzaev and Mc. Clements,2000; Longin et al., 1998; Greenwood et al., 2006; Saggin andCoupland, 2004). Thus, it is usually not recommended to assesssolutions at low soluble solids content (SSC). Recently, it has been

shown that ultrasonic longitudinal waves can lead to reliable shearviscosity estimation for Newtonian mango juices at low sugarcontent (Laux et al., 2013). Even though such method is not new,its application on juices seems to be an innovation. Since coconutwater has a Newtonian behavior but can exhibit high sugar contentduring concentration process, our goal was to investigate therelevancy of this ultrasonic method being applied on coconutwater within a large sugar concentration range. Furthermore, thesepreliminary data could be of a great interest for the scientific com-munity related to coconut water processing, considering the lownumber of works reported on coconut water.

2. Materials and methods

2.1. Samples

Seven samples were prepared as follows. Coconut water wasextracted from 180 immature coconut fruits harvested on a GreenDwarf variety of Thailand (Cock Brand, Thailand). CW extractswere collected together and the juice was dispatched in 1 L glassbottles prior to immediate freezing at �50 �C and storage at�18 �C until processing. CW concentration was performed withboth thermal and non-thermal technologies. Thermal concentra-tion was performed using a vacuum concentrator (Auriol,Marmande, France) at 60 �C, 70 �C, and 80 �C. Non-thermal concen-tration was performed using an Ederna Lab Unit by osmoticdehydration (Ederna, Toulouse, France) at ambient temperature.Soluble solids content (SSC) was measured on native and concen-trated samples using a digital hand-held PAL refractometer 0–85�

Fig. 1. Ultrasonic velocity measured in coconut water versus SSC.

Fig. 2. Ultrasonic attenuation versus (2p2f2)/(qVL3) for two samples (SSC = 6.6 and

59.6 �Brix).

Fig. 3. Longitudinal viscosity versus shear viscosity in coconut water.

D. Laux et al. / Journal of Food Engineering 126 (2014) 62–64 63

Brix (ATAGO Corp., LTD, Japan). CW samples were concentrated 5to 10-fold, leading to viscosities from 1.28 ± 0.01 to 40.00± 0.06 mPa s.

2.2. Flow tests

Apparent shear viscosity measurements were carried out usinga Physica MCR301 Rheometer (Anton Paar France, Courtaboeuf,France) equipped with a Couette flow measuring cell (Ref. DG27/T2000/SS). Sample temperature was achieved at 20 ± 0.1 �C usinga Peltier system and a fluid circulator Viscotherm VT 2 controlleddirectly from the Physica MCR. After 5 min of thermal stabilization,each 11 mL sample was submitted to a flow test in the 10–400 s�1

shear rate range.

2.3. Ultrasonic method based on longitudinal waves

In order to measure the ultrasonic velocity and attenuation incoconut water, the method described in (Laux et al., 2013) wasapplied. The principle consists in initiating a zo distance betweenan ultrasonic plane transducer and the bottom of a tank containingthe coconut water and to gradually reduce zo with a high resolutionstepper motor. At each position, the ultrasonic echo reflected fromthe bottom of the container is acquired. As the travel is graduallyreducing, the time needed for propagation in the coconut wateris also reduced. In addition, the amplitude of the echo acquiredat each position rises during the experiment since the travel isreduced and the ultrasonic signal propagates within a smallertravel in the absorbent viscoelastic medium. From a practical pointof view, a broadband Panametrics VM 324-SM sensor excited witha Sofranel Panametrics 5800 pulse generator was used. Itsfrequency of resonance was 25 MHz with a bandwidth around10 MHz. All displacements were ensured with a Microcontrol�stepper motor with an accuracy of ±1 lm. The echoes weredisplayed on a LT374M Lecroy� oscilloscope. After Fast Fourier’stransform calculation (for each echo acquired for frequencies pres-ent in the sensor bandwidth), the FFT modulus amplitude versus zwas plot. This lead to the attenuation a of the ultrasonic signal cal-culation versus frequency. Then, the velocity was also evaluatedversus frequency using the FFT phase at each frequency. If f isthe frequency of the wave, a the attenuation in Neper or m�1, qthe density, VL the longitudinal velocity, it can be shown (Dukhinand Goetz, 2009) that the longitudinal viscosity is given by:

gL �2aLqV3

L

ð2pf Þ2ð1Þ

Thus, a versus (2p2f2)/(qV3L ) was plotted. The slope was then equal

to gL.

3. Results

As a natural fluid mainly composed of soluble sugars, minerals,few proteins and traces of lipids, fresh coconut water obviouslybehaves as a Newtonian fluid. All performed flow tests haveconfirmed this behavior for shear rates from 10 to 400 s�1.

Fig. 1 shows the evolution of ultrasonic velocity versus SSC forwater coconut. Fig. 2 illustrates a versus (2p2f2)/(qV3

L ) for two sam-ples with extreme SSC equal to 6.6� and 59.6 �Brix. The linearregressions were very good with R2 greater than 0.99. Such regres-sions have been made for all the samples and also led to R2 > 0.99.From these regressions we found gL values between 6.4 and199.3 mPa s. The relative accuracy on gL evaluation is 4%. InFig. 3, the relation of gL versus gs (measured with the rheometer)was illustrated. The linear correlation was optimal and gavegL = 4.98gs with R2 � 0.99. This proportionality between gL and gs

is not surprising. Indeed, for Newtonian fluids it is possible to writethe following relationship : gL � gb þ 4

3 gS, where gs is the ‘‘usual’’dynamic shear viscosity, gb is the bulk viscosity usually propor-tional to gs. So, gb = Kgs. The factor K is generally ranging between1 and 3. For instance for water, (K) is around 3 and is equal to 1 forglycerol. But it seems that this rule is not universal and that (K)could be even greater. More details concerning gL and gb can befound in (Dukhin and Goetz, 2009).

4. Discussion: possible adaptation on pipelines during process

Concerning a possible adaptation and use in line, the velocityand attenuation measurements will have to be performed in a

64 D. Laux et al. / Journal of Food Engineering 126 (2014) 62–64

different way. Indeed, using plane waves is not suitable regardingthe cylindrical geometry of the pipelines. The experiment could yetbe done with a transducer in contact with fluid in pipeline thanksto a small window designed in the pipeline wall. However, in thiscase, the non destructive advantage of using ultrasonic waves islost because one should have to make modifications on thepipeline. So, regarding classical dimensions of the pipelines (typi-cally 1 or 2 cm diameter and 1 mm thickness pipelines for CWconcentration in our labs) we propose to use two cylindrical trans-ducers working in transmission mode in direct contact with thetube. The matching between the transducers and the tube can beensured with water, gel or viscous material such as honey. Thethickness of this matching layer can be calculated to act as anadaptation layer in order to transmit the maximum of energy intothe pipeline. After propagation through the pipeline, multipleechoes will be generated in the fluid and again collected by the sec-ond transducer acting as receptor, positioned at the opposite side.

Regarding the small thickness of steel, high frequency ultra-sound will correctly propagate through the pipeline wall. But incase the signal amplitude is too small it will be suitable to reducethe frequency. Indeed, for our experiments with plane waves andplane transducers we have chosen 25 MHz waves in order tolimitate diffraction effects due to the finite size of the sensor. Fora cylindrical geometry and cylindrical transducers, it will be possi-ble to work at lower frequencies (5–10 MHz) because diffractioneffects are naturally reduced due to the focusing of the waves. Infact, this method, developed in our lab and patented with EDF(Electricity of France Company) is already in use for pressure andcomposition of gases assessment in Zircaloy tubes (cladding ofnuclear rods) used in nuclear power plants. More informationconcerning sensors matching to the tube and signal processing inorder to accurately measure the velocity and attenuation of cylin-drical waves inside the tube (double fast Fourier’s transform. . .)can be found in (Ferrandis et al., 2007; Rosenkrantz et al., 2009).

5. Conclusions

On the basis of the coconut water samples analyzed, the highfrequency chosen in order to limit the effects of the diffractiondue to the non infinite size of the sensor and the signal processing

with Fourier analysis, led to a very good correlation between thelongitudinal and the shear viscosities. Thus, it seems promisingto some extend to substitute the use of a rheometer by ultrasoundwhile investigating coconut water viscosity. These preliminaryresults will have to be supported by other measurements on alarger number of samples in order to confirm the first linear corre-lation reported in the present paper.

Acknowledgements

The authors would like to thank the Ederna company (Toulouse,France), and especially, Fabrice Gascons Viladomat for providing ussome of the concentrated samples. We also thank Hiba Ben Slamafor her technical support.

References

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