In situ electrical conductivity measurement of select liquid foods under hydrostatic pressure to 800 MPa

Journal of Food Engineering 82 (2007) 489–497 www.elsevier.com/locate/jfoodeng

In situ electrical conductivity measurement of select liquid foods under hydrostatic pressure to 800 MPa Stephen Min a, S.K. Sastry a,*, V.M. Balasubramaniam b a

Department of Food, Agricultural and Biological Engineering, The Ohio State University, 590 Woody Hayes Drive, Columbus, OH 43210-1057, United States b Department of Food Science and Technology, The Ohio State University, 2015 Fyffe Road, Columbus, OH 43210-1007, United States Received 4 January 2007; received in revised form 2 March 2007; accepted 5 March 2007 Available online 13 March 2007

Abstract Electrical conductivity of select liquid foods and salt solutions was measured in situ during high pressure processing using a specially designed parallel electrode conductivity cell. Cell constants at atmospheric pressure were determined with KCl standards and calculated against standard data, while cell constants under pressure were estimated assuming isotropic compression. Measured conductivities of NaCl solutions under pressure were within 5.7% of previously reported data at pressures up to 800 MPa and temperatures to 61 °C. Electrical conductivity of NaCl and KCl solutions, orange juice, apple juice, tomato juice, and soybean oil were measured in triplicate in 100 MPa increments from 0.1 to 800 MPa. 0.01 m salt solutions were measured at 25 and 50 °C; 0.1 m salt solutions, juice and oil samples were measured at 25 °C. Results show conductivity of salt solutions and juice samples increased as a function of pressure, peaking between 200 and 500 MPa and decreasing above 500 MPa. Except for soybean oil, pressure had a significant effect (p < 0.01) on electrical conductivity for all samples. Temperature had a significant effect (p < 0.01) on electrical conductivity of 0.01 m salt solutions at all pressures. Conductivity of soybean oil was too low to be measured at atmospheric and pressurized conditions. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: High pressure processing; Electrical conductivity; Juice; Salts; In situ measurement; Sensor

1. Introduction High pressure processing (HPP) has shown the ability to inactivate microorganisms, and produce foods that retain fresh-like nutritional and sensory profiles and extended shelf life (Matser, Krebbers, van den Berg, & Bartels, 2004). Potential applications include pasteurization, sterilization, texture modification, enzyme inactivation, dehydration pretreatment, blanching, freezing and thawing (Welti-Chanes et al., 2005). While a number of commercially available HPP products exist (Sizer, Balasubramaniam, & Ting, 2002), little information exists on properties of foods measured in situ. Understanding effects of pressure on thermodynamic and physical properties of foods *

Corresponding author. Tel.: +1 614 292 3508; fax: +1 614 292 9448. E-mail address: [email protected] (S.K. Sastry).

0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.03.003

has been identified as a key research need (Barbosa-Ca´novas & Rodrı´guez, 2005) and may assist in product and process development. The electrical conductivity (EC) of a material represents its ability to transport electric charge, and its measurement provides a direct measurement of ionic behavior in electrolyte solutions (Kissinger & Heineman, 1996). Electrical conductivity can be influenced by temperature (Palaniappan & Sastry, 1991), electrolyte concentration, chemical content (Rieger, 1994), viscosity (Hamann, Hamnett, & Vielstich, 1998), suspended solids (Palaniappan & Sastry, 1991), electrolytic strength (Hamann et al., 1998), and presence of cell structure (Wang, Kuo, Kuo-Huang, & Wu, 2001). Sensitivity to such variables makes EC an indicator of process-induced changes in food systems. Wang and Sastry (1997) reported using EC measurement to detect gelatinization in starch solutions in situ during ohmic heat-

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Nomenclature A

CSA f I L m n s u V

circular cross sectional area between electrodes, as determined by inner diameter of polycarbonate electrode holder tube (m2) cross sectional area (m2) frequency (Hz) current (A) distance between electrodes (m) molal (mol solute/kg solvent) number of observations standard deviation uncertainty voltage (V)

ing. Bauer and Knorr (2004) reported electrical conductivity of starch solutions, measured after processing, was linearly related to degree of pressure induced starch gelatinization. Angersbach, Heinz, and Knorr (2002) used impedance measurements after HPP to characterize pressure induced cell membrane damage to potatoes and beets. In situ electrical conductivity measurement of foods may allow indirect measurement of pressure induced physicochemical changes during processing. EC data could be used to help develop hurdle processes combining pulsed electric field processing or ohmic heating with high pressure. Currently, no in situ sensors have been reported for electrical conductivity measurement of food or biological materials during pressure processing. Electrical conductivity of chemicals exposed to high pressures has been measured for metals (Bridgman, 1921), salt solutions (Quist & Marshall, 1968), acids (Quist, Marshall, & Jolley, 1965), and organic compounds (Scaife, 1974). Unfortunately, previous cell designs for EC measurement under pressure are not suitable for food and biological samples, since electrical properties of biomaterials are field strength dependent (Cima & Mir, 2004). Electrode configurations in prior work did not minimize fringe currents and did not have mostly uniform electric field strength distributions, likely because sample conductivities, usually electrolytes, were independent of electric field distribution. The objectives of this work were to design and assemble a conductivity cell with nearly uniform electric field strength, to determine cell constants at atmospheric and pressurized conditions, and to measure electrical conductivity of salt, juice and oil samples during high pressure processing. 2. Materials and methods 2.1. Pressure generating system and electrical feed through A hydrostatic pressure test system (Harwood Engineering, 26190, Walpole, MA), rated to 1000 MPa and 150 °C, was used for this work. The pressure transfer medium was

V r X r

volume (m3) radius (m) mean of independent observations, Xk electrical conductivity (S/m)

Subscript atm atmospheric pressure i inner radius k observation k of n total observations p pressurized condition

50:50 propylene glycol (Houghton Chemical, Safe-TTherm, Allston, MA): distilled, deionized water (OSU Chemistry Store, 97801, Columbus, OH). Pressure was supplied by a diaphragm pump (SC Hydraulic, SC-676000W-100, Brea, CA), hydraulic vane pump (Denison Hydraulics, TMB-002-21R-1-00, Marysville, OH) and intensifier (Harwood Engineering, SA-10-6-.875FGD-150 K, Walpole, MA). The cylindrical, jacketed vessel interior dimensions were 25.4 mm diameter and 153 mm depth; the vessel top closure housed electrically insulated copper and Type K thermocouple wire feed throughs that allowed for electrical signal and temperature measurement in the vessel. The top closure housed a valve that allowed air removal in the vessel prior to pressurization. A temperature controlled propylene glycol bath (Busch Electronics LLC, Minnesota) recirculated through a jacket around the vessel and allowed for control of sample temperature. Pressurization rates were approximately 20 MPa/s. 2.2. Electrical conductivity cell An axial cross section of the cylindrical electrical conductivity cell is shown in Fig. 1. The top closure and pressure vessel are not shown to emphasize the cell and measurement components. Differentiating features of this EC cell from previous designs include: (1) use of an insulated electric field, important in minimizing fringe currents (Schiefelbein, Fried, Rhoads, & Sadoway, 1998) and allowing estimation of cell constant under pressure; (2) presence of a well defined, mostly uniform electric field, necessary for differentiating between pressure and electromagnetic induced changes to food and biological samples (Cima & Mir, 2004); (3) allowance of uniform Joule heating of the sample under pressure, to allow research on combined effects of pressure, temperature and electric field strength on conductivity; (4) adaptability of the cell to any appropriately sized hydrostatic pressure vessel with proper signal and thermocouple feed throughs. The design is based on two polycarbonate sample holders both positioned vertically in the pressure chamber. The

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Fig. 1. Cross section of electrical conductivity cell. (1) Outer polycarbonate sample holder; (2) outer sample holder plug; (3) signal wires; (4) thermocouple wires; (5) epoxy barrier; (6) O-ring seal; (7) outer sample holder piston; (8) screw tightened O-ring seal (9) piston o-ring; (10) inner polycarbonate sample holder; (11) electrodes; (12) O-ring seal; (13) UltemÒ electrode holder plug; (14) VitonÒ packing material; (15) bored nylon screw; (16) O-ring; (17) 0.8 mm hole; (18) epoxy fixture.

outer sample holder, a 19 mm diameter polycarbonate tube (US Plastics, 43104, Lima, OH), had a bored plug (GE, machined UltemÒ, Pittsfield, MA) at one end; the signal and Type K thermocouple wires were fed through the hole in the plug, and the hole was sealed with epoxy (Devcon, 1233, Glenview, IL) to prevent pressure medium from entering the sample holder. The bottom of the plug had a grooved ledge, which when fitted with an O-ring, sealed against the polycarbonate tube. The 20 gauge copper and 24 gauge Type K thermocouple wire exiting the plug were soldered to the high pressure feed through wires and electrically insulated with TeflonÒ shrink tube and insulating epoxy (Devcon, 1233, Glenview, IL). The other end of the outer polycarbonate sample holder housed a removable, free moving polycarbonate piston, which allowed pressure to transmit from pressure medium to sample. The piston was installed after sample loading and had a screw-tightened O-ring seal that allowed gas removal when positioned vertically and prevented mixing of pressure media and sample. A lubricated piston O-ring allowed the piston to move freely and prevented pressure medium from entering the sample holder. The inner sample holder, a 12.7 mm diameter polycarbonate tube, contained the electrodes. The platinum-plated titanium cylindrical electrodes were 12.6 mm diameter on the face. The back half of each electrode stepped down to 6.9 mm diameter and housed an O-ring that allowed the

491

back half of each electrode to fit securely in an UltemÒ plug. The 20 gauge electrode lead wires were soldered to the platinum electrode coating and run through the UltemÒ plug. To prevent current leakage from the rear of the plug, the electrodes were insulated on the back end with VitonÒ packing material, a bored nylon screw and epoxy. Each electrode/plug combination fit securely in the inner sample holder with a tight fitting O-ring. The inner sample holder had two 0.8 mm holes at the midpoint. One hole positioned a Type K TeflonÒ coated thermocouple between the electrodes; the other allowed pressure equilibration of the sample between the inner and outer sample holders. The electrode/plug closest to the top closure was adhered against the outer sample holder plug with an epoxy fixture, creating a rigid assembly. The distance between the electrodes was adjustable by using a different length of polycarbonate tube for the inner sample holder and ranged from 10 to 24 mm. A simulation of electric field distribution between the electrodes was performed in Matlab (Mathworks, R2006B, Natick, MA) using a 2D finite element solution to the Laplace equation in cylindrical coordinates. For a 24 mm electrode gap and 6 V applied potential, the simulation confirmed the field is mostly uniform, around 250 V/ m. Fringe currents and higher field strengths, only seen close to the electrode–wall intersection, occupy minimal sample volume; thus making this cell suitable for future studies on biological and cellular materials.

2.3. Conductance measurements and data acquisition Electrical conductivity was measured according to Rieger (1994) r¼

LI : AV

ð1Þ

A signal generator (Hewlett Packard, 3314A, Palo Alto, CA) provided a 1–6 V RMS sinusoidal potential to the electrode circuit; lower voltages were used for higher conductivity solutions to minimize heat generation. Voltage was measured directly across the electrode leads, and current was measured as voltage across a verified 1 X resistor (PRC, PVS-1, Largo, FL) in series with the electrodes. Pressure was measured with a strain cell (Harwood Engineering, ML-1431, Walpole, MA); temperature was measured with a type K thermocouple (ThermoElectric, NN2H-K, Chicago, IL). Voltage, current, pressure and temperature were recorded twice per second with a data logger (Agilent, 34970, 22 Bit Board Resolution, Palo Alto, CA) connected to a computer for collection and data analysis. All wires were insulated, twisted pair with a grounded foil sheath to minimize induced voltage and other electrical noise. Because measured resistance included the resistance of the sample between the electrodes plus the accompanying circuit, a correction was made to determine sample resis-

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tance, V/I. The resistance of the total circuit, with electrodes in contact, was measured with an LCZ meter (Hewlett Packard, 4276A, Palo Alto, CA) at 2 kHz to be 1.1 X at atmospheric pressure. The resistance of the circuit exposed to pressure inside the sample holder was measured at 0.04 X at atmospheric pressure. Bridgman (1970) reported effect of pressure on resistance of copper, platinum and titanium to be less than 1.6% up to 800 MPa; thus corrections for pressure effect on resistance of the circuit exposed to pressure were assumed negligible at less than 1 mX. To report resistance of samples between the electrodes, 1.1 X was subtracted from the total measured resistance of the circuit for all resistance calculations in the study, including the V/I term in Eq. (1). To understand how resistance was affected by frequency, the resistance of 0.1 and 0.01 molal KCl solutions were measured from 10 to 20,000 Hz and plotted against (frequency)0.5; and the plots extrapolated to infinite frequency. Similar testing was performed at 400 and 800 MPa and at temperatures of 25 °C and 50 °C for all samples as per operational procedures outlined in Section 2.5.

2.4. Cell constant 2.4.1. Cell constant at atmospheric pressure The cell constant, L/A (/m), was determined at atmospheric pressure by measuring resistance of 0.01 and 0.1 molal KCl (Fisher Chemical, Certified ACS, Pittsburgh, PA) at 2000 Hz at 25 and 40 °C. The cell constant was calculated using published values (Lide, 2006) for the conductivity of KCl solutions and the measured resistance. For samples with conductivities less than 0.7 S/m at 0.1 MPa and 25 °C, the electrode gap distance was adjusted to 10 mm; samples with a greater conductivity used a 24 mm gap distance. This yielded resistances between 100 and 1000 X. Temperature was controlled by immersing the sample in a controlled water bath (Neslab, 10310101, USA). Cell constant determination for both samples was performed with five replicates. 2.4.2. Cell constant under pressure The cell constant (L/A) under pressure was calculated from volumetric compression data (Warfield, 1967) of polycarbonate, assuming isotropic compression, according to:   L Lp ¼ ð2Þ A p Ap  1=3 Vp Lp ¼ Latm ð3Þ V atm Ap ¼ p  r2i;p

ð4Þ

The value of r2i;p ðmÞ was determined by assuming the polycarbonate tube wall cross sectional area changes according to:

 CSAp ¼

Vp V atm

2=3 CSAatm

ð5Þ

2.5. Operation for electrical conductivity determination under pressure For each run, the cell and sample holder were cleaned, rinsed with double distilled, deionized water and dried. The sample was held in a cool water bath at 15 ± 2 °C prior to loading. Sample loading without presence of air bubbles was ensured with a strict sample loading procedure and absence of air was visually verified for each run. External to the pressure vessel, the entire apparatus was positioned vertically, with the top closure on the lower end. The electrode closest to the top closure was pressed into the lower end of the inner sample holder. The sample was gently poured into the inner and outer sample holder. When the depth of the sample reached the top of the inner holding tube, the second electrode was gently wetted with sample and pressed into the inner holding tube. Visual observation ensured no air bubbles were present between the electrodes or on the electrode surface. The sample was filled to the top of the outer sample holder, and the outer sample holder piston was inserted without the screw tightened o-ring seal. Due to vertical positioning, any air in the large sample holder rose to the top and was expelled from the central hole on the piston by pressing in on the piston. When all air was removed, the screw-tightened oring seal was installed. A final visual observation ensured no visible air in the sample. Thus, no bubbles would be expected under pressure, following Henry’s law (Ebbing, 1985). Additionally, the use of platinized titanium electrodes and high frequency AC signals minimizes gas formation from electrolysis; and the sample was observed after pressurization to ensure no gas formation occurred under pressure. Because sample loading at room temperature took up to 6 min, the sample warmed to 20 ± 3 °C prior to sealing in the pressure vessel. The sample was sealed in the vessel, and signal and thermocouple wires were connected to leadthrough wires with pin and socket connectors. For samples reported at 25 °C, 13° ± 3 °C media circulated through the vessel walls. The sample cooled in the vessel to a temperature, such that after compression heating, the sample temperature would overshoot 25 °C by 1–3 °C for pressures below 400 MPa, and overshoot up to 14 °C for 800 MPa. Higher overshoot at higher pressures was due to inability to cool the sample to lower temperatures prior to pressurization. After pressurization, the sample was cooled to 24.8 °C before depressurizing; cooling times were between 1 and 10 min depending on temperature overshoot due to compression heating. Four voltage and current measurements were made at 2000 Hz as the temperature cooled from 25.2 to 24.8 °C. These values were averaged and used to calculate resistance and EC at 25 °C.

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For samples reported at 50 °C, 60 ± 3 °C media circulated through the vessel walls. Prior to pressurization, the sample in the vessel was allowed to warm to a temperature, such that after compression heating, would be at 46 ± 3 °C. The sample was warmed under pressure and four voltage and current measurements were made at 2000 Hz as the sample temperature increased from 49.8 to 50.2 °C. These values were averaged and used to calculate resistance and EC at 50 °C. A similar approach was taken for 0.01 m NaCl samples at 61, 54 and 51 °C. Measurements at 0.1 MPa were made similarly in the vessel, without applied pressure. Taking into account the standard deviation of the temperatures at which the samples were measured and the maximum error of the datalogger, temperature values were reported as 25 and 50 °C ± 1.2 °C. After measurements were made, pressure was released. Because variation between samples in temperature history and pressure exposure time prior to making measurements could potentially alter the sample, preliminary tests to check these effects were performed with each sample. The sample was pressurized, cooled to 20 °C and allowed to heat to 60 °C and cool back to 20 °C while making measurements; a lack of difference (p = 0.4) in EC values between increasing and decreasing temperatures indicate these samples were not affected by time and temperature history. Despite efforts to obtain readings over a very narrow temperature range, a temperature gradient likely existed in the samples. The low standard deviation in resistances between 25.2 and 24.8 °C indicated this gradient was likely small. Additionally, similarity of EC values of 0.01 m NaCl compared with previously reported data, indicate the gradient may have minimal effect on the measured value. Unfortunately, the lack of multiple thermocouple leadthroughs makes measuring this gradient difficult. 2.6. Salt solutions and juice sample measurement Electrical conductivity of 0.01 m and 0.1 m NaCl (Fisher Chemical, Certified ACS, Pittsburgh, PA) and KCl (Fisher Chemical, Certified ACS, Pittsburgh, PA) solutions, apple juice, orange juice, tomato juice, and soybean oil was measured at pressures up to 800 MPa in 100 MPa increments at 25° ± 1.2 °C according to the procedure described in the previous section. Juice and oil samples were purchased from a local supermarket. 0.01 m KCl and NaCl were measured at 50° ± 1.2 °C from 0.1 to 800 MPa in 100 MPa increments.

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significance of the effect of pressure and temperature on electrical conductivity. 2.8. Uncertainty analyses Type A, Type B and combined uncertainty analyses were performed for electrical conductivity measurements of each sample at each pressure–temperature combination based on NIST (Taylor & Kuyatt, 1994) guidelines for evaluating uncertainty for measurement results. Type A uncertainty was considered for EC measurement random variability and electrode gap distance, caused by variability in sample loading. The Type A uncertainty component, u(x) was estimated by calculating the standard deviation of the mean, sðX Þ, of a group of (n) independent observations Xk of (x) at the same measurement conditions: !1=2 n X 1 2 uðxÞ ¼ sðX Þ ¼ ðX k  X Þ ð6Þ nðn  1Þ k¼1 Type B uncertainties were recorded for voltage and current measurements from manufacturer specifications. The combined standard uncertainty, uc(r) was estimated as the positive square root of the estimated variance of electrical conductivity, u2c ðrÞ, obtained using the law of propagation of uncertainty (Taylor & Kuyatt, 1994): 2 N  X oðrÞ 2 uc ðrÞ ¼ u2 ðxi Þ; ð7Þ ox i i¼1 where u(xi) indicates Type A or B uncertainty of EC measurement random variability, electrode gap distance, voltage and current. 3. Results and discussion 3.1. Frequency dependence on measured resistance Table 1 summarizes frequency dependence on measured resistance for all samples. Resistance decreased as frequency increased; likely causes of higher resistances at lower frequencies are described by Braunstein and Robbins (1967) as double layer capacitive reactance, due to charge build up on electrodes, and Faradaic processes, a short circuiting of the double layer and slow electron transfer at the electrodes. Measured resistances at 2000 Hz were within 1.7% of values extrapolated to infinite frequency for all samples. For conductivity measurements beyond frequency effect testing, 2000 Hz was applied.

2.7. Statistical analysis 3.2. Cell constant determination Each test condition was repeated three times, and means and standard deviations determined for each condition. Standard deviations were calculated from 12 readings, four values from each of three replicates. ANOVA analysis was performed with JMP 5.1 (JMP, Cary, NC) to determine

Table 2 summarizes cell constant values as a function of temperature and KCl concentration. The atmospheric cell constants, determined by geometric L/A calculation, were 79.6/m for L = 10 mm and 191/m for L = 24 mm. Cali-

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Table 1 Frequency dependence of resistance for samples tested Sample

Temperature (°C)a

Resistance (X)b 0.1 MPa 2000 Hz

400 MPa Infinite fc

% Difference

2000 Hz

800 MPa Infinite fc

% Difference

2000 Hz

Infinite fc

% Difference

0.01 m KCl

25 50

558.9 ± 1.3 553.8 361.9 ± 1.6 359

0.9 0.8

490.4 ± 0.6 485 328.8 ± 1.7 324.8

1.1 1.2

521 ± 2.6 517.3 360.3 ± 2.4 357

0.7 0.9

0.1 m KCl

25

146.9 ± 1.1 144.4

1.7

125.5 ± 1.8 123.8

1.4

142.2 ± 1.7 140.2

1.4

0.01 m NaCl

25 50

692.1 ± 1.8 686.4 487 ± 2 483.1

0.8 0.8

600.8 ± 3.8 594.9 444.1 ± 1.8 439.6

1 1

650.3 ± 2.5 643.3 480.1 ± 2.1 474.2

1.1 1.2

0.1 m NaCl Orange juice Apple juice Tomato juice

25 25

171.8 ± 1.2 169.5 194.5 ± 2.1 192.4

1.3 1.1

143.2 ± 2 141.2 171.9 ± 1.2 170.2

1.4 1

162.9 ± 1.9 160.6 189.2 ± 1.6 187.5

1.4 0.9

25 25

309 ± 4.6 305.8 311.2 ± 1.8 307

1.1 1.4

266.3 ± 1.1 264 277.2 ± 5 275

0.9 0.8

294.8 0.6 292.1 305.2 ± 8.0 301.7

0.9 1.1

a

Temperature variability is ±1.2 °C. Values for resistance are average of 12 total values (three replicates with four readings/replicate) ± standard deviation of the 12 values. c Values for infinite frequency are extrapolated from a plot of resistance vs. frequency0.5. Infinite frequency was considered linear y-intercept from data points at 2000, 4000, 8000, 12,000, 16,000 and 20,000 Hz. b

brated cell constants, based on measurements with 0.01 and 0.1 molal KCl, were within 1% of the geometrically calculated cell constant L/A. The variation in each set of replicates was ±1.4%, indicating acceptable reproducibility at atmospheric pressure. The geometric cell constant was used for subsequent calculations, as it allowed estimation of the cell constant under pressure. Table 3 summarizes the estimated cell constant as a function of pressure and temperature. 3.3. Electrical conductivity values under pressure Values for 0.01 and 0.1 m NaCl at 25 °C at 100, 200, 300 and 400 MPa were within 5.7% of values extracted from Quist and Marshall (1968). Additional data points at 61 °C and 400 MPa, 54 °C and 600 MPa and 51 °C and 800 MPa were within 1.9% of data obtained by Xu et al. (1997). Table 4 summarizes comparisons of 0.01 m NaCl data with previously reported data; these results indicate this sensor yields similar data to sensors of different design.

Fig. 2 shows average values of three replicates for electrical conductivity of 0.01 m NaCl and KCl as a function of pressure and temperature at 25 and 50 °C. Fig. 3 shows values of three replicates for electrical conductivity of 0.1 m NaCl and KCl as a function of pressure at 25 °C. Fig. 4 shows average values of three replicates for electrical conductivity of locally purchased tomato, orange and apple juices at 25 °C. Electrical conductivity of soybean oil was not measurable at atmospheric or under any pressure tested, indicating that pressure did not increase the conductivity of oil to be measurable by the sensor. For all salt solutions and juices, pressure had a significant effect (p < 0.01) on mean electrical conductivity. Similarly to atmospheric pressure, temperature had a significant effect (p < 0.01) on 0.01 m NaCl and KCl at each pressure tested. Qualitatively, sample conductivity increased as a function of pressure, peaking between 200 and 500 MPa and decreasing between 500 and 800 MPa. Electrical conductivity of liquids, including juices, is affected by ionic composition, ionic movement and viscosity of the liquid. Our data for juices at atmospheric pres-

Table 2 Comparison of calculated geometric cell constant and KCl verified cell constant KCl concentration (mol/kg H2O)

Temperature (°C)a

0.01

25 40

0.1

25 40

a

Geometric cell constantb 79.6

191

rreference (S/m)c

Measured resistance for reference (X)d

Cell constant based on reference measurement (/m)

Difference (%) between geometric and KCl verified cell constant

0.141 0.183

558.8 439.3

78.8 80.4

1.0 1.0

1.283 1.659

147.4 114.6

192.1 190.1

1.0 0.5

Temperature variability is ±1.2 °C. Geometric cell constant is calculated from L (m)/A (m2) for the confined current path between electrodes, where A is the cross sectional area of the polycarbonate tube that separates the electrodes at length L. c Lide (2006). d Average of five replicates (maximum standard deviation ±1.4%). b

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Table 3 Estimated cell constant, L/A, (/m) as a function of pressure and temperature Pressure (MPa)

0.1 100 200 300 400 500 600 700 800 a

25 °C

50 °C

Electrode gap distance = 0.01 ma (/m)

Electrode gap distance = 0.024 ma (/m)

Electrode gap distance = 0.01 ma (/m)

Electrode gap distance = 0.024 ma (/m)

79.6 78.4 77.8 77.4 76.8 76.1 75.8 75.3 75

191 188.1 186.7 185.7 184.4 182.7 181.9 180.7 180

79.6 78.6 78 77.6 77.2 76.3 75.9 75.6 75.1

191 188.7 187.1 186.2 185.2 183.2 182.2 181.3 180.3

Electrode gap distance at 0.1 MPa and 25 °C.

Table 4 Electrical conductivity of 0.01 m NaCl under pressure compared to previously reported values Pressure (MPa) 100 200 300 400 400 600 800 a b c d

Temperature (°C)a 25 25 25 25 61 54 51

Electrical conductivity, reference (S/m) c

0.131 0.132c 0.134c 0.13c 0.219d 0.22d 0.206d

Electrical conductivity, measured (S/m)b

Difference (%)

0.13 ± .002 0.14 ± .001 0.13 ± .001 0.13 ± .001 0.22 ± .002 0.22 ± .002 0.21 ± .001

0.76 5.7 3 0 0.5 0 1.9

Temperature variability is ±1.2 °C. Measured conductivities are average of 12 values (three replicates with four readings/replicate) ± standard deviation of 12 values. Quist and Marshall (1968). Xu et al. (1997).

Fig. 2. Electrical conductivity of 0.01 m KCl and NaCl from 0.1 to 800 MPa at 25 and 50 °C.

Fig. 3. Electrical conductivity of 0.1 m KCl and NaCl from 0.1 to 800 MPa at 25 °C.

sure qualitatively agrees with that of Ruhlman, Jin, and Zhang (2001) and Palaniappan and Sastry (1991) in that conductivity of tomato juice is highest, followed by orange juice, then apple juice. Although a juice chemical analysis and viscosity determination were outside the scope of this work, reports from a nutrient and composition database (USDA ARS, 2006) yield some explanation suggesting why the conductivities are different. They report mineral

content, the primary ionic constituents in juice, of tomato juice at 278 mg minerals/100 g juice, followed by orange juice with 210 mg minerals/100 g juice, followed by apple juice with 139.5 mg minerals/100 g juice. Further, nonconductive sugars have been shown to suppress electrical conductivity of liquids (Castro, Teixeira, Salengke, Sastry, & Vicente, 2003). In further support of our conductivity data, the USDA ARS nutrient database (2006) shows apple juice

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NaCl, 25 °C (0.01 S/m), orange juice (0.01 S/m), apple juice (0.008 S/m), and tomato juice (0.009 S/m). The uncertainty of electrode gap distance, measured as the standard deviation of the mean of 20 measurements of gap distance was 0.00025 m.

Fig. 4. Electrical conductivity of orange juice, apple juice and tomato juice from 0.1 to 800 MPa at 25 °C.

to have 10.9 g sugars/100 g, followed by orange juice with 8.4 g/100 g, followed by tomato juice with 3.6 g/100 g. Bridgman (1970) notes that the peaking behavior of EC as a function of pressure cannot be calculated based on ion concentration and viscosity changes alone. He hypothesizes that there may be a distortional effect on the ions that may hinder mobility at high pressures. Electrolyte concentration increases as a function of pressure, promoting increased conductivity. Viscosity of water, which should be similar to viscosity of these salt solutions, initially decreases and then increases at around 60 MPa at 25 °C (Linstrom & Mallard, 2005) and continues to increase, promoting an initial increase in conductivity followed by a decrease. However, the range of pressures in which conductivity peaks is between 300 and 500 MPa and is significantly higher than 60 MPa, thus suggesting influential effects beyond viscosity and concentration. Madura, Pettitt, and Calef (1988) suggest that increasing numbers of hydrogen bonds as a function of pressure can reduce ion activity, thus providing a counter effect on conductivity under pressure. Although no data are available describing viscosity of juices under pressure, their primary constituent is water at 88%, 89% and 94% for apple, orange and tomato juices, respectively. The effect of pressure on electrical conductivity of juices suggests they are experiencing similar effects as salt solutions under pressure. Data on viscosity of juices and understanding of ionic mobilities under pressure would be helpful in assessing their influences on EC under pressure. Future work will focus on understanding ionic mobility, from a mechanistic perspective, under pressure. 3.4. Uncertainty analyses 3.4.1. Type A analysis The maximum standard deviations of the mean for each sample were: 0.01 m KCl, 25 °C (0.003 S/m), 0.01 m KCl, 50 °C (0.006 S/m), 0.1 m KCl (0.03 S/m), 0.01 m NaCl, 25 °C (0.003 S/m), 0.01 m NaCl, 50 °C (0.005 S/m), 0.1 m

3.4.2. Type B analysis Key sources of Type B measurement uncertainty for these experiments are measurement of voltage and measurement of current. Measurement uncertainty for voltage and current, reported by the manufacturer (Agilent Technologies, Palo Alto, CA) is ±0.04%. Additional Type B uncertainties not considered in Eq. (7) are reporting of pressure and temperature. The uncertainty of temperature measurement, based on thermocouple and data logger specifications, is estimated at 1 °C, while the estimated uncertainty of pressure measurement, based on strain cell and data logger specifications, is 0.3%. 3.4.3. Combined standard uncertainty Estimated maximum combined standard uncertainties are: 0.01 m KCl, 25 °C (0.004 S/m), 0.01 m KCl, 50 °C (0.006 S/m), 0.1 m KCl (0.03 S/m), 0.01 m NaCl, 25 °C (0.004 S/m), 0.01 m NaCl, 50 °C (0.005 S/m), 0.1 m NaCl, 25 °C (0.02 S/m), orange juice (0.006 S/m), apple juice (0.007 S/m), and tomato juice (0.008 S/m). 3.5. Conclusion These results indicate this cell and method are adequate for measurement of electrical conductivity of liquids from 0.14 to 1.65 S/m, a typical range for foods and biomaterials. NaCl and KCl salt solutions and three juices show a significant effect of pressure on electrical conductivity. Electrical conductivity, as a function of pressure, shows an increase to a maximum value from 200 to 500 MPa, followed by a decrease as a function of pressure between 0.1 and 800 MPa. The conductivity cell has a well defined current path, with nearly uniform electric field strength. Future work will focus on using this cell to study pressure induced physicochemical changes to foods and determining a better mechanistic understanding of EC changes as a function of pressure. Acknowledgements Support for this research was provided by USDACSREES-NRICGP Grant 2005-35503-15365, by the Ohio Agricultural Research and Development Center (OARDC), The Ohio State University and by the Center for Advanced Processing and Packaging Studies (CAPPS). The authors acknowledge Carl Cooper, Ken Ayotte and Brian Heskitt for assistance with equipment machining and set up. References to commercial products or trade names are made with understanding that no endorsement or discrimination by The Ohio State University is implied.

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