High Pressure Differantial Scanning Calorimetry

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Journal of Food Process Engineering 27 (2004) 359376.All Rights Reserved.

Copyright 2004, Blackwell Publishing 359

HIGH-PRESSURE DIFFERENTIAL SCANNING CALORIMETRY

(DSC): EQUIPMENT AND TECHNIQUE VALIDATION USING

WATERICE PHASE-TRANSITION DATA

S. ZHU1

, S. BULUT2

, A. LE BAIL2

and H.S. RAMASWAMY1,3

1Department of Food Science and Agricultural ChemistryMcGill University

21111 Lakeshore Road,Ste Anne de Bellevue,

Quebec H9X3V9Canada2GEPA-ENITIAA (UMR CNRS 6144-SPI)

Rue de la Graudire BP 82225,F-44322 Nantes Cedex 03, France

Accepted for Publication June 18, 2004

ABSTRACT

Understanding phase transition during high-pressure (HP) processing of

foods is important both with respect to optimizing the process and improve-

ment of product quality, but scientific information available in this area is very

limited. In this study, the phase-transition behavior of water was evaluated

using a HP differential scanning calorimetry (DSC). Tests were carried out

under both isothermal pressure-scan (P-scan) and isobaric temperature-scan

(T-scan) modes with distilled water prefrozen in the sample cell. P-scan was

carried out at 0.3 MPa/min at two temperatures, -10 and-20C, and T-scan

was carried out at 0.15C/min at two pressures, 0.1 and 115 MPa. The

pressure-induced phase transition of water was accurately reproduced by

the P-scan test. Ice melting latent heat during P-scan showed no significant

difference (P>0.05) from the available reference data in literature. The

relationship between P-scan tested (Lm) and reference latent heat wasLm=0.987 L (R2=0.99, n =6) suggesting a mean error less than 2%. T-scan

mode was less appropriate and did not yield promising result. Measured

values were less accurate than P-scan probably because of the influence of

large heat capacity of sample cell. However, reliable and reproducible results

obtained under P-scan mode suggested that the HP DSC can be used for the

calorimetric determination of pressure-dependent water-phase transition in

real food systems during HP freezing/thawing operations.

Blackwell Science, LtdOxford, UKJFPEJournal of Food Process Engineering0145-8876Copyright 2004 by Food & Nutrition Press, Inc., Trumbull, Connecticut.2004275359376Original Article HIGH-PRESSURE DSC: EQUIPMENT/TECH-

NIQUE VALIDATION S. ZHU

ET AL.

3 Corresponding author. TEL: +1-514-398-7919; FAX: +1-514-398-7977; EMAIL:[email protected]


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360 S. ZHUET AL.

INTRODUCTION

High-pressure (HP) process is receiving considerable attention because

of its advantages and potential benefits for the preservation and modification

of foods. Phase transition is one of the major domains that are of potential

interest in HP processing of foods. Pressure-induced phase transitions, such

as freezing/thawing of high-moisture foods and crystallization of lipids, offer

numerous opportunities for process or product development (Knorr 1999).

Water undergoes different types of phase transitions when subjected to differ-

ent pressuretemperature domains (Bridgman 1912). This phenomenon

allows carrying out different freezing/thawing applications such as pressure

shift freezing, HP freezing and HP thawing (Cheftel et al. 2000). HP in this

context generally refers to pressures in excess of several hundred atmospheres

(>50 MPa).Calorimetry is a powerful tool to study thermodynamic properties of

materials and phase-change phenomena. Normally, calorimetric experiments

are carried out with a differential scanning calorimeter (DSC) at a constant

(usually atmospheric) pressure using temperature as the working parameter.

Bridgman (1912) developed experimental apparatus to measure phase transi-

tion and volume change of pure water under pressure, and to estimate latent

heat of water using Clapeyron equation. Hogenboom et al. (1995) developed

a similar apparatus with a novel piezometer to measure volume variation and

phase diagram of an aqueous magnesium sulfate solution.

In HP calorimetry, the cell for holding the sample should be strong

enough to resist high pressure. Thus, the cell becomes large in terms of mass

and heat capacity, which presents a major challenge to obtain accurate results

for temperature scan (T-scan) tests. To overcome this difficulty, an alternative

measure of pressure scan (P-scan) at a constant temperature is adopted in HP

calorimetry. The pressure can be changed either as a step change (Pruzan et al.

1979) or at a constant rate (Randzio et al. 1994). Several studies have been

carried out using P-scan tests for gathering thermal properties of water and

aqueous solution (Chourot et al. 2000; Le Bail et al. 2001) and some organic

liquids such as hexane, butane-1, benzene, toluene (Fuchs et al. 1979; Pruzan

et al. 1979; TerMinassian et al. 1988; Grolier and Randzio 1997) under HP

conditions. Two types of calorimetric apparatus have been used in terms of

working conditions: constant mass or constant volume. Most of the studies in

literature are related to liquids and used constant volume technique (Pruzan

et al. 1979; Randzio et al. 1994; Le Bail et al. 2001).

Most foods and food products contain a high percentage of moisture.

Thermo-physical properties of foods have been generally linked to moisture

content, temperature, phase and pressure. Phase transition of water in these

foods during HP process can be more complicated than that of pure water.


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HIGH-PRESSURE DSC: EQUIPMENT/TECHNIQUE VALIDATION 361

Scientific information related to HP calorimetry of foods is scarce and poorly

documented. The objective of this study was to evaluate the performance of

HP DSC for characterizing the phase-transition behavior of water at high

pressures by employing two modes: (1) the traditional T-Scan and (2) the

more recommended P-scan. A specific objective was to validate the tech-

nique by comparing the experimental results on phase-transition tempera-

tures and latent heat of pure water with published data and thereby pave way

for the use of the technique for gathering data on HP water-phase transition

in foods.

MATERIALS AND METHODS

HP Calorimetric System

A HP DSC (Fig. 1) was used in this study. The experimental system

consisted of a differential calorimetry head, a refrigerated circulator, two HP

cells, a HP compressor and a computer. The calorimetry head (Pass 27, Sceres,

Orsay, France) had two cavities (20 mm diameter 95 mm depth) used forholding the sample and the reference cells. There were 220 thermocouple

junctions installed between the two cavities to amplify the generated temper-

ature differential signal. Two platinum thermometers (Pt 100) were placed

under each of the cavities to monitor/control temperature of the calorimeter.

The temperature of the calorimetry head was controlled through a copper coil

winding around the contour of the head (Fig. 1a) and an oven around the

system. The copper coil was connected to the refrigerated circulator (Huber

CC250, Offenburg, Germany) with a water-glycol medium. During the exper-

iment, the calorimetric head together with copper coil was thermally insulated

in plastic box containing foamed plastics.

The HP cell (made of beryllium copper) contained two plugs with nitrile

O-ring and a threaded bolt for sealing on one side (Fig. 2). It was connected

to a pressure tube (3.2 mm diameter) on the other side with miniature fittings

(M2 Serie, 100 MPa, Harwood Engineering, MA). The pressure within the

system was achieved through the HP compressor (400 MPa, 5 cm3, Nova-

Swiss, Effretikon, CH) driven by a step motor (MO63-LE09, Mijno, Fenwick,

France) and controlled by the computer. A pressure sensor (200 or 400 MPa,

Asco Instruments, Chateaufort, France) was used to monitor and control the

pressure. When the pressure required for a test exceeded 180 MPa, the 400-

MPa sensor was used during the experiment. A software (Labview 6, National

instruments, Austin, TX) was used for system control and data recording

(temperature, pressure and heat flow). Pentane (Sigma, Fallavier, France) was

used as pressurization medium in the system because of its stability on the


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362 S. ZHUET AL.

pressuretemperature domain tested and its properties (low viscosity, no phase

change) (Bridgman 1970; Scaife and Lyons 1980).

Calibration

The pressure sensor was calibrated against a reference pressure gauge

(Bourdon, France). Temperature of the calorimeter was calibrated against a

FIG. 1. EXPERIMENTAL SET-UP OF HP DIFFERENTIAL SCANNING CALORIMETER:

(a) schematic diagram and (b) photograph of the calorimetric head and the cells.

Calorimetric head

Copper coil

HP cell

HP tube

(b)

Refrigerated

circulator

Computer

Cell

HP compressor

Medium

reservoir

HPsensor

Calorimetric head

Coppercoil

(a)


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K-type thermocouple (0.1 mm diameter, Omega, Stamford, CT) placed in the

sample cell at selected temperature between -20 and 20C. The calorimeterwas calibrated using a 100 W resistance settled in the sample cell. A voltagebetween 1 and 4.2 V was supplied to this resistance by a power source (220

programmable voltage source, Keithley Instruments SARL, Saint-Aubin,

France) for period between 30 and 90 min. The heat generated from theresistance was evaluated with the voltage and the current measured with a

voltmeter (7055 Voltmeter, Solartron Mobrey SA, Saint-Christophe, France)

and an ampere meter (195A multimeter, Keithley Instruments, SARL, Saint-

Aubin, France). The calorimetry differential temperature signal (mV) attrib-

uted to the electric heating was recorded every 5 s. The peak area was then

integrated to calculate the ratio of heat power to calorimetric voltage at

selected temperatures. The calibration tests were carried out from -20 to 20C.A linear fit of the experimental data was obtained as:

(1)

where q is heat flow rate (mW); Sis calorimetric signal (mV); kis the ratio

of heat flow rate to calorimetric signal (mW/mV); Tis the temperature of the

calibration tests performed (C). Similar calibration procedures have beenused in the study by Chourot et al. (2000) and by Le Bail et al. (2001).

Sample Preparation

All experiments were carried out using distilled water. About 0.5 g of

pure water (weight accurately measured up to milligram level; example

0.5712 g) was vacuum-packaged in a polyethylene bag (80-mm-thick multi-player film) (La Bovida, Nice, France). For the test, the sample was installed

q kS T S R n= = +( ) = =( )37 1 0 080 0 978 82. . . , ,

FIG. 2. SCHEME OF THE HIGH-PRESSURE CELL AND SAMPLE INSTALLATION


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364 S. ZHUET AL.

in the sample cell (see Figs. 1 and 2). The reference cell was prepared in the

same way as the sample cell but without the sample. Air bubble was carefully

removed from the cell during the preparation of the two-calorimeter cells.

Isothermal Pressure-Scan (P-Scan)

After sample installation, the sample and reference cells were placed in

the sample and reference cavities, respectively. The sample was frozen at the

set temperature (-10 or -20C) in the calorimeter. Once the calorimeter signalshowed a stable baseline (close to zero), the pressure was increased from the

atmospheric level (0.1 MPa) at a constant rate (0.3 MPa/min) up to a level

high enough for completion of the melting process (e.g., 160 MPa for the P-scan at -10C) and heat flow signal was recorded every 5 s (Fig. 3). When thepressure reached the corresponding phase-change temperature, ice started

melting (see point 1 in Fig. 3), resulting in a peak of heat flow to compensate

for the temperature drop. A low P-scan rate was employed in this study, as

compared to 1 MPa/min in some previous studies (Le Bail et al. 2001), in

order to obtain a well defined span of calorimetric peak.

Isobaric Temperature-Scan (T-Scan)

T-scan tests were carried out at constant pressure (Fig. 4) by increasing

the temperature at a prescribed rate. Before each T-scan test, the test sample

was frozen in the calorimeter and then stabilized at 0.1 MPa or 115 MPa,

corresponding to a phase-change temperature of about 0 or -10C, respectively(Bridgman 1912). When calorimetric signals approached a balance under a

constant pressure, the temperature was increased from lower than -15C at aconstant rate (about 0.15C/min) and the calorimeter signals were recorded

every 5 s.

RESULTS AND DISCUSSION

Isothermal P-scan

Figure 5 shows a typical experimental differential thermograph of an

isothermal P-scan with pure ice at the calorimetric temperature of-10C. Bycalibration using Eq. (1), calorimetric data is converted into heat flow rate.

Figure 6a demonstrates two peaks of heat flow rate against pressure during P-

scan of the ice samples at calorimetric temperature -10 and -20C, respec-tively. The HP DSC heat flow signal appeared a straight baseline as pressure

was increased at a constant rate. As the pressure level increased, the signal

started deviating from the base line (1 in Figs. 3 and 5). This transition was


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FIG. 3. SCHEMATIC DESCRIPTION OF ISOTHERMAL PRESSURE-SCAN: (a) water

phase-change diagram and pressurization and (b) corresponding calorimetric signal (Tc is calorimetric

temperture [C]).

21

0

Tem

perature(C)

Ice-I

Water

TC

2100.1

Pressure (MPa)

P-scan

P1 P3P2

1

2

3

(a)

P1 P2 P30.1

Calorimetricsignal(endodown) 0

Peak

(melting end)

Peak onset

(melting start)

Peak end

Peak baseline

Pressure (MPa)(b)

1 3

2


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366 S. ZHUET AL.

FIG. 4. SCHEMATIC DESCRIPTION OF ISOBARIC TEMPERATURE-SCAN WITH WATER

PHASE-CHANGE DIAGRAM

21

0

T

emperature(C)

Ice-I

Water

2100.1Pressure (MPa)

T-scan

FIG. 5. A TYPICAL MEASUREMENT OF ISOTHERMAL PRESSURE-SCAN (0.3 MPa/min) OF

PURE ICE (0.5712 g) WITH CALORIMETRIC TEMPERATURE AT -10C

1.8

1.5

1.20.9

0.6

0.3

0.0

0.3

0 60 120 180 240 300 360

Time (min)

Heatflo

wsignal(mV)

20

40

6080

100

120

140

160

Press

ure(MPa)

Pressure

Heat flow signal 1 3

2

t1 t2 t3


9/18


defined as onset temperature in this study. In the case of P-scan test, this

onset refers to the pressure that is associated with the beginning of the melting

of test sample (ice). For P-scan at -10 and -20C, the onset pressure was111 1.8 (mean SD, n= 3) and 194 1.5 MPa (n= 3) (Fig. 6a), respec-tively, which matched well with phase-change diagram of waterice I

(Bridgman 1912), as empirically expressed by Bridgman (1912) as follows:

(2)

where TP is phase-change temperature of water (C); P is pressure (MPa)(Chourot et al. 1997).

After onset, the temperature of the ice/water mixture decreases with a

further increase in pressure (12 in Fig. 3a), following pressuretemperature

relationship of the phase change of waterice I (Bridgman 1912; Le Bail et al.

2002). This phenomenon enhances the heat transfer between sample and its

surroundings (cell, calorimeter, etc.) because the HP DSC was set at constant

temperature during P-scan tests. Thus the absolute value of calorimetric signal

increases until complete melting of the sample (2 in Figs. 3 and 5) and then

decreases back to the initial baseline (23 in Figs. 3 and 5) representing an

inert or nonevent (melting) phase. Therefore, a pressure-dependent endot-

hermic calorimetric peak can be obtained for each P-scan experiment

(Fig. 6a). For the sample mass and the scan rate (0.3 MPa/min) used, the

endothermic peak (heat flow) spanned over 35 1.4 (n= 3) and 27 1.1 MPa(n= 3) with the summit occurring at 126 2.3 (n= 3) and 205 1.6 MPa(n= 3) for P-scan at -10 and -20C, respectively. The peak at the lowertemperature (-20C) occurred at a higher pressure and also had a lower summitvalue and a shorter bandwidth (Fig. 6a). This is because of the pressure-

dependent characteristics of ice Iwater phase change (Eq. 2) and latent heat

as empirically determined by Bridgman (1912):

(3)

whereL is latent heat of waterice I (J/g) (Chourot et al. 1997).

The shape and size of the P-scan endothermic (heat flow rate) peak

depends on several factors, such as sample mass, P-scan rate, thermo-physical

properties of the cell and pressure medium and so on. As in conventional DSC

experiments, a higher scan rate (either T-scan or P-scan) yields a wider span

of the peak. During a conventional DSC test, the sample span (usually very

small and made of out of light weight aluminum) is the only barrier to the

heat-transfer in the chamber between the heating medium and test/reference

samples. A larger heat capacity or a lower thermal conductivity of test cells

implies a lower heat transfer rate and therefore a longer duration to reach and

T P PP = - -0 0722 0 0001552

. .

L P P= - -333 0 399 0 000388 2. .


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368 S. ZHUET AL.

FIG. 6. PRESSURE-SCAN (0.3 MPa/min) OF PURE ICE WITH CALORIMETRIC

TEMPERATURE AT -10 AND -20C: (A) THAWING HEAT FLOW RATE AND (B) THAWINGLATENT HEAT

100

80

60

40

20

0

50 100 150 200 250

Pressure (MPa)

Heatflowrate(mW/g)

10C 20C

10C 20C

(a)

0

50

100

150

200

250

300

50 100 150 200 250

Pressure (MPa)

Thawinglaten

theat(J/g)

(b)


11/18


pass the peak, thus resulting in a wide peak span. Attributed to higher pres-

sures associated with HP DSC, the cell system itself is large and constructed

of more sturdy material. The large heat capacity of the cell system strongly

affects the span of the P-scan signal. Hence an appropriate calibration is vital

for obtaining realistic data from these cells. When using a similar HP colo-

rimeter with 1 g water and P-scan at 1 MPa/min, Le Bail et al. (2001) found

the peak to span over 60 MPa. Randzio et al. (1994) reported a peak span of

70 MPa for solidliquid transition in benzene at a P-scan at 0.3 MPa/min. The

peak span observed was smaller than these reported values because the sample

mass was smaller and further a lower P-scan rate was used in this study.

The onset and summit of the P-scan endothermic peak is related to the

start and end of ice melting, respectively (Le Bail et al. 2002). A weighted-

average pressure ( 1-2) was used to compare results from the P-scan tests

with the reference diagram of Bridgman (1912), which was mathematically

defined as:

(4)

where qb is the baseline of heat flow rate (mW); tis time (s); t1 and t2 are the

time at peak onset and summit (s) (Fig. 5). The average pressure during ice

melting was 119 2.2 (n= 3) and 200 1.6 MPa (n= 3) for P-scan at -10and -20C, respectively (Table 1), as compared with the reference values of112 1.4 and 195 1.0 MPa, respectively (Bridgman 1912) (Fig. 7).

The area of endothermic heat flow rate peak represents the quantity of

the heat transfer during the HP DSC test. The latent heat was evaluated by

P

PP q q t

q q t

t

t1 21

1

2

2-=

-( )

-( )

b

b

d

d

TABLE 1.

CHARACTERIZATION OF CALORIMETRIC SIGNAL PEAK (MEAN SD) FROMISOTHERMAL PRESSURE-SCAN (0.3 MPa/min) EXPERIMENTS USING DISTILLED WATER

Calorimetric temperature -10 0.2C (n= 3) -20 0.2C (n= 3)

Pressure span of peak (MPa) 34.8 1.4 25.9 1.1Pressure at peak onset (P1, MPa) 111 1.8 194 1.5Reference phase-change pressure (MPa)* 112 1.4 195 1.0Average pressure (

1-2, MPa) 119 2.2 200 1.6

Pressure at peak summit (P2, MPa) 126 2.3 205 1.6Measured latent heat (Lm, J/g) 278 1.0 233 1.9Calculated latent heat (L( 1-2), J/g)* 280 1.1 237 1.4

* Values were calculated using Eq. (2) or (3).

P

P


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370 S. ZHUET AL.

integrating the calorimetric peak calibrated with the conversion ratio (Eq. 1)

and subtracting the baseline:

(5)

where Lm is measured latent heat of waterice I (J/g); m is water mass (g).

Figure 6b shows the accumulation of thawing latent heat during the P-scan of

test using pure water sample (ice). The top value of each curve in Fig. 6b

indicates the total amount of absorbed or the latent heat released by the test

sample, which was 278 1.0 (n= 3) and 233 1.9 J/g (n= 3) for P-scan at-10 and -20C, respectively. By substituting the average pressure ( 1-2) intoEq. (3) (Bridgman 1912), the latent heat [L( 1-2) = 333 - 0.399 1-2-0.000388 1-2] as 280 1.1 and 237 1.4 J/g for P-scan at -10 and -20C,respectively, was obtained (Table 1). For all P-scan tests combined, statistical

analysis (t-test: paired two sample for means) indicated no significant differ-

ence (P> 0.05, d.f. = 1/6) between experimentally evaluated latent heat (Lm)from this study and the calculated reference [L( 1-2)] latent heat (Bridgman

1912). Based on the regression relationship between the values from the two

Lm

q q tm b d= -( )1

1000

P

P P

P

P

FIG. 7. PHASE TRANSITION BETWEEN PURE WATER AND ICE I OBSERVED DURING

PRESSURE-SCAN TESTS AS COMPARED TO THE REFERENCE DIAGRAM

(BRIDGMAN 1912)

25

20

15

10

5

0

5

50 100 150 200 250

Pressure (MPa)

Temperature(C)

Average ( 12P )

Ice-I

Water

Reference diagram

Peak summit (melting end)

0.1


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methods, the following equation was obtainedLm= 0.987L( 1-2) (R2= 0.99,n= 6) for P-scan tests (Fig. 8).

Isobaric T-scan

Figure 9 is a typical calorimetric signal plot of a T-scan experiment

(0.15C/min, 115 MPa, 0.5712 g ice). Figure 10 shows two peaks of heat flow

rate against temperature during T-scan at atmospheric conditions (0.1 MPa)

and at 115 MPa, respectively. Table 2 summarizes the results from isobaric T-

scan tests. The endothermic peak at the higher pressure occurred at a lower

temperature with a smaller peak area (Fig. 10a) because of the pressure

related thermo-properties of water. The peak-onset temperature was -0.2 0.2and -10.5 0.3C (n= 3) at 0.1 and 115 MPa, respectively (Table 2), as com-pared with calculated phase-change temperatures of 0 0.0 and -10.4 0.1Cby Eq. (2), which correspond very well.

Compared with isothermal P-scan peak, T-scan peak appeared larger in

signal value because ice-melting duration in T-scan (Fig. 9) was shorter than

that in P-scan (Fig. 5). However, T-scan rate (0.15C/min) in this study was

quite small as compared with that used in conventional DSC test with very

tiny sample (usually 1030 mg). HP DSC requires pressure resistant cells to

P

FIG. 8. MELTING LATENT HEAT OF PURE ICE MEASURED BY PRESSURE-SCAN (P-SCAN)

TESTS AS COMPARE TO THE REFERENCE DATA (BRIDGMAN 1912)

200

250

300

350

400

0 50 100 150 200 250

Pressure (MPa)

Latentheat(J/g)

Reference

P-scan


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372 S. ZHUET AL.

hold sample. Thus sample size should be large enough to reduce the ratio of

noise to HP calorimetric signal (Le Bail et al. 2001). A large sample can

strengthen the signal, but tend to increase the span of the peak. Because of

the large mass of the HP cell and sample, a low T-scan rate was selected to

obtain a peak span as narrow as possible. The peak span was 10.2 0.5 (n= 3)and 9.8 0.4C (n= 3), showing peek summit at 3.8 0.3 and -6.8 0.3C,for T-scan (0.15C/min) of the ice sample at 0.1 and 115 MPa, respectively

(Table 2). Although a low T-scan rate was used, the peak was much delayed

FIG. 9. A TYPICAL MEASUREMENT OF ISOBARIC TEMPERATURE-SCAN (0.15C/min) OF

PURE ICE (0.5712 g) AT 115 MPa

3

2

1

0

1

0 20 40 60 80 100 120

Time (min)

Heatflowsignal(mV)

15

10

5

0

5

Calo

rimetrictemperature(C)Heat flow signal

Temperature

TABLE 2.

CHARACTERIZATION OF CALORIMETRIC SIGNAL PEAK (MEAN SD) FROM ISOBARICTEMPERATURE-SCAN (0.15C/min) EXPERIMENTS USING DISTILLED WATER

Calorimeter pressure 0.1 0.0 MPa (n= 3) 115 0.9 MPa (n= 3)

Temperature at peak onset (C) -0.2 0.2 -10.5 0.3Phase-change temperature (C)* 0 0.0 -10.4 0.1Temperature at peak summit (C) 3.8 0.3 -6.8 0.3

Temperature span of peak (C) 10.2 0.5 9.8 0.4Measured latent heat (J/g) 318 2.1 277 2.4Calculated latent heat (J/g)* 333 0.0 282 0.5

* Values were calculated using Eq. (2) or (3).


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FIG. 10. TEMPERATURE SCAN (0.15C/min) OF PURE ICE AT 0.1 AND 115 MPa:

(a) thawing heat flow rate and (b) thawing latent heat

200

150

100

50

0

50

Hea

tflowrate(mW/g)

(a)

115 MPa 0.1 MPa

0

50

100

150

200

250

300

350

15 10 5 0 5 10 15 20

Temperature (C)

15 10 5 0 5 10 15 20

Temperature (C)

Thawinglatentheat(J/g)

(b)

115 MPa0.1 MPa


16/18

374 S. ZHUET AL.

because of the large heat quantity required to keep temperature increase in

sample and reference cells. The phase transition of water couldnt be well

demonstrated during T-scan test under current experimental conditions.

The latent heat was estimated using a similar integrating method to the

P-scan peak analysis. The value was 318 2.1 and 277 2.4 J/g for T-scan at0.1 and 115 MPa, respectively (Fig. 10b). Mathematically, the relationship

between measured (Lm) and reference latent heat (L) was expressed as

Lm= 0.961L (R2= 0.95, n= 6). Although the regression agreement was good,and the values seem to be pretty close to each other, statistical analysis (t-test:

paired two sample for means) demonstrated that measured value was signifi-

cantly (P < 0.05, d.f. = 1/6) lower than reference value of Bridgman (1912):318 versus 333 and 277 versus 282 J/g, at atmospheric and HP conditions,

respectively. T-scan test generally showed results that are slightly less accurate

than P-scan probably attributed to influence of large heat capacity of the mass

of the cell and pressure medium during T-scan (temperature increase). While

in P-scan test, the HP DSC worked at constant temperature. Overall the

difference between the values from the two methods and those published in

some classical literature was within a 10% error margin.

CONCLUSIONS

A HP DSC was examined for the evaluation of HP phase-transition

behavior of pure water (in the melting mode). Two techniques (T-scan and P-

scan) were used. Results obtained were statistical differences between the

techniques as well as between the experimental and some published models.

The pressure induced phase transition of ice Iwater was well pictured by P-

scan. The latent heat of ice melting obtained by isothermal P-scan showed no

significant difference (P> 0.05) from reference data. This good agreementvalidated the reliability and accuracy of the HP DSC system during the

isothermal P-scan test. HP calorimetry requires large cell mass to resist pres-

sure, which could reduce the accuracy/efficiency of calorimetric signal during

isobaric T-scan test. T-scan tests produced good and consistent results but were

slightly less accurate than P-scan counterparts or published models. The

results obtained showed that the system could be successfully used for gath-

ering HP phase-transition information, which is essential for the better under-

standing of pressure-dependent phase transition during HP freezing/thawing

of foods.

ACKNOWLEDGMENTS

This study was partially funded by the European Project of Safe Ice

(QLK1-CT-2002-02230) and the Strategic Grants Program of the Natural


17/18


Sciences and Engineering Research Council of Canada. Experiments were

carried out at ENITIAA, Nantes, France and analyses were performed at

McGill. The authors wish to thank L. Guihard and J. Laurenceau for their

technical support.

REFERENCES

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High Pressure Differantial Scanning Calorimetry