www.elsevier.com/locate/jfoodeng

Journal of Food Engineering 69 (2005) 425–436

Desalination of mussel cooking juices by electrodialysis: effecton the aroma profile

S. Cros a,b,*, B. Lignot a, P. Bourseau a, P. Jaouen b, C. Prost c

a Centre de Recherche, Universite de Bretagne Sud, L2P, BP 92116, 56321 Lorient Cedex, Franceb Universite de Nantes, GEPEA-ISOMer, UMR CNRS 6144, Faculte des Sciences, BP 92208, 44322 Nantes Cedex 3, France

c QM2A, ENITIAA, rue de la Geraudiere, BP 82225, 44322 Nantes Cedex, France

Received 31 May 2004; accepted 18 August 2004

Available online 12 October 2004

Abstract

The production of aromatic concentrates from seafood cooking juices can be done by reverse osmosis. Nevertheless, the high

osmotic pressure of these juices makes necessary a preliminary step of desalination by electrodialysis.

The aim of this work was to evaluate the impact of electrodialysis has on the aromatic quality of a mussel cooking juice. This

evaluation was done by sensory analysis and by gas chromatography coupled with mass spectrometry or with olfactometry. Results

show that the salt concentration can be decreased from 20g l�1 to 2.8g l�1 without aromatic profile modification. Under this critical

salt concentration, a significant modification of the sensory and chromatographic profiles was observed. This critical salt concen-

tration was obtained when the current density reached its limiting value. It�s well known that at this value, solutions located near

the membrane surface undergo physico-chemical changes (water-splitting, pH change. . .).� 2004 Elsevier Ltd. All rights reserved.

Keywords: Electrodialysis; Limiting current density; Marine aromas; Waste waters

1. Introduction

Water is a staple good, but of the earth�s water only0.00065% is available freshwater and it is not homogene-ous assessed. As countries become more industrialized

they divert greater amounts of their water to industrial

use. Volume of effluents rejected by the agrifood indus-

tries represents 20% of the whole industry. It is now

becoming a common decision that the water manage-

ment must be improved: either by improving processes

management to use less water, and/or by recycling waste

water. The first answer has permitted to decrease by 30%the water consumption in the tuna processing. Waste

0260-8774/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2004.08.036

* Corresponding author. Tel.: +33 297 87 45 87; fax: +33 297 87 45

88.

E-mail address: severine.cros@univ-ubs.fr (S. Cros).

water recycling is more difficult to implement as it gen-

erate high investment costs. But these costs could be re-

duced by recovering valuable products presents in these

effluents. In this way, membrane processes constitute amore interesting concentration (and separation) technol-

ogy than the thermal ones, because they are less energy-

consuming and avoid heat degradation of labile

substances. They are also more suitable to the continu-

ous treatment of such high volume and diluted streams.

Finally, the simplicity and the modularity of plant de-

sign and operation, limit installation space of membrane

processes versus thermal processes.Seafood processing industries have used membrane

filtration treatment since the end of the 1970s to recover

valuable peptides or proteins in defrosting, washing or

cooking waters (DeWitt & Morrissey, 2002a; DeWitt

& Morrissey, 2002b; Huidobro, Montero, & Borderias,

1998; Jaouen & Quemeneur, 1992). Other studies have

426 S. Cros et al. / Journal of Food Engineering 69 (2005) 425–436

shown that taste or aroma compounds can also be

recovered in cooking juices by reverse osmosis (RO)

(Cros, Bourseau, Vandanjon, Quemeneur, & Jaouen,

2003a; Cros, Vandanjon, Jaouen, & Bourseau, 2003b;

Cros, Lignot, Razafintsalama, Jaouen, & Bourseau,

Bour, 2004; Jayarajah & Lee, 1999; Vandanjon, Cros,Jaouen, Quemeneur, & Bourseau, 2002). But, the great

amount of salts in these marine effluents implies high os-

motic pressure which decreases considerably the perme-

ation fluxes of RO systems (Cros et al., 2003a), making

necessary a desalination step prior to concentration.

Electrodialysis process was previously shown to be more

adapted than diafiltration process to desalinate mussel

cooking juices while limiting aroma loss and preservingthe aroma profile (Cros et al., 2003b). Similar conclu-

sions were drawn by Lin and Chiang (1993) who com-

pared the performances of these two same methods on

the basis of desalting and taste compounds retention

from shrimp processing waste water. Electrodialysis is

also used to remove tartrate contained in wine. Goncal-

ves, Fernandes, Cameira dos Santos, and Norberta de

Pinho (2003) have shown only by sensory analysis thataromas contained in wine were well conserved during

electrodialysis. On the contrary, Gomez Benitez,

Palacios Macias, Szekely Gorostiaga, Veas Lopez, and

Perez Rodriguez (2003) have pointed up, also by sensory

analysis, that upon electrodialysis a significant aromas

loss was perceived at a tartre elimination rate of 30%.

But these studies should merit to be thorough to under-

stand and to control the aroma transfer through electro-dialysis membrane.

The first purpose of this paper is to evaluate the im-

pact of electrodialysis on the aroma quality of mussel

cooking juices. Therefore, the global aroma intensity

and aroma profile of raw and desalinated juices have

been characterized by sensory analyses. Their volatil

and odor active compounds contents have also been

evaluated respectively by gas chromatography coupledwith mass spectrometry and gas chromatography cou-

pled with olfactometry analyses; the combination of

these two techniques has allowed to identify the odorant

molecules involved in the evolution of the sensory pro-

file. The second part of this work is aiming both to iden-

tify the factors influencing aromas retention and

evaluate the mechanism of aroma transfer through the

electrodialysis membranes.

2. Materials and methods

2.1. Materials

Mussel (Mytilus edulis) cooking juices were provided

by Atlantic Seafood Ingredients Company (La Baule,France). These juices were produced and frozen

(�20 �C) in Vigo (Spain), and transported in refrigerated

containers to the Nantes-Saint Nazaire trade harbour

(France).

Before analysis, samples were thawed at 4�C over-

night. After thawing, juices were centrifuged at 1000·g(5min) to eliminate suspended matters which can in-

crease membrane fouling.Hydrochloric acid and sodium hydroxide used for

membrane cleaning, anhydrous sodium sulfate, dichlo-

romethane (purity 99.8%), and chemical standard aro-

mas used during aroma extraction and analyses were

provided by Acr�os (Noisy-le-Grand, France).

2.2. Electrodialysis

2.2.1. Process

The experiments (repeated twice) were carried out

with an electrodialyzer (P1; EIVS, Le Vesinet, France)

consisting of 20 cells with alternating CMV cation and

AMV anion exchange membranes (Asahi Glass Corpo-

ration, Tokyo, Japan). The total effective membrane

area was 0.138m2. The ED process involved three inde-

pendent circuits: diluting stream, concentrating stream(NaCl initial concentration, 10g l�1), and electrode rins-

ing stream (NaCl, 10g l�1). Three centrifugal pumps

were used to circulate the process and the electrode

streams up to 220 lh�1.

2.2.2. Set-up of operating conditions

When the voltage is applied in the electrodialysis

cells, the ions concentration on the diluate side decreaseswith respect to the bulk concentration whereas it in-

creases in the concentrate side of the membrane. When

the current increases, the concentration near the mem-

brane surface on the diluate side decreases. At a certain

current density this concentration reaches zero and a

limiting current density is reached. A drastic increase

in the cell�s electrical resistance occurred owing to the

lack of species conducting the current.Electrodialysis performances are much lower when

current density exceeds the LCD because of the increas-

ing electrical resistance of the solution. Moreover, the

resulting concentration polarization leads to water dis-

sociation and membrane damage (Valerdi-Perez &

Ibaoez-Mengual, 2001). To avoid these problems, the

LCD was determined before ED experiments, using

the unprocessed juice, by the Cowan and Brown method(1959). The voltage was increased and the corresponding

current density noted. A typical plot of U/I versus 1/I is

shown in Fig. 1. The LCD is obtained from the intersec-

tion of the two straight lines fitting the two branches of

the curve. Then, the current density was adjusted by the

voltage to 13Am�2 at the beginning of a batch run,

which correspond at a rate at less inferior to 80% of

the LCD. Thereafter, the voltage was kept constant.The ED system was operated in a batch recirculation

mode with a constant voltage and a constant velocity

Table 1

Odor references of the descriptors used to elaborate sensory profiles

Descriptor Odor reference

Green cis-3-Hexenol

Shellfish Juice of raw oyster

Stock Beef cooking juice

Cooked mussel Cooked mussel

Seaweed/grassy Alga of the Lorient harbor

Earthy Mud of the Lorient harbor

Limiting current density

U/I

1/i

Fig. 1. Schematic diagram illustrating the determination of the

limiting current density by plotting the total resistance of a cell pair

versus 1/i.

S. Cros et al. / Journal of Food Engineering 69 (2005) 425–436 427

(respectively 20V and 175 lh�1). The temperature of the

solution was maintained at 20 ± 2 �C.

2.2.3. Cleaning procedure

After each experiment, membranes were cleaned on

the spot by circulating a 0.1M HCl solution for

20min, followed by rinsing with deionized water and

an alkaline washing (3g l�1 NaOH solution, 20min).

The stack was finally rinsed thoroughly with deionized

water to neutral pH.

2.3. Aroma analysis

2.3.1. Sensory evaluation

Samples were sniffed by a trained panel of 10 judges,

selected from staff members and students of the ‘‘Uni-

versite de Bretagne Sud’’ (panel 1), taking into account

their acquaintance with the material to be analyzed,

their sensitivity, and their ability to reproduce judg-

ments. The panellists were trained as recommended byMulton (1998). Evaluations were performed in individ-

ual booths, by presenting 70ml of samples at the same

temperature (approximately 30 �C) in 3-digit coded

capped glass cups of 100ml. All the tests were carried

out between 11 and 12a.m.

Triangular tests were performed to detect an eventual

difference between the odor of the unprocessed and the

desalinated mussel cooking juices. For this test, the desa-linated juice was salted as the same rate than the unproc-

essed juice (20g l�1), so that the eventual difference in

sensory analysis could be attributed exclusively to the

aroma content. The differences between samples were

considered significant when the number of correct

answers was 7, 8 and 9, respectively, for a significance le-

vel of 5%, 1% and 0.1% (Meilgaard, Civille, &Carr, 1991).

Ranking tests were performed to observe an eventualchange of the global odor intensity during the desalina-

tion step. Ranking data were analysed with the Fried-

man rank test (Multon, 1998) to find if there were

significant global odor intensity differences between

samples, and with the method proposed by Conover

(1999) to establish if a sample differed from another.

Sensory profiles were established on two samples per

session. The panelists were asked to evaluate the inten-

sity of the different odorant descriptors (green, shellfish,

stock, cooked mussel, seaweed/grassy and earthy) on a

4-points scale (0: no odor; 3: high odor). The reference

odors for all the descriptors are detailed in Table 1.

These descriptors were identified as the more perceptible

and the more easily identifiable by the panelists in themussel cooking juice. One session was organized per

week. Analyses were carried out two times on each sam-

ple obtained after the process. Statistical analysis was

performed using the Wilcoxon rank sum test (Wilcoxon

signed test). Values of p < 5% were considered as statis-

tically significant.

2.3.2. Simultaneous steam distillation-solvent extraction

(SDE)

As for sensory analyses, the desalinated juices were

salted in the same rate than the raw juice (20g l�1), be-

fore extraction of the volatile compounds.

Mussel cooking juice (70ml), added with 0.5ml of

internal standard (4-methylthiazole, 19.5lgml�1), was

then extracted with 4 mL of redistilled dichloromethane

for 1h using a Godefroot apparatus (Godefroot,Sandra, & Verzele, 1983). Sample and solvent were both

heated to their boiling point. SDE extracts were dried

over anhydrous sodium sulfate and reduced to exactly

0.1ml under a gentle stream of nitrogen. Extracts were

stored at �20 �C until analysis.

2.3.3. Gas chromatography–mass spectrometry

(GC–MS)

A GC/mass-selective detector (Fison, Arcueil,

France) was used to analyse SDE extracts. Volatile com-

pounds were separated using a capillary column (Stabil-

Wax with a stationary phase of polyethylene glycol:

60m length · 0.25mm internal diameter · 0.25lm film

thickness; Restek, Bellefonte, Pennsylvania, USA). GC

conditions were finalized as follows: 1.5ll splitless injec-tion (60s valve delay); injector temperature, 240 �C; he-lium carrier gas flow of 1.0ml/min; oven programmed

from 40 to 240 �C at a rate of 3 �C/min, with initial

and final hold times of 5 and 10min. The quadrupole

mass selective detector, with electronic impact ionization

(ionization energy, 70eV) and an electron multiplier

voltage of 2000V, operated in the scan mode, with a

Table 2

Deionization degree (DD) obtained for the samples A–F

A B C D E F

DD (%) 0 51 63 86 90 91

428 S. Cros et al. / Journal of Food Engineering 69 (2005) 425–436

mass range of 30–300 atomic mass unity, at 0.45scan/s.

The detector interface temperature was set at 250 �C,with the temperature in the MS source reaching 245 �C.

Compound identifications were based on the compar-

ison of experimental retention indices (RI) (Van den

Dool & Kratz, 1963) with the RI found in the literatureand on the comparison of mass spectra with standard

MS spectra databases (NIST and LIBTX).

Concentration of each compound was calculated by

comparison of total peak area with that of the internal

standard. Averages and standard deviations were calcu-

lated on the three extracts, each being injected twice.

The results obtained were analysed for significance of

differences between samples using the ANOVA test(comparison of more than two samples). Then, the mean

were compared 2 per 2 with the adjusted t-test. Values of

p < 5% were considered as statistically significant.

2.3.4. Gas chromatography–flame ionization detection-

olfactometry (GC/FID-O)

The GC/FID-O system consisted of a 3400 Star GC

(Varian, Palo Alto, CA) equipped with a capillary col-umn StabilWax (polyethylene glycol) with a 60m

length · 0.25mm i.d. · 0.25lm film thickness (Restek,

Bellefonte, Pennsylvania, USA), and FID at 280 �C,and a sniffing port supplied with humidified air at

40 �C (flow rate: 600 mlmin�1). GC effluent (1.5ll ofthe SDE extract) was split 1/1 between the FID and

the sniffing port. Oven temperature was programmed

from 40 �C for 5min to 240 �C at 5 �C.min�1 and a finaltime of 10min.

A panel of eight judges trained in odour recognition

at the QM2A laboratory (panel 2) and with experience

in GC/O was selected. Sniffing was divided into two

parts of 18min. Each person participated in the sniffing

of both parts but during two distinct sessions to avoid

tiredness as recommended by Le Guen, Prost, and

Demaimay (2000). The panelists were asked to assignodour properties to each compound detected. Detection

of an odor at the sniffing port by fewer than three of

eight assessors was considered as noise (Pollien et al.,

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35

time (min)

ash

cont

ent (

g.L-1

)

A

B

C

DFE

Fig. 2. Conductivity during electrodialysis of the centrifugated raw

mussel cooking juice (juice volume: 1.7 l; voltage applied: 20V and flow

rate: 175 lh�1). A, B, C, D, E, and F being six samples which the aroma

quality was elevated by sensory analysis.

1997). A detection frequency was then calculated for

each compound.

3. Results and discussion

3.1. Influence of the electrodialysis process on the

odorant properties of mussel cooking juices

3.1.1. Modification of the global aroma intensity

The ED operating parameters are described in the

experimental part of this paper. At the beginning of

the experiment, voltage was adjusted to 20V so that

the current density (initially equal to 13Am�2) remainedbelow the LCD throughout the run.

The variation in the mussel cooking juice conductiv-

ity with respect to the operating time is given in Fig.

2. Conductivity in the diluate compartment initially de-

creased sharply. After 25–30min, conductivity levelled

off, indicating that the desalination rate strongly de-

creased and the ED experiment was therefore stopped.

At increasing values of ED running times, six samples(A–F) of the juice were collected. The ashes content

was measured as previously described (Cros et al.,

2004). The deionization degrees of the six samples were

calculated as follows and are reported in Table 2.

Deionization degreeðDDÞ ¼ ðashesÞinitial � ðashesÞfinalðashesÞinitial

Change in aroma intensity was assessed through senso-

rial analysis at each deionization degrees. The panelists

were asked to classify the six samples (A–F) according

to their decreasing global aroma intensity. The corre-

sponding results are assembled in Table 3.

Table 3

Ranking of the six samples by decreasing global aroma intensity given

by the panel 1, and groups of identical odor intensity samples based on

Friedman�s test (p < 5%)

Judges A B C D E F

1 1 3 2 4 5 6

2 2 1 4 3 5 6

3 4 3 1 2 6 5

4 3 2 1 4 6 5

5 3 1 2 5 4 6

6 1 4 2 3 5 6

7 2 4 3 1 6 5

8 4 2 3 1 5 6

9 1 4 2 6 5 3

10 4 2 3 1 5 6

Rank sum 25 26 23 30 52 54

0.00

1.00

2.00

3.00green

shellfish

stock

cooked mussel

seaweed/grassy

earthy

unprocessed juice desalinated juice

Fig. 3. Sensory profiles of the unprocessed and desalinated juices (ash

content: 1.9g l�1). Value of p < 5% (Wilcoxon test) were considered as

significantly different (*).

S. Cros et al. / Journal of Food Engineering 69 (2005) 425–436 429

According to the Friedman and Conover tests

(p < 5%), samples A to D were found as very similar

in intensity whereas samples E and F distinguished by

their much lower aroma intensity. Thus, whereas the

drop in salt content from 20 to 3g l�1 did not alter the

global aroma intensity of the juice, a drastic loss sud-denly occurred at a critical salt concentration comprised

between those of the samples D and E. New ED exper-

iments were performed in order to assess more accu-

rately this critical salt concentration. Desalting was

therefore processed until varied salt concentrations

(around the critical point) were reached: 2.8, 2.3 and

1.9g l�1. Triangular tests were performed by sniffing

the samples in order to identify those having a globalaroma significantly different than the unprocessed juice.

The results given by the panelists are gathered in Table

4. This table indicates the number of judges (on 10) hav-

ing correctly identified the odd sample among 3. At

numbers of 9, 8 and 7 correct answers, the samples are

considered to have significantly different odours (levels

respectively of 0.1%, 1% and 5%). So, the global aroma

intensity was shown to be significantly reduced (p < 5%)at an ash content comprised between 2.8 and 2.3g l�1.

Desalination of mussel cooking juices by electrodial-

ysis is intended to be a pretreatment in a process of mak-

ing marine aromas concentrates, before the

concentration of these juices by reverse osmosis (Cros

et al., 2004). So, aroma loss during desalination by elec-

trodialysis must be limited. The results reported here

indicate that electrodialysis must be stopped when theash content reaches 2.8g l�1.

3.1.2. Modification of the aroma profile

In addition to the global aroma intensity, the influ-

ence of the desalination degree on the aroma profile

was also examined. The panelists evaluated the odor

intensity of the different descriptors on the unprocessed

juice and on the juice desalinated beyond the critical saltconcentration (Fig. 3). Among the different descriptors

characteristic of the mussel cooking juice, the most po-

tent notes in the unprocessed juice were the ‘‘stock’’

and ‘‘cooked mussel’’ notes and the seaweed and earthy

notes. When the cooking juice was desalinated by elec-

trodialysis down to an ash content of 2.3g l�1 (beyond

the critical salt concentration), the stock, cooked mussel

Table 4

Detection of changes in the global aroma of the mussel cooking juice

by triangular test (panel of 10 judges)

Unprocessed juice Desalinated juice: ash content

2.8g l�1 2.3g l�1 1.9g l�1

Unprocessed – – – –

2.8g l�1 4 – – –

2.3g l�1 7 (*) 7 (*) – –

1.9g l�1 9 (***) 8 (**) 3 –

Significance level: *** < 0.1%, ** < 1%, * < 5%.

and earthy notes were significantly less perceived

(Wilcoxon test; p < 5%). The contrary holds for the‘‘shellfish’’ note.

3.1.3. Modification of the composition in odor active

compounds

Both samples were analysed by GC–MS (Table 5) to

characterize the volatile compounds present in the juice

and by GC–O (Table 6) to identify among them the

odor active ones.To account for the modification of the aroma profile

during ED operation, the odor active compounds whose

concentration was shown to be modified by GC–MS

analysis (Table 5), were first considered.

Upon prolonged electrodialysis (after the critical ash

content), the concentration of some odor active mole-

cules was significantly decreased (ANOVA test,

p < 5%), among them are one alcohol (phenol), onealdehyde ((Z)-4-heptenal), 2 ketones (2,3-butanedione,

2,6,6-trimethyl-2-cyclohexen-1,4-dione), and one N-con-

taining compounds (pyridine). The loss of the molecules

such as (Z)-4-heptenal, 2,3-butanedione and 2,6,6-

trimethyl-2-cyclohexen-1,4-dione respectively give charac-

teristic boiled potato, buttery and milky smells are thus

likely to contribute to the ‘‘stock’’ or ‘‘cooked mussel’’

note of the juice. Their partial loss during ED may thusexplain the decrease in both notes intensity.

On the opposite, the concentration of some odor ac-

tive molecules increased significantly upon ED (Table

4, ANOVA test, p < 5%). This is the case of three alcoh-

ols (2-ethyl-1-hexanol, 1-octen-3-ol, benzyl alcohol), two

aldehydes (octanal and (E)-2-heptenal), and one ketone

(3,5,5-trimethyl-3-cyclohexen-1-one). In general, alcoh-

ols are not odor active compounds unless they arepresent in high quantity or unsaturated (Heath &

Reineccius, 1986). Thus, 2-ethyl-1-hexanol was the only

saturated alcohol identified as odor active in the desali-

nated juice owing to its great accumulation upon electro-

dialysing (by a factor of 90). This compound giving a

boiled potato smell may contribute to the ‘‘cooked’’ note

as well as the ketone and 1-octen-3-ol both characterized

Table 5

Concentrations and standard deviation of the volatile compounds identified in the unprocessed and desalinated juices

Compound RI Method of identification Estimated concentration (g equiv. I.S./l of juice)

Unprocessed juice Desalinated juices

[salts] = 2.8g l�1 [salts] = 1.9g l�1

Aromatic hydrocarbons

Toluene 1029 MS, standard, RIa 7.8 ± 5.7 6.2 ± 2.5 % 96.1 ± 96.2

Styrene 1248 MS, standard, RIb 1.6 ± 0.3 % 27.3 ± 3.3 % 93.1 ± 56.3

Naphthalene 1749 MS, standard, RIa 11.7 ± 1.9 11.5 ± 2.2 6.5 ± 4.7

2-Methylnaphthalene 1856 MS, RIa 2.9 ± 1.8 1.5 ± 0.9 0.4 ± 0.1

Alcohols

1-Propanol 1025 MS, standard, RIa 1.7 ± 0.2 1.5 ± 0.4 & 1.1 ± 0.2

1-Butanol 1132 MS, RIb 3.9 ± 0.3 % 4.4 ± 0.5 % 5.3 ± 0.9

1-Penten-3-ol 1150 MS, RIb 2.2 ± 0.1 2.1 ± 0.5 2.2 ± 0.3

1-Pentanol 1238 MS, standard, RIb 1.0 ± 0.2 % 1.5 ± 0.2 1.4 ± 0.3

3-Octanol 1383 MS 1.1 ± 0.1 1.0 ± 0.1 1.0 ± 0.1

2-Butoxy ethanol 1392 MS, standard, 11.7 ± 1.1 12.6 ± 1.7 14.0 ± 2.3

1-Octen-3-ol 1443 MS, RIa 0.5 ± 0.2 0.5 ± 0.1 % 0.6 ± 0.1

Heptanol 1447 MS, standard, RIa 1.1 ± 0.1 % 1.7 ± 0.2 1.5 ± 0.3

2-Ethyl-1-hexanol 1484 MS, standard, RIa 4.6 ± 1.2 % 326.4 ± 23.4 % 397.6 ± 46.3

Benzyl alcohol 1886 MS, standard, RIc 11.5 ± 1.5 11.6 ± 1.9 % 15.4 ± 3.0

Phenol 2019 MS, standard, RIa 11.6 ± 4.1 11.7 ± 3.3 & 7.9 ± 1.0

2,4-bis(1,1-Dimethylethyl)phenol 2327 MS, RIa 81.9 ± 12.9 62.4 ± 14.7 67.1 ± 5.6

1-Hexadecanol 2387 MS 8.5 ± 2.1 6.8 ± 2.7 15.8 ± 8.5

Aldehydes

(E)-2-butenal 1002 MS nd nd nd

Hexanal 1073 MS, standard, RIb 5.7 ± 0.7 6.0 ± 0.8 % 9.4 ± 1.2

(E)-2-Methyl-2-butenal 1082 MS, RIb 1.5 ± 0.2 1.7 ± 0.2 1.6 ± 0.1

Heptanal 1180 MS, standard, RIb 3.5 ± 0.8 4.5 ± 0.7 5.4 ± 0.8

(E)-2-Hexenal 1212 MS, RIb 1.4 ± 0.3 1.1 ± 0.1 1.4 ± 0.1

(Z)-4-Heptenal 1234 MS, standard, RIb 2.7 ± 0.1 2.5 ± 0.1 & 1.2 ± 0.4

Octanal 1277 MS, standard, RIb 3.5 ± 0.5 % 8.3 ± 0.8 % 16.2 ± 0.7

(E)-2-Heptenal 1309 MS, RIb 3.1 ± 0.3 2.9 ± 0.4 % 4.1 ± 0.3

Nonanal 1387 MS, standard, RIb 13.8 ± 3.0 12.8 ± 1.2 14.0 ± 3.2

(E)-2-Octenal 1425 MS, RIb 1.6 ± 0.5 1.8 ± 0.2 1.9 ± 0.3

(E,E)-2,4-Heptadienal 1492 MS, standard, RIb 2.5 ± 1.1 1.9 ± 0.3 1.8 ± 0.3

Decanal 1497 MS, standard, RIa 7.5 ± 3.5 7.1 ± 2.0 11.2 ± 3.6

Benzaldehyde 1524 MS, RIa 13.6 ± 4.5 15.5 ± 1.5 13.4 ± 2.8

2-Nonenal 1534 MS 1.4 ± 0.2 1.2 ± 0.1 1.3 ± 0.4

(E,Z)-2,6-Nonadienal 1585 MS, RIa 4.9 ± 3.3 3.4 ± 0.3 3.7 ± 0.5

(E,E)-2,4-Decadienal 1827 MS, RIb 1.2 ± 0.3 1.3 ± 0.3 1.3 ± 0.4

Pentadecanal 2041 MS 8.5 ± 3.9 4.5 ± 1.5 7.0 ± 3.5

Aldehyde 2260 MS 16.4 ± 6.5 9.3 ± 2.7 30.8 ± 7.2

Aldehyde 2392 MS 1.6 ± 1.1 0.6 ± 0.3 % 1.5 ± 0.2

Carboxylic acids

Acetic acid 1453 MS, standard, RId 3.5 ± 3.3 2.3 ± 0.4 % 16.7 ± 7.1

Hexanoic acid 1848 MS, RId 5.2 ± 2.4 4.3 ± 1.3 5.9 ± 1.1

Octanoic acid 2068 MS, standard, RId 3.3 ± 0.7 3.4 ± 1.8 % 18.5 ± 3.8

Nonanoic acid 2173 MS, RId 13.3 ± 6.8 11.2 ± 4.5 % 33.2 ± 6.9

Ketones

2,3-Butanedione <1000 MS, standard, RIa 16.7 ± 1.7 18.6 ± 7.0 & 6.0 ± 1.5

3-Octanone 1244 MS, RIa 0.2 ± 0.1 0.3 ± 0.0 0.2 ± 0.1

2-Nonanone 1380 MS, RIa 1.0 ± 0.3 1.0 ± 0.2 0.9 ± 0.1

3,5,5-Trimethyl-3-cyclohexen-1-one 1402 MS, RIe 18.3 ± 2.5 15.4 ± 2.0 % 32.9 ± 3.5

3-Octen-2-one 1408 MS, RIa 21.3 ± 3.1 20.2 ± 5.7 & 10.9 ± 0.6

3,5,5-Trimethyl-2-cyclohexen-1-one 1611 MS, RIe 38.9 ± 6.1 34.4 ± 9.7 24.3 ± 5.8

1-Methyl-2-pyrrolidinone 1660 MS nd nd nd

2,6,6-Trimethyl-2-cyclohexen-1,4-dione 1710 MS, RIe 7.5 ± 0.9 5.5 ± 0.9 5.1 ± 0.5

Ketone 2142 MS 5.2 ± 1.4 3.1 ± 1.4 5.4 ± 1.9

Furan

2-Pentylfuran 1224 MS, RIa 0.7 ± 0.2 0.7 ± 0.6 2.2 ± 0.2

430 S. Cros et al. / Journal of Food Engineering 69 (2005) 425–436

Table 5 (continued)

Compound RI Method of identification Estimated concentration (g equiv. I.S./l of juice)

Unprocessed juice Desalinated juices

[salts] = 2.8g l�1 [salts] = 1.9g l�1

Nitrogen or sulfur-containing compounds

Pyridine 1170 MS, standard, RIa 3.0 ± 1.0 & 1.7 ± 0.9 & 0.4 ± 0.5

Methylpyrazine 1252 MS, standard, RIa 1.5 ± 0.1 & 1.2 ± 0.5 1.0 ± 0.1

2,5-Dimethylpyrazine 1303 MS, RIa 9.4 ± 1.1 8.8 ± 1.5 8.3 ± 0.8

Methional 1482 MS, standard, RIa tr tr 0.0

Dimethyl sulfoxide 1560 MS, standard 2.2 ± 1.1 2.9 ± 0.9 1.8 ± 0.4

Benzothiazole 1982 MS, RIa nd nd nd

Terpene

Limonene 1186 MS, RIa 0.7 ± 0.1 0.6 ± 0.0 & 0.4 ± 0.1

Unknown (odor actives, Table 6)

1010 MS 6.3 ± 1.4 7.1 ± 2.6 % 12.7 ± 2.1

1101 MS tr tr tr

1297 MS 3.2 ± 1.1 4.1 ± 1.9 2.9 ± 0.7

1340 MS 5.6 ± 1.2 5.3 ± 2.2 4.9 ± 1.8

1430 MS nd nd nd

1448 MS nd nd nd

1721 MS tr tr tr

1742 MS tr tr tr

2087 MS 15.2 ± 3.2 13.7 ± 2.5 & 7.8 ± 1.9

2100 MS nd nd nd

& and %: Significant decrease and increase of the volatile compound (ANOVA; p < 5%); tr: traces; nd: not determined.

References for Retention Index (RI):a Chung et al. (2002).b Cha and Cadwallader (1995).c Prost et al. (1998).d Sekiwa et al. (1997).e Le Guen et al. (2000).

S. Cros et al. / Journal of Food Engineering 69 (2005) 425–436 431

by their mushroom odor. However, these compounds

do not seem to accumulate enough to compensate the

loss of the other compounds contributing to this note.

Benzyl alcohol and the two aldehydes (octanal and (E)-

2-heptenal) give characteristic green smells (Table 6).

Although no significant increase in the ‘‘green’’ note

was detected by the panelists (Fig. 3), the accumulation

of these molecules may explain the increase in fresh notessuch as the ‘‘shellfish’’ note.

Among the 30 odor active compounds detected in the

unprocessed and desalinated juices by GC/O, 13 were

not detected or identified by GC–MS (Table 5). Indeed,

some aroma compounds present at the trace level were

not detected by GC–MS and yet perceived by GC/O

due to their very low odor thresholds. A drastic change

in the odor intensity of eight compounds exclusively de-tected by GC–O was evidenced, their detection fre-

quency equaling zero either in the unprocessed or in

the electrodialyzed juice (Table 6). Thus some molecules

giving ‘‘cooked’’ notes such as unknown compounds

with retention indexes (RI) of 2087 and 2100, and ben-

zothiazole were exclusively detected in the unprocessed

juice. Their decrease upon ED processing can be tied to

the decrease in the ‘‘stock’’ and ‘‘mussel cooked’’ note.On the opposite, 3 compounds giving a green smell (E-

2-butenal, unknown (RI 1006), and pentadecanal) were

only perceived in the desalinated juice but not in the

unprocessed juice. Their accumulation upon electrodial-

ysis may explain the increase in the shellfish note.

3.2. Investigation of the factors inducing volatile

compounds loss

Volatile compounds may be continuously lost in thecourse of electrodialyzing process owing to their diffusion

or electrotransport through the ion-exchangemembranes

(IEMs) or to their adsorption onto the membranes. The

sudden decrease in some volatile compounds concentra-

tion at a critical salt concentration (between an ash con-

tent of 2.8 and 2.3g l�1) suggests that an event occurs

which modifies one or more of these properties.

Different phenomena could account for the volatilecompounds loss at a critical salt concentration including

(a) the salting-out of amino acids inducing the co-pre-

cipitation of hydrophobic aroma compounds (b) the

ionization of volatile compounds and water splitting

occurring when ion species available for current trans-

port are fewer. Indeed, when the current density exceeds

the LCD, H+ and OH� generated by water splitting and

other ions resulting from the ionization of volatile com-pounds start participating in current transport. More-

over, ions being not abundant enough to carry all the

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 2 4 6 8 10 12 14 16ash content (g/L)

Opt

ical

den

sity

535

nm

7.35

7.4

7.45

7.5

7.55

7.6

pH

Fig. 4. Protein or peptide content (h) (measured by the Biuret

method; Gornall et al., 1949) and pH (j) evolution during desalina-

tion by electrodialysis.

Table 6

Odor active compounds detected in the unprocessed and the desalinated mussel cooking juices by olfactometry

RI Compound Odor description Detection frequency (/8 judges)

Unprocessed juice Desalinated juice (2.3g l�1)

983 2,3-Butanedione Buttery a,std 7 8

1002 (E)-2-Butenal Woody, musty, sulfury – 7*

1010 Unknown Green, sulfury, amine, mushroom – 6*

1101 Unknown Plastic 5 8*

1143 1-Penten-3-ol Marine, sulfury, plastic 3 –*

1168 Pyridine Sulfury, chimicalg 5 8*

1242 (Z)-4-Heptenal Fish,b boiled potato,a amine 7 7

1280 Octanal Mint, floral, fruit,a resin – 3*

1297 Unknown Mushroom 7 7

1332 (E)-2-Heptenal Woody,h grassy, green, moss 6 8

1340 Unknown Grilled 7 8

1378 Nonanal Green,std,e grassy, moss 8 4*

1393 2-Butoxy Ethanol Sulfury, mushroom, grassy 8 6

1408 3,5,5-Trimethyl-3-cyclohexen-1-one Mushroom 4 4

1430 Unknown Leather, grilled 4 4

1448 Unknown Grilled – 4*

1450 1-Octen-3-ol Woody, floral, mushroomd 3 –*

1463 Methional Boiled potatoa,std 8 8

1485 2-Ethyl-1-hexanol Boiled potato, medicalstd – 3*

1598 (E,Z)-2,6-Nonadienal Green,b floral 6 7

1660 1-Methyl-2-pyrrolidinone Green, grassy, hay 5 –*

1689 2,6,6-trimethyl-2-cyclohexene-1,4-dione Milky, medical – 5*

1721 Unknown Leather, chimical 4 –*

1742 Unknown Floral, chimical 4 4

1840 (E,E)-2,4-Decadienal Buttery, greenc 6 –*

1895 Benzyl alcohol Boiled potato, green 3 3

2020 phenol + pentadecanal Green, moss, sulfury, medicale – 3*

2087 Unknown Green, woody, marine 5 –*

2100 Unknown Plastic, marine 3 –*

1982 Benzothiazole Grilled,f resin, marine 4 –*

Correspondence aroma–odor also found by:a Le Guen et al. (2000).b Cha et al. (1998).c Prost et al. (1998).d Piveteau et al. (2000).e Tanchotikul and Hsieh (1989).f Machiels et al. (2003).g Schiffman et al. (2001).h Lopez et al. (2004).std Injection of standard.* Difference between detection frequency of unprocessed juice and desalinated juice P3.

432 S. Cros et al. / Journal of Food Engineering 69 (2005) 425–436

current, this creates a resistance to current flow which

induces local heating at the membrane surface. As a

consequence, a thermal degradation of the aroma com-

pounds can also be thought about when the feed solu-

tion passes through the cells.

These different hypotheses are tested below.

3.2.1. Interaction with amino acids

To test the first hypothesis, amino acids concentra-

tion in the diluate stream was measured all along the

desalination experiment. As shown in Fig. 4, the amino

acids content was actually reduced by 90% during ED.

However their loss occurred in the initial stage of desa-

lination, well before the observed drastic change in the

juice aroma profile. The aroma loss observed at the crit-

S. Cros et al. / Journal of Food Engineering 69 (2005) 425–436 433

ical salt concentration is thus unlikely to result from

their co-precipitation with amino acids.

The observed decrease in amino acids concentration

could result either from their transfer through IEMs

or from their adsorption on the membrane surface or

both. During the demineralization process, the pH ofthe diluate remains in the [7.4–7.6] range. At this pH,

most amino acids are present in their zwitterionic form

and should not transfer the electric current through

the IEMs. However, transfer of amino acids by an elec-

tric power along with mineral components were previ-

ously shown to occur during conventional ED (Peers,

1958) which was accounted for by the existence of a

small concentration of the cationic and anionic formcapable to be transferred.

Electrostatic interactions between amino acids and

the charged membranes may also lead to their adsorp-

tion onto the membrane surface and induce a decrease

in their bulk concentration. The fouling of the mem-

branes by the organic matter present in the juice was

investigated by measuring the LCD using the cooking

juice and comparing it to those established with a20g l�1 NaCl solution (Fig. 5). At each salt concentration

tested, the LCD obtained with the cooking juice was sys-

tematically much lower. Strathmann (1992) explain this

phenomenon by an increase of the membrane resistance

due to its fouling by amino acids and/or peptides.

3.2.2. Limiting current density

Strathmann (1992) indicates that when the LCD isreached, a drastic decrease in the pH value of the diluate

is observed. The pH value was thus measured all along

the ED experiment (Fig. 4). At the beginning of the

ED cycle, the bulk pH slightly increased. This can be

due to the liberation of hydroxyl ions remaining ad-

sorbed on the membranes after the cleaning operation.

After a few minutes stabilization period, a decrease in

the pH value occurred when the critical salt concentra-tion was reached, which indicates the current density ex-

ceeded the LCD. The drastic aroma loss observed at this

point can thus be related to the phenomenon occurring

when exceeding the LCD, in particular to water splitting.

0

2

4

6

8

10

12

14

0 2 4 6 8 10

ash content (g.L-1

)

ilim

(A

.m-2

)

Fig. 5. ilim evolution with the ash content for the mussel cooking juice

(j) and an NaCl solution (h).

As revealed by GC–MS analyses, the concentration

of eight compounds diminished upon desalination by

electrodialysis. The loss of the non ionizable molecules

((Z)-4-heptenal, 2,3-butanedione, 3-octen-2-one and

limonene) only occurred when the critical salt concen-

tration was reached, excluding the possibilities of theirprogressive transfer by diffusion from the diluate to

the concentrate compartments or of their adsorption

onto the membranes. As the current density exceeded

the LCD, these unsaturated compounds may undergo

thermal degradation or be chemically altered by the

products resulting from water splitting. This hypothesis

is supported by the fact that none of these aromas was

recovered in the saline brine (concentrated stream). Asan example, a-dicarbonyl compound such as 2,3-butan-

edione may undergo a molecular rearrangement leading

to the formation of a charged species able to cross the

IEMs. (Z)-4-heptenal may be hydrogenized in heptanal,

which was shown to accumulate in the juice.

Molecules partially ionized when dissolved in water

such as pyridin (pKa = 5.25) and methylpyrazine

(pKa � 0.6) were continuously removed all along theED run (Table 5). These compounds may either cross

the membrane by diffusion or electrotransport or may

be adsorbed through hydrophobic and ionic interactions

at the membrane surface. Their absence in the saline

brine suggested the third hypothesis may be true. In

the case of phenol (pKa � 10) and 1-propanol

(pKa � 16) which are fairly weak acids, the loss occurred

only after reaching the LCD. These compounds maythen undergo thermal degradation or their ionization

may be favoured due to the lack of species conducting

the current. To identify the truth, a desalination experi-

ment by ED was performed on a model solution com-

posed of phenol (50lg l�1) and NaCl in the same

hydrodynamic and temperature conditions, the current

being initially set up under the LCD (Fig. 6). A moder-

ate phenol loss (10%) was observed when the NaCl con-centration was reduced from 20 to 5g l�1. A further

decrease in NaCl concentration induced a much more

pronounced phenol loss (54% at a salt content of

2g l�1), similar to the drastic aroma loss observed on

the cooking juice at a critical salt content. Moreover,

0

10

20

30

40

50

60

0 5 10 15 20 25

[NaCl] (g.L-1

)

[phe

nol]

(µg

.L-1

)

[salt]initial = 3g/L

[salt]initial = 20g/L

Fig. 6. Evolution of the phenol concentration in the diluate stream,

when the initial salt concentration was 20 and 3g l�1.

434 S. Cros et al. / Journal of Food Engineering 69 (2005) 425–436

as phenol was recovered in the concentrate compart-

ment, the hypothesis of a possible phenol thermal dena-

turation can be rejected. Above the LCD, the ionization

of phenol would rather be favoured. This could facilitate

its membrane crossing by electrotransport.

3.3. Investigation of the factors inducing the apparent

accumulation of volatile compounds

According to GC–MS analyses, 17 volatile com-

pounds accumulated in the juice during the ED operation

including six alcohols (1-butanol, 1-pentanol, heptanol,

2-ethyl-1-hexanol, 1-octen-3-ol, benzyl alcohol), four

aldehydes (hexanal, heptanal, octanal, (E)-2-heptenal),one unsaturated ketone (3,5,5-trimethyl-3-cyclohexen-1-

one), 3 carboxylic acids (acetic, octanoic, nonanoic

acids), two aromatic hydrocarbons (toluene, styrene)

and one furan (2-pentylfuran).

Among these compounds, three (2-ethyl-1-hexanol,

toluene, styrene) are suspected to be membrane degrada-

tion products which could explain their abnormalous

high accumulation in the juice.The apparent accumulation of the other volatile mol-

ecules in the diluate stream may partially result from the

volume variation in the diluate compartment as a result

of electroosmosis: indeed, charged species migrating

through the membranes are accompanied by a shell of

molecules of solvent (Lee, Sarfert, Strathmann, &Moon,

2002). In the present case, the volume in the diluate

stream was decreased from 1800ml to 1750ml. However,the concentration of octanal, for example, increased in

the diluate stream to an extent much greater than can

be accounted for by the small change in volume.

Another hypothesis is that the extraction of volatile

compounds may be facilitated by a decrease in their sol-

ubility as a result of peptide loss. Their accumulation

should then occur at the beginning of the ED experi-

ment which is only the case of 1-butanol and octanal.When approaching the LCD, a change in their ioniza-

tion state may also occur, leading to a decrease in their

solubility. However, except carboxylic acids, no other

ionized solutes were found to accumulate. Unexpec-

tingly, none of the carboxylic acids appeared to cross

the ED membranes (not present in the concentrated

stream) although they were present at the ionized state

at the pH of the experiment. The concentration of threeof them (acetic, octanoic, nonanoic acids) was even

greatly increased when approaching the LCD (Table

5). Though partially ionized, steric hindrance may limit

the mobility of carboxylate ions in the membranes or

even prevent these large molecules to penetrate them.

Due to their high hydrophobicity (low solubility in

water) and consequent great affinity for the hydrocar-

bon skeleton of the polymeric membranes, they arelikely to preferentially adsorb onto the IEMs

(Lindstrand, Jonsson, & Sundstrom, 2000a). Owing to

the attractive electrostatic interactions, the positively

charged anion-exchange membranes were previously re-

ported to be more severely fouled by carboxylate ions

than the cation-exchange ones (Lindstrand, Jonsson, &

Sundstrom, 2000b). When the current density exceeds

its limiting value, water splitting occurs. The anion-exchange-membrane provides the electric current by

OH� ions (the cation-exchange membrane by H+ ions)

which results in the accumulation of H+ ions at the sur-

face of anion exchange membrane (OH� ions at the sur-

face of cation-exchange membrane) in the desalting

compartment. On the way to the anode, carboxylate

ions reach the water dissociation layer of anion-

exchange membrane, and react with H+ ions, being thusconverted into their corresponding neutral forms which

do not migrate in the electric field (Kameche, Xu,

Innocent, & Pourcelly, 2003). Long chain organic acids

(octanoic and nonanoic acids) being much less soluble in

water than their corresponding salts, their extraction

from the aqueous medium would therefore be facilitated

accounting for their apparent accumulation. However,

this explanation does not hold for acetic acid due toits small size and high solubility in water. Although

accumulating in the diluate stream, this compound

was also recovered in the saline brine showing its capac-

ity to cross the anion-exchange membranes.

The remaining molecules shown to accumulate may

also be generated by the thermal or electrochemical deg-

radation of other compounds present in the juice such as

amino acids, lipids, or carbohydrates.

4. Conclusion

This study proves the feasibility of eliminating 85% of

salts from a mussel cooking juice without altering signif-

icantly its aroma profile. On the contrary, when desali-

nation is processed further, a significant aroma losswas evidenced, which was related to a phenomenon

occurring above the limiting current density. Higher

desalination rates could be possibly reached, while pre-

serving the aroma profile, by selecting another mem-

brane material on which amino acids adsorption is

limited. Indeed, the reduced fouling of the membrane

would decrease its electrical resistance thereby increas-

ing the limiting current density and thus delaying theoccurrence of water splitting.

Even if the volatile compounds loss was shown to be

independent of the drastic amino acids loss observed in

the initial stage of electrodialysis, the latter organic com-

pounds are flavourants which highly contribute to the

taste and organoleptic quality of the aroma juice. Their

recovery could be advantageously improved by the use

of membranes develop to desalinate amino acids solu-tions (mosaic membranes: Sato, Sakairi, Yonemoto, &

Tadaki, 1995).

S. Cros et al. / Journal of Food Engineering 69 (2005) 425–436 435

Acknowledgement

Regions ‘‘Bretagne’’ and ‘‘Pays de la Loire’’ (France)

are gratefully acknowledged for their financial support.

We also thank the following: our colleagues for their

contribution to sensory analyses; Maryse Derouiniot(CRTT, Saint Nazaire, France) and Francoise Peresse

(Universite de Bretagne Sud, Lorient, France) for their

technical support; Corinne Razafintsalama (senior flav-

ourist) for her cooperation and discussion on sensory

analysis during the preparation of this work; and Atlan-

tic Seafood Ingredients Company (La Baule, France) for

discussion on the project and for providing us with large

volumes of mussel cooking juices.

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Further reading

Greiter, M., Novalin, S., Wendland, M., Kulbe, K.-D., & Fischer, J.

(2002). Desalination of whey by electrodialysis and ion exchange

resins: analysis of both processes with regard to sustainability by

calculating their cumulative energy demand. Journal of Membrane

Science, 210, 91–102.

Guerif, G. (1993). Electrodialysis applied to tartaric stabilisation of

wines. Revue des Oenologues et des Techniques Viticoles etŒnolog-

iques, 69, 39–42.

Le Guen, S., Prost, C., & Demaimay, M. (2001). Evaluation of the

representativeness of the odor of cooked mussel extracts and the

relationship between sensory descriptors and potent odorants.

Journal of Agricultural and Food Chemistry, 49(3), 1321–1327.

Perez, A., Andres, L. J., Alvarez, R., Coca, J., & Hill, C. G. (1994).

Electrodialysis of whey permeates and retentates obtained by

ultrafiltration. Journal of Food Production Engineering, 17,

177–190.

Shin, T.S., Ahn, C.B., Kim, H.R., 2001. Effect of desalinization of

oyster cooking effluent with electrodialyzer on flavor and sensory.

In IFT annual meeting, 73F-21.

Yasuhara, A. (1987). Comparison of volatile components between

fresh and rotten mussels by gas chromatography–mass spectrom-

etry. Journal of Chromatography, 409, 251–258.