Thermal and pulsed electric fields pasteurization of apple juice were compared. Both methods effectively reduced spoilage microorganisms like Lactobacillus brevis and Saccharomyces cerevisiae. However, pulsed electric field (PEF) pasteurization had less effect on physicochemical properties and volatile flavor compounds compared to thermal pasteurization. Specifically, pH and total acidity were minimally affected by both methods, but phenolic content and concentrations of volatile compounds showed greater retention after PEF treatment versus thermal treatment. PEF pasteurization is a promising non-thermal alternative for maintaining higher quality attributes in apple juice.

1. Thermal and pulsed electric fields pasteurization of apple juice:
Effects on physicochemical properties and flavour compounds
S.F. Aguilar-Rosas a
, M.L. Ballinas-Casarrubias a
, G.V. Nevarez-Moorillon a
,
O. Martin-Belloso b
, E. Ortega-Rivas a,*
a
Food and Chemical Engineering Programme, Autonomous University of Chihuahua, University Campus I, Chihuahua, Chih. 31170, Mexico
b
Department of Food Science and Technology, University of Lleida, Av. Alcalde Rovira Roure 177, 25198 Lleida, Spain
Received 6 September 2006; received in revised form 14 December 2006; accepted 15 December 2006
Available online 11 January 2007
Abstract
Apple juice, extracted from golden delicious fruits, was pasteurized using a pulsed electric field (PEF) treatment and compared with a
conventional high temperature-short time (HTST) method. The PEF treatment was carried out using a PEF laboratory unit, set with a
bipolar pulse (4 ls wide), an intensity of 35 kV/cm, and a frequency of 1200 pulses per second (pps). The thermal pasteurization was
performed at 90 °C for 30 s with an adapted laboratory set-up. Effects of variables of both treatments on pH, total acidity, phenolics
content, and volatile compounds were investigated. While minimal variability was observed in pH and no significant changes were
detected in acidity, phenolics content and volatile compounds concentration showed statistical significant differences between treatments.
In general, these measured variables were less affected by the PEF treatment than by the thermal pasteurization.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Apple juice; Thermal pasteurization; High voltage pulsed electric fields (PEF); High temperature-short time (HTST) pasteurization; Sensory
attributes
1. Introduction
Apple juice has been traditionally pasteurized by ther-
mal means. Both batch and continuous methods are used
in apple juice pasteurization and the treatment may be car-
ried out before or after packing the product in the con-
tainer. In batch pasteurization, individual volumes are
treated in jacketed stainless steel vessels. The jacket may
be used both for heating (with steam or hot water) and
cooling (with chilled water or brine). Continuous pasteuri-
zation may be carried out by passing the juice through
plate heat exchangers, which usually comprise the stages
of pre-heating, heating, holding and cooling. Currently,
high temperature-short time (HTST) pasteurization is a
commonly used method for heat treatment of apple juice.
In HTST pasteurization, the temperature used is 76.6–
87.7 °C for a holding time between 25 and 30 s (Moyer &
Aitken, 1980).
Thermal pasteurization is quite efficient in preventing
microbial spoilage of apple juice but the applied heat
may also cause undesirable biochemical and nutritious
changes which may affect overall quality of the final prod-
uct. Alternative methods of pasteurization that do not
include direct heat have been investigated in order to
obtain a product safe for consumption, but with sensory
attributes similar to the untreated juice. High voltage
pulsed electric fields (PEF) treatment is a promising non-
thermal processing method that may radically change
liquid food preservation technology. Treating liquid foods
with PEF may inactivate micro-organisms and enzymes
with only a small increase in temperature, simultaneously
providing consumers with safe, nutritious, and fresh-like
0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2006.12.011
*
Corresponding author. Tel./fax: +52 614 4241868.
E-mail address: eortegar@uach.mx (E. Ortega-Rivas).
www.elsevier.com/locate/jfoodeng
Journal of Food Engineering 83 (2007) 41–46

2. quality foods. PEF treatment is conducted at ambient tem-
perature for a short time (in microseconds), and energy lost
due to heating of foods is minimized (Jeyamkondan, Jayas,
& Holley, 1999).
In terms of microbial safety and energy efficiency, a
study of PEF inactivation demonstrated that, for achieving
a seven log reduction in survivability of Saccharomyces
cerevisiae in apple juice, PEF utilized less than 10% of
the electric energy for heat treatment (Qin, Zhang, Barb-
osa-Ca´novas, Swanson, & Pedrow, 1994). It has also been
reported (Mittal, 1998) that a PEF low energy pulser with
an instant-charge-reversal pulse waveform was successfully
used in apple cider treatment. The consumed energy was as
low as 5.76 J/ml at 20 °C, compared with the 50 kJ/kg nor-
mally required in conventional thermal processing. Micro-
bial inactivation, coupled with quality retention, has also
been reported for apple juice pasteurization using non-
thermal methods of preservation (Ortega-Rivas, Za´rate-
Rodrı´guez, & Barbosa-Ca´novas, 1998). A comparison of
ultrafiltration (UF) and PEF in apple juice pasteurization
reported six log reductions in survivability of total aerobic
micro-organisms using the indigenous flora of the juice
(Ortega-Rivas et al., 1998). In terms of quality aspects, sol-
uble solids, pH and acidity were reported practically
impaired by both techniques. Colour, however, suffered
changes such as browning for UF and fading for PEF (Ort-
ega-Rivas et al., 1998; Za´rate-Rodrı´guez, Ortega-Rivas, &
Barbosa-Ca´novas, 2000).
Flavour components in apple juice are numerous, and
flavour identification is considered quite complex due to
the aromatic nature of apples. Eight odour-active volatiles
have been, however, identified as the most important con-
tributors for the aroma–flavour authenticity of apple juice
(Cunningham, Acree, Barnard, Butts, & Braell, 1986).
Apparently, there are not reported studies of PEF effects
on volatile compounds in apple juice. Several reports have
appeared for orange juice (Jia, Zhang, & Min, 1999; Yeom,
Streaker, Zhang, & Min, 2000) focusing on effects of PEF
on quality aspects. The PEF-treated juice was compared
with juice pasteurized by heat at 94.6 °C for 30 s. The juice
treated by PEF retained greater amounts of vitamin C and
some representative flavour compounds, than the juice pas-
teurized by heat during storage at 4 °C. In terms of specific
flavour compounds, it was found that 40% of decanal was
lost by heat treatment at 90 °C for 3 min while no loss was
observed by PEF treatment at 30 kV/cm, either at 240 or
480 ls (Jia et al., 1999). Octanal showed a loss of 9.9%
for the heat treatment and 0% for any of the two PEF
treatments. Some compounds suffered losses for the PEF
treatments, but always in less proportion than the heat pas-
teurized juice. For example, 5.1% and 9.7% of ethyl buty-
rate were lost for the 240 ls and 480 ls treatments,
respectively, but 22.4% was lost in the thermal process
(Jia et al., 1999).
As discussed above, PEF has been challenged against
many spoiling micro-organisms in apple juice, with encour-
aging results. Also, pertaining quality, there are studies
looking at effects on physicochemical properties and some
sensory attributes, with results also being promising. For
example, Evrendilek et al. (2000) reported no apparent
changes in physical and chemical properties directly caused
by PEF treatment in apple juice and cider, while Barbosa-
Canovas, Pothakamury, Palou, and Swanson (1998) found
that pH and vitamin C concentration were not significantly
affected by PEF treatment of fresh apple juice and apple
juice reconstituted from concentrate. There is, however, a
dearth of information in the literature related to actual
effects of PEF on composition of volatile chemical com-
pounds responsible for odour and flavour of apple juice.
There are neither many direct comparisons of PEF and
HTST treatments, in terms of quality attributes in general.
This paper presents an investigation of a direct comparison
of PEF and HTST in pasteurization of apple juice, focused
on retention of volatile compounds, which have been iden-
tified as responsible for its characteristic aroma and tasteful
flavour.
2. Materials and methods
Freshly squeezed apple juice, from golden delicious
apple variety, was extracted with a domestic juice extrac-
tor. The juice was pre-filtered across a bag filter and stored
at 4 °C prior to treatment.
For conventional heat treatment, an experimental set-up
was constructed (Fig. 1). As can be observed, it consisted of
sanitary containers to hold heating and cooling fluids, coils
for juice passage, a centrifugal sanitary pump to circulate
the juice, and thermocouples to record the temperature.
A pasteurization temperature of 90 °C was tested for a
holding time of 30 s, which was virtually the maximum
range suggested in the literature (Moyer & Aitken, 1980).
Also, it was sufficient to achieve pasteurization conditions
using Lactobacillus brevis and S. cerevisiae, common spoil-
age micro-organisms in apple juice, as contaminating spe-
cies. As shown in Fig. 2, inoculates of L. brevis and S.
cerevisiae, expressed in colony forming units per millilitre
(cfu/ml) were properly reduced.
A high voltage pulsed electric field unit, designed and
constructed at Ohio State University (Columbus, OH,
USA) was used for the PEF treatment. As shown in
Fig. 3, this test apparatus consists of a high voltage power
Feed
Pre-heating Holding Cooling
Product
Fig. 1. Experimental set-up used for heat pasteurization of apple juice.
42 S.F. Aguilar-Rosas et al. / Journal of Food Engineering 83 (2007) 41–46

3. supply, a high voltage pulse generator, a series of treatment
chambers, and sample cooling and delivery devices. Fol-
lowing recommendations in the literature (Heinz, Toepfl,
& Knorr, 2003), a 4 ls bipolar pulse with an electric field
strength of 35 kV/cm was chosen to destroy the same spoil-
age micro-organisms mentioned above and select the
appropriate frequency for further treatment. Such
microbes were inoculated and inactivated using the previ-
ously mentioned conditions at different repetition rates.
Using 1200 pps, 6.3 and 4.2 log reduction cycles were
achieved for the bacterium and yeast, respectively
(Fig. 4), which were considered appropriate so this fre-
quency was used for experimentation.
The pH was measured by direct reading at 25 °C in an
Orion Benchtop pH/ISE-meter Model 420 A (Orion
Research Inc., Boston MA, USA). Acidity was measured
by titration with 0.1 N NaOH to a pH end-point of 8.2,
the result being expressed as g malic acid/l of sample
(AOAC, 1998).
Total phenol content was determined by the Folin–Cio-
calteu method (Singleton & Rossi, 1965), reading samples
in a HP 845 A UV/visible spectrophotometer (Hewlett–
Packard Inc., Palo Alto, CA, USA) at 760 nm. Samples
were centrifuged at 2000g (4 °C for 5 min) and diluted by
a factor of 10 with distilled water. Results were expressed
as mg of gallic acid/l of juice.
The flavour compounds in the headspace of the apple
juice were analysed by solid-phase microextraction (SPME)
and gas chromatography (Buchholz & Pawliszyn, 1994).
Samples of 10 ml of apple juice were transferred into
30 ml vials. The SPME fibre coated with 100 lm poly-
dimethylsiloxane was inserted into the headspace of the
juice and heated at 50 °C for 30 min. The SPME sample
was removed from the sample vial and inserted into a gas
chromatograph (GC) injection port, and held for 4 min
at 250 °C to desorb the flavour compounds absorbed on
the SPME coating. The desorbed flavour compounds were
separated using an Agilent 5973 Network GC/MS equip-
ment (Agilent Technologies, Palo Alto, CA, USA)
equipped with a capillary column of 0.25 mm internal
diameter Â30 m length, and coated with 0.25 lm thick
diphenylpolysiloxane. Helium was the carrying gas at a
rate of 1.5 ml/min. The GC oven temperature was pro-
grammed from 40 to 250 °C at 20 °C/min and held
10 min at the final temperature. At the end of the experi-
mental run, volatiles were qualified using the library in
the program of the instrument. After identification of vol-
atile compounds, calibration curves were derived for every
one using the authentic volatiles. The presence of the main
volatiles reported to be present in apple juice were con-
firmed by comparing the retention times of gas chromato-
graphic peaks to those of authentic compounds.
The results were interpreted using simple analyses of
variance (ANOVAS). For the volatile compounds, a Stu-
0
1
2
3
4
5
6
7
8
9
L. brevis S. cerevisiae
Microbial Inactivation by HTST Method
log(cfu/ml)
Initial count
Final count
Fig. 2. Inactivation of Lactobacillus brevis and Saccharomyces cerevisiae
by HTST pasteurization (90 °C for 30 s).
High voltage
power supply
High voltage
pulse
generator
Water bath
Treatment
chambers
HV
GND
T1T2
Feed
Product
Fig. 3. Diagram of pulsed electric field treatment operation.
0
1
2
3
4
5
6
7
8
9
0 400 800 1200 1600
Pulses per second of PEF treatment
log(cfu/ml)
L. brevis
S. cerevisiae
Fig. 4. Inactivation of Lactobacillus brevis and Saccharomyces cerevisiae
by PEF pasteurization (pulse frequency at 35 kV/cm).
S.F. Aguilar-Rosas et al. / Journal of Food Engineering 83 (2007) 41–46 43

4. dent t-test for independent samples was used. Means were
differentiated by Tukey’s tests. Significance of differences
was defined at p < 0.05. The tests were performed in tripli-
cate. The statistic system SAS Version 8 (SAS Institute
Inc., Cary, NC, USA) was used for actual calculations.
3. Results and discussion
An ANOVA for pH determinations showed statistical
significant difference (F = 130.40, p = 0.0001) between the
untreated apple juice and both the HTST-pasteurized and
the PEF-treated samples, as illustrated in Fig. 5. However,
by inspecting such figure, it can be observed that such dif-
ferences may be considered negligible for practical pur-
poses, since the measured pH in the three samples, only
vary between 3.8 and 3.9. The observed small discrepancies
could be, possibly, attributed to experimental error. These
findings agree with an investigation by Heinz et al. (2003)
who reported that PEF-pasteurized apple juice did not
show practical difference in pH. On the other hand,
Charles-Rodrı´guez (2002) found that thermally-treated
apple juice presented an increase in pH directly related with
temperature, reaching a value of 4.01 at the extreme pas-
teurization conditions of 85 °C and 27 s. This author also
reported that PEF-pasteurized juice did not show variabil-
ity in pH at different electric field strengths and frequencies.
It is known that maintaining pH on low values prevent
pathogenic microbial growth in fruit juices, so PEF treat-
ment gives, apparently, more stability to pH in apple juice
than HTST pasteurization.
In terms of acidity, no significant statistical difference
was observed for any of the treatments (F = 0.94,
p = 0.4404), as shown in Fig. 6. These results are in agree-
ment with the study of Heinz et al. (2003) already men-
tioned, who also reported no significant changes in
acidity of PEF-pasteurized apple juice. Ortega-Rivas
et al. (1998) presented results of a comparison of UF and
PEF in some physicochemical properties of apple juice,
finding that acidity did not present significant variability.
Acidity in apple juice is an important sensory attribute
associated with its characteristic flavour and astringency.
Apparently, PEF-pasteurized apple juice do not affect acid-
ity, so this important feature remains practically intact with
the consequently advantage in overall quality of the
product.
Considering the results previously discussed, apparently,
PEF maintained the physicochemical properties of the ori-
ginal juice, while HTST affected the pH and in a less extent
the acidity of the juice, as shown in Figs. 5 and 6. Apart
from the experimental error that may have caused some
discrepancies in readings, the changes observed in the ther-
mal method could be attributed to the evaporative effect of
organic acids as a function of temperature increase.
Contents of total phenol compounds presented variabil-
ity for the two compared pasteurization methods. The
ANOVA in this case indicated statistical significant differ-
ence (F = 44.4, p = 0.0003) between both treatments and
the control (Fig. 7). A Tukey test confirmed the difference
of means for the three samples. It can be observed in Fig. 7,
however, that the HTST treatment caused a considerable
3.74
3.76
3.78
3.8
3.82
3.84
3.86
3.88
3.9
3.92
3.94
Untreated PEF HTST
Treatment
pH
Fig. 5. Effect of treatment method on pH of pasteurized apple juice.
0.31
0.315
0.32
0.325
0.33
0.335
0.34
0.345
0.35
0.355
Untreated PEF HTST
Treatment
gmalicacid/litre
Fig. 6. Effect of treatment method on acidity of pasteurized apple juice.
0
20
40
60
80
100
120
Untreated PEF HTST
Treatment
ppmofgallicacid
Fig. 7. Effect of treatment method on total phenol compounds of
pasteurized apple juice.
44 S.F. Aguilar-Rosas et al. / Journal of Food Engineering 83 (2007) 41–46

5. lost of phenols (32.2%) when compared with the PEF treat-
ment, which only caused a 14.49% reduction. These results
agree with Spanos and Wrolstad (1992), who reported that
total phenol concentration is reduced up to 50% in apple
juice pasteurized thermally at 80 °C for 15 min. Gardner,
White, McPhail, and Duthie (2000) observed also consider-
able losses in phenolics in apple juice pasteurized by ther-
mal means. Phenol compounds are secondary metabolites
in plants known to play an important role in colour and
flavour development in fruit juices and wine. Phenols are
important constituents of pear, grapes and apple and
may be categorized into two groups: phenolic acids and
flavonoids (Spanos & Wrolstad, 1992). The combined
odour–flavour characteristics in apple and apple products
are due in part to phenol compounds. Phenols are also used
as indicators of physiological state and potential damage in
quality of fruit products (Blanco, Fraga, & Mangas, 2001).
Phenol compounds are, thus, important biochemical sub-
stances in apple juice. Their lost or decrease in concentra-
tion will, therefore, impair seriously apple juice sensory
attributes. PEF treatment with minimal losses (Fig. 7)
would represent an obvious advantage over HTST pasteur-
ization, in terms of concentration of these chemicals in
apple juice.
As previously stated, the flavour of apple juice consists
of many chemical compounds, but the literature indicates
8–23 compounds most responsible for the odour–flavour
attribute (Ko¨nig & Schreier, 1999; Rao, Acree, Cooley, &
Ennis, 1987). Eight volatile compounds were properly iden-
tified in the apple juice, fresh and processed by any of the
treatments, in this work. Table 1 compares the percentage
in concentration decrease for those mentioned volatiles in
the pasteurization methods investigated. All volatile con-
centrations showed statistical significant differences for
both treatments, as compared with the untreated sample,
by a Student t-test for independent samples (p < 0.05,
n = 3). In all compounds, and particularly in one (ethyl
acetate), the decrease was considerable higher for the
HTST treatment than for the PEF method (Table 1). Some
of the volatiles in the PEF treatment were almost retained.
Hexanal and hexyl acetate were only lost in 7% and 8.4%,
respectively. It is worth to mention (Table 1) that acetic
acid was completely lost in HTST treatment.
To the extent of the literature survey of this work,
there are not reported studies of PEF effects on volatiles
in apple juice. As previously discussed, several reports
have appeared for orange juice (Jia et al., 1999; Yeom
et al., 2000) focusing on effects of PEF on quality aspects.
Although orange juice and apple juice are different prod-
ucts, the studies mentioned for orange juice, along with
the results of this work, may be indicative of a definite
evaporative effect of thermal pasteurization methods in
fruit juice pasteurization in general. Thus, in a very broad
way, it may be considered that non-thermal pasteurization
techniques will represent a better choice for processing of
fruit juices in general, and apple juice in particular. Vola-
tile chemical compounds responsible for colour and fla-
vour of fruit juices are retained in a highest ratio, when
compared with fresh untreated samples, by the use of
non-thermal pasteurization techniques such as high volt-
age pulsed electric fields. PEF pasteurization of apple
juice may be, therefore, considered a feasible alternative
for fruit processors, in order to obtain a premium quality
product.
4. Conclusions
PEF, a thermal preservation technique to pasteurize
apple juice, proved to be efficient in microbial inactivation,
as well as in preserving some quality attributes. Conven-
tional HTST pasteurization, on the other hand, produced
significant losses in phenolic compounds and in volatiles
responsible for flavour. PEF-treated juice retained better
most of the volatile compounds responsible for colour
and flavour of the apple juice. Further studies on the chem-
istry of flavour components, to try to preserve them even
better, are advisable. It is also recommended to use sensory
evaluation to define quality differences of apple juice trea-
ted by non-conventional methods. The use of PEF as an
alternative to heat pasteurization of apple juice may be
considered a strategically important action to obtain a sen-
sory impaired product, highly competitive in global
markets.
Acknowledgements
An experimental part of this project was carried out at
the Department of Food Technology of University of
Lleida, Spain. The authors wish to express their gratitude
for the assistance provided by technical and academic
staff. Funding for the project was provided by the Na-
tional Council of Science and Technology (CONACyT,
Me´xico).
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