COMPARISON OF THERMAL PROCESSING AND PULSED ELECTRIC FIELDS TREATMENT IN PASTEURIZATION OF APPLE JUICE

COMPARISON OF THERMAL PROCESSING
AND PULSED ELECTRIC FIELDS TREATMENT
IN PASTEURIZATION OF APPLE JUICE
A. V. Charles-Rodr?´guez1, G. V. Neva´rez-Moorillo´n1, Q. H. Zhang2 and
E. Ortega-Rivas1,Ã
1Postgraduate Programme in Food Science and Technology, Autonomous University of Chihuahua,
Chihuahua, Me´xico.
2Department of Food Science and Technology, Ohio State University, Columbus, OH, USA.
Abstract: Apple juice is a popular beverage worldwide, which is perceived as a wholesome and
nutritious product. Overall quality of apple juice is an important factor to consider in processing,
since some attributes, such as aroma, colour and ?avour, are well appreciated by the ?nal con-
sumer, and are associated with freshness and authenticity. Non-thermal processing was tested
in apple juice pasteurization in order to verify its feasibility in microbial inactivation, as well as its
possibility of rendering a product impaired in terms of some sensory attributes. The non-thermal
technique of high voltage pulsed electric ?elds (PEF) treatment, was compared with conventional
high temperature-short time (HTST) pasteurization. Effects of process variables, such as voltage
intensity and frequency for the PEF treatment, as well as temperature and time for the HTST pas-
teurization were investigated over inactivation of Escherichia coli and changes of pH and colour.
Both techniques achieved more than ?ve log reductions in microbial inactivation, normally con-
sidered the standard for fruit juices pasteurization. Apparently, PEF preserved better the pH than
HTST as the thermal treatment showed an increase in this physicochemical property. Some
variability was observed in terms of colour for all the treatments.
Keywords: apple juice; heat pasteurization; non-thermal pasteurization; high voltage pulsed
electric ?elds.
INTRODUCTION
Ortega-Rivas et al., 1998; Evrendilek et al.,
à Correspondence to:
1999; Za´rate-Rodrl´guez et al., 2000).
Dr E. Ortega-Rivas,
Apple juice has been traditionally pasteurized
Although fundamental research on PEF is
Postgraduate Programme in
by thermal means using continuous pasteuri-
advancing while its commercial application
Food Science and
zation, which may be carried out by passage
has
been
reported
feasible
(Braakman,
Technology, Autonomous
University of Chihuahua,
through plate heat exchangers, and by
2003), there are only few comparative studies
Chihuahua, Mexico.
tunnel pasteurizers. Currently, HTST pasteur-
of PEF against conventional thermal treat-
E-mail: eortegar@uach.mx
ization is the mode commonly used for heat
ment in the literature (e.g., Wouters et al.,
treatment of apple juice. In HTST pasteuriza-
1999). More comparative studies are needed
DOI: 10.1205/fbp06045
tion the temperature used is 76.6 – 87.78C for
in order to determine whether PEF treatment
0960–3085/07/
a holding time between 25 and 30 s (Moyer
may produce apple juice safe and natural,
$30.00 þ 0.00
and Aitken, 1980). Thermal processing inacti-
under the same conditions of the traditional,
vates spoiling micro-organisms ef?ciently, but
thermal pasteurization method. This paper
Food and Bioproducts
may also degrade taste, colour, ?avour and
presents the results of an investigation
Processing
nutritional quality of foods (Qin et al., 1995a).
aimed at comparing aspects of microbial
Trans IChemE,
PEF treatment is a promising non-thermal
safety and quality assurance of apple juice
Part C, June 2007
processing
method
that
may
radically
treated by HTST and PEF.
change liquid food preservation technology.
# 2007 Institution
PEF
inactivates
micro-organisms
and
of Chemical Engineers
enzymes with only a small increase in temp-
MATERIALS AND METHODS
erature, simultaneously providing consumers
with safe, nutritious and fresh-like quality
Freshly squeezed apple juice of the Gala
foods. Several studies of PEF treatment of
variety was obtained using a domestic juice
apple juice have appeared in the literature
extractor. The juice was stored for 24 h at
(Qin et al., 1994, 1995b; Mittal, 1998;
48C, then pre-?ltered across a bag ?lter
93
Vol 85 (C2) 93 – 97

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94
CHARLES-RODRI´GUEZ et al.
(1 mm pore size), and ?nally processed by the pasteurization
methods previously described. Escherichia coli 8739 was
grown in tryphtic soy broth (TSB) and incubated at 358C
until reaching an absorbance of 1.8 at 600 nm, corresponding
to an end logarithmic growth phase with a viable count of
80 Â 107 colony forming units per millilitre (cfuml21). E. coli
was chosen as a main indicator for microbial inactivation as
outbreaks of this bacterium were observed in the USA in
1996 (Heinz et al., 2003). The grown micro-organism was
suspended in the apple juice 30 min before treatment. The
viability of E. coli before and after testing by HTST and PEF
was assayed by counting colony forming units on violent
red brilliant agar (VRBA). One microlitre of the treated juice
was used, serially diluted with 0.1% sterile peptone solution,
and 100 ml of dilution plated on VRBA at 358C for 48 h. The
treated ?uids were diluted to obtain colony forming units
between 20 and 200. The mean was calculated from four
plates.
For
conventional
heat
treatment,
an
experimental
set-up was constructed. It consisted of sanitary containers
to hold heating and cooling ?uids, coils for juice passage, a
centrifugal sanitary pump to circulate the juice, and thermo-
couples to record the temperature. Pasteurization tempera-
tures of 73, 80 and 838C were tested at a holding time of
27 s.
Figure 1. Diagram of pulsed electric ?eld treatment operation.
A Bench Scale Pulsed Electric Field Processor Model OSU
4-H (Ohio State University, Columbus, OH, USA) was used
for the PEF treatment. Its operating details are described in
Boston, MA, USA) was used. Colour components L (bright-
Table 1 and a schematic diagram illustrated in Figure 1. Elec-
ness/whiteness), a (redness) and b (yellowness), based on
tric ?eld strengths of 12, 24 and 36 kV cm21 and replication
the Hunter system, were measured with a Hunter Lab Ultra
rates of 400, 600 and 800 pulses per second (pps) were
Scan XE (Hunter Laboratory, Reston, VA, USA) colorimeter.
studied.
To analyse the results multiple linear regression was used
The effects of all the variables mentioned above on the
for each effect studied based on correlation values and model
pasteurization methods were evaluated for effects on pH
signi?cant levels. All means were subjected to Student’s
and colour of the pasteurized juice. To measure pH an
t-test to verify signi?cant differences between the quality attri-
Orion Benchtop pH-meter Model 370 (Orion Research Inc.,
butes investigated. Signi?cance of differences was de?ned at
P , 0.05. The actual calculations were carried out utilizing
the Statistica package (Statsoft Inc., Tulsa OK, USA).
Table 1. Characteristics of PEF unit (OSU 4-H).
RESULTS AND DISCUSSION
Description
Operating conditions
As mentioned above, multiple linear regression was used
A Pulse generator module
to ?t the experimental data. For the case of HTST testing,
Operating voltage
0 – 12 kV
regression equations could be ?tted to describe properly an
Peak current
0 – 100 A
Polarity
Bipolar or unipolar
inactivation of E. coli and an increase in pH, both directly
Wave shape
Square wave
related to temperature. Pertaining colour, no regression
Pulse duration
1 – 10 ms
expression could be ?tted at any signi?cant degree of
Repetition rate
Single shot to 2000 ppsÃ
correlation.
B Treatment chamber module
High correlation coef?cients were found for the regression
Number of chambers
2 – 8
Cooling capacity
108C per chamber pair
models ?tted, having values of 0.9725 for the survival ratio
System ?ow rate
0.5 – 8 ml s21
and 0.9909 for the pH effect. In terms of pH, an increase
Number of pulses
9 (under 1500 pps
directly related with temperature was observed (Figure 2),
per chamber
unipolar output with
which could be explained in terms of the evaporative effect
2 ml s21 ?ow rate)
Total treatment time
190 ms (using eight
of organic acids. This variation of pH with temperature
chambers, 1500 pps,
could have an effect in shelf life, since higher pH could
2 ml s21 ?ow rate, and
raise the possibility of yeasts growth or enzyme activity.
2.5 ms pulse duration)
As previously mentioned, no regression expression could
C Fluid handling module
be ?tted at any signi?cant degree of correlation for colour
Batch size
60 ml in syringes
Type of ?uid
Liquid, viscosity ,0.2 Pas
change. Although colour differences were not highly signi?-
Back pressure
1.034 bar
cant, the graphical variation of colour parameters L, a and
Syringe pump for
b as a function of temperature, as given in Figure 3,
automated operation
showed a similar trend somewhat. The parameter L rep-
ÃPulses per second.
resents brightness/whiteness of a given sample, so it can
Trans IChemE, Part C, Food and Bioproducts Processing, 2007, 85(C2): 93 – 97

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COMPARISON OF THERMAL PROCESSING AND PULSED ELECTRIC FIELDS
95
Figure 4. Survivor counts of E. coli as a function of electric ?eld
strength.
Figure 2. pH as a function of pasteurization temperature.
the 858C treatment is not totally understood, but may have
be considered an indicator of lightness, taking values
been due to improper operation, which may have caused
between L ¼ 100 (totally white) and L ¼ 0 (totally black).
oxygen uptake by the juice, with a consequent browning
A decrease in L value is, therefore, a direct indicative of
due to combination of enzymic and non-enzymic reactions.
browning (Petriella et al., 1985; Sapers et al., 1989). As
Treatment using PEF was also ef?cient in terms of
can be observed, a slight drop in all parameter values for
microbial inactivation. Figures 4 and 5 present survivability
the minimum pasteurizing temperature was followed by a
of E. coli against electric ?eld strength and pulse rates
rise for the intermediate treatment temperature and then by
respectively. In terms of intensity, as observed in Figure 4,
a ?nal drop, again, to approximate original values. Colour
seven log reductions were obtained at the highest applied
changes in treated fruit juices are complex and cannot be
strength of 36 kV cm21. Using this strength, up to six log
explained by a single processing factor. Colour deterioration
reductions were achieved at the maximum frequency of
in fresh and processed fruits have been normally related to
800 pps. These results are in agreement with previous
enzymic browning reactions (Giner et al., 2001; Van Loey
studies of E. coli inactivation in apple juice, con?rming that
et al., 2002), although non-enzymic Miallard type reactions,
PEF is a viable, non-thermal alternative for pasteurizing
can also in?uence colour changes (Moyer and Aitken,
apple juice (Zhang et al., 1994).
1980). The above mentioned trend could be explained in
In contrast with the variability observed in the conventional
terms of non-enzymic browning, which is known to be
thermal treatment, neither pH nor colour presented de?nite
caused by reactions of amino acids, sugars and organic
trends for the PEF pasteurization. Figure 6 shows values of
acids (Moyer and Aitken, 1980). Since the juice was, inevita-
pH against pulse frequency for the optimum inactivation elec-
bly, aerated during extraction, some enzymic browning could
trical ?eld strength of 36 kV cm21. As can be observed, pH
have also developed prior to treatment. The moderate temp-
value does not vary at any treatment con?rming that variation
erature in the ?rst treatment possibly did not triggered chemi-
for the case of thermal treatment could has been caused by
cal reactions responsible for browning. The apparent clarity
evaporative effects. Pertaining colour change as a conse-
of the juice treated at 808C, compared with the colour of
quence of browning, values of colour parameters are plotted
the untreated one, may have been caused by speci?c reac-
against pulse frequency, for the optimum inactivation strength
tions at that range of temperature. The slight browning at
value of 36 kV cm21, as before. The values of colour
Figure 3. Colour parameters as a function of pasteurization
temperature.
Figure 5. Survivor counts of E. coli as a function of pulse frequency.
Trans IChemE, Part C, Food and Bioproducts Processing, 2007, 85(C2): 93 – 97

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96
CHARLES-RODRI´GUEZ et al.
Table 2. Effect of treatment mode on some quality attributes of
pasteurized apple juice.a
Attribute
Untreated
PEF
HTST
pH
3.75b
3.73 + 0.01b
3.99 + 0.02c
L
37.28b
36.70 + 1.36b
36.07 + 4.70b
Colour parameters:
a
9.22b
9.63 + 0.98c
9.62 + 2.38c
b
6.72b
6.73 + 0.91c
6.97 + 2.50d
aMeans + standard deviation (n ¼ 4).
b – dAny two means in the same row followed by the same letter are
not signi?cantly (P , 0.05) different by Student’s t-test.
control sample might be due to enzymic browning in the
unprocessed juice, as a function of time. More research is
suggested to investigate the colour fading effect of the
Figure 6. pH as a function of pulse frequency for the optimum electric
?eld strength of 36 kV cm21.
juices treated by high voltage pulsed electric ?elds.
Comparison of means of the above discussed effects of
pasteurizing techniques on quality attributes are presented
parameters L, a and b remained virtually unchanged
in Table 2. As can be observed, means are, generally, signi?-
(Figure 7) as a function of applied frequency.
cantly different only for the pH of the thermally treated juice
Despite the above discussed colour stability of PEF treat-
compared with the control and the non-thermal processes.
ments, regression equations could be properly ?tted to rela-
The observed browning of the HTST process is considered
tive colour changes as a function of ?eld strength. In this
a quality problem, so the apparent ef?ciency of the PEF treat-
case, the mentioned model described a trend of the juice to
ment in avoiding browning is an aspect that should be inves-
become lighter as a function of ?eld strength. The colour
tigated further.
difference between the PEF-pasteurized juice and the
untreated sample was not, however, statistically signi?cant.
This apparent colour fading could be explained in terms of
CONCLUSIONS
enzymic and non-enzymic browning of apple juice. Enzyme
PEF, used as non-thermal preservation technique to pas-
inactivation by PEF has been investigated in milk (Vega-Mer-
teurize apple juice, proved to be ef?cient in microbial inacti-
cado et al., 1995) being reported that plasmin in a simulated
vation, as well as in preserving some quality attributes.
milk ultra?ltrate was reduced 90% after 50 pulses at 30 and
Conventional HTST pasteurization, on the other hand, had
45 kV cm21. A review paper (Castro et al., 1993) has also
apparent effects on pH and browning increase. Browning
reported enzyme inactivation by PEF in milk. Furthermore,
effects on thermally pasteurized juice presented, however,
effects of PEF processing on inactivation of polyphenol oxi-
contrasting results. The colour of the obtained juice pre-
dase (PPO) using extracts from apple and pear, have also
sented changes which were statistically signi?cant for both
been reported (Giner et al., 2001). These authors report
treatments, but PEF seemed to conserve more the colour
reduction of PPO activities up to 38% for both fruit extracts
of the natural product. More research is needed to under-
and were able to ?x a ?rst order kinetics model for the inac-
stand colour changes, which are very important in overall
tivation process. Thus, in this study PEF may have presented
quality of apple juice. Further studies on the chemistry of
enzyme inactivation capability and the observed difference
?avour components, by instrumental methods, would be advi-
in luminescence L between the PEF-treated juice and the
sable. It is also recommended to use sensory evaluation to
de?ne quality differences of apple juice treated by non-
conventional methods. The applicability of PEF as a non-
thermal preservation technology for pasteurization of fruit
juices, such as apple juice, has been shown in many
research reports. There is a perception, however, that high
operating costs and relative small scale may inhibit its indus-
trial exploitation. There are, nevertheless, industrial-scale
equipment available, at affordable running costs. Wide com-
mercialization may depend, therefore, on convincing inves-
tors with studies such as the one reported here, in which
direct comparisons with traditional industrial processes may
con?rm that PEF is a feasible alternative to obtain safe and
fresh-like fruit juices.
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Figure 7. Colour parameters as a function of pulse frequency for the
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Trans IChemE, Part C, Food and Bioproducts Processing, 2007, 85(C2): 93 – 97

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ACKNOWLEDGEMENTS
activity: Color changes due to nonenzymatic browning, J Food
Sci, 50(3): 622 – 626.
The main experimental part of this project was carried out at the
Qin, B., Zhang, Q., Barbosa-Ca´novas, G.V., Swanson, B.G. and
Department of Food Science of Ohio State University, USA. The
Pedrow, P.D., 1994, Inactivation of microorganisms by pulsed elec-
authors wish to express their gratitude for the assistance provided
tric ?elds of different voltage waveforms, IEEE Trans Dielec Elect
by technical and academic staff. The stay of author Charles-
Insul, 1(6): 1047– 1050.
Rodr?´guez in Ohio State University was possible thanks to funding
Qin, B.L., Pothakamury, H., Vega, H., Martin, O., Barbosa-
provided, as a MSc scholarship grant, by the National Council of
Canovas, G.V. and Swanson, B.G., 1995a, Food pasteurization
Science and Technology (CONACyT, Me´xico).
using high intensity pulsed electric ?elds, Food Technol, 49(12):
55 – 60.
The manuscript was received 12 July 2006 and accepted for
Qin, B.L., Chang, F.J., Barbosa-Canovas, G.V. and Swanson, B.G.,
publication 12 December 2006. The paper was published online
1995b, Nonthermal inactivation of Saccharomyces cerevisiae in
ahead of print 18 May 2007.
Trans IChemE, Part C, Food and Bioproducts Processing, 2007, 85(C2): 93 – 97

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Document Outline

• COMPARISON OF THERMAL PROCESSINGAND PULSED ELECTRIC FIELDS TREATMENTIN PASTEURIZATION OF APPLE JUICE
  • INTRODUCTION
  • MATERIALS AND METHODS
  • RESULTS AND DISCUSSION
  • CONCLUSIONS
  • REFERENCES
  • ACKNOWLEDGEMENTS

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