Food
Akhtar et al. J Food Process Technol 2011, 2:2
http://dx.doi.org/10.4172/2157-7110.1000108
Processing & Technology
Research Article
Open Access
Concentration of Apple Juice Using Spinning Disc Reactor Technology
Mahmood Akhtar*, Philip Chan, Novi Safriani, Brent Murray and Graham Clayton
School of Food Science and Nutrition, University of Leeds, Leeds LS2 9JT, UK
Abstract
The use of spinning disc reactor (SDR) technology for the concentration of apple juice was investigated. The
apple juice was passed over the SDR disc spinning at 2000rpm, heated at 90-120°C, and at a flow rate of 7mL s-1.
Experimental results showed that the SDR has no significant detrimental effects on the physicochemical properties
and quality of the apple juice after the concentration process, despite using high processing temperatures (90120°C). Due to the short residence time of the SDR, the thermally-induced colour change of the concentrates has
been minimized. All apple juice concentrate samples exhibited narrow particle size distributions with the average
particle size in the range 0.1 to 12µm. The SDR-made reconstituted apple juices are comparable to both the original
pure-pressed apple juice sample and the commercial reconstituted product. Volatile components (ester, aldehyde
and alcohol) collected during the concentration process were analyzed with GC-MS, and the analysis suggests the
formation of new aroma compounds (ethyl acetate, n-butyl alcohol and 2-hexenal) which can be added back to the
reconstituted apple juice to enhance its sensory quality.
Keywords: Concentration; Spinning disc reactor; Apple juice
concentrate; Physicochemical properties; GC-MS
was also retained. No significant colour change was observed in the
SDR-processed juice compared to the fresh juice sample.
Introduction
The SDR (type P201) used in this study (Figure 1) is comprised
of a 20 cm diameter disc with a heating and cooling facility in the
range of + 250°C to – 20°C by using heat transfer fluid (Paratherm
OR) in a recirculation bath. The spinning disc has a speed range of
100 to 3000rpm with sample feed rates in the range 0.5 to 7mL s-1. The
main vessel has been mechanically designed to withstand pressures
up to 5bar. A selection of two standard pumps can be incorporated
to the main controller, depending on the viscosity of the feed material
or the liquid sample to be used. The SDR technology works on the
principle that the feed liquid passes across the surface of a metal disc
which can be programmed to spin at controlled speeds, subjected to
heating and cooling at according to the product requirements. The
centrifugal force causes the liquid to form a very thin (typically several
µm in thickness) film on the disc, which gives very high heat transfer
coefficients between the disc and the liquid, as well as very high mass
transfer between the liquid and the gas phase above the liquid film. The
residence time on the disc is short, typically less than a second, so that
rapid pasteurization, followed by rapid cooling on the walls beyond the
disc edge, can be achieved. An inert carrier gas can also be introduced
to the system during the concentration process to facilitate evaporation
(Figure 1). In addition, the SDR method can be applied as a continuous
process on industrial scale.
Fruit juices are often concentrated to reduce their weight and
volume, and thus to reduce their packaging, transportation and storage
costs. Concentrated juices are biochemically stable and have a long shelf
life due to the reduction in the water activity [1]. However, conventional
thermal concentration techniques are known to cause losses of flavour
and aroma compounds, and have consequent undesirable effects on the
flavour characteristics of the juice products [2].
The aroma components of apple juice usually comprise a mixture
of volatile organic compounds, principally esters, aldehydes and
alcohols, but also ethers, fatty acids, lactones, terpenes and ketones
[3]. In general, the concentration of individual aroma components in
the apple juice ranges from less than 1 to 20ppm [4,5]. Each of these
compounds gives a typical character to the apple juice flavour. During
conventional thermal concentration processing, many of these volatile
compounds present in the apple juice are transformed (due to high
temperature) or lost (via water vaporization). The detrimental effects
were also reported in the manufacturing process of other fruit juice
such as orange juice [6,7] and pineapple juice [8]. These undesirable
effects can be significant, Peredi [9] reported more than half of the
tested volatile compounds were lost compared to the unprocessed
apple juice and Su and Wiley [10] demonstrated that almost all volatile
compounds in apple juice are lost during the thermal processing.
To generate a flavourful product, these aroma compounds must
be recovered and added back to the concentrate [11]. Therefore, it
is important to select the suitable technology for food processing,
especially for heat sensitive compounds, to produce high nutritive
value and appreciable organoleptic quality of the products.
In light of maximizing the efficiency of the concentration process
while maintaining the nutritional and sensory qualities of the final
product, the spinning disc reactor (SDR) could be an alternative
technology in the production of fruit juice concentrates. The SDR
has been widely used in emulsification, chemical and pharmaceutical
industries [12-15]. Recently, we have used the SDR technology in
pasteurizing carrot juice [12]. The results showed that the pasteurization
process is more controllable than the conventional technologies. Not
only was the processing time much reduced, but the quality of the juice
J Food Process Technol
ISSN:2157-7110 JFPT, an open access journal
In order to explore the potential of this new technology for the food
and drink industry, the aim of this work was to employ SDR for the
production of apple juice concentrate and to evaluate the efficiency of
*Corresponding author: Mahmood Akhtar, School of Food Science and Nutrition,
University of Leeds, Leeds LS2 9JT, UK, Tel:+44 (0) 113 3432952; fax: +44 (0)
1133432982; E-mail: [email protected]
Received January 20, 2011; Accepted March 25, 2011; Published March 27,
2011
Citation: Akhtar M, Chan P, Safriani N, Murray B, Clayton G (2011) Concentration
of Apple Juice Using Spinning Disc Reactor Technology. J Food Process Technol
2:108. doi:10.4172/2157-7110.1000108
Copyright: © 2011 Akhtar M, et al. This is an open-access article distributed under
the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited.
Volume 2 • Issue 2 • 1000108
Citation: Akhtar M, Chan P, Safriani N, Murray B, Clayton G (2011) Concentration of Apple Juice Using Spinning Disc Reactor Technology. J Food
Process Technol 2:108. doi:10.4172/2157-7110.1000108
Page 2 of 6
the concentration process. We also investigated the effect of processing
temperature on the physicochemical properties of the apple juice
concentrates, and characterized the SDR-made reconstituted apple
juice compared to a pure-pressed sample and a commercial product.
The objective of this study was to achieve a cost and energy efficient
concentration process without compromising the quality of the
product.
water to the concentrate II to make up to 500mL, i.e., same volume as
the unprocessed apple juice sample. The physicochemical properties of
the reconstituted juice were then compared to another commercially
manufactured apple juice (made from concentrate), containing a
measured value of 10.6°Brix, 3.02% acidity and a pH of 3.80.
Physicochemical properties
A 100% pure-pressed (not reconstituted) apple juice, purchased at
a local supermarket (Sainsbury’s Supermarkets Ltd., UK.), containing a
measured value of 10.5°Brix, 2.07% acidity, pH 4.0 and a trace amount
of added ascorbic acid, was used for this study. The sample had already
been pasteurized by the manufacturer and was stored in a refrigerator
at 5°C prior to testing.
A digital pH meter (Hanna Instruments Ltd., Bedfordshire, England
and UK.) was used to measure the pH of the apple juice samples at
5°C. The acid content of the juices was determined using an automatic
titrator, Titration Excellence T50 (Mettler-Toledo AG, Schwerzenbach,
Switzerland), equipped with a DGi111-SC titration sensor. An aliquot
(40mL) of the sample was titrated against a 0.1M NaOH to an endpoint of 8.1. The soluble solids, expressed in °Brix, were measured in
a digital refractometer (Bellingham+Stanley Ltd., Kent, England, UK.)
with 0.1 resolution. Each physicochemical parameter was measured in
triplicate.
Concentration of apple juice
Rheological measurement
A 500mL sample of apple juice was fed into the SDR at a flow
rate of 7mL s-1, and passed continuously over the SDR disc rotating
at 2000 rpm, set at 90°C – 120°C. A counter current nitrogen carrier
gas (99.99% purity; 0.1psi) was applied to the system in order to
facilitate water evaporation from the liquid sample, reducing the
processing time. Throughout the concentration process, samples of
water vapour containing the volatile aroma compounds of the apple
juice were collected from the SDR unit with deionised water for
gas chromatography-mass spectrometry (GC-MS) analysis. Apple
juice concentrates, labelled as concentrate I and concentrate II, were
collected at times when the volume of the apple juice sample had
reduced to 350mL and 150mL (i.e., to 30% and 70% of the original
volume), respectively. The times required to achieve such volume
reduction were around 5 to 15 and 9 to 25min, respectively, depending
on the processing temperature used. All samples were stored in airtight antiseptic containers at 5°C before analyses.
Steady-state viscosity of apple juices was determined using a
Bohlin C-VOR rheometer (Malvern Instruments Ltd., Worcestershire,
England, UK.), with a C25 cup and bob geometry. The sample was
poured into the rheometer cell, which is surrounded by a temperature
controlled vessel, and allowed to equilibrate at 5°C for 10 min prior to
the measurement. Apparent viscosity was measured at shear-rates in
the range 0.1–200 s-1 using continuous shear, with a 30s delay time and
a 30s integration time at each shear rate. Duplicate measurements were
made to check the reproducibility.
Materials and Methods
Apple juice
Reconstituted apple juice
Reconstituted apple juice was prepared by adding 350mL distilled
Particle size analysis
Particle size distributions of apple juice concentrates were
measured using a Malvern Mastersizer MS2000 laser light-scattering
analyser (Malvern Instruments Ltd., Worcestershire, England, UK.)
with absorption parameter value of 0.01 and refractive index ratio
of 1.53. The particle size measurements are reported as the volumeweighted mean diameter (d43 = Σi ni di4 / Σi ni di3), where ni is the number
of particles of diameter di.. The d43 value was used to monitor changes in
the particle-size distribution of the juice concentrates at 5°C.
Figure 1: Schematic diagram of the collection of volatile components during SDR-processing.
J Food Process Technol
ISSN:2157-7110 JFPT, an open access journal
Volume 2 • Issue 2 • 1000108
Citation: Akhtar M, Chan P, Safriani N, Murray B, Clayton G (2011) Concentration of Apple Juice Using Spinning Disc Reactor Technology. J Food
Process Technol 2:108. doi:10.4172/2157-7110.1000108
Page 3 of 6
Gas chromatography-mass spectrometry analysis
The aroma profile of the apple juice samples was investigated by
headspace solid-phase micro-extraction (HS-SPME) combined with
gas chromatography-mass spectrometry (GC-MS). Samples of 5g were
weighed into 18mL SPME vials and placed onto a CTC CombiPal
auto-sampler (CTC Analytics AG, Zwingen, Switzerland), attached
to a Varian C3800 gas chromatograph coupled to a Saturn 2000 mass
spectrometer (Varian Ltd., Palo Alto, CA). HS-SPME of the preheated
samples (40°C for 15 min) was conducted under agitation for 2min
using a 65µm PDMS/DVB fibre (Supelco, Bellefonte, PA). The SPME
fibres were then introduced into the injector port of the GC for 10min
at 250°C, where the volatiles extracted by the fibres were desorbed
thermally and introduced into the capillary column. The GC was
performed on a DB-WAXetr capillary column (60m, 0.25mm i.d.,
0.25µm film thickness), which was programmed to have an initial oven
temperature set at 40°C, held for 5min, and increased to 200°C at 8°C/
min and finally to 250°C at 10°C/min, with a constant helium flow of
1.0mL/min. Blank runs were conducted between each sample analysis
to eliminate any remaining sample in the system.
Analysis was carried out in duplicate and selected ions were used for
quantification of the individual components. Aroma compounds were
identified using the US National Institute of Standards and Technology
(NIST) 08 library of mass spectra and gas chromatographic retention
indicates reported of standard compounds.
Results and Discussion
Physicochemical characterization
The physicochemical characteristics including the pH, acidity
(expressed as malic acid), and the soluble solids (°Brix) of the
pure-pressed apple juice, the SDR-made concentrate, SDR-made
reconstituted juice and the commercial reconstituted sample are
shown in Table 1. The results indicate that the pH of the apple juice
decreases in the juice concentrates due to an increase in the relative
amount of acid in samples. It was also found that the decrease in the pH
of the concentrates is dependent on the processing temperature, which
Pure-pressed apple juice
pH
Acidity
(malic
acid/100g)
Soluble solids
(°Brix)
4.00
2.07
10.5
3.81
3.81
3.80
3.79
3.08
3.19
3.28
3.26
12.0
12.4
12.6
12.6
Apple juice concentrate I*
- at processing temperature:
90 °C
100 °C
110 °C
120 °C
Apple juice concentrate II†
- at processing temperature:
90 °C
100 °C
110 °C
120 °C
3.60
3.59
3.58
3.58
3.38
3.51
3.57
3.64
19.8
20.2
20.3
20.5
3.94
3.93
3.94
3.93
2.12
2.21
2.18
2.30
9.5
9.6
9.8
9.8
Commercial reconstituted apple juice 3.80
3.02
10.6
Reconstituted apple juice#
- from concentrate processed at:
90 °C
100 °C
110 °C
120 °C
Apple juice concentrate with 30% volume reduction
†
Apple juice concentrate with 70% volume reduction
#
Reconstituted apple juice made from apple juice concentrate II
*
Table 1: Physicochemical properties of apple juice and concentrates.
J Food Process Technol
ISSN:2157-7110 JFPT, an open access journal
could be explained as a result of evaporative effect during processing.
Meanwhile, there is no significant difference in the pH between the
SDR-made reconstituted juices and the pure-pressed sample (original).
All measured pH values presented in this study have a recorded
standard deviation of ± 0.07.
The tartness of pure fruit juices is largely due to organic acids: in
apple juice malic acid is the principal acid. The results show that the
acid content of the SDR-made apple juice increases with increased
concentration of the juice. The slight increase of the acidity with the
processing temperature may be caused by the release of the bound
forms of acid in the juice. Organic acids are normally present in fruits
in free and bound forms. The higher the processing temperature, the
higher amount of bound acid is released. However, due to the short
residence time of the SDR in the concentration process (0.4s for single
pass over the disc), this thermal effect on the acidity change of the
sample is not significant. Regarding the reconstituted apple juices,
the acidity values were similar to those of a pure-pressed sample. The
relatively high acidity of the commercial reconstituted juice is likely
due to the presence of the ascorbic acid added by the manufacturers,
although the exact amount added is not known. In general, despite a
small variation, there was a correlation between the higher acidity and
the lower pH values amongst the SDR-made apple juice concentrates.
Soluble solids, sometimes called total soluble solids (TSS), in fruit
juice comprise not only the sugars but also include many other soluble
substances such as salts, acids and tannins. However, sugars (mostly
sucrose) are the principal solids constituents and are usually measured
for all practical purposes. The soluble solids, expressed in degrees Brix
(ºBrix), affect the physical properties such as density, viscosity, and
boiling point elevation, of the juice products. Similar to the acidity
results, it was found that the °Brix value of the concentrates increased
with concentration. The increased °Brix value with the processing
temperature could be attributed to the evaporative effect [16]. In terms
of the reconstituted apple juice, there was slight decrease in the soluble
solids compared to the pure-pressed apple juice but the change was not
significant.
Particle-size distribution
The particle-size distribution is an important factor determining
the stability of a juice concentrate. Particles in a cloudy juice can
adhere together and form aggregates of increasing size (flocculation)
which may settle because of gravity. Flocs might undergo coagulation
and produce a much denser form, which is an irreversible process.
The particle-size distributions of the apple juice samples are shown in
Figure 2.
Both the pure-pressed apple juice and the SDR-made apple juice
concentrates (concentrate II) show fairly narrow distributions with the
average particle size d43 in the range 4 to 12µm. Thus, the concentration
process using the SDR does not have detrimental effect on the particle
size of the product.
Rheological properties
Figures 3a and 3b show that all the apple juice samples exhibited
shear-thinning behaviour over the entire range of shear rate tested
(0.1 – 200s-1). The results are consistent with the general observations
reported in the literature [17,18]. Shear-thinning behaviour is a
common type of flow behaviour in food systems due to a breakdown of
structure under the influence of the shear forces [19,20]. The viscosity
profiles of the pure-pressed apple juice and the commercial sample
Volume 2 • Issue 2 • 1000108
Citation: Akhtar M, Chan P, Safriani N, Murray B, Clayton G (2011) Concentration of Apple Juice Using Spinning Disc Reactor Technology. J Food
Process Technol 2:108. doi:10.4172/2157-7110.1000108
Page 4 of 6
are very similar. The viscosity of the concentrates increases with the
concentration of the apple juice as expected. It was observed that the
apple juice concentrates II (70% volume reduction) have a higher
viscosity than the apple juice concentrates I (30% volume reduction)
due to the higher sugar content of the latter. The results also showed
that the viscosity increases slightly with the processing temperature,
possibly due to evaporative effect. Generally, there was no significant
dependence of the viscosity profile on the processing temperature in
agreement with no significant effects of processing temperature on the
particle size distributions (Figure 2).
8
Pure pressed apple juice
Apple juice concentrate II processed at 90 oC
Apple juice concentrate II processed at 100 oC
Apple juice concentrate II processed at 110 oC
Volume (%)
6
4
2
0
0.01
0.1
1
10
100
1000 3000
Particle Size (µm)
Figure 2: The particle size distributions of the pure-pressed apple juice and
the SDR-made apple juice concentrates II processed at different temperatures.
Figure 3c: The viscosity profiles of pure-pressed apple juice, commercial
reconstituted apple juice, and reconstituted apple juices prepared from SDRprocessed concentrates II. All measurements were carried out at 5 °C.
Figure 3c presents the viscosity profiles of the reconstituted apple
juice made from concentrate II processed at different temperatures.
The viscosities of the pure-pressed apple juice and the commercial
reconstituted juice sample are also presented as references. The results
show that the viscosity curves of all these samples tended to be the same.
Thus, the SDR can also produce concentrates that do not significantly
alter the rheological properties of the reconstituted products.
Aroma component analysis with gas chromatography-mass
spectrometry (GC-MS)
Aroma in fruit juice comprises a mixture of hundreds of different
organic compounds present at very low amounts, ranging from ppm
(ormL/L) to ppb (or µg/L) levels. However, these aroma compounds
are highly volatile, and traditional thermal concentration methods
using either plate or tubular heat exchange units cause massive losses
of the aroma compounds to the vapour phase.
Figure 3a: The viscosity profiles of pure-pressed apple juice, commercial
reconstituted apple juice, and SDR-made apple juice concentrate I processed
at 90-120 °C. All measurements were carried out at 5 °C.
Figure 4 shows GC-MS chromatograms of samples recovered from
the SDR and the pure-pressed apple juice. Compounds of the ester,
aldehyde and alcohol groups that contribute to the fruity flavour of
the apple juice were identified in both the volatile component and the
concentrate (Table 2). It is noteworthy that these recovered volatile
aroma components can be added back to the reconstituted juice or to
enhance the sensory quality of other food products [3,4]. Along with
the identified aroma components, new peaks were also found in both
the GC chromatograms of the volatile and the concentrate, indicating
the possible development of new aroma compounds via the SDR
processing. Although these compounds have not yet been identified
and are awaiting further investigation, there are reports in the literature
verifying the production of new aromas or the changes in flavours of
foods and beverages with novel heat treatments/thermal processing
[10,21-24].
Visual observations
Figure 3b: The viscosity profiles of pure-pressed apple juice, commercial
reconstituted apple juice, and SDR-made apple juice concentrate II processed
at 90-120 °C. All measurements were carried out at 5 °C.
J Food Process Technol
ISSN:2157-7110 JFPT, an open access journal
Colour is a major factor determining the acceptability of processed
juice products. Concentration using thermal treatment can induce
darkening, which affects the quality of product that leads to consumer
dissatisfaction. Due to the increase in solid concentration and the
reduction in water activity, non-enzymatic browning reactions (e.g.
Maillard reaction) and pigment destruction have been found to be
major causes of such problems in apple juice [25,26]. These chemical
changes are dependent upon the processing time and temperature as
well as the storage time [8,27].
Volume 2 • Issue 2 • 1000108
Citation: Akhtar M, Chan P, Safriani N, Murray B, Clayton G (2011) Concentration of Apple Juice Using Spinning Disc Reactor Technology. J Food
Process Technol 2:108. doi:10.4172/2157-7110.1000108
Page 5 of 6
Figure 4: Flavour analysis of (A) pure-pressed apple juice, (B) volatile components collected during concentration process, and (C) apple juice concentrate II.
The advantage of using the SDR for the concentration of apple
juice is the efficient heat and mass transfer of the system, allowing the
concentration process to be achieved with a much shorter residence
time, reducing the detrimental thermal effects on the colour of the final
product. Visual observations of the SDR-made apple juice concentrates
are illustrated in Figure 5. The more concentrated samples have a
longer residence time on the SDR disc, so the colour of the apple juice
concentrates II appears to be slightly darker than that of the apple juice
concentrates I. This colour change could also be related to the low
pH and high malic acid content which is associated to the kinetics of
Maillard reaction [28]. Overall, however, all the samples have a very
similar colour compared to the pure-pressed apple juice, which once
again verifies that the SDR can be used for the concentration of apple
juice without compromising the appeal of the product.
Peak
Retention time (min)
1
2
3
4
5
6
7
8
9
10
11
7.884
13.206
14.729
17.005
18.244
21.872
24.317
25.173
40.549
43.470
46.903
Flavour component
Ethyl acetate
Ethyl iso-butyrate
Iso-butyl acetate
2-methyl butyl acetate
N-butyl alcohol
2-hexenal
Hexyl ester
n-undecyl aldehyde
(E)-9-tetradecan-1-ol
Caxyophyllene
3-methyl cyclohexene
Table 2: Flavour identifications according to the GC-MS analyses.
J Food Process Technol
ISSN:2157-7110 JFPT, an open access journal
Figure 5: Visual observations of the apple juice concentrates (I & II represent
for 30% and 70% volume reductions respectively) processed at 90-120 °C. The
pure-pressed apple juice is shown as reference.
Conclusion
The spinning disc reactor (SDR) technology has been used for the
concentration of apple juice, and the physicochemical properties of the
concentrates were investigated. It can be concluded that this novel SDR
technology is capable of producing apple juice concentrates efficiently
without compromising the quality of the juice products. Compared
to traditional concentration methods using thermal treatment, the
Volume 2 • Issue 2 • 1000108
Citation: Akhtar M, Chan P, Safriani N, Murray B, Clayton G (2011) Concentration of Apple Juice Using Spinning Disc Reactor Technology. J Food
Process Technol 2:108. doi:10.4172/2157-7110.1000108
Page 6 of 6
SDR allows more efficient heat and mass transfer that facilitates the
concentration process. In addition, the short residence time of the juice
sample on the SDR disc minimises the heat damage to the product. The
experimental results indicate that the processing temperature has no
significant detrimental effects on the physicochemical properties and
the quality of the apple juice concentrates. The SDR-made reconstituted
apple juices are comparable to both the original pure-pressed apple
juice sample and the commercial reconstituted product. The new peaks
found in the GC chromatograms of both the volatile component and
the concentrate suggest the development of new aroma compounds
during the concentration process, which can be recovered and added
back to the reconstituted apple juice to enhance the flavour of the
product.
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51: 1073-1074.
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J Food Process Technol
ISSN:2157-7110 JFPT, an open access journal
Volume 2 • Issue 2 • 1000108