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85 July / August 2020 (Vol. 73)
Yearbook 2006
The scientifi c organ
of the Weihenstephan Scientifi c Centre of the TU Munich
of the Versuchs- und Lehranstalt für Brauerei in Berlin (VLB)
of the Scientifi c Station for Breweries in Munich
of the Veritas laboratory in Zurich
of Doemens wba Technikum GmbH in Graefelfi ng/Munich www.brauwissenschaft.de
BrewingScience
Monatsschrift für Brauwissenschaft
Authors
https://doi.org/10.23763/BrSc20-13katsch
L. Katsch, F.-J. Methner and J. Schneider
Kinetic studies of L-ascorbic acid
degradation in fruit juices for the
improvement of pasteurization plants
Pasteurization especially high-temperature short time (HTST) heating is a widely used preservation method
which inactivates microorganisms and enzymes, but also degrades compounds as L-ascorbic acid. For a
gentle dimensioning of a pasteurization plant the knowledge of the kinetic figures is important. Activation
energy, reaction order and pre-exponential factor of the L-ascorbic acid degradation in a model solution,
apple, orange and black currant juice were determined. Lines of equal effects, which indicate different
time-temperature combinations for the degradation, could be derived and compared with the lethal effect
on microorganisms. The activation energies were located in the area of 25 to 44 kJ/mol for all samples
except of orange juice (74 kJ/mol) in the range of 40 90 °C with a zeroth reaction order. Based on these
values, the lines of equal effects showed a lesser degradation at higher temperatures and shorter holding
times even in the typical setting range of pasteurization plants.
Descriptors: pasteurization, kinetic parameters, non-isothermal, line of equal effect, modelling
1 Introduction
1.1 Preservation of juice
The emerging demand for safe and nutritious juices has led to
the development of various non-thermal preservation techniques
such as pulsed electric fields (PEF), high pressure treatment
(HP), pressure change technologies, ozone treatment, irradiation,
manosonication and modified thermal processes as microwave
or ohmic heating [1–5]. The new techniques are supposed to be
gentler to heat sensitive substances like volatiles and phenolic
compounds than thermal pasteurization [6]. In comparison with
equivalent degree of microbial inactivation, which is necessary
for healthy and unspoiled juices, there were only differences in
residual enzyme activities [7]. Pasteurization is more effective on
the inhibition of peroxidase than HP and PEF [7] and polyphenol
oxidase requires temperatures more than 80 °C for inhibition [8].
The residual enzyme activities can cause amongst others a lower
cloud stability, a decrease of polyphenols with a subsequent colour
or odour change [9–11]. However, due to their limitations in plant
capacity, type of package or because of inefficacies in damaging
specific microorganisms the pasteurization particularly in terms
of HTST is still the most common method in the industry for juice
preservation [12]. Therefore, there is high interest in improving
conventional pasteurization processes aiming for a gentle treat-
ment and low quality losses. In practice, temperatures of 76.6 °C
to 87.7 °C and time in the holding tube between 25 and 30 s are
typically applied in HTST for fruit juices [13]. Since the lethal heat
effect on microorganisms increases faster with raising temperature
than some chemical reactions, which was mainly explored in dairy
products [14], higher temperatures and shorter times could be
favourable for a gentle pasteurization.
1.2 Quality of juice
Juice consists of a large number of potentially reactive compounds
like sugars, organic and amino acids and vitamins as well as colour
compounds and in a small amount phytochemicals [12, 15]. Several
works listed in table 1 (see page 86) investigated kinetic figures
of different compounds in food with an isothermal heat treatment
and under different storage and heating conditions for the deg-
radation of L-ascorbic acid. With the knowledge of the activation
energy EA and the pre-exponential factor k0, the conversion of the
compounds can be defined with the Arrhenius equation (Eq. 3) at
different temperatures.
Non-isothermal methods were employed for the investigation of
colour change kinetics of grape juice by Rhim et al. [16] or for di-
methyl sulphide degradation in beer by Huang et al. [17]. In many
cases the L-ascorbic acid degradation and colour change to an
undesired colour impression were examined in regard to quality
deterioration in juice [18]. Apart of juice, also model solutions were
investigated to operate with a defined and reproducible system [19].
Linda Katsch, OstwestfalenLippe University of Applied Sciences and Arts,
Institute of Food Technology.NRW, Lemgo, Germany; Frank-Jürgen Methner,
Department of Food Technology and Food Chemistry, Chair of Brewing
Science, Technische Universität Berlin, Berlin, Germany; Jan Schneider,
OstwestfalenLippe University of Applied Sciences and Arts, Institute of
Food Technology.NRW, Lemgo, Germany; corresponding author: linda.
katsch@th-owl.de
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of Doemens wba Technikum GmbH in Graefelfi ng/Munich www.brauwissenschaft.de
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1.3 L-ascorbic acid
L-ascorbic acid is an important vitamin for human nutrition and
frequently found in fruits [15]. In juices it can be added as an anti-
oxidant, to stabilize the turbidity and to increase the viscosity [20].
The stability of L-ascorbic acid depends on ambient conditions
as pH value, temperature, presence of metal ions and oxygen
content [21–23]. At pH values lower than its pk1 (4,04) it is more
stable than above [24]. Heat treatment promotes the degradation
generally but it is comparatively lower by HTST-treatment [24]. The
degradation pattern of L-ascorbic acid differs depending on the
oxygen content, whereby the reaction can be catalysed by metal
ions during aerobic degradation [23, 24]. Due
to rising temperature and larger °Brix-values,
a lower oxygen solubility is expected [25].
Polyphenols can have an influence on the sta-
bility of L-ascorbic acid [26]. It was described
that flavonoids are able to impair L-ascorbic
acid degradation [12]. Clegg et al. showed
that flavonols exhibit a higher potential for
the protection of L-ascorbic acid against
degradation than anthocyanins, which can
even accelerate the oxidation [26].
The nutritional value of L-ascorbic acid
consists among others of its effect as an
antioxidant in human blood plasma for the
protection against degenerative processes
from oxidant stress [27], as an agent against
scurvy [28] and for the improvement of iron
absorption [29]. The loss of its vitamin activity
occurs when L-ascorbic acid is degraded to
2,3-diketogulonic acid [24]. The degradation
products of L-ascorbic acid can react with
or without amino acids to browning prod-
ucts [24]. Bharate & Bharate showed a path-
way for the degradation of L-ascorbic acid and
the formation of brown pigments [30]. Smuda
& Glomb showed the degradation products
which incorporate in Maillard reaction [31].
Shinoda et al. revealed different compounds
in a model solution of orange juice that affect
the formation of several reaction products re-
sponsible for the non-enzymatic browning. As
important chemical precursors an interaction
of L-ascorbic acid, amino acids and different
fruit sugars has been found whereas chela-
tors and radical scavengers could inhibit the
browning reactions [19].
Based on this importance for a consistent
product quality, information about the tem-
perature depending kinetic figures as reaction
order, activation energy and pre-exponential
factor of L-ascorbic acid degradation in juices
is of great interest for the comparison with
the microbial inactivation figures. With this
information, optimized time-temperature
parametrization can finally be elaborated with different time-
temperature-combinations in a diagram of lines of equal effects
on quality determining properties, which results in an improvement
in current pasteurization practice.
2 Materials and methods
2.1 Materials
The model solution (MS) was prepared immediately before the
experiment started in order to diminish a previous reaction of the
Table 1 Kinetic data reported for L-ascorbic acid degradation determined with isothermal
methods
Matrix EA
[kJ/mol]
Thermal
treatment
[°C]
k0
[min-1]
Reaction
order Reference
Orange juice
52.74 65 – 90 19.95*105pseudo 1 [5]
71.0 ± 3.8 20 – 45 (6.3 ± 0.6)*1021 [49]
56.02 ± 29.83 4 – 45 not specified pseudo 0 [41]
Mango pulp 39 ± 14 80 – 150 (1.3 ± 0.5)*10-1 biphasic; 1 [50]
Model system 66.94† 61 – 105 not specified 0 [42]
† Data for aw = 0.9
Fig. 1 Reaction of L-ascorbic acid with MTT [33]
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ingredients. The compounds of the model solution were adapted
to the composition of apple juice [32]. The solution consisted of
17 g/L sucrose (Südzucker, Mannheim), 64 g/L fructose (Da-
nisco, Kotka) and 26.4 g/L dextrosemonohydrate (Cargill, Kre-
feld). The pH-value was adjusted to 3.5 with malic acid (Merck,
Darmstadt).
Clear and cloudy apple juice (AJ) (Lagenser Fruchtsäfte™), black
currant juice (BCJ) (dm™), fresh oranges (OJ) (Valensina®) and
fresh apples (variety 1: Elstar; variety 2: orchard) were purchased
in a local supermarket. For the fresh juices, fruits were cut,
pressed, filtered and all apple juices were spiked with 350 mg/L
L-ascorbic acid (Chemsolute, Renningen). The content of soluble
solids content (SSC) was measured with the refractometer J157
(Rudolph Research Analytical, Hackettstown), the pH-value with
the pHSensor SE 101-MS (Knick, Berlin).
2.2 Sugar and organic acids
Sugar and organic acid composition was determined by liquid
chromatography (Perkin Elmer Flexar LC, Waltham) equipped with
an UV-VIS (210 nm) and RI detector. A Nucleogel Sugar 810 H
column (7.8 mm ID, 300 mm; MachereyNagel, Düren) was used
with an isocratic mobile phase (25 mmol/L sulphuric acid in water;
Fluka, Seelze) at a flow rate of 0.6 mL/min and a temperature of
35 °C. Samples of 5 µL were injected directly after a filtration with
a 0.25 µm syringe filter.
2.3 Determination of L-ascorbic acid
The automatic photometer Gallery (Thermo Scientific, Newington)
was used for the photometric measurement of L-ascorbic acid
(AA). Cloudy samples were centrifuged (10 min, 5450 g) prior
further analysis. As shown in figure 1 the photometric method is
based on the reduction of tetrazolium salt 3-(4,5-dimethylthiazol-
2-yl)-2,5-diphenyltetrazolium bromide (MTT, 0.1 mmol/L) by means
of the electron carrier phenazine methosulfate (PMS; 1 mmol/L)
to a coloured formazan by L-ascorbic acid and other reducing
substances [33]. The formazan was measured at a wavelength
of 575 nm. For the blank value L-ascorbic acid were oxidized by
ascorbate oxidase (10 KU/L) to dehydroascorbic acid (DHAA) and
only the remaining reducing substances in the analysis solution
were measured in order to calculate the L-ascorbic acid content [33].
Stock solution of L-ascorbic acid (Chemsolute, Renningen) was
prepared in 1.5 % w/v meta-phosphoric acid (Roth, Karlsruhe)
with a pH-value of 3.5 [34] and a test kit (Thermo Fisher Scientific,
Vantaa) was used for quantification.
2.4 Determination of the kinetic figures
2.4.1 Reaction order
The reaction order is required for the calculation of the activation
energy and pre-exponential factor [35]. Therefore, isothermal
experiments were carried out. According to Fink, the term
comprising the initial concentration c0, the concentration at a vari-
able time ct and the reaction order n was plotted against time t [36].
The highest coefficient of determination (R2) expresses the most
appropriate reaction order.
2.4.2 Activation energy
For the calculation of the activation energy, a non-isothermal method
according to Coats & Redfern with a linear heating rate was ap-
plied. This method requires only one defined heating rate [35]. The
conversion of a component in chemical reactions can be described
with equation 1 [35]
(Eq. 1)
with the reaction rate constant k. α describes the fraction of the
decomposed component (Eq. 2) [37] where cEnd is 0 mg/L.
(Eq. 2)
The temperature dependency of chemicals reactions can be de-
scribed with the Arrhenius equation 3 with the frequency factor k0
and the activation energy EA [35]
(Eq. 3)
The linear heating rate β for non-isothermal experiments can be
described by equation 4 [35]
(Eq. 4)
The combination of the equations 1, 3 and 4 leads to equation 5 [35]
(Eq. 5)
The integral on the right hand side has no explicit solution but it
can be replaced with an approximation shown in Coats & Redfern
as well as Huang et al. [17, 35]. The calculation of the activation
energy can be carried out for n 1 with equation 6 [17].
(Eq. 6)
A pre-processing for smoothing was applied. The experimentally
obtained data were plotted in a concentration-time diagram and a
function was obtained by a quadratic polynomial fit. More complex
fitting function such as higher polynomials or exponential function
did not provide better results in a here relevant extent. With this func-
tion, the concentration can be calculated for any time-temperature
combination and experimental fluctuations can be compensated.
2.4.3 Pre-exponential factor
Theoretically, the pre-exponential factor can be calculated from the
intercept of the y-axis of the linear equation 6. If the coefficient of
determination of the linear equation showed a non-linear course
(R2 < 1) the resulting error can lead to a deviation. Therefore, the
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Fig. 2 Exemplary schematic graphs of lines with equal effect for
typical chemical degradation and typical microbial inacti-
vation, grey area represents the preferred pasteurization
area
Table 2 z-values for the inactivation of selected microorganisms in model solution (MS),
apple juice (AJ) and orange juice (OJ)
Microorganism Matrix cell
conditions pH-value Brix
°Brix
z-value
°C Reference
Saccharomyces
cerevisiae MS vegetative
cells 2.8 13.5 5.8 [51]
Escherichia coli
O157:H7 AJ vegetative
cells 3.5 11.7 5.6† [52]
Alicyclobacillus
acidoterrestris OJ bacterial
spores 3.5 11.7 7.8 [53]
† Calculated from logD against temperature [51]
Fig. 3 Schematic drawing of the experimental setup
pre-exponential factor can be calculated
as shown in equation 8 with the help of the
rate law of the zeroth reaction order (Eq. 7),
the Arrhenius equation (Eq. 3), taking the
isothermal measured data for the change of
the degraded L-ascorbic acid Δc in a time
interval Δt [14].
(Eq. 7)
(Eq. 8)
Here Δc = c1-c2 and Δt = t2-t1 (t2 > t1).
2.4.4 Lines of equal L-ascorbic acid degradation
A diagram of lines with equal effects represents different time-
temperature-combinations for the degraded amount of L-ascorbic
acid and the microbiological lethal effect. The comparison of micro-
biological and chemical lines of equal effects allows the identification
of temperature-time combinations with the lowest nutritive losses.
The working area of pasteurization must be located on or above
the line of applied pasteurization units and below the accepted
quality decline. A schematic graph for illustration is shown in fig-
ure 2. The time-temperature-combinations for the microbiological
lethal values can be calculated via the Pasteurization units (PU)
with equation 9 [38].
(Eq. 9)
The applied temperature is T, the reference temperature TB for
fruit juices is by convention 80 °C [39]. The z-values indicates the
required temperature increase that is necessary to obtain the same
effect in a tenth of the time [14]. Exemplary selected z-values of
fruit juice relevant spoiling microorganisms can be obtained from
table 2. To calculate a line of equal effect for chemical degrada-
tion, a concentration change must be defined. For this particular
concentration change, e.g. L-ascorbic acid degradation, for any
temperature the corresponding time that is required can be deter-
mined with equation 8 with the previous calculated kinetic values
of EA and k0. If this temperature-time combination is used as refer-
ence point (TC, tC), all temperature-time combinations at a specific
degraded concentration can be calculated using equation 10 and
plotted in a diagram as a line of equal effects.
(Eq. 10)
2.4.5 Thermal processing
The isothermal and non-isothermal experiments were performed in
a 500 mL-double-walled beaker glass tempered by a thermostatic
water bath (Versacool 7, Thermo Scientific, Newington) mixed
with a magnetic stirrer (300 rpm) and controlled by a temperature-
measuring sensor (Pt-100) as shown in figure 3. The temperature
applied for the isothermal experiments was 85 °C for 8000 s and the
non-isothermal experiments were conducted with a linear heating
rate of 0.25 °C/min with the temperature range of 40 to 90 °C. Be-
fore the experiments were carried out, the heating rate was tested
to be linear and the constancy of the temperature was verified for
the isothermal experiments. The temperature range and heating
rate were chosen in a way that an adequate oxidation of L-ascorbic
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acid could be achieved. However, also a realistic pasteurization
temperature range was considered. The experimental design with
a double-walled reaction vessel was chosen to achieve a uniform
temperature distribution through permanent stirring and to perform
accurate temperature control directly in the heating medium.
The samples were immediately cooled in ice water to decelerate
chemical reactions and were subsequently analysed. Temperature,
time and the L-ascorbic acid concentration were recorded for the
kinetic calculation.
2.4.6 Statistical analysis
The experiments were carried out in triplicate. The values were
given as an arithmetic mean with the minimum-maximum span. In
order to determine the differences between the different matrices,
a one-way analysis of variance (ANOVA) was carried out. Least
Significant Difference test (LSD; p < 0.05) was used to determine
the significant difference between the juices.
2.4.7 Software
Microsoft Excel was used for kinetic and the statistical data cal-
culation. Origin was used for diagrams and regression analysis,
RI-CAD for the process flowsheet and Chemsketch for the chemi-
cal equation.
3 Results and Discussion
3.1 Composition of juices
Table 3 shows the composition of the different juices. A similar
concentration of L-ascorbic acid was added to each of the spiked
juices (target minimum 350 mg/L). Because of setting time of
the temperature in the thermostat and the dissolution time of the
L-ascorbic acid, a variation of the initial content was unavoidable.
3.2 Reaction order
For the calculation of the kinetic figures, the reaction order has to
be known. Regarding the literature, for the L-ascorbic acid degra-
Table 3 Composition of the model solution (MS) and juices (AJ-apple, OJ-orange, BCJ-black currant) in the non-isothermal experiments
(n = 3)
pH SSC Glucose Fructose Sucrose Malic acid Citric acid L-ascorbic
acid
°Brix [g/L] [g/L] [g/L] [g/L] [g/L] [mg/L]
MS 3.15 – 3.53 9.73 – 12.75 23.63 – 24.11 63.21 – 65.59 15.77 – 16.67 n.d. 290 – 374†
clear AJ 3.38 – 3.48 10.96 – 13.36 25.28 – 26.61 66.85 – 70.59 12.03 – 12.52 7.31 – 7.50 n.d. 341 – 365†
cloudy AJ 3.35 – 3.51 11.68 – 13.41 23.47 – 24.58 65.20 – 67.46 15.42 – 15.93 6.58 – 6.75 n.d. 556 – 584‡
fresh AJ (var. 1) 3.98 – 4.02 15.51 – 15.63 21.45 – 21.84 73.83 – 74.18 51.32 – 52.99 6.35 – 6.50 n.d. 333 – 367†
fresh AJ (var. 2) 3.28 – 3.38 13.72 – 14.05 20.78 – 21.43 68.96 – 70.00 37.16 – 38.31 8.26 – 9.30 n.d. 233 – 281†
OJ 3.76 – 3.85 11.95 – 12.40 21.75 – 23.03 22.40 – 23.59 54.68 – 57.22 2.17 – 2.26 8.83 – 9.06 423 – 460§
BCJ 2.88 – 2.99 16.61 – 17.97 44.01 – 45.21 65.65 – 67.23 n.d. 9.38 – 10.09 31.06 – 31.69 928 – 991§
n.d. not determinable †spiked ‡spiked & native content §native content
Fig. 4 Coefficients of determination R2 for all matrices (without
BCJ), used for the selection of the reaction order; Boxplot
with arithmetic mean, median, 25/75 % quartile and span
Table 4 R2 selection of the reaction order in whole numbers for the
L-ascorbic degradation with the help of spans; isothermal
experiments at 85 °C and 8000 s (MS-model solution, AJ-
apple juice, OJ-orange juice, BCJ-black currant juice,
var.-variety, n = 3)
012
R2
MS 0.9948 – 0.9996 0.9171 – 0.9439 0.6626 – 0.7752
clear AJ 0.9957 – 0.9991 0.9219 – 0.9763 0.676 – 0.8999
cloudy AJ 0.9973 – 0.9992 0.9645 – 0.9762 0.8568 – 0.9116
fresh AJ (var. 1) 0.9970 – 0.9993 0.9432 – 0.9873 0.8000 – 0.9505
fresh AJ (var. 2) 0.9923 – 0.9990 0.9656 – 0.9828 0.8150 – 0.9041
OJ 0.9933 – 0.9989 0.9469 – 0.9847 0.8066 – 0.9347
BCJ 0.9274 – 0.9932 0.9448 – 0.9965 0.9586 – 0.9976
dation, a zeroth- or more frequently first-order reaction is assumed
(Table 1). Own experiments, obtained from the isothermal trials
at 85 °C, tended clearly to the zeroth order. Figure 4 sums up the
coefficients of determination (R2) in dependency on the reaction
order for all samples, except black currant juice (BCJ) due to the
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Fig. 5 Degraded L-ascorbic acid during isothermal processing
at 85 °C, arithmetic mean with span (n = 3)
Fig. 6 Degraded L-ascorbic acid during non-isothermal heat
treatment, arithmetic mean with span (n = 3)
Fig. 7 Activation energy for the L-ascorbic acid degradation in
different juices, arithmetic mean with span (n = 3). Labels
indicate the mean value [kJ/mol] and different letters the
significant differences by LSD test (p < 0.05)
deviation from the other juices, which would result in a wider range.
Apparently, the L-ascorbic acid degradation behaved similarly in all
matrices. With increasing reaction order, the R2-values changed for
the worse and had a wider spreading. Table 4 shows, here including
BCJ, the evaluation of the reaction order broken down for whole
numbers (0, 1, and 2). The results revealed that for all samples
with exception of the BCJ the zeroth reaction order (n = 0) was
most applicable. The BCJ however seem so far to be an exception.
The R2 for the BCJ differed only slightly for the different reaction
orders. A reason was the high native content of L-ascorbic acid
(cf. Table 3). While the absolute amount of degraded L-ascorbic
acid ct is in a similar magnitude for all juices (Fig. 5), the amount
in relation to the initial amount c0 is comparably small in case of
the BCJ. The reaction order cannot be determined with sufficient
accuracy because of the similar course for the different reaction
orders for at low conversion rates [14]. As intermedi-
ate result, the zeroth reaction order was selected for the further
calculations which corresponds to the results reported by several
works [40–42].
3.3 Activation energy
The kinetic values obtained by non-isothermal heating are ex-
pected to be more accurate due to the lesser individual errors of
the method [43]. In figure 6, the amount of L-ascorbic acid loss
over the reaction time and the time related temperature increase
is displayed for the selected juices with non-isothermal treatment.
The interrelation of degradation and time was rather linear. Excep-
tions were the cloudy apple juice and even more clearly in case
of the orange juice. In addition, it is visible that L-ascorbic acid
degradation was more enhanced in orange juice at higher tem-
peratures compared to other juices. Figure 7 shows the activation
energy for the L-ascorbic acid degradation (R2 > 0.9). The activation
energy in orange juice was significantly higher than in the other
juices but coincided nearly with the values given in table 1. The
higher activation energy indicates the different behaviour of the
degradation in orange juice with increasing temperature explained
by the Arrhenius equation. Therefrom derived equation 11 [44]
can be used to calculate the rate constant k2 for any temperature
T2 when activation energy and rate constant k1 is known for one
defined temperature T1.
(Eq. 11)
With a larger activation energy, the rate constant increases at higher
temperatures. This means that a larger amount of L-ascorbic acid
will be degraded at higher temperatures. This effect was identifi-
able for orange juice in the concentration-time diagram (Fig. 6).
According to Miller & Joslyn, the L-ascorbic acid oxidation is impaired
by an increasing sugar content and a decreasing pH-value [45].
However, no correlation of the degradation rate of L-ascorbic acid
with the sugar content nor with the pH-value could be found.
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Orange juice contains a high amount of the flavanone-glycoside
hesperidin [15] which could be a reason for significantly higher
activation energy due to the protective function against the oxida-
tion of L-ascorbic acid. In contrast, black currant juice contains
mainly anthocyanins [26] and apple juice contains hydroxycin-
namic acids [46]. There was a significantly higher activation
energy for cloudy apple juice compared to clear apple juice. The
lower content of polyphenols in clear apple juice compared to
cloudy apple juice [46] hardly seems to have any influence on
the hindrance of the degradation, since the activation energy of
the model solution is on the same level. In case of the self-made
(fresh) juices, a higher oxygen absorption could have catalysed
the degradation. It is visible that the freshly pressed apple juices
had a different activation energy. According to Kahle et al., there
are significant differences in the polyphenol content between
dessert apples (var. 1) and cider apples (var. 2) [46]. This differ-
ence could explain the deviant activation energy. Purchased juice
are assumed to have a lower polyphenol content than freshly
produced [46], but other factors such as metal ions and oxygen
input during production can have an impact on the stability of
L-ascorbic acid [21, 23].
Disadvantageous for the determination of the activation energy
could be an oxygen input whilst stirring and production. The oxy-
gen concentration has an influence on the reaction mechanism
and may thus affect the found reaction order [24]. At lower oxygen
concentrations (< 0.63 %), a zeroth reaction order is expected and
with a higher oxygen content a faster oxidation is evident [21].
Figure 8 shows the correlation of degraded L-ascorbic acid in the
defined experimental set-up and the activation energy. The plot
allows the assumption of an exponential relation confirming the
applicability of the Arrhenius equation.
3.4 Pre-exponential factor
The pre-exponential factor is necessary for the calculation of any
time-temperature combinations related to an arbitrarily selected
concentration change (Eq. 8) and for the calculation of the reaction
rate constant k. Unlike the activation energy, the pre-exponential
factor cannot be satisfactorily determined with the non-isothermal
Coats & Redfern method. The reason is the need of an extrapola-
tion to find an axis intercept. The extrapolation is highly sensitive to
even minor deviation occurring in the non-isothermal experiments.
Therefore, as previously for the identification reaction order, the
isothermal method was used with equation 8. Figure 9 reveals
exemplary evidently better suitability of the isothermally found axis
intercept (representing the pre-exponential factor) compared with
non-isothermal method since this corresponds approximately to
the course of the measuring points. The isothermally calculated k0-
values for all matrices are shown in table 5. These figures coincide
fairly with literature data shown in table 1. The pre-exponential factor
is temperature-dependent, but for practical reasons it is assumed
to be constant in the Arrhenius equation [44].
3.4.1 Lines of equal L-ascorbic acid degradation
The heat load of real pasteurization conditions provides only a
rather minor extent of L-ascorbic acid degradation. In figure 10
Fig. 8
Absolute amount of degraded L-ascorbic acid at 6000 s,
obtained from the defined non-isothermal test arrangement
versus activation energy, arithmetic mean with span (n = 3)
Fig. 9 Coats and Redfern Plot of cloudy apple juice with non-
isothermal and calculated isothermal intercept
Table 5 Pre-exponential factor k0 of L-ascorbic acid degradation
in different juices, arithmetic mean and span at 85 °C
(n = 3), time span 8000 s with intervals of 500 s
Matrix k0 [molL-1s-1]
MS (1.52; 1.31 – 1.56)*10-2
clear AJ (1.50; 1.49 – 1.57)*10-2
cloudy AJ (3.75; 3.67 – 3.76)*10-1
fresh AJ (Var. 1) (8.15; 8.02 – 8.83)*10-4
fresh AJ (Var. 2) (2.77; 2.74 – 2.94)*10-2
OJ (9.47; 8.18 – 9.71)*103
BCJ (1.20; 1.06 – 1.91)*10-2
(see page 92) a technically possible heating range is shown. The
steeper curves represent the lines of equal inactivation of different
and juice relevant microorganisms in an extent of 10 PU. The flatter
curves are the lines of equal degradation (20 mg/L) of L-ascorbic
acid. The different steepness is a result of different activation ener-
gies and z-values respectively.
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In spite of the comparably small contribution for L-ascorbic acid
protection, it seems nonetheless worth to improve the current pas-
teurization practice. At conventional pasteurization temperatures
of 76.6 87.7 °C [13], there is a higher product damage due to the
loss of L-ascorbic acid than at higher temperatures and further de-
terioration is more probable due to the reaction of the degradation
products of L-ascorbic acid with other juice ingredients.
According to Labuza & Riboh, the extrapolation for the shelf life
prediction is influenced by different factors such as analytical preci-
sion, selection of an appropriate reaction order or the preference
of different reactions at different temperatures [47]. Therefore,
the course shown in figure 10 should be seen as an approxima-
tion and not as an exact calculation for the degraded amount of
L-ascorbic acid.
In addition, it is more advantageous for the sensory quality of
juices that the enzymes get inactivated at higher pasteurization
temperatures, since e.g. polyphenol oxidase is only inactivated at
temperatures above 80 °C [8–11]. Due to the small differences dur-
ing pasteurization, subsequent effects on the pasteurized product,
such as filling, transport and storage, are important influencing
factors in order to maintain the quality.
The low degradation refutes the assumption that pasteurization
is highly damaging process for L-ascorbic acid. Nevertheless, a
further optimization is possible by including the heat impact of the
recuperation zones in the calculation of the pasteurization units
and using higher temperatures with a shorter or no holding time
due to the short holding times at high temperatures. This has to
be evaluated for each beverage individually since certain reactions
occur only at higher temperatures, such as caramelization above
a temperature of 120 °C [48]. For this optimized pasteurization
practice the lines of equal effects have to be flatter for the chemical
reactions than the lines of equal microorganism inactivation and
the range of least deterioration must be selected.
4 Conclusion
The activation energy of L-ascorbic acid deg-
radation in juices can be sufficiently calculated
using the non-isothermal Coats & Redfern
method with a zeroth reaction order. For
different types of juice, significantly different
activation energies are calculated. However,
in relation to the large difference to the micro-
bial inactivation figures, the activation energy
variations among the juices have a compara-
tively small influence when comparing with
the lines of equal effects. As expected by the
initial hypothesis, the comparisons of the lines
of equal effect reveal a potential for a process
improvement in terms of a gentler process.
However, the results show also that the usual
pasteurization conditions in practice may not
cause severe damages to the product, but
it is still possible to optimize the process by
setting higher temperatures and shorter hold-
ing times and considering the recuperation
zones with their heat load contribution. This
knowledge can also be used in the beer pasteurization in general
or for beer-based mixed drinks. An application in the development
of quality-influencing reaction products or aroma and undesired
colour components is also conceivable. HMF or Thiobarbituric Acid
Index might be exemplary indicators. An important precondition
for this is that the lines of equal effects are flatter compared to
the lines of equal inactivation of microorganisms for a successful
application of higher pasteurization temperatures.
Acknowledgments
The project 005-1703-0012 is funded within the program “FH Zeit
für Forschung” of the Ministry of Culture and Science of the Ger-
man State of North Rhine-Westphalia.
Conflict of Interest
The Authors declare that there is no conflict of interest.
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Received 19 June 2020, accepted 7 August 2020
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