This document investigates the effects of drying temperature on the degradation kinetics of ascorbic acid (vitamin C) in tomatoes during the drying process. It studies ascorbic acid degradation in whole tomatoes that underwent caustic peeling (with or without prior osmotic pretreatment) as well as halved tomatoes at drying temperatures of 50, 60, and 70°C. The degradation rates were found to depend on both the sample pretreatment and drying temperature, with lower degradation observed in osmotically pretreated whole tomatoes and higher degradation in halved tomatoes. Increasing the drying temperature also increased the degradation rates.

1. Ascorbic acid degradation kinetics in tomatoes at different
drying conditions
P.H.M. Marfil, E.M. Santos, V.R.N. Telis*
UNESP - Departamento de Engenharia e Tecnologia de Alimentos, Universidade Estadual Paulista, 15054-000, S~ao Jose´ do Rio Preto, SP, Brazil
Received 21 May 2007; received in revised form 25 October 2007; accepted 6 November 2007
Abstract
High temperatures and long drying times used in hot air drying can negatively affect the nutritional quality of the final product. It is generally
observed that, if ascorbic acid is well retained, other components are also well retained. Hence, ascorbic acid can be taken as an index of nutrient
quality of foods. The interest in dried tomato has increased since its use as ingredients for pizza and various vegetable and spicy dishes has
became popular. Tomatoes are usually dried in slices or halves, after seeds and parenchyma removal with a resulting large amount of wastes
and an important nutrient loss. The objective of this work was to investigate the effects of drying temperatures on ascorbic acid degradation
kinetics in caustic-peeled whole tomatoes (with or without osmotic pre-treatment) and in halved and drained tomatoes. The degradation rates
were dependent on samples treatment before drying, as well as on drying temperature. Lower degradation rates were observed in osmotically
pre-treated whole tomatoes, whereas higher degradation rates occurred in halved tomatoes. Increasing drying temperature led to higher
degradation rates.
Ó 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.
Keywords: Convective drying; Osmotic dehydration; Vitamin C; Weibull model
1. Introduction
The interest in dried tomato products is increasing since
their use as ingredients for pizza and various vegetable and
spicy dishes became popular. Nevertheless, high temperatures
and long drying times found in hot air drying can negatively
affect the nutritional quality of final product. An osmotic de-
hydration step prior to air drying of fruits and vegetables has
been suggested by a number of authors to yield good quality,
fully dehydrated or intermediate moisture products of im-
proved stability (Alvarez et al., 1995; Nieto, Salvatori, Castro,
& Alzamora, 1998; Nsonzi & Ramaswamy, 1998; Sankat,
Castaigne, & Maharaj, 1996).
Up to the present moment, drying of whole tomatoes has
not yet been the object of an intensive study. The only use
for whole tomatoes is in caustic peeling technique in canned
tomato. Azoubel and Murr (2004), Shi, Le Maguer, Wang,
and Liptay (1997) studied osmotic drying of whole tomatoes
without complete peeling. With the purpose of reducing resis-
tance to mass transfer, the authors punched small holes in
tomato skin with a needle, what would be impracticable in
industrial scale.
Shi et al. (1997) investigated the osmotic dehydration
of whole tomatoes submitted to chemical and physical pre-
treatments to increase skin permeability. Lewick, Le, and
Pomaran´ska-Lazuka (2002) studied the effect of calcium chlo-
ride and osmotic dehydration on the kinetics of tomato drying
and on product rehydration properties. In these studies the os-
motic dehydration was carried out in sucrose/water solutions,
but according to Bohuon, Collignan, Rios, and Raoult-Wack
(1998), the use of ternary sucrose/NaCl/water solutions present
some advantages in osmotic dehydration, such as higher levels
of dehydration without over-salting the product, as well as the
possibility of increasing the total solute concentration without
attaining the saturation limits. In fact, Telis, Murari, and
Yamashita (2004) showed that, during osmotic dehydration
of tomato quarters in sucrose/NaCl aqueous solutions, the
* Corresponding author. Tel.: þ55 17 3221 2255.
E-mail address: vanianic@ibilce.unesp.br (V.R.N. Telis).
0023-6438/$34.00 Ó 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.lwt.2007.11.003
Available online at www.sciencedirect.com
LWT - Food Science and Technology 41 (2008) 1642e1647
www.elsevier.com/locate/lwt

2. NaCl diffusivities increased with decreasing sucrose concen-
tration, whereas sucrose diffusivities were higher when solu-
tions with lower NaCl concentrations were used, reinforcing
the interactive character of salt/sugar in osmotic dehydration.
In modern food technology, the trend is to maximize nutri-
ents retention during processing and storage. The increasing
interest in the antioxidant activity of lycopene e the most
abundant carotenoid in tomatoes e has been promoting several
research activities on fresh tomato and tomato products
(Chang, Lin, Chang, & Liu, 2006; Shi, Le Maguer, Kakuda,
Liptay, & Niekamp, 1999; Tavares & Rodriguez-Amaya,
1994). On the other hand, it is generally observed that, if
ascorbic acid is well retained, other nutrients are also well
retained. Hence, ascorbic acid can be taken as an index of
nutrient quality of foods (Gregory, 1996). Ascorbic acid is
known to be a labile vitamin that lose activity due to a number
of factors, including pH, moisture content, oxygen, tempera-
ture and metal ion catalysis (Uddin, Hawlader, & Zhou, 2001).
Several works concerning ascorbic acid degradation in
foods have suggested first order decay kinetics and the Bige-
low equation has been applied in modeling (Uddin, Hawlader,
Ding, & Mujumdar, 2002; Vieira, Teixeira, & Silva, 2001). It
is common to characterize first order reactions in terms of D
and z values (thermal death time concept). Singh and Lund
(1984) developed a mathematical model to describe the ascor-
bic acid degradation in stored apple as function of temperature
and water activity. Akinyele, Keshinro, and Akinnawo (1990)
investigated nutrient losses during and after processing of
pineapples and oranges and a number of authors studied vita-
min C degradation in various foodstuffs (Prado, Chandra, &
Bicalho, 1995; Vieira, Teixeira, & Silva, 2000; Yamashita,
Benassi, & Kieckbusch, 1999).
In drying process, the loss of ascorbic acid is affected spe-
cially by high temperatures. According to Zanoni, Peri, Nani,
and Lavelli (1999), degradation rate of vitamin C in tomatoes,
at 80 and 110
C, was dependent of temperature and moisture
and vitamin C was not detected in samples dried at 110
C to
50% of moisture. Nevertheless, these authors found a 10%
residue of vitamin C in samples dried at 80
C until 10% of
moisture. Erenturk, Gulaboglu, and Gultekin (2005) investi-
gated degradation kinetics of ascorbic acid during air drying
of whole rosehip. These authors found that temperature depen-
dency could be described by Arrhenius relationship, while the
activation energy and reaction rate constant could be determined
as functions of moisture content. Goula and Adamopoulos
(2006) determined a mathematical model for the rate of vitamin
C loss in a drying process of tomato halves and tomato pulp.
They observed that the reaction constant depended on moisture
content of the product, in addition to temperature. Furthermore,
the maximum rate constant was observed when the moisture
content was between 65 and 70%. These effects were expressed
by a linear relationship between temperature, moisture content
and natural logarithm of rate constant.
Tomatoes are usually dried in slices or in halves. In the last
case, seeds and parenchyma are removed and discarded with
a resulting large amount of wastes and an important nutrient
loss. Drying whole tomatoes, submitted to a chemical pre-
treatment in sodium hydroxide solution to withdraw the
skin, would be a feasible alternative to reduce the volume of
solid wastes, and could also contribute to decrease the rates
of nutrients loss, although with an important increase in or-
ganic and chemical pollution of wastewaters.
Based on the above considerations, the objective of this
work was to investigate the effects of drying temperatures
on ascorbic acid degradation kinetics in caustic-peeled whole
tomatoes (with or without osmotic pre-treatment in NaCl/
sucrose solution) and in halved and drained tomatoes, at the
same drying conditions.
2. Material and methods
2.1. Raw material
Ripe fresh tomatoes (Lycopersicon esculentum Mill.) of
industrial, pear-shaped type were purchased at local market.
The fruits were sorted visually for color, size and physical
damage and rinsed in fresh water. Whole tomatoes were sub-
mitted to caustic peeling by immersion in a NaOH solution
(6 g NaOH/100 g solution) at 30
C for 30 min (Santos, Mar-
fil, Telis-Romero, Telis, 2005). Halved tomatoes were not
peeled but had seeds and parenchyma manually removed.
2.2. Osmotic treatment
Sucrose (food grade) and NaCl (analytical grade) dissolved
in distilled water were used as osmotic agents. Whole peeled
tomatoes were immersed in a NaCl/sucrose solution (10 g
NaCl/100 g solution and 35 g sucrose/100 g solution), for
60 min, at 30
C, maintaining a 1:10 (w/w) tomato/solution
ratio (Telis et al., 2004). The treated samples were drained
for 1 min, rinsed with fresh running water to withdraw excess
solution and slightly wiped with an absorbent paper.
2.3. Convective drying
Drying was accomplished with air velocity of 1 m/s, at 50,
60 and 70
C. In general, food is dried at 60
C, a temperature
level that provides sufficiently high rates of water removal and
results in products without excessive loss of nutrients and with
pleasant texture. The other temperatures were chosen in order
to deviate in Æ10
C from the central level.
The equipment was a pilot scale tray drier with parallel air-
flow, which consists of an airflow rate control system, a drying
air heating section and a drying chamber. Once the desired op-
eration conditions were achieved, the tomatoes were inserted
into the dryer cabinet. The initial moisture contents were
determined gravimetrically using a vacuum oven at 60
C
for 48 h, and drying kinetics was determined by weighing
samples at regular time intervals.
2.4. Ascorbic acid analysis
At regular time intervals, three tomatoes were removed
from the dryer, cut and grounded, and a sample of 25 g was
1643P.H.M. Marfil et al. / LWT - Food Science and Technology 41 (2008) 1642e1647

3. homogenized with 50 g of the extraction solution (2 g oxalic
acid/100 g solution). An aliquot of 20 g was taken and diluted
to 50 ml with the extraction solution in a volumetric flask and
then vacuum filtered. Aliquots of 10 ml of the filtrated were
taken for titration with 2,6-dichlorophenolindophenol (0.01 g/
100 g solution). The titration end point was detected visually
and all analyses were conducted in duplicated (Benassi
Antunes, 1988).
In order to improve the uniformity of drying conditions of
all samples in the dryer, the drying tray was rotated at each
time that samples were removed from the tray to be subjected
to ascorbic acid analysis.
3. Results and discussion
3.1. Drying kinetics
In the present work, the Page model (Eq. (1)) was used to
fit experimental data of moisture content versus drying time.
The parameters of the model were calculated by non-linear
regression ( p 0.05) and the results can be seen in Table 1.
The correlation coefficient (R2
) and the sum of squared resid-
uals, SSR (Eq. (2)), were considered to evaluate the quality of
fittings.
M ¼
X
X0
¼ expðÀktn
Þ ð1Þ
SSR ¼ S
ÀÀ
Mexp À Mpred
Á2Á
ð2Þ
In Eq. (1), M (dimensionless) is the ratio between the moisture
content at time t (X ) and the initial moisture content (X0), and
k and n are the Page drying coefficients, which determine the
precise shape of the drying curve. While neither of these pa-
rameters have a direct physical significance, empirical regres-
sion equations have been developed relating both parameters
to drying conditions and raw material moisture content (Hos-
sain Bala, 2002; Queiroz, Gabas, Telis, 2004; Wang,
2002).
As expected, results showed that temperature was the main
variable affecting drying kinetics. Higher drying temperatures
led to lower drying times necessary to attain certain moisture
content. The osmotic pre-treatment also contributed for de-
creasing drying times of whole peeled tomatoes (Table 1).
The parameter n in the Page model is a behavior index,
related to the dependence of drying rate on the drying time.
Table 1 shows that n values increased with drying temperature
and were higher for halved tomatoes. When n 1 the drying
rate increases with time and contributes to reducing the neces-
sary drying time.
3.2. Ascorbic acid degradation kinetics
The average vitamin C content of fresh, whole tomatoes
was of 4.00 Æ 0.30 mg ascorbic/g dry matter (that corresponds
to 20.5 Æ 3.5 mg ascorbic/100 g fresh matter), whereas after
caustic peeling, this content felt to 3.36 Æ 0.56 mg ascorbic/g
dry matter, representing a decrease of about 16.0% in the
nutrient content. On the other hand, the osmotic treatment
in NaCl/sucrose solutions caused additional degradation of as-
corbic acid, reducing its content to 2.19 Æ 0.24 mg ascorbic/g
dry matter: a decrease of 35.0% in relation to peeled tomatoes
non-submitted to osmotic treatment. Probably, the reduction
of ascorbic acid content observed during osmotic dehydration
is related to the extraction of vitamin C by the osmotic solu-
tion. Similar results were obtained by Abushita, Daood, and
Biacs (2000) when analyzing the content of this nutrient in
tomatoes. Sablani, Opara, and Al-Balushi (2006), using the
same analytical method in tomatoes soon after harvesting,
found values of about 28 mg ascorbic/100 g fresh matter.
These authors, studying the vitamin C loss during storage,
found that, after a week at 25
C, ascorbic acid content was
reduced to 22 mg/100 g fresh matter.
Toor and Savage (2005) determined the major antioxidants
and antioxidant activity in different fractions (skin, seeds and
pulp) of three tomato cultivars and observed that skin fraction
of all cultivars had significantly higher levels of ascorbic acid
than pulp and seed fractions. These authors pointed out that re-
moval of skin and seeds of tomatoes during home cooking and
processing results in a significant loss of major antioxidants,
since according to their results these parts of the fruit contains
43% of the total ascorbic acid.
Table 1
Page parameters for tomato drying curves at different conditions
Sample Drying
temperature (
C)
K (hÀ1
) n Drying
time (h)*
SSR R2
Tomato halves 50 0.17 Æ 0.01 1.01 Æ 0.02 16.73 0.003 0.998
60 0.18 Æ 0.01 1.20 Æ 0.02 10.20 0.001 0.999
70 0.21 Æ 0.02 1.35 Æ 0.08 7.09 0.011 0.992
Whole peeled tomatoes 50 0.19 Æ 0.02 0.83 Æ 0.05 27.79 0.015 0.985
60 0.20 Æ 0.03 0.89 Æ 0.07 20.56 0.042 0.965
70 0.18 Æ 0.02 1.09 Æ 0.04 12.55 0.009 0.993
Osmotically pre-treated,
whole peeled tomatoes
50 0.14 Æ 0.00 0.96 Æ 0.01 17.90 0.001 0.999
60 0.22 Æ 0.00 0.85 Æ 0.01 16.81 0.0003 0.999
70 0.26 Æ 0.01 0.92 Æ 0.01 10.52 0.001 0.999
*Drying time necessary to attain 50% moisture (wb) estimated using Eq. (1) with parameters k and n from Table 1.
1644 P.H.M. Marfil et al. / LWT - Food Science and Technology 41 (2008) 1642e1647

4. Considering as reference the ascorbic acid content at the
beginning of air drying process, degradation curves were
obtained for each drying temperature and samples treatment.
Fig. 1 shows the results obtained for peeled, whole tomatoes.
Similar plots (Figs. 2 and 3) were obtained for halved toma-
toes and osmotically treated, peeled, whole tomatoes, respec-
tively. A great dispersion of data was detected, but this could
be attributed to the complexity and heterogeneity of natural
samples, as well as to the practical difficulty of assuring uni-
form drying conditions in the tray dryer during long time
periods. Even though, it is possible to observe a clear trend
of faster degradation of ascorbic acid with increasing drying
temperature.
The solid lines included in Figs. 1e3 represent the adjust-
ment of the Weibull model, given by Eq. (3), to experimental
data.
Ct
C0
¼ exp À
t
a
b
!
ð3Þ
In Eq. (3), symbols Ct and C0 refer, respectively, to ascorbic
acid concentration at a certain time, t, and at the zero time
of air drying, while a and b are the fitting parameters of the
model.
Eq. (3) was originally presented in 1939 by W. Weibull to
describe the collapse of stressed materials. Since then it was al-
ready successfully applied to describe kinetics of chemical, en-
zymatic or microbiological degradation processes, which also
lead the system to collapse. The parameter a can be interpreted
as a kinetic reaction constant and represents the characteristic
time to collapse, or specifically, the time when concentration
Ct attains a value corresponding to 36.8% (1/e) of C0. The con-
stant b represents a behavior index and, when b ¼ 1, the model
is reduced to a first order kinetics, with a constant degradation
rate. When b 1 the reaction rate increases with time and the
degradation curve assumes a sigmoidal shape. On the other
hand, if b 1 the reaction rate decreases with time and degra-
dation rate higher than the exponential is observed at the
process beginning (Cunha, Oliveira, Oliveira, 1998). Manso,
Oliveira, Oliveira, and Frias (2001) obtained good results
describing vitamin C degradation in orange juice and non-
enzymatic browning kinetics by the Weibull model.
Eq. (3) was fitted to experimental data by non-linear regres-
sion and the quality of the adjustment was evaluated through
the statistical parameters R2
and SSR. Except for tomato
halves fitting of Eq. (3) to ascorbic acid degradation curves
gave better results when adopting b ¼ 1, i.e. assuming a first
order decay kinetics. This result is in agreement with the
works of Erenturk et al. (2005) and Goula and Adamopoulos
(2006), which had also observed a first order decay for ascor-
bic acid degradation. Nevertheless, for tomato halves, b was
greater than unity, indicating higher degradation rates at the
end periods of drying. This fact could be related to the drying
behavior of tomato halves, which was also different from that
observed for whole tomatoes, as shown in Table 1. The n
0 2 4 6 8 10 12 14 16 18 20 22
0,0
0,2
0,4
0,6
0,8
1,0
1,2
Ascorbicacid(Ct/C0)
Time (hours)
Fig. 1. Ascorbic acid degradation curves for whole peeled tomatoes at different
drying temperatures (-, Tair ¼ 50
C; B, Tair ¼ 60
C; , Tair ¼ 70
C). The
solid lines correspond to the adjustment of the Weibull model.
0 2 4 6 8 10 12 14 16 18 20 22
0,0
0,2
0,4
0,6
0,8
1,0
1,2
Ascorbicacid(Ct/C0)
Time (hours)
Fig. 2. Ascorbic acid degradation curves for halved tomatoes at different dry-
ing temperatures (-, Tair ¼ 50
C; B, Tair ¼ 60
C; , Tair ¼ 70
C). The
solid lines correspond to the adjustment of the Weibull model.
0 2 4 6 8 10 12 14 16 18 20 22
0,0
0,2
0,4
0,6
0,8
1,0
1,2
Acidascorbic(Ct/C0)
Time (hours)
Fig. 3. Ascorbic acid degradation curves for osmotically treated, peeled whole
tomatoes at different drying temperatures (-, Tair ¼ 50
C; B, Tair ¼ 60
C;
, Tair ¼ 70
C). The solid lines correspond to the adjustment of the Weibull
model.
1645P.H.M. Marfil et al. / LWT - Food Science and Technology 41 (2008) 1642e1647

5. values for Page model were also greater than unity only for to-
mato halves.
The parameter a was dependent on temperature and on the
samples treatment. The temperature dependence could be
described by an Arrhenius type equation (Eq. (4)), as shown
in Fig. 4, where plots of ln a versus the reciprocal of drying
temperature in absolute degrees resulted in straight lines.
Higher a values indicate lower degradation rates or, in other
words, longer time to the nutrient collapse. Fig. 4 shows that
drying of whole tomatoes led to a better retention of ascorbic
acid and this retention was improved by the osmotic treatment.
ln a ¼ ln A À
Ea
RT
ð4Þ
Erenturk et al. (2005) observed that the raising of temperature
decreased the retention of vitamin C for fruits cut into pieces,
especially at the beginning of the drying. When surface area
exposed to air was increased, loss of vitamin C also increased.
This also was observed to increasing oxygen content in the
aireCO2 mixtures used as a drying medium. The results
showed that the degradation of vitamin C could be reduced
by using an inert gas.
In Eq. (4), the parameter Ea is the activation energy, R is the
universal gas constant, 8.314 J/mol K, and A is the linear plot
interception with vertical axis. Higher activation energies
indicate a greater temperature dependence of the reaction
rate. Except for drying of osmotically treated, whole peeled to-
matoes at 50
C, Eq. (4) resulted in good fitting to the Weibull
parameter a. This assay was repeated twice and the results
were practically the same, what reduced the probability of
experimental error. A possible explanation would be that
combination of osmotic treatment with the long drying time
necessary to dry the samples at the low temperature (50
C)
accelerated the ascorbic acid degradation.
The slopes of the obtained plots for drying of whole toma-
toes with or without osmotic pre-treatment were similar, show-
ing a similar dependency on temperature, whereas the slope of
the plot corresponding to halved tomatoes was smaller. As
expected, the lower exposition to oxygen in case of whole
tomatoes was able to retard ascorbic acid degradation.
Conclusions
Caustic peeling reduced the initial content of ascorbic acid
in about 16.0%, whereas osmotic pre-treatment reduced this
initial content in about 45.0%. The ascorbic acid degradation
rates during drying were dependent on samples treatment
before drying, as well as on drying temperature. Although
caustic peeling and osmotic pre-treatment caused a significant
reduction in the initial ascorbic acid content of the product,
during drying lower degradation rates were observed in osmot-
ically pre-treated whole tomatoes, whereas higher degradation
rates occurred in halved tomatoes. Higher drying temperatures
increased vitamin C degradation rates. It must be considered,
however, that the increase in ascorbic acid retention may not
compensate the greater costs and longer processing times
due to peeling and osmotic treatment.
Acknowledgements
Authors thank CNPq (Proc. 502883/03-0 and PIBIC) for
financial support.
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