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Desulphiting dried apricots by exposure to hotair flowMehmet Ozkan and Bekir Cemeroglu*Department of Food Engineering, Faculty of Agriculture, Ankara University, Dıskapı 06110, Ankara, Turkey
Abstract: Sulphited dried apricots were exposed to hot air flows at 40, 50 and 60°C and the removal of
SO2 was investigated as their moisture content fell from an initial value of 193.2g kg�1 to a final value of
80–90g kg�1. A first-order kinetic model was found for the removal of SO2 between 40 and 60°C.
Temperature quotients (Q10) for the removal of SO2 were 2.84 between 40 and 50°C and 4.93 between 50
and 60°C; the activation energy (Ea) was 114.40kJmol�1 between 40 and 60°C. Analysis of the kinetic
data also suggested a first-order reaction for non-enzymatic browning, withQ10 values of 2.34 between
40 and 50°C and 5.36 between 50 and 60°C and an Ea value of 109.36kJmol�1 between 40 and 60°C.
Exposure of dried apricots to a 60°C air flow resulted in a rate constant for brown pigment formation
that was 12 and 5 times higher than those at 40 and 50°C respectively.
# 2002 Society of Chemical Industry
Keywords: desulphiting; non-enzymatic browning; surface colour; dried apricots; reaction kinetics
INTRODUCTIONSulphites are the most widely used chemicals in the
production of dried apricots, mainly to preserve the
characteristic golden yellow colour during storage.
This preservative effect is due to the ability of sulphites
to reduce the extent of complex non-enzymatic
browning reactions during drying and storage.1–3
Traditionally, apricots in Turkey are treated with
fumes of burning sulphur in an enclosed room before
sun drying, mostly by the apricot producers them-
selves.
The maximum legal limit of 2000mg kg�1 SO2 in
dried apricots is accepted by most countries, including
Turkey.4 Traditionally sulphited apricots can have
SO2 contents ranging from less than 1000 to over
6000mg kg�1.5 This is because many factors affect the
absorption and retention of SO2 by fresh apricots,
including variety, maturity, form of apricot (whole,
pitted or cut), concentration of SO2 in the sulphur
house and exposure time.6,7 Because of the complexity
of the sulphiting process, dried apricots may some-
times contain SO2 levels much higher than the legal
limit. Therefore it becomes necessary to decrease the
final SO2 level to prevent loss of marketability.
The safety of sulphites has been questioned on the
grounds of their alleged role in initiating asthmatic
reactions in some sensitive individuals.8 Concerns over
safety resulted in the revocation of the GRAS
(generally recognised as safe) status of sulphites for
use in fresh fruits and vegetables by the FDA in 1986.9
As a result of continuous pressure from consumers and
the medical community, the legal limits for sulphited
foods, including dried apricots, are expected to be
lowered.
Desulphiting various fruit products preserved with
sulphites has long been practised.1 For example,
Maraschino cherries are commonly preserved with
sulphites, which are removed by a combination of
leaching with water and boiling. Fruits for jam making
used to be preserved with sulphites, which were
removed by heating during jam making. Similarly,
the removal of SO2 from sulphited fruit juices and
purees used to be carried out by heating under
vacuum.10 The effect of heating on the removal of
sulphites has been attributed to decomposition of the
sulphites bound to food constituents and removal of
the resulting free SO2 by volatilisation.1 Aside from
physical methods, hydrogen peroxide (H2O2) has been
recommended for desulphiting fruit juices and dried
fruits.1 Detailed experimental data were given on the
possible use of H2O2 for desulphiting pickling cucum-
bers preserved in sulphite brine.11 Moreover, observa-
tions on excessively sulphited dried apricots showed
that H2O2 could successfully be used for desulphit-
ing.12
No experimental data have been found on desulfit-
ing dried apricots by hot air flow. Only a few studies
have appeared in the literature on the loss of SO2 from
dried apricots during storage.3,13 Therefore this study
was undertaken to provide experimental data on
desulphiting dried apricots exposed to hot air flow.
Specific objectives were to determine the kinetics of
(Received 14 February 2002; revised version received 5 June 2002; accepted 12 August 2002)
* Correspondence to: Bekir Cemeroglu, Department of Food Engineering, Faculty of Agriculture, Ankara University, Diskapi 06110, Ankara,TurkeyE-mail: [email protected]/grant sponsor: Research Foundation of Ankara University, Turkey; contract/grant number: 96.25.00.12
# 2002 Society of Chemical Industry. J Sci Food Agric 0022–5142/2002/$30.00 1823
Journal of the Science of Food and Agriculture J Sci Food Agric 82:1823–1828 (online: 2002)DOI: 10.1002/jsfa.1266
the removal of sulphites from dried apricots and to
assess the effects of hot air on the colour of dried
apricots.
MATERIALS AND METHODSMaterialsCommercially sun-dried and excessively sulphited
(between 4000 and 4800mg kg�1 dry weight) apricots
(Prunus armenica L) were provided by the Apricot
Research Center, Malatya, Turkey. The apricots were
mixed thoroughly to minimise variations in the bulk
sample and left in an enclosed container at room
temperature for 2 weeks to equilibrate for moisture. At
the end of this period the apricots were placed into
polyethylene freezer bags and stored in a freezer at
�35°C until used for analysis.
Exposure of apricots to hot airApricots prepared as above were exposed to hot air
flows at 40, 50 and 60°C for 308.5, 144 and 100.5h
respectively. This study was carried out in a forced air
oven (Memmert ULM 500, Schwabach, Germany)
consisting of a fan and a drying chamber equipped
with an electronic temperature controller (�1°C).
The air exchange rate was approximately 75 times the
total oven volume (108 l) per hour. Approximately
1kg of dried apricots were spread uniformly on a
stainless steel tray in a single layer, placed in the oven
and exposed to hot air flow at a given temperature.
Samples of 100g were removed from the oven at
various intervals during drying, placed in hermetically
sealed jars and stored in a freezer at �35°C until used
for analysis. In these samples, SO2, surface colour and
non-enzymatic browning were determined.
Moisture analysisThe initial moisture content of dried apricots was
determined in quadruplicate under vacuum using
AOAC method 934.06.14 For this purpose, 400g of
apricots were taken from the bulk sample equilibrated
for moisture and ground through a plate with 4mm
orifices. This was repeated four times to obtain a
homogenised mixture. Drying was carried out first on
a boiling water bath and then in a vacuum oven at
70�1°C for 7 and 3h periods respectively.
A separate experiment was carried out to determine
the moisture content of apricot samples removed from
the oven at various intervals. For this purpose, eight
dried apricots were weighed and placed in the oven.
The weight loss due to drying was determined for each
time interval until the end of the drying period.
Weighing was carried out using an electronic balance
(Scaltech SBA 51, Heiligenstadt, Germany) with a
sensitivity of �0.01g. The moisture content of
samples was calculated from the weight loss.
Sulphur dioxide analysisSulphur dioxide was determined in duplicate by the
modified Monier Williams distillation method.15 The
homogenised sample prepared for moisture analysis
was used. The SO2 content of dried apricots was
expressed as mg kg�1 dry weight.
Non-enyzmatic browning measurementsNon-enzymatic browning was determined in duplicate
according to the method described previously.16
Extraction of the water-soluble brown pigment was
carried out with 20g l�1 acetic acid containing 10g l�1
formaldehyde. Interfering carotenoid pigments were
removed by the use of lead acetate and ethanol.
Formaldehyde was used to remove the interfering
SO2. A sample of 5g was taken from the sample
prepared for moisture analysis and rehydrated in 65ml
of 20g l�1 acetic acid containing 10g l�1 formaldehyde
at 4°C overnight prior to extraction. This mixture was
then homogenised in a Waring (New Hartford, CT,
USA) blender for 3min. The resulting cloudy extract
was centrifuged (Hettich EBA 12, Tuttlingen, Ger-
many) at 4020�g for 10min. The supernatant was
first clarified by adding 5ml of 100g l�1 lead acetate.
The resulting extract was stirred vigorously with a glass
rod and made up to volume with 20g l�1 acetic acid
containing 10g l�1 formaldehyde. After centrifugation
at 4020�g for 10min the supernatant was then
clarified by adding an equal volume of ethanol and
the mixture was centrifuged again as described above.
Absorbances were recorded at 420 and 600nm. The
browning was calculated by subtracting the absor-
bance at 600nm (for turbidity) from that at 420nm.
Surface colour measurementsThe surface colour of dried apricots was measured
with a Minolta CR-300 Reflektans colorimeter
equipped with DP-300 data processor (Minolta,
Osaka, Japan). The colorimeter had an 8mm diameter
viewing area and was calibrated with a white tile
(L*=97.26, a*=þ0.13, b*=þ1.71). Measurements
were recorded in L* (lightness), þa* (redness), þb*(yellowness) CIE (Commission Internationale de
I’Eclairage) colour co-ordinates. Chroma (C*) and
hue (h*) values were calculated from a* and b* values
using the equations
C* ¼ ða�2 þ b�2Þ1=2 ð1Þ
h* ¼ tan�1ðb*=a*Þ ð2Þ
Colour measurements were performed on both
ground and whole apricots immediately after drying
and rehydration.
Statistical analysisColour values were analysed using a paired t-test.
Minitab for Windows (v 12, 1998) and MSTAT
(v 3.00, 1985) software packages were used to analyse
the results.
RESULTS AND DISCUSSIONThe initial moisture and SO2 contents of dried apricots
1824 J Sci Food Agric 82:1823–1828 (online: 2002)
M Ozkan, B Cemeroglu
were 193.2g kg�1 and between 3967 and
4880mg kg�1 dry weight respectively. CIE colour
values of dried apricots were initially in the following
ranges: L*=32.62–44.28, a*=2.22–9.07, b*=16.28–
32.56, C*=16.76–33.44 and h*=69.44–84.65.
Effect of hot air on SO2 contentThe removal of SO2 was studied in dried apricots
exposed to hot air flows at 40, 50 and 60°C. Table 1
shows that exposure to hot air flow was effective in
reducing the SO2 content. For example, the SO2
content decreased from 4577 to 3019mg kg�1 dry
weight after 144h of drying at 50°C, a reduction of
34%. In Fig 1 the SO2 contents of dried apricots
exposed to various air temperatures are plotted as a
function of time. The linear relation indicates that the
removal of SO2 followed first-order kinetics. Reaction
rate constants (k) were calculated at a given tempera-
ture using the equation
ln ðCt=C0Þ ¼ �kt ð3Þ
where C0 is the initial SO2 concentration and Ct is the
SO2 concentration after tmin of exposure to hot air at a
given temperature.
As expected, more SO2 was removed at higher air
temperatures. For example, the SO2 content of dried
apricots after 96h of drying declined by 19.6% at
40°C, 27.6% at 50°C and 64.2% at 60°C. At 40 and
50°C, biphasic first-order plots were obtained (Fig 1).
The SO2 content declined rapidly at 40 and 50°C for
the first 40 and 25h of drying respectively. In contrast,
the removal of SO2 at 60°C followed a monophasic
pattern throughout the drying period. The biphasic
pattern may have occurred as a consequence of the
easy removal of SO2 initially from surface tissues at the
comparatively low temperatures of 40 and 50°C. The
transportation of SO2 from the centre of the fruit to the
surface may have been the rate–limiting factor at 40
and 50°C after this initial period, as it appears to have
been throughout at 60°C.
The temperature dependence of SO2 removal by hot
air flow was determined by calculating the activation
energy (Ea) and temperature quotients (Q10) between
40 and 60°C from the equations
k ¼ k0e�Ea=RT ð4Þ
Q10 ¼ ðk2=k1Þ10=ðT2�T1Þ ð5Þ
Ea is a measure of the temperature sensitivity of a
process. High-Ea processes are highly sensitive to
temperature changes. The Q10 value reflects the
change in the rate of a process for a 10°C rise in
temperature. For quality-deteriorative processes the
Q10 value is generally assumed to be equal to 2, ie the
rate of reaction doubles for each 10°C rise in
temperature.
The k values were calculated only from the second
part of the biphasic curves at 40 and 50°C. The Ea and
Q10 values are presented in Table 2. Between 40 and
60°C the Ea value was 114.40kJmol�1; the Q10 values
Table 1. SO2 contents of dried apricots treated with hot air at varioustemperatures
Temp
(°C)Time
(h)
SO2 content
(mg kg�1 dry wt)
Decrease in
SO2 content
(%)
40 0 4836�63 0
40 4114�289 14.9
88.5 3904�64 19.3
137.5 3680�23 23.9
185 3599�37 25.6
237 3508�91 27.5
280.5 3375�39 30.2
308.5 3242�28 33.0
50 0 4577�28 0
24 3965�200 13.4
48 3619�28 20.9
72 3530�23 22.9
96 3364�30 26.5
120 3036�80 33.7
144 3019�38 34.0
60 0 4005�54 0
15.5 3501�168 12.6
24.5 3360�48 16.1
38.5 2908�9 27.4
63.5 2270�130 43.3
72.5 1784�62 55.5
100.5 1298�58 67.6
Figure 1. Effect of various air temperatures on SO2 content of driedapricots.
Table 2. Effect of temperature on removal of SO2 from dried apricots
Temp
(°C)k
(103h�1)
Ea
(kJmol�1)
Q10
40–50°C 50–60°C
40 0.81 (0.978)a
50 2.30 (0.959) 114.40 2.84 4.93
60 11.35 (0.978)
a Numbers in parentheses are coefficients of determination.
J Sci Food Agric 82:1823–1828 (online: 2002) 1825
Desulphiting dried apricots
were 2.84 between 40 and 50°C and 4.93 between 50
and 60°C. Comparing the Q10 values, the temperature
quotient for the rate of SO2 removal between 50 and
60°C was almost twice as high as that between 40 and
50°C.
Non-enzymatic browningA linear relation was found between the logarithms of
absorbance values measured at 420nm for extracts of
apricot tissues and time, representing first-order
reaction kinetics (Fig 2). The rate of browning was
increased greatly by raising the air temperature.
Similarly, a considerable increase in browning of dried
apricots stored at room temperature for 6 months was
previously observed.17 In the same study, apricot
samples stored at 4 and 11°C for the same time period
did not suffer much colour loss.
The limit for acceptability of colour of dried apricots
was defined as an absorbance value of 0.3 at 440nm.3
Similarly, the storage life of dried apricots was defined
as the time at which they had lost 65% of their initial
SO2 content.13 In our study, absorbance values for
browning measurements at 420nm never reached 0.3
at 40 and 50°C, but this limit was reached in apricot
samples exposed to a 60°C air flow for 64, 73 and
100h, with values of 0.46, 0.70 and 1.24 respectively.
The removal of SO2 was 33% at 40°C for 309h and
34% at 50°C for 144h (Table 1). In contrast, the
decrease in SO2 content was 50, 61 and 72% for 64, 73
and 101h of drying respectively at 60°C. Based on
these results, the colour of dried apricots exposed to a
60°C air flow for over 64h cannot be accepted.
The Q10 values for browning of apricots were 2.24
between 40 and 50°C and 5.36 between 50 and 60°Cand the Ea value was 109.36kJmol�1 between 40 and
60°C. (Table 3). Similar Ea values were found in the
literature. For example, the browning of apricots
stored at 22.2–49°C followed first-order kinetics, with
a Q10 value of 3.9 and an Ea value of 108.86kJmol�1.13
A typical Ea value for non-enzymatic browning in
foods is said to be 125kJmol�1.18 In contrast, the
browning of apricots containing 800mg kg�1 SO2 and
dried at 50, 65 and 80°C followed zero-order kinetics,
with an Ea value of 41.87kJmol�1.19 This value is
typical of physical processes, and the authors sug-
gested that the rate of browning was controlled by
water diffusion.
Since dried apricots contain approximately
800g kg�1 dry matter, the Ea values for browning of
dried apricots can also be compared with the Ea values
for browning of fruit juice concentrates. For example,
Ea values for non-enzymatic browning were
127.33kJmol�1 in grapefruit juice concentrates of
62°Bx,20 126.53kJmol�1 in apple juice concentrates
of 70°Bx21 and 97.14kJmol�1 in pear juice concen-
trates of 72.5°Bx.22 Based on these comparisons, our
Ea result correlates well with those for non-enzymatic
browning of juice concentrates exposed to various
temperatures.
Surface colour measurementsThe change in colour of dried apricots exposed to hot
air to reduce the SO2 level was observed by measuring
the colour of both ground and whole apricots. The
colour of ground and whole apricots immediately after
drying could not be measured properly, because the
formation of a hard crust during drying resulted in an
uneven surface, which caused the reflection of light.
Covering the surface of ground apricots with poly-
ethylene film did not help to correct this. Because of
the difficulties in measuring the surface colour of dried
apricots with moisture contents as low as 90g kg�1, the
apricots exposed to hot air flow were rehydrated to
their initial moisture contents, thus allowing the colour
to be measured properly.
The change in colour of the apricots was expressed
by subtracting the CIE colour values of hot air-
exposed samples from those of control samples (Table
4). L*, b*, C* and h* values increased in apricot
samples exposed to hot air at 40 and 50°C and
subsequently rehydrated. The L* values increased by
3–4 units in apricots dried at 40 and 50°C and
subsequently rehydrated to their initial moisture
levels. Only the a* values decreased at 40 and 50°C.
At 60°C, all colour values decreased, except the a*value. The adverse effect of heat on colour was
especially apparent in apricots exposed to a 60°C air
flow for over 64h. Experiments carried out on dried
apricots containing 300mg kg�1 SO2 and stored at
room temperature for 6 months showed a decrease inFigure 2. Effect of various air temperatures on non-enzymatic browning ofdried apricots.
Table 3. Effect of temperature on non-enzymatic browning of dried apricots
Temp
(°C)k
(103h�1)
Ea
(kJmol�1)
Q10
40–50°C 50–60°C
40 1.77 (0.813)a
50 3.96 (0.996) 109.36 2.24 5.36
60 21.23 (0.975)
a Numbers in parentheses are coefficients of determination.
1826 J Sci Food Agric 82:1823–1828 (online: 2002)
M Ozkan, B Cemeroglu
Hunter L*, a* and b* values: L* from 39.7 to 26.4, a*from 25.7 to 10.8 and b* from 36.1 to 14.3.17
Hot air was effective in reducing the SO2 content of
oversulphited dried apricots. Although the SO2 con-
tents were decreased substantially by hot air flow, the
levels were still above the legal limit of 2000mg kg�1
(dry weight). Therefore this method is particularly
recommended for dried apricots with SO2 contents
between 2000 and 3000mg kg�1 (dry weight) at the
applied air temperatures and exposure times. In
particular, air temperatures of 40–50°C are strongly
recommended. At these temperatures the surface
colour parameters L* and b* increased in dried
apricots after rehydration to their initial moisture
levels in comparison with the L* and b* values of dried
apricots prior to hot air exposure. Also, brown pigment
formation was below the recommended limits at 40
and 50°C.
CONCLUSIONThe results from this study suggest that the SO2
content of dried apricots sulphited a little over the
legal limit can be reduced by exposure to hot air flow.
The storage life was also increased owing to the
substantial decrease in moisture content. Bin dryers,
commonly used in food dehydration, can easily be
used for desulphiting dried apricots. This study also
showed that surface colour measurements should be
carried out after rehydrating the dried apricots to their
initial moisture contents. To avoid the need for the
removal of sulphites from oversulphited dried apricots,
we recommend either that traditional sulphiting
practice be optimised or that new methods of
sulphiting be established.
ACKNOWLEDGEMENTSThis study forms part of M Ozkan’s PhD thesis and
was funded by the Research Foundation of Ankara
University, Turkey under grant 96.25.00.12. The
authors thank the Apricot Research Center, Malatya,
Turkey for providing the dried apricots.
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Table 4. Comparison of mean (40 measurements) colour values of rehydrated desulphited dried apricots with reference to their initial values
Temp
(°C)Time
(h)
Moisture
(g kg�1) L* a* b* C* h*
40 0 193.2 38.57 4.93 22.13 22.73 77.36
308.5 195.0a 43.22 4.26 25.00 25.41 80.21
Difference of means 4.650** �0.663 2.868** 2.685** 2.846**
50 0 193.2 39.26 5.42 24.18 24.81 77.44
144 194.2a 42.06 5.12 25.50 26.05 78.67
Difference of means 2.797** �0.304 1.326 1.240 1.231
60 0 193.2 38.16 5.35 22.19 22.87 76.49
100.5 187.5a 36.49 6.39 17.86 19.03 69.75
Difference of means �1.668** 1.038** �4.330** �3.841** �6.739**
a Moisture content of dried apricots after drying and subsequent rehydration.
** P<0.01.
J Sci Food Agric 82:1823–1828 (online: 2002) 1827
Desulphiting dried apricots
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M Ozkan, B Cemeroglu
