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Optimization of drying parameters and color changes of pretreated organic apple slices
STELA JOKIĆ*, JASMINA LUKINAC, DARKO VELIĆ, MATE BILIĆ, DAMIR MAGDIĆ,
MIRELA PLANINIĆ
Department of Process Engineering, Faculty of Food Technology, University J.J. Strossmayer of Osijek,
F. Kuhaca 18, P.O. Box 709, 31000 Osijek, Croatia
* Corresponding author. Tel.: +385 98 1666629, fax: +385 31 207 115, E-mail address: [email protected]
ABSTRACT
The aim in this research was to determine the optimal drying parameters for the production of dried
apple slices of good texture, good rehydration ability and suitable color. Organic apple samples variety “Florina”
were pretreated and dried in laboratory tray drier at different temperatures. Different chemical pretreatments
were applied on samples (dipping in 0.5% ascorbic acid solution; 0.3% L–cysteine solution; 0.1% 4–hexyl
resorcinol solution and 1% sodium metabisulfite solution). Drying temperatures for nontreated samples were 50,
60, and 70 °C at airflow velocity of 1.5 ms-1
. Color changes were measured by chromameter and digital image
analysis. The Page’s mathematical model was used to calculate the drying kinetic parameters. The obtained
results showed a good agreement with experimental data. According to drying time, rehydration and color
characteristics the optimal drying temperature was found to be 60 °C. The best results were achieved when
samples were pretreated with 4–hexyl resorcinol.
Keywords: Drying kinetics; Color; Rehydration; Pretreatment; Organic apple; Image analysis
ABBREVIATIONS
a red-green
AK ascorbic acid solution
b yellow-blue
B blue
BL blanching in hot water
G green
k, n parameters in model (6)
L lightness-darkness
LC L–cysteine solution
1
m weight (g)
NaB sodium metabisulfite solution
NT non treated samples
R red
RR rehydration ratio
t drying time (min)
T temperature (C)
W weight (g)
X moisture (kgwkgdb-1
)
X´ dimensionless moisture
dX´/dt drying rate (min-1
)
4H 4–hexyl resorcinol solution
∆ELab color changes in CIE Lab color model
∆ERGB color changes in RGB color model
Subscripts
d dried sample
db dry basis
e equilibrium
r rehydrated sample
w water
0 initial
2
INTRODUCTION
During the last decade there is a strong tendency towards organic apple cultivation. Defining the
optimal preservation and storage conditions for fresh apples is beneficial since unsuitable preservation and
storage methods cause losses of fruits and vegetables that range from 10% to 30% (Yaldiz and Ertekin 2001).
The development of new, high quality and consumer attractive dried fruit products is necessary to widen product
availability and diversify markets, particularly since fresh fruit consumption is generally below the levels
recommended in normal diet (Contreras et al. 2008).
Color is an important fruit quality attribute of fruit which occurs in the interaction among light,
observed object and observer (Yam and Papadakis 2004). Color changes are mostly related to browning
reactions that take place during drying of fruits and vegetables. There are many studies about pretreatments of
fruit in order to minimize adverse changes occurring during drying and rehydration (Son et al. 2001; Guerrero-
Beltran et al. 2005; Doymaz, 2006 and 2007). The browning of fruits and vegetables during drying appears due
to both enzymatic and non-enzymatic reactions (Vadivambal and Jayas 2007). To define and display color it is
necessary to select a color space which is a mathematical representation of a set of colors. The three most
common color spaces are: RGB (used for television, computer screens, scanners and digital cameras), CMYK
(used by the printing industry) and the CIE Lab space (used in laboratory colorimeters). Colorimeters measure
color parameters on small rounded area and give nonobjective results on colored samples with different color
area. Most objective color assessment can be obtained using image analysis of all visible surface of analyzed
sample. These techniques can be applied on both sides of apple, reddish and greenish to ensure more objective
results because almost 100% of apple surface is captured in an image. Color changes measured in RGB color
model can be separated in color channels with intensity values for red, green and blue color from 0 to 255
(Magdić and Dobričević 2007).
For the food technologist properties such as color, shape (shrinkage) and rehydration capacity are
determinant for the quality of the dried product (Fernandez et al. 2005). Drying is probably one of the oldest
methods of food preservation and also well researched scientific area (Lewicki and Jakubczyk 2004; Velić et al.
2004; Lewicki 2006; Sacilik and Elicin 2006; Kaya et al. 2007; Margaris and Ghiaus 2007; Doymaz 2008;
Lertworasirikul 2008). Thousands of years of experience and trial-error methods as well as research done during
the last hundred years resulted in development of a variety of drying methods and drying equipment.
The aim of this research was to investigate the impact of drying kinetics and different chemical
pretreatments on color changes of organic apple slices during the drying process. The effect of temperatures and
3
pretreatments on the quality of dried apple samples was determined on the basis of color and rehydration
characteristics. For this purpose color changes were measured by two different methods and results were
compared.
4
MATERIAL AND METHODS
Material
Organically grown apples (var. Florina) were obtained from the local small family farm and stored at +4
°C. After stabilization at the ambient temperature, apples were hand peeled and cut into tube-shaped samples, 20
mm diameter and 5 mm height.
Drying method
Drying was performed in a pilot plant tray dryer (UOP 8 Tray Dryer, Armfield, UK). The dryer operates
on the thermogravimetric principle. The dryer (Fig. 1) is equipped with controllers for controlling the
temperature and airflow velocity. Air was drawn into the duct through a diffuser by a motor driven axial flow fan
impeller. In the tunnel of the dryer there were carriers for trays with samples, which were connected to a balance.
The balance was placed outside the dryer and continuously determined and displayed the sample weight.
FIG. 1.
The drying temperatures for nontreated apple samples had varied from 50 °C, 60 °C and 70 °C. The
dryer was operated at air velocity of 1.5 ms-1
. Prior to drying at temperature of 60 °C, apple samples were treated
for four groups of analysis as follows: dipping in 0.5% ascorbic acid solution; dipping in 0.3% L–cysteine
solution; dipping in 0.1% 4–hexyl resorcinol solution and dipping in 1 % sodium metabisulfite solution. The
apple samples on trays were placed into the tunnel of the dryer and the measurement started from this point.
“Testo 350” probes, placed into the drying chamber, were used to measure the drying air temperature. Sample
weight loss and airflow velocity were recorded in five minutes interval during the drying process using a digital
balance (Ohaus, Explorer, USA) and digital anemometer (Armfield, UK). Dehydration lasted until a moisture
content of about 12% (wet base) was achieved. Dried samples were kept in airtight glass jars until the beginning
of rehydration experiments.
Determination of the total solid/moisture content
The moisture content of the dried samples was determined using a standard laboratory method. Small
quantities of each sample were dried in a vacuum oven (24 hours at 105 °C). Time dependent moisture content
of the samples was calculated from the sample weight and dry basis weight. Weight loss data allowed the
moisture content to be calculated such as follows:
w
db
mX(t)=
m (1)
5
Color measurements
The color characteristics were used as quality parameter of the dried apple samples. Color measurement
was done using Minolta CR-400 Chromameter and image analysis system. Data were stored in CIE Lab and
RGB color models and color changes during this period were evaluated. The total color difference in CIE Lab
color model was calculated as follows:
( ) ( ) ( )2 2 2
Lab∆E = ∆L + ∆a + ∆b
(2)
Parameter L refers to the lightness of the samples, and ranges from black = 0 to white = 100. A negative value of
parameter a indicates green, while a positive one indicates red–purple color. Positive value of parameter b
indicates yellow while negative value indicates blue color. Samples were placed on the measure head of CR-400
and measurements of color were performed for all prepared samples. A standard white color was used for
calibration.
Color changes in RGB color model were followed by image analysis. Basic elements of image analysis
system shown in Fig 2. were lightening chamber with low voltage halogen lamps with reflector (provided
illumination of sample area of 760±5 Lux), digital camera (Panasonic Lumix DMC-FZ30) and software for
image preprocessing and analysis (IrfanView, Adobe Photoshop, Global Lab Image/2). Samples for imaging
were placed at 60 cm from camera.
FIG 2.
Images were stored in bitmap (BMP) graphic format with 8-bit pallet (28 = 256 colors) and after that were
processed and analyzed. This graphic format stores information about colors in RGB-triplets for every pixel on
the image where red (R), green (G) and blue (B) are intensities of mentioned colors in range from 0 to 255.
Program calculated average percentage of red (R), green (G) and blue (B) color on a sample area. An average
share of each color on sample surface was presented as the final result. Color changes in RGB color model were
defined as:
( ) ( ) ( )2 2 2
∆E = ∆R + ∆G + ∆BRGB
(3)
where ∆R, ∆G and ∆B were differences between color values of fresh apple samples and color values of dried
samples. Average values of color and color changes of apples were calculated for both color models.
6
Rehydration
Rehydration characteristics of the dried products were used as a quality index and they indicated the
physical and chemical changes that occurred during the drying and were influenced by processing conditions,
sample compositions, sample preparation and extent of structural and chemical disruptions induced by drying
(Krokida and Maroulis 2001).
Approximately 3 g (±0.01 g) of dried samples were placed in a 250 ml laboratory glass (two
measurements for each sample), 150 ml distilled water was added, the glass was covered and heated for 3 min up
to the boiling point. The content of the laboratory glass was then cooked for 10 min by mild boiling and cooled.
Cooled content was filtered for 5 min under vacuum and weighted. The rehydration ratio (RR) was used to
express ability of the dried material to absorb water. It was determined by the following equation:
r
d
WRR=
W (4)
Drying rate curve determination
Page's exponential model successfully describes the drying kinetics of food materials (Velić et al. 2004;
Simal et al. 2005; Bozkir 2006; Wang et al. 2007; Singh et al. 2008). The authors also used this model to
describe the changes of moisture content and drying rates. To avoid some ambiguity in results due to differences
in initial sample moisture, the sample moisture was expressed as dimensionless moisture ratio ( 0X'=X(t)/X ) .
The drying curve for each experiment was obtained by plotting the dimensionless moisture of the sample vs. the
drying time. For approximation of the experimental data and calculating drying curves (Eq. 5) and drying rate
curves (Eq. 6), the simplified model was used, as follows:
( ) )(-ktn
etX' = (5)
( ) ( )tX'tnk
dt
dX' 1n⋅⋅⋅=−
− (6)
The parameters k and n were calculated by non-linear regression method (Quasi-Newton) using
Statistica 6.0 computer program. The correlation coefficient (r2) was used as a measure of model adequacy.
7
RESULTS AND DISCUSSION
Fig. 3 shows experimental data of moisture contents at 50 °C, 60 °C and 70 °C and the fitting to Page's
model (mod_50, mod_60 and mod_70) versus drying time at different temperatures. It can be seen that a good
agreement exists between the experimental data and the chosen mathematical model (Page's model), which is
confirmed by high values of correlation coefficient in all runs (R2 = 0.99981 - 0.99989). Results show that the
temperatures had a significant effect on the drying rates of apple. The variation of moisture content and drying
time was obtained at each drying temperature. Drying of apple samples at higher temperature resulted in shorter
drying time, as it was expected.
FIG. 3.
Fig. 4 shows typical drying curves versus drying time for different drying temperatures. Apples did not
exhibit a constant rate period of drying. The entire drying took place in the falling rate period.
FIG. 4.
Fig. 5 shows experimental moisture content versus drying time for different pretreatments at air velocity
of 1.5 m s-1
and drying temperature at 60 °C.
FIG. 5.
Fig. 6 shows drying rate vs. drying time for different pretreatments at air velocity of 1.5 m s-1
and
drying temperature at 60 °C. It can be seen that different pretreatments decrease the drying time compared to
nontreated apple samples.
FIG. 6.
Fig. 7 shows total color difference in both color models CIE Lab and RGB versus different drying
temperatures for nontreated apple samples. ∆ELab values varied from 9.4 to 11.42, whereas ∆ERGB values varied
from 12.39 to 21.74. The smaller color changes were observed where apple samples were dried at 60 °C.
FIG. 7.
Fig. 8 shows total color changes in both color models CIE Lab and RGB of pretreated apple samples
versus different pretreatments at air velocity of 1.5 m s-1
and drying temperature at 60 °C. ∆ELab values for
pretreated apple samples varied from 5.72 to 9.38, whereas ∆ERGB values varied from 2.01 to 39.22. Chemical
pretreatment with 0.1% 4–hexyl resorcinol give the smaller change of color in both color models. It can be seen
the big difference in color changes between two chosen color models in the case of pretreatments with ascorbic
acid. It depends on chosen area of analyzed apple sample surface. Lab values differs because of small diameter
8
of measuring head of instrument Meanwhile, using image analysis in RGB color model all area of analyzed
apple sample is included.
FIG. 8.
Fig. 9 shows RGB color space in pixels vs. luminosity of dried apples for different pretreatments and
drying temperature at 60 °C. The biggest luminosity changes were observed on samples pretreated with ascorbic
acid. Color content is expressed in nuances from 0 to 255. Samples closer to nuance 255 were lighter and the
samples closer to 0 showed darker color.
FIG. 9.
Fig. 10 shows rehydration ratio versus different drying temperatures for drying of nontreated apple
samples. The rehydration ratio was affected significantly by the drying temperatures. Rehydration ratio for
nontreated apple samples decreased as the drying temperature increased. The results show that the longer the
exposure time of apple samples to certain temperature is the higher are the irreversible degradation changes.
FIG. 10.
Fig. 11 shows rehydration ratio versus different pretreatments of apple samples. It can be seen that
dipping in 0.1 % 4–hexyl resorcinol solution resulted in the highest rehydration, compared to other
pretreatments.
FIG. 11.
9
CONCLUSIONS
Air drying of apple samples could be modeled using Page’s equation. The results of the estimation
exhibited correspondence to the experimental results. Increase in the drying air temperature caused a decrease in
the drying time and an increase in the drying rate. Rehydration rates and color characteristic of apple samples
were found to be dependent on drying conditions. The rehydration ratio decreased as the drying temperature
increased. Also, rehydration ratio of all treated samples was higher as compared to nontreated samples. In view
of the color measurements for nontreated apple samples calculated correlation among used color models was
found to be 0.97, and for pretreated apple samples correlation was equal to 0.37.
According to drying time, rehydration and color characteristics the optimal drying temperature was
found to be 60 °C. The best results (reduced drying time, high rehydration ratio and minimum color change)
were achieved when samples were pretreated with 4–hexyl resorcinol.
Today’s consumer expectation for better food quality drives research and improvement of drying
technologies. The observed differences in the drying kinetics and pretreatments should be considered when
selecting the best drying condition in order to improve the final product quality. The investigation of economic
parameters of different drying pretreatments should be considered as well.
ACKNOWLEDGMENTS
This work was financially supported by Ministry of Science, Education and Sports of the Republic of Croatia,
projects 113-0000000-3497 and 113-1130471-0592.
10
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12
Figures
Fig. 1. Schematic diagram of the convection drying equipment
Air outlet Air inlet
Digital
anemometer Trays
Heat power control
Fan speed
control
Digital
balance
Heaters
Thermocouples to PC
Relative humidity couples
to «Testo 350»
Door
Thermocouples
to «Testo 350»
13
1. Lightning chamber
2. Light source
3. Digital camera
4. Background for sample
5. Sample for analysis
6. Computer
Fig. 2. Image analysis system
14
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 50 100 150 200 250 300 350 400
t [min]
X'
50 °C
60 °C
70 °C
mod_50 °C
mod_60 °C
mod_70 °C
Fig. 3. Experimental and approximated moisture content of nontreated dried apple samples
15
0.000
0.005
0.010
0.015
0 100 200 300 400
t [min]
-dX
'/dt [m
in -1
]
50 °C
60 °C
70 °C
Fig. 4. Drying rate dynamics of nontreated apple samples at different temperatures
16
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 50 100 150 200 250 300
t [min]
X'
NT
AK
LC
4H
NaB
Fig. 5. Changes of apple moisture content at 60 °C drying temperature after different pretreatments
17
0.000
0.003
0.006
0.009
0.012
0 40 80 120 160 200 240 280
t [min]
-dX
'/dt
[m
in -1
]
NT
AK
LC
4H
NaB
1
Fig. 6. Drying rate dynamics of pretreated apple samples at 60 °C drying temperature
18
0
5
10
15
20
25
50 ºC 60 ºC 70 ºC
∆E
Lab
RGB
Fig.7. Total color changes (∆ELab and ∆ERGB) of nontreated apple samples at different drying temperatures
19
0
5
10
15
20
25
30
35
40
45
NT AK LC 4H NaB
∆E
Lab
RGB
Fig. 8. Total color changes (∆ELab and ∆ERGB) of pretreated apple samples at 60 °C drying temperature
20
0
500
1000
1500
2000
2500
3000
3500
4000
4500
100 120 140 160 180 200 220 240
luminosity
pix
els
1_NT
2_4H
3_AK
4_LC
5_NaB
1
23
4
5
Fig. 9. Dried apples luminosity after different pretreatments at 60 °C drying temperature
21
5.20
5.40
5.60
5.80
6.00
6.20
6.40
6.60
6.80
50 °C 60 °C 70 °C
RR
Fig. 10. Rehydration ratio (RR) of nontreated apple samples at different drying temperatures
22
0
1
2
3
4
5
6
7
8
9
10
NT AK LC 4H NaB
RR
Fig. 11. Rehydration ratio (RR) of pretreated apple samples at 60 °C drying temperature