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Journal of Engineering Science Vol. XXV, no. 3 (2018), pp. 100 - 110
Fascicle Food Engineering ISSN 2587-3474
Topic Food technologies and Food Processes eISSN 2587-3482
Journal of Engineering Science November, 2018, Vol. XXV (3)
PEACHES CONVECTIVE DRYING
Vitali Vişanu
Technical University of Moldova, Faculty of Mechanical Engineering and Transport,
9/8, Studentilor Str., MD-2045, Chisinau, Republic of Moldova
Visanu Vitali: visanuvitali@pmai.utm.md, (+373) 069675995
Received: July, 31, 2018
Accepted: October, 05, 2018
Abstract. This article deals with convective drying kinetics process of peaches that were
collected in Republic of Moldova climatic area, namely SPRINGCREST, CARDINAL and
REDHAVEN varieties. The kinetics study was performed for different temperatures: 50°C,
60°C, 70°C, 80°C, 90°C, for different drying agent velocities: 0.5 m. s-1, 1.0 m. s-1, 1.5 m. s-1,
2.0 m. s-1, 2.5 m. s-1, and for different thicknesses 2.10-3m, 4.10-3m, 6.10-3m, 8.10-3m, 10.10-
3m. For the determination of the optimal quality and conditions of drying parameters there
was also performed an appearance analysis for each dried sample.
Keywords: drying process, convective drying kinetics, drying agent temperature, velocity,
humidity, drying duration, peaches.
Introduction
Currently, globally, the peach is the third after the apple and the plum in terms of
surface and production volume. [1] Republic of Moldova exports about 5-7 k tons of fresh
peaches per year. [18]. Peaches are highly appreciated thanks to their excellent taste, which
is determined, by a fine pulp and pleasant aroma. The high food value of peaches is due to
a complex and equilibrated composition consisting of 87.5% water, 12.49% of total dry
substances and (10.54 %) soluble dry substances. Sugar content is 8.4 g.100-1 g of product
as well as a treatable acidity of 0.5% (pH=4).
The chemical composition is generally represented by proteins (0.9 g .100-1g of
product), lipids (0.30 g.100-1 g of product), carbohydrate (9.90g.100-1 g of product); The
minerals are represented by an increased content of K (190 mg.100-1 g of product), P (20
mg.100-1g of product), Mg (9 mg.100-1 g of product) and Ca (6 mg.100-1g of product). Peaches
contain as well a variety of vitamins such as A (326 IU), C (6.6 mg.100-1g of product), K (2.60
mg.100-1g of product), E (0.70 mg.100-1g of product), B3 (0.8 mg.100-1g of product), B5 (0.20
mg.100-1g of product), B8 (6.10 mg.100-1g of product), Betaine (0.3 mg.100-1g of product). [2-
7].
Being a seasonal product, there are important quantities of peaches that remain
unvalued as those have both short harvest and storing period. One of the most efficient
method to preserve their value is drying. This processing method has many conveniences
like reduced storing spaces, increased preservation terms and furthermore the obtaining of
DOI: 10.5281/zenodo.2557337
CZU 664.854:634.25
Vitali Vişanu 101
Journal of Engineering Science November, 2018, Vol. XXV (3)
a new product bringing health benefits. Containing a lot of vitamins, fresh and dry peaches
are low in calories and reach in sugars; they are a good source of carbohydrates,
phytonutrients, antioxidants, carotenoids – that are of great importance for the healthy of
eyes, flavonoids that protect against cancer and heart diseases.
Besides those listed, there are other health benefits like stimulating immune system,
normalizing the intestinal transit, stimulating gastric juice secretion, as well as helping in
different diseases treatment like gastritis, anemia, high blood pressure, asthma and
bronchitis, renal lithiasis, etc. [8-13]. In the research process, three varieties of peaches
cultivated in the Republic of Moldova were used, with different characteristics, as follows:
SPRING CREST tree, originally from Fort-Valley, Georgia, USA, is vigorous, high
cropping and abundant blossom. The fruit is medium sized (70÷100 g), regulate and round
with glossy, strong red blush over a yellow background peel, as well as low pubescence. The
flesh is semi-freestone, yellow, melting with a medium texture, good aroma and no red
infiltrations around the stone. Its maturity comes 22-24 days earlier than Red haven (first
decade of August). As technological particularities, one can mention its early, high
productivity and good transportation resistance, but alas, it requires heavy thinning to
attain a commercial size and because of its early blossom it could be affected by the late
spring frosts. Since 2015, it was approved for the Republic of Moldova Central, South and
Southeast fruit-growing zones.
CARDINAL tree, originally from Fort-Valley, Georgia, USA, is medium vigorous, having
mixed branches and, abundant, relative early blossom.
The fruit is medium sized (80÷140 g), asymmetric with claret red blush over a yellow
background peel, as well as low pubescence. The flesh is clingstone, orange-yellow, medium
melting with a fine consistent texture, good aroma and great sweet and sour flavor. Its
maturity comes in the third decade of July. As technological particularities, one can mention
its high productivity, good resistance, and medium frosts and drought resistance; prefers
fertile and irrigated soils. Since 1980, it was approved for the Republic of Moldova Central
and Southeast fruit-growing zones.
REDHAVEN tree, originally from Agricultural Experiment Station, Michigan, USA, is
medium to vigorous, very reliable cropping, mid-season bloom and becomes fertile in the
3rd year after planting.
The fruit is medium to very large (130÷170 g), round or rounded-ovate with red
streaks and red blush over 90% of the surface over an orange-yellow background peel,
along with a low pubescence.
The flesh is semi-freestone, orange-yellow, medium melting, good aroma and good
sweet and sour flavor, with small red infiltrations around the stone. Its maturity comes in
the first decade of August. As technological particularities, one can mention its high regular
productivity, being considered as etalon and one of the best peaches varieties; prefers
fertile and irrigated soils, medium drought resistance, and somewhat susceptible to
bacterial spot. Since 1980, it was approved for the Republic of Moldova Central and
Southeast fruit-growing zones. [1]
Materials and methods
To study the drying process, the following peach varieties served as raw material:
Springcrest, Cardinal and Redhaven. Fresh peach fruit was characterized by firmness, dry
substance content and initial humidity. Table 1.
102 Peaches convective drying
Journal of Engineering Science November, 2018, Vol. XXV (3)
Table 1
Fresh peach fruit characteristic
Peach varieties
Firmness (Kg. f/cm2)
Dry substances (%)
Humidity (%)
Springcrest
1,22
10,65
89,35
Cardinal
1,07
10,52
89,48
Redhaven
0,88
11,33
88,67
Drying installation (DRYER)
All experiments were performed applying the laboratory drying installation (figure 1)
that permits convective drying process study. As drying agent, one can use air or other
gases, like CO2. Using the heater (4), the drying agent can reach a temperature spectrum of
20÷100°C, applying the temperature convertor (6) one maintains the needed temperature.
In our case, we used five temperatures for the drying agent, specifically 50±0.5°C, 60±0.8°C,
70±1.0°C, 80±1.2°C and 90±1.5°C. To register drying agent‖s data, we used a group of
temperature (DALLAS 8820 – error ± 0.1°C) and humidity sensors (DALLAS 8820 – error ±
0.5 %) (8), installed right before and after the connection between the agent recycling pipe
and the drying chamber (1).
Figure 1. Experimental drying installation. 1 – drying chamber, 2 – electric motor, 3 – fan, 4 –
electric resistors (heater), 5 – frequency converter, 6 – temperature controller, 7 – auxiliary
device, 8 – temperature and humidity sensors, 9 – switches, 10 – electronic scale, 11 – control
block SHF, 12 – electronic device for monitoring and recording of input and output data.
The drying agent is being recycled with the help of a 0.16 kW ventilator (VORTICE
SPA C15/2T) (3), which assures an up 3.0 m/s airflow velocity. In our case, we used five
drying agent velocities, namely 0.5±0.05 m. s-1, 1.0±0.05 m. s-1, 1.5±0.06 m. s-1, 2.0±0.07 m. s-
1and 2.5±0.08 m. s-1. Using frequency convertor (5), we can change ventilator (3) speed thus
modifying and maintaining drying agent velocity. Agent drying velocity was measured
applying an anemometer (AM50 – error ± 3.0%). Inside the working chamber, the peaches
are arranged on a support installed on top of an electronic scale (G&G JJ2000 – error ±0.01
g) (10) this way permitting an online registering of drying process product mass dropping.
For product surface temperature measurement during the drying process, one used an
infrared thermometer (IR laser – error ±2.0°C or 2.0%). All listed sensors are connected to a
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Journal of Engineering Science November, 2018, Vol. XXV (3)
PC software IgiCOM & UTM Drier – V 2.0 thus granting us the possibility to create an online
registration database of product drying mass dropping, drying agent temperature and
humidity (figure 2). This way allowing us to monitor and control agent drying velocity and
product surface temperature through the entire drying process.
Figure 2. Data electronic processing. 1 – dial (indicates product’s mass dropping curve),
2 – dial (indicates drying chamber input and output drying agent temperature)
3 – dial (indicates drying chamber input and output drying agent humidity).
For the experimental drying process, there were selected 89.5±0.75% initial humidity
ripe peaches. After being water washed and dried at room temperature, all the samples
were tested for the technical requirements correspondence, including visual and tactile
examination, and after confirming the fruits were sliced in well defined (3÷4 mm) rings. A
100±0.5 g portion of slices was arranged on the drying tray.
To study the kinetics of the drying process of peaches, the "convection drying" method
was used, as it is a known and applicable method in research. [14, 15, 16]
There are multiple, technological process (velocity, humidity, drying agent temperature,
etc.) and drying product, the peach, properties (thermal conductivity, porosity, density,
geometrical parameters, etc.) parameters that affect the process of drying kinetics. [17, 22]
To study the kinetic curves of the drying process: the following thermal agent
temperatures were used: 50°C, 60°C, 70°C, 80°C, and 90°C for all three varieties of peaches;
different thicknesses of the product layer (2.10-3m, 4.10-3m, 6.10-3m, 8.10-3m, 10.10-3m) and
different working agent speeds (0.5 m. s-1, 1.0 m. s-1, 1.5 m. s-1, 2.0 m. s-1, 2.5 m. s-1). To achieve
high-precision experimental results, each experience was performed three times,
maintaining the same technical conditions (temperature, humidity and speed of the working
agent, temperature, pressure, and environmental humidity).
The processing of experimental data and the development of the electronic curves of
kinetics of the drying curves was based on the IgiCOM & UTM Drier - V 2.0 PC software.
Results and discussions
Processed by convective method and different thermal agent temperatures, peaches
drying curves shows a standard form, displaying stable moisture per time diminution (figure
3). [15, 21] From initial 89.5% to final 18.0% humidity drop duration depends on the drying
104 Peaches convective drying
Journal of Engineering Science November, 2018, Vol. XXV (3)
agent temperature. Thus for the same 2.0 m. s-1 drying agent velocity and initial 89.46%
humidity, but different temperatures, the drying period will be: for 50°C a 270 min length,
60°C a 225 min length, 70°C a 185 min length, 80°C a 160 min length and for 90°C a 110
min length.
Figure 3. SPRINGCREST peaches different thermal agent temperatures drying curves
(Thermal agent velocity 2.0 m. s-1, thermal agent relative humidity 60.0%,
slices thickness 3.10-3m).
Figure 4 shows peaches different thermal agent temperatures drying velocity curves.
Their form also corresponds to the classical one, described in references. [19, 20, 21]
There are presented as well the three drying periods, namely 1 – of product heating,
2 – of constant drying velocity and 3 – of decreasing drying velocity.
Figure 4. SPRINGCREST peaches different thermal agent temperatures drying velocity
curves (Thermal agent velocity 2.0 m. s-1, thermal agent relative humidity 60.0%,
slices thickness 3.10-3m)
Graphics show that as thermal agent temperature increases from 50°C to 90°C the
drying speed increases as well. Moreover, for the drying agent speed of 2.0 m/s and its
0
10
20
30
40
50
60
70
80
90
020 40 60 80 100 120 140 160 180 200 220 240 260 280
Humidity (%)
Drying time τ (min)
50°C
60°C
70°C
80°C
90°C
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
1,1
1,2
15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
Drying velocity (du/dt)
Humidity (%)
50°C
60°C
70°C
80°C
90°C
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Journal of Engineering Science November, 2018, Vol. XXV (3)
relative air humidity of 60%, the drying rate increases from 0.35 m. s-1 to 1.05 m. s-1
according to a polynomial law Eq. (1):
At the same time, there has been observed some correlation between the duration of
the constant drying rate and the temperature of the thermal agent. Thus, with the
temperature increase of the agent, in the range of 50÷90°C, the duration of the second
period – of the constant drying rate decreased from 155 min. to 70 min., which in turn
reduces the duration of the drying process.
The mass transfer in the product is largely influenced by the humidity and
temperature gradients, as well as by the thickness of the product layer that the humidity
needs to pass. [22] This way, the kinetics of the peach drying process was as well studied at
different thickness of the slices, namely 2.10-3m, 4.10-3m, 6.10-3m, 8.10-3m, and 10.10-3m.
(according to figures 5 to 7).
At convective drying of SPRINGCREST peaches at thermal agent‖s 2.0 m.s-1 speed,
60°C and relative humidity of 65%, the drying curves for different thickness of the slices
bear the same character (Figure 5), different being only their inclination angle.
Moreover, the correlation between slices thickness variation and drying curves
inclination angle is inversely proportional, which determines that the reduction of slices
thickness accelerates the drying process, as confirmed by the drying velocity curves (Figure
6) showing an increase in drying rate while reducing the slices thickness. Thus, at 10.10-3m
slices thickness, a drying rate of 0.21 ± 0,024 %/min was recorded, while at 2.10-3m
thickness of – 0.47 ± 0.063 %. min-1.
The decrease of the drying velocity with the increase of the peach slices thickness
within 2÷10.10-3m takes place according to Eq. (2):
Figure 5. SPRINGCREST peaches different slices thickness drying curves (Thermal agent
velocity 2.0 m. s-1, thermal agent relative humidity 65.0%, thermal agent temperature 60°C).
0
10
20
30
40
50
60
70
80
90
040 80 120 160 200 240 280 320 360 400 440
Humidity (%)
Drying time τ (min)
10 mm
8 mm
6 mm
4 mm
2 mm
106 Peaches convective drying
Journal of Engineering Science November, 2018, Vol. XXV (3)
Figure 6. SPRINGCREST peaches different slices thickness drying velocity curves (Thermal
agent velocity 2.0 m. s-1, thermal agent relative humidity 65.0%,
thermal agent temperature 60°C).
As mentioned earlier, the decrease in the peach slices thickness reduces the drying
time, as shown in Figure 7. Using convective drying in a flow of 2.0 m/s, 60°C and 60%
initial humidity air, one can observe, that 2.10-3m thick peaches slices, have a minimum
drying time of 220 min., 4.10-3m thick has a drying time of 280 min., for a thickness of 6.10-
3m the duration is 320 min., for 8.10-3m a 360 min. and for a thickness of 10.10-3m the
drying time is 440 min. The correlation between the drying time and the thickness of the
product layer, of slices, within 2÷10.10-3m bears a linear character Eq. (3):
Figure 7. Correlation between SPRINGCREST peaches slices thickness and drying time
(Thermal agent velocity 2.0 m. s-1 thermal agent relative humidity 60.0%,
thermal agent temperature 60°C).
0,0
0,1
0,1
0,2
0,2
0,3
0,3
0,4
0,4
0,5
0,5
10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00 90,00
Drying velocity (du/dt)
Humidity (%)
10 mm
8 mm
6 mm
4 mm
2 mm
0
2
4
6
8
10
12
200 220 240 260 280 300 320 340 360 380 400 420 440 460
Slicess thickness (mm)
Drying time τ (min)
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Journal of Engineering Science November, 2018, Vol. XXV (3)
Wet products drying process, particularly peaches, among others are greatly
influenced by the speed of the thermal agent. Figure 8 shows the drying curves and in
Figure 9 curves of the drying speed of peaches cut in 3.10-3m thickness rounds using
thermal agent temperature of 60°C for different speeds. Both the drying curves and the
drying speeds curves indicate an intensification of the process as the thermal agent speed
increases [15, 17].
Figure 8. Peaches different thermal agent velocities drying curves (Thermal agent relative
humidity 65.0%, thermal agent temperature 60°C, slices thickness 3.10-3m).
Figure 9. Peaches different thermal agent velocities drying velocity curves (Thermal agent
relative humidity 65.0%, thermal agent temperature 60°C, slices thickness 3.10-3m).
The following results were obtained from the analysis of Figure 8 and Figure 9: at
the speed of 0.5 m. s-1, a drying time of 440 min was obtained, at 1.0 m. s-1 – 380 min., at
speed of 1.5 m. s-1– 340 min., at of 2.0 m. s-1– 280 min., and for 2.5 m. s-1 for 200 min.
0
10
20
30
40
50
60
70
80
90
030 60 90 120 150 180 210 240 270 300 330 360 390 420 450
Humidity (%)
Drying time τ (min)
0,5 m/s
1,0 m/s
1,5 m/s
2,0 m/s
2,5 m/s
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00 90,00
Drying velocity (du/dt)
Humidity (%)
0,5 m/s
1,0 m/s
1,5 m/s
2,0 m/s
2,5 m/s
108 Peaches convective drying
Journal of Engineering Science November, 2018, Vol. XXV (3)
According to Figure 10, the dependence of the drying time of the thermal agent
speed (t = f (v)) in the range of 0.5÷2.5 m. s-1 is linear Eq. (4)
Figure 10. Correlation between peaches drying time and thermal agent velocity (Thermal
agent relative humidity 65.0%, thermal agent temperature 60°C, slices thickness 3.10-3m).
During the kinetics research of the peach drying process, such varieties as
SPRINGCREST, CARDINAL and REDHAVEN were studied. Both drying and drying velocity
curves have the same character, indicating that small differences in physical, mechanical
and thermal properties of different varieties have little influence on the transfer
phenomena in the drying process. The drying times of different varieties differ in average by
±10÷15 min., which is 8÷6% (Figure 11).
Figure 11. Correlation between peaches variety and drying time (τ = f (variety)).
As a result of drying process kinetics study, dry product samples were obtained for
different temperatures and speeds of the thermal agent, and different thickness of the
rounds. Of particular interest is the external appearance of dried peaches at different
temperatures, since drying, as a thermal process, may be accompanied by various unwanted
0,0
0,5
1,0
1,5
2,0
2,5
3,0
180 220 260 300 340 380 420 460
Thermal agent velocity
(m/s)
Drying time τ (min)
0
50
100
150
200
250
300
50°C 60°C 70°C 80°C 90°C
Drying time τ (min)
Temperature, °C
springcrest
cardinal
redhaven
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Journal of Engineering Science November, 2018, Vol. XXV (3)
effects such as sugar caramelization, browning polyphenols, etc. Figure 12 shows
SPRINGCREST dry peach samples with a thickness of 3.0 .10-3 m, the drying being carried
out by convection at different thermal agent temperatures at a speed of 2.0 m. s-1 and a
relative humidity of 60%.
a) b) c)
d) e)
Figure 12. SPRINGCREST peaches aspect dried using: thermal agent 2.0 m/s velocity and
60.0% relative humidity: a) 50°C; b) 60°C; c) 70°C; d) 80°C; e) 90°C temperature.
The pictures show that dried peaches at temperatures between 50°C and 60°C are
more attractive than those dried at temperatures of 70°C, 80°C and 90°C, indicating that at
the given temperatures the undesirable caramelization phenomena and peach browning do
not take place yet.
Conclusion
The study of peaches convective drying kinetics at the temperature of the thermal
agent in the range of 50÷90°C, speed of 0.5÷2.5 m.s-1 revealed that the increase both
thermal agent temperature, speed and decreasing the thickness of the rolls from 10 to 1.10-
3m, leads to an intensification of the process. However, temperatures above 60°C cause an
acceleration of the undesirable sugar caramelization and browning phenomena, Figure 12.
Therefore, for the convective drying of peaches, the temperature of 60°C with the speed of
the heating agent 2.0 m. s-1 and the thickness of the rolls 3 10-3m are recommended for
getting an optimal drying process. The character of the drying curves is classical and does
not differ from that of the fruits and vegetables described in the specialized literature [14-
16, 19-22].
Bibliography
Books:
1. Popa S., Braghiş A., Manziuc V., Cumpanici A. Manual tehnologic Producerea piersicilor. Chişinău 2016. pag. 5,
7, 10 - 13, 173;
2. Tatarov P. Chimia produselor alimentare. Chişinău 2017. pag. 5-12, 48-50, 64-66, 200-215, 313-316;
Web references:
Nutrition information of fresh peaches:
3. https://skipthepie.org/fruits-and-fruit-juices/peaches-raw/;
110 Peaches convective drying
Journal of Engineering Science November, 2018, Vol. XXV (3)
4. https://dieta.romedic.ro/aliment/piersici;
5. http://cesamancam.ro/valori_nutritive_fructe.html;
6. https://www.nutrition-and-you.com/peaches.html;
7. https://draxe.com/peach-nutrition/;
Health benefits of peaches:
8. https://www.nutrition-and-you.com/peaches.html;
9. https://www.medicalnewstoday.com/articles/274620.php;
10. https://draxe.com/peach-nutrition/;
11. https://www.stylecraze.com/articles/benefits-of-peaches/#gref;
Health benefits of dried peaches:
12. http://montagudriedfruitnuts.co.za/7-health-benefits-of-cling-peaches/
13. https://nuts.com/driedfruit/peaches/jumbo.html
Studies of peaches drying process
14. https://www.researchgate.net/publication/275650176_Different_Drying_Methods_Their_Applications_and_R
ecent_Advances
15. https://www.researchgate.net/publication/292947307_Drying_studies_on_peach_and_strawberry_slices
16. https://www.researchgate.net/publication/275650090_COMPARATIVE_STUDY_OF_EFFECT_OF_DIFFERENT_
DRYING_METHODS_ON_NUTRITIONAL_QUALITY_OF_PEACH_CULTIVARS_DURING_STORAGE
Symposia volumes:
17. Țislinscaia N., Bernic M., Malezhyk I., Buleandra A. Mathematical model of drying process velocity factor. /
Proceedings of the International Conference “Modern Technologies in the Food Industry-2016”, Chișinău”
Bonus Offices”, 2016. p. 99-104.;
Journal published papers:
18. Agricultura Republicii Moldovei. Departament statistică şi sociologie al Republicii Moldova - Statistica
Moldovei, 2003.;
19. Golisz1 E, Jaros M, Kalicka M. Analysis of convectional drying process of peach. Faculty of production
engineering, Warsaw University of life sciences. Received 21 July 2013; accepted 27 October 2013;
available on line 15 November 2013.;
20. Naseer A., Jagmohan S., Rajkumari K., Parshant B., Anisa M., Harleen K., Harmeet C., Prerna G., Comparative
study of effect of different drying methods on nutritional quality of peach cultivars during storage. Special issue,
vol. vi: 01-06: 2014 an International quarterly journal of environmental sciences, ISSN: 0974 – 0376;
21. Bernic M., Răducan M., Ciobanu E. Drying Kinetics of Sunflower Seeds using Pulsed UHF Energy Intake. TEM
Journal, 2(4), November 2013, pp. 305-308.;
22. Bernic M. Aspecte teoretice ale fenomenelor de transfer în procesele de uscare. // Meridian Ingineresc. –
Chişinău: UTM, 2008. – Nr. 2. – р. 32 – 35.