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Air drying characteristics of solid waste (pomace) of olive oil processing
Journal of Food Engineering 72 (2006) 378–382 www.elsevier.com/locate/jfoodeng
Air drying characteristics of solid waste (pomace) of olive oil processing Fahrettin Go¨g˘u¨ßs *, Medeni Maskan Department of Food Engineering, University of Gaziantep, Gaziantep 27310, Turkey Received 25 June 2004; accepted 21 December 2004 Available online 19 February 2005
Abstract Drying of olive pomace was studied at 60, 70, 80 C and constant air velocity of 1.5 m/s for various sample thickness and particle sizes. Drying of olive pomace took place in the falling rate period at all temperatures. Drying time changed from 140 to 65 min for 80 C, 170 to 80 min for 70 C and 240 to 125 min for 60 C by changing thickness from 12 to 6 mm. Effective diffusivity values increased with increasing sample thickness and air temperature and found to be in the range of 1.84 · 107–3.94 · 107 m2/s. The activation energy was 25.4, 25.7 and 29.2 kJ/mol for 6, 9 and 12 mm thickness respectively. 2005 Elsevier Ltd. All rights reserved. Keywords: Olive pomace; Air drying; Diffusion
1. Introduction Olive oil may be produced from olives either by means of conventional systems using hydraulic presses or by resorting to modern horizontal axis centrifuges. Both systems produce olive pomaces. Olive pomace has been evaluated as animal feed (Haddadin, AlNatour, Al-Qsous, & Robinson, 2002) and a raw material for glycolipidsÕ biosynthesis (Bednarski, Adamczak, Tomasik, & Plaszczyk, 2004). Olive pomace is the solid phase that remains after pressing olives, which contains on average 5–8% of residual oil with 25–55% of vegetable water, the rest being solid matter. The residual oil could be economically recovered in oil extraction plants after reducing the moisture content to about 5–8% (wet basis). The extracted oil is known as crude pomace oil. It is considered an inferior grade and is used for soap
*
Corresponding author. Fax: +90 3423 60 11 05. E-mail address: [email protected] (F. Go¨g˘u¨ßs ).
0260-8774/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.12.018
making or industrial purpose. Refined olive-pomace oil is that obtained from crude pomace oil by arefiningprocess. Because of the industrial drying and of behaviour of the solvent, crude pomace oil contains unsaponifiable matter and free fatty acid at higher levels than those found in olive oils. In general, crude pomace oil is of an intense green color, with a typical odor. The common operation to dry the pomace usually carried out in countercurrent rotary dryers. The hot flue gases produced by using the residual pomace, obtained from the extraction of crude pomace oil, used as fuel in direct-fired systems. However it has been found that the high temperature applications such as 250 C of drying medium causes pomace hydrolysis and reduces the quality of oil produced (Freire, Figueiredo, & Ferrao, 1999). Gomes and Caponio (1997) have dried the olive pomace with hot air flowing through inside-bladed rotating cylinders. They also have found that most of the oxidation occurs during pomace drying. Air drying of solid materials involves vaporisation of water contained by the solid material, and removal of vapor in a stream of air. The phenomenon is one of
F. Go¨g˘u¨ßs , M. Maskan / Journal of Food Engineering 72 (2006) 378–382
2. Materials and methods The drying experiments were performed in a pilot plant tray dryer (UOP 8 tray dryer, Armfield Ltd, UK), which was operated at an air velocity of 1.5 m/s, temperatures 60–80 C and relative humidities 6–12%. The olive pomace was kindly obtained from Gu¨venal Olive Oil Company, Gaziantep, Turkey. The pomace had an oil content of 8% and a moisture content of 58% (dry basis). The samples were evenly spread, completely covering the base, on the drying pan (27.5 cm · 18.5 cm · 1.2 cm) to have different initial thickness with their deviations (6 ± 0.32 mm, 9 ± 0.46 mm, 12 ± 0.53 mm). Therefore, the drying took place only from the top surface. Air was blown into the dryer by means of a centrifugal fan with adjustable flow rate parallel to the drying surface of the sample. Dry bulb and wet bulb temperatures were monitored in the drying chamber. Drying data (average moisture content X vs. time t) were obtained by periodic weighing of the samples with an Avery Berkel CC62D balance, placed on the top of the dryer. Pomace samples were dried until equilibrium was reached. The other set of additional experiments had the objective of studying the effect of sample size on the drying characteristics. The standard screens were used to separate the original sample into three classes according to their particle size in the size ranges of greater than 2 mm, between 1 mm and 2 mm, and less than 1 mm. Sieves were made of metals and the openings were circu-
lar in shape. The separated samples were dried in the same manner as described above.
3. Results and discussion 3.1. The effect of thickness Fig. 1 shows the effect of thickness on drying of the olive pomace at 80 C. The similar drying behaviour was observed at 60 and 70 C. As it is seen the decrease in thickness results in decrease in the time of drying of olive pomace. The drying time was 125, 160 and 240 min at 60 C; 80, 100 and 170 min at 70 C; and 65, 85, and 140 min at 80 C for 6, 9 and 12 mm respectively. The shortest drying time was observed for a thickness of 6 mm at all temperatures. The drying mechanism might be responsible for the longer times for thick samples and shorter times for thin samples for the air-drying of olive pomace. The longer moisture diffusion path might explain the longer drying time for thick sample. In the drying of wet porous materials, internal moisture transport starts mainly by capillary flow and it is controlled by the diffusion for the later stages of drying (Ferrao et al., 1998). There is a very sharp decrease in 6 mm thick sample almost down to the equilibrium moisture content. However the decrease is not sharp for 9 and 12 mm samples. This behavior is because of change of drying mechanism from capillary to diffusion in the thick samples. Go¨g˘u¨ßs and Maskan (2001) recorded that there is no significant effect of thickness on the drying of olive pomace by microwave oven. This result shows that the microwave drying technique is carried out in a capillary drying mechanism region. Because it is a fast drying method, it does not allow for continuing with a diffusional process. So, the samples with various thicknesses
0.7
Moisture Content (g water/g dry solid)
the simultaneous heat and mass transfer, and is affected by several internal and external conditions, such as material properties and air conditions. Many porous structural materials show two main periods during drying: the constant rate period and falling rate period. The falling rate period is the most important in a dehydration process and the mechanisms in this period are more complex. FickÕs Law of diffusion has been used to describe this period (Vergara, Amezaga, Barcenas, & Welti, 1997). Although there are some studies on air drying (Arjona, Garcia, & Ollero, 1999; Ferrao, Figueiredo, & Freire, 1998; Freire et al., 1999) and microwave-fan assisted convection oven drying properties of olive pomace (Go¨g˘u¨ßs & Maskan, 2001), a survey of recent literature shows that the available information on drying characteristics of olive pomace is rather scarce especially at low temperatures to prevent the possible deterioration of the olive-pomace oil. The objective of this work is to investigate the drying characteristics of this potentially valuable by-product as a function of sample thickness, particle size and air temperature.
379
12 mm 9 mm 6 mm
0.6
0.5
0.4
0.3
0.2
0.1
0.0 0
20
40
60
80
100
120
140
160
Time (min)
Fig. 1. Effect of sample thickness at the constant temperature of 80 C on drying behaviour of olive pomace.
F. Go¨g˘u¨ßs , M. Maskan / Journal of Food Engineering 72 (2006) 378–382
380
dry almost at the same time in microwave drying process.
0
In industry the olive pomace is dried with hot air at about 260 C flowing through inside-bladed rotating cylinders. The substantial rise in the temperature may enhance the degree of oxidation of the oil the pomaces contain. There are very few reports on this topic in the literature (Gomes & Caponio, 1997; Gomes & Caponio, 2001). In particular, little is known about the extent of thermal damage caused by the drying process or about the influence of solvent removal after oil extraction. The temperature on the drying of olive pomace is an important parameter to obtain a high quality pomace oil. Fig. 2 shows the effect of temperature on the rate of drying at 9 mm thickness of pomace sample. The similar drying behaviour was observed for 6 and 12 mm thicknesses. As it is seen in Fig. 2, the increase in temperature results in decrease in time of drying. The shortest time needed to reach equilibrium was achieved at a temperature of 80 C. The results found by Vaccarezza, Lombardi, and Chirife (1974) for sugar beet root and Maskan and Go¨g˘u¨ßs (1998) for mulberry are in agreement with those found in this study. A constant rate period was not observed in any case of olive pomace drying. Arjona et al. (1999) examined the drying of olive pomace and concluded that the constant rate drying period was not observed during olive pomace drying. Vaccarezza et al. (1974) investigated the kinetics of moisture movement of sugar beet root. They also observed that no constant rate period for sugar beet root. For the olive pomace a large range of falling rate periods was observed. That is the critical moisture content is equal to the initial moisture contents shown in Fig. 3 for a 9 mm thick sample. The results are
Moisture content (g water/g dry solid)
0.7 60 oC 70 oC 80 oC
0.6
0.5
0.4
0.3
0.2
0.1
0.0 20
40
60
80
100
120
140
160
180
Time (min)
Fig. 2. Effect of temperature at the constant sample thickness of 9 mm on drying behaviour of olive pomace.
ln((X-Xe)/(X0-Xe))
3.2. Effect of temperature
0
60 °C 70 °C 80 °C
-1 -2 -3 -4 -5 -6 -7 0
20
40
60
80
100
120
140
160
Time (min)
Fig. 3. Logarithmic drying curves at various temperatures at a thickness of 9 mm for olive pomace.
an indication of the drying process, which is controlled by the internal mass transfer resistance. In the light of results and the literature surveyed, the Fickian diffusion model assumption was suitable to analyze the drying behaviour of oil pomace. Assuming uniform internal moisture distribution, long drying times and negligible external resistance, the solution for the slab expressed in terms of the average moisture content is (Crank, 1975); ðX X e Þ 8 ¼ expðp2 Deff t=L2 Þ ðX 0 X e Þ p2
ð1Þ
where X is the average moisture content (kg water/kg dry solids), Xe is the equilibrium moisture content (kg water/kg dry solids), X0 is the initial moisture content (kg water/kg dry solids), L is the thickness of slab (m) for drying from one side, Deff is the effective moisture diffusivity (m2/s) and t is the drying time(s). As it is seen in Fig. 3, ln[(X Xe)/(X0 Xe)] as a function of time plot shows that there are more than one falling rate periods for 9 mm thick sample of pomace at various temperatures. It was also observed that 12 mm thick sample had the falling rate periods more than one and divisible into two or three distinct phases. However, it was found that there is only one falling rate drying period for 6 mm thick sample. Similar results with two or three falling rate periods were also observed by Maskan and Go¨g˘u¨ßs (1998) for mulberry and Go¨g˘u¨ßs and Maskan (1999) for okra. A linear regression analysis was employed to calculate Deff values from the slopes of straight lines. From the logarithmic drying curves, the average Deff values were obtained for each case at changing slopes (Table 1). Except the drying of 6 mm thick samples, two or three Deff values were obtained. However only first falling rate periods were considered to be valuable to compare with the others. Because, the main part of the
F. Go¨g˘u¨ßs , M. Maskan / Journal of Food Engineering 72 (2006) 378–382
Thickness (mm) 6 6 6 9 9 9 12 12 12
2
Temperature (C)
Deff (m /s) 7
1.84 · 10 3.03 · 107 3.42 · 107 2.17 · 107 2.98 · 107 3.67 · 107 2.18 · 107 3.22 · 107 3.94 · 107
60 70 80 60 70 80 60 70 80
2
R
0.998 0.994 0.995 0.999 0.998 0.997 0.998 0.998 0.995
0.7
Moisture content (g water/ g dry solid)
Table 1 Effective diffusivity and R2 values (m2/s) for drying of olive pomace at various temperatures and sample thicknesses
381
ps > 2 mm 1 mm < ps < 2 mm ps < 1 mm
0.6
0.5
0.4
0.3
0.2
0.1
0.0 0
falling rates is in the first falling rate period. Other than this, Deff values were found to be increased with increasing time. This may be because of the cracks occurred in the later stage of drying and this does not show a proper diffusional migration. Therefore, it was decided to use the first falling rate periods in each case. Deff values from the slopes of the straight line parts of the curves and their R2 values are given in Table 1. As it is seen from Table 1, there is a clear effect of both temperature and olive pomace thickness on the effective diffusivity values of olive pomace. The diffusion coefficient increases with increasing temperature due to rapid movement of water at high temperatures. Natural logarithms of the diffusivity values calculated for the first falling rate period at 60, 70 and 80 C was plotted against the inverse of the absolute temperature to find the activation energy using an Arrhenius type relationship (Eq. (2)), Deff ¼ D0 exp½Ea =ðRg T Þ
ð2Þ
where D0 is a constant, Ea is the energy of activation for diffusion in kJ/mol, T is the absolute temperature and Rg is the gas constant (8.314 kJ/mol K). The plot showed a linear behaviour with R2 values greater than 0.970. The activation energies and their R2 values for the olive pomace samples at different thicknesses are shown in Table 2. The activation energy increases with increasing pomace thickness. This is a clear indication of the higher energy needed to remove the water from 12 mm sample compared to the 9 and 6 mm samples. Similar activation energy values have been found in the same range of temperature (60–80 C) by Go¨g˘u¨ßs and Maskan (1999) as 32.4 kJ/mol for okra and by Vaccarezza et al. (1974) as 28.8 kJ/mol for sugar beet root. Table 2 Energy of activation with respect to the thickness
20
40
60
80
100
120
140
Time (min)
Fig. 4. The effect of particle size on the drying behaviour of olive pomace at 80 C and 12 mm sample thickness (ps: particle size).
3.3. The effect of particle size As shown in Fig. 4 the drying rate of olive pomace also affected by the particle size. The drying time decreased with increasing particle size. The olive pomace with no particle size separation showed a different drying behavior. So, the time of drying was different from the samples with certain particle sizes. The results show that the samples with small particle sizes are packed to obtain an almost nonporous structure. However, large particles resulted in a porous structure and the rate of removal of water in such a solid system is easier than the nonporous systems. Other than the structural nature of the pomace samples, their composition might be an important parameter on the rate of drying. Because large particles are mainly from the pits of the olive and they contain higher amounts of cellulose compared to the smaller particles. However, the small particles are from olive fruit itself and contain more oil than pits pomace. The oil may resist water movement. Hence, the drying takes place in a longer time. This compositional difference also affects the equilibrium moisture content. The drying behaviour of olive pomace by hot air was found to be different when compared with that of microwave. In microwave drying, there was no effect of sample thickness and porosity on the rate of drying (Go¨g˘u¨ßs & Maskan, 2001). However, as it was discussed above, in hot air drying both sample thickness and porosity had a considerable effect. 4. Conclusions
Thickness (mm)
Ea (kJ/mol)
R2
6 9 12
25.4 25.7 29.2
0.997 0.989 0.972
In conclusion, the results from this study showed that temperature, thickness and particle size affected the rate of drying of olive pomace. The increase in both temperature and particle size resulted in the decrease in time of
382
F. Go¨g˘u¨ßs , M. Maskan / Journal of Food Engineering 72 (2006) 378–382
drying. On the other hand, thick samples compared to the thin ones dried in a longer time at all temperatures.
References Arjona, R., Garcia, A., & Ollero, P. (1999). The drying of alpeorujo, a waste product of the olive oil mill industry. Journal of Food Engineering, 41, 229–234. Bednarski, W., Adamczak, M., Tomasik, J., & Plaszczyk, M. (2004). Application of oil refinery waste in the biosynthesis of glycolipids by yeast. Bioresource Technology, 92, 15–18. Crank, J. (1975). Mathematics of diffusion. Oxford, England: Clarendon Press. Ferrao, P., Figueiredo, A., & Freire, F. (1998). Experimental analysis of the drying kinetics of a food product. Drying Technology, 16, 1687–1702. Freire, F., Figueiredo, A., & Ferrao, P. (1999). Thermal analysis and drying kinetics of olive bagasse. Drying Technology, 17, 895–907. Gomes, T., & Caponio, F. (1997). Evaluation of the state of oxidation of crude olive-pomace oils. Influence of olive-pomace drying and oil extraction with solvent. Journal of Agricultural and Food Chemistry, 45, 1381–1384.
Gomes, T., & Caponio, F. (2001). Possibility of improving the quality characteristics of olive-pomace oil and enhancing its differentiation from refined olive-pomace oil. Journal of the Science of Food and Agriculture, 81, 62–67. Go¨g˘u¨ßs , F., & Maskan, M. (1999). Water adsorption and drying characteristics of okra (Hibiscus Esculentus L.). Drying Technology, 17, 883–894. Go¨g˘u¨ßs , F., & Maskan, M. (2001). Drying of olive pomace by a combined microwave-fan assisted convection oven. Nahrung, 45, 129–132. Haddadin, M. S., Al-Natour, R., Al-Qsous, S., & Robinson, R. K. (2002). Bio-degradation of lignin in olive pomace by freshlyisolated species of Basidiomycete. Bioresource Technology, 82, 131–137. Maskan, M., & Go¨g˘u¨ßs , F. (1998). Sorption isotherms and drying characteristics of Mulberr (Morus alba). Journal of Food Engineering, 37, 437–449. Vaccarezza, L. M., Lombardi, J. L., & Chirife, J. (1974). Kinetics of moisture movement during air drying of sugar beet root. Journal of Food Technology, 9, 317–327. Vergara, F., Amezaga, E., Barcenas, M. E., & Welti, J. (1997). Analysis of the drying processes of osmotically dehydrated apple using the characteristic curve model. Drying Technology, 15, 949–963.
Air drying characteristics of solid waste (pomace) of olive oil processing Fahrettin Go¨g˘u¨ßs *, Medeni Maskan Department of Food Engineering, University of Gaziantep, Gaziantep 27310, Turkey Received 25 June 2004; accepted 21 December 2004 Available online 19 February 2005
Abstract Drying of olive pomace was studied at 60, 70, 80 C and constant air velocity of 1.5 m/s for various sample thickness and particle sizes. Drying of olive pomace took place in the falling rate period at all temperatures. Drying time changed from 140 to 65 min for 80 C, 170 to 80 min for 70 C and 240 to 125 min for 60 C by changing thickness from 12 to 6 mm. Effective diffusivity values increased with increasing sample thickness and air temperature and found to be in the range of 1.84 · 107–3.94 · 107 m2/s. The activation energy was 25.4, 25.7 and 29.2 kJ/mol for 6, 9 and 12 mm thickness respectively. 2005 Elsevier Ltd. All rights reserved. Keywords: Olive pomace; Air drying; Diffusion
1. Introduction Olive oil may be produced from olives either by means of conventional systems using hydraulic presses or by resorting to modern horizontal axis centrifuges. Both systems produce olive pomaces. Olive pomace has been evaluated as animal feed (Haddadin, AlNatour, Al-Qsous, & Robinson, 2002) and a raw material for glycolipidsÕ biosynthesis (Bednarski, Adamczak, Tomasik, & Plaszczyk, 2004). Olive pomace is the solid phase that remains after pressing olives, which contains on average 5–8% of residual oil with 25–55% of vegetable water, the rest being solid matter. The residual oil could be economically recovered in oil extraction plants after reducing the moisture content to about 5–8% (wet basis). The extracted oil is known as crude pomace oil. It is considered an inferior grade and is used for soap
*
Corresponding author. Fax: +90 3423 60 11 05. E-mail address: [email protected] (F. Go¨g˘u¨ßs ).
0260-8774/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.12.018
making or industrial purpose. Refined olive-pomace oil is that obtained from crude pomace oil by arefiningprocess. Because of the industrial drying and of behaviour of the solvent, crude pomace oil contains unsaponifiable matter and free fatty acid at higher levels than those found in olive oils. In general, crude pomace oil is of an intense green color, with a typical odor. The common operation to dry the pomace usually carried out in countercurrent rotary dryers. The hot flue gases produced by using the residual pomace, obtained from the extraction of crude pomace oil, used as fuel in direct-fired systems. However it has been found that the high temperature applications such as 250 C of drying medium causes pomace hydrolysis and reduces the quality of oil produced (Freire, Figueiredo, & Ferrao, 1999). Gomes and Caponio (1997) have dried the olive pomace with hot air flowing through inside-bladed rotating cylinders. They also have found that most of the oxidation occurs during pomace drying. Air drying of solid materials involves vaporisation of water contained by the solid material, and removal of vapor in a stream of air. The phenomenon is one of
F. Go¨g˘u¨ßs , M. Maskan / Journal of Food Engineering 72 (2006) 378–382
2. Materials and methods The drying experiments were performed in a pilot plant tray dryer (UOP 8 tray dryer, Armfield Ltd, UK), which was operated at an air velocity of 1.5 m/s, temperatures 60–80 C and relative humidities 6–12%. The olive pomace was kindly obtained from Gu¨venal Olive Oil Company, Gaziantep, Turkey. The pomace had an oil content of 8% and a moisture content of 58% (dry basis). The samples were evenly spread, completely covering the base, on the drying pan (27.5 cm · 18.5 cm · 1.2 cm) to have different initial thickness with their deviations (6 ± 0.32 mm, 9 ± 0.46 mm, 12 ± 0.53 mm). Therefore, the drying took place only from the top surface. Air was blown into the dryer by means of a centrifugal fan with adjustable flow rate parallel to the drying surface of the sample. Dry bulb and wet bulb temperatures were monitored in the drying chamber. Drying data (average moisture content X vs. time t) were obtained by periodic weighing of the samples with an Avery Berkel CC62D balance, placed on the top of the dryer. Pomace samples were dried until equilibrium was reached. The other set of additional experiments had the objective of studying the effect of sample size on the drying characteristics. The standard screens were used to separate the original sample into three classes according to their particle size in the size ranges of greater than 2 mm, between 1 mm and 2 mm, and less than 1 mm. Sieves were made of metals and the openings were circu-
lar in shape. The separated samples were dried in the same manner as described above.
3. Results and discussion 3.1. The effect of thickness Fig. 1 shows the effect of thickness on drying of the olive pomace at 80 C. The similar drying behaviour was observed at 60 and 70 C. As it is seen the decrease in thickness results in decrease in the time of drying of olive pomace. The drying time was 125, 160 and 240 min at 60 C; 80, 100 and 170 min at 70 C; and 65, 85, and 140 min at 80 C for 6, 9 and 12 mm respectively. The shortest drying time was observed for a thickness of 6 mm at all temperatures. The drying mechanism might be responsible for the longer times for thick samples and shorter times for thin samples for the air-drying of olive pomace. The longer moisture diffusion path might explain the longer drying time for thick sample. In the drying of wet porous materials, internal moisture transport starts mainly by capillary flow and it is controlled by the diffusion for the later stages of drying (Ferrao et al., 1998). There is a very sharp decrease in 6 mm thick sample almost down to the equilibrium moisture content. However the decrease is not sharp for 9 and 12 mm samples. This behavior is because of change of drying mechanism from capillary to diffusion in the thick samples. Go¨g˘u¨ßs and Maskan (2001) recorded that there is no significant effect of thickness on the drying of olive pomace by microwave oven. This result shows that the microwave drying technique is carried out in a capillary drying mechanism region. Because it is a fast drying method, it does not allow for continuing with a diffusional process. So, the samples with various thicknesses
0.7
Moisture Content (g water/g dry solid)
the simultaneous heat and mass transfer, and is affected by several internal and external conditions, such as material properties and air conditions. Many porous structural materials show two main periods during drying: the constant rate period and falling rate period. The falling rate period is the most important in a dehydration process and the mechanisms in this period are more complex. FickÕs Law of diffusion has been used to describe this period (Vergara, Amezaga, Barcenas, & Welti, 1997). Although there are some studies on air drying (Arjona, Garcia, & Ollero, 1999; Ferrao, Figueiredo, & Freire, 1998; Freire et al., 1999) and microwave-fan assisted convection oven drying properties of olive pomace (Go¨g˘u¨ßs & Maskan, 2001), a survey of recent literature shows that the available information on drying characteristics of olive pomace is rather scarce especially at low temperatures to prevent the possible deterioration of the olive-pomace oil. The objective of this work is to investigate the drying characteristics of this potentially valuable by-product as a function of sample thickness, particle size and air temperature.
379
12 mm 9 mm 6 mm
0.6
0.5
0.4
0.3
0.2
0.1
0.0 0
20
40
60
80
100
120
140
160
Time (min)
Fig. 1. Effect of sample thickness at the constant temperature of 80 C on drying behaviour of olive pomace.
F. Go¨g˘u¨ßs , M. Maskan / Journal of Food Engineering 72 (2006) 378–382
380
dry almost at the same time in microwave drying process.
0
In industry the olive pomace is dried with hot air at about 260 C flowing through inside-bladed rotating cylinders. The substantial rise in the temperature may enhance the degree of oxidation of the oil the pomaces contain. There are very few reports on this topic in the literature (Gomes & Caponio, 1997; Gomes & Caponio, 2001). In particular, little is known about the extent of thermal damage caused by the drying process or about the influence of solvent removal after oil extraction. The temperature on the drying of olive pomace is an important parameter to obtain a high quality pomace oil. Fig. 2 shows the effect of temperature on the rate of drying at 9 mm thickness of pomace sample. The similar drying behaviour was observed for 6 and 12 mm thicknesses. As it is seen in Fig. 2, the increase in temperature results in decrease in time of drying. The shortest time needed to reach equilibrium was achieved at a temperature of 80 C. The results found by Vaccarezza, Lombardi, and Chirife (1974) for sugar beet root and Maskan and Go¨g˘u¨ßs (1998) for mulberry are in agreement with those found in this study. A constant rate period was not observed in any case of olive pomace drying. Arjona et al. (1999) examined the drying of olive pomace and concluded that the constant rate drying period was not observed during olive pomace drying. Vaccarezza et al. (1974) investigated the kinetics of moisture movement of sugar beet root. They also observed that no constant rate period for sugar beet root. For the olive pomace a large range of falling rate periods was observed. That is the critical moisture content is equal to the initial moisture contents shown in Fig. 3 for a 9 mm thick sample. The results are
Moisture content (g water/g dry solid)
0.7 60 oC 70 oC 80 oC
0.6
0.5
0.4
0.3
0.2
0.1
0.0 20
40
60
80
100
120
140
160
180
Time (min)
Fig. 2. Effect of temperature at the constant sample thickness of 9 mm on drying behaviour of olive pomace.
ln((X-Xe)/(X0-Xe))
3.2. Effect of temperature
0
60 °C 70 °C 80 °C
-1 -2 -3 -4 -5 -6 -7 0
20
40
60
80
100
120
140
160
Time (min)
Fig. 3. Logarithmic drying curves at various temperatures at a thickness of 9 mm for olive pomace.
an indication of the drying process, which is controlled by the internal mass transfer resistance. In the light of results and the literature surveyed, the Fickian diffusion model assumption was suitable to analyze the drying behaviour of oil pomace. Assuming uniform internal moisture distribution, long drying times and negligible external resistance, the solution for the slab expressed in terms of the average moisture content is (Crank, 1975); ðX X e Þ 8 ¼ expðp2 Deff t=L2 Þ ðX 0 X e Þ p2
ð1Þ
where X is the average moisture content (kg water/kg dry solids), Xe is the equilibrium moisture content (kg water/kg dry solids), X0 is the initial moisture content (kg water/kg dry solids), L is the thickness of slab (m) for drying from one side, Deff is the effective moisture diffusivity (m2/s) and t is the drying time(s). As it is seen in Fig. 3, ln[(X Xe)/(X0 Xe)] as a function of time plot shows that there are more than one falling rate periods for 9 mm thick sample of pomace at various temperatures. It was also observed that 12 mm thick sample had the falling rate periods more than one and divisible into two or three distinct phases. However, it was found that there is only one falling rate drying period for 6 mm thick sample. Similar results with two or three falling rate periods were also observed by Maskan and Go¨g˘u¨ßs (1998) for mulberry and Go¨g˘u¨ßs and Maskan (1999) for okra. A linear regression analysis was employed to calculate Deff values from the slopes of straight lines. From the logarithmic drying curves, the average Deff values were obtained for each case at changing slopes (Table 1). Except the drying of 6 mm thick samples, two or three Deff values were obtained. However only first falling rate periods were considered to be valuable to compare with the others. Because, the main part of the
F. Go¨g˘u¨ßs , M. Maskan / Journal of Food Engineering 72 (2006) 378–382
Thickness (mm) 6 6 6 9 9 9 12 12 12
2
Temperature (C)
Deff (m /s) 7
1.84 · 10 3.03 · 107 3.42 · 107 2.17 · 107 2.98 · 107 3.67 · 107 2.18 · 107 3.22 · 107 3.94 · 107
60 70 80 60 70 80 60 70 80
2
R
0.998 0.994 0.995 0.999 0.998 0.997 0.998 0.998 0.995
0.7
Moisture content (g water/ g dry solid)
Table 1 Effective diffusivity and R2 values (m2/s) for drying of olive pomace at various temperatures and sample thicknesses
381
ps > 2 mm 1 mm < ps < 2 mm ps < 1 mm
0.6
0.5
0.4
0.3
0.2
0.1
0.0 0
falling rates is in the first falling rate period. Other than this, Deff values were found to be increased with increasing time. This may be because of the cracks occurred in the later stage of drying and this does not show a proper diffusional migration. Therefore, it was decided to use the first falling rate periods in each case. Deff values from the slopes of the straight line parts of the curves and their R2 values are given in Table 1. As it is seen from Table 1, there is a clear effect of both temperature and olive pomace thickness on the effective diffusivity values of olive pomace. The diffusion coefficient increases with increasing temperature due to rapid movement of water at high temperatures. Natural logarithms of the diffusivity values calculated for the first falling rate period at 60, 70 and 80 C was plotted against the inverse of the absolute temperature to find the activation energy using an Arrhenius type relationship (Eq. (2)), Deff ¼ D0 exp½Ea =ðRg T Þ
ð2Þ
where D0 is a constant, Ea is the energy of activation for diffusion in kJ/mol, T is the absolute temperature and Rg is the gas constant (8.314 kJ/mol K). The plot showed a linear behaviour with R2 values greater than 0.970. The activation energies and their R2 values for the olive pomace samples at different thicknesses are shown in Table 2. The activation energy increases with increasing pomace thickness. This is a clear indication of the higher energy needed to remove the water from 12 mm sample compared to the 9 and 6 mm samples. Similar activation energy values have been found in the same range of temperature (60–80 C) by Go¨g˘u¨ßs and Maskan (1999) as 32.4 kJ/mol for okra and by Vaccarezza et al. (1974) as 28.8 kJ/mol for sugar beet root. Table 2 Energy of activation with respect to the thickness
20
40
60
80
100
120
140
Time (min)
Fig. 4. The effect of particle size on the drying behaviour of olive pomace at 80 C and 12 mm sample thickness (ps: particle size).
3.3. The effect of particle size As shown in Fig. 4 the drying rate of olive pomace also affected by the particle size. The drying time decreased with increasing particle size. The olive pomace with no particle size separation showed a different drying behavior. So, the time of drying was different from the samples with certain particle sizes. The results show that the samples with small particle sizes are packed to obtain an almost nonporous structure. However, large particles resulted in a porous structure and the rate of removal of water in such a solid system is easier than the nonporous systems. Other than the structural nature of the pomace samples, their composition might be an important parameter on the rate of drying. Because large particles are mainly from the pits of the olive and they contain higher amounts of cellulose compared to the smaller particles. However, the small particles are from olive fruit itself and contain more oil than pits pomace. The oil may resist water movement. Hence, the drying takes place in a longer time. This compositional difference also affects the equilibrium moisture content. The drying behaviour of olive pomace by hot air was found to be different when compared with that of microwave. In microwave drying, there was no effect of sample thickness and porosity on the rate of drying (Go¨g˘u¨ßs & Maskan, 2001). However, as it was discussed above, in hot air drying both sample thickness and porosity had a considerable effect. 4. Conclusions
Thickness (mm)
Ea (kJ/mol)
R2
6 9 12
25.4 25.7 29.2
0.997 0.989 0.972
In conclusion, the results from this study showed that temperature, thickness and particle size affected the rate of drying of olive pomace. The increase in both temperature and particle size resulted in the decrease in time of
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drying. On the other hand, thick samples compared to the thin ones dried in a longer time at all temperatures.
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