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Volatility of apples during air and freeze drying
Journal of Food Engineering 73 (2006) 135–141 www.elsevier.com/locate/jfoodeng
Volatility of apples during air and freeze drying M.K. Krokida *, C. Philippopoulos School of Chemical Engineering, National Technical University of Athens, Zografou Campus, 15780 Athens, Greece Received 18 November 2003; accepted 13 January 2005 Available online 7 April 2005
Abstract The volatility of some representative aroma compounds during the freeze and convective drying of apples was analysed by gas chromatography/mass spectrometry (GC/MS). Among the 36 aroma compounds identified by GC–MS, three representative components of the apple flavor, ethyl acetate, ethyl butyrate and methyl anthranilate, were examined in detail. The retention of the above compounds and moisture content retention during freeze and conventional drying at three different temperatures was investigated. Drying kinetics of volatile compounds and moisture content were studied by introducing a first order kinetic model, involving a characteristic parameter (drying constant), as a function of the drying temperature. Most of the losses of the flavor compounds occurred during the early stages of drying. Retention of aroma was affected by the temperature of drying and the drying method used. Lower drying temperatures and freeze drying method are suggested for maximum retention of flavor in the dried product. 2005 Elsevier Ltd. All rights reserved. Keywords: Apples; GC–MS; Volatiles, drying kinetics; Ethyl acetate; Ethyl butyrate; Methyl anthranilate
1. Introduction The taste of apples is partially determined by the biogenesis of a great number of volatile compounds (Brackmann, Streif, & Bangerth, 1993; Drawert, 1975; Saravacos & Moyer, 1968; Valero, Sanz, & MartinezCastro, 1999). More than 300 compounds have been identified as aroma substances from various cultivars of apples (Dimick & Hoskin, 1983). However, only a few of the compounds emanating from apples have been determined to have a decisive impact on the sensory quality and are, therefore, designated as impact compounds in apple fruits (Cunningham, Acree, Barnard, Butts, & Braell, 1986). Among the most commonly found impact compounds in apples are ethyl acetate, ethyl butyrate and methyl anthranilate (Kakiuchi et al., 1986). A significant portion of these compounds
*
Corresponding author. Fax: +30 1 772 3155. E-mail address: [email protected] (M.K. Krokida).
0260-8774/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.01.012
may evaporate during the drying process, resulting in a product of poor flavor, varying between different drying methods and conditions. A quantitative knowledge of the losses of aroma compounds during drying of fruits is also essential for improving the essence recovery in various commercial operations. Retention of aroma is influenced by a number of factors, including the vapor pressure of each compound, its relative volatility, the drying temperature and the composition of the food product (Saravacos & Maroulis, 2001; Mattheis, Fellman, Chen, & Patterson, 1991; Saravacos & Moyer, 1966). It has been demonstrated by a number of researchers that very substantial losses of volatile compounds occur in the immediate vicinity of the atomizer, due to the high surface areas and mass transfer coefficients involved in atomization (Kieckbusch & King, 1980). Once drops are formed and the surfaces of the drops have been dried sufficiently, selective diffusion comes into effect as the diffusion coefficients of trace solutes in the surface region become much lower than that of the water bulk
136
M.K. Krokida, C. Philippopoulos / Journal of Food Engineering 73 (2006) 135–141
solute system (Etzel & King, 1984). The surface thereby becomes selectively permeable to water (Chandrasekaran & King, 1972; King, 1988; Thijssen, 1971; Thijssen & Rulkens, 1968). It has been shown that substantial loss of volatile substances occurs in addition to those attributable to simple diffusion from drops (Alexander & King, 1985; Kerkhof & Thijssen, 1977). Conventional air drying is the most frequently used dehydration operation in the food and conventional chemical industry. In this case, drying kinetics are greatly affected by the air temperature and material characteristic dimension, while all other process factors exert practically negligible influence (Maroulis & Saravacos, 2003). Volatile retention is of special importance in fruit dehydration since a significant portion of volatile compounds, such as esters, acids and alcohols evaporate during air drying process, resulting in a product of poor flavor (Song & Bangerth, 1996). Freeze drying is one of the most sophisticated dehydration methods. Freeze-dried foods are considered as having a quality higher than other dehydrated products mainly because they can be reconstituted with water rapidly to products closely resembling the original food. Another important advantage is that freeze drying is accomplished at relatively low temperatures and the various heat-sensitive biological compounds are not damaged. It is generally accepted that the flavor of freeze dried foods is better than the air dehydrated products. It provides dried products of porous structure (Karathanos, Anglea, & Karel, 1996; Karathanos, Maroulis, Marinos-Kouris, & Saravacos, 1996) and little or no shrinkage, superior taste retention, better rehydration properties, compared to products of alternative drying processes. However, its advantages are directly weighed against its corresponding high treatment cost. In the present work, an investigation on the quantitative retention of major flavor compounds during freeze and convective drying of apples is presented. The qualitative and quantitative measurement of these compounds and the effect of air temperature during convective drying have been measured using the GC–MS (gas chromatography/mass spectrometry) method.
Table 1 GC–MS analysis details Apparatus Column Length Internal diameter Carrier gas Total flow Mode Injector temperature Detector temperature Oven temperature
Hewlett Packard GC 6890-MSD 5973 HP-INOWAX 30 m 250 · 106 m Helium 2.4 ml min1 Pulsed split (1:10) 250 C MS Quad: 150 C MS source: 230 C Initial: 60 C for 3 min 60 C ! 230 C, 20 C min1 Final: 230 C for 6 min
tile compounds was by comparison of peak-height with a known standard. 2.2. Drying kinetics Drying kinetics describe the mechanisms of heat and mass transport phenomena and investigate the influence that certain process variables exert on moisture and volatile compounds removal processes. It forms the most essential part of the actual mathematical model of any dehydration operation, which seeks a proper estimation of the drying time involved as well as the related behavior of all corresponding operational factors playing an important role in the design and optimization of dryers. Drying kinetics is investigated through experimental studies of material moisture content and volatile compound removal. The measurement of material moisture content and volatile compounds content as a function of time under constant drying conditions constitutes the so-called drying curve.
2.1. Materials and methods
2.2.1. Convective drying The samples were dehydrated in an experimental airdryer, which consists of four basic sections: air flow rate control, heat control, humidity control and drying test compartments. Convective drying experiments were carried out at various levels of drying air temperature: 30, 50 and 70 C. A total of 9 experiments were performed.
Delicious red apples harvested in October were used in these experiments. Apples (approximately 10 g) were reconstituted overnight in 20 g of water. A liquid sample (1 ll) was injected into a Hewlett Packard GC/MS equipment with an HP-INNOWax column using an injector split. The analysis details are presented in Table 1. Volatile analyses were replicated two to three times using different sets of apples. Quantification of the vola-
2.2.2. Freeze drying Freeze dried materials were frozen at 35 C for 48 h, tempered for 1 h in liquid N2, and freeze dried using a Lyovac Gt 2 laboratory freeze dryer. Freeze drying was performed under constant vacuum conditions, to obtain initial sample temperatures, below the glass transition temperature of the tissues (Tg 0 = 30 C) (Anglea, Karathanos, & Karel, 1993). The vacuum
2. Experimental procedure
M.K. Krokida, C. Philippopoulos / Journal of Food Engineering 73 (2006) 135–141
was reduced by leaking air through one of the pressure release valves. The temperature of the sample was measured in the center of the sample during freeze drying using a thermocouple. Analysis of flavor compounds was made on samples before during and after drying procedures, in order to obtain the drying curves for moisture and volatile compounds during freeze and convective drying.
137
Table 3 Drying kinetics Mathematical model
dX ¼ kðX X e Þ dt
where: X moisture content (kg/kg db) Xe equilibrium material moisture content (kg/kg db) t time (h) Parameter k drying constant
3. Mathematical modelling
Parameter equation
The mathematical models describing drying kinetics, rather than being strictly mechanistic, are often quasimechanistic and sometimes, mostly empirical. A complete description of the actual mechanisms involved, is usually not obtainable, and would certainly be hopelessly complex (Carbonell, Pinaga, Yusa, & Pena, 1986; Hawlader, Uddin, Ho, & Teng, 1991; Islam & Flink, 1982; Mazza & Lemaguer, 1980). Empirical models can be deduced from detailed mechanistic ones under certain assumptions, or can be evaluated empirically, in the sense that they should at least account for the basic mechanisms in the process examined (Kiranoudis, Maroulis, & Marinos-Kouris, 1990). The empirical model chosen to describe moisture and volatile compounds transfer is presented in Tables 2 and 3. It has Table 2 Volatile substances of apple identified by GC–MS No.
Aroma compounds
Peak area *103 1001 g fresh weight
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Methylacetate Butanol Ethylacetate Propylacetate Methylbutanoete 2-Methylpropylacetate Ethyl butyrate Propylpropionate Butylacetate 2-Methylbutylacetate Butanol Butylpropionate Methyl anthranilate Pentyacetate 2-Methylpropylacetate 2-Methylbutanol Butylpentanoate Ethylexanoate Hexylacetate Hexylpropionate Hexanol Butylhexanoate Octylacetate Propyloctanoate Hexylhexanoate 4-Methoxyallylbenzene a-Farnesene
0.5 0.7 5 1 0.2 0.5 4.5 0.9 0.6 0.8 0.2 0.3 4.7 0.5 0.05 0.07 0.3 0.6 0.9 1 0.4 0.3 0.2 0.1 0.05 0.04 0.02
Tr 1 k ¼ h1 exp h2 T E0 h1 ¼ k 0 exp RT r
h2 ¼
E0 RT r
where: k is the drying constant (h1) k0 is the Arrhenius factor (h1) R is the ideal gas content (kJ/mol K) T is the absolute temperature (K) Tr is the reference temperature (K) E0 activation energy
the form of a general linear ordinary differential equation, in which the right hand side contains an empirical mass or volatile compound transfer coefficient multiplied by the corresponding driving force. The effect of the process temperature on the drying constant is expressed using the following Arrhenius type equation: E0 k ¼ k 0 exp ð1Þ RT where k is the drying constant (h1), k0 is the preexponential factor (h1), R is the ideal gas content (kJ/kmol K) and T is the absolute temperature (K). The accurate determination of the ‘‘activation energy’’ term for moisture and volatile compounds diffusion E0, is considered important in the understanding of diffusion phenomena. Parameter transformations were used by Esposito and Floudas (1998), which resulted in the following drying constant expression: Tr k ¼ h1 exp h2 1 ð2Þ T E0 E0 where h1 ¼ k 0 exp RT , Tr is the referand h2 ¼ RT r r ence temperature (350 C). This correlation assures that the two parameters are not correlated.
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4. Results and discussion Among the substances found, 36 could be identified by GC–MS (Table 2). Standards used during the course of these examinations were used for quantifications and closer examinations. Three representative components of apple flavor, ethyl acetate, ethyl butyrate and methyl anthranilate, were examined in detail. Typical drying curves of moisture content and the three volatile components examined are presented in Figs. 1 and 2. The recorded temperatures represent the temperature of the center of the samples during freeze drying are also presented in Fig. 1. The proposed empirical equation was fitted to these data and the results for drying kinetics and drying constants are presented in Figs. 3 and 4. The parameter estimates are presented in Table 4. During the early stages of freeze drying, volatiles were evaporated at a faster rate than in the last period of drying. At the initial temperatures of freeze drying these compounds had vapour pressures higher than ice and this resulted in rapid evaporation of the flavour compounds from the surface and the interior of the frozen material. In general the more volatile the compound the smaller the retention in the freeze drying material. Freeze dried materials seem to have the maximum retention of volatile compounds in comparison to that obtained during convective drying. Under the freeze drying conditions of low temperatures the loss of flavour compounds during the sublimation of ice was similar to the losses observed at the initial stage of the other compounds. During the advanced stages of freeze drying the partially dried layer of the material influences to an increasing degree the retention of flavour compounds by sorption or locking. The temperature of the porous layer
Fig. 2. Kinetics of volatile components during drying.
10.0
300
280
Freeze Drying
260 240
220
Moisture Content (kg/kg db)
Sample Temperature (°C)
320
Freeze Drying 50°C
1.0 0
5
10
15
20
70°C
Air Drying
200 0
5
10
Drying time (h)
15
20
0.1
Drying Time (h)
Fig. 1. Sample temperature during freeze drying and moisture content kinetics during air and freeze drying.
M.K. Krokida, C. Philippopoulos / Journal of Food Engineering 73 (2006) 135–141 1.2
Ethyl Acetate
139
Ethyl Butyrate
1.2
0.8
Drying Constant (h-1)
Drying Constant (h-1)
Freeze Drying
0.4
0.8
Air Drying
0.4
Freeze Drying Air Drying 0 240
290
340
0 240
390
290
Temperature (K)
340
390
Temperature (K) 1.2
1.2
Methyl Anthranilate
Moisture Content
Freeze Drying Freeze Drying
Drying Constant (h-1)
Drying Constant (h-1)
Air Drying 0.8
Air Drying
0.4
0 240
290
340
390
0.8
0.4
0 240
290
340
390
Temperature (K)
Temperature (K)
Fig. 3. Drying constant of volatiles versus temperature.
increases gradually with the corresponding rise of the vapour pressure of flavour compounds. Convective drying takes place at higher temperatures, and as a result the loss of flavour compounds is rapid. It is obvious that temperature increment during convective drying has an adverse effect on the flavour compound retention. The thickness and shape of the pieces of food materials are expected to have a significant effect on the retention of flavour compounds. As it can be concluded from the estimated model parameters, the distribution of the flavour compounds during freeze and convective drying processes. More precisely the flavour removal rate of methylanthranilate and ethylbutyrate is higher than that of the moisture
content and ethylacetate for both methods varying with different process temperatures.
5. Conclusions The effect of the drying method and corresponding process conditions on drying kinetics of moisture content and volatile compound content of dehydrated apple was investigated for convective and freeze drying. Factors affecting drying kinetics were found to be air temperature for convective drying and sample temperature during freeze drying. The retention of flavor in freeze dried materials was higher than in the convective drying experiments. It is obvious that the higher retention of
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M.K. Krokida, C. Philippopoulos / Journal of Food Engineering 73 (2006) 135–141
Freeze Drying
1.2
Ethyl Butyrate
Methyl Anthranilate 0.8 Moisture Content
Drying Constant (h-1)
Drying Constant (h-1)
1.2
Air Drying
Methyl Anthranilate 0.8 Ethyl Butyrate
0.4
0.4
Moisture Content Ethyl Acetate Ethyl Acetate 0
0 240
260
280
300
320
240
290
340
390
Temperature (K)
Temperature (K)
Fig. 4. Drying constant of volatile components during air and freeze drying.
Table 4 Parameter estimates h1
h2
Convective drying Moisture content Ethyl acetate Ethyl butyrate Methylanthranilate
0.5 0.16 0.69 0.77
7.1 6.2 5.4 5.8
Freeze drying Moisture content Ethyl acetate Ethyl butyrate Methylanthranilate
2.7 0.19 3.7 3.5
7.1 0.8 7.2 8.1
aroma in freeze drying is caused mainly by a lower product temperature.
References Alexander, K., & King, C. J. (1985). Factors governing surface morphology of spray-dried amorphous substances. Drying Technology, 3, 321–348. Anglea, S. A., Karathanos, V. T., & Karel, M. (1993). Low temperature transitions in fresh and osmotically dehydrated plant materials. Biotechnology Progress, 9, 204–209. Brackmann, A., Streif, J., & Bangerth, F. (1993). Relationship between a reduced aroma production and lipid metabolism of apple after long Term Controlled Atmosphere Storage. Journal of American Society for Horticultural Science, 118, 243–247. Carbonell, J. V., Pinaga, F., Yusa, V., & Pena, J. L. (1986). The dehydration of paprika with ambient heated air and the kinetics of colour degradation during storage. Journal of Food Engineering, 5, 179–193.
Chandrasekaran, S. K., & King, C. J. (1972). Volatiles retention during drying of food liquids. AIChe Journal, 18, 520–526. Cunningham, D. G., Acree, T. E., Barnard, J., Butts, R. M., & Braell, P. A. (1986). Charm analysis of apple volatiles. Food Chemistry, 19, 137–147. Dimick, P. S., & Hoskin, J. C. (1983). Review of apple flavor-state of the art CRC critical review. Food Science Nutrition, 18, 387– 409. Drawert, F. (1975). Formation des aromes a different stages de lÕ evolution du fruit; engymes intervenant dans cette formation. Facteurs et Regulation de la Maturation des Fruits. Colloques Internationaux C.N.R.S. No. 238, 309–318. Esposito, W. R., & Floudas, C. A. (1998). Global optimization in parameter estimation of non linear algebric models via the error in variables approach. Industrial and Engineering Chemistry Research, 37, 1841–1858. Etzel, M. R., & King, C. J. (1984). Loss of volatile trace organics during spray drying. Industrial and Engineering Chemistry Process Design and Development, 23, 705–710. Hawlader, M. N. A., Uddin, M. S., Ho, J. C., & Teng, A. B. W. (1991). Drying Characteristics of Tomatoes. Journal of Food Engineering, 14, 259–268. Islam, M. N., & Flink, J. M. (1982). Dehydration of potato II. Osmotic concentration and its effects on air drying behavior. Journal of Food Technology, 17, 387–403. Kakiuchi, N., Moriguchi, S., Fukuda, H., Ichimura, N., Kato, Y., & Banda, Y. (1986). Composition of volatile compounds of apple fruits in relation to cultivars. Journal of Japan Society Horticultural Science, 55, 280–289. Karathanos, V. T., Anglea, S. A., & Karel, M. (1996). Structure collapse of plant materials during freeze drying. Journal of Thermal Analysis, 46(5), 1541–1551. Karathanos, V. T., Maroulis, Z. B., Marinos-Kouris, D., & Saravacos, G. D. (1996). Hygrothemal and quality properties applicable to drying. Data sources and measurement techniques. Drying Technology, 14(6), 1403–1418. Kerkhof, P. J. A. M., & Thijssen, H. A. C. (1977). Quantitative Study of the effects of process variables on aroma retention during the drying of liquid foods. AICHE Symposium Series, 73, 33–46.
M.K. Krokida, C. Philippopoulos / Journal of Food Engineering 73 (2006) 135–141 Kieckbusch, T. G., & King, C. J. (1980). Volatiles loss during atomization in spray drying. AICHE Journal, 26, 718–725. King, C. J. (1988). Spray drying of food liquids and volatiles retention. In Bruin (Ed.), Preconcentration and drying of food materials (pp. 147–162). Amsterdam: Elsevier. Kiranoudis, C. T., Maroulis, Z. B., & Marinos-Kouris, D. (1990). Mass transfer modelling for virginia tobacco curing. Drying Technology, 8(2), 351–366. Maroulis, Z., & Saravacos, G. D. (2003). Food process design. New York: Marcel Dekker. Mattheis, J. P., Fellman, J. K., Chen, P. M., & Patterson, M. E. (1991). Changes in headspace volatiles during physiological development of Bisbee Delicious Apple Fruit. Journal of Agricultural Food Chemistry, 39, 1902–1906. Mazza, G., & Lemaguer, M. (1980). Dehydration of onion: some theoretical and practical consideration. Journal of Food Engineering, 14, 241–255. Saravacos, G. D., & Maroulis, Z. (2001). Transport properties of foods. New York: Marcel Dekker.
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Saravacos, G. D., & Moyer, J. C. (1966). Volatility of some aroma compounds during vacuum-drying of fruit juices. Food Technology, 22(5), 89–93. Saravacos, G. D., & Moyer, J. C. (1968). Volatility of some flavor compounds during freeze-drying of foods. Chemical Engineering Progress Symposium Series, 64(86), 37–42. Song, J., & Bangerth, F. (1996). The effect of harvest date on aroma compound production from ‘‘Golden Delicious’’ apple fruit and relationship to respiration and ethylene production. Postharvest Biology and Technology, 8, 259–269. Thijssen, H. A. C. (1971). Flavor retention in drying preconcentrated food liquids. Journal Applied Chemistry and Biochemistry, 21, 372–377. Thijssen, H. A. C., & Rulkens, W. H. (1968). Retention of aromas in drying food liquids. Ingenieur, 80, 45. Valero, E., Sanz, J., & Martinez-Castro, I. (1999). Volatile components in microwave and conventionally heated milk. Food Chemistry, 66(3), 333–338.
Volatility of apples during air and freeze drying M.K. Krokida *, C. Philippopoulos School of Chemical Engineering, National Technical University of Athens, Zografou Campus, 15780 Athens, Greece Received 18 November 2003; accepted 13 January 2005 Available online 7 April 2005
Abstract The volatility of some representative aroma compounds during the freeze and convective drying of apples was analysed by gas chromatography/mass spectrometry (GC/MS). Among the 36 aroma compounds identified by GC–MS, three representative components of the apple flavor, ethyl acetate, ethyl butyrate and methyl anthranilate, were examined in detail. The retention of the above compounds and moisture content retention during freeze and conventional drying at three different temperatures was investigated. Drying kinetics of volatile compounds and moisture content were studied by introducing a first order kinetic model, involving a characteristic parameter (drying constant), as a function of the drying temperature. Most of the losses of the flavor compounds occurred during the early stages of drying. Retention of aroma was affected by the temperature of drying and the drying method used. Lower drying temperatures and freeze drying method are suggested for maximum retention of flavor in the dried product. 2005 Elsevier Ltd. All rights reserved. Keywords: Apples; GC–MS; Volatiles, drying kinetics; Ethyl acetate; Ethyl butyrate; Methyl anthranilate
1. Introduction The taste of apples is partially determined by the biogenesis of a great number of volatile compounds (Brackmann, Streif, & Bangerth, 1993; Drawert, 1975; Saravacos & Moyer, 1968; Valero, Sanz, & MartinezCastro, 1999). More than 300 compounds have been identified as aroma substances from various cultivars of apples (Dimick & Hoskin, 1983). However, only a few of the compounds emanating from apples have been determined to have a decisive impact on the sensory quality and are, therefore, designated as impact compounds in apple fruits (Cunningham, Acree, Barnard, Butts, & Braell, 1986). Among the most commonly found impact compounds in apples are ethyl acetate, ethyl butyrate and methyl anthranilate (Kakiuchi et al., 1986). A significant portion of these compounds
*
Corresponding author. Fax: +30 1 772 3155. E-mail address: [email protected] (M.K. Krokida).
0260-8774/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.01.012
may evaporate during the drying process, resulting in a product of poor flavor, varying between different drying methods and conditions. A quantitative knowledge of the losses of aroma compounds during drying of fruits is also essential for improving the essence recovery in various commercial operations. Retention of aroma is influenced by a number of factors, including the vapor pressure of each compound, its relative volatility, the drying temperature and the composition of the food product (Saravacos & Maroulis, 2001; Mattheis, Fellman, Chen, & Patterson, 1991; Saravacos & Moyer, 1966). It has been demonstrated by a number of researchers that very substantial losses of volatile compounds occur in the immediate vicinity of the atomizer, due to the high surface areas and mass transfer coefficients involved in atomization (Kieckbusch & King, 1980). Once drops are formed and the surfaces of the drops have been dried sufficiently, selective diffusion comes into effect as the diffusion coefficients of trace solutes in the surface region become much lower than that of the water bulk
136
M.K. Krokida, C. Philippopoulos / Journal of Food Engineering 73 (2006) 135–141
solute system (Etzel & King, 1984). The surface thereby becomes selectively permeable to water (Chandrasekaran & King, 1972; King, 1988; Thijssen, 1971; Thijssen & Rulkens, 1968). It has been shown that substantial loss of volatile substances occurs in addition to those attributable to simple diffusion from drops (Alexander & King, 1985; Kerkhof & Thijssen, 1977). Conventional air drying is the most frequently used dehydration operation in the food and conventional chemical industry. In this case, drying kinetics are greatly affected by the air temperature and material characteristic dimension, while all other process factors exert practically negligible influence (Maroulis & Saravacos, 2003). Volatile retention is of special importance in fruit dehydration since a significant portion of volatile compounds, such as esters, acids and alcohols evaporate during air drying process, resulting in a product of poor flavor (Song & Bangerth, 1996). Freeze drying is one of the most sophisticated dehydration methods. Freeze-dried foods are considered as having a quality higher than other dehydrated products mainly because they can be reconstituted with water rapidly to products closely resembling the original food. Another important advantage is that freeze drying is accomplished at relatively low temperatures and the various heat-sensitive biological compounds are not damaged. It is generally accepted that the flavor of freeze dried foods is better than the air dehydrated products. It provides dried products of porous structure (Karathanos, Anglea, & Karel, 1996; Karathanos, Maroulis, Marinos-Kouris, & Saravacos, 1996) and little or no shrinkage, superior taste retention, better rehydration properties, compared to products of alternative drying processes. However, its advantages are directly weighed against its corresponding high treatment cost. In the present work, an investigation on the quantitative retention of major flavor compounds during freeze and convective drying of apples is presented. The qualitative and quantitative measurement of these compounds and the effect of air temperature during convective drying have been measured using the GC–MS (gas chromatography/mass spectrometry) method.
Table 1 GC–MS analysis details Apparatus Column Length Internal diameter Carrier gas Total flow Mode Injector temperature Detector temperature Oven temperature
Hewlett Packard GC 6890-MSD 5973 HP-INOWAX 30 m 250 · 106 m Helium 2.4 ml min1 Pulsed split (1:10) 250 C MS Quad: 150 C MS source: 230 C Initial: 60 C for 3 min 60 C ! 230 C, 20 C min1 Final: 230 C for 6 min
tile compounds was by comparison of peak-height with a known standard. 2.2. Drying kinetics Drying kinetics describe the mechanisms of heat and mass transport phenomena and investigate the influence that certain process variables exert on moisture and volatile compounds removal processes. It forms the most essential part of the actual mathematical model of any dehydration operation, which seeks a proper estimation of the drying time involved as well as the related behavior of all corresponding operational factors playing an important role in the design and optimization of dryers. Drying kinetics is investigated through experimental studies of material moisture content and volatile compound removal. The measurement of material moisture content and volatile compounds content as a function of time under constant drying conditions constitutes the so-called drying curve.
2.1. Materials and methods
2.2.1. Convective drying The samples were dehydrated in an experimental airdryer, which consists of four basic sections: air flow rate control, heat control, humidity control and drying test compartments. Convective drying experiments were carried out at various levels of drying air temperature: 30, 50 and 70 C. A total of 9 experiments were performed.
Delicious red apples harvested in October were used in these experiments. Apples (approximately 10 g) were reconstituted overnight in 20 g of water. A liquid sample (1 ll) was injected into a Hewlett Packard GC/MS equipment with an HP-INNOWax column using an injector split. The analysis details are presented in Table 1. Volatile analyses were replicated two to three times using different sets of apples. Quantification of the vola-
2.2.2. Freeze drying Freeze dried materials were frozen at 35 C for 48 h, tempered for 1 h in liquid N2, and freeze dried using a Lyovac Gt 2 laboratory freeze dryer. Freeze drying was performed under constant vacuum conditions, to obtain initial sample temperatures, below the glass transition temperature of the tissues (Tg 0 = 30 C) (Anglea, Karathanos, & Karel, 1993). The vacuum
2. Experimental procedure
M.K. Krokida, C. Philippopoulos / Journal of Food Engineering 73 (2006) 135–141
was reduced by leaking air through one of the pressure release valves. The temperature of the sample was measured in the center of the sample during freeze drying using a thermocouple. Analysis of flavor compounds was made on samples before during and after drying procedures, in order to obtain the drying curves for moisture and volatile compounds during freeze and convective drying.
137
Table 3 Drying kinetics Mathematical model
dX ¼ kðX X e Þ dt
where: X moisture content (kg/kg db) Xe equilibrium material moisture content (kg/kg db) t time (h) Parameter k drying constant
3. Mathematical modelling
Parameter equation
The mathematical models describing drying kinetics, rather than being strictly mechanistic, are often quasimechanistic and sometimes, mostly empirical. A complete description of the actual mechanisms involved, is usually not obtainable, and would certainly be hopelessly complex (Carbonell, Pinaga, Yusa, & Pena, 1986; Hawlader, Uddin, Ho, & Teng, 1991; Islam & Flink, 1982; Mazza & Lemaguer, 1980). Empirical models can be deduced from detailed mechanistic ones under certain assumptions, or can be evaluated empirically, in the sense that they should at least account for the basic mechanisms in the process examined (Kiranoudis, Maroulis, & Marinos-Kouris, 1990). The empirical model chosen to describe moisture and volatile compounds transfer is presented in Tables 2 and 3. It has Table 2 Volatile substances of apple identified by GC–MS No.
Aroma compounds
Peak area *103 1001 g fresh weight
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Methylacetate Butanol Ethylacetate Propylacetate Methylbutanoete 2-Methylpropylacetate Ethyl butyrate Propylpropionate Butylacetate 2-Methylbutylacetate Butanol Butylpropionate Methyl anthranilate Pentyacetate 2-Methylpropylacetate 2-Methylbutanol Butylpentanoate Ethylexanoate Hexylacetate Hexylpropionate Hexanol Butylhexanoate Octylacetate Propyloctanoate Hexylhexanoate 4-Methoxyallylbenzene a-Farnesene
0.5 0.7 5 1 0.2 0.5 4.5 0.9 0.6 0.8 0.2 0.3 4.7 0.5 0.05 0.07 0.3 0.6 0.9 1 0.4 0.3 0.2 0.1 0.05 0.04 0.02
Tr 1 k ¼ h1 exp h2 T E0 h1 ¼ k 0 exp RT r
h2 ¼
E0 RT r
where: k is the drying constant (h1) k0 is the Arrhenius factor (h1) R is the ideal gas content (kJ/mol K) T is the absolute temperature (K) Tr is the reference temperature (K) E0 activation energy
the form of a general linear ordinary differential equation, in which the right hand side contains an empirical mass or volatile compound transfer coefficient multiplied by the corresponding driving force. The effect of the process temperature on the drying constant is expressed using the following Arrhenius type equation: E0 k ¼ k 0 exp ð1Þ RT where k is the drying constant (h1), k0 is the preexponential factor (h1), R is the ideal gas content (kJ/kmol K) and T is the absolute temperature (K). The accurate determination of the ‘‘activation energy’’ term for moisture and volatile compounds diffusion E0, is considered important in the understanding of diffusion phenomena. Parameter transformations were used by Esposito and Floudas (1998), which resulted in the following drying constant expression: Tr k ¼ h1 exp h2 1 ð2Þ T E0 E0 where h1 ¼ k 0 exp RT , Tr is the referand h2 ¼ RT r r ence temperature (350 C). This correlation assures that the two parameters are not correlated.
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4. Results and discussion Among the substances found, 36 could be identified by GC–MS (Table 2). Standards used during the course of these examinations were used for quantifications and closer examinations. Three representative components of apple flavor, ethyl acetate, ethyl butyrate and methyl anthranilate, were examined in detail. Typical drying curves of moisture content and the three volatile components examined are presented in Figs. 1 and 2. The recorded temperatures represent the temperature of the center of the samples during freeze drying are also presented in Fig. 1. The proposed empirical equation was fitted to these data and the results for drying kinetics and drying constants are presented in Figs. 3 and 4. The parameter estimates are presented in Table 4. During the early stages of freeze drying, volatiles were evaporated at a faster rate than in the last period of drying. At the initial temperatures of freeze drying these compounds had vapour pressures higher than ice and this resulted in rapid evaporation of the flavour compounds from the surface and the interior of the frozen material. In general the more volatile the compound the smaller the retention in the freeze drying material. Freeze dried materials seem to have the maximum retention of volatile compounds in comparison to that obtained during convective drying. Under the freeze drying conditions of low temperatures the loss of flavour compounds during the sublimation of ice was similar to the losses observed at the initial stage of the other compounds. During the advanced stages of freeze drying the partially dried layer of the material influences to an increasing degree the retention of flavour compounds by sorption or locking. The temperature of the porous layer
Fig. 2. Kinetics of volatile components during drying.
10.0
300
280
Freeze Drying
260 240
220
Moisture Content (kg/kg db)
Sample Temperature (°C)
320
Freeze Drying 50°C
1.0 0
5
10
15
20
70°C
Air Drying
200 0
5
10
Drying time (h)
15
20
0.1
Drying Time (h)
Fig. 1. Sample temperature during freeze drying and moisture content kinetics during air and freeze drying.
M.K. Krokida, C. Philippopoulos / Journal of Food Engineering 73 (2006) 135–141 1.2
Ethyl Acetate
139
Ethyl Butyrate
1.2
0.8
Drying Constant (h-1)
Drying Constant (h-1)
Freeze Drying
0.4
0.8
Air Drying
0.4
Freeze Drying Air Drying 0 240
290
340
0 240
390
290
Temperature (K)
340
390
Temperature (K) 1.2
1.2
Methyl Anthranilate
Moisture Content
Freeze Drying Freeze Drying
Drying Constant (h-1)
Drying Constant (h-1)
Air Drying 0.8
Air Drying
0.4
0 240
290
340
390
0.8
0.4
0 240
290
340
390
Temperature (K)
Temperature (K)
Fig. 3. Drying constant of volatiles versus temperature.
increases gradually with the corresponding rise of the vapour pressure of flavour compounds. Convective drying takes place at higher temperatures, and as a result the loss of flavour compounds is rapid. It is obvious that temperature increment during convective drying has an adverse effect on the flavour compound retention. The thickness and shape of the pieces of food materials are expected to have a significant effect on the retention of flavour compounds. As it can be concluded from the estimated model parameters, the distribution of the flavour compounds during freeze and convective drying processes. More precisely the flavour removal rate of methylanthranilate and ethylbutyrate is higher than that of the moisture
content and ethylacetate for both methods varying with different process temperatures.
5. Conclusions The effect of the drying method and corresponding process conditions on drying kinetics of moisture content and volatile compound content of dehydrated apple was investigated for convective and freeze drying. Factors affecting drying kinetics were found to be air temperature for convective drying and sample temperature during freeze drying. The retention of flavor in freeze dried materials was higher than in the convective drying experiments. It is obvious that the higher retention of
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Freeze Drying
1.2
Ethyl Butyrate
Methyl Anthranilate 0.8 Moisture Content
Drying Constant (h-1)
Drying Constant (h-1)
1.2
Air Drying
Methyl Anthranilate 0.8 Ethyl Butyrate
0.4
0.4
Moisture Content Ethyl Acetate Ethyl Acetate 0
0 240
260
280
300
320
240
290
340
390
Temperature (K)
Temperature (K)
Fig. 4. Drying constant of volatile components during air and freeze drying.
Table 4 Parameter estimates h1
h2
Convective drying Moisture content Ethyl acetate Ethyl butyrate Methylanthranilate
0.5 0.16 0.69 0.77
7.1 6.2 5.4 5.8
Freeze drying Moisture content Ethyl acetate Ethyl butyrate Methylanthranilate
2.7 0.19 3.7 3.5
7.1 0.8 7.2 8.1
aroma in freeze drying is caused mainly by a lower product temperature.
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