- Email: [email protected]
The effects of dipping pretreatments on air-drying rates of the seedless grapes
Journal of Food Engineering 52 (2002) 413–417 www.elsevier.com/locate/jfoodeng
The effects of dipping pretreatments on air-drying rates of the seedless grapes I_ brahim Doymaz *, Mehmet Pala Yildiz Technical University, Chemical Engineering Department, Davutpasa Cad No. 127, 34210 Esenler, Istanbul, Turkey Received 20 April 2001; accepted 30 June 2001
Abstract Turkey has a significant potential for fruit and vegetable production. Grape is an economically important item for Turkey’s export. However, problems encountered during drying of grapes have not yet been resolved. The effects of different dipping solutions on hot air drying of grapes were studied. Seedless grapes pretreated with alkaline emulsion of ethyl oleate (AEEO) resulted in comparatively good quality raisins. Grapes dipped into ethyl oleate solution prior to drying showed shorter drying times than those untreated, or pretreated with potassium carbonate solution. Colour analysis of grapes showed that the best results are obtained in grapes which were pretreated with an AEEO and dried with air at 60 C. The drying rates of grapes were modeled by the Page and Exponential equations. The model results agreed with experimental results. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Grape; Hot-air drying; Pretreatment solution; Mathematical models
1. Introduction Dried grapes, commonly known as raisins, have a great economic importance for Turkey (250,000 t were produced in 1998). In Turkey, the majority of seedless grapes, including Sultana and Thompson varieties, are cultivated in the Aegean region. In this region, harvested grapes are traditionally dipped into pretreatment solutions (5–6% (v/v) potassium carbonate in 0.5% (v/v) olive oil solution) and sun-dried on canvas sheets or concrete surfaces. In grape drying, the rate of moisture diffusion through the berries is controlled by the waxy cuticle of the grapes (Pangavhane, Sawhney, & Sarsavadia, 1999). A number of authors have reported the effects of pretreatments on the drying rates and quality parameters of various foodstuffs. Some experimental results on grape drying are reported in the literature (Aguilera, Oppermann, & Sanchez, 1987; Bolin, Petrucci, & Fuller, 1975; Guadagni, Stafford, & Fuller, 1975; Karathanos & Belessiotis, 1997; Mahmutoglu, Emir, & Saygi, 1996; Pala,
*
Corresponding author. Tel.: +90-212-449-1718; fax: +90-212-4491895. E-mail address: [email protected] (I_ . Doymaz).
Saygi, & Sadıkoglu, 1993; Raouzeos & Saravacos, 1986; Saravacos, Marousis, & Raouzeos, 1988; Tulasidas, Raghavan, & Norris, 1996). Dipping in hot water or the use of chemicals such as sulphur, caustic and ethyl or methyl oleate emulsions are some of pretreatments widely used for grape drying. The aim of using those pretreatment solutions is to increase drying rates and to produce raisins of the desired quality level. Grapes were surface treated by dipping in ethyl oleate, which greatly increases the drying rate by altering the waxy layer structure at the grape surface, thus reducing the internal resistance to water diffusion (Di Matteo, Cinquanta, Galiero, & Crescitelli, 2000; Doymaz, 1998; Petrucci, Canata, Bolin, Fuller, & Stafford, 1973; Ponting & McBean, 1970; Riva & Peri, 1986). Ethyl oleate evidently acts on grape skin by dissolving the waxy components which offer a high resistance to moisture transfer, yet higher alkali concentrations and longer dipping times can cause adverse changes in the quality of dried grapes (Saravacos et al., 1988). In this study, the rate and time of drying in a hot-air drier and the colour of the final product were investigated. Furthermore, the influence of dipping solutions such as ethyl oleate and potassium carbonate were studied. Also, drying time and colour quality of dried untreated grapes were compared with those pretreated with dipping solutions.
0260-8774/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 1 ) 0 0 1 3 3 - 9
I_ . Doymaz, M. Pala / Journal of Food Engineering 52 (2002) 413–417
414
Nomenclature c Deff K M Me
constant in Eq. (2) effective moisture transfer diffusion coefficient ðm2 =sÞ constant in Eq. (2) moisture content (kg water/kg dry mass) equilibrium moisture content (kg water/kg dry mass)
1.1. Mathematical modeling For a sphere, the solution of Fick’s second law, with the assumptions of moisture migration being by diffusion, negligible shrinkage, constant temperature and diffusion coefficients and long drying times is as given below (Di Matteo et al., 2000; Mahmutoglu et al., 1996; T€ ut€ unc€ u & Labuza, 1996): 2 M Me 6 p Deff t ln ¼ ln : ð1Þ p2 R2 M0 Me Eq. (1) can also be written in a more simplified form as: M Me ¼ K expðctÞ: M0 Me
ð2Þ
M0 R t x y
initial moisture content (kg water/kg mass) radius (m) drying time (min) constant in Eq. (3) constant in Eq. (3)
size (1.8–2.4 g) were used. The initial moisture content of grapes varied from 77.3% to 80.5%, wet basis. Chemicals used for dipping grape were technical grade. 2.2. Methods Drying of Sultana grapes was carried out in cabinet drier produced by APV&PASILAC firm (England). During drying experiments, the moisture content of fresh and dried grapes, air flow rate and colour parameters L (lightness), +a (redness) and +b (yellowness) were measured according to AOAC (1990), TESTO 440 Vane probe Anemometer and Hunter Lab Colour D 25 D2P, respectively.
Eq. (2) is known to be the Exponential equation (Exp.). An alternative approach to analysis of thin layer drying has been to use empirical relationships. One equation that has been widely used in thin layer drying studies is Page’s equation (Diamante & Munro, 1993; Madamba, Driscoll, & Buckle, 1996).
2.3. Drying experiments
M Me ¼ expðxty Þ: M0 Me
• POTAS: Traditional potassium carbonate solution: This solution is prepared by dissolving 0:5 kg K2 CO3 in 10 l water and adding 0.05 kg olive oil. • AEEO: Alkaline emulsion of ethyl oleate: AEEO is prepared by dissolving 0:5 kg K2 CO3 in 10 l water and adding 0.2 kg ethyl oleate.
ð3Þ
The values of the equilibrium moisture content, Me , are relatively small compared to M or M0 . Thus ðM Me Þ=ðM0 Me Þ is simplified to M=M0 M ¼ K expðctÞ; M0
ð4Þ
M ¼ expðxty Þ: M0
ð5Þ
To calculate diffusion coefficients ðDeff Þ, the slope of lnðM=M0 Þ versus time, as given by Eq. (1), was used (Lomauro, Bakshi, & Labuza, 1985; T€ ut€ unc€ u & Labuza, 1996).
2. Materials and methods 2.1. Materials The Sultana seedless (Vitis vinifera L.) variety of grapes was used. Generally, grape berries of uniform
Grapes were dipped in several solutions and dried afterward. Pretreatment solutions and the associated code numbers are given below:
Dipping time in the pretreatment solutions was about 1 min at ambient temperature. Then, grapes were dried as a single layer in a batch drier. In this drier, air flow is perpendicular to the grape berries. Grapes were dried at air temperatures of 50 C, 55 C, 60 C and 70 C and the air velocity was 1.2 m/s. Also untreated grapes were dried (code NAT) at 60 C and 70 C air temperatures. In each experiment, 1 kg of fresh grapes was used. Weight loss of the drying grapes was measured by means of a load cell (REVERE SHBXM CC). The drying was concluded when the final moisture content was 0.2 kg/kg dry mass. The product was cooled and packed in low density polyethylene (LDPE) bags which were heatsealed. The experiments was repeated twice and the average of the moisture ratio at each value was used for drawing the drying curves.
I_ . Doymaz, M. Pala / Journal of Food Engineering 52 (2002) 413–417
415
3. Results and discussion Drying time of each experiment, diffusion and correlation coefficients are given in Table 1. Fig. 1 shows drying curves of grapes pretreated differently and dried at 60 C. Grapes dipped in AEEO or POTAS solution prior to drying were found to have a shorter drying time compared to untreated grapes (Table 1). Grapes dipped in AEEO and POTAS solutions and dried at 60 C air temperature had a drying time of 1230 and 1320 min, respectively, whereas untreated grapes had a drying time of 2880 min (Table 1, Fig. 1). These results demonstrated that AEEO grapes had drying times that were 7.3% shorter than POTAS code and 134.1% shorter than NAT code grapes. Similarly, when the drying times of grapes with POTAS code and NAT code grapes were compared, the drying time for POTAS code grapes was 118% lower than that for NAT code grapes. Consequently, AEEO solution is much more effective than POTAS solution in grape drying. One of the most important criteria of foods is colour. Undesirable changes in colour of food may lead to a decrease in its quality and marketing value. The results of the drying experiments have shown that pretreatments have also an important effect on the colour parameters of the final dried products (Table 2). In terms of desired colour properties, higher L and lower a=b ratio are preferred. Hunter L (lightness) values of grapes dipped in AEEO solution were in most cases higher than those for grapes dipped in POTAS and naturally dried grapes. For example, the values of L were 21.03, 20.29 and 17.74 for grapes dried at 60 C, respectively. The value of a=b, which is a measurement of the redness/yellowness colour, was 0.74, 0.81 and 1.05, respectively. Therefore, grapes with AEEO and POTAS codes are lighter and brighter than those with NAT code by 18.54% and 14.3%, respectively. The values reported in Table 2 show that L values increase with increasing temperature, whereas the a=b
Fig. 1. Drying curves of grapes dried at 60 C after different pretreatments.
values did not show this correlation. Lowest values for a=b were found for AEEO pretreated grapes dried at 60 C. In order to estimate the moisture content as a function of drying time, the empirical Page and Exponential equations were fitted and correlation coefficients were calculated in this study. The values of r2 obtained from the Page Eq. (5) are higher than those attained from the Exponential Eq. (4). The r2 values of the Page equation vary between 0.995 and 0.999, and between 0.973 and 0.995 for the Exponential equation (Table 1). Both equations represent the correlation between the moisture content and drying time. Fig. 2 shows the experimental and fitted drying curves. The diffusion coefficients of grapes dipped in AEEO were higher than those with POTAS and NAT codes. For example, the diffusion coefficients ðDeff Þ of grapes dried at 60 C were 2:01 109 , 1:82 109 , and 9:10 1010 m2 =s, respectively (Table 1). Based on these results, 2%AEEO þ 5%K2 CO3 solution (AEEO) was found to be the most effective dipping solution in increasing the drying rate compared with a 5%K2 CO3 þ 0:5% olive oil solution (POTAS).
Table 1 Drying times of experiments, diffusion and correlation coefficients Code
Air temperature (C)
Drying time (min)
r2
Deff ðm2 =sÞ
Page
Exp.
AEEO
50 55 60 70
2040 1740 1230 810
0.996 0.997 0.997 0.999
0.975 0.985 0.985 0.995
8:49 1010 1:32 109 2:01 109 3:50 109
POTAS
50 55 60 70
2430 1830 1320 870
0.995 0.995 0.998 0.995
0.973 0.973 0.991 0.990
7:91 1010 1:19 109 1:82 109 2:54 109
NAT
60 70
2880 1080
0.997 0.995
0.987 0.993
9:10 1010 1:92 109
I_ . Doymaz, M. Pala / Journal of Food Engineering 52 (2002) 413–417
416 Table 2 Colour analysis of dried grapes Code
Air temperature (C)
Hunter colour values L
+a
+b
a=b
AEEO
50 55 60 70
18.95 19.30 21.03 22.75
3.04 3.99 5.08 5.23
3.84 4.80 6.84 6.65
0.79 0.83 0.74 0.78
POTAS
50 55 60 70
17.57 19.56 20.29 22.31
3.84 3.97 4.55 4.53
4.28 4.32 5.57 5.29
0.89 0.91 0.81 0.86
NAT
60 70
17.74 19.55
2.29 4.74
2.18 5.43
1.05 0.87
that of the untreated grapes. When the drying rates of grapes are modeled by the Page and Exponential equations, the results agree satisfactorily with experimental results.
Acknowledgements This research work was supported by Yildiz University Research Fund under the grant number 92-B-07-01-01. References Fig. 2. Experimental and fitted drying curves for grapes dipped in AEEO solution and dried at 60 C.
Higher drying rates are observed for all grapes dipped into ethyl oleate solution. The reason can be ascribed to the ethyl oleate. Ethyl oleate destroys the resistance of waxy layer which prevents moisture transfer and increases the drying rate.
4. Conclusions The effect of AEEO and potassium carbonate (POTAS) solutions on hot air drying of grapes was studied. Grapes dipped in AEEO or POTAS solution were found to have shorter drying times compared with untreated grapes. The AEEO dipping increased the drying rates more than the potassium carbonate pretreatment, and improved the quality and appearance of the dried grapes. Pretreatment with the AEEO solution is effective in increasing the drying rate. However, it should be noted that this pretreatment is more effective in increasing the drying rate during the early period of drying. The use of ethyl oleate as pretreatment solution for the drying of grapes leads to a better colour (L and a=b values). Colour of the treated grapes was lighter than
Aguilera, J. M., Oppermann, K., & Sanchez, F. (1987). Kinetics of browning of Sultana grapes. Journal of Food Science, 52, 990–993. AOAC (1990). Official methods of analysis. In K. Heldrich (Ed.), Association of official analytical chemists (15th ed.). Washington, USA, p. 1298. Bolin, H. R., Petrucci, V., & Fuller, G. (1975). Characteristics of mechanically harvested raisins produced by dehydration and by field drying. Journal of Food Science, 40, 1036–1038. Diamante, L. M., & Munro, P. A. (1993). Mathematical modeling of the thin layer solar drying of sweet potato slices. Solar Energy, 51(4), 271–276. Di Matteo, M., Cinquanta, L., Galiero, G., & Crescitelli, S. (2000). Effect of a novel physical pretreatment process on the drying kinetics of seedless grapes. Journal of Food Engineering, 46, 83–89. Doymaz, I_ . (1998). Investigation of drying characteristics of grape and Kahramanmaras pepper. Ph.D. Thesis, Science Institute, Yildiz Technical University, Istanbul. Guadagni, D. G., Stafford, A. E., & Fuller, G. (1975). Taste thresholds of fatty acid esters in raisins and raisin paste. Journal of Food Science, 40, 780–783. Karathanos, V. T., & Belessiotis, V. G. (1997). Sun and artificial air drying kinetics of some agricultural products. Journal of Food Engineering, 31, 35–46. Lomauro, C. J., Bakshi, A. S., & Labuza, T. P. (1985). Moisture transfer properties of dry and semimoist foods. Journal of Food Science, 50, 397–400. Madamba, P. S., Driscoll, R. H., & Buckle, K. A. (1996). The thinlayer drying characteristics of garlic slices. Journal of Food Engineering, 29, 75–97. Mahmutoglu, T., Emir, F., & Saygi, Y. B. (1996). Sun/Solar drying of differently treated grapes and storage stability of dried grapes. Journal of Food Engineering, 29, 289–300.
I_ . Doymaz, M. Pala / Journal of Food Engineering 52 (2002) 413–417 Pala, M., Saygi, Y. B., & Sadıkoglu, H. (1993). A study on the drying of Sultana grapes by different techniques and effective parameters. In G. Charalambous (Ed.). Food flavors, ingredients and composition (pp. 437–444). Amsterdam. Pangavhane, D. R., Sawhney, R. L., & Sarsavadia, P. N. (1999). Effect of various dipping pretreatment on drying kinetics of Thompson seedless grapes. Journal of Food Engineering, 39, 211–216. Petrucci, V., Canata, N., Bolin, H. R., Fuller, G., & Stafford, A. E. (1973). Use of oleic acid derivatives to accelerate drying of Thompson seedless grapes. Journal of The American Oil Chemists’ Society, 51, 77–80. Ponting, J. D., & Mc Bean, D. M. (1970). Temperature and dipping treatment effects on drying rates and drying times of grapes, prunes and other waxy fruits. Food Technology, 24, 85–88.
417
Raouzeos, G. S., & Saravacos, G. D. (1986). Solar drying of raisins. Drying Technology, 4(4), 633–649. Riva, M., & Peri, C. (1986). Kinetics of sun and air drying of different varieties of seedless grapes. Journal of Food Technology, 21, 199– 208. Saravacos, G. D., Marousis, S. N., & Raouzeos, G. S. (1988). Effect of ethyl oleate on the rate of air-drying of foods. Journal of Food Engineering, 7, 263–270. Tulasidas, T. N., Raghavan, G. S. V., & Norris, E. R. (1996). Effects of dipping and washing pre-treatments on microwave drying of grapes. Journal of Food Process Engineering, 19, 15–25. T€ ut€ unc€ u, A. M., & Labuza, T. P. (1996). Effect of geometry on the effective moisture transfer diffusion coefficient. Journal of Food Engineering, 30, 433–447.
The effects of dipping pretreatments on air-drying rates of the seedless grapes I_ brahim Doymaz *, Mehmet Pala Yildiz Technical University, Chemical Engineering Department, Davutpasa Cad No. 127, 34210 Esenler, Istanbul, Turkey Received 20 April 2001; accepted 30 June 2001
Abstract Turkey has a significant potential for fruit and vegetable production. Grape is an economically important item for Turkey’s export. However, problems encountered during drying of grapes have not yet been resolved. The effects of different dipping solutions on hot air drying of grapes were studied. Seedless grapes pretreated with alkaline emulsion of ethyl oleate (AEEO) resulted in comparatively good quality raisins. Grapes dipped into ethyl oleate solution prior to drying showed shorter drying times than those untreated, or pretreated with potassium carbonate solution. Colour analysis of grapes showed that the best results are obtained in grapes which were pretreated with an AEEO and dried with air at 60 C. The drying rates of grapes were modeled by the Page and Exponential equations. The model results agreed with experimental results. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Grape; Hot-air drying; Pretreatment solution; Mathematical models
1. Introduction Dried grapes, commonly known as raisins, have a great economic importance for Turkey (250,000 t were produced in 1998). In Turkey, the majority of seedless grapes, including Sultana and Thompson varieties, are cultivated in the Aegean region. In this region, harvested grapes are traditionally dipped into pretreatment solutions (5–6% (v/v) potassium carbonate in 0.5% (v/v) olive oil solution) and sun-dried on canvas sheets or concrete surfaces. In grape drying, the rate of moisture diffusion through the berries is controlled by the waxy cuticle of the grapes (Pangavhane, Sawhney, & Sarsavadia, 1999). A number of authors have reported the effects of pretreatments on the drying rates and quality parameters of various foodstuffs. Some experimental results on grape drying are reported in the literature (Aguilera, Oppermann, & Sanchez, 1987; Bolin, Petrucci, & Fuller, 1975; Guadagni, Stafford, & Fuller, 1975; Karathanos & Belessiotis, 1997; Mahmutoglu, Emir, & Saygi, 1996; Pala,
*
Corresponding author. Tel.: +90-212-449-1718; fax: +90-212-4491895. E-mail address: [email protected] (I_ . Doymaz).
Saygi, & Sadıkoglu, 1993; Raouzeos & Saravacos, 1986; Saravacos, Marousis, & Raouzeos, 1988; Tulasidas, Raghavan, & Norris, 1996). Dipping in hot water or the use of chemicals such as sulphur, caustic and ethyl or methyl oleate emulsions are some of pretreatments widely used for grape drying. The aim of using those pretreatment solutions is to increase drying rates and to produce raisins of the desired quality level. Grapes were surface treated by dipping in ethyl oleate, which greatly increases the drying rate by altering the waxy layer structure at the grape surface, thus reducing the internal resistance to water diffusion (Di Matteo, Cinquanta, Galiero, & Crescitelli, 2000; Doymaz, 1998; Petrucci, Canata, Bolin, Fuller, & Stafford, 1973; Ponting & McBean, 1970; Riva & Peri, 1986). Ethyl oleate evidently acts on grape skin by dissolving the waxy components which offer a high resistance to moisture transfer, yet higher alkali concentrations and longer dipping times can cause adverse changes in the quality of dried grapes (Saravacos et al., 1988). In this study, the rate and time of drying in a hot-air drier and the colour of the final product were investigated. Furthermore, the influence of dipping solutions such as ethyl oleate and potassium carbonate were studied. Also, drying time and colour quality of dried untreated grapes were compared with those pretreated with dipping solutions.
0260-8774/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 1 ) 0 0 1 3 3 - 9
I_ . Doymaz, M. Pala / Journal of Food Engineering 52 (2002) 413–417
414
Nomenclature c Deff K M Me
constant in Eq. (2) effective moisture transfer diffusion coefficient ðm2 =sÞ constant in Eq. (2) moisture content (kg water/kg dry mass) equilibrium moisture content (kg water/kg dry mass)
1.1. Mathematical modeling For a sphere, the solution of Fick’s second law, with the assumptions of moisture migration being by diffusion, negligible shrinkage, constant temperature and diffusion coefficients and long drying times is as given below (Di Matteo et al., 2000; Mahmutoglu et al., 1996; T€ ut€ unc€ u & Labuza, 1996): 2 M Me 6 p Deff t ln ¼ ln : ð1Þ p2 R2 M0 Me Eq. (1) can also be written in a more simplified form as: M Me ¼ K expðctÞ: M0 Me
ð2Þ
M0 R t x y
initial moisture content (kg water/kg mass) radius (m) drying time (min) constant in Eq. (3) constant in Eq. (3)
size (1.8–2.4 g) were used. The initial moisture content of grapes varied from 77.3% to 80.5%, wet basis. Chemicals used for dipping grape were technical grade. 2.2. Methods Drying of Sultana grapes was carried out in cabinet drier produced by APV&PASILAC firm (England). During drying experiments, the moisture content of fresh and dried grapes, air flow rate and colour parameters L (lightness), +a (redness) and +b (yellowness) were measured according to AOAC (1990), TESTO 440 Vane probe Anemometer and Hunter Lab Colour D 25 D2P, respectively.
Eq. (2) is known to be the Exponential equation (Exp.). An alternative approach to analysis of thin layer drying has been to use empirical relationships. One equation that has been widely used in thin layer drying studies is Page’s equation (Diamante & Munro, 1993; Madamba, Driscoll, & Buckle, 1996).
2.3. Drying experiments
M Me ¼ expðxty Þ: M0 Me
• POTAS: Traditional potassium carbonate solution: This solution is prepared by dissolving 0:5 kg K2 CO3 in 10 l water and adding 0.05 kg olive oil. • AEEO: Alkaline emulsion of ethyl oleate: AEEO is prepared by dissolving 0:5 kg K2 CO3 in 10 l water and adding 0.2 kg ethyl oleate.
ð3Þ
The values of the equilibrium moisture content, Me , are relatively small compared to M or M0 . Thus ðM Me Þ=ðM0 Me Þ is simplified to M=M0 M ¼ K expðctÞ; M0
ð4Þ
M ¼ expðxty Þ: M0
ð5Þ
To calculate diffusion coefficients ðDeff Þ, the slope of lnðM=M0 Þ versus time, as given by Eq. (1), was used (Lomauro, Bakshi, & Labuza, 1985; T€ ut€ unc€ u & Labuza, 1996).
2. Materials and methods 2.1. Materials The Sultana seedless (Vitis vinifera L.) variety of grapes was used. Generally, grape berries of uniform
Grapes were dipped in several solutions and dried afterward. Pretreatment solutions and the associated code numbers are given below:
Dipping time in the pretreatment solutions was about 1 min at ambient temperature. Then, grapes were dried as a single layer in a batch drier. In this drier, air flow is perpendicular to the grape berries. Grapes were dried at air temperatures of 50 C, 55 C, 60 C and 70 C and the air velocity was 1.2 m/s. Also untreated grapes were dried (code NAT) at 60 C and 70 C air temperatures. In each experiment, 1 kg of fresh grapes was used. Weight loss of the drying grapes was measured by means of a load cell (REVERE SHBXM CC). The drying was concluded when the final moisture content was 0.2 kg/kg dry mass. The product was cooled and packed in low density polyethylene (LDPE) bags which were heatsealed. The experiments was repeated twice and the average of the moisture ratio at each value was used for drawing the drying curves.
I_ . Doymaz, M. Pala / Journal of Food Engineering 52 (2002) 413–417
415
3. Results and discussion Drying time of each experiment, diffusion and correlation coefficients are given in Table 1. Fig. 1 shows drying curves of grapes pretreated differently and dried at 60 C. Grapes dipped in AEEO or POTAS solution prior to drying were found to have a shorter drying time compared to untreated grapes (Table 1). Grapes dipped in AEEO and POTAS solutions and dried at 60 C air temperature had a drying time of 1230 and 1320 min, respectively, whereas untreated grapes had a drying time of 2880 min (Table 1, Fig. 1). These results demonstrated that AEEO grapes had drying times that were 7.3% shorter than POTAS code and 134.1% shorter than NAT code grapes. Similarly, when the drying times of grapes with POTAS code and NAT code grapes were compared, the drying time for POTAS code grapes was 118% lower than that for NAT code grapes. Consequently, AEEO solution is much more effective than POTAS solution in grape drying. One of the most important criteria of foods is colour. Undesirable changes in colour of food may lead to a decrease in its quality and marketing value. The results of the drying experiments have shown that pretreatments have also an important effect on the colour parameters of the final dried products (Table 2). In terms of desired colour properties, higher L and lower a=b ratio are preferred. Hunter L (lightness) values of grapes dipped in AEEO solution were in most cases higher than those for grapes dipped in POTAS and naturally dried grapes. For example, the values of L were 21.03, 20.29 and 17.74 for grapes dried at 60 C, respectively. The value of a=b, which is a measurement of the redness/yellowness colour, was 0.74, 0.81 and 1.05, respectively. Therefore, grapes with AEEO and POTAS codes are lighter and brighter than those with NAT code by 18.54% and 14.3%, respectively. The values reported in Table 2 show that L values increase with increasing temperature, whereas the a=b
Fig. 1. Drying curves of grapes dried at 60 C after different pretreatments.
values did not show this correlation. Lowest values for a=b were found for AEEO pretreated grapes dried at 60 C. In order to estimate the moisture content as a function of drying time, the empirical Page and Exponential equations were fitted and correlation coefficients were calculated in this study. The values of r2 obtained from the Page Eq. (5) are higher than those attained from the Exponential Eq. (4). The r2 values of the Page equation vary between 0.995 and 0.999, and between 0.973 and 0.995 for the Exponential equation (Table 1). Both equations represent the correlation between the moisture content and drying time. Fig. 2 shows the experimental and fitted drying curves. The diffusion coefficients of grapes dipped in AEEO were higher than those with POTAS and NAT codes. For example, the diffusion coefficients ðDeff Þ of grapes dried at 60 C were 2:01 109 , 1:82 109 , and 9:10 1010 m2 =s, respectively (Table 1). Based on these results, 2%AEEO þ 5%K2 CO3 solution (AEEO) was found to be the most effective dipping solution in increasing the drying rate compared with a 5%K2 CO3 þ 0:5% olive oil solution (POTAS).
Table 1 Drying times of experiments, diffusion and correlation coefficients Code
Air temperature (C)
Drying time (min)
r2
Deff ðm2 =sÞ
Page
Exp.
AEEO
50 55 60 70
2040 1740 1230 810
0.996 0.997 0.997 0.999
0.975 0.985 0.985 0.995
8:49 1010 1:32 109 2:01 109 3:50 109
POTAS
50 55 60 70
2430 1830 1320 870
0.995 0.995 0.998 0.995
0.973 0.973 0.991 0.990
7:91 1010 1:19 109 1:82 109 2:54 109
NAT
60 70
2880 1080
0.997 0.995
0.987 0.993
9:10 1010 1:92 109
I_ . Doymaz, M. Pala / Journal of Food Engineering 52 (2002) 413–417
416 Table 2 Colour analysis of dried grapes Code
Air temperature (C)
Hunter colour values L
+a
+b
a=b
AEEO
50 55 60 70
18.95 19.30 21.03 22.75
3.04 3.99 5.08 5.23
3.84 4.80 6.84 6.65
0.79 0.83 0.74 0.78
POTAS
50 55 60 70
17.57 19.56 20.29 22.31
3.84 3.97 4.55 4.53
4.28 4.32 5.57 5.29
0.89 0.91 0.81 0.86
NAT
60 70
17.74 19.55
2.29 4.74
2.18 5.43
1.05 0.87
that of the untreated grapes. When the drying rates of grapes are modeled by the Page and Exponential equations, the results agree satisfactorily with experimental results.
Acknowledgements This research work was supported by Yildiz University Research Fund under the grant number 92-B-07-01-01. References Fig. 2. Experimental and fitted drying curves for grapes dipped in AEEO solution and dried at 60 C.
Higher drying rates are observed for all grapes dipped into ethyl oleate solution. The reason can be ascribed to the ethyl oleate. Ethyl oleate destroys the resistance of waxy layer which prevents moisture transfer and increases the drying rate.
4. Conclusions The effect of AEEO and potassium carbonate (POTAS) solutions on hot air drying of grapes was studied. Grapes dipped in AEEO or POTAS solution were found to have shorter drying times compared with untreated grapes. The AEEO dipping increased the drying rates more than the potassium carbonate pretreatment, and improved the quality and appearance of the dried grapes. Pretreatment with the AEEO solution is effective in increasing the drying rate. However, it should be noted that this pretreatment is more effective in increasing the drying rate during the early period of drying. The use of ethyl oleate as pretreatment solution for the drying of grapes leads to a better colour (L and a=b values). Colour of the treated grapes was lighter than
Aguilera, J. M., Oppermann, K., & Sanchez, F. (1987). Kinetics of browning of Sultana grapes. Journal of Food Science, 52, 990–993. AOAC (1990). Official methods of analysis. In K. Heldrich (Ed.), Association of official analytical chemists (15th ed.). Washington, USA, p. 1298. Bolin, H. R., Petrucci, V., & Fuller, G. (1975). Characteristics of mechanically harvested raisins produced by dehydration and by field drying. Journal of Food Science, 40, 1036–1038. Diamante, L. M., & Munro, P. A. (1993). Mathematical modeling of the thin layer solar drying of sweet potato slices. Solar Energy, 51(4), 271–276. Di Matteo, M., Cinquanta, L., Galiero, G., & Crescitelli, S. (2000). Effect of a novel physical pretreatment process on the drying kinetics of seedless grapes. Journal of Food Engineering, 46, 83–89. Doymaz, I_ . (1998). Investigation of drying characteristics of grape and Kahramanmaras pepper. Ph.D. Thesis, Science Institute, Yildiz Technical University, Istanbul. Guadagni, D. G., Stafford, A. E., & Fuller, G. (1975). Taste thresholds of fatty acid esters in raisins and raisin paste. Journal of Food Science, 40, 780–783. Karathanos, V. T., & Belessiotis, V. G. (1997). Sun and artificial air drying kinetics of some agricultural products. Journal of Food Engineering, 31, 35–46. Lomauro, C. J., Bakshi, A. S., & Labuza, T. P. (1985). Moisture transfer properties of dry and semimoist foods. Journal of Food Science, 50, 397–400. Madamba, P. S., Driscoll, R. H., & Buckle, K. A. (1996). The thinlayer drying characteristics of garlic slices. Journal of Food Engineering, 29, 75–97. Mahmutoglu, T., Emir, F., & Saygi, Y. B. (1996). Sun/Solar drying of differently treated grapes and storage stability of dried grapes. Journal of Food Engineering, 29, 289–300.
I_ . Doymaz, M. Pala / Journal of Food Engineering 52 (2002) 413–417 Pala, M., Saygi, Y. B., & Sadıkoglu, H. (1993). A study on the drying of Sultana grapes by different techniques and effective parameters. In G. Charalambous (Ed.). Food flavors, ingredients and composition (pp. 437–444). Amsterdam. Pangavhane, D. R., Sawhney, R. L., & Sarsavadia, P. N. (1999). Effect of various dipping pretreatment on drying kinetics of Thompson seedless grapes. Journal of Food Engineering, 39, 211–216. Petrucci, V., Canata, N., Bolin, H. R., Fuller, G., & Stafford, A. E. (1973). Use of oleic acid derivatives to accelerate drying of Thompson seedless grapes. Journal of The American Oil Chemists’ Society, 51, 77–80. Ponting, J. D., & Mc Bean, D. M. (1970). Temperature and dipping treatment effects on drying rates and drying times of grapes, prunes and other waxy fruits. Food Technology, 24, 85–88.
417
Raouzeos, G. S., & Saravacos, G. D. (1986). Solar drying of raisins. Drying Technology, 4(4), 633–649. Riva, M., & Peri, C. (1986). Kinetics of sun and air drying of different varieties of seedless grapes. Journal of Food Technology, 21, 199– 208. Saravacos, G. D., Marousis, S. N., & Raouzeos, G. S. (1988). Effect of ethyl oleate on the rate of air-drying of foods. Journal of Food Engineering, 7, 263–270. Tulasidas, T. N., Raghavan, G. S. V., & Norris, E. R. (1996). Effects of dipping and washing pre-treatments on microwave drying of grapes. Journal of Food Process Engineering, 19, 15–25. T€ ut€ unc€ u, A. M., & Labuza, T. P. (1996). Effect of geometry on the effective moisture transfer diffusion coefficient. Journal of Food Engineering, 30, 433–447.