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Yield improvement in progressive freeze-concentration by partial melting of ice
Journal of Food Engineering 108 (2012) 377–382
Contents lists available at SciVerse ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Yield improvement in progressive freeze-concentration by partial melting of ice Osato Miyawaki ⇑, Sho Kato, Kanako Watabe Department of Food Science, Ishikawa Prefectural University, 1-308 Suematsu, Nonoichi, Ishikawa 921-8836, Japan
a r t i c l e
i n f o
Article history: Received 25 July 2011 Received in revised form 3 September 2011 Accepted 8 September 2011 Available online 24 September 2011 Keywords: Progressive freeze-concentration Yield of solute Partial melting of ice Apparent partition coefficient Osmotic pressure
a b s t r a c t Progressive freeze-concentration (PFC) was applied for the concentration of sucrose solutions with various concentrations (0.3–40%), a model pear (La France) flavor solution, tomato juice (5.3 Brix) and apple juice (12–15 Brix). When the solute concentration was low with low osmotic pressure as in the case of the model pear flavor solution, the apparent partition coefficient (Kapp) between the ice and the concentrated liquid phase was low to give a high yield. For samples with high solute concentrations, however, Kapp increased with an increase in solute concentration to reduce the yield in PFC operation. In this case, the partial melting of ice was effective. In this method, the ice crystal formed after PFC was melted gradually with time to collect the melted fractions with intervals. Then the initial fractions were found to contain the higher amount of solute. Therefore, the yield could be improved to a necessary level (>90%) by recovering these fractions with the higher solute concentration. The recovered melted ice fraction may be mixed with the feed in the succeeding batch of PFC operation to avoid dilution of the concentrate. The partial ice melting will extend the practical applicability of PFC substantially. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction There are three methods for the concentration of liquid food: evaporation, reverse osmosis, and freeze concentration. Among these, the freeze concentration is known to give the best quality (Deshpande et al., 1982). The conventional method of freeze concentration is based on the suspension crystallization (Huige and Thijssen, 1972), in which many small ice crystals are formed and grown large spending long time through the Ostwald ripening mechanism. This method needs very complicated system composed of a surface-scraper heat-exchanger for generation of seed ice, a recrystallization vessel for ice crystal growth, and a washing tower for separation of ice crystals from the concentrated mother solution. This complex system requires very high initial investment for the process. On the contrary, progressive freeze-concentration (PFC) have been proposed (Matthews and Coggeshall, 1959; Shapiro, 1961; Bae et al., 1994; Miyawaki et al., 1998), in which a single ice crystal is formed in the system so that the process is expected much simpler causing much lower initial investment as compared with the conventional suspension crystallization method. We used a small vertical freezing test apparatus for PFC (Bae et al., 1994). This is composed of a cylindrical sample vessel, a cooling bath, and a driving system to plunge the sample vessel into the cooling bath at a constant speed to control the ice growth rate.
⇑ Corresponding author. Tel./fax: +81 762277465. E-mail address: [email protected] (O. Miyawaki). 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.09.013
The sample vessel was equipped with a propeller to stir the solution at the ice–liquid interface. In this system, the ice crystal is allowed to grow vertically from the bottom to the top in the sample vessel. By using this vertical freezing system, we have shown that model solutions including glucose and Blue dextran were effectively concentrated (Liu et al., 1997) and a high quality concentration of tomato juice was feasible (Liu et al., 1999). Similar system was applied for the concentration of Andes berry (Ramos et al., 2005). Although PFC is proved to be effective for the high quality concentration of liquid food, the production scale of this method by the vertical freezing system is much lower as compared with the suspension crystallization method. For a large scale production, a falling film reactor was proposed and applied for waste water treatment (Muller and Sekoulov, 1992), sucrose solution (Flesland, 1995), apple and pear juices (Hernandez et al., 2009), orange juice (Sanchez et al., 2010), must (Hernandez et al., 2010), and whey (Sanchez et al., 2011). In PFC, the mass transfer at the ice–liquid interface has been proved important to reduce the incorporation of solute into the ice phase (Miyawaki et al., 1998). In the falling film system, the mass transfer at the ice–liquid interface is limited because of the limited flow rate. Therefore, a tubular ice system with circulating flow by a pump was proposed (Shirai et al., 1999; Wakisaka et al., 2001; Miyawaki et al., 2005), in which the ice crystal grows on the inside surface of a pipe being cooled by a coolant. In this way, the production scale was easily increased simply by increasing the surface area of the cooling plate by increasing the number of pipes bundled together and interconnected in series.
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O. Miyawaki et al. / Journal of Food Engineering 108 (2012) 377–382
In PFC, a solute in the mother solution is separated at the ice–liquid interface so that the effective partition coefficient of the solute between the ice and liquid phase is very important. This partition phenomenon has been theoretically analyzed by the concentration polarization model (Miyawaki et al., 1998) and the effective partition coefficient was proved to be dependent on the ice growth rate and the mass transfer at the ice–liquid interface. In this model, the limiting partition coefficient is a key process parameter, which corresponds to the partition coefficient at the infinitesimal ice growth rate and/or the infinite mass transfer rate at the ice–liquid interface. The limiting partition coefficient could be obtained by the effective partition coefficient under the various operating conditions of ice growth rate and the mass-transfer rate at the ice–liquid interface (Pradistsuwana et al., 2003; Gu et al., 2008). The limiting partition coefficient thus obtained was proved to be strongly dependent on the osmotic pressure of the system (Gu et al., 2005) so that the solute components are inevitably incorporated into the ice phase when the initial concentration or osmotic pressure is high. Therefore, the yield in PFC is not so high for the high concentration sample, which have been considered to be the major drawback of PFC. In this paper, we applied the partial melting of ice to improve the yield in PFC for the highly concentrated system.
make the operation simpler because the yield could be improved by the partial melting of ice. 2.3. Partial melting of ice After PFC operation, the concentrated solution phase and the ice phase were separated with each other. Then, ice phase was removed form the sample vessel and put in a funnel and left at the room temperature (25 °C) to be melted. The melted fractions were collected at an interval, typically 30 min, and analyzed. 2.4. Analytical method Sucrose concentration was analyzed by a refractometer (APAL1, AsOne, Tokyo, Japan). For the analysis of the flavor components in a model solution of pear flavor and its concentrate, 5 ml sample with 20 ppm methyl butanoate added as an internal standard, was extracted by 3 ml diethyl ether, which was analyzed by gas chromatograph (G-3900, Hitachi, Tokyo, Japan) with a capillary column (InertCap WAX, GL Science, Tokyo, Japan) and FID detector. The column temperature was kept at 40 °C for 5 min, then raised at 10 °C/min until it reached 220 °C. Injector and detector temperature was kept at 200 and 250 °C, respectively. For tomato and apple juices, concentration as Brix was measured by a refractometer.
2. Experimental method 3. Results and discussions 2.1. Materials 3.1. Progressive freeze-concentration of sucrose solutions Sucrose, diethylether, butyl aldehyde, ethyl acetate, ethanol, methyl butanoate, 1-propanol, butyl acetate, hexyl aldehyde, 1butanol, hexyl acetate, 1-hexanol, and diethyl ether were purchased from Kanto Kagaku (Tokyo, Japan). To prepare a model solution of pear (La France) flavor, two aldehydes (butyl aldehyde and hexyl aldehyde), three alcohols (1-propanol, 1-butanol, and 1-hexanol), and three esters (ethyl acetate, butyl acetate, and hexyl acetate), all 20 mg each, were dissolved into 1.29 ml ethanol, which was dissolved into 1000 ml distilled water. Tomato and apple juices were kindly gifted by Research Institute, Kagome Co., Ltd. (Tochigi, Japan). 2.2. Progressive freeze-concentration For a small scale experiment, a vertical freezing test apparatus (Fig. 1A: Miyawaki et al., 1998) was used. This was composed of a sample vessel (50 mmu, 170 mmH), a cooling bath (15 °C), and a driving system to plunge the sample vessel into the cooling bath at a constant speed (1 cm/h). The sample solution (150 ml) was precooled down to the temperature close to the freezing point. In the sample vessel set in the cooling bath, 1 ml pure water was initially added to form a seed ice crystal at the bottom of the vessel, then the precooled sample solution was applied to start PFC. The sample solution was stirred (1000 rpm) at the ice–liquid interface. For the larger scale concentration (10 l), a tubular ice system with circulating flow was used (Fig. 1B: Miyawaki et al., 2005). The apparatus was composed of two straight pipes, 35.7 mm in diameter and 240 cm long, bent pipes at the top and the bottom, and pump for circulation. Straight pipes were cooled down from outside by a coolant. Bent pipes were thermally insulated to minimize the heat transfer from the environment. The coolant temperature was initially kept at -3 °C for 10 min then cooled down to -9 °C at a rate of -1 °C/10 min by a program. Initial circulation flow rate was controlled at 0.93 m/s. For the tubular ice system for PFC, the seed ice lining is recommended before the sample application to avoid initial supercooling (Miyawaki et al., 2005). In the present case, however, no ice lining was applied to
Sucrose solutions at various concentrations were freeze-concentrated by a vertical freezing test apparatus. Results were summarized in Table 1. In this table, Kapp is the apparent partition coefficient of solute between the ice and the liquid phases defined by the following equation
K app ¼ C ice =C final
ð1Þ
where Cice and Cfinal are solute concentrations in the ice phase and in the concentrated solution after PFC, respectively. When the initial sucrose concentration was lower than 1%, the purity of ice was satisfactory so that Kapp was kept low and the yield was higher than 98%. With an increase in the initial sucrose concentration, however, the purity of ice decreased so that Kapp increased and the yield decreased. When the initial sucrose concentration was 40%, sucrose was concentrated up to 51.3% but the yield was as low as 72.6%. Wakisaka et al. (2001) also applied PFC for waste water treatment with glucose as a solute and reported the similar increase in Kapp with an increase in the solute concentration. With an increase in sucrose concentration, the osmotic pressure increases so that the incorporation ratio of solute into the ice phase will increase to decrease the yield theoretically (Gu et al., 2005). This was supposed to be a weak point of PFC as compared with the suspension crystallization. For the improvement in the yield at the higher solute concentrations, we applied the partial melting technique of ice. In this method, the ice crystal formed after PFC operation was melted gradually with time to collect the melted fractions with intervals. Then the initially melted fractions were found to contain the higher amount of solute. Therefore, the yield improvement could be possible by recovering these fractions with the higher solute concentration. Fig. 2 shows the typical result of the application the partial melting of ice to 1% sucrose solution. In this figure, the concentration of each melted ice fraction and the final yield after recovering the melted ice fraction are plotted as a function of the melted ice fraction. In this case, the osmotic stress was not so high so that
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A
B Stirrer
Ice crystal Motor
Sample vessel
Unfrozen fraction
Ice fraction
Coolant
Pump
Cooling bath
Fig. 1. Apparatus for progressive freeze-concentration. ((A) Vertical freezing test apparatus: (B) Tubular ice system with circulating flow).
Table 1 Progressive freeze-concentration of sucrose solutions by vertical test apparus. Cinitial (%)
Cfinal (%)
Cice (%)
Vinitial (ml)
Vfinal (ml)
Kapp
Yield
0.3 1 3 10 20 30 40
0.702 2.10 7.60 17.9 32.0 42.3 51.3
0.0061 0.0289 0.702 3.76 8.96 17.3 25.9
149.2 147 153 151.9 154.1 150.9 148.3
63 69 51 65 74.5 72.5 84
0.0087 0.0138 0.0924 0.210 0.280 0.409 0.504
0.988 0.986 0.844 0.765 0.774 0.677 0.726
0.5
1 3.2. Progressive freeze-concentration of pear (La France) flavor model solution
0.3
0.98
0.2
0.97
0.1
0.96
0
0.2
0.4
0.6
0.8
1
0.95
Melted ice fraction (-) Fig. 2. Partial melting of ice formed after progressive freeze-concentration of 1% sucrose solution.
the ice purity was good to give the yield at 98.6%. Even in this case, the first melted fraction contained the solute at a high concentration of 0.22% so that the yield could be improved from 98.6% to 99.8% by recovering this fraction. For 3% and 10% sucrose solutions, the yield could be improved higher than 90% by recovering the initial 10% and 20% melted fractions, respectively (Figs. 3 and 4). For
Fig. 6 shows the chromatogram of the pear flavor model solution. All the peaks observed were identified as shown in the figure.
1
5
4
0.95
Concentration 3
Yield
0.9
2
Yield (-)
0.99 Yield
Yield (-)
Concentration (%)
0.4
Concentration (%)
Concentration
0
40% sucrose solution, the solution was concentrated up to 51.3% but the yield was 72.6%, which could be also improved up to 90% by recovering the initial 50% melted fraction (Fig. 5). Thus the yield was proved to be improved to the necessary level in PFC by applying the partial melting of ice. In the literature, the freezing-thawing technique (Yee et al., 2003) has been reported to be effective as a method for freeze concentration. In this case, however, the whole sample was frozen at first then the partially melted part was simply collected for the higher concentration fraction, the concentration of which is uncontrolled. In the present case, the necessary concentration level of solute is attained by PFC and then the yield was improved by the partial melting of ice.
0.85 1
0
0.8 0
0.2
0.4
0.6
0.8
1
Melted ice fraction (-) Fig. 3. Partial melting of ice formed after progressive freeze-concentration of 3% sucrose solution.
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15
100
1
80
40
0.9
yl
ol an
ex
yl
H
ac
ex
et at e
ta n
ol
de
Bu
al de
ac ty l
hy
ol et at e
l
ex H
H
op
an
no
Pr
Bu
Bu
ty l
0.8
Et ha
hy
de
ta te
0
ce
5
20
la
0.85
al de
Yield
Et hy
Concentration
10
60
Yield (-)
Concentration (%)
0.95
0.75 100
0
0
0.2
0.4
0.6
0.8
1
0.7
80
Melted ice fraction (-)
60
Fig. 4. Partial melting of ice formed after progressive freeze-concentration of 10% sucrose solution.
40 20
0.8
20
0.7
0.6
0
0
0.2
0.4
0.6
0.8
1
0.5
Melted ice fraction (-) Fig. 5. Partial melting of ice formed after progressive freeze-concentration of 40% sucrose solution.
H
ex
an
ol
e et
ac yl
ex H
Fig. 7. Yield of each flavor component after progressive freeze-concentration of pear flavor model solution. (Upper; Concentrated liquid phase: Lower; Ice phase).
A 150 ml flavor model solution was freeze-concentrated to 34 ml by a vertical freezing test apparatus. The flavor components in the original solution, in the concentrate, and the ice phase were analyzed and the yields were obtained for each component. The result is shown in Fig. 7. In this case, the incorporation of components into the ice phase was very low because of the low osmotic pressure of the sample and the yield of the most components was higher than 80%. Esters showed relatively the lower yield but the incorporation into the ice phase was also low, except ethyl acetate, so that these might been vaporized and lost into the air during the freeze-concentration process because of the open-air structure of the sample vessel presently used. In this case, Kapp was considered
Butyl aldehyde Diethyl ether
at
l ta
no
de
Bu
de al
H
ex
yl
ty
Bu
hy
ta ce
la
an op
te
ol
l no
Et
ha
ta ce
la
Et
Pr
e yd
Concentration 10
hy
eh ld ty
la
30
Bu
0.9
Yield
Yield (-)
Concentration (%)
40
te
0
1
50
1-Hexanol
Ethyl acetate Ethanol
Hexyl acetate Hexyl aldehyde Butyl acetate Methyl butanoate 1-Butanol 1-Propanol
Fig. 6. Chromatogram of pear (La France) flavor model solution.
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O. Miyawaki et al. / Journal of Food Engineering 108 (2012) 377–382 Table 2 Progressive freeze-concentration of fruits juices by tubular system. Sample
Cinitial (Brix)
Cfinal (Brix)
Cice (Brix)
Vinitial (ml)
Vfinal (ml)
Kapp
Yield
Tomato Apple(1) Apple(2)
5.3 11.9 15.3
7.9 21.3 28.4
1.16 5.29 7.51
10,000 10,000 10,000
6129 4101 3733
0.147 0.248 0.265
0.915 0.736 0.692
1
8
0.95 Concentration
6 Yield
0.9
4 0.85
2
0
Yield (-)
Concentration (Brix)
10
0
0.2
0.4
0.6
0.8
1
Apple juice of 11.9 Brix could be concentrated to 21.3 Brix with the yield of 73.6%, which could be improved to 90% by recovering the initial 30% melted fraction (data not shown). Apple juice of 15.3 Brix could be concentrated up to 28.4 Brix as shown in Table 2. In this case, the yield was as low as 69.2%, which also could be improved to 90% by recovering the initial 40% melted fraction (Fig. 9). The partial melting of ice was thus proved to be effective to improve the yield in PFC to overcome the solute incorporation of solute into the ice phase for the high concentration sample, which was supposed to be a major drawback of PFC. The melted fraction to be recovered to attain the necessary yield increased with an increase in the solute concentration because of the increase in Kapp. The recovered melted ice fraction may be mixed with the feed in the succeeding batch of PFC operation to avoid dilution of the concentrate.
0.8 4. Conclusion
Melted ice fraction (-) Fig. 8. Partial melting of ice formed after progressive freeze-concentration of tomato juice (5.3 Brix) by tubular ice system.
1
30
0.9 Concentration
20
0.8 Yield
15
0.7
Yield (-)
Concentration (Brix)
25
10 0.6
5 0
PFC was effective for the low solute concentration sample with low osmotic pressure. In this case, the apparent partition coefficient of solute between the ice and the liquid phases was low enough to give a high yield in the process. For samples with high solute concentrations, however, the yield in PFC decreased with an increase in the solute concentration because of the higher incorporation rate of the solute into the ice phase. In this case, the application of the partial melting of ice was proved to be very effective. By recovering the initial fractions with high concentrations in the partial melting process, the yield could be improved to a necessary level (>90%). This means that the major drawback in PFC, the lower yield for the higher concentrated sample, could be overcome by the partial melting of ice to extend the practical applicability of PFC substantially. The possible industrial applications of the present method are widely expected in the high quality concentration of fruits and vegetable juices, coffee and tea extracts, seasoning solution, milk products, etc. Acknowledgment
0
0.2
0.4
0.6
0.8
1
0.5
Melted ice fraction (-) Fig. 9. Partial melting of ice formed after progressive freeze-concentration of apple juice (15.3 Brix) by tubular ice system.
to be very low because of the low osmotic pressure of the sample so that the application of the partial melting was not necessary.
3.3. Progressive freeze-concentration of fruits juices by tubular ice system with circulating flow For the freeze-concentration of fruits juices, a tubular ice system with circulating flow was used. Results are summarized in Table 2. Tomato juice of 5.3 Brix could be concentrated to 7.9 Brix with a good yield of 91.5% because of the low osmotic pressure. In this case, the yield could be improved to 95% by recovering the initial 20% melted fraction by applying the partial melting of ice (Fig. 8).
We would like to acknowledge Kagome Co. Ltd., for the kind gift of tomato and apple juices. Technical help by Ms. M. Tatsuno is highly appreciated. References Bae, S.K., Miyawaki, O., Arai, S., 1994. Control of freezing front structure and its effect on the concentration efficiency in progressive freeze-concentration. Cryobiology and Cytotechnology 40, 29–32. Deshpande, S.S., Bolin, H.R., Salunkhe, D.K. 1982. Freeze concentration of fruit juices. Food Technology, 68–82. Flesland, O., 1995. Freeze concentration by layer crystallization. Drying Technology 13, 1713–1739. Gu, X., Suzuki, T., Miyawaki, O., 2005. Limiting partition coefficient in progressive freeze-concentration. Journal of Food Science 70, E546–551. Gu, X., Watanabe, M., Suzuki, T., Miyawaki, O., 2008. Limiting partition coefficient in a tubular ice system for progressive freeze-concentration. Food Science and Technology Research 14, 249–252. Hernandez, E., Raventos, M., Auleda, J.M., Ibarz, A., 2009. Concentration of apple and pear juices in a multi-plate freeze concentrator. Innovative Food Science and Emerging Technologies 10, 348–355. Hernandez, E., Raventos, M., Auleda, J.M., Ibarz, A., 2010. Freeze concentration of must in a pilot plant falling film cryoconcentrator. Innovative Food Science and Emerging Technologies 11, 130–136.
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Huige, N.J.J., Thijssen, H.A.C., 1972. Production of large crystals by continuous ripening in a stirred tank. Journal of Crystal Growth 13 (14), 483–487. Liu, L., Miyawaki, O., Nakamura, K., 1997. Progressive freeze-concentration of model liquid food. Food Science and Technology International Tokyo 3, 348–352. Liu, L., Miyawaki, O., Hayakawa, K., 1999. Progressive freeze-concentration of tomato juice. Food Science and Technology Research 5, 108–112. Matthews, J.S., Coggeshall, N.D., 1959. Concentration of impurities from organic compounds by progressive freezing. Analytical Chemistry 31, 1124 (1959). Miyawaki, O., Liu, L., Nakamura, K., 1998. Effective partition constant of solute between ice and liquid phases in progressive freeze-concentration. Journal of Food Science 63, 756–758. Miyawaki, O., Liu, L., Shirai, Y., Sakashita, S., Kagitani, K., 2005. Tubular ice system for scale-up of progressive freeze-concentration. Journal of Food Engineering 69, 107–113. Muller, M., Sekoulov, I., 1992. Waste water reuse by freeze concentration with a falling film reactor. Water Science and Technology 26, 1475–1482. Pradistsuwana, C., Theprugsa, P., Miyawaki, O., 2003. Measurement of limiting partition coefficient in progressive freeze-concentration. Food Science and Technology Research 9, 190–192.
Ramos, F.A., Delgado, J.L., Bautista, E., Morales, A.L., Duque, C., 2005. Journal of Food Engineering 69, 291–297. Sanchez, J., Ruiz, M., Raventos, M., Auleda, J.M., Hernandez, E.M., 2010. Progressive freeze concentration of orange juice in a pilot plant falling film. Innovative Food Science and Emerging Technologies 11, 644–651. Sanchez, J., Hernandez, E., Auleda, J.M., Raventos, M., 2011. Freeze concentration of whey in a falling-film based pilot plant: process and characterization. Journal of Food Engineering 103, 147–155. Shapiro, J., 1961. Freezing-out, a safe technique for concentration of dilute solutions. Science 133, 2063–2064. Shirai, Y., Wakisaka, M., Miyawaki, O., Sakashita, S., 1999. Effect of seed ice on formation of tube ice with high purity for a freeze waste water treatment system with a bubble-flow circulator. Water Research 33, 1325–1329. Wakisaka, M., Shirai, Y., Sakashita, S., 2001. Ice crystallization in a pilot-scale freeze wastewater treatment system. Chemical Engineering and Processing 40, 201– 208. Yee, P.L., Wakisaka, M., Shirai, Y., Hassan, M.A., 2003. Effects of single food components on freeze concentration by freezing and thawing technique. Japan Journal of Food Engineering 4, 77–82.
Contents lists available at SciVerse ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Yield improvement in progressive freeze-concentration by partial melting of ice Osato Miyawaki ⇑, Sho Kato, Kanako Watabe Department of Food Science, Ishikawa Prefectural University, 1-308 Suematsu, Nonoichi, Ishikawa 921-8836, Japan
a r t i c l e
i n f o
Article history: Received 25 July 2011 Received in revised form 3 September 2011 Accepted 8 September 2011 Available online 24 September 2011 Keywords: Progressive freeze-concentration Yield of solute Partial melting of ice Apparent partition coefficient Osmotic pressure
a b s t r a c t Progressive freeze-concentration (PFC) was applied for the concentration of sucrose solutions with various concentrations (0.3–40%), a model pear (La France) flavor solution, tomato juice (5.3 Brix) and apple juice (12–15 Brix). When the solute concentration was low with low osmotic pressure as in the case of the model pear flavor solution, the apparent partition coefficient (Kapp) between the ice and the concentrated liquid phase was low to give a high yield. For samples with high solute concentrations, however, Kapp increased with an increase in solute concentration to reduce the yield in PFC operation. In this case, the partial melting of ice was effective. In this method, the ice crystal formed after PFC was melted gradually with time to collect the melted fractions with intervals. Then the initial fractions were found to contain the higher amount of solute. Therefore, the yield could be improved to a necessary level (>90%) by recovering these fractions with the higher solute concentration. The recovered melted ice fraction may be mixed with the feed in the succeeding batch of PFC operation to avoid dilution of the concentrate. The partial ice melting will extend the practical applicability of PFC substantially. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction There are three methods for the concentration of liquid food: evaporation, reverse osmosis, and freeze concentration. Among these, the freeze concentration is known to give the best quality (Deshpande et al., 1982). The conventional method of freeze concentration is based on the suspension crystallization (Huige and Thijssen, 1972), in which many small ice crystals are formed and grown large spending long time through the Ostwald ripening mechanism. This method needs very complicated system composed of a surface-scraper heat-exchanger for generation of seed ice, a recrystallization vessel for ice crystal growth, and a washing tower for separation of ice crystals from the concentrated mother solution. This complex system requires very high initial investment for the process. On the contrary, progressive freeze-concentration (PFC) have been proposed (Matthews and Coggeshall, 1959; Shapiro, 1961; Bae et al., 1994; Miyawaki et al., 1998), in which a single ice crystal is formed in the system so that the process is expected much simpler causing much lower initial investment as compared with the conventional suspension crystallization method. We used a small vertical freezing test apparatus for PFC (Bae et al., 1994). This is composed of a cylindrical sample vessel, a cooling bath, and a driving system to plunge the sample vessel into the cooling bath at a constant speed to control the ice growth rate.
⇑ Corresponding author. Tel./fax: +81 762277465. E-mail address: [email protected] (O. Miyawaki). 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.09.013
The sample vessel was equipped with a propeller to stir the solution at the ice–liquid interface. In this system, the ice crystal is allowed to grow vertically from the bottom to the top in the sample vessel. By using this vertical freezing system, we have shown that model solutions including glucose and Blue dextran were effectively concentrated (Liu et al., 1997) and a high quality concentration of tomato juice was feasible (Liu et al., 1999). Similar system was applied for the concentration of Andes berry (Ramos et al., 2005). Although PFC is proved to be effective for the high quality concentration of liquid food, the production scale of this method by the vertical freezing system is much lower as compared with the suspension crystallization method. For a large scale production, a falling film reactor was proposed and applied for waste water treatment (Muller and Sekoulov, 1992), sucrose solution (Flesland, 1995), apple and pear juices (Hernandez et al., 2009), orange juice (Sanchez et al., 2010), must (Hernandez et al., 2010), and whey (Sanchez et al., 2011). In PFC, the mass transfer at the ice–liquid interface has been proved important to reduce the incorporation of solute into the ice phase (Miyawaki et al., 1998). In the falling film system, the mass transfer at the ice–liquid interface is limited because of the limited flow rate. Therefore, a tubular ice system with circulating flow by a pump was proposed (Shirai et al., 1999; Wakisaka et al., 2001; Miyawaki et al., 2005), in which the ice crystal grows on the inside surface of a pipe being cooled by a coolant. In this way, the production scale was easily increased simply by increasing the surface area of the cooling plate by increasing the number of pipes bundled together and interconnected in series.
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O. Miyawaki et al. / Journal of Food Engineering 108 (2012) 377–382
In PFC, a solute in the mother solution is separated at the ice–liquid interface so that the effective partition coefficient of the solute between the ice and liquid phase is very important. This partition phenomenon has been theoretically analyzed by the concentration polarization model (Miyawaki et al., 1998) and the effective partition coefficient was proved to be dependent on the ice growth rate and the mass transfer at the ice–liquid interface. In this model, the limiting partition coefficient is a key process parameter, which corresponds to the partition coefficient at the infinitesimal ice growth rate and/or the infinite mass transfer rate at the ice–liquid interface. The limiting partition coefficient could be obtained by the effective partition coefficient under the various operating conditions of ice growth rate and the mass-transfer rate at the ice–liquid interface (Pradistsuwana et al., 2003; Gu et al., 2008). The limiting partition coefficient thus obtained was proved to be strongly dependent on the osmotic pressure of the system (Gu et al., 2005) so that the solute components are inevitably incorporated into the ice phase when the initial concentration or osmotic pressure is high. Therefore, the yield in PFC is not so high for the high concentration sample, which have been considered to be the major drawback of PFC. In this paper, we applied the partial melting of ice to improve the yield in PFC for the highly concentrated system.
make the operation simpler because the yield could be improved by the partial melting of ice. 2.3. Partial melting of ice After PFC operation, the concentrated solution phase and the ice phase were separated with each other. Then, ice phase was removed form the sample vessel and put in a funnel and left at the room temperature (25 °C) to be melted. The melted fractions were collected at an interval, typically 30 min, and analyzed. 2.4. Analytical method Sucrose concentration was analyzed by a refractometer (APAL1, AsOne, Tokyo, Japan). For the analysis of the flavor components in a model solution of pear flavor and its concentrate, 5 ml sample with 20 ppm methyl butanoate added as an internal standard, was extracted by 3 ml diethyl ether, which was analyzed by gas chromatograph (G-3900, Hitachi, Tokyo, Japan) with a capillary column (InertCap WAX, GL Science, Tokyo, Japan) and FID detector. The column temperature was kept at 40 °C for 5 min, then raised at 10 °C/min until it reached 220 °C. Injector and detector temperature was kept at 200 and 250 °C, respectively. For tomato and apple juices, concentration as Brix was measured by a refractometer.
2. Experimental method 3. Results and discussions 2.1. Materials 3.1. Progressive freeze-concentration of sucrose solutions Sucrose, diethylether, butyl aldehyde, ethyl acetate, ethanol, methyl butanoate, 1-propanol, butyl acetate, hexyl aldehyde, 1butanol, hexyl acetate, 1-hexanol, and diethyl ether were purchased from Kanto Kagaku (Tokyo, Japan). To prepare a model solution of pear (La France) flavor, two aldehydes (butyl aldehyde and hexyl aldehyde), three alcohols (1-propanol, 1-butanol, and 1-hexanol), and three esters (ethyl acetate, butyl acetate, and hexyl acetate), all 20 mg each, were dissolved into 1.29 ml ethanol, which was dissolved into 1000 ml distilled water. Tomato and apple juices were kindly gifted by Research Institute, Kagome Co., Ltd. (Tochigi, Japan). 2.2. Progressive freeze-concentration For a small scale experiment, a vertical freezing test apparatus (Fig. 1A: Miyawaki et al., 1998) was used. This was composed of a sample vessel (50 mmu, 170 mmH), a cooling bath (15 °C), and a driving system to plunge the sample vessel into the cooling bath at a constant speed (1 cm/h). The sample solution (150 ml) was precooled down to the temperature close to the freezing point. In the sample vessel set in the cooling bath, 1 ml pure water was initially added to form a seed ice crystal at the bottom of the vessel, then the precooled sample solution was applied to start PFC. The sample solution was stirred (1000 rpm) at the ice–liquid interface. For the larger scale concentration (10 l), a tubular ice system with circulating flow was used (Fig. 1B: Miyawaki et al., 2005). The apparatus was composed of two straight pipes, 35.7 mm in diameter and 240 cm long, bent pipes at the top and the bottom, and pump for circulation. Straight pipes were cooled down from outside by a coolant. Bent pipes were thermally insulated to minimize the heat transfer from the environment. The coolant temperature was initially kept at -3 °C for 10 min then cooled down to -9 °C at a rate of -1 °C/10 min by a program. Initial circulation flow rate was controlled at 0.93 m/s. For the tubular ice system for PFC, the seed ice lining is recommended before the sample application to avoid initial supercooling (Miyawaki et al., 2005). In the present case, however, no ice lining was applied to
Sucrose solutions at various concentrations were freeze-concentrated by a vertical freezing test apparatus. Results were summarized in Table 1. In this table, Kapp is the apparent partition coefficient of solute between the ice and the liquid phases defined by the following equation
K app ¼ C ice =C final
ð1Þ
where Cice and Cfinal are solute concentrations in the ice phase and in the concentrated solution after PFC, respectively. When the initial sucrose concentration was lower than 1%, the purity of ice was satisfactory so that Kapp was kept low and the yield was higher than 98%. With an increase in the initial sucrose concentration, however, the purity of ice decreased so that Kapp increased and the yield decreased. When the initial sucrose concentration was 40%, sucrose was concentrated up to 51.3% but the yield was as low as 72.6%. Wakisaka et al. (2001) also applied PFC for waste water treatment with glucose as a solute and reported the similar increase in Kapp with an increase in the solute concentration. With an increase in sucrose concentration, the osmotic pressure increases so that the incorporation ratio of solute into the ice phase will increase to decrease the yield theoretically (Gu et al., 2005). This was supposed to be a weak point of PFC as compared with the suspension crystallization. For the improvement in the yield at the higher solute concentrations, we applied the partial melting technique of ice. In this method, the ice crystal formed after PFC operation was melted gradually with time to collect the melted fractions with intervals. Then the initially melted fractions were found to contain the higher amount of solute. Therefore, the yield improvement could be possible by recovering these fractions with the higher solute concentration. Fig. 2 shows the typical result of the application the partial melting of ice to 1% sucrose solution. In this figure, the concentration of each melted ice fraction and the final yield after recovering the melted ice fraction are plotted as a function of the melted ice fraction. In this case, the osmotic stress was not so high so that
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A
B Stirrer
Ice crystal Motor
Sample vessel
Unfrozen fraction
Ice fraction
Coolant
Pump
Cooling bath
Fig. 1. Apparatus for progressive freeze-concentration. ((A) Vertical freezing test apparatus: (B) Tubular ice system with circulating flow).
Table 1 Progressive freeze-concentration of sucrose solutions by vertical test apparus. Cinitial (%)
Cfinal (%)
Cice (%)
Vinitial (ml)
Vfinal (ml)
Kapp
Yield
0.3 1 3 10 20 30 40
0.702 2.10 7.60 17.9 32.0 42.3 51.3
0.0061 0.0289 0.702 3.76 8.96 17.3 25.9
149.2 147 153 151.9 154.1 150.9 148.3
63 69 51 65 74.5 72.5 84
0.0087 0.0138 0.0924 0.210 0.280 0.409 0.504
0.988 0.986 0.844 0.765 0.774 0.677 0.726
0.5
1 3.2. Progressive freeze-concentration of pear (La France) flavor model solution
0.3
0.98
0.2
0.97
0.1
0.96
0
0.2
0.4
0.6
0.8
1
0.95
Melted ice fraction (-) Fig. 2. Partial melting of ice formed after progressive freeze-concentration of 1% sucrose solution.
the ice purity was good to give the yield at 98.6%. Even in this case, the first melted fraction contained the solute at a high concentration of 0.22% so that the yield could be improved from 98.6% to 99.8% by recovering this fraction. For 3% and 10% sucrose solutions, the yield could be improved higher than 90% by recovering the initial 10% and 20% melted fractions, respectively (Figs. 3 and 4). For
Fig. 6 shows the chromatogram of the pear flavor model solution. All the peaks observed were identified as shown in the figure.
1
5
4
0.95
Concentration 3
Yield
0.9
2
Yield (-)
0.99 Yield
Yield (-)
Concentration (%)
0.4
Concentration (%)
Concentration
0
40% sucrose solution, the solution was concentrated up to 51.3% but the yield was 72.6%, which could be also improved up to 90% by recovering the initial 50% melted fraction (Fig. 5). Thus the yield was proved to be improved to the necessary level in PFC by applying the partial melting of ice. In the literature, the freezing-thawing technique (Yee et al., 2003) has been reported to be effective as a method for freeze concentration. In this case, however, the whole sample was frozen at first then the partially melted part was simply collected for the higher concentration fraction, the concentration of which is uncontrolled. In the present case, the necessary concentration level of solute is attained by PFC and then the yield was improved by the partial melting of ice.
0.85 1
0
0.8 0
0.2
0.4
0.6
0.8
1
Melted ice fraction (-) Fig. 3. Partial melting of ice formed after progressive freeze-concentration of 3% sucrose solution.
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15
100
1
80
40
0.9
yl
ol an
ex
yl
H
ac
ex
et at e
ta n
ol
de
Bu
al de
ac ty l
hy
ol et at e
l
ex H
H
op
an
no
Pr
Bu
Bu
ty l
0.8
Et ha
hy
de
ta te
0
ce
5
20
la
0.85
al de
Yield
Et hy
Concentration
10
60
Yield (-)
Concentration (%)
0.95
0.75 100
0
0
0.2
0.4
0.6
0.8
1
0.7
80
Melted ice fraction (-)
60
Fig. 4. Partial melting of ice formed after progressive freeze-concentration of 10% sucrose solution.
40 20
0.8
20
0.7
0.6
0
0
0.2
0.4
0.6
0.8
1
0.5
Melted ice fraction (-) Fig. 5. Partial melting of ice formed after progressive freeze-concentration of 40% sucrose solution.
H
ex
an
ol
e et
ac yl
ex H
Fig. 7. Yield of each flavor component after progressive freeze-concentration of pear flavor model solution. (Upper; Concentrated liquid phase: Lower; Ice phase).
A 150 ml flavor model solution was freeze-concentrated to 34 ml by a vertical freezing test apparatus. The flavor components in the original solution, in the concentrate, and the ice phase were analyzed and the yields were obtained for each component. The result is shown in Fig. 7. In this case, the incorporation of components into the ice phase was very low because of the low osmotic pressure of the sample and the yield of the most components was higher than 80%. Esters showed relatively the lower yield but the incorporation into the ice phase was also low, except ethyl acetate, so that these might been vaporized and lost into the air during the freeze-concentration process because of the open-air structure of the sample vessel presently used. In this case, Kapp was considered
Butyl aldehyde Diethyl ether
at
l ta
no
de
Bu
de al
H
ex
yl
ty
Bu
hy
ta ce
la
an op
te
ol
l no
Et
ha
ta ce
la
Et
Pr
e yd
Concentration 10
hy
eh ld ty
la
30
Bu
0.9
Yield
Yield (-)
Concentration (%)
40
te
0
1
50
1-Hexanol
Ethyl acetate Ethanol
Hexyl acetate Hexyl aldehyde Butyl acetate Methyl butanoate 1-Butanol 1-Propanol
Fig. 6. Chromatogram of pear (La France) flavor model solution.
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O. Miyawaki et al. / Journal of Food Engineering 108 (2012) 377–382 Table 2 Progressive freeze-concentration of fruits juices by tubular system. Sample
Cinitial (Brix)
Cfinal (Brix)
Cice (Brix)
Vinitial (ml)
Vfinal (ml)
Kapp
Yield
Tomato Apple(1) Apple(2)
5.3 11.9 15.3
7.9 21.3 28.4
1.16 5.29 7.51
10,000 10,000 10,000
6129 4101 3733
0.147 0.248 0.265
0.915 0.736 0.692
1
8
0.95 Concentration
6 Yield
0.9
4 0.85
2
0
Yield (-)
Concentration (Brix)
10
0
0.2
0.4
0.6
0.8
1
Apple juice of 11.9 Brix could be concentrated to 21.3 Brix with the yield of 73.6%, which could be improved to 90% by recovering the initial 30% melted fraction (data not shown). Apple juice of 15.3 Brix could be concentrated up to 28.4 Brix as shown in Table 2. In this case, the yield was as low as 69.2%, which also could be improved to 90% by recovering the initial 40% melted fraction (Fig. 9). The partial melting of ice was thus proved to be effective to improve the yield in PFC to overcome the solute incorporation of solute into the ice phase for the high concentration sample, which was supposed to be a major drawback of PFC. The melted fraction to be recovered to attain the necessary yield increased with an increase in the solute concentration because of the increase in Kapp. The recovered melted ice fraction may be mixed with the feed in the succeeding batch of PFC operation to avoid dilution of the concentrate.
0.8 4. Conclusion
Melted ice fraction (-) Fig. 8. Partial melting of ice formed after progressive freeze-concentration of tomato juice (5.3 Brix) by tubular ice system.
1
30
0.9 Concentration
20
0.8 Yield
15
0.7
Yield (-)
Concentration (Brix)
25
10 0.6
5 0
PFC was effective for the low solute concentration sample with low osmotic pressure. In this case, the apparent partition coefficient of solute between the ice and the liquid phases was low enough to give a high yield in the process. For samples with high solute concentrations, however, the yield in PFC decreased with an increase in the solute concentration because of the higher incorporation rate of the solute into the ice phase. In this case, the application of the partial melting of ice was proved to be very effective. By recovering the initial fractions with high concentrations in the partial melting process, the yield could be improved to a necessary level (>90%). This means that the major drawback in PFC, the lower yield for the higher concentrated sample, could be overcome by the partial melting of ice to extend the practical applicability of PFC substantially. The possible industrial applications of the present method are widely expected in the high quality concentration of fruits and vegetable juices, coffee and tea extracts, seasoning solution, milk products, etc. Acknowledgment
0
0.2
0.4
0.6
0.8
1
0.5
Melted ice fraction (-) Fig. 9. Partial melting of ice formed after progressive freeze-concentration of apple juice (15.3 Brix) by tubular ice system.
to be very low because of the low osmotic pressure of the sample so that the application of the partial melting was not necessary.
3.3. Progressive freeze-concentration of fruits juices by tubular ice system with circulating flow For the freeze-concentration of fruits juices, a tubular ice system with circulating flow was used. Results are summarized in Table 2. Tomato juice of 5.3 Brix could be concentrated to 7.9 Brix with a good yield of 91.5% because of the low osmotic pressure. In this case, the yield could be improved to 95% by recovering the initial 20% melted fraction by applying the partial melting of ice (Fig. 8).
We would like to acknowledge Kagome Co. Ltd., for the kind gift of tomato and apple juices. Technical help by Ms. M. Tatsuno is highly appreciated. References Bae, S.K., Miyawaki, O., Arai, S., 1994. Control of freezing front structure and its effect on the concentration efficiency in progressive freeze-concentration. Cryobiology and Cytotechnology 40, 29–32. Deshpande, S.S., Bolin, H.R., Salunkhe, D.K. 1982. Freeze concentration of fruit juices. Food Technology, 68–82. Flesland, O., 1995. Freeze concentration by layer crystallization. Drying Technology 13, 1713–1739. Gu, X., Suzuki, T., Miyawaki, O., 2005. Limiting partition coefficient in progressive freeze-concentration. Journal of Food Science 70, E546–551. Gu, X., Watanabe, M., Suzuki, T., Miyawaki, O., 2008. Limiting partition coefficient in a tubular ice system for progressive freeze-concentration. Food Science and Technology Research 14, 249–252. Hernandez, E., Raventos, M., Auleda, J.M., Ibarz, A., 2009. Concentration of apple and pear juices in a multi-plate freeze concentrator. Innovative Food Science and Emerging Technologies 10, 348–355. Hernandez, E., Raventos, M., Auleda, J.M., Ibarz, A., 2010. Freeze concentration of must in a pilot plant falling film cryoconcentrator. Innovative Food Science and Emerging Technologies 11, 130–136.
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