Concentration of Sour Cherry Juice Using Osmotic Distillation
G. Racz1, N. Papp3, A. Hegedűs2, E. Bányai3, G. Vatai1
Corvinus University of Budapest, Department of Food Engineering
2
Corvinus University of Budapest, Department of Genetics and Plant Breeding
3
Corvinus University of Budapest, Department of Applied Chemistry
Hungary3
[email protected]
1
Sour cherry is one of the most famous and characteristic Hungarian fruit what is commonly
used for in food production as colour additive. Valuable compounds in fruit juice –vitamins,
polyphyenols, etc.- are heat and mechanical sensitive molecules and during the concentrate
process should have to avoid from degradation. Osmotic distillation
seems suitable option
to product high quality of sour cherry juice because this process not requires high temperature or
pressure. Raw juice with approximately 15°Brix was used for the experiment and tried to
concentrate up to 60°Brix where the water activity enough low to inhibit the microbiological
effects. Before and after the process TPC (total phenolic compounds) and AC (antioxidant
capacity) was measured by spectrophotometric method to determine the effect of the osmotic
distillation. Results point out that osmotic distillation is a promising method to make
concentrated sour cherry juice while the valuable compounds could avoid the degradation.
1. Introduction
Osmotic distillation is a kind of mass transfer driven membrane process where the driving
force is the vapour pressure difference between two solutions. Similar that membrane distillation
(MD) in case of OD is also used hyrdophobic, porous, polymeric membranes. For the osmotic
distillation process usually high concentration of osmotic agent, mostly salt solution (NaCl,
CaCl2, K2HPO4, K–acetate) or some kind of organic solutions (polyethylene-glycol, glycerol,
etc.) is used wich can keep and sustain very low value of vapour pressure during the process.
These osmotic agents able to performe the suitable vapour pressure difference with the high
concentration and low vapour pressure value between the two sloutions (Bailey et al. 2000;
Cassano et al. 2007/a; Gostoli et al.1999; Hongvaleerat et al. 2008). OD process based on a
phenomenon: between two aqueous solutions wich have different water actity occurs a volatile
compound stream to equalize the vapour pressure different of the two solution. In case of OD a
microporous hydrophobic membrane in present between these solutions wich is separate totally
them from each other because of the hydrophobicity and surface energy. Thus aqueous solutions
cannot penetrate into the pores than two liquid-vapour interface form at the entrances of the
pores with different water activity. As a result, volatile compound molecules (water) evaporate
from the higher water activity liquid-vapour interface (liquid food side), cross the pores by
diffusion and condense at the lower water activity vapour-liquid interface (osmotic agent side).
Figure 1. shows the flow sheet of the osmotic distillation process (Bélafi-Bakó et al. 2007;
Courel et al. 2000/b; Mansuori et al. 1999; Thanedgunbaworn et al. 2007/a.)
Figure 1. Flow sheet of osmotic distillation
Hydrophobe, porous
membrane
cFm
PFm»PPm
cPm
cP
Saturated
salt
solution
cF
Water vapour
Sour
cherry
juice
Low operating temperature (ambient temperature) and atmospheric pressure can be facilitate
to conecntrate of diluted aqueous solutions wich contain valuable heat sensitive compounds. In
case of fruit juice production several scientific journal has published worldwide the applications
and possibilies of osmotic distillation replacing high energy demand evaporation. However the
implementation of this technique in industrial scale is not realized yet, variety of fruit juice
concentration by using OD were performed experimentally and showed the potential of osmotic
distillation Cassano et al. examined concentration of kiwi juice, Thanedgunbaworn et al. clarified
passion fruit juice for the experiments. Rodriges et al. performed to concentrate camu-camu juice
and Hongvaleerat et al. concentrated pineapple juice with osmotic distillation. Experimental
results showed that operation temperature has sigificant effect on distillate flux. Moreover the
distillate flux decreased during the process wich is due to the increasing of total soluble solid on
the liquid food side wich is comes from the decreasing of the driving force.Similarly, like in
other membrane separation processes in case of osmotic distillation temperature increases the
distillate flux. A commonly use rule of thumb says 1°C increase cause ~3% increase of distillate
flux. A limit is exist in temperature-increasing which is due to temperature tolerance of the
applied membrane materials (polymeric membranes) and of course the heat degradation of
valuable compounds. Increasing of the recirculating flow rate both side of membrane decreasing
the thickness of the boundary layer and of course the effect of the concentration polarization is
also decreased. As a result it is possible to increase the distillate flux with the increasing
recirculating flow rate to a limited value.
2. Materials and methods
For the concentration experiments it was used pressured „Oblacsinkszka” species sour cherry
juice. The initial concentration of juice was 14.7 °Brix total solid content (mainly saccharides),
the initial mass was 2500 g. Juice was circulating in the capillars. Saturated (43 m/m %) CaCl2
solution was used for osmotic agent.
•
Membrane properties were: 0.2 m2 membrane area, 0.1µm pore size.
•
Recirculating flow-rate: 36l/h each side
•
Operation temperature: 30°C each side
Figure 2. shows the laboratory Osmotic distillation eqipment and illustrates the operation of
the concentration process. The vapour pressure difference between the juice and osmotic agent
along the membrane two side is the driving force of the process and of coure the solutions could
not penetrate the membrane. Two thermostates were used to keep stagnant temperature both
solutions. The osmotic solution’s increasing mass during the process was measured by digital
balance and registred by PC.
Figure 2. Laboratory scale osmotic distillation equipment
1. membrane module, 2. peristaltic pumps, 3. sour cherry juice,
4. saturated CaCl2 solution 5. digital balance, 6. heat-exchanger,
7-8. thermostate , 9. PC.
To evaluate registered data the flux values was calculated to determine the efficient of the
process according to the following equation:
J=m/(A*t)
(1)
where
m is the mass of the distillate acrossed the membrane (water), kg
A is the membrane area, m2
t is the operation time, h
The concentration process was performed in 3 separated steps and after every step the
osmotic solution had to be refreshed to keep higher driving force. One step performed one day.
Figure 3. shows the amount an the total solid content of the juice and osmotic solution during the
concentration process.
Analitical measurements – spectrophotometric methods - were used to determine the effect
of the concentration process on phenolic compounds and antioxidant capacity (FRAP-method).
Figure 3. Concentration steps of sour cherry juice by osmotic distillation
Pressured sour cherry juice:
14, 7°Brix
2500 g
End of 1. day :
19, 3°Brix
End of 2. day:
32, 3°Brix
End of conc.:
60,8°Brix
350 g
3. Results and discussions
3.1. Concetration process
Figure 4. shows the whole concentration process of sour cherry juice. The system worked in
batch process, the treatment’s total time was 13,3 h. The relatively low values of flux results
from the low membrane area because the flux strongly depends on that. It is observed also that
flux curves in all steps show decreasing tends because of the dilution of the osmotic solution and
the increasing the viscosity of juice. The final concentration of sour cherry juice was 60.8°Brix
and the final viscosity was 70 mPas therefore to move the concentrated juice was much more
difficult than the initial time.
Figure 4. Concentration process of sour cherry juice
Global process:
Jav=0,86 kg/m2h
t=13,3 h
14,7°BrixÎ60,8°Brix: 4x concentration
2,00
2
J, kg/m h
1,50
1,00
0,50
0,00
0
1
2
3
4
Analytical measurements
5
6
7
8
9
10
11
12
13
14
15
t, h
To determine the effect of the osmotic distillation process analytical methods were use.
Figure 5. illustrates the antioxidant capacity before and after the treatment (in unit of mmol
ascorbic acid/l). For this it was used FRAP-method. It is well osberved that the process did not
have any kind of decreasing effect of the antioxidant capacity wich is anticipate osmotic
distillation could be graet possibility for conecntration sour cherry juice.
Figure 5. Results of FRAP-method to determine the effect of osmotic distillation on sour cherry
concentration
FRAP (mmolAS/L)
13
11,12
SD=0,2578
11,10
SD=0,5757
11
9
7
5
3
1
OD előtt
Before
OD
OD után
After
OD
Another interesting results is well observed in Figure 6. wich can be seen that the total
phenolic compounds were not change during the concentration process (in unit mmol gallic
acid/l).
Phenolic content (mmol GS/L)
Összfenoltart.
Figure 6. Results of FRAP-method to determine the effect of osmotic distillation on sour cherry
concentration
12
10
9,27 SD=0,1273
9,19 SD=0,4763
8
6
4
2
0
Before
OD
OD előtt
After
OD
OD
után
4. Conlusion
The final concentration of sour cherry juice was 60,8 °Brix therefore the process was
suitable for to concentrate sour cherry juice high concentration range. The phenolic content and
antioxidant capacity was the same before and after the osmotic distillation, so osmotic distillation
is a siutable process to concentrate sour cherry juice without decreasing any analyzed valuable
compounds content.
Acknowledgement
TÁMOP-4.2.1/B-09/1/KMR-
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