Production of Low Alcohol Content Apple Cider by Reverse Osmosis

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Production of Low Alcohol Content Apple Cider by Reverse Osmosis Mariola Lo´ pez, Silvia Alvarez, Francisco A. Riera,* and Ricardo Alvarez Department of Chemical Engineering and Environmental Technology, University of Oviedo, C/Julia´ n Claverı´a, 8, 33 006 Oviedo, Spain

In this work alcohol removal from apple cider using the membrane technique of reverse osmosis was studied. Several aromatic polyamide and cellulose acetate membranes were tested. Experiments were carried out at different transmembrane pressures (15-50 bar), while the temperature was set at 15 °C and feed flow at 200 L/h. Permeate flux and rejection were observed to increase with transmembrane pressure, and aromatic polyamide membranes showed higher retention and higher selectivity for the permeation of ethanol vs other aroma compounds than cellulose acetate membranes. Experimental results were explained using the preferential sorptioncapillary flow model. Batch and diafiltration configurations were compared for cider dealcoholization. Diafiltration presented higher values of permeate flux and ethanol removal than the batch process. About 75% of ethanol can be removed from the cider without significant losses of the main aroma compounds. Removal of other undesirable compounds, such as methanol and acetic acid, is also high. 1. Introduction Nonalcoholic and low-alcoholic content fermented beverages, such as cider, beer, and wine, have become of great interest because of the health and social problems related to alcohol. Several methods for obtaining low-alcohol beverages have been reported in the references. These processes were usually developed for the production of beer or wine. Distillation is the most frequently used process, but it has shown significant losses in taste and aroma because of the alteration of the aroma profile from the original beverage due to the high temperature.1,2 Some years ago, low-alcohol cider was produced and commercialized in France and Switzerland using the controlled fermentation technique. According to this method, when the fermentation has produced the desired alcoholic content, the process is stopped.3 The main problem of this technique is the small amount in the final product of certain aroma compounds, which are produced during a complete fermentation cycle and are important for taste. The membrane separation process of reverse osmosis (RO) can be used to remove or reduce the alcohol content in naturally fermented beverages such as wine, beer, or cider. As the process is carried out at low temperatures and fully fermented product is used as feed, the main drawbacks of conventional processes can be avoided. The objective of the process is to remove ethanol while retaining low molecular weight alcohols, which are, with ethanol, the most important compounds in cider flavor.4-8 Cider aroma is highly complex with more than 190 volatiles being found in its composition. Several authors reported ethanol, superior alcohols, and esters as the most important components of apple cider flavor.9-14 Other compounds, such as methanol and acetic acid, were negatively related to the quality of the product. The organoleptic characteristics of apple cider depend on several factors, such as apple ripeness, microorgan* To whom correspondence should be addressed. Tel.: +34 8 5103436. Fax: +34 8 5103434. E-mail: [email protected]. uniovi.es.

isms employed in the fermentation step, apple variety, and processing procedures. Some of these volatiles and their common concentration in apple cider are reported in Table 1. The compounds shown in Table 1 can be considered the most important in cider aroma. Thus, the quality of low-alcohol cider depends on an adequate retention of these compounds, which may permeate the membrane. Some authors have tried to make a correlation between their concentration and the quality of the cider: intensity, fruity aroma, sweet aroma, scented aroma, fermented aroma, acidic aroma, sulfur aroma, and others, but final results were not very reproducible.11,12 In addition, some compounds in Table 1 (methanol and acetic acid) are confirmed as undesirable because of their negative effect on the final organoleptic properties. The permeation of these compounds together with ethanol could improve the dealcoholized cider flavor. In this paper, low-alcohol cider has been produced by reverse osmosis. Permeate flux, alcohol permeation, and aroma compound retention were tested using different cellulose acetate (CA) and aromatic polyamide (PA) membranes. A mathematical model was used to explain the experimental results on solvent and solute permeation through the membranes. Finally, two different operation modes (batch and diafiltration) were evaluated for cider dealcoholization. 2. Materials and Methods 2.1. Feed Solution. Fresh apple cider supplied by the Spanish company Valle, Ballina and Fernandez, S.A. was used as feed. The composition of the feed solution is shown in Table 1. 2.2. Equipment. Experiments were performed in a commercial DSS Lab unit 20 plate and frame ultrafiltration-reverse osmosis system supplied by Danish Separation Systems (DSS, Denmark). The system was modified for use with plate and frame and with tubular membranes. Cellulose acetate and aromatic polyamide membranes supplied by DSS and Paterson Candy International

10.1021/ie020155a CCC: $22.00 © 2002 American Chemical Society Published on Web 11/06/2002

Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002 6601 Table 1. Apple Cider Composition Reported by Several Authors9-14 compound (ppm) ethanola methanol 1-propanol isobutanol 1-butanol isoamyl alcohol hexanol acetaldehyde acetic acid ethyl acetate a

Beech9 (1972)

Beech et al.10 (1977)

Le´guerinel et al.11 (1987)

Mangas et al.12 (1996)

3.6 130 10 12 11 84

7.2 147 15 33

6.2 40 11 11

6-7 76 13 15

8.0 165 25 36

97

57 76

84

18 775 49

37

102 63 29 2528 52

6.66 4-47 14-74 4-32 16-39 2-17 20-100

12 24-82 3-6 113-176 2-29

Medina and Martinez13 (1997)

De la Roza14 (1999)

27 3734

25

62

feed solution

Ethanol content in % (v/v).

Table 2. Characteristics of the Membranes Used membrane

configuration

material

pHmax

Tmax (°C)

Pmax (bar)

water permeability (L/h‚m2‚bar)

NaCl S (%)

CA 865 PP CA 995 PP HR 95 HR 98 AFC 80 AFC 99

plate and frame plate and frame plate and frame plate and frame tubular tubular

CA CA AP AP AP AP

2-8 2-8 2-11 2-11 3-9.5 3-11

30 30 60 60 60 70

40 60 60 60 60 70

0.797 0.537 0.414 0.387 0.778 0.470

26-34 >94 >95 >97.5 80 >99

(PCI, England) were used in this work. Their main characteristics are indicated in Table 2. The surface area of the plate and frame and tubular membranes were 0.072 and 0.047 m2, respectively. Permeate and retentate were recycled back to the feed tank in the first set of experiments, where solvent and solute permeations through each membrane were tested. Following this, two different operating modes were considered: (a) Permeate containing water, ethanol, and other volatiles was removed from the process so that the volume of the feed solution in the tank decreased and its concentration increased (batch mode). (b) The level in the feed tank is maintained constant by adding water until the desired alcoholic content was obtained (diafiltration mode, DF). Experiments were carried out at pressures ranging from 1.0 to 5.0 MPa. The temperature was set at 15 °C to minimize aroma losses.15 Considerable losses of aroma compounds that are important for the taste of the product occur at high temperature. Moreover, heatinduced changes could be observed in the product. Due to limitations of the experimental equipment (i.e., limitations due to the pump), all the experiments were carried out at 200 L/h. Permeate flux was measured gravimetrically. 2.3. Analytical Procedures. Aroma compounds concentration in feed, permeate, and concentrate were analyzed by gas chromatography (GC). Analyses were carried out with a Konik 3000 gas chromatograph equipped with a flame ionization detector and using a Spectra-Physics SP4290 integrator and a CTCA2000S liquid autosampler (Konik-Tech, Spain). A Supelcowax 10 fused silica capillary column (Supelco Inc., USA) of 60 m × 0.25 mm i.d. was used for the analysis. Helium was employed as carrier gas at a flow rate of 2.0 mL/ min using a 95:1 split ratio. The temperature ramp was programmed from 40 to 90 °C at a rate of 4 °C/min and then from 90 to 110 °C at a rate of 35 °C/min, with an initial, intermediate, and final hold of 20, 10, and 15 min, respectively. The injector and detector were set to 190 and 200 °C, respectively. Samples of 1 µL were injected into the gas chromatograph. The experimental error was lower than 10% for all the chromatographic

Figure 1. Concentration polarization phenomenon.

determinations and it was lower than 4% for most of the runs. 3. Reverse Osmosis Model Several mechanisms have been proposed to explain the selectivity of RO membranes.16 In this work, the preferential sorption-capillary flow model was used.17-18 This model has been successfully applied by the authors in a previous work to the RO concentration of apple juice model solutions.19 The equations describing solute and solvent flux are well-known,

NA )

DAM (C - CA3) Kδ A2

NB ) A[∆P - ∆Π]

(1) (2)

where NA and NB are the flux of solute and solvent, respectively, through the membrane, A is the solvent permeability of the membrane, ∆P is the pressure difference across the membrane, ∆Π is the osmotic pressure difference at both sides of the membrane, CA2 and CA3 are the solute concentrations on the membrane surface at the feed side and at the permeate side, respectively, and DAM/Kδ is the solute transport parameter, which is usually an unknown parameter. Due to the concentration polarization phenomenon, the concentration at the membrane surface (CA2) is higher than the concentration in the bulk solution (CA1), as illustrated in Figure 1. To predict the concentration

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of solute on the membrane surface at the feed side, the film theory can be used,

NA + NB ) kAC1 ln

[

]

XA2 - XA3 XA1 - XA3

Table 3. Physical Meaning of Polar and Steric Taft Numbers and Modified Small Number

(3)

where kA is the mass-transfer coefficient, C1 is the molar density of the bulk solution, and XA1, XA2, and XA3 are the mole fractions of the solute in the bulk solution, on the membrane surface at the feed side, and in the permeate, respectively. As the concentration of the aroma compounds in the feed solution is very low, NA can be overlooked in eq 3. The mole fraction of the solute in the permeate can be calculated as

XA3 )

NA NA ≈ NA + NB NB

(4)

The “apparent rejection coefficient” of the aroma compounds (S) can be determined by the following expression,

(

)

CA3 S (%) ) 1 × 100 CA1

(5)

CA1 being the concentration of solute in the bulk solution. The “intrinsic rejection coefficient” could not be determined due to lack of data for estimating the concentration of the aroma compounds at the membrane surface (CA2). Anyway, S is the observed rejection for practical purposes. According to Sourirajan and Matsuura, the solute transport parameter is a function of the chemical nature of membrane and solute (polar and nonpolar effects) and the size of membrane pores and solute molecules (steric effect); thus,18

ln

( )

DAM ∝ Fσ + φEs + ωs* Kδ

Table 4. Physicochemical Data for Some Organic Solutes in Apple Cider

compound

boiling point (°C)

molecular weight

σ* (25 °C)

Es (25 °C)

s*/1000 (25 °C)

ethanol methanol 1-propanol isobutanol isoamyl alcohol 1-hexanol acetaldehyde acetic acid ethyl acetate

78.6 64.7 97.8 107 130 157 20.2 118 77

46.1 32.0 60.1 74.1 88.15 102.18 44.1 60.1 88.1

-0.100 0 -0.115 -0.200 -0.202 -0.130 0 0 -0.100

-0.07 0 -0.36 -0.93 -0.34 -0.40 0 0 -0.07

∼0 ∼0 ∼0 0.035 0.169 0.339 ∼0 ∼0 0.067

(6)

where σ is the polar Taft number, Es is the steric Taft number, s* is the modified Small number, and F, φ, and ω are empirical coefficients that represent the relative importance of each number and depend on the functional group of the solute and the chemical nature of the membrane. The polar Taft number measures the extent of the acidity of the solute. A more negative Taft number results in an increase of the basicity of the molecule. The steric Taft number measures the steric hindrance of molecules due to repulsions between nonbonded atoms. The more negative this number, the higher the steric hindrance of the molecule. The modified small number measures the nonpolar or hydrophobic nature of the organic solute molecule. The higher this number, the more hydrophobic the compound. According to Sourirajan and Matsuura, this number can be overlooked for very polar solutes, that is, those solute molecules containing no more than three straight chain carbon atoms not associated with a polar functional group.18 So, within the compounds considered in this research, this effect could have a certain importance only in the case of hexanol. The physical meaning of these numbers is indicated in Table 3.

Figure 2. Influence of pressure on permeate flux (temperature, 15 °C; feed flow, 200 L/h).

Numerical data on these three numbers for various organic molecules and structural groups are available in the literature. Table 4 shows the value of these numbers for the main volatile compounds in apple cider at 25 °C in aqueous solution.18,20 4. Results and Discussion The first set of results was obtained with recirculation of both permeate and concentrate to the feed tank. Figure 2 shows the influence of pressure on permeate flux for several PA and CA membranes. It can be observed that permeate flux increases with this variable for all the membranes considered, according to eq 2. The increase of flux with pressure is fairly linear. However, for the CA 865 PP membrane a loss of linearity can be observed at pressures higher than 35 bar. This can be due to membrane compactation18 (this effect is more noticeable for the CA membranes than for the PA

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Figure 3. Influence of pressure on aroma compounds retention (membrane AFC 99).

Figure 4. Aroma compounds retention at 25 bar, 15 °C, and 200 L/h feed flow.

membranes), but also to the concentration polarization phenomenon, according to the film theory. Next, the effect of pressure on ethanol and other aroma compounds permeation was studied. Only the results obtained with the AFC99 membrane are reported (Figure 3), while similar conclusions were observed for the rest of the membranes. It can be noted that rejection increases with transmembrane pressure. As pressure increases, the transport of solutes toward the membrane surface rises, thus increasing their permeation through the membrane. As already mentioned, solvent flux also rises with transmembrane pressure. The increase of rejection with pressure means that solvent flux is more affected by this variable than solute flux; that is, the NB/NA ratio rises. High pressure is recommended as lower losses of aroma compounds are obtained, ethanol removal being high enough. Figure 4 compares aroma compounds retention at certain operating conditions using different membranes. In this figure, the bar is missing for certain compounds and membranes, meaning 0% retention for this particular case. It can be observed that PA membranes present higher retention than CA membranes with similar NaCl retention (HR95 vs CA995PP, for example, which also present the same configuration), and even higher than CA membranes with higher NaCl retention (such as AFC80 vs CA995). This result can be explained by the fact that cellulose acetate membranes are more polar than polyamide membranes. Most of the cider flavor compounds are very polar. Thus, a stronger polar membrane would concentrate the polar organic solutes at the interfacial layer near the membrane and higher permeation of these solutes would be expected. A more hydrophobic membrane shows less affinity for polar organic solutes and a greater rejection is obtained.

These results agree with previous reports by other authors using different feed solutions.15,18-22 No significant change was observed in the results with time. Therefore, it was supposed that the aroma compounds did not cause any significant modification in the membrane transport properties, probably due to their low concentration in the feed solution. In the literature, these effects were observed, however, when nonaqueous solutions prepared in nonpolar solvents were filtered using organic membranes.23 Retention of the individual aroma compounds can also be explained using the preferential sorption-capillary flow mechanism. CA membranes act as a base, with a net negative charge; thus, the more basic the solute (the more negative the Taft number), the higher the rejection should appear. On the other hand, PA membranes seem to act as a very weak acid, with a net positive charge. The solvent (water) presents much higher acidity (σ ) 0.49); thus, basic solutes are attracted by the higher acidity of water in the bulk solution instead of being attracted by the membrane. Therefore, for both CA and PA membranes, the more negative the value of the solute polar Taft number, the greater the rejection should be.18 For this reason methanol, acetic acid, and ethanol, which present the lower negative Taft numbers, show the lowest rejection. Furthermore, compounds with a more negative Taft number, such as isoamyl alcohol and isobutanol, show high rejection. However, acetaldehyde, whose Taft number is also one of the less negative (σ ) 0), presents, especially in the case of CA membranes, surprisingly high rejections. Acetaldehyde is a very volatile compound, with a boiling point as low as 20 °C, so part of this solute can be lost during the process or while handling the samples. This could be a reason for the apparently low rejection observed. Solute rejection is also influenced by the steric Taft number. The more negative this number, the greater the steric hindrance to the permeation of solutes and, therefore, the higher their rejection should be. The compounds with the lower negative steric Taft number (such as methanol, ethanol, and acetic acid) show low rejection, while compounds with a more negative steric Taft number (such as isobutanol) present high rejection. Once again acetaldehyde rejection is higher than expected when analyzed from the steric effect prespective. The hydrophobic effect can be overlooked for all the aroma compounds except hexanol, which presents the longest hydrocarbon chain. Due to its nonpolar character, hexanol would be attracted to the hydrophobic part of the membrane and its permeation through the membrane would increase. This effect can be better observed for the PA membranes, which are more hydrophobic. It can be noted that hexanol rejection is lower than the rejection of other compounds with similar steric and polar Taft numbers (such as propanol). On the basis of the above results, the aromatic polyamide membrane AFC99 was selected to carry out the dealcoholization process because the highest rejection of aroma compounds was observed, while its permeability is higher than the permeability of HR95 and HR98 membranes, which also present high rejection. The alcohol removal rate is slower than that reached with CA membranes, but better quality apple cider will be produced. Low-temperature operation is recommended to avoid losses and alterations of the aroma. High pressure is also recommended to increase permeate flux and aroma retention.

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Figure 5. Influence of the VCR on ethanol flux and rejection in batch configuration.

Two operating modes, batch and diafiltration, were considered to reduce ethanol content in apple cider. In batch configuration, as the permeate is continuously removed from the system, the concentration of ethanol and other compounds in the feed stream increases, thus increasing the osmotic pressure of the solutions. According to eq 2, this means a reduction of the driving force for permeation and, therefore, a decrease in permeate flux. At a certain volume concentration ratio (VCR), which is defined as the relationship between initial and final volume in the feed tank, the process has to be stopped because permeate flux is very low, as is the ethanol removal rate. At VCR 3 permeate flux is only 1.4 L/m2‚h. Figure 5 shows the influence of the VCR on ethanol permeation and rejection. As the VCR increases, the feed concentration rises and the concentration of solutes on the membrane surface is higher. However, it can be observed that ethanol flux through the membrane decreases with the VCR. This can be explained using eq 1. The driving force for ethanol permeation is the concentration difference between both sides of the membrane (CA2-CA3). Thus, the reduction of ethanol flux with the VCR is due to a decrease in (CA2-CA3) with this parameter. Therefore, ethanol concentration in the permeate, CA3, increases with the VCR more than its concentration on the membrane surface, CA2. From eq 4, CA3 can be estimated as the relationship between solute and permeate flux. It was experimentally observed that both magnitudes decrease with the VCR. The high rise of CA3 with this variable indicates that permeate flux is much more affected by concentration than ethanol flux. The same argument can be used to explain the decrease in ethanol rejection with the VCR, which can also be observed in Figure 5. Different results were obtained with the DF configuration. Due to a reduction in concentration and osmotic pressure with dilution volume (DV), permeate flux increases with this variable. Dilution volume was calculated as the water volume added to the feed tank divided by the initial volume. Figure 6 shows the influence of DV on ethanol flux and retention. It can be observed that ethanol flux through the membrane decreases with DV due to the reduction of solute concentration at the high-pressure side (CA2). As solute flux decreases and solvent flux increases with DV, ethanol retention was observed to increase with this variable until a steady value of about 70% ethanol retention was reached. During the DF process, certain amounts of aroma compounds permeate the membrane. Figure 7 shows the

Figure 6. Influence of dilution volume on ethanol permeation and rejection in diafiltration configuration.

Figure 7. Aroma compounds removal with the dilution volume for the diafiltration configuration. Table 5. Aroma Compounds Removed in Batch and Diafiltration Configurations (Membrane, AFC99; Temperature, 15 °C; Pressure, 45 bar; Feed Flow, 200 L/h) removed (%) aroma compound

batch (VCR: 3)

diafiltration (DV: 3.2)

ethanol methanol 1-propanol isobutanol isoamyl alcohol 1-hexanol acetaldehyde acetic acid ethyl acetate

50 50 20 0 2 5 20 25 25

75 94 0 0 0 2 43 18.5 28

amount of each compound removed from the cider as a function of the dilution volume. It can be noted that the elimination of the solutes is very slow at the end of the process due to their low concentration in the feed tank. It can also be observed that those compounds with very high polar and steric Taft numbers (ethanol, methanol, acetic acid, and acetaldehyde) permeate the membrane more easily, while those compounds with much more negative Taft numbers (isobutanol and isoamyl alcohol) are almost 100% retained. Table 5 compares the amount of aroma compounds removed in both batch and DF operation modes. It can be observed that methanol is the compound that is most easily removed from the apple cider. This is beneficial for the final taste of the product as methanol has been negatively related to apple flavor by several authors.11 Moreover, methanol is a harmful compound whose high concentration in food products can cause severe health problems. In DF configuration, methanol is almost completely removed from the cider, while in batch mode only 50% of methanol is removed at VCR 3. The amount of ethanol

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eliminated in the DF configuration is also higher than the amount eliminated in batch configuration. On the other hand, the rejection of other volatiles which are important for the flavor of the final product is very high, specially in DF operation mode. Several compounds, such as 1-propanol, isobutanol, and isoamyl alcohol, were observed to be completely retained by the membrane. Taking these results into account, DF operation mode is recommended over the batch mode. An improvement in the elimination of ethanol and methanol and in the retention of desirable aroma compounds was found. 5. Conclusions Reverse osmosis has proven to be an efficient technique for alcohol removal from apple cider. The experimental results on flux and retention can be explained using the preferential sorption-capillary flow model. Aromatic polyamide membranes show higher retention than CA membranes, which can be explained in terms of their higher hydrophobicity, according to this model. The higher the basicity (the more negative the polar Taft number) and the higher the steric hindrance to permeation (the more negative the steric Taft number) the solute presents, the more it is retained by the membrane. Methanol and ethanol are the compounds presenting the lowest retention. The permeation of the rest of the compounds through the membrane is very low. Permeate flux and retention were observed to increase with transmembrane pressure. The diafiltration configuration is recommended over batch configuration as the higher removal of methanol and ethanol and the higher retention of other desirable compounds can be achieved. Acknowledgment The authors also thank the apple processing factory Valle, Ballina and Ferna´ndez, S.A. for the cider supplied. List of Symbols A ) solvent permeability of the membrane, kmol/s‚m2‚MPa C ) concentration, kmol/m3 D ) diffusion coefficient, m2/s DAM/Kδ ) solute transport parameter, m/s DV ) dilution volume Es ) steric Taft number k ) mass-transfer coefficient, m/s N ) flux through the membrane, kmol/s‚m2 ∆P ) transmembrane pressure, MPa S ) apparent rejection coefficient, % s* ) modified Small number VCR ) volume concentration ratio X ) mole fraction Greek Letters δ ) membrane width, m φ ) empirical coeficient related to the steric Taft number ∆Π ) osmotic pressure difference at both sides of the membrane, MPa F ) empirical coefficient related to the polar Taft number, kg/m3 σ ) polar Taft number ω ) empirical coeficient related to the modified Small number

Subscripts A ) solute B ) solvent M ) membrane 1 ) bulk solution 2 ) membrane surface at feed side 3 ) permeate

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Received for review February 21, 2002 Revised manuscript received August 22, 2002 Accepted August 28, 2002 IE020155A