Recovery of Valuable Tea Aroma Components by Pervaporation

permselective to the aroma compounds than the PDMS membrane, and the POMS membrane was also more efficient for the separation of ETH than ETB...
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Ind. Eng. Chem. Res. 2003, 42, 6924-6932

Recovery of Valuable Tea Aroma Components by Pervaporation Dharmesh M. Kanani,† Bhaurao P. Nikhade,† Padma Balakrishnan,‡ Gurmeet Singh,‡ and Vishwas G. Pangarkar*,† Chemical Engineering Department, University Institute of Chemical Technology (formerly UDCT), Matunga, Mumbai 400019, India, and Unilever Research India, 64 Main Road, Whitefield P.O., Bangalore 560066, India

The present work describes the possibility of using pervaporation to recover the tea aroma compounds from tea aroma condensate generated in the manufacturing of tea or instant tea, as well as directly from tea extract. Eight compounds that make a significant contribution to tea aroma, namely, trans-2-hexenal, linalool, cis-3-hexenol, 3-methylbutanal, 2-methylpropanal, benzyl alcohol, phenylacetaldehyde, and β-ionone, were studied in this work. Permeation studies for all of these compounds with poly(octyl methyl siloxane) (POMS) and poly(dimethylsiloxane) (PDMS) membranes in a batch-type vacuum pervaporation system were carried out, first, in their aqueous solutions (binary mixtures); second, in a model solution containing all of the abovementioned compounds; and last, with an actual tea extract. In this work, mainly the effect of the feed concentration on the organic flux and separation factor was studied. The permeation studies with the mixture revealed that β-ionone, trans-2-hexenal, linalool, cis-3-hexenol, and 3-methylbutanal offered very good selectivity, whereas phenylacetaldehyde, 2-methylpropanal, and benzyl alcohol gave moderate selectivity. The results indicate that pervaporation is an attractive technology for the recovery of tea aroma compounds from tea aroma condensate as it (i) yields good separation and (ii) operates under mild conditions. However, it should be noted that, because the selectivity offered by pervaporation varies considerably from compound to compound, attempts to concentrate a solution of volatiles to a high degree can result in significant alteration in the profile of the aroma. Thus, the commercial utility of this approach will need to be ascertained on a case-by-case basis. Introduction Flavor is a key attribute of foods and beverages. Volatile organic constituents of foods and beverages, generally referred to as aroma compounds, contribute significantly to the flavor. However, most of these volatile compounds are lost during the processing of food. Extraction of volatile compounds prior to processing by means such as steam distillation or their recovery from the process exhaust streams by condensation for eventual reconstitution into the finished product is difficult because of their low concentrations in the aqueous solutions obtained. Typical concentrations of volatile aroma compounds in the aqueous solutions obtained from steam distillation or exhaust condensate are in parts per million range. Concentration of such highly diluted solutions remains a major limitation to the industrial development of techniques for the recovery and/or production of natural aroma. The challenge is made tougher by the constraint of conducting the aroma recovery and concentration under mild conditions because of the heat sensitivities of aroma compounds. In view of the above considerations, pervaporation, which operates under mild conditions and can provide good separation, can be considered as an alternative to the conventional separation processes. The use of pervaporation for extraction and concentration of volatile aroma compounds is not new. Voilley * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +91-22-414 5616. Fax: +91-22-414 5614. † University Institute of Chemical Technology. ‡ Unilever Research India.

et al.1 suggested one such application in 1988. Most research on aroma recovery by organophilic pervaporation has been conducted using aqueous solutions of single aroma compounds as model systems. The recovery of individual aromas relevant to the flavor of grapes or wine has been reported by several authors.2,3 Karlsson and Tragardh4 have provided an extensive literature review on pervaporation modeling studies and applications of pervaporation to aroma compound recovery. Organophilic pervaporation linked to fermentation has been studied for the recovery of individual aroma compounds or inhibiting metabolic products.5 A number of pervaporation studies reported in the literature have focused on comparisons of different membranes with respect to selectivity and flux. Voilley et al.1,6,7 studied the recovery of volatile aroma compounds (1-octene-3-ol and 2,5-dimethylpyrazine) from model fermentation broths. The water solutions of each aroma compound were sweep gas pervaporated with two different membranes, microporous polypropylene and homogeneous poly(dimethylsiloxane) (PDMS). These studies showed that PDMS membranes were more suitable for the extraction of aroma compounds than polypropylene membranes. Lamer and Voilley8 studied the aroma compound extraction of 1-octene-3-ol, ethyl ethanoate, ethyl butanoate, and ethyl hexanoate with vacuum pervaporation through PDMS membranes. In comparison with previous work, their results showed slightly higher fluxes for homogeneous PDMS membranes than for zeolite-filled ones, as expected.9 Sampranpiboon et al.10 employed pervaporation to recover aroma compounds from ethyl butanoate (ETB)-H2O, ethyl hexanoate (ETH)-H2O, and ETB-ETH-H2O

10.1021/ie0340185 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/12/2003

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mixtures using poly(octyl methyl siloxane) (POMS) and poly(dimethylsiloxane) (PDMS) membranes. In general, the POMS membrane was found to be more permselective to the aroma compounds than the PDMS membrane, and the POMS membrane was also more efficient for the separation of ETH than ETB. A number of papers that study the effects of the physical properties of the compounds on their selectivities in pervaporation are available in the literature. Pervaporation of natural apple essence through poly(dimethylsiloxane) membrane at 22 °C and a permeate pressure of 3 mbar was studied by Zhang and Matsuura.11 When enrichment factors for different aroma compounds were plotted versus their boiling points, a decreasing trend with increasing boiling point was found for the enrichment factor. An enrichment factor above 20 was achieved for aroma compounds with boiling points below 100 °C. Baudot and Marin12 analyzed aroma compounds recovery by pervaporation through an exhaustive review of the literature. These authors discussed mainly the effective selectivities of organophilic membranes in terms of the vapor-liquid equilibria (VLE) of the aroma compounds and the importance of the nature of the aroma compounds to be extracted in relation to the type of membrane used. They found that some chemical groups of molecules are better separated through the available pervaporation membranes. The effect of the chemical functionality of volatile compounds on their selectivity has also been an important focus of research. Bengsten et al.,13 in their experiments on the concentration of aroma compounds from fruit juice using poly(dimethylsiloxane) (PDMS) on a polyamide-polyimide copolymer support for pervaporation, concluded that the separation factors for different aroma compounds vary considerably with the type of compound. The alcohols were found to have the smallest enrichment factors, ranging between 5 and 13, whereas esters were concentrated up to 100-fold. The enrichment factors for the aldehydes were between 40 and 60. Baudot et al.14 attempted to use all of the available information to derive a universal law for pervaporation. They conducted pervaporation experiments with four aroma compounds (covering a range of physicochemical properties, from hydrophilic low-boilers to extremely hydrophobic high-boilers) diluted in model binary aqueous solutions through three kinds of commercial organophilic membranes on a plate-and-frame module. They found that, regardless of the nature of the membranearoma compound associations, the transmembrane transfer coefficient model resulted in an accurate prediction of the selectivity of the pervaporation operation over a large range of total permeate pressures. A few papers on the industrial scale-up of the pervaporation process are also available. Kaschmekat et al.15 studied the pervaporation of an effluent stream from an apple juice plant. On the basis of their data, a pervaporation system with the capacity to remove 90% of the aroma compounds from an effluent stream was designed. The cost analysis showed that the value of the permeate obtained exceeded the operating cost of the system and that the system would have an overall payback time of 4-5 months. Despite such evidence, the application of pervaporation in industry for the extraction and concentration of aroma compounds has been quite limited. The prime reasons for this are the novelty

of the technique, the cost of the membrane, and the minimal knowledge about membrane lifetimes under actual process conditions. However, an equally important reason is the lack of information on the effect of the technique with respect to the sensory properties of the flavor being concentrated. A key requirement of a flavor is its quality and a sensorial match with the original food flavor. Because the selectivities of the various compounds making up a food flavor will be different for any given concentration technique, the resulting concentrated flavor will, in turn, be different in composition from the original. Thus, in addition to physical selectivity, we have tried to emphasize the sensorial aspect of the concentrated flavor. As much as the use of model systems containing one volatile aroma compound or a mixture of volatile aroma compounds is sensible for the detailed analysis of process performance and optimization, such an approach does not allow for an evaluation of the actual capacity of a process to recover a flavor. A food flavor consists of a multitude of compounds, many of which differ in regard to concentration; physical, chemical, and organoleptic properties; and contribution to the overall aroma profile. Thus, studies with actual aroma solutions are for the data to be interpretable in sensorial terms. The system selected for the study in this paper is tea. Tea is a popular beverage around the world. The aroma of tea comprises more than 600 volatile compounds. As it is impractical to undertake a study of all of these components, we have restricted ourselves to eight compounds, namely, trans-2-hexenal, linalool, cis-3hexenol, 3-methylbutanal, 2-methylpropanal, benzyl alcohol, phenylacetaldehyde, and β-ionone. Studies were conducted with single compounds in aqueous solution, mixtures of the eight compounds, and tea aroma obtained from extracts of tea that contained all of the other tea components, in addition to the eight selected for this study. Thus, the effects of the interactions of the compounds not considered are also incorporated in the results. It should be noted that the eight compounds selected for this study were not chosen at random. Rather, these compounds were selected to encompass different physical properties, chemical functionalities, and sensorial profiles. Moreover, in the literature, these compounds have been highlighted as having an important influence on the overall aroma of tea.16,17 All of the selected tea aroma constituents are partially, slightly, or very slightly soluble in water, which signifies pronounced hydrophobic behavior. Therefore, PDMS and POMS membranes were considered for use in this study. Both of these membranes are hydrophobic in nature and likely to sorb the tea volatile compounds readily. However, these membranes have different diffusional cross sections, and hence, their membranephase diffusivities are different, giving different overall selectivities. POMS contains octyl groups, which is hydrophobic in nature, as side chains, and hence, POMS membranes might yield good sorption selectivities at the expense of diffusional hindrance. Experimental Section Materials. Elastosil LR 7600 A and B were kindly supplied by Wacker Chemie, Munich, Germany, for preparation of the PDMS membrane. The solvent for the polymer, isooctane, was procured from S.D. Fine Chemicals, Mumbai, India. Composite poly(octyl methyl siloxane) (POMS) membrane (skin layer thickness ) 5

6926 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 Table 1. Physical Properties of the Compounds19

µm), supported on poly(ether imide) (PEI) microporous support was kindly supplied by GKSS, Germany. The authentic samples of aroma compounds, namely, trans2-hexenal, 3-methylbutanal, 2-methylpropanal, phenylacetaldehyde, benzyl alcohol, linalool, cis-3-hexenol, and β-ionone, were supplied by Unilever Research India, Bangalore, India. Various physical properties, including boiling point, molar volume, molecular weight, specific gravity, and chemical structure, of the above-mentioned eight organic compounds are listed in the Table 1. Table 2 reports the solubility parameters due to dispersion (δd), polar (δp), and hydrogen bonding (δh) and the total solubility parameters (δtotal) for all of these compounds. Membrane Preparation. Elastosil LR 7600 A (polymer) and B (cross-linker) were mixed in a 9:1 proportion, and a 10% solution of this mixture in isooctane was prepared. The solution was spread on a glass plate with a bar coater and then cured at 80 °C for 8 h.9 This

Table 2. Solubility Parameters (MPa1/2)a trans-2-hexenal cis-3-hexenol 2-methylpropanal 3-methylbutanal phenylacetaldehyde benzyl alcohol linalool β-ionone waterb PDMSc

δd

δp

δh

δtotal

15.85 15.80 15.42 15.42 14.92 18.4 16.45 17.69 15.5 16

6.93 4.30 8.88 7.43 6.86 6.3 2.79 3.80 16 0.1

6.24 13.10 7.07 6.47 6.21 13.7 10.56 3.14 42.4 4.7

18.39 20.97 19.14 18.29 17.56 23.79 19.75 18.36 47.9 16.6

a Unless otherwise indicated, all δ values were calculated by the van Krevelen group contribution method.20 b δ value reported by Grulke.20 c δ value reported by Mulder.21

process yielded a stable PDMS membrane with an average thickness of 160 µm. Experimental Setup and Procedure. Pervaporation experiments in the present work were carried out

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in batch mode. The setup used was the same as that used by Mandal and Pangarkar.18 It consisted of a pervaporation cell, a cooling trap, and a vacuum pump. The effective membrane separation area was 38.5 cm2. Two openings were provided at the top. A propeller (axial flow, directed toward the membrane surface) was introduced through the central opening to agitate the feed liquid, whereas the other opening was used to feed the solution. To ensure the desired feed temperature, a thermowell was also placed in the feed compartment, the closed end of which was just above the membrane surface. A glass cooling trap dipped in liquid nitrogen was used to condense the permeate. The feed solution was kept in contact with the membrane overnight for equilibrium to become established. During the experiments, the upstream side was at atmospheric pressure, and the low downstream pressure was maintained at 5 mmHg through application of a vacuum. Pervaporation experiments were carried out at a constant temperature of 30 °C. To ensure adequate mixing of the liquid feed so as to eliminate any concentration gradient, continuous stirring of the feed liquid was done by means of propellertype downward-flow impeller. The permeate was condensed by means of a liquid nitrogen cold trap. Analysis. The feed, permeate, and retentate samples of tea aroma were analyzed by gas chromatography (Perkin Elmer model XL GC and head space Turbomatrix 40) with flame ionization detection (FID) at Unilever Research India, Bangalore, India. The permeate was a heterogeneous mixture consisting of an organic phase and an organic-rich water phase. The total permeate was weighed first and then dissolved in a known quantity of distilled water before analysis. The analysis was done by headspace gas chromatography. (The analytical column phase was CP-Sil 8 CB (Varian) with helium as the carrier gas and FID). Theory. Pervaporation has been popularly described in terms of the solution-diffusion model. According to this model, in the absence of concentration polarization, the flux of a solute is given by

J)

D(Cmf - Cp) δ

(1)

where Cmf is the membrane-phase concentration of solute, Cp is vapor-phase concentration of permeate solute, D is the diffusion coefficient, and δ is the membrane thickness. Because Cp ≈ 0 (the vapor-phase concentration is very small), eq 1 becomes

( )

J)D

Cmf δ

(2)

Cmf can be expressed in terms of the feed liquid concentration Clf using the sorption coefficient S as

Cmf ) SClf

(3)

where Clf is the feed concentration of solute and S is the sorption coefficient. Combining eq 2 and 3 gives

J)

(DS)Clf δ

(4)

J)

P Clf δ

(5)

where P is defined as the permeability. When the flux (J) vs Clf plot is linear, the permeability is constant over the particular concentration range covered and is equal to the slope of this plot. It will be seen later that, for the dilute range of aroma compounds used in this work, the permeability is constant. Further, because the permeability is independent of the concentration, it is useful for comparison of the selectivities for different feed concentrations of the different aroma compounds. Pervaporation performance parameters. The separating ability of a pervaporation membrane is generally characterized in terms of the separation factor and flux. The separation factor is defined as the ratio between the concentrations of solute in the permeate and in the feed. The fluxes were determined directly by weighing the permeates

flux )

weight of permeate (area)(time)

To compare selectivity of a pervaporation membrane toward an aroma compound, VLE separation factors were calculated, the detailed procedure for which is given by Baudot and Marin.12 Results and Discussion Initially, permeation studies for trans-2-hexenal, 3-methylbutanal, 2-methylpropanal, phenylacetaldehyde, benzyl alcohol, linalool, cis-3-hexenol, and β-ionone in their aqueous solutions (binary mixtures) were carried out with POMS and PDMS membranes. Later, permeation studies with model solutions containing all of the above-mentioned compounds in proportions similar to those expected in actual tea aroma condensate generated in the manufacturing of instant tea and with an actual hot-water tea extract were carried out. In this work, mainly the effect of the feed concentration on the partial flux and separation factor was studied. In the pervaporation of organic compounds from their dilute solutions, concentration polarization in the liquid film can develop. Under these conditions, the true membrane permeation behavior is masked by the external diffusion (concentration polarization) resistance. In the present case, the upper compartment of the cell containing the feed liquid was provided with a propeller (axial flow directed toward the membrane surface). Preliminary experiments were carried out at varying speeds of agitation, and the flux values were determined. It was found that starting with a low speed (1 revolution/s) the flux values increased and then became constant at and above 4 revolutions/s. In the experimental data reported in this work, all experiments were carried out at 5 revolutions/s. For comparison of membrane performance, the total and partial (organic) fluxes were normalized to 5-µm thickness for the PDMS membrane. The observations and results obtained from these experiments are discussed in the following sections. Permeation Studies with Aqueous Solutions of Individual Aroma Compounds (Binary Mixtures). In the concentration range studied, all eight organic solutes permeated preferentially relative to water. Also, the permeate concentration exceeded the solubility limit of each respective aroma compound, and milky solutions

6928 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003

Figure 1. Variation of separation factor with the logarithm of the mean feed concentration for aldehydes in binary system.

Figure 2. Variation of separation factor with the logarithm of the mean feed concentration for alcohols in binary system.

were obtained in the permeate because of the formation of an emulsion. As the feed concentration increased, the permeate concentration also increased, showing higher extraction rates with higher solute contents in the feed. For some compounds, the difference between the feed and retentate concentrations was large; therefore, logarithmic mean (LMC) of the feed and retentate concentrations was considered instead of the feed concentration itself. Figures 1 and 2 show the effect of the feed concentration on the separation factor for aldehydes and alcohols, respectively, whereas Figure 3 shows the effect of the feed concentration on the partial organic flux for alcohols. For all of the compounds investigated, the partial flux increases linearly with increasing feed concentration. On the other hand, as the feed concentration increased, the separation factor decreased. Table 3 lists the separation factors and organic fluxes for the concentration ranges studied for all of the compounds studied. Comparisons between the VLE and pervaporation separation factors for aldehydes and alcohols (with β-ionone included) are shown in Figures 4 and 5, respectively. In the alcohols studied, cis-3-hexenol showed the highest permeability, followed by linalool and benzyl alcohol, with both the POMS and PDMS membranes. This type of behavior can be explained in terms of molecular size and hydrophobicity. cis-3-Hexenol is much smaller than linalool, but slightly larger than

Figure 3. Variation of organic flux with the logarithm of the mean feed concentration for alcohols in binary system.

benzyl alcohol, as indicated by molar volumes reported in Table 1. However, benzyl alcohol is the least hydrophobic of the three alcohols and, hence, is likely to sorb less. Therefore, considering both molecular size and hydrophobicity, cis-3-hexenol showed the highest permeability, followed by linalool and benzyl alcohol. POMS gave better separation factors and organic fluxes than PDMS for linalool and cis-3-hexenol because of its higher hydrophobicity, whereas for benzyl alcohol, the two membranes gave nearly the same separation factors, but PDMS gave a higher organic flux because of its very flexible chains. The separations achieved by pervaporation for these compounds, except linalool, were above the corresponding VLE separations. Of the aldehydes studied, 3-methylbutanal showed the highest permeability, the followed by trans-2hexenal, phenylacetaldehyde, and 2-methylpropanal with POMS membrane. 2-Methylpropanal is less hydrophobic than the other three aldehydes, and hence, it might be sorbed less. Even though 3-methylbutanal is less hydrophobic than trans-2-hexenal and phenylacetaldehyde, it showed a higher permeability because of its smaller molecular size, whereas with PDMS membrane, trans-2-hexenal showed the highest permeability, followed by 3-methylbutanal, phenylacetaldehyde, and 2-methylpropanal. This type of behavior can be explained in terms of the hydrophobic nature of the PDMS membrane, as well as its very flexible chains. Overall, PDMS gave better separation than POMS for the aldehydes. All of the aldehydes except 2-methylpropanal showed pervaporation separation factors above those obtained by VLE. Permeation Studies with Model Aroma Mixtures. Figures 6 and 7 show the effects of the feed concentration on the separation factor and partial flux, respectively, for linalool, phenylacetaldehyde, and benzyl alcohol in a model aqueous solution containing all eight compounds in the total feed concentration range of 38-69 mmol/L for the POMS and PDMS membranes. Because the concentration ranges for the binary and model aroma mixture solutions were different, to eliminate the effect of feed concentration, the permeabilities were calculated using eq 5. The resulting values are included in Table 4. Table 5 lists the separation factors and organic fluxes for the concentration ranges studied for the above eight compounds in the above-mentioned mixtures. Partial flux increases with feed concentration for all of these compounds. It is evident that the

Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6929 Table 3. Separation Factors and Organic Fluxes for the Concentration Ranges Studied for Eight Tea Aroma Compounds in Their Binary Aqueous Solutions POMS

trans-2-hexenal 2-methylpropanal 3-methylbutanal phenyl acetaldehyde benzyl alcohol cis-3-hexenol linalool β-ionone

PDMS

feed conc (mmol/L)

separation factor

partial flux (kg/m2‚h)

feed conc (mmol/L)

separation factor

partial flux (kg/m2‚h)

0.08-3.08 0.04-3.70 1.45-13.35 0.79-2.00 0.46-1.94 0.25-1.90 0.26-1.39 0.21-0.75

46-36 17-6 114-54 21-14.5 4.6-2.9 295-105 66-48 4-3

(5-123) × 10-5 (7-214) × 10-6 (2-10) × 10-3 (2.8-4.8) × 10-4 (3.4-9.5) × 10-5 (7.8-29) × 10-4 (3.3-12) × 10-4 (2.2-7.2) × 10-5

1.07-2.54 0.69-3.33 2.09-11.38 0.41-2.08 0.51-2.31 0.40-1.80 0.23-1.04 0.21-0.73

415-245 21-7 195-122 27-9 6.4-3.4 95-48 30-12 2.7-1.8

(2.5-5.5) × 10-2 (4.3-15.8) × 10-4 (2.3-20) × 10-1 (4.2-18) × 10-4 (2-4.6) × 10-4 (3.8-15.5) × 10-4 (2.6-0.2) × 10-4 (6.3-22.5) × 10-5

Figure 4. Comparison between separation factors of pervaporation and vapor liquid equilibrium for aldehydes.

Figure 5. Comparison between separation factors of pervaporation and vapor liquid equilibrium for alcohols and β-ionone.

alcohols, which have stronger hydrogen-bonding capacities than the aldehydes (as indicated by the higher δh values for alcohols than aldehydes in Table 2), show an increase in permeability in model multicomponent mixtures as compared to the binary mixtures. This is likely to be due to the fact that a smaller hydrophobic alcohol molecule, which can readily sorb in the hydrophobic membrane, can also invite a larger alcohol molecule through hydrogen bonding. Although the feed is dilute in terms of concentration, the preferential sorption increases the membrane-phase concentration. Additionally, the hydrogen bonding between alcohols explained above can increase the overall alcohol concentration in the membrane phase. All of the alcohols showed higher permeabilities than in the binary mixture. However, in POMS, cis-3-hexenol showed an

Figure 6. Variation of separation factor with the logarithm of the mean feed concentration for linalool, phenylacetaldehyde, and benzyl alcohol in a multicomponent mixture.

Figure 7. Variation of organic fluxes with the logarithm of the mean feed concentration for linalool, phenylacetaldehyde, and benzyl alcohol in a multicomponent mixture.

exceptionally different trend in terms of a lower permeability than in the binary mixture that is difficult to explain. On the other hand, all of the aldehydes except 2-methylpropanal showed lower permeabilities than in the corresponding binary mixtures. The behavior of 2-methylpropanal is again difficult to explain. The lower permeabilties of the aldehydes can be attributed to the fact that the alcohols “crowd them out” through their stronger hydrogen bonding, which allows them to occupy a large fraction of the free volume in the membrane. This implies that the free volume available for the aldehydes is lower, and hence, the result should be both lower sorption and lower diffusion and, consequently, lower permeability. Actual Tea Extract. Finally, permeation studies were carried out with an actual tea extract with hot water to check the feasibility of direct extraction and

6930 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 Table 4. Permeabilities (cm3‚cm/cm2‚h) of the Components in Binary and Model Multicomponent Mixture POMS

trans-2-hexenal 2-methylpropanal 3-methylbutanal phenylacetaldehyde benzyl alcohol cis-3-hexenol Linalool β-ionone

PDMS

binary

model multicomponent

binary

model multicomponent

2.0 × 10-4 4.0 × 10-5 4.5 × 10-4 1.0 × 10-4 2.5 × 10-5 8.0 × 10-4 3.0 × 10-4 2.5 × 10-5

1.5 × 10-4 5.0 × 10-5 2.0 × 10-4 5.0 × 10-5 5.0 × 10-5 3.0 × 10-4 5.5 × 10-4 8.0 × 10-4

1.2 × 10-2 3.5 × 10-4 9.6 × 10-3 4.0 × 10-4 1.0 × 10-4 4.5 × 10-4 3.0 × 10-4 1.0 × 10-4

2.9 × 10-3 9.0 × 10-4 1.9 × 10-3 3.0 × 10-4 2.5 × 10-4 1.6 × 10-3 2.2 × 10-3 1.5 × 10-3

Table 5. Separation Factors and Organic Fluxes for the Concentration Ranges Studied for Eight Tea Aroma Compounds in the Multicomponent Mixture POMS

trans-2-hexenal 2-methyl propanal 3-methylbutanal phenyl acetaldehyde benzyl alcohol cis-3-hexenol linalool β-ionone

PDMS

feed conc (mmol/L)

separation factor

partial flux (kg/m2‚h)

feed conc (mmol/L)

separation factor

partial flux (kg/m2‚h)

3.30-5.40 0.002-0.005 34.82-60.36 0.1-0.17 0.16-0.26 0.17-0.28 0.06-0.1 0.0001-0.008

80-61 10-7 18-29 6-7.5 8.4-8.0 36-50 65-74 250-90

(4.2-5.0) × 10-3 (1.6-3.4) × 10-7 (9.2-19) × 10-3 (1.2-2.4) × 10-5 (2.3-3.5) × 10-5 (1-1.8) × 10-4 (1-1.7) × 10-4 (7.4-206) × 10-7

4.38-5.91 0.002-0.005 47.60-61.53 0.12-0.17 0.18-0.28 0.25-0.35 0.08-0.11 0.004-0.009

88-65 20-29 35-60 7-9 6-8 35-50 66-55 45-35

(2.3-3.3) × 10-2 (2-6.7) × 10-6 (1.1-2.2) × 10-1 (8.3-13.7) × 10-5 (9.7-16.5) × 10-5 (7.6-11) × 10-4 (5.4-7.4) × 10-4 (2.5-5.6) × 10-5

Table 6. Separation Factors and Organic Fluxes for All of the Compounds Studied with Tea Extract POMS

trans-2-hexenal 2-methylpropanal 3-methylbutanal phenylacetaldehyde Benzyl alcohol cis-3-hexenol Linalool β-ionone

PDMS

feed conc (mmol/L)

separation factor

partial flux (kg/m2‚h)

separation factor

partial flux (kg/m2‚h)

2.10 × 10-3 2.77 × 10-5 1.04 × 10-4 7.99 × 10-4 1.27 × 10-3 2.30 × 10-4 4.27 × 10-4 2.60 × 10-5

22 7 69 26 24 900 31 1787

6.95 × 10-7 1.60 × 10-9 7.42 × 10-8 3.79 × 10-7 5.03 × 10-7 2.44 × 10-6 3.16 × 10-7 1.25 × 10-6

23 65 7 117 74 1113 173 990

1.05 × 10-7 2.13 × 10-9 1.13 × 10-9 2.48 × 10-7 2.25 × 10-7 4.42 × 10-7 2.51 × 10-7 9.98 × 10-8

Table 7. Threshold Values and Odor Active Values odor active value permeate

compound

sensory attribute

threshhold limit (mg/L)

feed

POMS

PDMS

trans-2-hexenal 2-methyl propanal 3-methylbutanal Phenyl acetaldehyde benzyl alcohol Linalool Cis-3-hexenol β-ionone

green apple chocolate/malty chocolate/malty rosy sweet lemon green grass woody

0.017 0.0012 0.0016 0.004 1 0.006 0.07 0.000 007

12.12 1.67 5.63 24 0.14 11 0.33 714.3

266 11 387 616 3 342 291 1 163 968

280 101 40 2807 10 1895 360 644 813

concentration of the aroma compounds from the tea extract generated in the manufacturing of instant tea. Table 6 shows the permeate concentrations, separation factors, and organic fluxes for all of the compounds studied at their feed concentration. There are large differences in the fluxes and separation factors of the compounds from the results obtained in the binary and model multicomponent solutions. This is because actual tea aroma contains more than 600 compounds, some of which are volatile and others nonvolatile. This makes the comparison with binary and multicomponent solutions difficult. Moreover, the concentrations of all of the remaining components are not known. We tried to predict the actual tea aroma fluxes from the binary and multicomponent data, but we found large differences in the calculated and predicted values. However, there is

clear trend of higher separation factors for the alcohols, the reason for which is again their hydrogen-bonding capability. The increase is much higher than in the model aroma mixture (exception linalool in POMS), which might be due to the presence of other unknown alcohols in the tea extract that aid the sorption of alcohol molecules. The aldehydes show mixed behavior, with some showing an increase and others showing a decrease in separation factor. Such behavior is difficult to explain. However, it is clear from the separation factor values that both POMS and PDMS yielded very good separations, with PDMS providing better separation than POMS. Effect on Flavor Profile. The results given in Tables 3, 5, and 6 indicate that pervaporation provides

Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6931

a good separation. However, the separation factors and fluxes for the eight compounds studied vary significantly. Consider the data given in Table 6. These results were obtained from pervaporation studies performed on a real tea solution. The separation factors for the compounds range from as low as 7 for 2-methyl propanal to 1787 for β-ionone. Similarly, the flux varies from 1.6 × 10-9 to 1.25 × 10-6 kg/(m2‚h). This can result in a product that has a sensory profile that is different from that of the feed. The importance of the contribution of each compound to the overall sensory profile of a feed system is typically judged by an odor active value (OAV) analysis. The odor active value is defined as the ratio of the concentration of a compound in a given food system to its odor threshold value. The threshold value of volatile compounds is the lowest concentration at which a compound can be detected by smell in its pure form. The threshold values and the odor active values for the eight compounds in the feed and permeate for the two membranes are reported in Table 7. If the odor active value of 2-methyl propanal in the permeate is lower than in the feed, it would result in lower chocolate/malty notes. In the tea sample selected, β-ionone had a very high odor active value compared to the other compounds and thus imparted an overall “woody” note to the tea. It is also the compound that has the highest flux and separation factor. Thus, the product obtained from pervaporation also has a very high odor active value for β-ionone, implying that the chief sensory attribute of the product would also be a woody note. The small changes in the OAVs of other compounds would result in a slight change in the overall profile, but the woody note would still dominate. Thus, although the profile would change at a molecular level, its sensory impact in this case would be minimal. However, this was only a chance case and might not happen all the time. Had 2-methylpropanal been the dominant contributor, the product would have been very different from the feed in sensory terms. The separation factors for various compounds are bound to be different, regardless of the concentration technology. Thus, the application of any separation technology should be evaluated from a sensory viewpoint and not just according to the degree of concentration it provides. The suitability of a technology would finally depend on the degree of concentration that it would provide within an acceptable range of sensory profile. Conclusion Permeation studies for eight tea aroma compounds, namely, trans-2-hexenal, 2-methylpropanal, 3-methylbutanal, phenylacetaldehyde, cis-3-hexenol, linalool, benzyl alcohol, and β-ionone, in binary, model multicomponent, and actual tea extract systems were conducted. It was found that, generally, alcohols showed higher separation factors when present in model multicomponent solutions than in binary systems with water. On the other hand, aldehydes showed an opposite trend. The actual tea aroma mixture showed a quite different behavior from the model aroma mixture, probably because of the presence of very large numbers of unknown compounds. Overall, the PDMS membrane with vinyl end groups used in this work showed higher separation factors and fluxes for most of the aroma compounds. Pervaporation is an attractive technology

for the recovery of tea aroma compounds from tea aroma condensate as it (i) yields good separations and (ii) operates under mild conditions. However, the wide range of selectivities for the different aroma compounds will result in alterations in the profile of the aroma recovered in the permeate. Commercial applicability of this technique will need to be evaluated on a case-bycase basis. Literature Cited (1) Voilley, A.; Schmidt, B.; Simatos, D.; Baudron, S. Extraction of aroma compounds by pervaporation technique. In Proceedings of the 3rd International Conference on Pervaporation Processes in the Chemical Industry; Bakish, R., Ed.; Bakish Materials Corporation: Englewood, NJ, 1988; p 429. (2) Karlsson, H. O. E.; Loureiro, S.; Tragardh, G. Aroma compound recovery with pervaporationsTemperature effects during pervaporation of Muscat wine. J. Food Eng. 1995, 26, 177. (3) Rajagopalan, N.; Cheryan, M. Pervaporation of grape fruit aroma. J. Membr. Sci. 1995, 104, 243. (4) Karlsson, H. O. E.; Tragardh, G. Pervaporation of dilute organic-water mixtures: A literature review on modeling studies and applications to aroma compound recovery. J. Membr. Sci. 1993, 76, 121. (5) Schafer, T.; Bengston, G.; Pingel, H.; Boddeker, K. W.; Crespo, J. P. S. G. Recovery of aroma compounds from wine-must fermentation by organophilic pervaporation. Biotechnol. Bioeng. 1999, 62, 412. (6) Voilley, A.; Lamer, T.; Nguyen, T.; Simatos, D. Extraction of aroma compounds by pervaporation. In Proceedings of the 4th International Conference on Pervaporation Processes in the Chemical Industry; Bakish, R., Ed.; Bakish Materials Corporation, Corporation: Englewood, NJ, 1989; p 332. (7) Voilley, A.; Charbit, G.; Gobert, F. Recovery and separation of 1-octene-3-ol from aqueous solutions by pervaporation through silicone membrane. J. Food Sci. 1990, 55, 1399. (8) Lamer, T.; Voilley, A. Influence of different parameters on the pervaporation of aroma compounds. In Proceedings of the 5th International Conference on Pervaporation Processes in the Chemical Industry; Bakish, R., Ed.; Bakish Materials Corporation, Corporation: Englewood, NJ, 1991; p 110. (9) Netke, S. A.; Sawant, S. B.; Joshi, J. B.; Pangarkar, V. G. Sorption and permeation of aqueous picolines in elastomeric membranes. J. Membr. Sci. 1994, 91, 163. (10) Sampranpiboon, P.; Jiraratananon, R.; Uttapap, D.; Feng, X.; Huang, R. Y. M. Separation of aroma compounds from aqueous solutions by pervaporation using poly-octylmethylsiloxane (POMS) and poly-dimethylsiloxane (PDMS) membranes. J. Membr. Sci. 2000, 174 (1), 55. (11) Zhang, S. Q.; Matsuura, T. Recovery and concentration of flavor compounds in apple essence by pervaporation. J. Food Proc. Eng. 1991, 14, 291. (12) Baudot, A.; Marin, M. Pervaporation of aroma compounds: Comparison of membrane performances with vaporliquid equilibria and engineering aspects of process improvement. Trans Inst. Chem. Eng. C 1997, 75, 117. (13) Bengtsson, E.; Tragardh, G.; Hallstrom, B. Recovery and concentration of apple juice aroma compounds by pervaporation. J. Food Sci. 1989, 10, 65. (14) Baudot, A.; Souchon, I.; Marin, M. Total permeate pressure influence on the selectivity of the pervaporation of aroma compounds. J. Membr. Sci. 2002, 142 (1), 129. (15) Kaschemekat, J.; Wijmans, J. G.; Baker, R. W. Removal of organic solvent contaminants from industrial effluent streams by pervaporation. In Proceedings of the 4th International Conference on Pervaporation Processes in the Chemical Industry; Bakish, R., Ed.; Bakish Materials Corporation, Corporation: Englewood, NJ, 1989; p 321. (16) Sakata, K. Tea chemistry with a special reference to aroma precursors. In Global Advances in Tea Science; Jain, N. K., Ed.; Aravali Books International Pvt. Ltd.: New Delhi, India, 1999; p 693. (17) Yamanishi, T. Tea Flavour. In Global Advances in Tea Science; Jain, N. K., Ed.; Aravali Books International Pvt. Ltd.: New Delhi, India, 1999; p 707.

6932 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 (18) Mandal, S.; Pangarkar, V. G. Effect of thermodynamics and morphological parameters on the separation of methanolbenzene and methanol-tolune mixtures by pervaporation. J. Memb. Sci. 2002, 201, 175. (19) Arctander, S. Perfume and Flavor Chemicals (Aroma Chemicals), 1st ed.; Published by the Author: Montclair, NJ, 1969. (20) Grulke, E. A. Solubility parameter values, In Polymer Handbook; Immergut, J., Grulke, E. A., Eds.; Wiley-Interscience Publications: New York, 1975; Vol. VII, p 675.

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Received for review July 24, 2003 Revised manuscript received October 6, 2003 Accepted October 9, 2003 IE0340185