Biotechnol. frog. 1995, 11, 214-220
214
Fractionation of Citrus Oils Using a Membrane-BasedExtraction Process Daniel J. Brose,?Mark B. Chidlaw,*,*Dwayne T. Friesen? Ed D. LaChapelle,*and Paul van EikerenG Bend Research, Inc., 64550 Research Road, Bend, Oregon 97701, Chemica Technologies, 20340 Empire Avenue, Suite E-9, Bend, Oregon 97701-5711, and Sepracor, Inc., 33 Locke Drive, Marlborough, Massachusetts 01752
A membrane-based extraction process is described for the fractionation of citrus oils to produce an oil stream enriched in oxygenated components. The process relies on the high selectivity of cyclodextrins (CDs) to preferentially bind the desirable oxygenates. In the process, low-temperature citrus oil flows on one side of a nonporous membrane, a n d an aqueous CD solution flows on the other side of the membrane. Oxygenates diffuse through the water-swollen hydrophilic membrane a n d preferentially partition into the aqueous CD solution. The aqueous CD solution is then heated a n d circulated past a second membrane, where the CD/oxygenate complex dissociates and the liberated oxygenates diffuse into a citrus oil solution. Disassociated CD is recycled to the first membrane. Data a r e presented on the effect of temperature and CD type on the partitioning of oxygenates from orange oil into aqueous CD solutions. Operation of a pilot-scale test loop t h a t capitalizes on the high selectivity of CDs and the temperature dependence of t h e CD binding is described for the production of oxygenate-enriched orange oil.
Introduction Citrus oils are obtained from the pressed peels of citrus fruits such as oranges, lemons, and limes. Typically, the pressed oils contain 96-98 wt % terpene hydrocarbons (mainly d-limonene) and 1-3 wt % oxygenates (aldehydes, alcohols, and esters). The terpenes contribute only slightly to the flavor o r fragrance of the oil, and they rapidly oxidize to form undesirable components; the oxygenates are the main flavor components of the oil (Temelli et al., 1988). It is common industrial practice to remove some or all of the terpenes to concentrate the oxygenates in the oil. These concentrated oils are referred to as folded oils. For example, a &fold oil is one that has been concentrated to one-fifth its original weight; it would typically contain 5-10 wt % oxygenates. Folded citrus oils are used by flavor chemists to impart natural flavor, freshness, and fragrance to beverages, icings, fruit snacks, and confections. Folded citrus oils are more stable during storage than raw citrus oils. Typically, folded oils are produced by vacuum distillation (Matthews and Braddock, 1987; Vora et al., 1983), but they have been produced by other methods, such as steam distillation, solvent extraction (Owusu-Yaw et al., 1986), adsorption (Kirchner and Miller, 19521, and supercritical fluid extraction (Temelli et al., 1988). However, these technologies have drawbacks. For instance, the heat required for distillation causes chemical changes in the oil, resulting in flavor notes in the folded product that are not present in the original oil (Braddock, 1980). The extraction and adsorption processes may introduce impurities into the folded oil, and supercritical fluid extraction has not been demonstrated to have sufficient selectivity to make it competitive (Temelli et al., 1988). We describe a new membrane-based process for the Chemica Technologies. Bend Research, Inc. 9 Sepracor, Inc. +
concentration of oxygenates in citrus oils (van Eikeren et al., 1991). This process relies upon the selectivity of inclusion complex formation between the oxygenates in the citrus oil and cyclodextrin (CD), a cyclic oligosaccharide with unique complexation properties (Szejtli, 1988). CDs are starch-derived macrocyclic polymers that contain glucose units attached in a ring structure. CDs can have either six glucose units (a-CD), seven glucose units (pCD), or eight glucose units (yCD). The internal diameters of a-, p-, and y-CD molecules are approximately 0.45, 0.70, and 0.85 nm, respectively (Szejtli, 1982). All glucose units are aligned such that the primary hydroxyls point toward one opening of the ring and the secondary hydroxyls point toward the other opening of the ring. This alignment gives CD molecules the shape of a truncated cone, or torus, with the wider opening formed by the secondary hydroxyls and the narrower opening formed by the primary hydroxyls. The locations of the hydroxyls make the exterior of the torus relatively hydrophilic and the interior, which is lined with CH groups, relatively hydrophobic (Saenger, 1980). CDs selectively form inclusion complexes (guesthost complexes) with molecules that have a size and polarity compatible with the inside of the hydrophobic CD cavity. The driving forces for the formation of inclusion complexes include (1)decrease of the ring strain resulting from complex formation and (2) removal of the highenergy water molecules from the CD cavity. It is welldocumented that in many cases CDs bind solutes better a t low temperatures than a t high temperatures; this is a result of the decrease in entropy upon complex formation (Szejtli, 1982; Gelb et al., 1979; Lammers et al., 1972). Due to the unique shape and chemical environment of the CD cavity, the formation of guesthost complexes offers a powerful tool for chemical separations. This usefulness of CDs has been clearly demonstrated on the analytical scale, where CD-bonded phases are routinely used for the separation and analysis of many chemicals, including flavor compounds, structural iso-
8756-7938/95/3011-0214$09.00/0 0 1995 American Chemical Society and American Institute of Chemical Engineers
215
Biotechnd. Pmg.., 1995, Vol. 11, No. 2 Oxygenate-
Hydrophmc Membrane
Hydrophilic
Ch
Enriched Orange Oil
-Heater
Figure 1. Membrane-based extraction process for the production of oxygenate-enriched orange oil. Potting
Material
\
Aqueous CD Solution out
Tr Orange-Oil Solution out
Orange-011 Solution In
\ Fibers Hollow
fl Aqueous CD Solution In
Figure 2. Hollow-fiber membrane contador module. mers, stereoisomers, and amino acids (Cyclobond Handbook, 1987;Szejtli, 1988). The membrane-based extraction process is shown conceptually in Figure 1 for the production of oxygenateenriched orange oil; it works as follows. Raw orange oil at low temperature is circulated on one side of a nonporous hydrophilic membrane (i.e., a membrane contactor). A cold aqueous CD solution is circulated on the opposite side of the membrane. The oxygenates from the orange oil diffise through the membrane from the cold orange oil phase to the cold aqueous CD phase, where the oxygenates preferentially form inclusion complexes with the CD. The aqueous CD solution, which becomes enriched in oxygenates, is then heated and circulated past a second membrane contactor, where it is contacted with hot raw orange oil. At the elevated temperature in the second membrane contactor, the oxygenates partition preferentially from the aqueous CD phase into the orange oil phase. As a result, there is a net transfer of oxygenates from the cold orange oil feed to the hot orange oil product solution. The nonporous hydrophilic membranes used in this process are wetted only by the aqueous CD solution and not by the orange oil solutions; thus, the membrane acts as a physical barrier that prevents the bulk mixing of aqueous and organic phases (Lee et al., 1976;Cooney and Jin, 1985). The membranes are configured as hollow fibers and arranged in modular form, as shown in Figure 2. In this configuration, the aqueous CD solution flows through the module on the exterior (or shell side) of the hollow fibers; the orange oil flows in the interior (or tube side) of the hollow fibers. Use of membrane contactors in the membrane-based extraction process offers the following potential advantages: (1)because the membrane acts as a physical barrier between the organic and aqueous phases, phase entrainment and the loss of CD to the organic phase are
minimized; (2) the flow rates of the aqueous and organic phases can be independently controlled, allowing the process to be semicontinuous; and (3)the membrane process is modular, simplifying scale-up and allowing modules to be combined in hybrid processes, if desired.
Materials and Methods Materials. CD derivatives were obtained from the
following sources: a-CD, /I-CD, and hydroxypropyl-/I-CD from American Maize Products Co. (Hammond, IN); maltosyl-CD (Isoeleat, mixture of 60% maltosyl-a-CD, and 30% maltosyl-/I-CD); and 10% maltosyl-y-CD from Ensuiko Sugar Refining Company (Yokohama, Japan). The orange oil used was cold-pressed, California-type orange oil from a single lot purchased from Sunkist Growers, Inc. (Sherman Oaks, CA). The composition of the orange oil, which contained 1.2 f 0.05 wt % oxygenates, did not change over the course of the study. The hollow-fiber modules contained regenerated cellulose membranes and were manufactured by Baxter Travenol (capillary flow dialyzer, Model 15.11,Deerfield, IL); they contained 1.0 m2 of membrane area. The original polycarbonate end caps on the modules were replaced with solvent-resistant end caps made of CPVC. Analytical M e t h o d s . Orange-oil components were analyzed by gas chromatography (GC) using a modification of a method reported in the literature (Johnson and Vora, 1983). Neat orange oil samples were diluted 50fold with hexane that was spiked with 1000 ppm octane as an internal standard; this was injected directly onto a 60-m fused-silica capillary GC column containing methylsilicone as the stationary phase. The temperature of the column was held at 40 "C for 11 min, raised from 40 "C to 200 at 6 "C/min, raised from 200 "C to 250 "C at 20 "C/min, and then held at 250 "C for 6 min. Solutes were detected using a flame-ionization detector operated at 250 "C. Using this method, we achieved baseline
Biotechnol. Prog., 1995, Vol. 11, No. 2
216 Table 1. Terpenes and Oxygenates Detected in Orange Oil Using the GC Procedure retention time (min) 22.9 24.3 24.5 24.9 25.0 26.5 27.5 28.6 30.1 31.4 31.7 32.6 33.3
terpene a-pinene sabinene /3-pinene
oxygenate
octanal myrcene limonene y-terpinene linalool citronellal a-terpineol decanal neral geranial
resolution of each of the terpenes and oxygenates shown in Table 1. Although these compounds represent only a fraction of those reported in orange oil, they are the major components and represent from 97% to 99% of the orange oil solutes (Mathews and Braddock, 1987). Partitioning Studies. The partitioning of organic solutes between orange oil and a n aqueous CD solution was determined by contacting the aqueous CD solution with an equal volume of raw orange oil. The mixture was equilibrated by vigorous mixing at a controlled temperature (ranging from 2 to 60 "C) for 10 min; the phases were then separated by centrifugation. The concentration of terpenes and oxygenates in the aqueous CD solution a t equilibrium was determined by first extracting organic solutes from the aqueous solution with hexane and then analyzing the extract by GC. In these experiments, each of the following CD derivatives was tested a t the indicated concentrations: a-CD (1 wt %), P-CD (2 wt %), hydroxypropyl-P-CD (5 wt %), and maltosyl-CD (5 wt %). Extraction Studies. Orange oil was enriched through multiple temperature swing extraction cycles with aqueous CD solutions using the following procedure. Raw orange oil (200 mL) a t 2 "C was mixed vigorously with 200 mL of 5 wt % CD in water (also at 2 "C). The phases were then separated, and the oxygenate-enriched CD solution was heated to 60 "C and mixed vigorously with 5 mL of fresh raw orange oil (also a t 60 "C). The phases were separated, and the oxygenate-depleted CD solution was again chilled and used to extract oxygenates from the original 200 mL of cold raw orange oil. By using this temperature swing cycle repeatedly, the oxygenates in the 200 mL of cold orange oil feed were depleted and the oxygenates in the 5 mL of hot orange oil product solution were enriched. This procedure was used with the following CD derivatives at the indicated aqueous concentrations: a-CD (2 wt %), P-CD (2 wt %), hydroxypropyl-P-CD (5 wt %), and maltosyl-CD (5 wt %). Permeability Studies. The permeability of solutes through the regenerated cellulose membrane contactor was determined using the following typical procedures. The orange oil reservoir was filled with 1500 mL of orange oil maintained a t 5 "C; this solution was pumped through the interiors of the hollow fibers a t 600 mumin. The aqueous CD reservoir was filled with 1000 mL of a solution of 5 wt % maltosyl-CD in water, maintained a t 5 "C; this solution was pumped on the exterior of the hollow fibers at 100 mumin. The pressure of the orange oil solution was maintained a t least 10 psi higher than that of the aqueous CD solution to prevent water from leaking through the hydrophilic membrane (Sirkar, 1988). In related experiments, the membrane contactor was tested for recovery of oxygenates from the aqueous CD solution into an orange oil product solution a t 60 "C.
Over the course of the experiments, the concentrations of organic solutes (terpenes and oxygenates) in the orange oil and aqueous CD solutions were measured using GC. Permeabilities were estimated by fitting the experimental data to calculated concentration versus time profiles. The calculated profiles were based on a mathematical model that describes the time dependence of solute concentrations in the feed or product reservoir as a function of the experimental conditions. Specifically, the mathematical model is based on the combination of a fundamental membrane transport equation with overall mass balance equations. The fundamental membrane transport equation, shown in eq l, states that solute flux (J,g/cm2/h)equals the product of permeability (P, cm/h) and the solute concentration difference across the membrane (AC, g/cm3):
J = PAC
(1)
[For discussion of this and related mass transfer equations, see Cussler (198411. In this equation, permeability is the overall mass transfer coefficient that combines the effect of mass transfer resistances due to diffusion across the oil boundary layer, the polymeric membrane, and the aqueous boundary layer. The solute concentration difference (AC) equals the solute (oxygenate) concentration in the orange oil feed solution (C*,il) minus the solute concentration in the aqueous CD solution (Caq),where is defined as an aqueous concentration that would be in equilibrium with the oil phase. The equilibrium relationship used to define C*"il in terms of the actual solute concentration in the oil phase (Coil)is '*oil
= CoiPoiliaq
(2)
This relationship, when combined with eq 1, results in the flux equation (Cooney and Jin, 1985):
J = P(Coil/Doi~aq - Caq)
(3)
where Doivsqis the oillaqueous distribution coefficient. Equation 3 is combined with the overall mass balance equation, Coil,t=oVoil = Coilvoil
+ Caqvaq
(4)
and with a differential equation describing mass balance in the oil reservoir.
to result in the following set of equations that relate solute concentrations in the oil and aqueous reservoirs as functions of operating time (these equations are for transport from a feed oil solution to a product aqueous solution):
and
where
Biotechnol. frog., 1995, Vol. 11, No. 2
217 fll
Aqmoua CD Product Rmorrdr
Hot-ON
Cdd-61 C o n Pump
Cow Pump
Figure 3. Pilot-scale test loop for the production of oxygenate-enriched orange oil.
t is time (h), A is membrane area (cm2), and the other variables are the same as defined earlier. When the transport of solute is from an aqueous feed solution to an oil product solution, as is the case during regeneration of the aqueous CD solution by stripping with an oil solution, a similar procedure is used to derive the following equations that relate solute concentrations in the oil and aqueous reservoirs as hnctions of operating time:
and
where
Therefore, for an experiment in which oxygenates were transported from an orange oil feed solution to a n aqueous CD product solution, permeabilities were determined by adjusting the value of P in eqs 6-8 until the curves (represented by eqs 6 and 7) provided a best fit to the data. Values for the distribution coefficient (Doivaq) were determined from the final equilibrium concentrations in the aqueous and oil phases. Similarly, for a n experiment in which oxygenates were transported from an aqueous CD feed solution to a n orange oil product solution, permeabilities were determined by adjusting the value of P in eqs 9-11 until the curves provided the best fit to the data. Pilot-PlantStudies. The pilot-scale test loop shown schematically in Figure 3 was built and operated. During operation, a n aqueous solution (2000 mL) containing 5 wt % maltosyl-CD was pumped on the exterior of hollow fibers in two modules; each module contained 1 m2 of regenerated cellulose membrane. One module was operated at 5 "C to extract oxygenates from the orange oil feed (1500 mL) and to load the circulating aqueous CD solution with oxygenates; the other module was operated at 60 "C to strip the oxygenates from the aqueous CD solution into the raw orange oil product solution (125 mL). The oil streams were circulated through the fiber interiors at a flow rate of 300 m u m i n and were main-
tained at pressures of 10-15 psi. The aqueous maltosylCD solution was circulated between the two modules at a flow rate of 200 m u m i n and maintained at a pressure of less than 5 psig.
Results and Discussion Partitioning Studies. In these studies, we measured the effect of both CD type and temperature on the partitioning of oxygenates between orange oil and aqueous CD solutions. We tested four different CD derivatives (a-CD, p-CD, maltosyl-CD, and hydroxypropyl-pCD) in water to determine their effectiveness in extracting oxygenates from orange oil. The temperature used in these experiments was varied from 2 to 60 "C, and from these experiments, we determined the effects of CD derivative type and temperature on oxygenate partitioning. The results of the partitioning studies are summarized in Figure 4, which shows the total oxygenate concentration in the aqueous phase as a function of temperature for each CD derivative tested. As shown by the bottom line in Figure 5, when no CD is added to the aqueous solution, the partitioning of oxygenates into water is low and is not affected by temperature. When CD derivatives are added to the aqueous phase, the partitioning of oxygenates into water increases dramatically, and the extent of partitioning is greater at lower temperatures, which is consistent with a decrease in entropy upon formation of the CD inclusion complex. Oxygenate partitioning as a function of temperature is given by the slopes of the lines in Figure 4. On this basis, the temperature dependence for oxygenate partitioning increases in the following order, with the slope of the corresponding line in Figure 4 shown in parentheses: p-CD (0.8ppm/"C) < a-CD (1.9 ppm/"C) < hydroxypropylp-CD (2.7 ppm/"C) < maltosyl-CD (5.1 ppm/"C). This order of temperature dependence undoubtedly is due to a number of factors, including the concentration of the CD derivative, the physical size of the CD cavity, and the chemical nature of the CD and its pendant groups. Hydroxypropyl-p-CD and maltosyl-CD were tested at higher concentrations than a-CD and p-CD due to their higher solubility in water; this likely contributes to their superior performance. The reasons for the near doubling in temperature dependence for maltosyl-CD versus hydroxypropyl-p-CD are not clearly understood. Analysis of this difference is complicated by the complex nature
Biofechnol. Prog., 1995,Vol. 11, No. 2
218
500 I
I
I Aqueous oxygenate 300 Concentration bpm)
-
o 5 Wm Maltowl-CD
II
cycle
\
initial 1 2 3
200
oxygenate concentrate oxygenate concentration in hot orange oil product solution (wt %) in cold orange oil feed (wt %)
1.23 1.15 1.13 1.09
1.23 2.62 3.34 4.81
Table 3. Oxygenate Concentrations in Hot Orange Oil Product Solutions after Three Temperature Swing Extraction Cycles Using Various CD Derivatives
100
0
I
I
20
40
60
Temperature PQ
Figure 4. Effect of temperature on the partitioning of oxygenates from orange oil into various aqueous CD solutions (equal volumes of oil and aqueous solutions were used in partitioning experiments; orange oil contained 1.2 wt % oxygenates).
Oxygenate Concentration In Aqueour CD Product Reaervoir lppm)
Table 2. Oxygenate Concentrations in Cold Orange Oil (200 mL) and Hot Orange Oil (5 mL) Using Temperature Swing Extraction with 5 w t % Maltosyl-CD
300
1
f
Bared On Beat-Fit CUN. Eqn. 7 and P=O.O9 cmlhr
?B OB
i
I
0
100
200
-Erparlmenlal
Data
loo
300
Time (min)
Figure 5. Time course data for the transport of oxygenates from orange oil into 5 wt % maltosyl-CD using a regenerated cellulose membrane contactor operating at 5 "C (concentrations shown are those in the aqueous CD product reservoir).
of orange oil and the likely interaction between components upon their extraction into aqueous CD solutions. Extraction Studies. As Figure 4 shows, aqueous CD solutions extract more oxygenates from citrus oils a t lower temperatures than a t higher temperatures. This property makes it possible to produce folded citrus oils by extracting oxygenates into an aqueous CD solution a t low temperature and then back-extracting oxygenates into a n orange oil product solution at high temperature (i.e., using a temperature swing process). We conducted multiple temperature swing extractions wherein oxygenates were extracted from an orange oil feed into an aqueous CD solution at low temperature, and then the oxygenates were back-extracted from the CD solution to an orange oil product solution a t high temperature, as described in the Materials and Methods section. Table 2 shows the oxygenate concentrations in the cold orange oil feed and hot orange oil product solution after each temperature swing cycle for a n experiment in which 5 wt % maltosyl-CD was used. The results in Table 2 show that the oxygenate concentration of the orange oil can be increased 4-fold after only three cycles of the temperature swing process. We performed similar temperature swing experiments using other CD derivatives. The final oxygenate concentrations after three extraction cycles are summarized in Table 3. This table shows that the other CD derivatives
CD derivative 2 wt % /3-CD 5 wt % hydroxypropyl-P-CD 2 wt % a-CD
5 wt % maltosyl-CD
oxygenate concentration (wt 70) 1.6 2.4 2.8 4.8
also effectively extract oxygenates, although maltosylCD produces the greatest oxygenate enrichment after three temperature swing cycles. These results are consistent with those reported in Figure 4. On the basis of these results, we used maltosyl-CD in subsequent experiments. Permeability Studies. In these studies, we measured the permeability of oxygenates through regenerated cellulose membrane contactors. Specifically, we measured the oxygenate permeability a t 5 "C for transport from orange oil to an aqueous maltosyl-CD solution and a t 60 "C for transport from the aqueous oxygenateloaded maltosyl-CD solution into orange oil. Data obtained from operation of the test apparatus a t 5 "C are shown in Figure 5, which shows the concentration of oxygenates in the aqueous CD product reservoir as a function of time. Also shown in this figure is the bestfit curve corresponding to the theoretical concentration versus time profile represented by eq 7 . This equation was fitted to the experimental data using membrane permeability (P)as the adjustable parameter; this is the method by which permeability is determined. As can be seen from the plot in Figure 5 , the oxygenate concentration in the 5 wt % maltosyl-CD solution increased to a n equilibrium concentration of about 480 ppm after 3 h, and the oxygenate permeability that provided the best fit of eq 7 is 0.09 c&. In a separate experiment, we operated the test apparatus to back-extract oxygenates from the aqueous CD solution into a raw product orange oil a t 60 "C. Figure 6 shows that the oxygenate concentration in the maltosylCD solution rapidly decreased to a n equilibrium concentration of about 130 ppm in less than 60 min, and the oxygenate permeability that results in the best fit of eq 10 is 0.66 cmih. From similar experiments, we determined oxygenate permeabilities for operation of the test loop a t higher aqueous flow rates; all permeability data are summarized in Table 4. As this table shows, the oxygenate permeability is about 7 times higher at 60 "C than a t 5 "C, and operation at a n aqueous flow rate of 300 m u m i n results in about a 15% improvement in oxygenate permeability a t a temperature of 60 "C. The increase in aqueous flow rate makes no difference in permeability for operation a t 5 "C. The temperature dependence of permeability, as shown in Table 4, is consistent with the mechanism of permeation being primarily by molecular diffusion, which is highly temperature-dependent. On the basis of the permeability results shown in Table 4,a continuous process to produce folded orange oil could
Biotechnol. Prog., 1995, Vol. 11, No. 2
219
" Oxygenate
Concontr8tlon In Aquoous CD Feed RO8WVOlr IppmJ
B..t.fii
CUNI
4
B.ud On Eqn. I U and W . S S cmlhr
2
100
/
I
O 0
20
40
60
80
100 120 140
0
I
I
I
50
100 Time Qlr)
150
Tim. lmlnl
Figure 6. Time course data for the transport of oxygenates from 5 w t % maltosyl-CD solution into orange oil using a regenerated cellulose membrane contactor operating at 60 "C (concentrations shown are those in the aqueous CD feed
reservoir).
Table 4. Summary of Permeability Results for the Transport of Oxygenates through a Regenerated Cellulose Membrane Contactor flow rates (mumin) oxygenate temDerature ("C) aaueous CD orange oil Dermeabilitv ( c m h ) 5 5 60 60
100 300 100 300
600 600 600 600
0.09 0.08 0.66 0.77
be designed such that the hot module has one-seventh the membrane area of the cold module. This would result in equal oxygenate transport across the two modules a t steady state, with equal oxygenate driving forces across the two modules. However, because the oxygenates in citrus oils are temperature-sensitive, it would be preferable to increase the size of the hot module to minimize the residence of oxygenates in the hot loop of the system. Operation of Pilot-Scale Test Loop. We operated a pilot-scale test loop that combined cold loading and hot stripping of the aqueous maltosyl-CD solution to produce oxygenate-enriched orange oil. The test loop was operated continuously for several days. During this time, oxygenate concentrations in the cold orange oil feed and the hot orange oil product solution were monitored. Figure 7, which plots these concentration versus time data, shows that, during the first 120 h of operation, the oxygenate concentration in the hot orange oil product solution increased from 1.2 wt % to about 4.2 wt % and the oxygenate concentration in the cold orange oil feed decreased to about 0.8 wt %. The depleted feed oil was then replaced with fresh orange oil feed, and the rate of oxygenate transfer to the orange oil product solution increased significantly. The orange oil feed was replaced again a t about 170 h, which resulted in a subsequent, although smaller, increase in the rate of oxygenate transfer to the hot orange oil product solution. The test was concluded a t about 200 h, a t which time the orange oil product solution contained 6.24 w t % total oxygenates-more than a 5-fold increase in oxygenates. The increase in the rate of oxygenate transport upon replacement of the orange oil feed solution (as shown in Figure 7) is due to a n increase in the oxygenate driving force with the addition of fresh feed oil. As oxygenates are depleted from the feed oil, the rate of transport drops (corresponding to a drop in the driving force) until a new feed solution is added to increase the driving force. The repeated replacement of the feed solution, which approximately models a continuous-flow process, has di-
200
Figure 7. Oxygenate concentrationsin the orange oil feed and product solution as functions of time during the operation of
the pilot-scale test loop (regenerated cellulose membranes).
Table 5. Chemical Composition of the Feed and Product Orange Oil chemical
component a-pinene sabinene ,%pinene
octanal myrcene limonene y -terpinene linalool
citronellal
a-terpineol decanal neral geranial total oxygertates
initial feed oil ( w t %)
final product oil (wt %)
0.38 0.37 0.05 0.44 1.88 95.99 0.04 0.33 0.07 0.04 0.27 0.04 0.06 1.22
0.49 0.19 0.07 2.31 1.41 91.29 0.03 1.56 0.02 0.28 2.13 0.08 0.14 6.24
minishing returns with each cycle, as a n overall equilibrium is approached between the fresh feed oil and the highly enriched product oil, which is not replaced when the feed oil is replaced. Although this experiment is not meant to completely model a continuous process, it does point to some of the equilibrium-dictated limitations of a continuous process. Specifically, when this CD derivative is used a t the feed and product temperatures described, the maximum oxygenate concentration that can be obtained in the product oil is approximately 6.5%. Table 5 summarizes the chemical compositions of the initial orange oil feed and the final orange oil product solution. As this table shows, all of the terpenes, except a- and P-pinene, were decreased in the final product orange oil, and all of the oxygenates were increased in the product oil. Furthermore, not all of the oxygenates increased in the same ratio, e.g., decanal increased 7.9fold, a-terpene01 increased 7.O-fold, octanal increased 5.2fold, and linalool increased 4.7-fold. Finally, although our GC analysis did not examine the effect of the process on enantiomeric enrichment of the oxygenates, it is likely that there was enrichment of oxygenate enantiomers due to the chirality of CD. Further analysis, both sensory and analytical, is needed to examine the effects of CD type, temperature, and residence time on the oil quality and its suitability for use as a flavoring agent.
Conclusions We investigated the use of a membrane-based extraction process to enrich the oxygenates in orange oil; maltosyl-CD was used in the aqueous phase as a selective complexing agent for the oxygenates. Our successful operation of the pilot-scale test loop to produce oxygenate-
Biotechnol. Prog., 1995, Vol. 11, No. 2
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enriched orange oil demonstrates the feasibility of using CD for large-scale fractionation of essential oils. In addition, the use of a membrane for organidaqueous phase contacting can improve the process in the following ways: (1)the membrane allows independent control of aqueous and organic flow rates, thereby making the process semicontinuous; (2) the membrane allows compartmentalization of the phases, avoiding phase entrainment; and (3) the membrane process is modular, allowing combination of modules in hybrid processes. However, the CD-based process is limited in its application to the production of oxygenate-enriched orange oils by the presence of a n upper limit on oxygenate concentration that can be attained-about 6.5%. In comparison to vacuum distillation, the most widely used method t o produce oxygenate-enriched orange oil, the CD-based process has a disadvantage in that it cannot produce highly enriched oxygenates from orange oil. A potential advantage of the membrane process is that oil is exposed to high temperatures for a shorter time than in vacuum distillation. However, further testing with a continuous membrane process is needed before this conclusion can be stated definitively.
Acknowledgment This work was supported in part through a Small Business Innovation Research (SBIR) contract from the U.S. Department of Energy (Contract No. DE-AC0387ER80467). We thank Dr. Mike Auerbach a t Pfizer Specialty Chemicals R&D for his assistance in obtaining sensory analyses of the enriched orange oil samples.
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Abstract published in Advance ACS Abstracts, March 1,1995.
