Environ. Sci. Technol. 2010, 44, 4019–4025
Microwave Assisted Extraction of Biodiesel Feedstock from the Seeds of Invasive Chinese Tallow Tree D O R I N B O L D O R , * ,† AKANKSHA KANITKAR,† BEATRICE G. TERIGAR,† CLAUDIA LEONARDI,‡ MARYBETH LIMA,† AND GARY A. BREITENBECK§ Department of Biological and Agricultural Engineering, Louisiana State University Agricultural Center, Baton Rouge, Louisiana 70803
Received January 14, 2010. Revised manuscript received April 17, 2010. Accepted April 19, 2010.
Chinese tallow tree (TT) seeds are a rich source of lipids and have the potential to be a biodiesel feedstock, but currently, its invasive nature does not favor large scale cultivation. Being a nonfood material, they have many advantages over conventional crops that are used for biodiesel production. The purpose of this study was to determine optimal oil extraction parameters in a batch-type and laboratory scale continuousflow microwave system to obtain maximum oil recovery from whole TT seeds using ethanol as the extracting solvent. For the batch system, extractions were carried out for different time-temperature combinations ranging from 60 to 120 °C for up to 20 min. The batch system was modified for continuous extractions, which were carried out at 50, 60, and 73 °C and maintained for various residence times of up to 20 min. Control runs were performed under similar extraction conditions and the results compared well, especially when accounting for extremely short extraction times (minutes vs hours). Maximum yields of 35.32% and 32.51% (by weight of dry mass) were obtained for the continuous and batch process, respectively. The major advantage of microwave assisted solvent extraction is the reduced time of extraction required to obtain total recoverable lipids, with corresponding reduction in energy consumption costs per unit of lipid extracted. This study indicates that microwave extraction using ethanol as a solvent can be used as a viable alternative to conventional lipid extraction techniques for TT seeds.
1. Introduction Chinese tallow tree (TT), Triadica sebifera, is an ancient and valuable oil seed-producing tree with a history of large scale commercial production in China and other parts of Asia. Although it is considered as an invasive species in the U.S., it has been regarded as a promising candidate for biomass production, due to its ability to resprout, its rapid growth rate and tolerance to both salt and drought (1). It is tolerant * Corresponding author phone: 225-578-7762; fax: 225-578-3492; e-mail:
[email protected]. † Biological & Agricultural Engineering Department, LSU Agricultural Center; ‡ Present address: Pennington Biomedical Research Center. § Present address: School of Plant, Environmental & Soil Sciences, LSU Agricultural Center. 10.1021/es100143z
2010 American Chemical Society
Published on Web 04/29/2010
of most soil conditions and is one of the few oil seed crops to tolerate high level of salts (2). It can thrive with no inputs of fertilizer or pesticides. A non food material, TT seeds are a good source of both a highly saturated fat as well as highly unsaturated oil (3). The seeds can contain more than 40% lipids (4), almost equally distributed in the external vegetable tallow coating and in the kernel as Stillingia oil (Figure 1), suitable for conversion into biodiesel (5, 6). Stillingia oil can also be utilized as drying oil (2) and vegetable tallow can be of interest to the confectionery industry due to the presence of 75% palmitic acid and 20-25% oleic acid (7, 8). Along with lipids, protein can also be separated from TT seeds (3, 9). Much less is known about the value-added products derived from TT seeds, though various products with some biological activity have been isolated from roots, barks, and leaves of the tree (10). It is one of nature’s most prolific producers of renewable hydrocarbons, yielding the equivalent of 500 gallons (12 barrels) of fats and oils per acre per year (4700 L · ha-1 · year-1), far exceeding other traditional oil seed crops (11). Among alternative, nonfood feedstocks considered for biodiesel production, Jatropha curcas is a fast growing, oil seed producing shrub that has received considerable attention. Similar to TT seeds, it has a high content (35-40%) of nonedible oil (12), low fertility requirements, and can be cultivated on saline, drought-prone, and marginal land without competing for food production (13-15). The tree’s physical appearance, its pest resistance, and colorful fall leaves make it a desirable ornamental, but its ability to compete against native plants (allelopathy) allows it to encroach on native lands giving it an invasive nature (16). Currently, its invasiveness does not necessarily favor large scale cultivation, as it tends to replace native species, resulting in large scale ecosystem modification. It can become dominant in vacant lots and abandoned agricultural lands, and once established it is almost impossible to eliminate it by known control methods. Owing to these factors, it was listed in “The Nature Conservancy’s list of America’s Least Wanted-The Dirty Dozen” (9). Means to control its dispersion must be considered in order to be used as a biomass and lipid feedstock. For example, a major pathway for invasiveness is via seed dispersion by birds which feed on seeds during winter, but only metabolize the external layer (kernel being eliminated in dropping). Removing the seeds as an energy crop eliminates this particular pathway, which would significantly reduce the species invasiveness. Various methods have been employed for extraction of oils from oilseeds (organic solvent, Soxhlet, supercritical fluid, ultrasound, mechanical pressing, aqueous) (17-23). However, the main commercial processes used for defattening and deoiling are mechanical pressing and organic solvent extraction. Disadvantages of these methods include long extraction times (Soxhlet extraction, ultrasonic extraction), low recovery (mechanical pressing), require large amounts of solvent (solvent extraction), are expensive (supercritical extraction) or energy intensive (pressurized liquid extraction) (20, 24, 25). One potential technique that can improve extraction efficiency of oil and other functional components from plant material is microwave processing that can either be used as a thermal pretreatment prior to solvent extraction or as a process enhancement during extraction (26). Recently, microwaves have been used to extract essential components, usually lipophilic (18, 21, 27, 28). In microwave assisted extraction (MAE), rapid generation of heat and pressure within the biological system force compounds out from VOL. 44, NO. 10, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. (a) Chinese TT seeds with coating and kernels and (b) seed cross section (USDA, 2000). biological matrices, producing good quality extracts with better target compound recovery (29, 30). Its advantages also include rapidity of extraction, lower energy consumption, reduced byproduct formation, and lower solvent usage (31). In conventional extractions, yields depend on components solubility in the extracting solvent, mass transfer kinetics, and solvent matrix interactions (32). In MAE, heating rate plays an important role. The extent to which a given matrix can absorb microwave energy is dependent on the dielectric properties of the plant material and extracting solvent. For a solvent-matrix to absorb microwaves, it has to preferably have a higher dielectric constant (potential for electric energy storage) as well as a higher dielectric loss (electric energy dissipation) (33). Microwave radiation can cause efficient internal (volumetric) heating of the mixture by interacting with the sample matrix at molecular levels via dipolar rotation and ionic conduction (34). At microwave frequencies dipoles in the sample align themselves with the direction of rapidly oscillating electric field and rotate, generating heat via frictional forces between randomly rotating polar molecules and surrounding media. In ionic conduction, dissolved charged particles oscillate with the changing electric field, dissipating their kinetic energy via friction as they slow down and change direction. This friction in turn leads to localized superheating and thereby high temperature and pressure gradients (35). Thus the major advantages of microwave assisted synthesis is that reaction can be performed quickly, efficiently, and safely (36). Hexane is most commonly used for oil extraction, but toxicological, environmental, and safety issues related to its usage (37) have led to a need for alternative solvents in MAE such as acetone, isopropanol, ethanol, methanol, and acetonitrile (29, 38, 39). The attractive features of using ethanol include its low cost, easy synthesis from a large variety of biological feedstocks, and less toxic nature, rendering the defatted protein-rich meal more suitable to be used as an animal feed (40). The present study was conducted to determine optimal oil extraction parameters using a batch type and a laboratory scale continuous flow microwave extraction system to obtain maximum oil recovery from whole TT seeds with ethanol as a solvent.
2. Materials and Methods 2.1. Sample Preparation. Tallow tree seeds were manually harvested from near Baton Rouge, LA between OctoberNovember 2008. After harvest, the seeds were air-dried for 2-3 days (∼5.2% mcdb) and processed through a thresher to separate seeds from adhering leaves and branches. These seeds were winnowed to remove any foreign material. The cleaned seeds were then ground using a blade-type coffee grinder and stored in freezer at -4 °C until further use. 2.2. Batch-Type Microwave Extraction of TT Seeds. Extractions were performed in an Ethos E batch microwave system (Milestone Inc., Monroe, CT), having a maximum power output of 1.6 kW. The system consisted of 250 mL 4020
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sealed Teflon(polyfluorotetraethylene) holders with magnetic stirrers and a built-in optical fiber temperature sensor for process monitoring and control. Twenty grams of ground TT seeds were placed in holders with 60 g of absolute, anhydrous ethanol (ACS/USP grade), (3:1 solvent:feedstock ratio). This mixture was subjected to microwave treatment at different temperatures ranging from 60-120 °C in increments of 20 °C for different extraction times (3, 9, 15, and 20 min). Heating ramp-up time was 5 min with a chamber vent time of 15 min. Yield at time zero (after ramp-up time) was extrapolated from data obtained at time zero in the CMAE process. 2.3. Continuous Microwave Assisted Extraction (CMAE) of TT Seeds. The batch system described above was modified for continuous operation (for details see Supporting Information (SI) page S2 and Figure S1). The mixtures were heated to 50, 60, and 73 °C at residence times of 4-20 min, at 4 min intervals. 2.4. Conventional Extraction (Control). The conventional extractions (CE) were carried out in a round-bottom flask equipped with a water condenser on a plate heater with a magnetic base. The flask was immersed in a constant temperature oil bath, and its content was mixed using a magnetic stirrer. Extractions were carried out at the same time-temperature combinations as the MAE and CMAE. Soxhlet extraction was also performed for 12 h to establish maximum oil content. 2.5. Solvent Separation and Oil Extraction. After extraction, samples were cooled and the oil-solvent mixture was vacuum-filtered through 1.2 µm Whatman filter paper (cat. no. GF/C 1822047). The solvent was evaporated in a vacuum centrifuge, and oil yield was computed as previously described (41, 42). Throughout the manuscript, lipid extraction yields are expressed as percent of dry mass. 2.6. Fatty Acid (FA) Composition. FA compositions were determined by quantifying the methyl esters using gas chromatography (IUPAC method 2.301). The detailed procedure in provided in SI page S2. 2.7. Statistical Analysis. Statistical analysis of oil yield data was carried out using SAS (version 9.1, SAS Institute, Cary, NC). Two-way ANOVA using Proc Mixed multiple comparison tests were performed by using Tukey-Kramer’s adjustment to determine significant differences between the two treatments at p < 0.05. Analysis was performed only across different times at the same temperatures and not among different temperatures. Statistical analysis of the batch vs continuous system was not performed due to the different residence times used.
3. Results and Discussions 3.1. FA Composition. FA composition of extracted oils was examined and lipid profiles were compared between both microwave technologies and Soxhlet extraction. Within the microwave technologies, the major component extracted was linolenic acid, followed by palmitic, oleic, and linoleic acids (Table 1). Eicosanoic acid was more predominantly present
TABLE 1. Composition of the Lipid Extracted Using Batch MAE and CMAE (Select Parameters Reported, Values Reported as %, unless Otherwise Noted) batch MAE 60 °C
CMAE 120 °C
50 °C
Soxhlet 73 °C
fatty acid
3 min
20 min
3 min
20 min
4 min
12 min
20 min
4 min
12 min
20 min
myristic, C14:0 pentadecanoic, C15:0 palmitic, C16:0 palmitoleic, C16:1 stearic, C18:0 oleic, C18:1 linoleic, C18:2 linolenic, C18:3 arachidic, C20:0 eicosanoic, C21:0 docosenoic, C22:1
0.26 0.02 16.14 2.68 1.85 15.34 16.54 35.79 0.00 10.89 0.49
0.13 0.01 17.03 0.85 1.22 13.22 13.30 42.65 0.00 10.92 0.68
0.09 0.01 11.18 0.17 2.59 14.15 36.21 28.03 0.22 6.28 0.33
0.15 0.02 16.36 0.45 1.57 19.69 16.57 33.82 0.00 10.62 0.76
0.00 0.00 14.22 2.72 1.55 17.20 15.15 48.58 0.58 0.00 0.00
0.00 0.00 13.49 3.03 1.27 15.71 22.31 42.58 0.00 1.62 0.00
0.00 0.00 13.20 0.43 1.36 13.13 24.43 47.45 0.00 0.00 0.00
0.00 0.00 15.91 1.13 1.19 16.41 19.82 43.94 0.00 1.60 0.00
0.00 0.00 18.96 0.34 0.82 13.70 14.21 50.11 0.00 1.86 0.00
0.00 0.00 22.03 0.00 0.91 12.54 13.43 51.09 0.00 0.00 0.00
a
ethanol
hexane
a
0.00 0.00 22.17 1.79 1.50 20.04 14.71 39.78 0.00 0.00 0.00
0.00 0.00 23.23 0.00 1.24 16.06 19.58 39.89 0.00 0.00 0.00
0.06 0.00 44.17 0.10 2.04 16.96 14.88 22.43 0.09 0.00 0.00
Average values (with hexane) from a sample of 22 trees from a very wide geographic region.
TABLE 2. TT seeds Lipid Yields Obtained Using CMAE, MAE, and Control Extraction (mean ± std) CMAE and CMAE control temp (°C)
extraction time (min)
50
0 4 8 12 16 20 0 4 8 12 16 20 0 4 8 12 16 20
60
73
a
lipid yielda, microwave extraction (% dry mass) 4.82 ( 1.29a 10.73 ( 0.82a 19.52 ( 1.58a 19.39 ( 0.25a 22.39 ( 0.07a 24.17 ( 0.60a 5.87 ( 1.27a 22.14 ( 0.89a 28.05 ( 0.63a 29.07 ( 0.41a 29.32 ( 1.27a 30.63 ( 1.76a 6.97 ( 0.34a 25.90 ( 0.64a 30.71 ( 0.76a 31.81 ( 1.21a 33.16 ( 0.95a 35.32 ( 1.53a Soxhlet extraction
MAE and MAE control lipid yielda, control extraction (% dry mass) 5.82 ( 0.53b 11.65 ( 0.35b 12.67 ( 0.60b 14.05 ( 0.08b 16.02 ( 0.03b 17.02 ( 1.37b 6.17 ( 0.17b 12.3 ( 0.35b 15.32 ( 0.38b 16.02 ( 0.38b 18.77 ( 0.17b 22.15 ( 0.28b 7.47 ( 0.45b 13.6 ( 0.42b 17.27 ( 0.10b 19.12 ( 0.38b 22.37 ( 0.88b 25.62 ( 0.31b
temp (°C)
extraction time (min)
lipid yielda, microwave extraction (% dry mass)
lipid yielda, control extraction (% dry mass)
60
3 9 15 20 3 9 15 20 3 9 15 20 3 9 15 20
26.12 ( 0.192a 26.17 ( 0.378a 27.66 ( 0.203a 28.73 ( 0.331a 26.01 ( 0.292a 27.28 ( 0.27a 28.88 ( 0.120a 29.99 ( 0.105a 29.35 ( 0.405a 30.27 ( 0.255a 30.95 ( 0.073a 31.81 ( 0.159a 30.39 ( 0.217a 30.62 ( 0.439a 31.66 ( 0.120a 32.51 ( 0.455a
11.52 ( 0.634b 12.45 ( 0.517b 13.41 ( 0.451b 14.74 ( 0.513b 12.72 ( 0.624b 15.38 ( 0.584b 16.34 ( 0.456b 20.76 ( 0.707b 15.78 ( 0.900b 17.54 ( 0.661b 20.85 ( 0.419b 25.34 ( 0.575b 18.74 ( 0.642b 20.13 ( 0.57b 22.77 ( 0.487b 26.34 ( 0.404b
80
100
120
33.68 ( 2.29
Different letters “a” and “b” imply statistical difference between the two treatments within the same row.
(10%) in the oil extracted with batch MAE, whereas in CMAE this amount was less than 2%. Similarly for myristic, pentadecanoic, and docosenoic acid, small percentages ( 0.92) with increasing temperatures for continuous and batch systems (Figure 2), as expected, mainly due to an increase in diffusion coefficients. Nevertheless, the temperature effect was not that profound in batch MAE (slopes 0.46, exception at 0 min). The smaller slope in the batch system can be attributed to the factors ascribed above to the lower yields (less contact between solvent and feedstock, intermittent microwave exposure, less mixing, and therefore overall energy into the system), but for most practical purposes these higher
FIGURE 3. Time dependency of lipid yields in the continuous (top) and batch (bottom) MAE systems.
FIGURE 2. TT seeds lipid yields as a function of temperature obtained from the continuous (top) and batch (bottom) microwave systems. temperatures are not likely to be achieved in a commercial system as they require more complex equipment (vacuum, more energy, pressure safety valves, etc.). For example, lipid yields of 28.73% were obtained at 60 °C-20 min and 29.99% at 80 °C-20 min. In terms of efficient extraction of lipid from TT seeds feedstock, CMAE proved to a better technique as compared to the batch system. For example under similar extraction temperatures, CMAE gave a lipid yield of 35.32% at 73 °C-20 min as opposed to 29.99% at 80 °C-20 min, respectively. Yields for batch MAE did increase at extraction temperatures of 100 and 120 °C, though they were substantially lower than those achieved by CMAE at an extraction temperature of 73 °C. The influence of microwave extraction temperature on the extraction yield of oil from other feedstocks such as rice bran has been validated by other researchers (38, 45). Duvernay et al. (38) concluded that at higher extraction temperatures, MAE provided increased rice bran oil yields when isopropanol was used, and better oil yields than conventional Soxhlet extraction for the same extraction times. 3.4. Effect of Extraction Time on Yields. At the same temperature, yields increased with an increase in extraction times (Figure 3). As expected, longer exposure time (from 3 min to 9, 12, 15, and 20 min in case of batch and from 4 min to 20 min in case of CMAE) of solvent-feedstock mixture will always increase the quantity of lipid extracted, with significant differences being observed between lipid yields among different extraction times for a given temperature in a majority of cases. The most important observation that can be made from the data presented is that the dynamic response of the
extraction process (yields vs time) follows extremely closely (with few exceptions, R2 > 0.95) a first order response to a step input (exponential rise from an initial to a maximum value). The rates of yield increase are given by exponent coefficients, with a higher number indicating a faster rate of oil recovery. From this perspective, both microwave systems performed much better than the respective conventional systems. In CMAE, even though theoretical final values (the sum of y0 and a) are similar at 34-35%, the rate in CMAE is 6-fold higher than controls. In other words, CE would require approximately 6 times more time to achieve the CMAE yields, which is directly observed in Table 1 (i.e., CE yield at 73 °C-20 min is the same as CMAE yield at 4 min). The same first-order behavior was observed for the batch MAE, with much higher rates of extraction than controls (in this case, differences between coefficients b were even higher than for CMAE). The difference in performance was so pronounced in the batch MAE system, than the longest times used in CE did not yield nearly the yields at the shortest times in MAE (i.e., at 100 °C, CE yields were 25.34% at 20 min compared to 29.53% at 3 min in MAE). Statistically, for batch MAE all lipid yields increased significantly with increasing extraction time except at the lowest temperature and lowest extraction time (24.67% at 60 °C-9 min and 23.39% at 60 °C-3 min). Similarly, for CMAE at 60 °C the amount of lipids extracted after 12 min did not significantly differ from 16 min. Similar trends in oil yields were observed in case of MAE of soybeans and rice bran oil in both batch and continuous configurations (41, 42) as well as in a separate study on soybeans only by Li et al. (44). In the present study, lipid recovery rate from TT seeds was highest at lowest times (0 min or 3 min) and slowly decreased as with time, but total amount of lipid recovered increased with time. Whereas the above relationships can be used either at individual temperatures or at individual times with good confidence, they do not necessarily account for the combination of both parameters. As such, a multiple linear VOL. 44, NO. 10, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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regression was performed according to the following eq 1: f(t, T) ) y0 + at + bt2 + cT + dT2
(1)
where y0 is the initial yield (% of dry mass), t is time (min), T is temperature (°C), and a, b, c, d are regression coefficients. The results (SI Table S2) indicate that good correlations can be obtained between any combinations of operating parameters in the ranges investigated in this study. These relationships, together with those determined at individual times and temperatures, provide powerful tools to scientists and engineers for predicting lipid extraction yields using these microwave technologies.
Acknowledgments We acknowledge the LSU Agricultural Center and LSU Biological and Agricultural Engineering Department for supporting this project. Funding was provided by the U.S. Department of Transportation, SunGrant Initiative, Award No. AB-5-61770.LSU1 and LA Soybean and Grain Research Board. Assistance was provided by Carlos Astete, Laura Picou, James Allen, and Sundar Balasubramanian. Published with the approval of the Director of Louisiana Agricultural Experiment Station as Manuscript No. 2010-232-4103.
Supporting Information Available Page S2 and Figure S1 contain detailed description of the CMAE system and its operation. Figure S2 shows snapshots of microwave power input into batch MAE and CMAE. Additional details for GC analysis and SEM imaging procedure are presented on page S3. Figure S3 shows the SEM results for TT seeds unprocessed and processed at 60 °C-20 min in the control, batch MAE, and CMAE systems. Page S4 and Table S1 show the results of the mass balances. Table S2 shows the coefficients of the multiple linear regressions. This material is available free of charge via the Internet at http:// pubs.acs.org.
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