Effective Antarctic krill oil extraction using switchable hydrophilicity solvents
Antarctic krill has been widely studied because of its abundant biomass, rich nutritional value, and great production potential. Notably, krill oil (KO) is rich in phospholipids (PLs), polyunsaturated fatty acids (PUFAs), and astaxanthin. A method based on a green switchable hydrophilicity solvent N,N-dimethylcyclohexylamine (DMCHA), which can reversibly change from oil soluble to water soluble in the presence of CO2 was used to extract KO from frozen Antarctic krill as it consumes less energy than traditional methods. We showed that DMCHA destroyed the surface structure of Antarctic krill and accelerated the dissolution of KO. In addition, this method enabled the PL extraction to reach up to 80.2% of total PLs, among which PC accounted for the highest proportion, up to 90.91% in PL. In fact, the astaxanthin extraction reached up to 81.44% of total astaxanthin while the fatty acid (FA) extraction up to 84.35%. The KO extracted through DMCHA was rich in PUFA, up to 47.74%, and the content of EPA + DHA reached 42.16% of total FA content. Furthermore, the amount of residual solvent in the lipid phase was just 0.23% of the DMCHA used for the extraction and the recovery rate of solvent was up to 93.2%. Our results demonstrated the high efficiency of oil extraction and the environmental friendliness of this method.
KeywordsKrill oil Switchable hydrophilicity solvents N,N-dimethylcyclohexylamine Phospholipids Polyunsaturated fatty acids Astaxanthin
Antarctic krill (Euphausia superba) is considered to be one of the richest biomasses in the world (Auerswald et al. 2015) and is known as a potential alternative source of protein, lipids, as well as chitin (Grantham 1977). Among them, KO has attracted increasing attention because of its unique composition and positive impact on human health. It contains abundant LC-ω-3 PUFAs, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (Haider et al. 2017) as well as powerful, naturally-occurring antioxidants, such as astaxanthin (Tou et al. 2007). The PUFAs in KO are easily absorbed into target organs as they are composed of both PLs and triglycerides (TGs), while the EPA and DHA in other supplements like fish oil are mainly in the form of TGs (Schuchardt et al. 2011). Moreover, astaxanthin, has effective antioxidant properties that can easily maintain the stability of PUFAs from oxidative damage (Lee et al. 2015). In addition, as an anti-inflammatory component, astaxanthin has been shown to play an active role in preventing diabetes, cancer, and supporting the immune system (Costanzo et al. 2016).
Supercritical carbon dioxide (SC-CO2) and organic solvents are commonly used to extract KO (Bruheim et al. 2015). Though the properties of components in KO, separated via the SC-CO2 method, can be improved in certain cases, high capital costs for improving batch extraction and engineering technology should be considered (Friedrich and Pryde 1984; Gigliotti et al. 2011). By contrast, organic solvents could be used to extract KO containing PLs more simply and economically, without expensive instrumentation (Xie et al. 2017). Acetone, ethanol, isopropanol, ethyl acetate, isohexane, and n-hexane are the traditional organic solvents used for KO extraction. However, these volatile solvents are ineffective to Antarctic krill with high water contents (Cooney et al. 2009) so an air drying step is required for Antarctic krill to obtain the final KO, which consumes energy (Jessop et al. 2010). Moreover, solvent recovery is not easy, and environmental pollution arises due to solvent evaporation and high energy consumption (Boyd et al. 2012a). Therefore, energy-efficient techniques must be developed to extract oil from frozen Antarctic krill.
Switchable solvents are liquids with hydrophilicity or polarity that can be reversibly switched from one form to another in the presence of CO2 (Jessop et al. 2005). This includes switchable hydrophilicity solvents (SHSs) (Jessop et al. 2010, 2011), and switchable polarity solvents (SPSs). Both are non-volatile and non-flammable, preventing volatilization or distillation during solvent recovery. However, SHSs are more attractive than SPSs, or even conventional solvents, as extraction solvents for KO because the SHS system works in the presence of water. Thus, the krill does not require drying prior to extraction, preventing the unwanted formation of the bicarbonate salt of 1,8-diazabicyclo[5.4.0] undec-7-ene (DBU), which is formed when using SPSs (Boyd et al. 2012b). In addition, SHSs are less expensive and more stable. Lastly, SHS-water miscibility is fully reversible because after CO2 is gently removed (i.e., with low heat, bubbling with an inert gas), they return to their original hydrophobic state (Samorì et al. 2017).
In this paper, DMCHA was chosen for its low volatility, intermediate polarity, low toxicity, and relatively low water solubility (18 g·L−1) (Vanderveen et al. 2014). DMCHA was used to extract the KO from the Antarctic krill, a type of shrimp. The KO was separated by bubbling CO2 into the system in the presence of water at normal pressure then the solvent was recovered after the release of CO2. Subsequently, the effects of the extraction conditions, including extraction time, temperature, and shrimp/solvent ratio, were investigated to better understand the extraction system. The content of PL, astaxanthin, as well as the unsaturated fatty acid (UFA) and EPA + DHA ratio of FA in extracted KO, were measured. The DMCHA ratio remaining in the lipid layer was also determined.
Results and discussion
Extraction efficiency of PL, astaxanthin, FA by DMCHA
The effect of temperature on the PL extraction efficiency as the reaction time increased is shown in Fig. 2b. The PL yield increased as the temperature increased. The extraction efficiency increased as the reaction time increased, reaching a maximum of 76.2% at 45 °C after 12 h then decreasing slightly after the optimum reaction time, similar to the results presented in Fig. 2a.
Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were the two main types of PL detected in all KO samples. The relative proportions of PC and PE at different extraction temperatures are shown in Fig. 2c. In general, as the temperature increased, the percentage of PC in PL decreased from 90.91% to 82.35% as temperature had a negative effect on the selective extraction of PC (Teberikler et al. 2001). Our results also showed that PC was the most abundant PL in KO, rendering KO a good source of naturally-occurring PC.
Figure 3c, d show the comparison of the astaxanthin esters in KO extracted by the Folch method and that by the DMCHA method. Unlike the astaxanthin esters extracted from Haematococcus pluvialis, the content of the astaxanthin diester is more than that of the astaxanthin monoester in the oil extracted from Antarctic krill (Maoka et al. 1985). The free astaxanthin accounted for a small part both in Fig. 3c, d. The astaxanthin diester extracted by the Folch method was significantly more than the astaxanthin monoester. This trend was also seen when the KO was extracted by DMCHA (Fig. 3d).
Figure 3e shows the effect of the shrimp/solvent ratio on the extraction efficiency of FA. The extraction efficiency of FA increased when the solvent volume was increased. Specifically, the solvent ratio played a small role in the extraction of FAs when the ratios ranged from 1:15 to 1:20. On the other hand, the FA yield increased gradually with increasing reaction time and was almost saturated after 12 h extraction with DMCHA. After that, the yield did not significantly increase, and an FA yield of up to 80.81% could be achieved. Figure 3f illustrates the effect of temperature on the FA extraction efficiency. In the process, an increase in temperature enhanced the mass transfer process; hence, increasing the FA yield. In addition, the FA yield further increased with longer reaction times, becoming saturated after a 12 h treatment with DMCHA. An FA yield of up to 84.35% could be achieved at a temperature of 45 °C.
The FA compositions of KO extracted at different temperatures
DMCHA 25 °C
DMCHA 35 °C
DMCHA 45 °C
∑EPA + DHA (%)
DMCHA breaks the surface structure of shrimp powder
Recovery rate and the amount of residual solvent
Mechanism of SHS-based extraction
SHSs are a second class of switchable solvents specifically designed for wet samples (Boyd et al. 2012a). Amidine and tertiary amine SHSs have been identified as switching between the two forms by the addition or removal of CO2 from the system. CO2 is preferred to initiate the switching process because it is non-toxic, benign, inexpensive, and easily removed (Jessop et al. 2010).
The overall process comprises an acid–base reaction with hydrated CO2. The native hydrophobic form of the DMCHA is shown in Fig. 5b. The DMCHA forms a bicarbonate salt that is water soluble. The DMCHA can be mixed with water samples and is easily separated from the aqueous phase by removing CO2 from solution by bubbling another gas. Alternatively, the hydrophilicity switch can be also triggered by altering the charge of the SHSs by pH shift, which is especially useful in the context of microextraction because it reduces the potential losses by evaporation or splashing during the CO2 removal process.
This behavior has been exploited as a method for removing solvent from products such as soybean oil (Jessop et al. 2010), algae oil (Samorì et al. 2013), and bitumen (Holland et al. 2012), astaxanthin (Huang et al. 2018) and high density polystyrene powder (Jessop et al. 2011). For the first time, we were able to extract lipids from Antarctic krill without any treatment for cell disruption while avoiding the use of volatile organic solvents and their associated evaporation energy costs for lipid recovery.
This study proved that DMCHA can be used to extract lipids from Antarctic krill in an environmentally friendly way. The extraction efficiency of PL, astaxanthin, and FA in the extracted KO is comparable to that of the chloroform/methanol extraction method. Furthermore, the solvent waste in the lipid layer is only 0.23% of the DMCHA used for the extraction. This research established an efficient technique for extracting lipids from Antarctic krill using green solvents with low operational costs and energy consumption. Thus, this technique will contribute to the cost-effective utilization of highly abundant Antarctic krill.
Materials and methods
Antarctic krill was harvested from the Antarctic area by China National Fisheries Co., Ltd., frozen, then transported back to Qingdao cold storage. The sample we used in this study was obtained after microwave thawing, heating at 95 °C for 5 min, and centrifugation.
DMCHA (industrial grade > 99%), triphenylphosphate, methylphenidate, methanol hydrochloride, n-hexane, n-hexadecanoic acid methyl ester, chloroform, dimethylterephthalate, dichloromethane, methyl tert-butyl ether, methanol, petroleum ether, ether, acetic acid, deuterated chloroform, and ethanol were purchased from Shanghai Beihe Chemical Co. Ltd (China). D-14 hexane was purchased from Omega (America).
Total lipid content
The total lipid content of Antarctic krill was measured by the Folch method after the Antarctic krill cells were thoroughly homogenized (Folch et al. 1957). Briefly, 1 g of freeze-dried Antarctic krill meal was weighed, and the lipids were extracted using 20 mL of a 2:1 chloroform/methanol (v/v) mixture.
Lipid extraction through DMCHA
To determine the effect of the shrimp/solvent ratio on the lipid extraction, three shrimp/solvent volume ratios, 1:20, 1:15, and 1:4, were established. Then, 1 g of Antarctic krill (water content of 70%) was weighed and the lipids extracted using DMCHA at room temperature. The filtrate was collected after extracting for 1, 6, 12, and 24 h, and an equal volume of water was added. The filtrate/water biphasic mixture was subjected to CO2 bubbling using a gas dispersion tube until the amine and water layers combined, leaving the lipid layer floating on the surface. After centrifugation, the oil was collected in a glass vessel for analysis.
To compare the effect of temperature, lipids were extracted at three temperatures, 25 °C, 35 °C, 45 °C. After extracting for 1, 3, 6, 12, and 24 h with a 1:15 shrimp/solvent ratio at each temperature, lipids were extracted into a glass vessel for analysis.
Thin-layer chromatography (TLC) for lipid class separation (Akanbi and Barrow 2017)
An aliquot of 10 mg of KO was suspended in 200 μL of dichloromethane and spotted onto TLC plates (20 cm × 20 cm) (Merck). Spots were visualized under ultraviolet light and lipid classes identified using standards to determine the type of lipid.
Surface structure changes of shrimp powders
The morphology of the Antarctic krill powders before and after DMCHA extraction was characterized using scanning electron microscopy (SEM) (Hitachi S-4800, Japan) at an accelerating voltage of 15 kV. The powders were sprinkled onto double-backed cellophane tape attached to an aluminum stub before coating with gold palladium in an argon atmosphere (Hondoh et al. 2016).
Determination of PLs in extracted KO by NMR
The phospholipid contents and compositions were determined by 31P nuclear magnetic resonance (NMR), as previously described by Li (2014) with slight modifications. All NMR experiments were conducted with a Bruker Avance Spectrometer 500 (Bruker, Germany) operating at 243 MHz and 25 °C. KO (5 mg) dissolved in 900 µL of CDCl3 and 100 µL of triphenylphosphate (TPP, 100 mmol/L) as an internal standard were added to the NMR tubes. The relative integrated intensity of each peak was applied to calculate the concentration and composition of the lipids. The PL yields were calculated according to the peak area ratios relative to that of the TPP reference.
Determination of astaxanthin in extracted KO by HPLC
A high-performance liquid chromatograph (HPLC, LC-20AT, Shimadzu, Kyoto, Japan) equipped with an ultraviolet detector (SPD-20A, Shimadzu, Kyoto, Japan) and YMC-Carotenoid-C30 (4.6 mm × 250 mm, 5 μm) (Japan YMC Co., Ltd.) were used to analyze astaxanthin based on the method described by Rao et al. (2013) with slight modifications. Briefly, the extracted KO was dissolved in 3 mL of dichloromethane. After filtration with a 0.22-µm filter, 0.5 mL of the product was placed into liquid vials. The resulting product was analyzed by HPLC. The resulting product (free astaxanthin, astaxanthin monoester, and astaxanthin diester) was analyzed by HPLC. LC conditions were optimized to analyze astaxanthin within 40 min at 30 °C, using mobile phases consisting of (A) methanol and (B) methylphenidate. Using a flow rate of 1 mL/min, the gradient consisted of a ramp from 100% A to 90% A over 10 min, then 10–20 min from 90% A to 60% A, followed by a hold at 60% A for 10 min, then from 60% A back to 90% A at 30–35 min, ending with a hold at 90% A for 5 min. The injection volume was 60 μL. The astaxanthin was detected at a wavelength of 476 nm, and the measured amount was based on the peak area of the standard astaxanthin.
Lipid transesterification and FAMES analysis by GC–MS
Variable amounts of extracted lipids (30–50 mg) from shrimp were converted into fatty acid methyl esters (FAMEs) through transesterification (Chiappe et al. 2016). Briefly, the extracted lipids were dissolved in 25 mL of ethanol, taking 5 mL into the centrifuge tube. Next, 2 mL of methanol hydrochloride was added then 3 mL of a chloroform/methanol (v/v 1:1) mixture was added. Methyl nonadecanoate was added as an internal standard before starting transesterification at 85 °C for 1 h. After cooling to room temperature, n-hexane was used to dilute the upper organic layer after centrifugation for 5 min at 10,000 r/min, which was then used for analysis.
The FAMEs that were recovered were analyzed with a Thermo Fisher Trace 1310 ISQ GC/MS equipped with a TG-5MS capillary column (30 m × 0.25 mm; coating thickness 0.25 μm). The FAME samples were finally identified by comparing the retention times of the mixture of FAME standards with those of the sample peaks. The FA contents are expressed as weight (µg) of every type of fatty acid (Yin et al. 2015).
Content of the residual solvent in the lipid layer
The water/DMCHA mixture was separated by bubbling with N2, with the mixture heated until the aqueous and amine layers were completely separated (Samorì et al. 2013). To determine the loss of DMCHA into the lipid phase, after switching DMCHA (5 mL) into DMCHAH + HCO3− using CO2 and H2O (5 mL), 1 mL of D-14 hexane was added to dissolve the lipid layer for analysis. In addition, dimethyllterephthalate was used as an internal standard to perform 1H NMR quantitative analysis. Spectra were recorded in D-14 hexane using a 5-mm probe on a VARIAN Mercury 400 spectrometer.
The oil extraction experiments were performed in triplicate (n = 3). For each triplicate, at least three measurements were performed. Statistical analysis was conducted using the statistical software SPSS version 20.0. The results were statistically evaluated by one-way analysis of variance (ANOVA) using Tukey’s test. Significant differences between means were assessed at P < 0.05.
Financial support provided by the National Natural Science Foundation of China (U606403): Marine Drugs and Biological Products was appreciated.
XJ, WH and CX designed this review. WS wrote the article. BS joined the experiment. All authors read and approved the final manuscript.
Compliance and ethical standards
Conflict of interest
The authors have declared no conflict of interest.
Animal and human rights statement
This article does not contain any studies with human participants or animals performed by any of the authors.
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