Evaluation of Oxidation Stability of Refined Mineral Oil Enriched with

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Evaluation of Oxidation Stability of Refined Mineral Oil Enriched with Carotenoids from Carrot Using Supercritical Carbon Dioxide Extraction Arwa Mustafa,†,§ J. Johan Verendel,‡ Charlotta Turner,§ and Per Wiklund*,‡ †

NuraGreen-Technical and Research Solutions, SE-245 64 Hjärup, Sweden Nynas AB, SE-149 82 Nynäshamn, Sweden § Centre of Analysis and Synthesis, Lund University, Box 124, SE-221 00 Lund, Sweden ‡

ABSTRACT: Refined mineral oil, intended for various technical applications, was enriched with carotenoids by supercritical carbon dioxide extraction using the oil itself as cosolvent. It was envisioned that the carotenoids could function as renewable oil additives, adding chemical functionality to the end product such as enhanced resistance to oxidation. In order to investigate such possible antioxidant activity, a testing protocol was developed in which oil samples were thermally aged in the presence of a controlled amount of oxygen, and the time-dependent hydroperoxide and keto functionality concentration was monitored. An indication of antioxidant activity was indeed found, and further experiments were undertaken in order to investigate whether this was caused by the main carotenoid found in carrots, β-carotene. This was not found to be the case, and other possible explanations for the observed oxidative behavior, still to be investigated, are discussed.

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INTRODUCTION To reach a sustainable, technically advanced, industrialized society, it is of an interest to address other uses of mineral crude oil than fuels. The modern society utilizes hydrocarbon based liquids for various equally vital functions such as in lubrication of moving parts,1 for complex product formulations such as plastics, rubber, adhesives, etc., and as circulating electrical insulating coolant (transformer oil).2 The combined global market for lubricating specialty oils was about 35 million tons in 2010 and is expected to grow.3 Although it is already clear that it is possible to produce such hydrocarbon liquids from renewable sources (biomass to liquid, BTL)4 or to recycle and rerefine used material,5 there is a clear lack of methods applicable in large scale to utilize naturally occurring antioxidants and other additive compounds necessary to improve or modify the material properties of the oils. Such additives6 are commonly derived from petrochemicals also in (technical) applications where vegetable oils are used. Many plant species produce compounds that exhibit antioxidant properties; however, most of these compounds have very limited solubility in nonaqueous solvents like hydrocarbonbased oils. Clear exceptions are carotenoids found in, for example, green leaves as well as in other plants, with carrots being one of the richest sources.7 Although carotenoids have been shown to be able to act both as anti- and pro-oxidants in biological systems depending on conditions,8 the high oil solubility and abundance of these compounds merit further investigation. Extraction of carotenoids is commonly carried out using traditional methods that require the use of large amounts of organic solvents and intensive heat treatment, which is not perfectly suitable for such heat and light sensitive compounds. The use of technologies such as supercritical fluid extraction (SFE) and pressurized liquid extraction (PLE) provides © 2014 American Chemical Society

promising alternatives to more conventional extraction technologies.9 These methods are highly efficient in terms of yield and solvent use10 and can therefore be regarded as more environmentally sustainable. An added bonus is that the extraction process is carried out in absence of light and oxygen, conditions that protect the compounds of interest from degradation. SFE using supercritical carbon dioxide as fluid has been shown to be highly suitable for the extraction of relatively heat sensitive lipophilic compounds such as carotenoids and lycopene,10,11 which because of the possibility of very mild, low temperature conditions (31 °C and 74 bar, the critical limit of CO2) are not degraded. SFE has been used for extraction of carotenoids with and without cosolvent present.12 These studies demonstrate that use of cosolvent with supercritical carbon dioxide extraction can enhance the extraction yield of carotenoids from carrots. The idea carried through the work reported here was to use carbon dioxide SFE on carrot byproducts with refined mineral oil as cosolvent and that the extracted material should then constitute a functional improving additive in the oil in its intended technical application, i.e., as antioxidant in transformer oil. Starting from earlier work by Mustafa et al.,13 this was first accomplished using mineral oil as a cosolvent to ethanol in PLE, the idea being that the main solvent could be evaporated away leaving the carotenoid enriched oil as the sole product. As this was found to be relatively inefficient, an SFE-protocol using carbon dioxide was then developed. The extract enriched oil was then subjected to a novel oxidation stability test protocol, including a GCMS-based method for determination of Received: Revised: Accepted: Published: 19028

September 5, 2014 November 18, 2014 November 22, 2014 November 23, 2014 dx.doi.org/10.1021/ie503524m | Ind. Eng. Chem. Res. 2014, 53, 19028−19033

Industrial & Engineering Chemistry Research

Article

injection volume was 10 μL. The β-carotene content was determined by light absorption at 450 nm. GCMS Determination of Peroxide Content in Oil. Peroxide content of oil samples before and after aging was analyzed using a robust GCMS-based method developed for use with light mineral oils in which a solution of triphenylphosphine (500 μL, 37 mM in dichloromethane) was added by syringe through a silicone septum to an oil sample (250 μL) enclosed in a standard 1.5 mL glass GC-vial equipped with an aluminum crimp lid. Over a period of 10−15 min peroxides were allowed to oxidize the triphenylphosphine into triphenylphosphine oxide (which is the analyte for GCMS). Thereafter, a solution (500 μL) of sulfur (55 mM) and the internal standard fluorene (6 mM) in dichloromethane was added by syringe through the septum into the vial. Sulfur will react with the surplus triphenylphosphine and prevents overdetermination of peroxide content because of the slower oxidation of triphenylphosphine by oxygen from the air. The two reagent solutions were prepared in volumetric flasks at 25 °C, deoxygenated by purging with nitrogen, and stored in glass serum flasks closed by aluminum crimp lid with silicone septa. Application of a slight nitrogen pressure from a needle inserted into the storage flask pushes the solution into a syringe at discharge of the reagent which can thus be handled under inert conditions. The subsequent determination of the triphenylphosphine oxide content (proportional to peroxide content of the sample) with GCMS by direct injection from the reaction vial is fully described by a previously published procedure (the difference here is use of dichloromethane instead of chloroform as the solvent).14 IR Spectroscopy Monitoring of Oil Oxidation. Monitoring of oxidation in lubricating oils by observation of carbonyl absorption (1670−1800 cm−1) in IR is described in several industrial standard methods such as ASTM D 7214. From laboratory experience with additive free mineral oils (prior to addition of conventional lubricant additives), we chose monitoring by peak height at 1710 cm−1, using a baseline drawn from 1640 to 1780 cm−1. The oil sample was charged into a NaCl cell with an optical length of 0.02 mm after which spectra were acquired by FTIR instrument (Nicolet iS10, Thermo Scientific). In the case of the SFE extracted oil a further peak at 1759 cm−1 was measured using a baseline from 1720 to 1780 cm−1. Oxidation Onset Temperature (OOT) by Pressurized Differential Scanning Calorimetry (PDSC). An aliquot of oil (3.0−3.3 mg) was weighed in an aluminum crucible (without lid) manufactured to fit in the heating chamber of a MettlerToledo HP DSC1 (PDSC). The oil-filled crucible and an empty crucible for reference were put into the heating chamber which was subsequently closed and sealed. A pressure of 35 bar of oxygen was applied with a constant gas flow of 50 mL/min. Measurement of OOT commenced during heating from 40 to 300 °C at a rate of 10 °C/min. This procedure is in line with ASTM E2009-08, Standard Test Method for Oxidation Onset Temperature of Hydrocarbons by Differential Scanning Calorimery.15 OOT is sometimes also referred to in the literature as OIT (oxidation induction temperature), but for clarity it is better to use this abbreviation for oxidation induction time, i.e., when temperature is kept constant until the exothermic reaction occurs. Oxidation Stability Test of Oil by Aging Followed by Peroxide Determination. An aging test methodology previously applied to insulating oils (transformer oil)16 was

peroxides in mineral oil by Wiklund et al.,14 in combination with IR carbonyl absorption measurement after aging of oil samples at elevated temperature. Conclusions from these tests were in part confirmed by application of oxygen pressurized differential scanning calorimetry (PDSC) to the oil samples.

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MATERIALS AND METHODS Materials. Two types of Swedish carrots were used: (i) fresh carrots purchased from a local grocery store and (ii) unsorted carrot byproducts obtained from a local carrot processing plant. The carrots were obtained fresh and were stored at −8 °C until the experimental work was carried out. Frozen samples were thawed, grinded, and homogenized using a food processor (MCM2054 Bosch, 240 W). The dispersing agents used were (a) Hydromatrix, an inert diatomaceous earth from Varian, (Palo Alto, CA, USA) and (b) glass beads of size 2 mm, which were purchased from VWR, Stockholm, Sweden. Two types of carotenoid standards were used: carotene-mixed isomers from carrots, minimum 95% HPLC, ∼2:1 β/(α- and βcarotene), minimum 95% by HPLC, which were obtained from Sigma-Aldrich (Steinheim, Germany). Severely hydrotreated light naphthenic mineral oil NS8 (Nynas AB, Nynäshamn, Sweden) with a density of 0.897 g/mL was used. Methanol (>99.9%, Honeywell Burdick & Jackson, Seezle, Germany) was used in SFC. Ethanol (99.7%, Solveco, Rosenberg, Sweden) was used in PLE and as a cosolvent in SFC, and CO2 (ultrapure 126, Air Products, Amsterdam, The Netherlands) was used for the SFE and SFC. Preparation of Carotenoid-Enriched Mineral Oil through SFE. Freeze-dried carrots were finely milled using a ball mill (oscillating mill MM 400, Retsch, Rheinische, Germany). The extraction experiments were done using an SFE extraction unit. The setup includes a supercritical fluid extractor (ISCO SFX 210) with a built-in temperature controller and a heating unit for the pressure restrictor, an ISCO 260D syringe pump (Teledyne Isco, Thousand Oaks, CA) that was used for pumping liquid CO2, an LC pump (Varian 9001, CA, USA) that was used for pumping the oil, and a cooling bath (Julabo F12-ED, Labortechnik GmbH, Seelbach, Germany) for cooling the CO2 pump. About 1 g of the freeze-dried carrots was weighed into a 10 mL extraction cell, in which the sample was dispersed with glass beads. The extraction parameters were as follows: the CO2 (more than 99% purity) was pumped at a pressure of 400 bar and a flow rate of about 5 mL/min; the mineral oil was pumped at a flow rate of 0.2 mL/min; the extraction was carried out in a continuous flow mode for 40 min at 60 °C. The temperature of the restrictor was about 40 °C. The extract, i.e., the carotenoid-enriched oil, was collected in a glass tube placed inside a dark icebox. The oil samples were found to have an acid concentration corresponding to around 0.10 mg of KOH/ g (as measured by ASTM D974). The oil samples produced by this method are hereafter called SFE-oils. Analysis and Testing. β-Carotene Determination in SFEOil through SFC-VIS. SFE extracts were analyzed using a SFC Investigator system (Waters Corporation, Milford, MA, USA) using a YMC-ODS column (250 mm × 4.0 mm i.d., 120 Å pore size, and 5 μm particle size) from YMC Europe GMHB, Schermbeck, Germany. Liquid CO2 and ethanol were used as a mobile phase. An isocratic elution was used to separate the carotenoids under the following conditions: Total flow rate was 5 mL/min, containing 15% of cosolvent (ethanol). The oven temperature was 40 °C. The backpressure was 100 bar, and the 19029

dx.doi.org/10.1021/ie503524m | Ind. Eng. Chem. Res. 2014, 53, 19028−19033


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half of the practical solubility limit of pure β-carotene (vide supra). Although β-carotene is the dominant species in lipophilic extractable compounds in carrots,7 there are others as well. In fact in the SFC−vis analysis to determine β-carotene content of the oil samples, α-carotene was also clearly identifiable (Figure 1) in quantities of around 30% of the β-carotene. Exact

adapted to a smaller scale and further developed. One complete test set consisted of 36 samples of oil (250 μL each) filled into standard 1.5 mL glass GC vials. The oil was charged into the vials using a calibrated (with the oil as gravimetric standard) automatic pipet. The vials were closed by aluminum crimp lids with silicone septa. A set of 6 vials were retained as nonaged samples (0 h), and the remaining 30 vials were placed in an oven at 120 ± 1 °C. Sets of 6 vials at a time were removed from the oven and placed at room temperature after 6, 9, 15, 24, and 48 h, respectively. For each aging time the peroxide content was determined in three vials, and the IR carbonyl absorption was determined in the remaining three vials. The average result and the standard deviation were determined for both measurements.

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RESULTS AND DISCUSSION Extraction and Yield. The aim of this study was to enrich mineral oil with carotenoids from carrot byproducts, using pressurized fluid technology, and to investigate the effects on mineral oil oxidation stability. Building on previous work, PLE with ethanol was initially used, but β-carotene is only sparingly soluble in ethanol and methanol at room temperature, whereas solubility in hexane is reported to be 1090 μg/mL in cold hexane,17 which is more like mineral oil in terms of solubility parameters. Practical experiments with the mineral oil, which is a complex mixture of hydrocarbons, gave at hand a solubility limit of about 360 μg/mL at room temperature. Considering these facts, SFE using carbon dioxide seemed more suitable than PLE for the purpose of extractive enrichment of lipophilic compounds such as carotenoids into a lipophilic matrix such as a mineral oil. In SFE, the possibility of altering the temperature and pressure can be used to optimize the solvent strength to the extraction at hand. With mineral oil, which is highly lipophilic, used as a cosolvent in a continuous manner the solubility of carotenoids should also increase. In the context of carotenoid extractions it has been reported that the solubility of oil, albeit vegetable oil, in supercritical CO2 is enhanced by a pressure increase (i.e., density).12,18 On the basis of this, the operating pressure for the extraction test was made at the highest pressure possible with the equipment at hand, i.e., 400 bar. The results show that when using mineral oil as a continuously added cosolvent in SFE for the extraction of carotenoids from carrot byproducts, the extractable content of β-carotene was around 270 μg/g dry weight of carrot, corresponding to about 35 μg/g fresh weight (Table 1). The

Figure 1. SFC chromatogram of freeze-dried carrots extracted by SFEoil, monitored at 450 nm.

determination of the α-carotene content was not carried out at the time because of lack of suitable pure reference material, but its presence shows that compounds other than the β-carotene were extracted as well. Furthermore, it cannot be excluded that other (similar) compounds not detectable at 450 nm or compounds that coelute with α- and β-carotene because of poor chromatographic resolution are in fact present in the SFEoils. A further indication of a more complex content is that the SFE-oils contained something acidic (see Materials and Methods). Taken together, this may explain why the oil content of the β-carotene apparently did not reach saturation despite the continuous extraction mode; i.e., the oil became saturated with other very similar compounds. The β-carotene content in Table 1 should be interpreted with these limitations in mind. Oxidation Stability Testing. Whereas there are many established screening methods for antioxidant effects in biological systems,19 few such methods for use with mineral oils are well developed. Hence, the procedure outlined above for oxidation stability testing by aging was developed. The data obtained on peroxide content over time during thermal aging for the oil samples are depicted in Figure 2. In the pure oil, which has very little resistance to oxidation, the oxygen in the air available inside the aging vial reacts rapidly with the hydrocarbons of the oil to form various peroxides (probably mainly hydroperoxides). Since peroxides are highly reactive species, they continuously form other oxygenated types of hydrocarbons (alcohols, aldehydes/ketones, carboxylic acids, and esters) and water through various radical and nonradical pathways.20 Thus, the peroxide content should reach a peak concentration while they are still formed and then level off as they react further. The limiting factor for peroxide concentration in a closed system should be oxygen availability. It was determined that the vials used for testing on average can hold a volume of 2009 μL in the capped state; i.e., after addition of

Table 1. β-Carotene Content in Mineral Oil Processed by SFE sample no.

μg/mL oil after extraction

μg/g dry weight of carrot

μg/g weight of fresh carrot

1 2 3 4 5

144.35 145.80 135.26 142.91 150.53

270.10 268.40 275.35 271.22 261.27

34.57 34.35 35.24 34.68 33.44

reported values in the literature are in the range 150−500 μg/g extractable β-carotene using SFE.10,12 It is, however, difficult to compare the content from different studies because of the huge discrepancy in carotenoids content as result of variation in carrot varieties. Table 1 also shows the content of β-carotene in the oil samples after SFE. It is about 140−150 μg/mL, about 19030



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peak diminished over time and likely represents a species that is also extracted from the carrots and that is consumed by the aging/oxidation. The initial orange-yellow color of the extracted oils gradually subsided, and after about 24 h the samples were indistinguishable from the colorless pure oil. In fact carotene bleaching is an often applied method to assess oxidation and/or antioxidant properties of specifically isolated chemical species,21 mainly from plants.22 In an attempt to verify that it was indeed the carotenoids, mainly β-carotene, that caused the apparent retardation of peroxide formation and reactivity in the oil aging experiments above, synthetic β-carotene (>95% from Sigma-Aldrich) was dissolved in the pure oil and the same aging experiment was repeated with the resulting oil solution. The results of this experiment in comparison to fresh oil and SFE-oil are depicted in Figure 4.

Figure 2. Peroxide content vs time during aging of oil samples. Error bars at each measurement point indicate standard deviation of three replicates.

250 μL of oil there is 1759 μL of air in a capped vial, 21% (atmospheric concentration) of which will be oxygen. By application of the gas law, the theoretical maximum concentration of any oxygenated species is found to be 61.2 mM (for compounds containing both the oxygen atoms of molecular oxygen) in the 250 μL of oil in the vial. Allowing for continuous consumption of peroxides formed early, the actual maximum peroxide content (45.6 mM) seems to indicate that all available oxygen is indeed consumed after around 10 h at 120 °C. Considering the curve for the SFE-oil in comparison, the time it takes to reach peak peroxide concentration is longer, and the peak concentration is lower. This indicates a retarded oxidation reaction. In addition, the subsidence of the peroxide concentration is slower (rate of consumption lower), indicating increased stability of the formed perioxides. Both effects seem to indicate an antioxidant activity. The antioxidant effect seen in the retardation of peroxide formation and consumption is confirmed also for more highly oxidized hydrocarbons containing carbonyl functions. Up to about 15 h at 120 °C the rate of carbonyl absorption increase at 1710 cm−1 is clearly slower (Figure 3), and after that point in time it is clearly lower until the end of the experiment. In the SFE-oil a further absorption at 1759 cm−1 was discovered, which could correspond to an α,β-unsaturated aldehyde. This

Figure 4. Effect of adding synthetic β-carotene to mineral oil (143 μg/ mL oil). Error bars indicate standard deviation of three replicates.

Surprisingly it appears that separate addition of synthetic βcarotene had virtually no effect on peroxide formation and only a very small effect on retardation of the decay of peroxides. The samples were totally discolored in a matter of a few hours, confirming the absence of antioxidant activity. This result opens up for the possibility that something else in the extracted oils is causing the observed antioxidant effects. For further investigation into the effects on oxidation stability of the oil samples we turned to PDSC with oxygen, a calorimetric fast oxidation stability test method. PDSC has recently been used to evaluate and optimize the addition of synthetic antioxidants to insulating oils.23 This is a very useful technique for probing oxidative stability of oils and polymers by measurement of oxidative induction time24 or oxidation onset temperature (OOT).25 Because of the low intrinsic oxidation stability of the highly refined oil (higher degree of refining gives low intrinsic oxidation stability but high response to addition of antioxidants)26 and the anticipated small effects of the extracted compounds, rapid ramping of temperature was chosen. Figure 5 depicts the results of such measurements for three samples of SFE-oil. It is apparent that under the high temperature/oxygen pressure conditions the SFE-oils have lower oxidation stabilities than the pure oil. The results also suggest that there may be a correlation to the concentration of β-carotene. In order to test this in a controlled system pure β-carotene was added to the pure oil at four different concentrations and the OOT was again

Figure 3. IR carbonyl absorption in oils after the aging test (peak height of the respective IR signals). Error bars at each measurement point indicate standard deviation of three replicates. 19031



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material has been achieved, resulting in higher oxidative stability.29 A small-scale protocol for testing of oxidation stability (antioxidant effect) based on aging by heat followed by determination of hydroperoxide and carbonyl development was developed to test for this possibility in our case. Although a small antioxidant effect, manifested in slower production of hydroperoxides (presumably by slower consumption of available oxygen) and slower degradation of formed peroxides into carbonyls, was seen in the SFE-oils, it appears this effect is not caused by the presence of β-carotene. On the contrary, separate addition of β-carotene to oil gave a clear pro-oxidant effect in the test protocol, something that was also confirmed by PDSC measurement of oxidation onset temperature. This means that the weak antioxidant effect in the SFE-oils must have been caused by something else. An intriguing possibility is that the effect is caused by enzymatic degradation products of carotenes. The presence of an α,β-unsaturated aldehyde like retinal would fit with the observed carbonyl IR absorptions. Retinal is formed by enzymatic oxidation of carotenes, and in a second oxidative step retinoic acid is formed.30 Perhaps this could explain the observed acidity of the SFE-oil. Our work will continue along these lines, not the least on the analytical methods applied here which apparently failed to identify the compounds causing the observed antioxidant effect.

Figure 5. OOT comparison of pure oil (left) and SFE-oils corresponding to entries 1−3 in Table 1. Error bars indicate standard deviation of three replicates.

tested for these samples. The results show a clear concentration dependency depicted in Figure 6.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Swedish Research Council Formas (Grant 229-2009-1527) is acknowledged for funding and Mariannes Farm for providing carrots.

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Figure 6. OOT of pure oil fortified with different concentrations of βcarotene. Error bars indicate standard deviation of three replicates.

REFERENCES

(1) Mortier, R. M.; Fox, M. F.; Orszulik, S. T. Chemistry and Technology of Lubricants; Springer: Dordrecht, The Netherlands, 2009. (2) Hirschler, M. M. Electrical Insulating Materials: International Issues; ASTM International: West Conshohocken, PA, U.S., 2000. (3) Wadle, H. Novel Efficient Base Oil Production: Overview, Potentials and Challenges; UNITI-Mineralöltechnologie GmBH: Berlin, Germany, 2012; pp 1−22. (4) Swain, P. K.; Das, L. M.; Naik, S. N. Biomass to liquid: a prospective challenge to research and development in 21st century. Renewable Sustainable Energy Rev. 2011, 15 (9), 4917−4933. (5) (a) Boughton, B.; Horvath, A. Environmental assessment of used oil management methods. Environ. Sci. Technol. 2004, 38 (2), 353− 358. (b) Pawlak, Z.; Urbaniak, W.; Kaldonski, T.; Styp-Rekowski, M. Energy conservation through recycling of used oil. Ecol. Eng. 2010, 36 (12), 1761−1764. (6) Herdan, J. M. Lubricating oil additives and the environmentan overview. Lubr. Sci. 1997, 9 (2), 161−172. (7) Sharma, K. D.; Karki, S.; Thakur, N. S.; Attri, S. Chemical composition, functional properties and processing of carrot-A review. J. Food Sci. Technol. (New Delhi, India) 2012, 49 (1), 22−32. (8) Palozza, P. Prooxidant actions of carotenoids in biologic systems. Nutr. Rev. 1998, 56 (9), 257−265. (9) (a) Camel, V. Recent extraction techniques for solid matrices supercritical fluid extraction, pressurized fluid extraction and microwave-assisted extraction: Their potential and pitfalls. Analyst (Cambridge, U. K.) 2001, 126 (7), 1182−1193. (b) Henry, M. C.; Yonker, C. R. Supercritical fluid chromatography, pressurized liquid extraction,

A concentration dependent pro-oxidant effect of β-carotene has also been found for oil in chlorophyll photosensitized oil-inwater emulsions.27 Also in this case the effect was confirmed by several methods. Whereas β-carotene rich diets appear to have a preventive effect against cancer, pro-oxidant effects have also been seen in cellular assays under certain conditions,28 something that may explain the failures in treating and preventing cancer by increased supplemental β-carotene.

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CONCLUSIONS In this work it was shown that SFE with carbon dioxide and oil as cosolvent can be a very efficient, convenient, and most likely scalable method for enrichment of oils for technical applications with substances occurring in plant material. Such substances can add functionality to the oil, such as protection against oxidation, without the need to extract and isolate specific compounds in separate steps. Although the application of PLE to the same type of extraction was initially envisioned, it must be concluded that SFE is both more efficient and practical for extraction and oil enrichment of highly lipophilic substances. As anticipated, in the case of extractions from carrots, carotenoid enriched oil could easily be obtained. For edible vegetables, oils enrichment with naturally occurring antioxidants through ethanol extraction of plant 19032



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Evaluation of Oxidation Stability of Refined Mineral Oil Enriched with

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