The use of modified vegetable oil from Crambe abyssinica as a lubricant base for the food industry

Crambe oil is an inedible seed oil extracted from the seeds of the Crambe abyssinica. This annual oilseed plant originated from the Mediterranean region, western Asia and eastern Africa and it occurs naturally in the Mediterranean Europe, Morocco and the Middle East [feedipedia.org/node/45]. The oil has got high content of erucic acid (50–60%, C22:1). Crambe is the cheapest source of erucic acid and it is considered to be a possible replacement for rapeseed oil in this capacity. Crambe abyssinica oil can be used as an industrial lubricant, as an corrosion inhibitor and as an ingredient to manufacture synthetic rubber. Abyssinian oil from ZielonyKlub company was used as a raw material, which is characterized by the following physicochemical properties: 0.906–0.911 g/cm³ density; 48.20 cSt kinematic viscosity at 40°C; 2.80 meq O₂/kg peroxide number; 83.48 g I₂/100 g iodine number; 170.50 mg KOH/g saponification number; and 0.34 mg KOH/g acid number.

Vegetable oil oxidation tests were carried out using the Design of Experiments technique. This method allowed to set up an optimal research plan, perform mathematical modelling based on the obtained results and finally verify the model's validity [Iłowska, Chrobak, Grabowski et al., 2018]. The use of this method entailed the reduction of the number of experiments and, as a result, facilitated obtaining products of the oxidation with the desired characteristics, especially viscosity, faster and more efficiently. The experiment plan was developed in accordance with Taguchi's approach, which assumes the ‘best nominal’ criteria. Dependent variables were defined as: temperature, pressure and catalyst content in accordance with the technological parameters of the oil modification process. The experimental plan was composed of nine systems according to the orthogonal array. The aim of the conducted research was to obtain VG 150 viscosity class oil and because of that, the kinematic viscosity range of 135–165 mm²/s was targeted. It was also assumed that the modified oil should reach a viscosity that is as close as possible to 150 mm²/s. Within the framework of the research, the modification of vegetable oil from Crambe abyssinica involved its oxidation in the presence of the catalyst and was carried out in the presence of supercritical carbon dioxide (as a solvent) in a pressurized reactor.

The oxidation processes of Abyssinian oil were conducted using equipment that was composed of three devices: a Büchi limbo pressure reactor (Uster, Switzerland), a Blue Shadow HPLC metering pump (Berlin, Germany), and a Julabo thermostat (Seelbach, Germany). The raw materials (Abyssinian oil and a specified amount of N-hydroxyphthalimide catalyst) were placed into the pressure reactor together with oxygen and carbon dioxide. The pressure was controlled during the process. An oxidation process initiator azobisisobutyronitrile (AIBN) was used in the amount of 0.05% (w/w).

The obtained modified oils were tested through the determination of kinematic viscosity with Ubbelohde viscometer; the peroxide number in accordance with PN-EN ISO3960:1996 norm; the iodine number in accordance with PN-87/C-04281 norm; the saponification number in accordance with PN-C-04288-07:1988 norm; and the acid number in accordance with PN-ISO 660:1998 norm.

The manometric respirometry method, approved by the European Union guidelines, was used to determine the biodegradability of the samples of modified vegetable oils. This methodology was suitable for substances insoluble in water, keeping the dispersion of the test substance in the system at a constant level by constant agitation.

The decomposition of the test substance was determined by the biological oxygen demand (BOD), which is the amount of oxygen required for the oxidation of organic compounds by microorganisms. The tested oils, which were the only source of carbon in the sample, were applied directly to the bottles of the Lovibond^® BOD sensor set with the inoculated medium. The initial concentration of the tested oils was 100 mg/dm³. The study was conducted in the dark for 28 days at 22 ± 2°C. BOD determination was based on the measurement of the pressure in the closed system. The microorganisms in the sample consumed oxygen and produced CO₂ that was absorbed by the solid KOH. A negative pressure was created, which correlated directly with BOD as a measurement value. Systems containing modified vegetable oils, systems containing reference material as well as systems that were blank tests containing only bacterial inoculum in the medium were prepared in at least two replicates. As a reference substance, characterized by high biodegradability, ethylene glycol was used.

The percentage of biodegradation was calculated on the basis of measuring the amount of oxygen uptake in the bottle with the test substance corrected by the uptake in a parallel blank sample, expressed as a percentage of the theoretical oxygen demand (ThOD).

In order to determine ThOD, the carbon and hydrogen content of the tested modified vegetable oils was determined by elemental microanalysis. ThOD was then determined based on the formula in accordance with the OECD 301 guidelines.

The Raman spectra were obtained using a NRS 5100 Jasco confocal grating Raman microspectrometer (Tokyo, Japan), which was equipped with a pumped laser with a wave length of 532 nm and CCD (Charge Coupled Device) detector. The operating conditions of the spectrometer were as follows: diffraction grating of 1800 lines/mm; laser power of 5.1 mW; numerical aperture of 4000 μm; resolution of 8.4 cm⁻¹; lens of magnification of 20×, and exposure time of 15 s.

Ethylene glycol was used as the reference substance. The activated sludge from the second outflow of the biological chamber of the municipal sewage treatment plant in Kędzierzyn-Koźle was used as the inoculum. Selected parameters characterizing the activated sludge are given in Table 2.

The test results, including oxygen uptake and the progress of biodegradation of the modified vegetable oils over time, are presented in Table 3 and Figure 1 [Studnik, Iłowska, Chrobak et al., 2018]. Curves illustrating the process of oxygenic biodegradation by means of manometric respirometry (OECD 301F) of modified vegetable oils showed the effect of chemical modification on the ability of their microbiological degradation. Pure vegetable oils are characterized by high, nearly 100%, biodegradability in the test according OECD 301F. The high temperature used in oil oxidation processes, in addition to positive effects such as increased stability and viscosity of the modified base oil, affected the biodegradation of vegetable oils. Despite the use of an oxidation reaction catalyst to reduce the process temperature, the degree of biodegradation has decreased to about 70% for modified oils. Thermal modification in the presence of oxygen was accompanied by oligomerization of fatty acids, which translated into a slower rate of biodegradation of such products by microorganisms.

Figure 1

Raman spectroscopy is an instrumental method of chemical analysis, which provides detailed information and therefore can be used to describe changes in the chemical structure of vegetable oils caused by the modification process. The necessity of quality assessment, involving the determination of the presence of C=C bond trans and cis isomers, was the main reason for using this technique. Spectra for the initial Abyssinian oil and the oils after the oxidation processes are presented below (Figure 2).

Figure 2

Changes in the chemical structure can be described by observing the intensities of the bands, before and after modification processes, in the ranges corresponding to different chemical bonds. For the Oil 1, which was modified in the presence of oxygen, the main change is the reduction of intensities of the bands at 3008 and 1655 cm⁻¹. The oxidation process, which was conducted in the presence of oxygen and carbon dioxide, caused an increase in the intensity of the bands at 2850 and 2931 cm⁻¹ and a decrease in the band intensity at 3008 cm⁻¹. Furthermore, it was observed that the intensity of the band at 1263 cm⁻¹ dropped completely to the baseline for both modified oils. The most important observation is the absence of the band at 1670 cm⁻¹ that is typical for undesired trans isomers. Harmful oxidation products, which are associated with negative health effects, are not formed during both modification processes. The band assignments of the Raman spectra of initial Abyssinian oil and modified oils are reported in Table 4 [Szmatoła, Chrobak, Grabowski et al., 2018].

In conclusion, the oxidation of vegetable oils didn’t have a negative effect on their chemical composition. No harmful carcinogenic structures derived from trans bonds as well as aromatic and polycyclic structures were found in the Raman spectra of modified vegetable oils.