In our study, seeds of four cruciferous species were kindly provided by the M.M. Hryshko National Botanical Garden at the National Academy of Sciences of Ukraine (Kyiv, Ukraine).

In particular, they were from ten C. sativa genotypes (breeding lines FEORZhYaF-1, FEORZhYaF-2, FEORZhYaF-3, FEORZhYaF-4, FEORZhYaF-5, FEORZhYaFD, FEORZhYaFCh, FEORZhYaFChP and varieties ‘Peremoha’, ‘Evro-12’), six winter (breeding lines EOSOFU, EOSOFGl, EOSOFDn, EOSOFVol and varieties ‘Oriana’, ‘Oriana-1’) and four spring genotypes (breeding lines EOSYaF-1, EOSYaF-2, EOSYaFR, Pioner-YuT) of B. campestris, eight R. sativus var. oleifera (breeding lines EORDOFL-2, EORDOFL-3, EORDOF-5, EORDOF-6, EORDOF-8 and varieties Rayduha’, ‘Tambovchanka’, ‘Kyyanochka’) and five genotypes of hybrid culture tyfon (breeding lines EOTFVS, EOTFV and varieties ‘Obriy’, ‘Fitopal’, ‘Orakam’).

Field trials of all the above-mentioned genotypes were conducted on testing plots at the M.M. Hryshko National Botanical Garden (50⁰32’′N, 30⁰33′E). Plot size was 4 m long and 1.5 m wide. Each genotype was cultivated on a separate testing plot (n=3) for 3 years from 2015 to 2018. The soil of the plots was classified as alfisol (рН 6.1-6.8, measured at 1:2 soil to water ratio). The average air temperature and precipitation in this area were +7.6^oC and 550-560 mm per year.

Seed total lipid content was measured using the Soxhlet extraction method [39]. Estimated seed oil yield (kg/ha) for each crop was calculated based on lipid content and seed yield data. Oil content was measured for each genotype (n=3) per species across the three harvesting seasons (2015-18) as well as seed yield and estimated oil yield.

Seed oil from the 2018 harvest was used for chromatography assay and further biodiesel production tests. Samples for chromatography assay were prepared in at least three replicates (n=3) per genotype, whereas seeds for each repetition were taken from a separate testing plot. Oil extraction from seed samples for GC-analysis was done using the manual press Prom-1 (Olis, Ukraine). Conditions were as follows: mass of press – 20 kg, maximum power gain – 12 t, glass volume for oil 200 cm³, the exposure time for seeds under pressure to get the final product – 5 min. Seeds from the 2018 harvest were taken to extract oil for gas chromatography assay. The obtained oil was transferred to 2 mL vials, samples were left to stand for 48 h at 8°C for precipitation of pigments and then cleaned oil was poured into new vials and used for fatty acids methyl esters (FAMEs) preparation.

In 10 mL glass tube, 60 mg of oil sample, 4 mL of isooctane and 200 μL of 2 M potassium hydroxide dissolved in methanol were added. Then, the tube was vortexed to obtain a clear solution, which meant the completion of reesterefication reaction. To neutralize potassium hydroxide, 1g of sodium sulfate monohydrate was added. After salt sedimentation, the upper layer was transferred into a new 4 mL tube. The obtained isooctane solution contained approximately 15 mg/mL of FAMEs.

FAMEs were separated and quantified by Gas Chromatography (GC) with Flame Ionization Detection (FID). The chromatographic analysis was performed with a GC–MS instrument (Agilent 7890B-7697, USA) equipped with a capillary column (60 m × 0.25 mm × 0.2 μm) Zebron ZB-FAMЕ (Phenomenex Inc., USA) using nitrogen as a carrier gas, H₂ and air for FID detector. Other chromatographic parameters were set as follows: the vaporizer temperature +250°С, column temperature +185°С. In total, the analysis lasted 40 min. The injection volume was 1 μL. The identification of the individual compounds was performed by frequent comparison with authentic standard mixtures (С₁₀–С₂₄, Supelco) analyzed under the same conditions.

Fatty acids composition was expressed as follows: MUFA –monounsaturated fatty acids, PUFA - polyunsaturated fatty acids, SFA - saturated fatty acids, SCFA - fatty acids with carbon chain length 18 or less, LCFA - fatty acids with carbon chain length more than 18.

The transesterifìcation reaction of oil was carried out using samples from C. sativa (FEORZhYaFD), B. campestris (cv. ‘Oriana’), R. sativus var. oleifera (cv. ‘Rayduha’) and tyfon hybrid (cv. ‘Fitopal’). All experiments were repeated three times (n=3) for each genotype at each of the different transesterification conditions. For each repetition, oil samples from different testing plots were used, where particular genotypes had been cultivated. Fatty acid ethyl esters (biodiesel) were produced via mixing the required amount of vegetable oil with an alcohol (bioethanol) in the presence of an alkaline catalyst and heating up to +60°C. The volume of the reactor used for transesterification (Fig. 1) was 500 cm³. Two types of experiments were conducted for each oil sample using different catalysts:

Fig. (1). Reaction of transesterification which occurs during biodiesel production.

Fig. (2). Scheme of facilities for biodiesel production. 1 – catalyst reactor; 2 – stirred biodiesel reactor; 3 – glycerol decanter; 4 – biodiesel washing decanter.

1. The plant seed oil was taken in a 1:6 molar ratio to dehydrated bioethanol (C₂H₅OH). As a catalyst, sodium hydroxide (NaOH) was added in ethanol in different quantities: 0.5, 1, 1.5, 2 and 2.5% (w/w) of plant oil that was used in the reaction;
2. The plant seed oil was also taken in a 1:6 molar ratio to dehydrated bioethanol (C₂H₅OH). Sodium ethoxide (C₂H₅ONa) as a catalyst was added in ethanol in different quantities from 0.5, 0.8, 1, 1.3, 1.5 to 2% (w/w) of plant oil that was used in the reaction.

The reaction of transesterification lasted 90 min for ‘type a)’ reaction and 30 min for ‘type b)’ reaction. After the reaction was completed, a two-phase system was formed in which higher density glycerol was in the lower phase and fatty acid esters and remainder of alcohol were in the upper layer. The phases were then separated by pouring glycerol (Fig. 2) and the catalyst leftovers were neutralized by adding 10% v/v solution of glacial acetic acid and then washed with water. After that, a new two-phase system was formed in another decanter, where saponificated fats were collated at the bottom. The phases were again separated and, as a result, pure fatty acid ethyl esters were obtained following the dehydration with sodium sulfate.

To improve the technology of FAEE production, other types of reactions were also tested. The main procedure remained as described above, except for the reaction conditions. Camelina oil (280 g) was preliminarily mixed with 70 g of mineral diesel (at 4:1 ratio). A similar approach was described in our patent, using sunflower oil [40]. Sodium hydroxide was used as a catalyst (0.5%, 0.8%, 1.0%, 1.3%; 1.5%, 2.0% as w/v percentage of plant oil amount) dissolved in dehydrated bioethanol, (molar ratio ethanol to plant oil 1:6).

Additionally, the fatty acids conversion efficiency was evaluated for reaction, based on another solvent. As a catalyst, NaOH was added in 0.8 % w/w quantity for production tests with the addition of polyoxyethylene (20) sorbitan monooleate (Tween 80, Polysorbate 80, E433), which was added in 0.5 % w/w to the oil. Similar to the previous reaction, camelina oil was preliminarily mixed with mineral diesel at a 4:1 ratio. The reaction time was 30 min.

Complex catalyst efficiency was tested using winter turnip rape (B. campestris f. biennis) oil. Furthermore, dehydrated bioethanol was used in oil:bioethanol molar ratio 1:6. As a complex catalyst, a 3:1 mixture of sodium ethoxide and monoethanolamine [41] was added in 0.05, 0.07 and 0.1% (w/w) of oil. The reaction time was 60 min. All mentioned productivity tests were conducted

All biodiesel samples obtained were tested according to Ukrainian regulations on diesel fuel quality (DSTU 7688:2015). The following criteria were used to evaluate the main technical characteristics of produced samples: cetane number, sulfur presence, presence of hydrogen sulfide, corrosive properties, acidity and ash content. Cetane number was identified according to the procedure described in ASTM D613-18a. Measurement of the sulfur amount was recorded by the methods from ASTM D4294-10, EN ISO 20846:2011, EN ISO 13032:2012, as well as the presence of hydrogen sulfide. Corrosive properties were tested on a copper plate as described in ASTM D130-12, the test was scored as ‘passed’ if no significant colour change of copper was observed (group 1a and 1b by ASTM D130-12 classification). Acidity was identified as the amount of KOH in mg per 100 cm³ (0.1 L) of fuel that is needed for sample neutralization. This procedure was performed according to the GOST 5985-59 and the GOST 14141-69 (technical standards issued in the former Soviet Union). Ash content was measured as described in ASTM D482-12.

All statistical processing of obtained data was conducted using OriginPro 9.1 software. Deviations of all means were calculated as the Standard Deviation (SD). To identify significance of differences in yield, oil content and estimated oil productivity values between studied genotypes, Fisher’s Least Significant Differences (LSDs) were calculated. The LSDs were used to identify homogeneous groups in yield, oil content and estimated oil productivity values separately for each studied species (with p<0.05 significance level). The same was done for fatty acids content data, for which homogeneous groups were identified separately for particular acid content. The LSDs-based comparison, in this case, was also done separately for each species. In terms of analysis of MUFA, PUFA, SFA, SCFA, LCFA content, LSDs were calculated, taking into account data obtained for all studied genotypes of four species.

Fig. (3). C. sativa in mass blooming phase.

Fig. (4). C. sativa in seeds ripening phase.

One of the most important traits that play a crucial role in the quality of diesel biofuel is the fatty acid composition of plant oil used for its production [4, 14, 64-66]. Moreover, the fatty acid profile allows not only to predict the quality of the fuel, but it also predetermines oil usage for other technical purposes [16, 17, 22, 67]. Thus, we have conducted a chromatographic analysis of seed lipid samples of each of the above-mentioned plant genotypes.

Properties and, consequently, the quality of biodiesel, are directly determined by the fatty acid composition of the plant feedstocks. In practical terms, it means presence/absence and proportions of particular fatty acids. It is well known that the carbohydrate chain length of a fatty acid molecule determines its melting point and ignition temperature as well as the products of the transesterification [1, 4, 12, 13, 76]. Additionally, the presence of a double bond in the fatty acid molecule increases compound volatility, but decreases oxidative stability [3]. In general, the ratio between different fatty acids’ amounts determines properties of oil and properties of the end-product, such as biodiesel, lubricant material, etc [21, 22, 67]. Since different structural characteristics of fatty acids (length of carbon chains and the number of double bonds) have a determining impact on biodiesel quality, we suggest to evaluate oil composition profiles by comparing total values of such groups of fatty acids [4, 67, 76].

Thus, based on the obtained data on the seed oil fatty acid composition, the levels of each main fatty acid category were calculated for all studied genotypes: MUFA, PUFA, SFA plus separate classification by fatty acid chain length: short-chained (C18 and less or SCFA) and long-chained (C18 and more or LCFA). All these features are presented in Table 11, while the average amounts of appropriate fatty acid class for each species are illustrated in Fig. (11).

Fig. (11). Mean values of MUFA, PUFA, SFA, SCFA and LCFA for camelina, winter and spring turnip rape, oil radish and tyfon genotypes based on their fatty acid profiles (M±SD, Table 11). The significance of differences (in Latin letters) is indicated above the bars separately for each parameter (MUFA, PUFA, etc.).

Our data in Fig. (11) show that tyfon and winter turnip rape fatty acid patterns are rather close, they both possess the highest values of long-chained fatty acids comparing to other species. The highest proportion of short-chained fatty acids has been identified in camelina and spring turnip rape oils, while false flax possesses at least two-fold the amount of PUFA comparing to other studied species. It is worth to note that camelina has the highest content of saturated fatty acids (SFA) probably because of the highest content (up to 11.08%) of palmitic (16:0) acid. The highest MUFA levels were found in tyfon and winter turnip rape genotypes.

Practically it is possible to obtain biodiesel via transesterification from the majority of plant oils and other lipid sources, e.g., animal fats [77]. The problem is that not all such ‘fuels’ are suitable for usage in engines. For normal engine performance, obtained biodiesel must correspond to the same technical requirements as traditional mineral diesel [1, 78, 79]. Besides, fatty acid esters usually demonstrate higher density and viscosity at low temperatures, which cause additional restrictions to the use of such fuel under cold weather conditions [76, 78, 80].

Since the goal of this investigation is to obtain FAEE using bioethanol instead of more toxic methanol, it poses stricter limitations for plant oil source, because FAEEs are less volatile than FAMEs [76, 81]. Therefore, feedstock for biodiesel production should contain a significantly higher proportion of SCFAs than LCFAs. At the same time, fatty acids should be unsaturated, preferably monounsaturated, although polyunsaturated fatty acids could also be suitable, as they do not negatively affect the storage time [1, 4, 80].

The problem of low oxidative stability could potentially be bypassed by creating special storage conditions, by mixing the biofuel of interest with fuel from other feedstock or by performing on some production stage oil hydroprocessing to saturate double bonds with H₂ [24, 26, 82]. However, the latter approach has not been well elaborated yet. Consequently, all these approaches, especially the last one, can lead to price increase and cause other unfavourable market conditions for the biofuel.

Oil feedstocks with high levels of LCFAs could also be used for biodiesel production, but, as it has been mentioned above, such fuel would have lower volatility at low temperatures and, as a consequence, lead to worse performance of the engine.

The other useful application for such seed lipids is the production of lubricant materials. In such a case, the presence of long-chained fatty acids is even desirable, but on the condition that the majority of fatty acids are monounsaturated; in an ideal situation, it could be just one dominating MUFA, e.g., erucic (22:1) acid [67, 76]. Moreover, the content of polyunsaturated fatty acids should be as low as possible, since oxidative stability is crucial for lubricants which are often exposed to high temperatures for a long time [22]. Having taken into account all the above-mentioned requirements, we decided to analyze particular genotypes of each studied species by comparing MUFA, PUFA, SFA, SCFA, LCFA values, given in Table 11.

Having considered the data in Fig. (11), it could be concluded that winter turnip rape and hybrid tyfon with dominating ‘heavy’ fatty acids and the highest MUFA content have better prospects for usage as lubricant feedstock. False flax, spring turnip rape and oil radish that contain high proportions of SCFAs could be a good oil source for biodiesel production, especially due to the high amounts of MUFA contained in spring turnip rape and oil radish. At the same time, the main disadvantage of B. campestris f. annua is its rather low productivity – only 737-998 kg of seed oil per hectare that makes its industrial cultivation financially unprofitable. Probably, further breeding of spring turnip rape with higher productivity crops could eliminate this obstacle.

In general, camelina seed lipids contain a large amount of PUFA, which can pose some difficulties for fuel storage. Low oxidation stability of C. sativa derived biodiesel can be improved by adding synthetic antioxidants, like butylated hydroxyanisole (BHA), 2,6-di-tert-butyl-4-methylphenol (BHT), propyl gallate (PrG) and tert-butylhydroquinone (TBHQ). It was shown that these antioxidants make a positive impact on fuel storage time when they are added to camelina-derived biodiesel in the amount of 500-3000 ppm [83]. On the other hand, oil of this crop is characterized by the highest content of short-chained FAs, including one at the highest level, that is C16:0, amongst the studied species

Industrial crop productivity of camelina is high enough compared to other studied crops, so it could be a very promising feedstock for biodiesel production. In addition, great attention of researchers to this crop creates more possibilities for further improvement of false flax using not only classical breeding approaches, but also by applying novel molecular genetic techniques [15, 50]. In particular, FEORZhYaFD can be considered as the most promising C. sativa genotype from the studied collection due to the highest content of short-chained fatty acids (85.37%), low amount of SFA (13.58%) and high crop productivity (1426 kg of oil per ha), as well as the highest oil content in seeds among all studied false flax genotypes – 42.6%. It was shown recently [84] that it was possible to increase the content of short-chained and very-short-chained (C10-C14) fatty acids in camelina oil by introducing seed-specific transgenes from Cuphea species to C. sativa and by RNAi silencing of genes for acyl-acyl carrier protein synthases. These modifications made it possible to produce false flax lines that were able to accumulate up to 28 mol% of C8:0, C10:0, C12:0, C14:0, with dominating capric (C10:0) acid – up to 25 mol% of total fatty acid content. Oil of such modified camelina could be easily converted either to biojet fuel or to biodiesel (probably with high volatility) [84].

Oil radish variety ‘Rayduha’ seems to be one of the most promising options for biofuel production due to rather favourable features, such as high crop productivity (3120 kg of seeds per ha), high SCFA content (70.31%), the lowest content of PUFA (24.7%) and the highest value of MUFA (65.36%) within all studied R. sativus var. oleifera genotypes.

Since winter turnip rape and tyfon have a significant amount of LCFA, they meet the criteria for lubricant feedstock. Variety ‘Oriana’ (B. campestris f. biennis) has a good potential as an oil source due to high crop productivity (1373 kg of oil per ha), the highest MUFA content (75.68%) and the lowest level of PUFA (19.36%) among all studied winter turnip rape genotypes. The same conclusion could be made for tyfon variety ‘Fitopal’ that produces high oil yield (1730 kg/ha) and contains 73.8% of MUFA.

This allows to conclude that the most appropriate genotypes for further biodiesel production tests are C. sativa – FEORZhYaFD; R. sativus var. oleifera - ‘Rayduha’; B. campestris f. biennis - ‘Oriana’; tyfon hybrid - ‘Fitopal’.

All our experiments were conducted using a pilot facility, which works according to the scheme illustrated in Fig. (12), except for procedures of glycerol purification and alcohol reuse, which were not included in our experiment, but suggested here as potential steps to be introduced on an industrial scale.

Presently, oil transesterifìcation (or reesterification), which occurs with alcohol (methanol, ethanol, etc.) in the presence of a catalyst, solvent or under supercritical conditions, represents the main technology for biodiesel production [4, 77, 78, 85]. The general scheme of this process and reaction formula are provided in Figs. (1 and 2).

There are two main technological approaches to obtaining biodiesel via transesterification. The first one, traditional, is based on a reaction induced by adding the catalyst (usually alkali or base catalyst) into a mixture of oil with alcohol in a molar ratio from 1:4 – 1:20. Such type of transesterification occurs at the temperatures below the methanol boiling point of 64.5^oC, if this alcohol is used. The reaction does not require extra pressure. The other methodology does not require a catalyst but involves other agents to facilitate the reaction: either the addition of a solvent or the creation of supercritical conditions. One of the most widely used solvents for reesterification is tetrahydrofuran, which is added being dissolved in alcohol. In this case, the reaction lasts for 5-10 min at +30 ^oC. The disadvantages of such a method are: increased cost of the end-product, high-flammability and volatility of tetrahydrofuran and, consequently, the requirement of special hermetic sealed equipment to work with. Another non-catalyst approach is based on creating supercritical conditions, that include significantly high temperatures (up to 350-400 ^oC), high pressure (up to 80 atm.) and usually high molar ratio of alcohol added to the oil (42:1) [1, 4, 5, 76, 86].

There are also enzymatic transesterification approaches which involve lipases of different origin instead of a catalyst [87, 88]. Alternatively, a novel method of biodiesel (biojet fuel) production, which is currently progressing very actively, is based on hydroprocessing (hydrocracking) of plant oils to obtain mainly n-alkanes suitable for fuel in jet engines [24, 26, 82]. According to a recent analysis, camelina-derived biojet fuel could provide up to 800 million gallons of fuel per year in the USA, which would potentially reduce greenhouse gas emission to 67-80% of its current level, compared with mineral oil-based jet fuel [89].

Fig. (12). General overview of the main steps of biodiesel production that takes place at the pilot facility used.

Fig. (13). Measurement of esters yield (conversion rate, %) depending on the catalyst (NaOH) amount (% w/w). * - difference among samples is not significant.

The ‘classic’ technologies for transesterification are still in demand due to their simplicity in implementation and a relatively predictable economic effect [85]. Thus, our study has been mostly focused on the catalytic reesterification of plant lipids, especially on FAEE production based on bioethanol. Currently, FAME is a common type of biodiesel, since the methanol-based reaction is faster, compared with ethanol-based, and it produces shorter chain ester from the same fatty acid, than ethanol or another alcohol, such as butanol, etc [4, 76]. Methanol is known to have high toxicity for the human body and could easily form highly-explosive mixtures with air. In addition, the possibility of diesel-biodiesel-alcohol blends is also discussed nowadays [4].

In our experiments, the first reaction type was based on NaOH usage as a catalyst (described in Section 2.6). Camelina oil (breeding line FEORZhYaFD) was used in the series of experiments to find out the optimal amount of catalyst that would allow obtaining maximal FAEE output. False flax was chosen as a model oil feedstock object, because the procedure for biodiesel production from its oil is well described and its fatty acid composition is very close to other common biofuel crop – soybean [90]. In each experiment, different quantities of NaOH (0.5, 1, 1.5, 1.6, 2 and 2.5% w/w of plant oil) were dissolved in ethyl alcohol, which was taken in a 6:1 ratio to plant oil. The curve of conversion rate was built based on the obtained results of these tests (Fig. 13). Fatty acid esters yield increased rapidly from 8.2% to 84.5%, following the increase of the catalyst amount up to 1.5%. The highest conversion rate reached was 87%, with 1.6% of the catalyst and started to decrease to an average of 85% of esters with 2.5% of NaOH.

In a subsequent step of our investigation, biodiesel samples were produced from oil of variety ‘Rayduha’ (R. sativus var. oleifera), v. ‘Oriana’ (B. campestris f. biennis) and v. ‘Fitopal’(tyfon). To do that, transesterification was carried out under the conditions that were considered optimal (1.5-1.6% w/w of NaOH) according to the results of the previous experiment. The obtained fuel samples were further examined for compliance with the requirements of the Ukrainian diesel fuel standards (Table 12). The highest conversion rate was registered for C. sativa oil – 87% w/w, while the lowest one was determined for winter B. campestris oil – 85%. The obtained fatty acid esters fit almost all main requirements of the Ukrainian standards, except for biodiesel samples produced from B. campestris and hybrid tyfon seed lipids, which had a lower cetane number than required: 46 vs 48. Other studies devoted to the production of biodiesel from the oil of R. sativus var. oleifera (FAMEs) reported that their obtained fuel samples had high acidity regardless of the previously mentioned FA profiles peculiarities [71, 74, 91]. In these papers, carbon residue amounted to 0.01-0.4% w/w levels, while in the present study, the ash content was only 0.015% w/w. Data on esters yield was reported in the other study [57], where it equaled 86% w/w (FAMEs), similar to the value in the present study, but for FAEE.

It was shown in the case of B. campestris oil that it was possible to reach 97% yield of FAME, if biodiesel is produced under the following conditions: 9:1 methanol to oil ratio, 1 wt% of NaOH as a catalyst and 75^oC reaction temperature, that is above the boiling temperature of this alcohol [92]. Authors of this research also mentioned that such fuel might fit all the ASTM D6751-15a requirements. Lately, it was established that it was possible to produce only 81% of biodiesel (also methyl esters-based) from turnip rape oil. On the other hand, this experiment has been carried out under different conditions (alcohol oil ratio – 6:1, 70 ^oC, unknown catalyst amount) that probably determined the decreased esters yield [93]. Alternatively, as reported earlier [94], the use of 2.5% w/w of sodium methoxide as a catalyst enabled to reach 84% of FAMEs yield from B. campestris oil. Authors of that study also identified that the B50 blend of that fuel had only 44 cetane number, while B100 showed it at a higher level – 53. Therefore, based on the described findings and data, presented in Table 12, we conclude that the technology of biodiesel production (either FAME or FAEE) from turnip rape oil requires further research and improvement.

The second reaction type was based on using sodium ethoxide (C₂H₅ONa) as a catalyst which was added in quantities of 0.5%, 0.8%, 1%, 1.3%, 1.5% and 2% w/w of plant oil weight. The general design of these experiments remained the same as it was described above for the other catalyst (NaOH). The mean values of the conversion rate of camelina seed lipids to FAEE with different catalyst amount are provided in Fig. (14). Ester yield at 0.5% w/w of the catalyst was very low, approximately 48%, but by adding a higher sodium ethoxide amount (0.8% w/w), it increased rapidly up to 90.1%. The higher conversion rate (avrg. 90.2%) was obtained with 1% w/w of the catalyst, which is not proved to be significantly different from the previous (0.8% w/w) sample. With more sodium ethoxide esters, the yield slowly decreased.

Fig. (14). Measurement of esters yield (conversion rate, %) depending on the catalyst (C2H5ONa) concentration (% w/w). * - difference among samples is not significant.

The following fuel samples were produced using oil of another studied oilseed species under the reaction conditions identified as optimal (0.8% w/w of C₂H₅ONa). The biodiesel derived from all these oil sources was tested to comply with the main requirements of the Ukrainian standard for diesel (Table 13). It is important to note that the cetane number increased up to 52 for the false flax sample (which at the same time possessed the highest acidity). The lowest cetane number was registered for winter B. campestris and tyfon samples. In general, the quality of the biodiesel samples, compared to the previous method, has improved. It is important to note that in both experiments, camelina oil-based biodiesel samples possessed the highest acidic value due to the formation of the highest amount of PUFA ethyl esters. This leads to low oxidation stability that negatively impacts the fuel storage lifespan. As it was reported earlier, camelina biodiesel shows the poorest oxidative stability if compared with fuel from other common feedstocks (e.g., soybean, palm, sunflower, etc.) [90]. Apart from that, other advantages of false flax-derived biodiesel make it still very attractive as a fuel for engines.

In other studies, it was shown that camelina biodiesel (FAMEs) possessed 42.7-52.8 cetane number [46, 51, 95, 96], which is consistent with our results. Different standards require various minimal values of this parameter. The strictest limit is described in EN 14214 2012+A1:2014, that does not accept cetane number below 51, while ASTM D6751-15a sets this limit at 47. Ukrainian diesel fuel standard DSTU 7688:2015, which is based on both these documents, defines the cetane number limit as ‘not lower than 48’, to which most of the produced biodiesel samples comply with (Tables 12 and 13).