A comparative study of apricot kernel oil yield using different extraction methods

Abstract

Apricot kernel was used as a raw material to compare and analyze the yield of apricot kernel oil using a pressing method, an ultrasonic-assisted extraction method, and a Soxhlet extraction method. The optimum extraction conditions were further verified through gas chromatography-mass spectrometry (GC-MS), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The fatty acid composition of the apricot kernel oil consisted of palmitic acid and stearic acid, and the total content of cis-oleic acid and cis-linolenic acid reached 93%. The apricot kernel oil obtained by Soxhlet extraction had the highest yield. However, the Soxhlet method has some limitations, such as high consumption of energy, long extraction time, and low efficiency. Ultrasound-assisted extraction has been developed by the industry to minimize these disadvantages. Finally, nuclear magnetic resonance (NMR) analysis was carried out on the ultrasonic-assisted extraction method, and it was again shown that the apricot kernel oil contained many unsaturated fatty acids.

INTRODUCTION

Biofuels have been introduced to replace non-renewable fuels. In addition, animal, vegetable, and waste oils can be used as fuels to replace traditional diesel fuels. As the main product of the apricot kernel synthesis process is a fatty acid, it is considered a renewable source of fuel (Jamilet al. 2020; Koletet al. 2020). Apricot trees are the largest nut trees in the world, belonging to the Rosaceae family, also known as wild apricot (Haciseferoğullari et al. 2007; Wirthensohn et al. 2008; Falcóet al. 2020; Özcan et al. 2020; Vichi et al. 2020). Apricots are rich in protein, fats, vitamins, and other nutrients, and they possess good medicinal properties. The many applications of apricots have attracted widespread attention (Yada et al. 2013; Zhou et al. 2019; Salinas et al. 2020).

In recent years, the research on apricot kernel oil has increased. The extraction methods of apricot kernel oil include hot pressing, cold pressing, Soxhlet extraction, ultrasonic extraction, and supercritical carbon dioxide (CO₂) fluid extraction. Zhu et al. (2015) extracted apricot kernel oil by the aqueous enzymatic method, the Soxhlet extraction method, and the supercritical CO₂ fluid extraction method, and the physical and chemical properties were analyzed. Falcó et al. (2020) studied the stability of apricot kernel fat by infrared spectroscopy. Furthermore, Zhou et al. (2019) explored the effect of subcritical fluid extraction on the quality of apricot kernel oil compared with conventional mechanical extrusion. Da et al. (2014) analyzed the quality of different apricot kernel oils from three sources: commercially available, low-temperature pressing, and solvent extraction.

Apricot kernels were used as the raw material in this study to further study their oil yield. The effects of the pressing, ultrasonic-assisted extraction, and Soxhlet extraction methods on the oil yield of apricot kernels were compared. The composition of the apricot kernel oil was analyzed by gas chromatography-mass spectroscopy (GC-MS) and its physicochemical properties were analyzed. A visual summary of the technical route used in this study can be seen in Fig. 1. This work provides a strong theoretical basis for the development and utilization of apricot kernel resources.

Fig. 1. Summary of the technical route used in this study

EXPERIMENTAL SECTION

Materials

The apricot kernels were purchased from Chifeng, China. The seeds were peeled, cleaned, dried, and broken into several pieces for storage.

Reagents and Instruments

Petroleum ether, glacial acetic acid, potassium hydroxide, sodium thiosulfate, anhydrous sodium carbonate, and potassium bromide were all analytically pure.

A digital display constant temperature water bath pot was obtained from Changzhou Jiangnan Experimental Instrument Factory (HH-1, Jiangsu, China). An ultrasonic disperser was obtained from Dandong Baite Instrument Co. (BT-50, Dangdong, China). An oil press was obtained from Zhengzhou Qixin Machine Co. (BOZY-01G, Henan, China). A 7890B GC system and an inductively coupled plasma emission spectrometer were obtained from Agilent Technologies (Santa Clara, CA, USA). A Fourier transform infrared spectrometer was obtained from Bruker (Tensor, Billerica, MA, USA), and a scanning electron microscope was obtained from Thermo Fischer Scientific (Phenom Pro, Waltham, MA, USA).

Methods

Pressing method

In this experiment, the apricot kernels were dried for 10 to 50 min using the cold pressing method, and the apricot oil was obtained by pressing the apricot kernels with an oil press (Güneşer and Yilmaz 2019; Ashirovet al. 2020). A diagram of the oil press can be seen in Fig. 2. The oil press contains material used for solid-liquid separation. When the oil press runs, the processed oil enters the chamber from the hopper. The spiral press continuously pushes the embryo forward to apply pressure.

Fig. 2. Diagram of the pressing method

Ultrasonic-assisted extraction

Twenty grams of apricot powder were weighed and placed in a beaker, and petroleum ether (60 to 90 °C) was added to the beaker according to a certain solid/liquid ratio (between 1:3 and 1:11). The beaker was placed in an ultrasonic disperser, a sketch of which can be seen in Fig. 3.

Fig. 3. Diagram of the ultrasonic-extraction method

The ultrasonic probe was immersed in one-third of the mixed liquid, and the extraction power (120 to 360 W) was set. The extraction time was set between 30 and 70 min. The extracted liquid was left to stand for 12 h, and the supernatant was poured out. The extract was removed by rotary evaporation. The obtained apricot oil was weighed and its yield was calculated. The ultrasonic-extraction method changes the physical and chemical properties of materials. Since ultrasonic extraction is based on the cavitation phenomenon, thermal effects, and mechanical effects, it can effectively destroy plant cell walls and accelerate the release, diffusion, and dissolution of intracellular substances (Liet al. 2004; Luque-García and Luque de Castroet al. 2004; Cravottoet al. 2008; Zhanget al. 2009; Liet al. 2015; Ninget al. 2020). Thus, the cavitation phenomenon can effectively accelerate the dissolution of oil in almonds and can improve the efficiency of the experiment.

Soxhlet method

In this experiment, the Soxhlet extraction method was used to extract almond oil from seeds. A diagram of the Soxhlet extraction device is shown in Fig. 4. The seeds were put into the flask, and petroleum ether and other additives were added. The bottle mouth was connected to the condenser tube. The water bath pot was adjusted to the required temperature to start the reaction until the required vegetable oil was obtained.

Fig. 4. Diagram of the Soxhlet extraction device

The apricot kernel oil was extracted by pressing, ultrasonic-assisted extraction, and Soxhlet extraction methods. The oil yields of the different methods were calculated according to Eq. 1 (Hao et al. 2011):

(1)

where Y_oilis the oil yield (%), M_oil is the mass of the extracted oil (g), and M_ds is the mass of the dried apricot kernel seeds (g).

The acid value determination, the refractive index determination, and the saponification value determination were conducted according to the GB/T 5530 (2005), 5527 (2010), and 5534 (2008) standards, respectively. The moisture and volatile determination and the relative density determination were conducted according to the GB/T 14489.1 (2008) and 5009.2 (2016) standards, respectively. The relative molecular weight was calculated according to the measured acid value and saponification value using Eq. 2:

(2)

All of the fatty acids were determined according to the third method in the GB 5009.168 (2016) standard. The determination of elements was determined according to the GB 5009.268 (2016) standard. The obtained apricot kernel oil was analyzed by GC with a running time of 52 min and a dilution factor of 1.0. The scanning wavenumber range was 4000 to 400 cm^-1 using the potassium bromide tableting method. The elements of the apricot kernel oil were analyzed by inductively coupled plasma mass spectroscopy (ICP-MS). According to the images obtained by scanning electron microscopy (SEM), the oil extracted from the almond seeds was compared and analyzed. The obtained data were recorded by Excel software (Microsoft, Redmond, WA, USA), and the images were processed using Origin software (Northampton, MA, USA).

RESULTS AND DISCUSSION

Pressing Method

As shown in Fig. 5, the oil yields at drying times of 10, 20, 30, 40, and 50 min were 31.0%, 32.9%, 33.3%, 32.4%, and 31.9%, respectively. To diminish the energy loss, 20 min was selected as the drying time.

Fig. 5. Effect of the drying time on the apricot kernel oil yield

Ultrasonic-Assisted Extraction

Extraction time

The effect of the extraction time on the apricot kernel oil yield was observed at a temperature of 80 °C, a solid/liquid ratio of 1:7, and a power of 240 W (Fig. 6a). The oil yield increased to the highest point and then decreased as the extraction time increased, which occurred because ultrasonic-assisted extraction can lead to plant cell rupture and can accelerate oil dissolution (Donget al. 2010). The oil yield reached the highest value when the extraction time was 50 min (Fig. 6a).

Fig. 6. The effect of different conditions on the apricot kernel oil yield obtained by ultrasonic-assisted extraction at (a) 50 min, (b) 1:7, and (c) 240 W

Solid-to-liquid ratio

The effect of the solid/liquid ratio on the apricot kernel oil yield was observed at a time of 50 min, a temperature of 80 °C, and a power of 240 W (Fig. 6b). The oil yield increased as the solvent content was increased. When the solid/liquid ratio was 1:7, the oil yield reached the highest point before it stabilized and slightly decreased. According to the mass transfer principle, the increase in the solvent content promotes the dissolution of oil. However, when the amount of solvent was sufficient, the oil in the apricot kernel was dissolved, and no further increase in the amount of solvent was needed (Zhanget al. 2009; Louet al. 2010). Therefore, 1:7 was selected as the optimal solid-to-liquid ratio.

Ultrasonic power

Using an extraction time of 50 min, a temperature of 80 °C, and a solid-to-liquid ratio of 1:7, the effect of ultrasonic power on the oil yield of the Apricot kernels was observed (Fig. 6c). As the ultrasonic power increased, the oil yield also gradually increased. The oil yield reached its highest point when the ultrasonic power was 240 W, after which it gradually decreased. Due to the enhancement of the ultrasonic power, the cavitation effect became stronger, leading to the rupture of the cell wall and the acceleration of the oil spill. Wang et al.(2019) found that when the ultrasonic power was greater than 240 W, the volatilization of solvent and the dissolution of oil were accelerated due to the enhancement of the thermal effect. Therefore, 240 W was selected as the optimal ultrasonic power.

Soxhlet Method

Extraction temperature

Under the condition of extracting for 3 h with the same mass of apricot kernel powder, the effect of the extraction temperature on the apricot kernel oil yield was investigated. The results of the 3-h extraction condition can be seen. It can be seen from Fig. 7a that at a temperature lower than 68 °C, the extraction temperature had no effect on the oil yield, after which it rose to the highest point before gradually stabilizing. This is because an extremely high temperature results in solvent volatilization and high energy consumption (Wanget al. 2019). Therefore, 76 °C was selected as the optimal extraction temperature.

Extraction time

The effect of the extraction time on the apricot kernel oil yield was explored at 76 °C with the same mass of apricot kernel powder. The results are shown in Fig. 7b. As the extraction time increased, the oil yield initially increased before it decreased. The oil yield reached its highest point at 3 h. Then, because the apricot kernel oil was completely extracted, the extension time did not increase the oil yield (Wanget al. 2019). Therefore, 3 h was selected as the optimal extraction time.

Fig. 7. The effect of different Soxhlet extraction conditions on the apricot kernel oil yield under conditions of (a) 76°C and (b) 3 h

Analysis of the Fatty Acid Composition of Apricot Kernel Oil

GC-MS analysis

Gas chromatography-mass spectrometry was used to analyze the effects of different extraction methods on the fatty acid composition of the apricot kernel oil. The fatty acid composition and the GC content of the apricot kernel oil are shown in Fig. 8 and Tables 1, 2, and 3. The fatty acid composition of the apricot kernel oil was the same across the different extraction methods. Apricot kernel oil mainly contains palmitic acid, stearic acid, cis-oleic acid, and cis-linoleic acid, among others. The cis-oleic acid and cis-linoleic acid comprise a large portion of the total fatty acid content, up to 93%. According to Hao et al.(2011), methyl oleate (C₁₉H₃₆O₂) is an ideal molecular compound that can replace diesel fuel. Other researchers (Harrington 1986; Xie and Zhou 2007) have reported that substances with a long carbon chain and a small number of double bonds are suitable alternatives to diesel fuel. Additionally, experiments have confirmed that oils rich in oleic acid and linoleic acid are the most suitable raw materials to produce biodiesel; meanwhile, the oil composition of apricot oil is precisely dominated by C18, which is similar to the molecular formula of ideal diesel. Therefore, preparing biodiesel from apricot kernel oil is feasible.

Fig. 8. The main fatty acids of apricot kernel oil extracted by (a) Soxhlet extraction, (b) ultrasonic-assisted extraction, and the (c) pressing method 1. hexadecanoic acid (palmitic acid), 2. cis-9-hexadecanoic acid (palmitic acid), 3. octadecanoic acid (stearic acid), 4. cis-9-octadecenoic acid (cis-oleic acid), and 5. cis-9,12-octadecadienoic acid (cis-linoleic acid)

Table 1. Fatty Acid Composition and GC Content of the Apricot Kernel Oil Obtained by Soxhlet Extraction

Table 2. Fatty Acid Composition and GC Content of the Apricot Kernel Oil Obtained by Ultrasonic-Assisted Extraction

Table 3. Fatty Acid Composition and GC Content of the Apricot Kernel Oil Obtained by Pressing

Physicochemical properties and elemental analysis

The fatty acid profile of almonds is mostly unsaturated with oleic acid and linoleic acid accounting for 600 to 700 g⋅kg^-1 and 150 to 250 g⋅kg^-1, respectively. The unsaturated nature of this oil makes almonds very interesting from a nutritional point of view but also quite susceptible to degradation (Falcóet al. 2020). The physicochemical properties are shown in Table 4.

Table 4. Comparison of the Physical and Chemical Properties of the Apricot Kernel Oil Obtained with Different Extraction Methods

The acid value of the Soxhlet extraction method was higher than that of the other two methods. This was attributed to the fact that the Soxhlet extraction method was too long, resulting in the hydrolysis of the generated glycerol and an increase in the acid value. Moreover, the high saponification value indicated that the free fatty acid content in the almond oil was high, and the relative density was in the normal range (Wei et al. 2009, 2011; Geng et al. 2018; Jedidi et al.2020; Maestri et al.2020).

Table 5 shows the element content of apricot oil obtained by ultrasonic extraction. The top five elements in the apricot oil were phosphorus, zinc, magnesium, potassium, and calcium, with contents of 43.8, 34.0, 14.9, 14.3, and 12.5 mg/kg, respectively. The element content in apricot kernel oil is extremely rich, and long-term consumption can be beneficial for physical and mental health.

Table 5. Element Composition and Content of the Apricot Kernel Oil

Fourier transform infrared spectroscopy (FTIR) analysis

The FTIR analysis of the apricot kernel oil extracted by different methods is shown in Fig. 9.

Fig. 9. The FTIR results of the apricot kernel oil extracted by the (a) ultrasonic-assisted, (b) Soxhlet extraction, and (c) pressing methods

The structure of the apricot kernel oil extracted by different methods remained the same. The stretching vibration peak of C-H appeared at 3007 cm^-1 in the apricot kernel oil, and C-H stretching vibration peaks of the saturated hydrocarbon group appeared at 2925 and 2854 cm^-1. The stretching vibration peak of C=O in the triglyceride ester bond appeared at 1747 cm^-1, whereas 1163 cm^-1was the C-O-C stretching vibration peak of the ester group, and 723 cm^-1 was the stretching vibration of the carbonyl group (Zheng et al. 2013). According to the FTIR results, the fatty acid structures in the apricot kernel oil obtained by different extraction methods are similar, which further correspond with the fatty acid composition and GC content.

SEM analysis

The surface morphology characteristics of the Prunus armeniaca seeds were analyzed by SEM (Guoet al. 2016; Komartinet al. 2021; Shiet al. 2021). As shown in Fig. 10a, the surface of the P. armeniaca seeds was smooth and complete, and the oil edge was clear. Oil were all attached to the cell surface in a spherical or elliptical shape, and they were closely arranged in cells. As shown in Fig. 10b, the cells in the seeds were destroyed, and a large amount of oil was dissolved in the cells, which clearly illustrates the reduction of oil in the cells. It was shown that the extraction rate of the seed oil was greatly improved by ultrasonic-assisted extraction. The seeds extracted by Soxhlet extraction are shown in Fig. 10c. Due to the effect of the solvent, a large amount of oil was extruded from the cell wall of the seeds, which accelerated the rapid dissolution of the oil. The pressed seeds are shown in Fig. 10d.

Fig. 10. The SEM images of almond seeds (a) before treatment and after (b) ultrasonic-assisted extraction, (c) Soxhlet extraction, and (d) pressing

The surface of the seeds was still flat, while the oil in the cells was randomly pressed into a flat shape. Some oils formed large liposomes, which were distributed on the surface of the cells. The pressing method did not increase the efficiency of oil production compared with the two previous methods.

¹H NMR and ¹³C NMR analyses of the apricot kernel oil

Firstly, according to the data obtained by a single factor experiment, the ultrasonic-assisted extraction method had high potential in promoting the oil yield of apricot oil. Secondly, GC-MC, physical and chemical property, elemental, and FTIR analyses of the apricot oil extracted by different methods were carried out.

It was found that there was no strong difference in the fatty acid composition and functional group structure of the apricot oil. Finally, the SEM analysis of apricot oil extracted by different methods was carried out. It was found that the seeds that were treated by ultrasonic-assisted extraction had a much higher oil yield. Therefore, the ultrasonic-assisted extraction of apricot oil was selected for the NMR analysis, which was done by adding an appropriate number of samples to the NMR tube, followed by deuterated chloroform (CDCl₃) and mixing evenly. The oil was then observed at frequencies of 400 and 101 MHz (Yanget al. 2015; Merchaket al. 2017; Fadhilet al. 2019; Xiaoet al. 2020).

As shown in Fig. 11, the ultrasonic-assisted extraction of apricot oil was characterized by ¹H NMR, and 7.28 ppm was selected as the solvent peak. At the chemical shift of δ 5.37 to 5.33 ppm, olefinic hydrogen appeared. Methylene appeared at δ values between 5.28 and 5.26 ppm and between 4.32 and 4.13 ppm. Methylene also appeared at δ 2.77 to 2.76 ppm in linoleic acid and at δ 2.31 to 2.30 ppm in glyceryl ester linked to the carbonyl group.

Methylene appeared at both ends of olefins in the ester chain at δ 2.02 to 2.01 ppm and the β position of the carbonyl group in the ester chain at δ 1.61 ppm. Lastly, methylene appeared at δ 1.43 ppm to 1.27 ppm in the rest of the ester chain, and at δ 0.89 ppm to 0.87 ppm in the glyceryl ester of palmitic acid, stearic acid, oleic acid, linoleic acid, and arachidic acid.

Because the hydrogen spectrum cannot be used to analyze the position specificity of fatty acids, and the NMR carbon spectrum has obvious advantages in analyzing fatty acids at different positions in apricot oil, the ¹³C NMR of apricot oil was characterized, and 77 ppm was selected as the solvent peak. The high resolution of the carbon spectrum led to the low overlap of peaks, which was more complicated than the hydrogen spectrum. In the chemical shift of δ 173.16 to 172.14 ppm, a small peak appeared. At δ 130.11 to 127.80 ppm, there was an unsaturated peak, and the strongest peak occurred at δ 29.04 to 28.97 ppm.

It was concluded that there are many unsaturated fatty acids in apricot oil, which can be supported by other tests.

Fig. 11. The (a) 1H NMR and (b) 13C NMR spectra graphs of the apricot kernel oil

CONCLUSIONS

1. Different extraction methods were investigated for apricot kernels, and it was found that the oil yield was greatly impacted by the extraction method. However, the extraction method did not have a large impact on the physical and chemical properties of the apricot kernels.

2. The results indicated that the ultrasonic-assisted extraction method was advantageous for increasing the oil extraction yield with a reduced processing time, using a lower amount of solvent, and maintaining the quality of the extracted oil.

3. The results from this study could be directly explored in future work, such as investigating the optimal process of ultrasonic-assisted extraction.

4. It was determined by GC analysis that the sum of cis-oleic acid and cis-linolic acid in the apricot kernel oil reached 93%. As such, using apricot kernel as a raw material in biodiesel fuel can help facilitate the production of sustainable fuels.

ACKNOWLEDGMENTS

This work was supported by the Transformation Fund for Science and Technology Achievements of Inner Mongolia Autonomous Region (No. 2019CG018).

References

Ashirov, M. Z., Datkhayev, U. M., Myrzakozha, D. A., Sato, H., Zhakipbekov, K. S., Rakhymbayev, N. A., and Sadykov, B. N. (2020). “Study of cold-pressed tobacco seed oil properties by gas chromatography method,” The Scientific World Journal 2020, 8852724. DOI: 10.1155/2020/8852724

Cravotto, G., Boffa, L., Mantegna, S., Perego, P., Avogadro, M., and Cintas, P. (2008). “Improved extraction of vegetable oils under high-intensity ultrasound and/or microwaves,” Ultrasonics Sonochemistry 15(5), 898-902. DOI: 10.1016/j.ultsonch.2007.10.009

Da, C., Hongzhi, L., Li, L., Hui, H., and Qiang, Q. (2014). “Quality comparison of almond oil extracted with different processes and correlation analysis of quality indexes,” China Journal of Grain and Oil 29(2), 47-52.

Dong, J., Liu, Y., Liang, Z., and Wang, W. (2010). “Investigation on ultrasound-assisted extraction of salvianolic acid B from Salvia miltiorrhiza root,” Ultrasonics Sonochemistry 17(1), 61-65. DOI: 10.1016/j.ultsonch.2009.05.006

Fadhil, A. B., Nayyef, A. W., and Sedeeq, S. H. (2019). “Valorization of mixed radish seed oil and Prunus armeniaca L. oil as a promising feedstock for biodiesel production: Evaluation and analysis of biodiesels,” Asia-Pacific Journal of Chemical Engineering 15(1). DOI: 10.1002/apj.2390

Falcó, I. P. R., Moya, M. S. P., Pérez, S. E. M., Carratalá, M. L. M, and Teruel, N. G. (2020). “Chemometric comparison of almond oxidation rates using kinetic parameters obtained by infrared spectroscopy,” Journal of the Science of Food and Agriculture 100(12), 4549-4557. DOI: 10.1002/jsfa.10513

Güneşer, B. A., and Yilmaz, E. (2019). “Comparing the effects of conventional and microwave roasting methods for bioactive composition and the sensory quality of cold-pressed orange seed oil,” Journal of Food Science and Technology 56(2), 634-642. DOI: 10.1007/s13197-018-3518-y

GB 5009.2 (2016). “National food safety standard. Determination of relative density of foods,” Standardization Administration of China, Beijing, China.

GB 5009.168 (2016). “National food safety standard. Determination of fatty acids in foods,” Standardization Administration of China, Beijing, China.

GB 5009.268 (2016). “National food safety standard. Determination of multi-elements in food,” Standardization Administration of China, Beijing, China.

GB/T 5527 (2010). “Animal and vegetable fats and oils. Determination of refractive index,” Standardization Administration of China, Beijing, China.

GB/T 5530 (2005). “Animal and vegetable fats and oils. Determination of acid value and acidity,” Standardization Administration of China, Beijing, China.

GB/T 5534 (2008). “Animal and vegetable fats and oils. Determination of saponification value,” Standardization Administration of China, Beijing, China.

GB/T 14489.1 (2008). “Oilseeds. Determination of moisture and volatile content,” Standardization Administration of China, Beijing, China.

Geng, P., Pen, Y., Hu, C., He, D. (2018). “Comparison of physicochemical properties and oil characteristics of walnuts from eight regions,” China Oils and Fats 43(9), 116-120.

Guo, W., Li, J.-x., Yue, Y., Li, L.-l., and Jin, H. (2016). “Microstructure observation of Xanthoceras sorbifolia bunge seed with different oil content by scanning electron microscope,” Heilongjiang Agricultural Science 10, 92-94.

Haciseferoğullari, H., Gezer, İ., Öezcan, M. M., and MuratAsma, B. (2007). “Post-harvest chemical and physical-mechanical properties of some apricot varieties cultivated in Turkey,” Journal of Food Engineering 79(1), 364-373. DOI: 10.1016/j.jfoodeng.2006.02.003

Hao, Y. N., Wang, X. M., and Ding, L. J. (2011). “Preparation of biodiesel from Xanthoceras sorbifolia bunge seed oil,” Advanced Materials Research 183-185, 1777-1782. DOI: 10.4028/www.scientific.net/AMR.183-185.1777

Harrington, K. J. (1986). “Chemical and physical properties of vegetable oil esters and their effect on diesel fuel performance,” Biomass 9(1), 1-17. DOI: 10.1016/0144-4565(86)90008-9

Jamil, U., Khoja, A. H., Liaquat, R., Naqvi, S. R., Omar, W. N. N. W., and Amin, N. A. S. (2020). “Copper and calcium-based metal organic framework (MOF) catalyst for biodiesel production from waste cooking oil: A process optimization study,” Energy Conversion and Management 215, 112934. DOI: 10.1016/j.enconman.2020.112934

Jedidi, B., Mokbli, S., Sbihi, H. M., Nehdi, I. A., Romdhani-Younes, M., and Al-Resayes, A. I. (2020). “Effect of extraction solvents on fatty acid composition and physicochemical properties of Tecoma stans seed oils,” Journal of King Saud University – Science 32(4), 2468-2473. DOI: 10.1016/j.jksus.2020.03.044

Kolet, M., Zerbib, D., Nakonechny, F., and Nisnevitch, M. (2020). “Production of biodiesel from brown grease,” Catalysts 10(10), 1189. DOI: 10.3390/catal10101189

Komartin, R. S., Stroescu, M., Chira, N., Stan, R., and Stoica-Guzun, A. (2021). “Optimization of oil extraction from Lallemantia iberica seeds using ultrasound-assisted extraction,” Journal of Food Measurement and Characterization 15, 1-11. DOI: 10.1007/s11694-020-00790-w

Li, H., Pordesimo, L., and Weiss, J. (2004). “High intensity ultrasound-assisted extraction of oil from soybeans,” Food Research International 37(7), 731-738. DOI: 10.1016/j.foodres.2004.02.016

Li, H.-Z., Zhang, Z.-J., Hou, T.-Y., Li, X-J., and Chen, T. (2015). “Optimization of ultrasound-assisted hexane extraction of perilla oil using response surface methodology,” Industrial Crops and Products 76, 18-24. DOI: 10.1016/j.indcrop.2015.06.021

Lou, Z., Wang, H., Zhang, M., and Wang, Z. (2010). “Improved extraction of oil from chickpea under ultrasound in a dynamic system,” Journal of Food Engineering 98(1), 13-18. DOI: 10.1016/j.jfoodeng.2009.11.015

Luque-Garcı́a, J. L., and Luque de Castro, M. D. (2004). “Ultrasound-assisted Soxhlet extraction: An expeditive approach for solid sample treatment,” Journal of Chromatography A 1034(1-2), 237-242. DOI: 10.1016/j.chroma.2004.02.020

Maestri, D., Cittadini, M. C., Bodoira, R., and Martínez, M. (2020). “Tree nut oils: Chemical profiles, extraction, stability, and quality concerns,” European Journal of Lipid Science and Technology 122(6), 1900450. DOI: 10.1002/ejlt.201900450

Merchak, N., Silvestre, V., Loquet, D., Rizk, T., Akoka, S., and Bejjani, J. (2017). “A strategy for simultaneous determination of fatty acid composition, fatty acid position, and position-specific isotope contents in triacylglycerol matrices by 13C-NMR,” Analytical and Bioanalytical Chemistry 409(1), 307-315. DOI: 10.1007/s00216-016-0005-z

Ning, X., Lu, Q., Ban, W., Luo, X., Luo, Z., Mo, Z., Du, W., and Wang, L. (2020). “Extraction technology optimization of shatian pomelo seed oil and its properties,” Journal of the Chinese Cereals and Oils Association 7, 66-71.

Özcan, M. M., Matthäus, B., Aljuhaimi, F., Ahmed, I. A. M., Ghafoor, K., Babiker, E. E., Osman, M. A., Gassem, M. A., and Alqah, H. A. S. (2020). “Effect of almond genotypes on fatty acid composition, tocopherols and mineral contents and bioactive properties of sweet almond (Prunus amygdalus Batsch spp. dulce) kernel and oils,” Journal of Food Science and Technology 57(3), 4182-4192. DOI: 10.1007/s13197-020-04456-9

Salinas, F., Vardanega, R., Espinosa-Álvarez, C., Jimenéz, D., Muñoz, W. B., Ruiz-Domínguez, M. C., Meireles, M. A. A., and Cerezal-Mezquita, P. (2020). “Supercritical fluid extraction of chañar (Geoffroea decorticans) almond oil: Global yield, kinetics and oil characterization,” The Journal of Supercritical Fluids 161, article no. 104824. DOI: 10.1016/j.supflu.2020.104824

Shi, Y., Wang, W., Zhu, X., Wang, B., Hao, Y., Wang, L., Yu, D., and Elfalleh, W. (2021). “Preparation and physicochemical stability of hemp seed oil liposomes,” Industrial Crops and Products 162, article no. 113283. DOI: 10.1016/j.indcrop.2021.113283

Vichi, S., Mayer, M. N., León-Cárdenas, M. G., Quintanilla-Casas, B., Tres, A., Guardiola, F., Batlle, I., and Romero, A. (2020). “Chemical markers to distinguish the homo- and heterozygous bitter genotype in sweet almond kernels,” Foods 9(6), 747. DOI: 10.3390/foods9060747

Wang, H., Chao, Y., Li, D., and Hu, B. (2019). “Comparative study on microwave-assisted and ultrasonic-assisted extraction of peach kernel oil,” Chemistry and Industry of Forest Products 39(6), 44-52. DOI: 10.3969/j.issn.0253-2417.2019.06.006

Wei, L.-p., Zhao, Z., Ma, X.-h., Li, K.-y., Ma, Y.-h., Guo, C.-j., Mao, Q.-z., and Qi, G.-g. (2009). “Technological condition of converting biodiesel from bitter almond oil,” Journal of Northwest Sci-Tech University of Agriculture and Forestry 3, 146-150.

Wei, L., Zhao, Z., Ma, Y., Ma, X., and Li, K. (2011). “Study on physical and chemical properties of biodiesel from bitter almond oil,” Journal of Anhui Agricultural Sciences 34, 21259-21261.

Wirthensohn, M. G., Chin, W. L., Franks, T. K., Baldock, G., Ford, C. M., and Sedgley, M. (2008). “Characterising the flavour phenotypes of almond (Prunus dulcis Mill.) kernels,” The Journal of Horticultural Science and Biotechnology 83(4), 462-468. DOI: 10.1080/14620316.2008.11512407

Xiao, K., Gong, C., Guo, Q.-S., and Xu, X. (2020). “Simultaneous determination of fatty acids at sn-1,3 and sn-2 of triglyceride in edible oils by quantitative 13C-nuclear magnetic resonance,” Chinese Journal of Analytical Chemistry 6, 802-810.

Xie, Z.-j., and Zhou, B. (2007). “Influence factors of exploiting biodiesel,” China Oils and Fats 32(9), 49-50.

Yada, S., Huang, G., and Lapsley, K. (2013). “Natural variability in the nutrient composition of California-grown almonds,” Journal of Food Composition and Analysis 30(2), 80-85. DOI: 10.1016/j.jfca.2013.01.008

Yang, Y., Yin, Y., Yin, F., Sun, X., and Li, W. (2015). “NMR spectra characteristics of several kinds of edible oils and quick discerning of swill-cooked dirty oil,” China Oils and Fats 40(7), 45-50. DOI: 10.3969/j.issn.1003-7969.2015.07.010

Zhang, Q.-A., Zhang, Z.-Q., Yue, X.-F., Fan, X.-H., Li, T., and Chen, S.-F. (2009). “Response surface optimization of ultrasound-assisted oil extraction from autoclaved almond powder,” Food Chemistry 116(2), 513-518. DOI: 10.1016/j.foodchem.2009.02.071

Zheng, Y., Huang, Y., and Zheng, Z. (2013). “Optimization of ultrasonic-assisted extraction of rubber seed oil,” China Oils and Fats 38(6), 1-4.

Zhou, Q., Xiao, J., Liu, Y., Wan, C., Zheng, C., Li, S., and Huang, F. (2019). “The effect of the subcritical fluid extraction on the quality of almond oils: Compared to conventional mechanical pressing method,” Food Science & Nutrition 7(7), 2231-2241. DOI: 10.1002/fsn3.1023

Zhu, Z., Liu, K., Yi, J., Liu, M., and Li, Y. (2015). “Comparative research on the properties of three different extraction methods of almonds oil from Prunus armeniaca Linn,” Journal of the Chinese Cereals and Oils Association 30(2), 57-60.