Palmitic acid (> 99%) and squalene (GC grade) came from Merck (Germany). Monopalmitin (99%), dipalmitin (99%) and tetradecane (99%) were supplied by Sigma (USA). Pyridine (99.8%), hexane (> 95%), acetone (> 99.8%), acetonitrile (HPLC grade) and N-methyl-N-trimethylsilyl-trifluoroacetamide were purchased from Fluka (Switzerland), Lab-Scan (Ireland), Riedel-de Häen (Germany), Prolabo (France), and Macherey-Nagel (Germany), respectively.

Ripe palm fruits (Elaeis guineensis) were from Carotech (Malaysia). The fruits were separated into skin, mesocarp (pulp), and kernel. The yellow part of the mesocarp was investigated in this work. The average particle size of the pulp ready for extraction was about 1 mm x 2 mm x 6 mm (Phan Tai and Brunner, 2019).

A standardized supercritical extraction system developed at the Institute of Thermal Separation Processes, Hamburg University of Technology was used as described in previous research (Phan Tai and Brunner, 2019). Fluid, propane or carbon dioxide (99.95% purity) was delivered from the reservoir tank by a Maximator pump (max. 600 bar) to the 100 mL steel extractor cell, which was loaded with 14.5 g of palm mesocarp for each run. A specific flow rate of 35 kg·h⁻¹ of gas per kg of sample was used and the pressure and temperature were monitored. The extracts were collected continuously in 10-mL glass vials, used as sample collectors at atmospheric pressure. Duplicate runs were carried out for each experimental condition with a reproducibility of ± 5%.

Statistical analysis was performed with JMP® version 10 software (SAS, USA). Data were expressed as the mean of triplicate measurements. One-way analysis of variance (ANOVA) and Tukey test (P < 0.05) were carried out to test any significant differences between means.

The composition of mesocarp varies with the size and age of the palm fruit. Table 1 presents the average composition of palm mesocarp used as material input for the extraction in this study. The results show that palm mesocarp is a good source to extract oil and valuable minor compounds like carotenes or tocochromanols. The concentrations of these components are in agreement with those reported by Phoon et al. (2018). However, the total mono and diacylglycerol contents in the studied palm fruit is rather high compared to 5% reported elsewhere (Tan and Nehdi, 2012). This can be attributed to the enzymatic hydrolysis of the oil under the influence of an endogenous lipase in the pulp (Doye R. Abigor, 1985) after long transportation and preservation of the palm fruit. The difference may be also due to the analysis method, maturation stage, and environmental growth variation of the palm fruit.

The course of a solid extraction of oil can be represented by an overall extraction curve, in which the amount of extract accumulated during the course of the extraction is plotted as a function of time. Figure 1 shows the extraction curves of palm oil with subcritical propane in comparison with SCCO₂. According to Brunner (1994), the first part of the overall extraction curve is linear, corresponding to a constant extraction rate. The gradient of this part may represent the equilibrium solubility of the extract in supercritical fluid. However, the straight line of the overall extraction curve could correspond to a constant mass transfer resistance. In the second part, the extraction rate is declining and the graph approaches a limiting value where all the extractible substances are removed from the input material.

The results show that free oil was more soluble in subcritical propane than in SCCO₂. However, extraction with SCCO₂ gave better total oil yields after 45 minutes when the available oil near the palm surface was depleted. Within the study conditions, palm oil could be recovered by up to 80% after 120 minutes with SCCO₂ at 400 bar and 70% with subcritical propane at 50 bar. The difference in oil recovery can be attributed to the structural change in palm fibers during the process. It was proven that SCCO₂ can affect the cellulose structure by increasing the accessible surface area of the cellulosic substrates (Kim and Hong, 2001; Putrino et al., 2020). Lau et al. (2006) reported a palm oil yield of 77.3% obtained with SCCO₂ at 300 bar and 80 ^oC for 8h. In another research, it was shown that flaxseed oil extraction yields using subcritical propane were lower compared to the result from using SCCO₂ (Piva et al., 2018).

The loading of solvent during the extraction can also be obtained from the first part of palm oil extraction curves. This value is commonly considered as apparent solubility and calculated from the extraction when palm oil is easily accessible throughout the fixed bed (at constant extraction rate). In the case of palm oil, SCCO₂ at 400 bar only had a loading capacity of 1.7 - 2.7%, while subcritical propane at 50 bar can reach an oil loading of up to 4.5% depending on the extraction condition as described in Figure 2. The same phenomena were also reported by Zanqui et al. (2016) in which the extraction of Sacha inchi (Plukenetia volubilis L.) oil using subcritical propane was faster compared to SCCO₂ due to the higher solubility of lipids in propane.

Palm oil is a very good source of carotenes and tocochromanols, which are interesting valuable minor compounds in supercritical fluid extraction. The concentrations of carotenes and tocochromanols in extracted palm oil by compressed propane at 50 bar are presented in Figure 3. It was observed that the concentration of these compounds varied moderately during extraction time with the subcritical propane. Using subcritical propane can co-extract these minor compounds with concentrations in the same range as a normal pressed palm oil. This is in agreement with Trentini et al. (2017) regarding extraction from macauba pulp, who assert that propane is more efficient to extract active compounds like tocopherols and carotenoids compared to CO₂. In the extraction of oil from perilla, a higher concentration in tocopherols was determined in oil resulting from compressed propane extraction compared to the classical Soxhlet method (Silva et al., 2015).

Palm oil is a complex product consisting of many components as presented in Table 1. Therefore, the concentration in carotenes and tocochromanols of extracted palm oil depends not only on their solubilities in the solvent but also on the solubilities of other compounds like mono, di- or triglycerols at the same time. A decrease in tocochromanol concentration was observed when the temperature was increased from 55 to 65 °C. A rather high temperature may decrease the content in tocochromanols because these compounds are sensitive to temperature. In contrast, it was observed that the concentration of carotenes increased with an increase in temperature. The condition of less solubility of tocochromanols in extracted palm oil can be more favourable for the solubility of other compounds like carotenes. As a result, the concentration in carotenes increased from ca 400 mg·kg⁻¹ to 500 mg·kg⁻¹ when temperature increased from 55 to 65 °C. Zanqui et al. (2016) also showed that temperature can influence the lipid composition of Sacha inchi oil extracted by subcritical propane. The subcritical propane of Maximiliana maripa pulp at 40 °C and 60 bar also provided fast extractions and high yields of oil enriched in beta-carotene (Turola Barbi et al., 2019).

It was reported that a high amount of carotenes and tocochromanols remained in the palm residue from extraction with SCCO₂ or screw pressing (Birtigh et al., 1995). However, subcritical propane stands out as a good solvent to recover these valuable compounds. As shown in Figure 4, there is a slight difference in tocochromanol and carotene concentrations in the oil extracted from palm mesocarp by subcritical propane and those of oil in the palm residue of extraction. To objectively evaluate the efficiency of recovery of tocochromanols and carotenes, a relative comparison of the concentrations of these compounds in the extracted oil and residue oil was used. Enrichment factor K of a component, as described in a previous study (Phan Tai and Brunner, 2019), is defined as the following equation (Eq. 1). As a result, extraction with a higher value of K(X) provides better potential to recover component X.

The results in Table 2 show that carotenes and tocochromanols can be recovered effectively by subcritical propane. The recovery efficiency of these minor compounds by using subcritical propane was better compared to other extraction methods by using SCCO₂ or traditional crew pressing. Phan Tai and Brunner (2019) reported that enrichment factors K(carotenoids) and K(tocochromanols) of palm oil extracted by SCCO₂ at 400 bar and temperature of 45-65 °C was only around 0.90-1.19 and 0.54-0.86, respectively. This also agrees with a previous study which confirmed that compressed propane has higher solvating power compared to SCCO₂, which results in a reduction in the consumption of solvent, higher efficiency and shorter extraction time (Silva et al., 2015). It was also reported that subcritical propane extraction was a suitable and selective method for the extraction of the foxtail millet bran oil in view of smaller times and lower pressures employed compared to SCCO₂ and revealed the possible high content in carotenoids and highest tocopherol content obtained (Shi et al., 2015).

Moreover, it was reported that the extraction of minor compounds from the palm-pressed fiber is not very practical (Chuang and Brunner, 2006). Therefore, the results of this study prove that using subcritical propane as an extraction solvent will bring more benefit because of better recovery of these valuable compounds. The subcritical propane extraction of palm oil, simultaneously recovering its high contents in carotenes and tocochromanols from the palm fruits appears as a promising alternative separation technique for palm oil processing.