The quantity of oil contained in palm kernel seed was measured using the Soxhlet extraction method with hexane as a solvent. The result was used as a divisor to determine the extraction efficiency of the CBE intensified palm kernel extraction using green solvent. The oil yield using the Soxhlet oil method was 39.53 ± 2.2% with acid value of 1.06 ± 0.2%. This result is in agreement with previous studies (Costa et al. 2019; Tarigan et al. 2017). The lauric acid dominated the fatty acid profile at a concentration of 47% while the oleic acid (15.6%) was the highest for unsaturated fatty acid. As mentioned, the residual oil remaining after screw press method is 10 – 20%; therefore the middle number was used as a comparison to calculate extraction efficiency based on the screw press.
Table 1. Fatty acid composition of palm kernel oil
Fatty Acid
|
Concentration (g/100 g)
|
Percentage
|
C6:0
|
Caproic
|
0.06
|
0.16
|
C8:0
|
Caprylic
|
1.84
|
4.66
|
C10:0
|
Capric
|
1.58
|
4.00
|
C12:0
|
Lauric
|
18.58
|
47.01
|
C14:0
|
Myristic
|
6.03
|
15.25
|
C16:0
|
Palmitic
|
3.30
|
8.35
|
C18:0
|
Stearic
|
0.92
|
2.33
|
C20:0
|
Arachidic
|
0.08
|
0.21
|
C18:1
|
Oleic
|
6.16
|
15.58
|
C18:2
|
Linoleic
|
0.97
|
2.46
|
Total fatty acids
|
39.53
|
|
Saturated
|
32.40
|
81.96
|
Monounsaturated
|
6.16
|
15.58
|
Polyunsaturated
|
0.97
|
2.46
|
3.1. Effect of different extraction solvents
Six green solvents such as water, ethanol, d-limonene, dimethyl carbonate, isopropyl alcohol and ethyl acetate were used as solvent extractors on CBE-intensified palm kernel oil extraction and the solvent with the highest extraction efficiency was selected to study the operational variable of the CBE. As comparison, hexane also used either as a solvent for traditional soxhlet extraction or CBE-intensified palm kernel oil extraction. The extraction condition conducted for this experiments were ratio of palm kernel : solvent of 1:3 (w/v), extraction time of 10 minutes and 5000 rpm rotational speed. Figure 2A shows that all green solvents could extract oil from the palm kernel with an average oil yield of 26% except water, which could not extract the lipid out of the cell walls. It is interesting to note that hexane, which has similar polarity with lipid, only extracts 24% of palm kernel oil in CBE-intensified extraction method. This is probably because the extracted oil in the hexane at that volume ratio had reached its saturation point. Increasing the volume of hexane used showed significantly increased oil yield. In addition, only ethanol had oil yields less than hexane. The highest oil yield was obtained using ethyl acetate (29.5 ± 1%). This result is in agreement with Cascant et al. who concluded that ethyl acetate has the potential to replace environmentally hazardous hexane as extracting agent due to its ability to extract all classes of lipid (Cascant et al. 2017).
The average extraction efficiency based on the soxhlet method was quite low (66%) while the screw press method’s extraction efficiency was 84%. Ethyl acetate as the best extracting agent in this study showed extraction efficiencies of 74.6 ± 2.5% and 95.9 ± 3.2% based on soxhlet and screw press, respectively. These results showed that the CBE-intensified palm kernel oil extraction using green solvents has lower performance than the soxhlet and screw press methods. However green solvent has the potential to improve the mechanical disruption method with further variations of the operational parameters of CBE using ethyl acetate.
The average concentration of MG, DG and TG extracted using CBE-intensified palm kernel oil was 4, 2.2 and 84.4 mol%, respectively (Fig. 3A). The highest concentrations of MG and DG were extracted using d-limonene of 9.9 ± 0.6 and 6.4 ± 1 mol%, respectively, which are 2.5 and 7.4 times higher than the soxhlet method. Even though the oil yield extracted using ethanol was the lowest, the highest TG concentration of 87.9 ± 1.1 mol% occurred using this green solvent. Overall the TG extraction efficiency of 99% showed that all the green solvents used have similar performance with the soxhlet-hexane method on TG extraction process.
3.2. Effect of rotational speed
The rotational speed of CBE is one of the important factors that impact the oil yield. Normally in chemical reactors, increasing the agitation / rotational speed could increase the mixing intensity of reactants yielding high mass transfer and enhancing reaction rates (Mueanmas et al. 2019). Therefore in this study 6 different rotational speeds starting from 3000 to 8000 rpm with increments of 1000 rpm were investigated in extraction conditions of ratio of palm kernel to ethyl acetate of 1:3 (w/v) and extraction time of 10 minutes. As shown in Figure 2B the rotational speed of CBE did not affect the oil yield. The average oil yield of 27.8% was obtained, which is less than the soxhlet and screw press method. The analysis of the variance test indicated a significant effect of changing the rotational speed for CBE-intensified extraction efficiency of both the soxhlet and the screw press method. The significant effect was mainly driven by rotational speed of 6000 and 8000 rpm which yielded the lowest extraction efficiencies of 66.5 ± 2.4% and 67.1 ± 0.5%, respectively for comparison with soxhlet method and 85.5 ± 3.1% and 86.3 ± 0.6% for extraction efficiency based on the screw press method. For all variables tested, the highest extraction efficiency was obtained under the extraction condition of 5000 rpm rotational speed.
The average concentrations of MG, DG and TG (3.86, 0.79 and 84.72 mol%) extracted using the CBE-intensified method were similar to the concentration extracted using the soxhlet method (Fig. 3B). However, for some rotational speeds the extraction efficiency of MG, DG and TG exceeded the concentration extracted using the soxhlet method. The highest extraction efficiencies of MG, DG and TG were 138, 125 and 103%, respectively observed at rotational speeds of 6000, 3000 and 4000 rpm, respectively. An ANOVA one-way test detected the significant effect either for MG, DG or TG. A Tukey test post-hoc analysis established that the significant effect was driven by a high extraction of MG at the rotational speed of 6000 rpm (5.5 ± 0.6 mol%) while, in contrast, the low extraction of DG at a rotational speeds of 6000 – 8000 rpm (0.36 ± 0.1 – 0.45 ± 0.1 mol%) causes the significant effect.
3.3. Effect of extraction time
It is noted that extraction time has a significant effect on oil yield (Nguyen et al. 2020). Furthermore regarding production cost, electric consumption and extraction / reaction time have the biggest impact, as prolonged extraction / reaction time could increase the electricity consumption of the reactor (Chia et al. 2018). Therefore finding the specific extraction time is necessary. In this present study the extraction time was determined as ranging from 5 to 30 minutes in the extraction condition of the ratio of palm kernel to ethyl acetate of 1:3 at a rotational speed of 5000 rpm (Fig. 2C). The average oil yield of 28.3% obtained from this parameter ranged from 25.2 ± 0.38% to 29.5 ± 0.2%. The average extraction efficiency of the soxhlet method was 71%, and 92% of the screw press method. The highest extraction efficiencies were observed after 10 minutes extraction time with values of 74.6 ± 2.5% and 95.9 ± 3.2% of the soxhlet and screw press methods, respectively. Significant effects were observed for this parameter both for the extraction efficiencies based on soxhlet and the screw press methods. The further Tukey post-hoc test did not detect the main effect.
Extraction time had a significant effect on the extraction efficiency of MG, DG and TG (Fig. 3C). The extraction of MG was significantly highest at 10 minutes extraction time achieving extraction efficiency of 99.5% whilst a 122% extraction efficiency of DG was achieved in the same extraction time. In contrast, the TG was completely extracted in 5 minutes extraction time compared to TG concentration using soxhlet-hexane.
3.4. Effect of ratio palm kernel to ethyl acetate
The effects of the ratio of palm kernel to ethyl acetate on CBE-intensified oil yield and MG, DG and TG extraction were investigated at seven different ratios in extraction condition of 5000 rpm rotational speed and 10 minutes extraction time. The average extraction efficiency based on the soxhlet method was 74% with the highest (86.53 ± 3.6%) achieved at a ratio of 1:7. In contrast, the extraction efficiency based on the screw press method exceeded 100% at a ratio of 1:6 and 1:7 achieved 108.7 ± 1.3% and 111.2 ± 4.6%, respectively (Fig. 2D). This result shows that the CBE-intensified is superior in palm kernel oil extraction against the screw press method and in fact this was done in only 10 minutes extraction time, which is faster than the traditional soxhlet and mechanical press methods. The univariate test of significance for both the extraction efficiency based on soxhlet and the screw press methods showed significant effects. The post-hoc Tukey test further detected that the significant effect was mainly drive by all the parameters tested. The results are similar to previous results in studies which used thin film devices in fatty acid to fatty acid methyl ester conversion efficiency of microalgae Chloroparva pannonica and fungi Mucor plumbeus biomass with a ratio of biomass to methanol at 1:6 to 1:18 (Sitepu et al. 2018a; Sitepu et al. 2018b; Sitepu et al. 2019).
Significant effects were determined for MG, DT and TG in this study. However the Tukey test post-hoc did not detect the main effect. The average concentrations of MG and DG obtained were quite similar with the concentrations extracted using the soxhlet-hexane method whilst the average TG concentration exceeded the standard. The highest concentrations of MG and DG of 3.96 ± 0.4 and 1.06 ± 0.02 mol% were observed at a ratio 1:3 at , respectively and a concentration of 90.5 ± 0.4 mol% was obtained for TG at a ratio of 1:4 (Fig. 3D).
3.5. Energy consumption comparisons
Energy consumption of CBE-intensified palm kernel oil extraction was determined as kilowatt hour electricity consumption per kilogram palm kernel used (Table 2). As comparison, the traditional Soxhlet-hexane extraction method was conducted using a hotplate magnetic stirrer as the heating source. The data of palm kernel oil extraction using the screw press method was obtained from published research (Ezeoha 2020). Another green extraction method using a digestive drug mainly composed of a mixture of amylase and protease enzymes was also used. This method used a shaking incubator to facilitate the hydrolysis reaction of palm kernel cell walls generating palm kernel oils (unpublished result). Due to insufficient data on the extraction of palm kernel oil using a subcritical fluid, the published result on the extraction Jatropha oil was used, as the oil content in Jatropha curcas is in a range similar to palm kernel (Liu et al. 2014).
Table 2. Comparison of energy consumption of some extraction methods.
Extraction Method
|
Seed
|
Extraction Condition
|
Energy Consumption (kWh Kg-1)
|
Ref.
|
CBE green solvent
|
Palm kernel
|
Palm kernel 20 gram, ratio palm kernel : ethyl acetate (1:7), rotational speed 5000 rpm, extraction time 10 minutes, oil yield = 34.2%
|
12.53
|
This study
|
Screw press
|
Palm kernel
|
Fabricated palm kernel screw press, capacity 101.7 kg h-1, speed 56 rpm, oil yield = 39%
|
45
|
(Ezeoha 2020)
|
Subcritical fluid
|
Jatropha curcas
|
Jatropha seed = 2.4 gram, ratio jatropha : solvent (1:20), temperature 90°C, pressure 0.5 MPa, 15 minutes extraction time, oil yield = 40.3%
|
57.3
|
(Liu et al. 2014)
|
Enzymatic
|
Palm kernel
|
Palm kernel 20 gram, ratio palm kernel : water (1:5), concentration of digestive drug 9% (wt/v), pH 9, incubation time 90 min and temperature 45°C, oil yield = 38.3%
|
74.25
|
Unpublished result
|
Soxhlet-hexane
|
Palm kernel
|
Palm kernel 20 gram, 150 ml Hexane, 30 minutes extraction time, oil yield = 39.5%
|
15
|
This study
|
As shown in Table 2 the energy consumption of CBE-intensified palm kernel oil using a green solvent was determined based on the maximum oil yield obtained in the extraction condition. The CBE consumed 1.5 kW to operate in room temperature. Therefore the total energy required for processing 20 g of palm kernel at a rotational speed of 5000 rpm for 10 minutes was 12.53 kWh kg-1, which is equal to 45.11 MJ kg-1 seed. The energy consumption of CBE-intensified palm kernel oil extraction was lower than for other extraction methods (Table 2). The CBE-intensified method provides an energy saving of 72, 78, 83 and 16% compared to the screw press, subcritical fluid, enzymatic and soxhlet-hexane processes, respectively. In addition, the CBE-intensified process could be conducted in only 10 minutes which is 33, 89 and 67% faster than subcritical fluid, enzymatic and soxhlet-hexane processes, respectively.