Effect of process duration and SCG-to-solvent ratio on lipid extraction

This section investigates the effect of solvent extraction parameters including duration, and SCG-to-solvent ratio on the efficiency of lipid recovery from SCG through the Soxhlet method (Section 3.2.3.1). Figure 5.1 shows the oil yields obtained from ICG1 on a dry weight basis with varying Soxhlet duration and a constant dry SCG-to-hexane ratio of 1:9.0 w/v, while a logarithmic best fit line has been added.

Figure 5.1: Oil yields per mass of dry ICG1 weight achieved at different Soxhlet extraction durations (0.5 to 24 hours). Error bars show the standard deviation of the mean calculated by three experimental repeats at each timing

setting.

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% Oil yield w/w

Duration (h)

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Figure 5.1 shows that the highest oil yield was achieved at 8 hours of solvent extraction, with a subsequent slight decrease of oil yields obtained at durations greater than 8 h. This is in agreement with Picard et al. (1984) and Sayyar et al. (2009) who investigated oil extraction via Soxhlet from roasted Robusta grounds and jatropha seeds respectively and found a duration of 8 hours to be ideal for oil recovery. Figure 5.1 also shows that the extracted oil yield reduces considerably when the duration of Soxhlet extraction is less than 2 hours. This is likely because the short duration of extraction does not allow sufficient time for the recirculating solvent to extract the total available oil from the SCG, and is in agreement with previous studies investigating the extraction of lipids from other oilseeds through Soxhlet which suggested that durations longer than 3.5 hours result in more efficient extractions (Kerbala University, 2010; Lawson et al., 2010).

Figure 5.2 shows the FFA content of ICG1 oil samples that were obtained at different durations of Soxhlet extraction with hexane at a SCG-to-solvent ratio of 1:9.0 w/v measured according to the method described in Section 3.2.6. A linear regression line has also been added.

Figure 5.2: Correlation of the % w/w FFA content of extracted coffee oil with the duration of solvent extraction (2-24 h). The error bars show the standard

deviation calculated from three titration measurements with oil samples extracted at different durations.

y = -0.2937x + 26.97 R² = 0.9684

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FFA content (%) w/w

Solvent extraction duration (h)

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Figure 5.2 shows that the FFA content of the extracted oil decreases with increasing duration of solvent extraction, with a R² value of 0.97. It is suggested that FFAs, as smaller molecules than triglycerides or diglycerides may have been preferentially desorbed from the SCG matrix before these larger molecules due to their lower Van der Waals forces, and consequently the fraction of these was higher when the duration of extraction was short.

However, as the duration of extraction increased, more triglycerides were recovered and therefore the percentage of FFAs in the collected oil decreased.

The effect of the dry ICG1 to solvent ratio on the oil yield was examined at a constant Soxhlet duration of 8 hours with hexane. Figure 5.3 shows the relationship between SCG-to-hexane ratio and obtained ICG1 oil yield, while a logarithmic best fit line has been added.

Figure 5.3: ICG1 oil yield achieved at varying SCG-to-solvent ratios. The error bars show the standard deviation calculated from three experimental repeats

at each SCG-to-solvent ratio setting.

It can be seen in Figure 5.3 that the variation of the SCG-to-solvent ratio, which ranged between 1:9.0 and 1:4.0 w/v, or 0.11 and 0.25 w/v as shown in the figure, caused a change in the obtained oil yield between approximately 24.6 % w/w and 17.6 % w/w, with a tendency for higher yield at the lower SCG-to-solvent ratios. The same relationship between decreasing seed to

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0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26

Oil yield (%) w/w

SCG-to-solvent ratio (w/v)

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solvent ratio and increasing oil yield was observed by Pichai and Krit (2015) and Sayyar et al. (2009) during oil extraction from SCG and jatropha seeds respectively. The slight difference in oil yield obtained at the three lower SCG-to-solvent ratios suggests that oil extraction from dry SCG can be successfully performed with a ratio of 1:5.5 w/v (or 0.18), with further increase of solvent quantity used only resulting in slightly improved lipid yields. The decrease of obtained oil yield with increase of SCG-to-solvent ratio above 1:5.5 w/v can be attributed to saturation of the solvent.

5.1.2 Effect of SCG particle diameter and moisture content on lipid extraction efficiency

5.1.2.1 Particle size determination

The coffee particles diameter distribution of SCG and FRCGs was calculated through the use of various test sieves as described in Section 3.2.2. Figure 5.4 shows the dry particle diameter distribution of various coffee samples.

Figure 5.4: Particle diameter distribution of various coffee samples after complete moisture removal. The error bars shown represent the standard

deviation of the mean calculated from 3 sizing repeats with the available sieves.

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80 230 380 530 680 830 980 1130 1280 1430 1580

Coffee particle size distribution (%) w/w

Average particle diameter (μm)

ICG1 ICG3 ICG4 RCG1 FRCG RCG2

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Figure 5.4 shows that the different coffee samples exhibit significant variation in particle diameter distribution; this can possibly be attributed to the grinding process of the roasted grounds, along with the different methods that they may have been subjected to during upstream processing and/or brewing.

Most of the SCG samples have a particle diameter less than 1700 μm and more than 500 μm, while ICG3 has the largest fraction of particles (57.41 % w/w) of diameter greater than 1700 μm.

Figure 5.4 also shows that 97 % w/w of the FRCG sample has a particle diameter between 500 μm and 850 μm, and is therefore the most homogeneous in terms of particle size distribution of all the coffee samples.

This low variance of particle diameter in FRCG can be likely attributed to the coffee machine grinder that reduces the roasted coffee beans to a predetermined size depending on the method by which they are to be brewed.

The particle size distribution of the same coffee grounds following brewing with hot water (RCG2) is significantly different, with 57 % w/w of the grounds now within a size of 500-850 μm, while 27 % w/w had a diameter greater than 850 μm and 15.5 % w/w a diameter less than 500 μm. These findings suggest that brewing process in an Espresso machine results in compression of some coffee particles into larger particle blocs, while fragmenting and reducing the size of others.

5.1.2.2 Effect of SCG particle diameter on solvent extraction efficiency Various size fractions of dry ICG1, ICG3, ICG4 and RCG1 samples were used at a fixed SCG-to-hexane ratio of 1:9.0 w/v, and extraction duration of 8 h.

Each of the SCG samples was split into fractions of the following particle diameter: < 0.5 mm, 0.5 mm < d < 0.85 mm and d > 1.7 mm. Figure 5.5 presents the oil yield achieved from different particle size fractions of each SCG sample.

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Figure 5.5: Oil yield of various SCG samples when different particle size fractions were used. The error bars represent the standard deviation (σ) of the

mean calculated from three experimental repeats.

Figure 5.5 shows that an intermediate particle diameter between 0.5 and 0.85 mm led to higher oil recoveries from all the SCG samples. This is in agreement with (Sayyar et al., 2009) who achieved the highest oil recoveries from jatropha seeds with particle size meal between 0.5 and 0.75 mm. With the exception of ICG1, solvent extraction from SCG particles of diameter larger than 1.7 mm resulted in higher oil recoveries relative to trials with SCG particles of diameter smaller than 0.5 mm, something that can possibly be attributed to the compaction and agglomeration of the fine particles that result in reduction of the surface area. This is in contrast with previous studies on found in Table 4.1), suggesting that a mix of different size particles facilitates the solvent and oil flow, therefore leading to higher oil recoveries.

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5.1.2.3 Effect of SCG moisture content on solvent extraction efficiency The effect of various ICG1 moisture levels on the crude extract yield was examined with hexane at a SCG-to-solvent ratio of 1:9.0 w/v, while three repeats were performed for every experiment. Figure 5.6 shows the relationship found between moisture content and achieved yield on a dry weight basis, while the various moisture content levels of the samples were achieved with thermal drying based on the method described in Section 3.2.1.

Figure 5.6: Crude extract yields on a dry weight basis versus the respective moisture content for 4 and 8 hours Soxhlet extractions with hexane.The error

bars represent standard deviation (σ) calculated from three experimental repeats at each timing setting.

Figure 5.6 shows that a moisture content of approximately 2 % w/w did not affect the solvent extraction process negatively and was actually beneficial as it resulted in a slightly higher crude extract yield relative to those obtained from SCG samples with lower moisture contents. Moisture contents greater than 2 % w/w resulted in lower crude extract yields, however a SCG sample with a moisture level of roughly 3.5 % w/w yielded ~20 % w/w at both the durations of 4 and 8 hours, which is not a significant drop from the peak yield of ~25 % w/w.

Dry weight crude extract yield (%) w/w

Moisture content (%) w/w

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The obtained results are consistent with previous studies that reported SCG oil yield decrease with increasing moisture presence when hexane was the solvent used (Caetano et al., 2014; Najdanovic-Visak et al., 2017). However, studies considering other oilseeds including soybeans found optimum oilseed moisture contents for solvent extraction between 9-11 % w/w, highlighting the differences in extraction characteristics between SCG and other feedstocks (KerbalaUniversity, 2010; Lajara, 1990; Lawson et al., 2010).

The effect of moisture on the extraction of crude extracts from SCG was also investigated with ethanol at a SCG-to-solvent ratio of 1:9.0 w/v for a constant duration of 8 hours. Figure 5.7 shows the dry weight crude extract yield extracted from ICG1 through Soxhlet with ethanol against the moisture content of the sample.

Figure 5.7: Crude extract yields on a dry weight basis versus the respective moisture content for 8 hours Soxhlet extractions with ethanol. Error bars represent standard deviation (σ=0.9) calculated from three experimental

repeats.

Figure 5.7 shows that a moisture content of ~2 % w/w was beneficial for extraction with ethanol, similar to experiments performed with hexane (Figure 5.6), while the relatively high crude extract yield (28.3 % w/w) obtained at this moisture content can be potentially attributed to enhanced extraction of polar

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Dry weight crude extract yield (%) w/w

Moisture content (%) w/w

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compounds (Johnson and Lusas, 1983). Increase of moisture content above this limit resulted in decreased yields, however, a comparison with the yields presented in Figure 5.6 indicates that ethanol was less sensitive to the presence of water relative to hydrophobic hexane which is insoluble in water.

In general, the presence of water in the feedstock is known to seal the micropores which contain the lipids and therefore prevent their extraction by not allowing contact with the solvent, while it is also responsible for emulsion formation (Matthäus and Brühl, 2001; Richter et al., 1996).