Effect of various solvent types
In this study, hexane, methanol, ethanol, acetone, isopropanol were
examined for the free fatty acid crystallization fractionation. These
solvents are permitted at parts-per-million (ppm) residual
concentrations in pharmaceuticals according to the United States
Pharmacopoeia (USP). Methanol is a Class 2 solvent associated with an
allowable concentration limit of 3000 ppm (Kameyama et al., 2019).
Acetone, a polar solvent, is commonly selected as crystallization
solvent for the separation of triglycerides and free fatty acids
(Vazquez and Akoh, 2012). It is a permitted solvent in food industry.
Hexane, a nonpolar solvent, is less toxic than methanol and widely used
as the extraction solvent in oilseed crushing plants (Zhu et al., 2017).
Figure 2a shows the effect of solvent type on the content and yield of
palmitoleic acid. Initially, crystallizations were performed with 5 g
SPOMFs and 60 mL solvent at -20 °C for 12 h to investigate the effect of
solvent type on palmitoleic acid enrichment. When methanol was used as
the crystallization solvent, the palmitoleic acid content was up to
38.55%, which was significantly higher than those with other solvents.
Therefore, methanol gave the highest content of palmitoleic acid in the
liquid fraction 1, followed by hexane, acetone, isopropanol and ethanol.
For acetone, isopropanol and ethanol, the increase of palmitoleic acid
in the fraction was very limited and insignificant differences in
palmitoleic acid level were found among these three solvents. Previous
study is consistent with the results of this experiment. It is concluded
that hexane was not the most suitable solvents for the concentration of
free fatty acids by low-temperature crystallization (Vazquez and Akoh,
2011).
Although the crystallization in methanol led to the highest proportion
of palmitoleic acid in the liquid fraction, the yield needs to be
improved. The yields of palmitoleic acid as affected by the treatments
applied were compared and the results are shown in Figure 2a. There is
no statistical difference in yield
among solvents. Thus, taking both purity and yield into account,
methanol was selected as an optimal solvent for further crystallization
treatments.