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.