Effect of the crystallization temperature
First, the effect of crystallization temperature (0 °C, -20 °C, and -40 °C) on palmitoleic acid content and yield was investigated. When the free fatty acids were mixed with a selected solvent and then stored in a reduced temperature, saturated fatty acids, which have higher melting points and lower solubility in solvent particularly at a low temperature, will undergo crystallization and unsaturated fatty acids will remain in the liquid fraction (Vazquez and Akoh, 2012). Table 2 shows that the content of palmitic acid in SPOMFs was 33.59%, and the total contents of saturated fatty acids was 35.48%. Therefore, the main goal of the crystallization process was to remove saturated fatty acids. In this study, two solvents (methanol and acetone) were selected to examine the effect of crystallization temperature. It was observed that only few crystals were formed at 0 °C. Although the yields of palmitoleic acid were high, the content of palmitoleic acid was only increased slightly in two solvents compared to SPOMFs, as shown in Figure 1a and b. When the crystallization was conducted at -20 °C, the levels of palmitoleic acid using methanol and acetone as solvents increased to 38.55% and 33.82%, respectively, which were significantly higher than the initial content. In addition, the palmitoleic acid product obtained at -20 °C had a significantly higher content compared to 0 °C regardless of which solvent was used. However, when the crystallization was further decreased to -40 °C, the content of palmitoleic acid decreased slightly compared to that at -20 °C. The results might be due to the crystallization of palmitoleic acid from the solution at a lower temperature, causing the removal of palmitoleic acid from the liquid fraction (Mu et al., 2016).
The yield of palmitoleic acid, as shown in Figure 1b, was the lowest at -40 °C compared to yields at other temperatures (-20 and 0 °C). The yield of palmitoleic acid showed a significant reduction with decreased temperature because more fatty acid crystals formed at the very low temperature. The results are in agreement with a previous study (Zhang et al., 2017). Based on these results, the crystallization at -20 °C that had high palmitoleic acid contents and reasonable yields was determined as the optimum temperature, and this temperature was used in the following crystallization process.