Effect of crystallization time
Time has an influence on the mass transfer between the solid and liquid phases in the crystallization process (Morales-Medina et al., 2016). Thus, four crystallization times (4 h, 8 h, 12 h and 16 h) were investigated with the SPOMFs to methanol ratio of 1:15 at -20 °C. It was observed that the change of crystallization time had a significant influence on the purity and yield of palmitoleic acid as shown in Figure 2c. Overall, the purity of palmitoleic acid increased with time, whereas the yield decreased with time. When the crystallization time ranged from 4 to 16 h, the purity of palmitoleic acid ranged from 27.17% to 41.55%. There were only slight increases in palmitoleic acid purity from 12 h to 16 h, suggesting that crystallization was almost complete at 16 h. Longer time will lead to the crystallization of the palmitoleic acid, reducing its yield significantly. Add what time is the best….
Secondary crystallization of liquid fraction 1
After completing the optimization of crystallization conditions at the first stage, the fatty acid composition of liquid fraction 1 was obtained under the optimal conditions. As shown in Table 2, the liquid fraction 1 contained low contents of saturated fatty acids, with palmitic acid being the most abundant, accounting for 9.37% of the total fatty acids. It is important that the fatty acids having ≤16 carbons are removed as much as possible from the liquid fraction because subsequent molecular distillation can only separate palmitoleic acid from the C18 fatty acids, but cannot separate palmitoleic acid from palmitic acid. However, it has been proven that multistage fractionation can further improve the purity (Jin et al., 2017). Based on these considerations, secondary crystallization of liquid fraction 1 was performed to remove palmitic acid.
A liquid fraction 2 was collected after re-crystallization at -40 °C. As shown in Table 2, the purity of palmitoleic acid in the liquid fraction 2 by secondary crystallization was 45.22%, with a yield of 95.49%. The content of palmitic acid was significantly reduced, from 9.37% to 2.13% in this fraction. Therefore, re-crystallization led to a significant increase of palmitoleic acid content in the liquid fraction, from 40.55% to 45.22%. In addition, the oleic acid content in the liquid fraction 1 significantly differed from that in the liquid fraction 2, and no statistically significant differences were found in the proportions of linoleic acid and linolenic acid in solution after secondary crystallization. It should be noted that these contents are relative percentages. If one fatty acid is removed, others will tend to increase in relative percentage values.
Overall, secondary crystallization significantly decreased the proportion of saturated fatty acids in the liquid phase, and thus the unsaturated fatty acid content in liquid fraction 2 reached 97.52% after two steps of crystallization. Finally, we had obtained sufficiently enriched material for further purification by molecular distillation.