Introduction
Palmitoleic acid, or (9Z )-hexadec-9-enoic acid, is a 16-carbon omega-7 monounsaturated fatty acid (Astudillo et al., 2018). This fatty acid has demonstrated a wide range of applications in nutrition, medicine and chemical industries (Luan et al., 2018). For example, in animal models of metabolic disease, adipose tissue has been shown to release palmitoleic acid, which suppresses hepatic steatosis and improves insulin sensitivity (Trico et al., 2019). Consequently, palmitoleic acid has been in the spotlight as a promising anti-inflammatory lipid that may help ameliorate metabolic disorders (Cao et al., 2008). In addition, palmitoleic acid is used in cosmetics to improve water retention and elasticity of the skin, delay the aging of skin, hair and nails, and improve eye health (Bal et al., 2011). At present many biopharmaceutical and nutrition companies are vigorously developing palmitoleic acid-based health products and pharmaceutical preparations; some of which have been successfully marketed.
Palmitoleic acid can be found in almost any oils of animal or plant origin, but usually in very low concentrations (Wu et al., 2012). At present, wild plants are the main sources of palmitoleic acid. The seed oil of cat’s claw (Doxantha unguis-cati L.), a woody vine native to the Amazon rainforest, South America and Central America, comprises 64% palmitoleic acid in the oil (Wu et al., 2012). Macadamia nut oil contains 24%-36% palmitoleic acid (Aquino-Bolaños et al., 2016), and the pulp oil from sea buckthorn contains up to 30% palmitoleic acid (Smida et al., 2019). Of these three plants, only sea buckthorn is widely distributed and has good cultivation development potential in China. From other wild plants that contain high proportions of palmitoleic acid, only low extraction yields of palmitoleic acid are obtained, and the geographical distribution of these plants is narrow, making them less suitable for commercial cultivation compared to sea buckthorn. Therefore, sea buckthorn pulp oil (SPO) is considered to be the best raw material for palmitoleic acid for enrichment.
Fatty acids can be separated by crystallization, urea complexation, supercritical fluid extraction, molecular distillation, enzymatic transesterification and preparative liquid chromatography (Lei et al., 2016; Magallanes et al., 2019; Wang et al., 2020). To date, however, very little attention has been paid to the preparation of palmitoleic acid concentrate from natural sources. Chemical synthesis is commonly used to obtain highly pure palmitoleic acid, but this creates partial trans- palmitoleic acid (Guillocheau et al., 2020). Klaas and Meurer (2004) reported the enrichment of palmitoleic acid from natural sources, in which the palmitoleic acid concentration was increased by approximately 50% in a process of transesterification, distillation and urea crystallization. However, although this led to a product that was highly enriched in the ester of palmitoleic acid (81.9%), the overall yield of this method was very low (~4%), and carcinogenic ethyl or methyl carbamate may be formed during urea inclusion (Solaesa et al., 2016), limiting the application of the extract in the food and pharmaceutical industries. In another study, Gutiérrez and Belkacemi (2008) crystallized SPO product (41.4% palmitoleic acid) at 15 °C in acetone, resulting in ~53% enrichment and a 20% yield of palmitoleic acid. However, the proportion of palmitoleic acid in the liquid fraction was increased by only 27% compared with the initial proportion in crude SPO.
Various methods are available in the literature for the concentration or separation of unsaturated fatty acids, but only a few are feasible for scalable preparation (Patil and Nag, 2010). In this study, crystallization and molecular distillation were used to enrich palmitoleic acid from sea buckthorn pulp oil mixed fatty acids (SPOMFs). The operating conditions, namely the crystallization temperature, solvent, ratio of SPOMFs to solvent (w/v), crystallization time, and the distillation temperature, were optimized to achieve an acceptable concentration and yield of palmitoleic acid. Importantly, these methods are suitable for the scalable production of palmitoleic acid from an inexpensive and accessible natural source.