INTRODUCTION

Surface active lipids such as partial acylglycerols, phospholipids, and glycolipids are amphiphilic molecules that lower either the surface tension of the medium in which they are dissolved or the interfacial tension between phases at which they are adsorbed. Due to their amphiphilicity, surface active lipids are able to self-assemble into various unique supramolecular structures ranging from micro- to nanoscales. These unique supramolecular structures are found to have enhanced physical properties ranging from increased gelation capability, guest molecules loading capacity, and in-vivo stability to targeted delivery and molecular recognition. Thus, surface active lipids have garnered huge interest as encapsulation agents and delivery vehicles in various industries including food, pharmaceutical, and cosmetics. In fact, work has also been directed towards synthesis of novel compounds and structural modification of existing surface active lipids. This chapter aims to provide a review on current state-of-the art design, synthesis, and purification of surface active lipids, namely partial acylglycerols, phospholipids, glycolipids, aminolipids and lipopeptides, phytosterol surfactant, and antioxidant esters. Physical properties of such lipids including self-assembling and formation of various supramolecular structures are elucidated. Some potential applications of the surface active lipids are also detailed.

PARTIAL ACYLGLYCEROLS

SYNTHESIS OF PARTIAL ACYLGLYCEROLS

Partial acylglycerols, namely monoacylglycerols (MAGs) and diacylglycerols (DAGs), are esters of glycerol with either one or two of their hydroxyl groups esterified with fatty acids, respectively. They exist in various isomers, namely sn-1 and 2 MAG and sn-1, 2 and 1, 3 DAG. At equilibrium, the ratio of sn-1 to 2 MAG and sn-1, 2 to 1, 3 DAG are 9 to 1 and 6 to 4, respectively (Nakajima et al., 2004). With hydrophilic hydroxyl groups in the structure, partial acylglycerols show interfacial properties and surface activity that renders them suitable for use as emulsifiers in various industries, particularly food and cosmetics (Macierzanka et al., 2009). DAGs also generated interest in the food industry as functional food with beneficial health effects including reducing postprandial lipid level, increasing β-oxidation of fat, and managing and preventing obesity (Flickinger & Matsuo, 2003).

Commercially, partial acylglycerols are produced either through chemical glycerolysis at high reaction temperatures (220–260°C) using inorganic alkaline catalysts (Jacobs et al., 2003; Yang et al., 2004) or two-steps steam hydrolysis and enzymatic esterification (Yamada et al., 2001). High temperature chemical glycerolysis is unsuitable for producing heat-sensitive polyunsaturated fatty acids enriched partial acylglycerols. It also causes development of off-flavors in the final product (Guo & Xu, 2005). In comparison, enzymatic synthesis of partial acylglycerols offers a number of advantages including reduced energy consumption, environmental-friendly operation, higher selectivity, and improved product quality. There are a number of enzymatic synthetic routes for partial acylglycerols, namely esterification, acidolysis, glycerolysis, alcoholysis, partial hydrolysis, and interesterification. Among these, extensive research has been done on glycerolysis (Damstrup et al., 2005; Kristensen et al., 2005; Yang et al., 2005), esterification (Weber & Mukherjee, 2004; Lo et al., 2007) and partial hydrolysis (Cheong et al., 2007; Babicz et al., 2010).

Enzymatic synthesis of partial acylglycerols has been extensively reviewed on several occasions (Lai et al., 2004; Lo et al., 2008). This section highlights the enzymatic glycerolysis that has the favorable product yield. Stoichiometrically, two moles of oils react with one mole of glycerol to yield three moles of partial acylglycerols. In order to produce a high yield of partial acylglycerols, phase homogeneity between the hydrophobic oil and hydrophilic glycerol is essential. Solvents such as n-hexane, n-heptane, acetone, tert-butanol and diethyl ether are sometimes used to improve the miscibility of reaction mixtures and, subsequently, the overall product yield. Tertiary alcohols, namely tert-butanol, tert-pentanol, or mixtures of these with hexane, were efficient reaction media for high yield of MAG (Damstrup et al., 2005). Recently, ionic liquids, which are considered eco-friendly solvents due to their negligible vapour pressure, have emerged as new reaction media. The benefits include adjustable solubility, enhanced lipases stability, positive effects on enzymes’ specificity, and facilitation of recoverability and recyclability of lipases. Binary ionic liquids system of TOMA.Tf2N/Ammoeng 102 served as a good reaction medium with at least 70% DAG formation (Kahveci et al., 2009). AOT surfactant (sodium di-2-ethylhexyl sulfosuccinate) is another example of “green” solvents which produced more than 50% yield of MAG (Fiametti et al., 2008). Despite the high efficiency, usage of solvents in the development of food-based products is generally deemed undesirable. Solvent-free enzymatic glycerolysis is found to be able to produce more than 50% DAG by using a stirred tank batch reactor, which constantly stirred the reaction substrate for phase homogeneity (Cheong et al., 2009; Kristensen et al., 2005). Another consideration for enzymatic glycerolysis is the selection of enzymes. It is best to select an enzyme that is immobilized on a hydrophobic carrier in order to prevent glycerol adsorption on the lipase, which will subsequently decrease the reaction rate. Lipases from Rhizomucor meihei and Candida antarctica immobilized on macroporous resin and acrylic resin, respectively, have been often used in enzymatic glycerolysis (Cheong et al., 2009; Kristensen et al., 2005).

The final product from enzymatic synthesis usually contains a mixture of glycerol, free fatty acids (FFA), MAG, DAG, and TAG. Thus, purification of the resultant product is necessary to obtain partial acylglycerols of high purity. Short path distillation (SPD) is a purification method where compounds with different boiling points are separated under vacuum, low evaporating temperature, and minimal residence time. This method is especially suitable for the separation of thermosensitive compounds such as polyunsaturated fatty acids enriched partial acylglycerols. In fact, SPD has been used industrially to purify partial acylglycerols up to a purity of more than 95% (Xu et al., 2002). In spite of this, other problems persist in regards to coloration and possible development of carcinogenic glycidol fatty acid esters during the refining process. The coloration of oil can be easily solved through adsorption or a bleaching process (Minoru, 2006). Thus, work is currently directed towards investigation of other purification methods or elimination of the glycidol fatty acid esters (Strijowski et al., 2010).