3.1.1. High pressure homogenization

3.1.1. High pressure homogenizationAstaxanthin like other carotenoids needs a specific temperature range for solubility, and to avoid crystallization and degradation (Ribeiro et al. 2005). Moreover, the kinetics of droplet breakup and stabilization by high pressure homogenization requires an optimum interfacial balance to avoid coalescence(Khalid et al. 2017b; McClements and Rao 2011). A two-step homogenization procedure is adopted to produce fine droplets of astaxanthin loaded emulsions (Fig. 3). The first step involves production of a premix emulsion either by rotor-stator homogenization or simply by stirring. This premix is later passed through high pressure homogenization in order to get fine emulsion droplets(Khalid et al. 2017b; Ribeiro et al. 2010; Affandi et al. 2011).The astaxanthin loaded dispersed phase is produced by dissolving astaxanthin in different vegetable oils or medium chain triacylglycerols (MCT) with heating to over 100 °C (Fig. 4a). Temperature reduction and fluctuation is an important step in avoiding degradation of astaxanthin, as high temperature results in interconversion of isomeric forms of astaxanthin (Fig. 4b). The reduction in temperature should be as fast as possible to increase the efficiency of encapsulation. A range of studies have been conducted to encapsulate astaxanthin using high pressure homogenization. Recently, Khalid et al. encapsulated different astaxanthin extracts in nanoemulsions via two step high pressure homogenization using modified lecithin and sodium caseinate as emulsifiers. The formulated nanoemulsions at 100 MPa have droplet sizes < 170 nm and have encapsulation efficiency > 70% after 30 days of storage(Khalid et al. 2017b). Sotomayor-Gerding et al. (2016) encapsulated 0.5% purified astaxanthin in linseed loaded O/W nanoemulsions with varying speed of homogenization from 5 to 100 MPa and significant reduction of droplet size was observed at 100 MPa with an average droplet diameter of 134 nm. The astaxanthin nanoemulsions were stable against a range of environmental stress. In another study, supercritical extracted astaxanthin was encapsu-lated in an O/W nanoemulsion using high pressure homogenization, by Kim et al. (2012).They formulated stable nanoemulsions with mean droplet diameter between 160 to 190 nm using different glyceryl esters. 10% (w/w) Astaxanthin extract from Fuji Chemicals was encapsulated in pure palm olein using a two-step homogenization process at 800 bars, with the resultant stability of nanoemulsions being dependent upon surfactant concentration, number of homogenization cycles and pressure (Affandi et al. 2011).High-pressure homogenization is an effective method for astaxanthin encapsulation, however the release profile, stability and bioavailability are dependent upon optimum emulsifier concentration and homogenization speed.3.1.2. Lipid nanodispersionsAstaxanthin is also encapsulated in different nanodipsersions, either using low energy methods or high energy methods. Affandi and co-workers did a comprehensive study of encapsulation of astaxanthin using nanodipsersions as the main carrier (Anarjan et al. 2014a; Anarjan et al. 2015; Anarjan et al. 2011a; Anarjan et al. 2014b; Anarjan et al. 2013; Anarjan and Ping Tan 2013; Anarjan and Tan 2013b; Anarjan and Tan 2013c; Anarjan and Tan 2013a; Anarjan and Tan 2013d; Anarjan et al. 2011b; Anarjan et al. 2012).In most of these studies, highly purified astaxanthin (> 90%) was used to prepare the nanodispersions using a solvent displacement method. In this solvent displacement method (Fig. 5) astaxanthin was dissolved in a mixture of dichloromethane and acetone, with the aqueous phase containing the emulsifiers and stabilizer mixtures. The organic and aqueous phases are mixed with different high shear homogenizers, followed by solvent removal using rotor evaporation. The astaxanthin nanodispersions with particle sizes of < 100 nm were stable against different pHs except near the isoelectric point, salt (NaCl, CaCl2) and at different temperatures (Anarjan et al. 2014a).Similarly, response surface methodology was used to investigate the effect of different homogenization times (0.5–20 min) and speeds (1,000–9,000 rpm) on nanodispersions via the solvent displacement method. The optimized conditions for astaxanthin encapsulation include homogenization at 6,000 rpm for 7 min (Anarjan et al. 2015).Astaxanthin is also stabilized in nanodispersions using natural emulsifiers like sodium caseinate, using similar methodology to the solvent displacement method described previously. Multipleresponse optimization predicted stable nanodispersions at 30 MPa with three passes (Anarjan et al. 2011a). The effect of different polysaccharides on stabilization of astaxanthin nanodispersions was investigated by Anarjan and Ping Tan (2013). They used 0.3% (w/w) methyl cellulose, pectin, xanthan gum and gum Arabica as the main polysaccharides to stabilize 0.3% (w/w) astaxanthin using a solvent ev