## 2 A General Summary of Food Biocompatible SLNs and NLCs: Composition and Preparation Methods
### Structural Composition
#### 2.1.1 Lipids  
The composition of the lipid matrix (as the core) is a crucial parameter when determining the properties, stability, and structure of SLNs and/or NLCs, especially the type of crystals that are formed upon cooling of the system. The solid lipid components used for SLNs and NLCs, which appear as solid state at ambient and body temperature, mainly consist of triacylglycerlogycerols (TAGs), fatty acids, monoacylglycerols, diacylglycerols, steroids, and waxes [19; 22]. TAGs are composed of three fatty acids bound to a glycerol molecule (Figure 2, top). These fatty acids can be saturated or unsaturated, e.g., palmitic (C16:0) and stearic (C18:0) acids are the main saturated fatty acids, while oleic (C18:1) and linoleic (C18:2) acids are the representative unsaturated fatty acids in vegetable oils. Saturated fatty acids are stable against oxidation and possess a higher melting temperature than unsaturated fatty acids with the same chain length. The TAGs containing saturated fatty acids also usually exhibit higher melting temperatures than those with unsaturated fatty acids, due to better packing of the fatty acid chains. Different solid lipids, as the core of SLNs or NLCs, exhibit different complex crystallization behaviors. During the cooling-induced solidification process, the crystallization of lipids takes places, and the polymorphic transitions of crystallized lipids further change during storage. To illustrate this, TAGs can be applied as an example. For TAGs, there are three main polymorphs (\(\alpha\), \(\beta^{\prime}\), and \(\beta\)) with different lipid molecular packing (Figure 2, bottom). The \(\alpha\)-form is metastable with a hexagonal chain stacking, which tends to rapidly transform to \(\beta^{\prime}\)- or \(\beta\)-forms with better packing and greater stability. Among all the forms of TAGs, the \(\beta\)-form is the most stable. Generally, a molten lipid (or lipid mixture) initially crystallizes as the \(\alpha\)-form during the solidification process, and the \(\alpha\)-form gradually and eventually transforms into the \(\beta\)-form via the \(\beta^{\prime}\)-form during storage. The polymorph  
Figure 2: Schematic representation of triacylglycerol molecules and lipid polymorphism with three common types of subcellular cells, referring to the polymorphs \(\alpha\), \(\beta^{\prime}\), and \(\beta\). phic transformation is an irreversible process, and is dependent on the cooling or storage temperature, time, and the degree of fatty acid heterogeneity of TAGs [19]. Stearic acid is a long-chain fatty acid, as a major component in many natural oils and fats, with high food compatibility and low toxicity. It has a high melting temperature of approximately 70 \({}^{\circ}\)C. This fatty acid and the corresponding TAGs (tritestrain), or lipid mixtures rich in these components, have been widely utilized as lipid matrices to prepare SLNs [19]. Natural waves, e.g., beeswax, can also be used as lipid matrices for food-compatible SLNs. They are commercially available, and have been approved as GRAS for food applications [19]. Regarding NLCs, liquid lipids with low melting temperatures are frequently mixed with solid lipids to form an imperfect crystalline structure, thus facilitating the drug or bioactive incorporation in the lipid matrix. The medium chain triacylglycerllycerols (MCT), oleic acid and triolein are the most frequently applied lipid components for preparation of NLCs [19]. Oleic acid (C18:1), with a crystallization temperature of 4 \({}^{\circ}\)C, is a major component of most edible plant oils and fats. It possesses many health effects and is much more stable against oxidation than linoleic and linoleic acids. Therefore, this fatty acid is widely used as a liquid lipid to formulate NLCs. Aside from purified oleic acid, a wide range of edible oils, such as sunflower oil, flaxseed oil, corn oil and soybean oil, can also be used for NLC formulations [19].