## 1 Introduction  
Over the past several decades, great progress has been achieved in the development of functional foods enriched with bioactive compounds, or nutraceuticalals, with potential benefits to human health. This came to pass, not only due to growing awareness about the health of customers, but also thanks to the development of innovative nanotechnology in the food science sector [1; 2; 3]. Quickly accumulating evidence has continued to support the supposition that the consumption of functional foods, or nutraceuticalals, via normal diet is associated with improved health conditions, as well as reduced risks of multiple chronic diseases [4]. However, many nutraceuticalals and bioactive compounds are poorly soluble and have low oral bioavailability, which greatly limits their applications in health-benefiting food formulations. It has been widely recognized that the usage of appropriate nano-delivery systems is one of the most effective strategies to improve the solubility (even in aqueous phase), stability, and bioavailability of many poorly soluble nutraceuticalals and bioactives, such as polyphenols, curcuminoids, carotenoids, phytosterools, and others [2; 5; 6; 7]. Among all the reported nano-delivery systems for nutraceuticalals, lipid-based nanocarriers seem to be the most appropriate delivery systems for hydrophobic or lipid-soluble nutraceuticalals, due to their biocompatibility with lipid matrices [8; 9]. The high specific surface area of lipid-based nanocarriers is favorable for lipid digestion, as well as absorption of the loaded bioactives from the intestinal tract. Oil-in-water nanoemulsions, less than 200 nm in diameter, are among the most promising lipid-based nanocarriers, with a wide range of applications in food, pharmacy, and cosme [10]. Despite the enhanced physical stability due to the strong Brownian motion of nanosized droplets against gravity, the bioactive compounds encapsulated in nanosized lipid droplets usually undergo a faster release and more rapid oxidation degradation, since they are short in diffusion distance to the interface of droplets and large in surface area. In order to overcome this limitation, solid lipid nanoparticles (SLNs), which use solid (rather than liquid) lipids to limit the diffusion of bioactives in lipid matrices and improve oxidative stability, were designed and developed at the beginning of the 1990s as alternative carriers for emulsions, liposomes, and polymeric nanoparticles in the drug industry [11]. SLNs, as drug carriers, have been confirmed to exhibit many advantageous features--non-toxicity, biocompatibility, high stability and drug loading capacity, prolonged drug release, superior cellular uptake, high bioavailability, and easy scale up, for instance--over traditional colloidal delivery systems, e.g., liposomes and polymeric nanoparticles [12; 13; 14; 15; 16; 17]. In addition to the drug formulation, SLNs exhibit great potential applications in a wide range of other fields, including cosmetic and dermatological preparations [11], as carriers for lipophilic bioactives in foods [8; 18; 19; 20], or as delivery systems for antimicrobials [21]. Thanks to the successful development of SLNs in the pharmaceutical field over the past several decades, a number of reviews were available in the literature that addressed different aspects of SLNs, including their chemical composition (lipids, emulsifiers or co-emulsifiers, and water) [13; 19], fabrication techniques, production methods [11; 12; 14; 15; 22], stability and related properties [14; 19; 23], digestion and absorption [16; 24; 25], drug vs. bioactive release [14; 23], administration routes and in vivo fate [22], and bioactivities of encapsulated bioactives (e.g., essential oils and flavonoids, [26]). In general, SLNs possess some common physicochemical and structural characteristics, e.g., particle size and morphology, stability, and release behavior, depending on their chemical composition and production processes, or whether some surface modifications have been performed.