Solid Lipid Nanoparticles: Effect of Carrier Oil and Emulsifier Type on Phase Behavior and Physical Stability

The impact of surfactant type and carrier oil type on the phase behavior and physical stability of emulsified tripalmitin was investigated. Solid lipid nanoparticles (SLNs) were prepared by homogenizing lipid and aqueous phases at a temperature (≈80 °C) above the melting point of tripalmitin, and then cooling the resulting oil-in-water emulsion to induce lipid droplet crystallization. When stored at 37 °C, tripalmitin particles had good long-term stability (d < 150 nm) when coated with Tween 20, but were prone to aggregation and gelation when coated with modified starch (MS). Conversely, when stored at ≤20 °C tripalmitin particles coated by MS were more stable to aggregation/gelation than those coated by Tween 20. Blending tripalmitin with low melting point lipids (either medium chain triglycerides or orange oil) prior to homogenization led to a considerable alteration in the SLN phase behavior and stability. DSC measurements indicated that the presence of the carrier oils reduced the crystallization temperature, melting temperature, and melting enthalpy of tripalmitin. In addition, the carrier oils improved the stability of SLNs to particle aggregation and gelation, although some particle coalescence still occurred. These results have important implications for formulating colloidal delivery systems for utilization within the food and other industries.     

Thermal properties of fats and oils

Summary Heat capacities of the α- and β-forms of trimyristin, tripalmitin, and tristearin, and the β-form of trilaurin were measured. The heats of fusion of the β-forms of these four compounds were determined. The heats of fusion of the α-forms of trimyristin, tripalmitin, and tristearin were calculated from heat content data. Calculations were also made of the heats of transition, α- to β-form. The molal entropy at 298.16° K. was calculated for the β-form of each compound. The experimental work reported herein was carried out in part by one of the authors and in part by G. D. Oliver under the direction of A. E. Bailey while the latter were employed with this laboratory. One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, U. S. Department of Agriculture.     

Kinetics of melt crystallization and transformation of tripalmitin polymorphs

The rates of melt crystallization and phase transformation of three polymorphs of tripalmitin were examined by optical microscopy, X-ray diffractometry and DSC with and without surfactant additives (sorbitan mono- and tristearates). The following results were obtained: (a) Crystallization rate increased in order ofα, β′ andβ; (b) transformation rate was slower than crystallization rate for each polymorph at the same temperature examined; (c) when the most stableβ form was recrystallized from the melt just after the melting ofα, its recrystallization rate was much higher than that by simple melt-cooling; (d) surfactant additives retarded both the crystallization and transformation of all the polymorphs, yetβ′ was influenced the most. A mechanistic interpretation based on the molecular structures both of the melt and of each polymorph is presented.     

Gamma-irradiation of individual cholesterol oxidation products

Cholesterol and seven of its oxidation products in aqueous suspensions of multilamellar vesicles or sonicated aqueous suspensions were subjected individually to γ-radiation (10 KGy) at 0–4°C in air, N₂ or N₂O. All compounds underwent some changes under the influence of radiation. β-Epoxide (cholesterol 5β,6β-epoxide) and, to a much lesser extent, α-epoxide (cholesterol 5α,6α-epoxide) were converted in low yield to 6-ketocholestanol (5α-cholestan-3β-ol-6-one). 7β-Hydroxycholesterol (cholest-5-ene-3β,7β-diol) and, to a lesser extent, 7α-hydroxycholesterol (cholest-5-ene-3β,7α-diol) gave low yields of 7-ketocholestanol (5α-cholestan-3β-ol-7-one). The latter compound also was obtained by irradiation of 7-ketocholesterol (cholest-5-ene-3β-ol-7-one). 6-Ketocholestanol and 7-ketocholestanol are potential biomarkers for irradiated meat and poultry.     

Study of the polymorphism and the crystallization kinetics of tripalmitin: A microscopic approach

The morphology and kinetics of crystallization of tripalmitin have been examined in detail by optical microscopy. The α-crystallization process is characterized by a fast heterogeneous nucleation and spherulitic growth, even at low undercooling, resulting in intense birefringence and smooth spherulitic entities. Four different β’-microstructures have been found—grainy, fibrous, feathery and lamellar. Around 47°C, a clear change from a grainy to a fibrous β’-microstructure is observed. This transition seems to take place without a drastic change in nucleation or in crystal growth. At 50°C, both nucleation and crystal growth exhibit a clear discontinuity, indicating interference from β-crystallization. Around 52°C, the β’-form changes again from a fibrous to a more feathery microstructure; the transition is accompanied by a distinct decrease in crystal growth rate. The lamellar β’-structure exhibits the highest stability and can be obtained onlyvia an accelerated nucleation at low temperature, followed by further growth at elevated temperature near the melting temperature of the β’-form. Determination of the β-form on the basis of its microstructure is not always precise, because the microstructure strongly depends on whether the β-crystals are obtained from a transformation of α or β’, or whether β-crystallization occurs directly from the melt. Clear confirmation of the polymorphic nature of the solid state can be obtained from melting point determination.     

Stability of Solid Lipid Nanoparticles in the Presence of Liquid Oil Emulsions

The dissolution of solid lipid nanoparticles (d ~200 nm, SLN) by mass transfer of lipid molecules from liquid oil emulsion droplets (d ~200 nm) was investigated by differential scanning calorimetry (DSC) and nuclear magnetic resonance spectroscopy (NMR). The mass transfer of n-tetradecane to n-eicosane SLN resulted in the dissolution of the SLN over the course of several hours. The rate of dissolution increased with the tetradecane droplet to eicosane SLN ratio and was greater in the presence of a micelle forming surfactant (i.e., polyoxyethylene sorbitan monolaurate) compared to a protein (i.e., sodium caseinate). The rate of mass transfer was slower in a triacyglycerol system (i.e., liquid droplets of Miglyol and solid droplets of palm stearin, SLN) but could be accelerated by the presence of isopropanol (1.4%) in the aqueous phase. This work shows that even if SLN can be stabilized against aggregation, they may still dissolve due to diffusion of the lipids through the aqueous phase. Even a partial dissolution of SLN can dramatically change their functionality as delivery systems.     

On the sub-α-form and the α-form in monoacylglycerols

The concentration of 2-monostearoyl-glycerol (2-MS) in 1-MS kept at 100°C reaches a plateau concentration of about 12% (w/w) according to ¹H nuclear magnetic resonance. The phase diagram of these two components, made by differential scanning calorimetry (DSC), showed that the temperature of the transition from the sub-α₁-form to the α-form and the melting point of the α-form decreased when the content of 2-MS increased above 9%. The temperature of transition from the sub-α₂-form to the sub-α₁-form, however, decreased at even lower contents of 2-MS. The reversible phase transition of mixtures of 2-MS and 1-MS from the sub-α-form to the α-form has been followed by DSC and X-ray diffraction. Concentrations of 2-MS above 9% change the tilt of the molecules in the mixture abruptly during the transition from the sub-α-form to the α-form. At higher temperature, the conformational disorder in the α-form of the mixture changes continuously. This conformational disorder is, however, unstable enough to change gradually into a more stable one. Even for pure 1-rac-MS, small changes were observed. In addition, monobehenoyl-glycerol and monopalmitoyl-glycerol, which also contained 2-monoacyl-glycerol, were also investigated. The longer fatty acid chains tend to give a higher degree of conformational disorder. The transition behavior has been analyzed on the basis of the crystal structures. Water swelling behavior has also been studied and compared for the sub-α-form and the α-form. The α-form can incorporate more water, probably because it has more disordered polar groups.     

Stabilization of water-in-oil emulsions by submicrocrystalline α-form fat particles

The results presented in this study confirm previous knowledge and stress the need for both hydrophobic emulsifiers and submicronial fat particles to stabilize water-in-vegetable oil emulsions. It was demonstrated that polyglycerol polyricinoleate (PGPR) is superior to glycerol monooleate and/or lecithin, but is incapable of stabilizing these fluid emulsions for sufficient storage periods. Fluid emulsions, unlike margarine, exhibit high droplet mobility and are susceptible to flocculation and coalescence. It was also demonstrated that submicronial α-form crystals of hydrogenated fat can be obtained in the oil phase by the flash-cooling process. The crystals are homogeneously almost mono-dispersed and exhibit insufficient stability against flocculation and phase separation. The use of an emulsifier (PGPR) in the fat crystallization process was very helpful in decreasing the aggregation and flocculation processes. The α-form (mixed with β′-form) submicronial crystals can stabilize water-in-oil emulsions only in the presence of food emulsifiers, provided the concentration of tristearin is limited to 1.0–2.0 wt% (to prevent phase separation and high viscosity) and the PGPR is added at sufficient concentrations (PGPR/tristearin ratio of 2.0 or more). Ideally stable (for over 6–8 wk) fluid emulsions can be formed in systems composed of fat submicrocrystalline hydrophilic particles and food-grade emulsifiers. These water-in-oil emulsions can serve as the basic preparation for any food-grade water-in-oil-in-water double emulsion.     

Application of infrared spectroscopy in the study of polymorphism of hydrogenated canola oil

Low temperature infrared spectroscopy was used to study the polymorphic transformations occurring in hydrogenated Canola oil. Hydrogenation of the oil was carried out under selective and nonselective conditions to an iodine value (IV) of 70 and 60, respectively. The four samples studied differed in their fatty acid composition, melting points andtrans fatty acid content. For the first part of the study, the samples were cooled rapidly using liquid nitrogen to −100 C. All the samples initially crystallized in the α-form and further transformation to the β′-form was detected only in some samples. In the second part of the study, the melted samples were crystallized at 10 C. The resulting spectra showed either a pure β′-form or a mixture of β′ and β-forms.     

β-Crystal formation of isotactic polypropylene induced by N, N’-dicyclohexylsuccinamide

N, N’-dicyclohexyl succinamide (DCS) was found to be a new β-nucleating agent for isotactic polypropylene (iPP) by means of wide angle X-ray diffraction (WAXD) and polarized light microscopy (PLM) measurements for the first time. The maximum proportion of β-form within iPP specimen was 79.1% with addition of 0.05% DCS. With increasing crystallization time, the proportion of β-form changed slightly when the nucleated iPP specimens crystallized at 120°C and 130°C for more than 20 min; but at 140°C, the content of β-form markedly decreased because β-α solid to solid transformation occurred. The analysis of the cell parameters of DCS and β-form iPP showed good lattice matching relationship between them.     

Stabilizing polymorphic transitions of tristearin using diacylglycerols and sucrose polyesters

The polymorphic transitions of synthesized tristearin in the presence of selected DAG or commercial sucrose polyesters (SPE) were investigated using DSC and X-ray diffractiometry. The stabilizing effects of DAG and SPE on α to β transitions of tristearin were dependent on the chemical structures of additives such as FA chain length, saturation of FA, positions and number of FA on backbones. The addition of 1,2-distearin (DS) or SPE containing 70% stearic acid with a hydrophile-lipophile balance value of 1 (S-170) to tristearin resulted in a significant stabilizing effect on the α to β transition during constant heating and storage of α forms at 53°C. The addition of 1,2-DS or S-170 also stabilized the β′ to β transitions of tristearin during constant heating and storage at 59°C. The addition of S-170 exhibited greater stabilizing effects than the addition of 1,2-DS during early stages of storage of α or β′ forms of tristearin. This study provides evidence of potential uses for SPE as additives to improve the quality and shelf life of foods containing fats by stabilizing the desirable α or β′ forms of fats.     

Polymorphism of milk fat studied by differential scanning calorimetry and real-time X-ray powder diffraction

The crystallization behavior of milk fat was investigated by varying the cooling rate and by isothermal solidification at various temperatures while monitoring the formation of crystals by differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD). Three different polymorphic crystal forms were observed in milk fat: γ, α, and β′. The β-form, occasionally observed in previous studies, was not found. The kind of polymorph formed during crystallization of milk fat from its melted state was dependent on the cooling rate and the final temperature. Moreover, transitions between the different polymorphic forms were shown to occur upon storing or heating the milk fat. The characteristic DSC heating curve of milk fat is interpreted on the basis of the XRD measurements, and appears to be a combined effect of selective crystallization of triglycerides and polymorphism.     

The rele of chain length and an emulsifier on the polymorphism of mixtures of triglycerides

The appearance of the β′ form in the α-β transformation in tristearin is hardly detectable. On the other hand, in mixtures of tristearin and tripalmitin at different ratios, β′ formation has been observed to be favored. This observation confirms the statement in the literature that in mixtures of different chain lengths orthorhombic packing is stabilized. When an emulsifier was added to the mixtures, both the β′ and β formation were inhibited. The effect caused by the addition of the emulsifier as an impurity to tristearin is compared to that caused by the addition of tripalmitin: their effects, although both kinetic, were very different. In spite of this difference, they both have implications in the confectionery and fats industries.     

A differential scanning calorimetry study of the crystallization kinetics of tristearin-tripalmitin mixtures

A comprehensive study of the isothermal crystallization kinetics of tripalmitin-tristearin mixtures was carried out using DSC, with data fitted to the Avrami equation. Polymorphs were identified by subsequent melting of samples in the differential scanning calorimeter, with additional confirmatory information obtained from wide-angle X-ray diffraction. It was found that α-, β′-, and β-forms require small (<1.0°C), moderate (3.5–8.5°C), and large (9.0–13.0°C) amounts of subcooling below their respective polymorph melting temperatures for nucleation to occur. Concurrent crystallization of β and β′ polymorphs was not observed. The β polymorphs exhibited sharper heat flow exotherms than β′, due to the higher crystallization driving forces experienced. Analysis of apparent induction times shows that the activation free energy of nucleation for the β-form is significantly higher than for the β′-form. Samples rich in either species crystallized faster (both shorter apparent induction times and sharper peaks) than samples with equivalent compositions. Driving-force arguments do not fully explain this behavior, strongly suggesting that mass transfer resistances (greatest for equivalent compositions) have a significant effect on kinetics. Multiple crystallization events were observed for 50–80% tristearin samples between 56 and 60°C and were attributed to a demixing of tripalmitin-rich and tristearin-rich β phases, in line with established phase diagrams.     

Properties and Stability of Solid Lipid Particle Dispersions Based on Canola Stearin and Poloxamer 188

Solid lipid particles (SLP) are one strategy for encapsulating lipophilic molecules, including for controlled release and enhanced bioavailability applications. SLP based on fully hydrogenated canola stearin (CaSt, 5, 10, 20, and 30 wt%) and the non-ionic surfactant Poloxamer 188 (P188, 0.0, 0.5, 1.0, 2.5, 5.0 and 10.0 wt%) were produced by high pressure melt homogenization using a microfluidizer. Spherical particles in the region of 140 nm were formed, depending on compostion and processing parameters. Surfactant concentration and pressure had a significant influence on particle diameter (P < 0.05), although number of homogenization cycles did not (P > 0.05). A maximum surfactant surface load of approximately 4 mg m⁻² was observed and, at or above 2.5% P188, excess surfactant was present in the continuous phase after production. P188 had the effect of decreasing particle size and facilitating transitions from the α to the β polymorph (P < 0.05) both through surface nucleation and size reduction effects. A stability study of the 10% CaSt SLP with 0.0, 1.0, or 5.0% P188 revealed particle growth for the 0.0 and 1.0% P188 SLP, especially at 20 versus 4 °C, but no changes in the 5.0% P188 SLP, which were exclusively in the β form, at both temperatures for up to 240 days.     

Transitions of saturated monoacid triglycerides: Modeling conformational change at glycerol during α→β′→β transformation

The glycerol region geometry of modeled saturated monoacid triglycerides was altered by bond rotations and minor angle distortions to convert theoretical α-forms into bent β′- and β-forms. Direct α to β conversion involves lateral disruption of fatty chain packing to generate side-chain character typical of the β-form. Such disruption, which could contribute to fat bloom, allows additional molecular movement and shifts in molecular mechanics energy (MME) that may approximate thermal changes observed by differential scanning calorimetry during α to β transformations. Energy calculations at 100 points throughout each transformation identified plausible conversion routes. A two-stage conversion, α to either of two stereospecific β′-forms bent at glycerol followed by subsequent conversion to β, showed less chain movement and more favorable MME than direct α to β conversion. The preferred path, based on energy profiles of each conversion, involves a β′-D form and rotation of carbonyl to α-carbon bonds in chain #2 and a side chain (chain #3).     

Thermotropic polymorphism of a nonphospholipid mixture: Lactylated fatty acid esters of glycerol and propylene glycol

Lactylated FA esters of glycerol and propylene glycol (LFEGPG) are a lipid blend that is commercially available and used in the food industry as an emulsifying agent. Because the mutual impact of the two different backbones on the lipid phase behavior was of particular interest, we fractionated the commercial lipid blend by column LC. Fractions with varying ratios of glycerol and propylene glycol esters were collected and characterized by MS. DSC and X-ray diffraction were applied to study the thermotropic phase behavior of the dry emulsifier and the derived lipid fractions. LFEGPG exhibited rich polymorphic behavior, adopting a sub-α-crystalline phase that converted to an α-crystalline phase. Concomitantly, a β-crystalline phase was formed by some components of this lipid mixture. We found that the fractions with the highest amounts of lipids bearing the less-polar propylene glycol as their backbone tended to form a β-crystalline phase. Also, a higher number of self-polymerized lactic acid molecules in the head group of the propylene glycol esters favored the formation of a β-crystalline phase.     

Parameters influencing cholesterol oxidation

The purpose of this study was to investigate the effects of temperature, oxidation time, presence of water, pH, type of buffer and form of substrate used on cholesterol oxidation. Microcrystalline cholesterol films, both solid and melted, and aqueous suspensions of film fragments were used as substrates. Use of dispersing agents was avoided. Quantitative analysis of the unaltered substrate and the products of its autoxidation was carried out by gas chromatography over the course of oxidation. Solid cholesterol films were found to be resistant to autoxidation in the dry state. However, when heated at 125°C, a sudden increase in oxidation rate occurred at a point coinciding with the visible melting followed by a plateau of the oxidation rate. All of the autoxidation products formed underwent further decomposition. Film fragments of cholesterol oxidized at a faster rate in aqueous suspensions than when oxidized in the dry state. In aqueous suspensions, the differences in the resistance of cholesterol to oxidation were not significant within the pH range 6.0–7.4, except for the early stages of oxidation. The 7-ketocholesterol/7-hydroxycholesterol ratio dropped significantly with increasing pH. However, at all pH levels tested, this ratio remained relatively constant during the 6 h of heating. While the 7β-hydroxycholesterol/7α-hydroxycholesterol ratio was not affected by pH in the range of 6.0–7.4, at pH 7.4 a high preference was observed for the cholesterol β-epoxide over its α-isomer. The β/α-epoxide ratio decreased with time of heating and with decreasing pH. The data show that the physical state of the substrate exerts a major influence on the oxidative behavior of cholesterol.     

在T型微通道内制备固体脂质纳米粒(SLN)的实验研究

研究了一种用T型微通道制备固体脂质纳米粒(solid lipid nanoparticles,SLN)的新方法.以softisan100(C_(10)～C_(18)的混合脂)丙酮溶液作为脂相,以Poloxamer 188水溶液为水相,用注射泵分别将脂相和水相注入T型通道的主道和支道内,两相在交叉口接触后形成明显的相界面,并继续沿主道向前流动.脂相中丙酮通过相界面迅速向水相扩散,随着流体的向前运动,脂相中脂的浓度不断增大至过饱和而形成固体脂质纳米粒(SLN).实验考察了两相流速和微通道尺寸对SLN粒径大小和粒径分布的影响.结果表明:在实验条件下,制得的SLN粒径在110～350 nm之间,多分散性指数小于0.24;T型通道交叉口的流场分布受两相相对流速的影响,并直接影响成粒规律,在不出现两相返混条件下,保持水相流速不变,SLN粒径随脂相流速增大而增大;保持脂相流速不变,粒径随水相流速的增大略有增大;通道尺寸越小所制得粒径也越小.     

Mechanistic considerations of polymorphic transformations of tristearin in the presence of emulsifiers

Fat polymorphs influence the quality of some food and cosmetic products. Emulsifiers traditionally have been added in order to retard undesired polymorphic transformations. The present study is an attempt to understand the role of selected emulsifiers on such transformations. Tristearin was heated or aged under controlled conditions using differential scanning calorimetry (DSC) and X-ray techniques, and the extent of transformation was evaluated in view of the possible pathways of α transforming into β. The temperature regime controls the extent of mobility of fat molecules, the local crystal imperfections and the degree of liquefaction. As a result, it dictates the kinetics of the polymorphic transformation. The surfactant added as an impurity does not have a straightforward effect, as thought previously, but rather varies with the kinetic conditions. During aging some selected solid emulsifiers will retard the α-β transformation while others still enhance it (during heating, all of them will inhibit β form crystallization). Their effect probably is related to different crystalline organizations and the creation of imperfections. Liquid emulsifiers in any case will enhance the α-β transformation, due probably to their weak structure compatibility with tristearin, which causes a higher mobility of triglyceride molecules.