α-Melt-mediated crystallization of 1-palmitoyl-2-oleoyl-3-stearoyl-sn-glycerol

The α-melt-mediated crystallization of 1-palmitoyl-2-oleoyl-3-stearoyl-sn-glycerol (POS) has been investigated by differential scanning calorimetry (DSC), combined with polarized-light microscopy. Starting from a completely liquid state, the melt was first cooled down and maintained at a temperature, T ₁, during a time, t ₁, where the α-phase formed. Then it was heated to a temperature, T ₂, above the melting point of α for isothermal solidification into a solid phase, which was identified as δ. Based upon DSC solidification peaks, the time-temperature-transformation (TTT) diagram of POS was constructed for these solidification conditions and was compared with the TTT diagram of direct crystallization from the melt. The α-melt-mediated solidification showed accelerated kinetics of the δ-phase. The effects of T ₁ and t ₁ were also studied: at short t ₁, crystallization was faster with a decreasing value of T ₁, whereas the opposite trend was observed for a longer plateau at T ₁. These tendencies were interpreted in terms of three competing phenomena: the density of δ-nuclei that can form during the plateau at T ₁, α-δ solid-state transformation, and memory effects of molecule arrangements in the α-remelted phase.     

Binary Phase Behavior of 1,3-Dipalmitoyl-2-oleoyl-<Emphasis Type="Italic">sn</Emphasis>-glycerol and 1,2-Dioleoyl-3-palmitoyl-<Emphasis Type="Italic">rac</Emphasis>-glycerol

1,3-Dipalmitoyl-2-oleoyl-sn-glycerol (POP) and 1,2-dioleoyl-3-palmitoyl-rac-glycerol (OOP) are two major molecular species that account for roughly half of the total triacylglycerols in palm oil. The binary phase behavior of a POP/OOP mixture plays an important role in the crystallization of palm oil. We conducted thermodynamic and kinetic studies of OOP and its mixtures with POP using differential scanning calorimetry and X-ray diffraction with a conventional generator and synchrotron radiation. We found that OOP has two polymorphs, α as a metastable form and β′ as the most stable form, and that the two forms are stacked in a triple-chain-length structure. The POP/OOP mixtures exhibited immiscible eutectic natures in both their metastable and their most stable states, in contrast to POP/1,2-dipalmitoyl-3-oleoyl-rac-glycerol and POP/1,3-dioleoyl-2-palmitoyl-sn-glycerol mixtures, in which molecular compounds of a double-chain-length structure were formed. A time-resolved synchrotron radiation X-ray diffraction study undertaken during the cooling and heating processes indicated that the α and β′ forms of the POP and OOP fractions crystallized and melted in separate manners, and that crystallization of the β′ form and the polymorphic transformation from α to β′ of POP and OOP are promoted in the presence of another component. The absence of molecular compound crystals in the binary mixtures of POP/OOP is explained by taking into account the molecular interactions of acyl chain packing, glycerol conformation, and methyl end stacking, among which glycerol conformation appeared to be most influential.     

Binary phase behavior of 1,3-distearoyl-2-oleoyl-sn-glycerol (SOS) and 1,3-distearoyl-2-linoleoyl-sn-glycerol (SLS)

The binary phase behavior of SOS (1,3-distearoyl-2-oleoyl-sn-glycerol) and SLS (1,3-distearoyl-2-linoleoyl-sn-glycerol) was examined by using DSC and conventional and synchrotron radiation X-ray diffraction. The solid-solution phases were observed in the metastable α and γ forms in all concentration ranges. Results indicated that the miscible γ form did not transform to the β′ form when the mixtures were subjected to simple cooling from a high-temperature liquid to a low-temperature solid phase. However, and α-melt-mediated transformation into β′ and β₂ resulted in the formation of immiscible phases in concentration ranges of SLS below 30%. By contrast, at SLS concentration ranges above 30%, the α-melt-mediated transformation caused crystallization of only the γ form, and β′ and β₂ crystals did not appear. These results show that the specific interactions between SOS and SLS are operative in the phase behavior of the mixture states of SOS and SLS.     

Polymorphism of POS. II. kinetics of melt crystallization

Melt crystallization of four polymorphs of POS, α,δ, pseudo-β′ andβ, was examined with pure samples (>99.9%). Induction time, τ, for newly occurring crystals was measured with a polarizing microscope equipped with a temperature-controlled growth cell. Rate of crystallization, 1/τ, was obtained for each polymorph, whose identification was done with x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Two modes of crystallization, melt cooling and melt mediation, were applied. From these experiments, the following conclusions were obtained: (i) The rate of melt-mediated crystallization was always higher than of simple melt cooling; (ii) the pseudo-β′ form was crystallized in a wider range of temperature than the less stable δ form; (iii) the occurrence behavior of the polymorphs differed between simple melt cooling and melt mediation; (iv) the δ form was crystallized only by simple melt cooling in a narrow range of temperature, 25.5°C∼28.3°C. This means that there is a possibility that δ may result from racemic compounds that are crystallized in a specific manner. The experimental results are discussed in comparison to 1,3-dipalmitoyl2-oleoylglycerol (POP), 1,3-distearoyl-2-oleoylglycerol (SOS) and cocoa butter.     

Crystallization kinetics of the pure triacylglycerols glycerol-1,3-dipalmitate-2-oleate,glycerol-1-palmitate-2-oleate-3-stearate,and glycerol-1,3-distearate-2-oleate

Crystallization kinetics of the three main components of cocoa butter, the triacylglycerols POP, POS, and SOS (where P, O, and S stand for palmitic, oleic, and stearic acids, respectively) were studied by combined differential scanning calorimetry and polarized light microscopy. The morphologies, nucleation kinetics, growth kinetics, and phases of the grains formed were identified with this system. The experimental data, as well as two different models to simulate crystallization and to predict behavior of the pure triacylglycerols, are presented. The first model is based on a macroscopical approach to solidification by using time-temperature-transformation (TTT) diagrams and the additivity principle. It allows prediction of the proportion of the different phases formed for any given thermal path imposed on the sample once the TTT diagram is known for the product. It is illustrated for SOS at constant cooling rates and is compared with experimental results. The second model directly simulates growth of the spherulites in the sample by using nucleation and growth rates that are determined experimentally. It provides a view of the structure as it would be observed with a microscope and shows evolution of the heat released in the sample. Isothermal solidification of POP at 15°C is displayed. The experiment and the model are in good agreement.     

Crystallization behavior of hydrogenated sunflowerseed oil: Kinetics and polymorphism

Kinetics of crystallization of hydrogenated sunflowerseed oil was studied by means of an optical method. Two different aspects were examined: the effects of preheating of the molten liquid on induction time of isothermal crystallization and the effects of cooling rate on the crystallization behavior. Induction time for crystallization was markedly dependent on the crystallization temperature and the cooling rate selected. Morphology, polymorphism and chemical composition of the crystals were examined. At all crystallization temperatures, β′-form was found for the first occurring crystals. Long spacings were also similar in all cases and corresponded to a double chainlength arrangement. The chemical composition of the crystals showed no differences at either cooling rate. However, the melting behavior was different. At a slow cooling rate, fractionation occurred, and differential scanning calorimetry diagrams had a broad second endotherm with three peaks, none of which were completely resolved. The polymorphic transformation rate from β′ to β was slower when induction times were longer.     

Polymorphism of pos. I. occurrence and polymorphic transformation

Polymorphic behavior of 1,3-rac-palmitoyl-stearoy 1-2-oleoylglycerol, 99.9% purity (POS) was examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC), solubility measurements and optical microscopy in comparison with 1,3-dipalmitoyl-2-oleoylglycerol (POP) and 1,3-distearoyl-2-oleoylglycerol (SOS). Melt crystallization and solvent crystallization were examined for the occurrence of metastable and stable polymorphs. The number of independent polymorphs was four; α,δ, pseudoβ′ andβ. The lowest melting form, α, was identical to that commonly observed in POP and SOS lowest melting forms. As to the highest melting form,β, the XRD shortspacing pattern was identical toβ ₁ of POP and SOS. This is consistent with crystal habit:β single crystals of POS showed the same shape as those of β₁ of POP and SOS. However, the melting point ofβ (POS), 35.9°C, was lower than those ofβ ₁ of POP, 36.7°C, and of SOS, 43.0°C. Correspondingly, solubility ofβ of POS was lower than that of β₁ of POP below about 13°C, but higher above 13°C. POS did not possessβ ₂ , which is the second stable form in POP and SOS. Two forms of6 and pseudoβ′ occurred, the latter being more stable. The structural properties ofδ showed thatδ is not identical toγ previously observed in POP and SOS. Transformation behavior from the metastable to stable polymorphs of POS showed some differences from those of POP and SOS. Presented at the AOCS annual meeting in Cincinnati, Ohio, in May 1989.     

Binary Phase Behavior of Diacid 1,3-Diacylglycerols

Fifteen phase diagrams were prepared using data from differential scanning calorimetry analysis of binary blends of representative diacid 1,3-DAG. The behavior observed in binary phase diagrams is related to the difference in T _m (ΔT _m) between system components—eutectic for ΔT _m < 26 °C and monotectic for ΔT _m > 30 °C. Binary blends were prepared using six diacid 1,3-DAG: 1,3-hexanoyl-lauroyl-rac-glycerol, 1,3-hexanoyl-palmitoyl-rac-glycerol, 1,3-hexanoyl-oleoyl-rac-glycerol, 1,3-lauroyl-palmitoyl-rac-glycerol, 1,3-lauroyl-oleoyl-rac-glycerol and 1,3-palmitoyl-oleoyl-rac-glycerol. Diacid 1,3-DAG were synthesized using representative FA: hexanoic (6:0)—short-chain FA; lauric (12:0)—medium-chain FA; palmitic (16:0)—long-chain FA; and oleic (18:1)—mono-unsaturated FA. In addition to the aforementioned phase diagrams, the physical chemistry of 1,3-hexanoyl-lauroyl-rac-glycerol, 1,3-hexanoyl-oleoyl-rac-glycerol and 1,3-lauroyl-oleoyl-rac-glycerol is reported.     

Phase Equilibrium and Optimization Tools: Application for Enhanced Structured Lipids for Foods

Solid–liquid phase equilibrium modeling of triacylglycerol mixtures is essential for lipids design. Considering the α polymorphism and liquid phase as ideal, the Margules 2-suffix excess Gibbs energy model with predictive binary parameter correlations describes the non ideal β and β′ solid polymorphs. Solving by direct optimization of the Gibbs free energy enables one to predict from a bulk mixture composition the phases composition at a given temperature and thus the SFC curve, the melting profile and the Differential Scanning Calorimetry (DSC) curve that are related to end-user lipid properties. Phase diagram, SFC and DSC curve experimental data are qualitatively and quantitatively well predicted for the binary mixture 1,3-dipalmitoyl-2-oleoyl-sn-glycerol (POP) and 1,2,3-tripalmitoyl-sn-glycerol (PPP), the ternary mixture 1,3-dimyristoyl-2-palmitoyl-sn-glycerol (MPM), 1,2-distearoyl-3-oleoyl-sn-glycerol (SSO) and 1,2,3-trioleoyl-sn-glycerol (OOO), for palm oil and cocoa butter. Then, addition to palm oil of Medium-Long-Medium type structured lipids is evaluated, using caprylic acid as medium chain and long chain fatty acids (EPA-eicosapentaenoic acid, DHA-docosahexaenoic acid, γ-linolenic-octadecatrienoic acid and AA-arachidonic acid), as sn-2 substitutes. EPA, DHA and AA increase the melting range on both the fusion and crystallization side. γ-linolenic shifts the melting range upwards. This predictive tool is useful for the pre-screening of lipids matching desired properties set a priori.     

Non-Isothermal Crystallization of Bovine Milk Fat

The crystallization kinetics of milk fat were studied under non-isothermal and simulated adiabatic conditions using pulsed NMR spectroscopy. Isothermal experiments confirmed that when milk fat is shock cooled to below the α melting point it crystallizes in two steps due to the different crystallization kinetics of α and β′ modifications. In non-isothermal experiments, the fat samples were heated early during the plateau between steps to a temperature above the α melting point and β′ crystals formed more rapidly than in isothermal conditions. Fresh α crystals are believed to melt and form lamellar units containing triglyceride molecules with high degrees of isomorphism and these units can accelerate the nucleation and growth rates of β′ polymorph crystals. The crystallization behavior changed when the heating occurred late in the plateau and the α crystals are believed to have demixed, which allowed them to transform to β′ crystals directly in the solid state. Under simulated adiabatic conditions the rate of β′ crystallization was increased by a factor of 2–3 over the isothermal case. These findings were used to infer approaches to process difficult fat blends in scraped-surface heat exchanger plants.     

Crystallization kinetics of fully hydrogenated palm oil in sunflower oil mixtures

The crystallization kinetics of mixtures of fully hydrogenated palm oil (HP) in sunflower oil (SF) was studied. The thermal properties and phase behavior of this model system were characterized by means of differential scanning calorimetry and X-ray diffraction. From the melting enthalpy and clear point of HP, it was possible to calculate the supersaturation at a given temperature for every composition of the model system. Supersaturation of the model system for the β′ but not for the α polymorph yielded the β′ polymorph, while supersaturation for the α polymorph yielded a mixture of mainly β and some β′ polymorphs. The crystallization kinetics of HP/SF mixtures were determined by pulsed wide-line proton nuclear magnetic resonance for various initial supersaturations in the β′ polymorph. The determined curves were modeled by a modified classical nucleation model and an empirical crystal growth function, which are both functions of supersaturation. Heterogeneous nucleation rates in the β′ polymorph yielded a surface Gibbs energy for heterogeneous nucleus formation of 3.8 mJ·m⁻². About 80% of the triglyceride was assumed to be in a suitable conformation for incorporation in a nucleus. Induction times for isothermal crystallization in the β′ polymorph yielded a surface free energy for heterogeneous nucleus formation of 3.4 to 3.9 mJ·⁻².     

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.     

Isothermal crystallization of tripalmitin in sesame oil

Crystallization of tripalmitin (TP) in sesame oil was investigated under isothermal conditions at a cooling rate similar to the one achieved in industrial crystallizers (1 K/min). The results obtained indicated that, at TP concentrations <0.98%, triacylglycerides of sesame oil developed mixed crystals with TP. However, at concentrations within the interval of 0.98 to 3.44%, tripalmitin crystallized independently from sesame oil. Within this concentration interval, discontinuities were observed in the behavior of the induction time of TP crystallization (T _i) in sesame oil as evidenced by differential scanning calorimetry, polarized microscopy studies, and determination of the Avrami index (n). In general, the discontinuities in T _i were associated with different polymorph states developed by TP in sesame oil as a function of its concentration and crystallization temperature. Thus, TP crystals obtained at temperatures above 296 K with 1.80 and 2.62% TP solutions had n values close to 3 and developed lamellar-shaped crystals that are characteristic of β tripalmitin. In contrast, the crystals obtained at temperatures of 296 K and below with 1.80% and 2.62% TP solutions provided n values close to 3. Axialite-shaped β′ TP crystals were obtained under these conditions. For the 0.98% TP solution, simultaneous production of α and β′ crystals occurred below 291 K. However, at temperatures above 291 K, a crystallization process with n=3 was obtained, and it developed a different polymorph state, i.e., β, with lamellar-shaped TP crystals.     

Synchrotron radiation X-ray diffraction study on phase behavior of PPP-POP binary mixtures

The phase behavior of thesn-1,3-dipalmitoyl-2-oleoylglycerol (PPP-POP) binary mixture system was studied by powder X-ray diffraction with synchrotron radiation and by differential scanning calorimetry. The results showed that the immiscible phases were observed in metastable and in the most stable forms. In particular, synchrotron X-ray diffraction enabled us to reveal the monotectic nature of α as a kinetic phase behavior. The equilibrium phase diagram of the PPP-POP mixture is divided into two regions. In POP concentration ratios below 40%, solid-state transformation from α to β was observed, indicating that the α-β transition of PPP was promoted in the presence of POP. By contrast, the polymorphic transition proceeds from α to β through the occurrence of the intermediate β′ form at POP concentration ratios above 50%.     

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.     

Crystallization and Polymorphism of 1,3-Acyl-Palmitoyl-<Emphasis Type="Italic">rac</Emphasis>-Glycerols

Crystallization and melting behavior, small-angle X-ray scattering, X-ray powder diffraction and infra-red absorbance were measured for nine 1,3-acyl-palmitoyl-rac-glycerols (1,3-acetoyl-, -butyroyl-, -hexanoyl-, -octanoyl-, -decanoyl-, -lauroyl, -myristoyl- and -oleoyl-palmitoyl-rac-glycerol and 1,3-dipalmitoyl-glycerol). All but one of the prepared 1,3-diacylglycerols (1,3-DAG) were β-stable with 1,3-acetoyl-palmitoyl-rac-glycerol being the exception (β′-stable). Small-angle X-ray scattering indicates that molecules in β-tending diacid 1,3-DAG adopt a herringbone-type configuration similar to monoacid 1,3-DAG. In this configuration acyl chains of the same length associate and regular chain-end matching between terminal methyl groups delineate lamellae. In contrast, molecules in crystalline 1,3-acetoyl-palmitoyl-rac-glycerol are oriented similar to those of 1(3)-monoacylglycerol. Interestingly, DSC curves indicate five of the nine diacid compounds have meta-stable forms—suggesting these forms are quite common for diacid 1,3-DAG. Meta-stable forms are observed in the melting curve when the difference in length between acyl chains is large (1,3-acetoyl-, -butyroyl- and -hexanoyl-palmitoyl-rac-glycerol), and in the crystallization curve when the difference is moderate (1,3-decanoyl- and -lauroyl-palmitoyl-rac-glycerol).     

The DSC thermal analysis of crystallization behavior in high erucic acid rapeseed oil

DSC isothermal analysis is used to investigate the crystallization behavior of high erucic acid rapeseed oil (HEAR oil) in conjunction with usual cooling and heating methods. The crystallization of HEAR oil is found to be in two stages and which are accounted for by the crystallization of the α form and its transformation to theβ form under isothermal cooling conditions. Theα form is also transformed to theβ form under constant heating rate conditions (10 K/min), but this transformation is governed by the heating rate. The crystallization behavior of HEAR oil appears to be dominated by its characteristic triglyceride structure as the transformation is completely altered by random interesterification. Further, when HEAR oil is blended with SBO, the crystallization is changed as the SBO component is increased.     

Real-time X-ray powder diffraction investigations on cocoa butter. I. temperature-dependent crystallization behavior

The crystallization behavior of cocoa butter has been investigated by means of real-time X-ray powder diffraction. Two procedures have been followed: cooling from 60°C at a constant rate until maximum solidification has taken place; and cooling from 60°C in 2 min to a constant solidification temperature. It appears that all polymorphic forms of cocoa butter, with the exception of the β form, can be formed from liquid. The solidification temperature appears to be the most important crystallization parameter.     

Polymorphism of POP and SOS. II. kinetics of melt crystallization

Melt crystallization of the polymorphs of SOS, α,γ, pseudo-β′, β₂ and β₁. and of POP, α,γ, pseudo-β′₂ pseudo-β ₁, β₂andβ ₁was examined using pure samples (99.9%). Induction time τ for newly occurring crystals in the melt phase was measured with a polarizing microscope equipped with a temperature-controlled growth cell. Rate of crystallization, 1/τ, was obtained for each polymorph of POP and SOS whose identification was done with X-ray diffraction and differential scanning calorimetry (DSC). Two modes of crystallization, melt-cooling and melt-mediation, yielded approximately the same results for POP and SOS: (a) The rates of crystallization were always higher in less stable than in more stable forms,β ₂only crystallized via a γ-melt mediation, butβ ₁did not occur by the melt crystallization; (b) the rate of meltmediated crystallization was always higher than the simple melt-cooling as examined at the same crystallization temperature; (c) the occurrence behavior of the polymorphs differed between the simple-cooling and meltmediation. The results were related to the solidification behavior of the polymorphs of cocoa butter. Presented at the AOCS Annual Meeting in Phoenix, Arizona in May 1988.     

DSC and synchrotron-radiation X-ray diffraction studies on crystallization and polymorphic behavior of palm stearin in bulk and oil-in-water emulsion states

The crystallization and polymorphic behavior of palm stearin (PS) in a bulk state and in oil-in-water (O/W) emulsion droplets (average diameter, 1.7±0.3 μm) was observed by using DSC, optical microscopy, and in situ X-ray diffraction with synchrotron radiation (SR-XRD). For the bulk sample the DSC measurements revealed three main exothermic peaks at approximately 31 (large), 21 (small) and 3°C (medium) on cooling, and broad endothermic peaks at approximate −3 (small), 8, 15 to 25 (medium), and 37 and 53°C upon heating. The SR-XRD patterns taken during cooling from 60 to −5°C clarified that the DSC exothermic peaks around 31 and 3°C corresponded to crystallization of the α form of high-melting and low-melting fractions, respectively, and that the occurrence of β′ corresponded to the small exothermic peak around 21°C. The XR-XRD patterns taken during heating from −5 to 60°C demonstrated that the DSC endothermic peaks corresponded to the following transformation processes: melting of α of the low-melting fraction (−3°C), melt-mediated transformation from α to ∇′ (15–25°C), melting of β′ (36°C), and melting of β (53°C) of the high-melting fraction. As for the O/W emulsion sample, the DSC and SR-XRD measurements during the cooling and heating processes exhibited basically the same behavior as that of PS in the bulk state, except that β′ did not crystallize during the cooling process, and the temperatures of crystallization of α, melt-mediated α→β′→β transformation, and melting of β were lower in the emulsion droplets than in the bulk state.