Double Chain Length Structure Research Articles

Formation and properties of dark chocolate prepared using fat mixtures of cocoa butter and symmetric/asymmetric stearic-oleic mixed-acid triacylglycerols: Impact of molecular compound crystals

This paper reports an experimental study of the formation and properties of dark chocolate prepared using novel CB alternative fats (CBAs): symmetric (OSatO) and asymmetric (SatSatO) mixed-acid triacylglycerols (TAGs), in which Sat and O represent saturated fatty acids (stearic:S + palmitic:P) and oleic acid moieties, respectively. It was found that the ternary fat mixtures of CB/SatSatO/OSatO with a ratio of CB/(SatSatO + OSatO) of 1:1 formed the most stable β-form of the double chain length (DCL) structure (β-2), which revealed sufficient hardness and sharp melting profiles around body temperature without tempering processes. Fat bloom formation was not observed in dark chocolate with CBAs at ratios of CB/SatSatO/OSatO of 50/20/30–50/0/50 during the one-year storage test at temperatures between 15 °C and 30 °C. Overall, the present study has shown that fat mixtures made of CB/SatSatO/OSatO, which are rich in oleic acid moieties, can be employed as a cocoa butter equivalent (CBE) fat without tempering procedures.

Effect of Fractionation and Chemical Characteristics on the Crystallization Behavior of Milk Fat.

The experiments reported in this study provided a more comprehensive insight into the effect of chemical composition on the crystallization behavior of milk fat (MF). MF was fractionated between 20 and 40 °C into nine fractions with different melting points and was first subjected to the heating step (L20, L30, L40, and S40) followed by the cooling phase (SS40, SL40, SS30, SL30, and LL40). Furthermore, the species of fatty acids (FAs) and triglycerides (TAGs) of the MF fractions were identified. The thermodynamics, crystallization behavior, and polymorphs were determined using differential scanning calorimetry, pulsed nuclear magnetic resonance, and X-ray diffraction, respectively. The results indicated that L40 yielded the highest percentage (∼35% of the total MF) of all the fractions. Enthalpies of the melting and crystallization processes of solid fat content in this study were related to the different FA and TAG compositions of MF and its fractions. High melting fractions (HMFs) were enriched with long-chain saturated fatty acids and tri-saturated (S3) TAGs, and low melting fractions (LMFs) were enriched with short-chain unsaturated FAs and tri-unsaturated (U3) TAGs. Moreover, the various nucleation mechanisms of MF fractions were identified according to the Avrami equation. The polymorphic transformation from a β' form of double chain length structures to a β form of triple chain length occurred in the native MF and HMFs, whereas the LMFs displayed almost no crystals. PRACTICAL APPLICATION: This study represented the first time that nine fractions were obtained using MF fractionation via a heating step, followed by a cooling phase. Furthermore, the chemical composition of MF fractions was investigated. The results obtained from this study might be of specific value in understanding the functional properties of fat-based dairy food in both storage conditions and real-time applications.

Comparative study on crystallization behaviors of physical blend- and interesterified blend-based special fats

Crystallization behavior of special fats has a crucial influence on their physical properties. A comparative study was carried out on the crystallization characteristics of physical blend- and interesterified blend-based fast-frozen special fats (PBSF and IBSF). The content of trisaturated TAGs (SSS) decreased from 27.3% for PBSF to 12.5% for IBSF. At 0–45 °C, the SFC of PBSF and IBSF varied from 60.2% to 16.3% and from 53.1% to 2.9%, respectively. XRD results indicated that PBSF and IBSF had β′ and β crystals with double chain length structures, but IBSF had more β′ crystal content. Isothermal and non-isothermal crystallization kinetics studied by pNMR and DSC revealed that the nucleation and growth mechanism of PBSF and IBSF was different during crystallization. For IBSF, there were a large amount of spherical crystals with small size present in the crystal network. However, rod-like crystals with big size were distributed throughout the crystal network of PBSF.

Thermal and structural changes of rapeseed oil during isothermal storage at low temperature

Abstract The crystallization behavior of rapeseed oil during isothermal storage was investigated using differential scanning calorimetry (DSC), and wide-angle and small-angle X-ray scattering analyses. Rapeseed oil is composed of many kinds of triacylglycerols. When this oil was cooled to its freezing temperature, a meta-stable phase was immediately formed. After a certain period, exothermal crystallization was induced. DSC analysis showed that the induction period, which is defined as the time from the beginning of isothermal storage until an exothermal change is detected, was affected by the storage temperature but not by cooling rate and sample size. Exothermal behavior after the induction period at −25 °C was different from those stored at −17 and −20 °C. X-ray scattering analysis clarified that this difference was due to differences in the formation of the meta-stable phase during the induction period. At −17 and −20 °C, the triple chain-length (3L) structure transformed to a double chain-length (2L) structure, where the β form was the dominant component. In contrast, at −25 °C, the 2L structure was formed while maintaining the 3L structure, and further structural changes were not observed during storage. It has been suggested that the induction period for the crystallization behavior of rapeseed oil during isothermal storage is governed by the kinetics of the meta-stable phase transition. It is important to understand this meta-stable phase transition for controlling the crystallization of oil in foods because the induction period are related to the subsequent crystallization behaviors that affects food quality, and is responsible for food deterioration.

Crystal structures and morphologies of fractionated milk fat in nanoemulsions

The triacylglycerol (TAG) crystal structures and morphologies of fractionated milk lipids in nanoemulsions were investigated at 4°C. Droplet size (0.17 versus 1.20μm), lipid composition (stearin versus olein) and cooling rate (1 versus 10°Cmin−1) had an influence on the structural properties. Five crystal polymorphs (α, β′1, β′2, β1, and β2) were formed with either triple and/or double chain length structures in the solid phases of the emulsified systems. X-ray scattering peak intensities were reduced with the nanoemulsion particles. The internal structure of TAG exhibited stacking of individual lamellar layers (3.8–4.2nm). Various anisometric shapes of fat nanoparticles were formed due to a highly sharp curvature of the nano-size droplets. The shape of olein nanoparticles was more polyhedral compared to the stearin. TAG crystals arranged in a planar-layered organisation at the slower cooling rate. These differences imply that the nanometric confinement of oil droplets modifies the fat crystal habit.

Comparative Analysis of Lipid Composition and Thermal, Polymorphic, and Crystallization Behaviors of Granular Crystals Formed in Beef Tallow and Palm Oil

Six rectangular block all beef tallow (BT)-based and all palm oil (PO)-based model shortenings prepared on a laboratory scale, respectively denoted BTMS and POMS, were stored under temperature fluctuation cycles of 5-20 °C until granular crystals were observed. The lipid composition and thermal, polymorphic, and isothermal crystallization behaviors of the granular crystals and their surrounding materials separated from BTMS and POMS, respectively, were evaluated. The changes of nanostructure including the aggregation of high-melting triacylglycerols (TAGs) and polymorphic transformation from β' form of double chain length structures to complicated crystal structures, in which the β and β' form crystals of triple and double chain length structures simultaneously coexist, had occurred in granular crystals compared with surrounding materials, whether in BTMS or in POMS. Consequently, a slower crystallization rate appeared in granular crystal parts of both model shortenings noted above, which would yield larger and fewer crystals indicated by the Avrami model analysis that would further aggregate to form large granular crystals.

Phase Behavior of a Binary Mixture of 1,3-Dipalmitoyl-2-oleoyl-sn-glycerol and 1,3-Dioleoyl-2-palmitoyl-sn-glycerol in n-Dodecane Solution

The phase behavior of a binary mixture of triacylglycerol (TAG), POP (1,3-dipalmitoyl-2-oleoyl-sn-glycerol), and OPO (1,3-dioleoyl-2-palmitoyl-sn-glycerol) in an organic solvent, n-dodecane, was examined by DSC and synchrotron radiation X-ray diffraction (SR-XRD). In our previous study on POP-OPO mixtures in neat liquid, the formation of molecular compound (MC) crystals of POP/OPO = 50/50 was observed (Minato et al. J. Am. Oil Chem. Soc. 1997, 74, 1213-1220). The purpose of the present study was to determine whether MC is formed in the POP-OPO mixture system in diluted solution when n-dodecane (C(12)) is added to the mixture as a solvent. Two solution systems having weight concentration ratios of POP and OPO to n-dodecane ((POP+OPO):C(12)) of 50:50 (50% solution) and 20:80 (20% solution) were prepared. We found that MC formed at a ratio of POP/OPO = 50/50 in both 50% solution and 20% solution in the stable and metastable states. Differences between the neat liquid and solution systems appeared in two results. In the first, the melting temperatures of MC crystals were lowered from 32.0 degrees C (neat liquid) to 20.4 degrees C (50% solution) and 18.0 degrees C (20% solution). In the second, the beta polymorph of the MC was always observed in the two solutions under the conditions examined in the present study, whereas alpha and beta polymorphs were observed in the neat liquid. These differences were caused by thermodynamic solvent effects. However, we conclude that aliphatic chain-chain interactions through palmitic and oleic acid moieties forming the MC of POP-OPO having a double chain length structure are not disturbed by solute-solvent interactions.

The binary phase behavior of 1,3-dilauroyl-2-stearoyl- sn-glycerol and 1,2-dilauroyl-3-stearoyl- sn-glycerol

The binary phase behavior of purified 1,3-dilauroyl-2-stearoyl- sn-glycerol (LSL) and 1,2-dilauroyl-3-stearoyl- sn-glycerol (LLS) was investigated at a slow (0.1 °C/min) and a relatively fast (3.0 °C/min) cooling rate in terms of melting and crystallization, polymorphism, solid fat content (SFC), hardness and microstructure. Much of the behavior of the system is explained by its polymorphism and the influence of thermal processing. The α-form and the β′-form of a double chain length structure were detected in the mixtures cooled at 3.0 °C/min, whereas only the β′-form was detected in those cooled at 0.1 °C/min. X-ray diffraction data as well as thermodynamic data propose that the most stable phases are promoted by the symmetrical LSL. The measured trends in structural characteristics, thermal properties, SFC, relative hardness and microstructure delimit three groups of mixtures which imply a competition between the stabilizing effect of LSL and disordering introduced by kinetic effects: (a) LLS-rich mixtures with LSL molar fractions ( X LSL) less than 0.3, (b) mixtures with X LSL clustered around 0.5 and (c) LSL-rich mixtures with X LSL ≥ 0.7. The balance between ordering and kinetic effects determines the polymorphism of the mixtures, which in turn determines the behavior of the LSL/LLS system. The kinetic phase diagram of the LSL/LLS binary system constructed using heating differential scanning calorimetry thermograms displayed a singularity at the 0.5 LSL molar fraction which delimits two distinct behaviors: eutectic behavior in one region and monotectic behavior in the other. The molecular interactions, as depicted by a non-ideality parameter of mixing obtained from a thermodynamic model based on the Hildebrand equation, suggests an almost ideal mixing behavior and a moderate tendency to the formation of unlike-pairs in the liquid state.

Microbeam X-ray Diffraction Study of Granular Crystals Formed in Water-in-Oil Emulsion

Margarine and fat spread contain typical water-in-oil emulsions, including semi-solid fats, as continuous oil phases. The application of palm oil, one of the most promising trans-fat alternatives, for semi-solid fats is increasing. However, granular crystals often occur in palm-oil-based solid fats and cause deterioration. In this study, we carried out differential scanning calorimetry (DSC), optical microscopy, and X-ray diffraction (XRD) experiments on granular crystals in margarine. A conventional laboratory-scale technique was applied to examine thermal properties of high-melting fat fractions in granular crystals. In addition, microstructures of the granular crystal were precisely observed with a synchrotron radiation small-angle XRD (SR-μ-SAXD) technique by using a microbeam having a cross section of 5 × 5 μm2. The following results were obtained. (1) DSC indicated that granular crystals are composed of high-melting fractions having a melting temperature of 23 °C. (2) Conventional XRD of granular crystals indicated that β-fat crystals of a triple chain length structure (β-3) melted below the melting of β′-fat of a double chain length structure (β′-2). (3) SR-μ-SAXD indicated that the fat crystals in normal margarine were β′-2. However, the fat crystals of the double and triple chain length structures were simultaneously observed at the center region of a granular crystal, whereas only the fat crystals of β-3 were observed at the outer region of a granular crystal. We analyzed the microstructures and formation processes of granular crystals in relation to the fractional crystallization of the β form of 1,3-dipalmitoyl-2-oleoyl-sn-glycerol promoted by crystallization and transformation of tripalmitin and tristearin fractions.

The binary phase behavior of 1, 3-dipalmitoyl-2-stearoyl- sn-glycerol and 1, 2-dipalmitoyl-3-stearoyl- sn-glycerol

The binary phase behavior of purified 1, 3-dipalmitoyl-2-stearoyl- sn-glycerol (PSP) and 1, 2-dipalmitoyl-3-stearoyl- sn-glycerol (PPS) was investigated at a very slow (0.1 °C/min) and a relatively fast (3.0 °C/min) cooling rate. Mixtures with molar fractions of 0.1 increments were studied in terms of melting and crystallization, polymorphism, solid fat content (SFC), hardness and microstructure. Only the α-form of a double chain length (DCL) structure was detected for all mixtures in both experiments. The kinetic phase diagram, constructed using heating DSC thermograms, displayed two distinct behaviors separated by a singularity at the 0.5 PSP composition: a eutectic in the X PSP ≤ 0.5 and a monotectic in the X PSP ≤ 0.5 concentration region. The singularity was attributed to the formation of a 1:1 (mol:mol) molecular compound. Apart from the segment from 0.0 PSP to the eutectic point, X E , the simulation of the liquidus line using a model based on the Hildebrand equation suggested that the molecular interactions are strong and tend to favor the formation of unlike pairs in the liquid state and that the miscibility is not significantly dependent on cooling rate. The kinetic effects are manifest in all measured properties, particularly dramatically in the X PSP ≤ X E concentration region. An analysis of induction time as measured by pulse nuclear magnetic resonance (pNMR) showed that PPS retards crystal growth, an effect which can explain the peculiarity of this concentration region. At both cooling rates, fit of the SFC (%) versus time curves to a modified form of the Avrami model revealed two common growth modes for all the mixtures. The polarized light microscope (PLM) of the PSP–PPS mixtures revealed networks made of spherulitic crystallites of size, growth direction and boundaries that are varied and sensitive to composition and cooling rate. The change in the microstructure and final SFC (%), particularly noticeable at compositions close to the eutectic, explain in part the differences seen in relative hardness.

Crystallization Properties and Polymorphism of Triacylglycerols in Goat’s Milk Fat Globules

The sensorial, functional, and nutritional properties of goat dairy products result from the specific fatty acid composition of goat's milk fat. However, information on the physical and thermal properties of goat's milk fat is scarce. In this study, crystallization of triacylglycerols (TG) in goat's milk fat globules was investigated using polarized light microscopy and the coupling of time-resolved synchrotron radiation X-ray diffraction (XRD) and high-sensitivity differential scanning calorimetry (DSC). The molecular organization of the solid fat phase was characterized for cooling rates between 3 and 0.1 degrees C/min. Quenching of goat's milk fat globules from 50 to -8 degrees C and 4 degrees C was also examined to identify the most unstable polymorphic forms of TG. Then, the melting behavior of fat crystals was studied on subsequent heating at 1 degrees C/min. Triple chain length (3L: 68.6-70 A) and double chain length (2L: 37-45.4 A) structures were characterized and 5 polymorphic forms, alpha, sub-alpha, beta' 1, beta' 2, and beta were identified. Polymorphic transitions were observed within goat's milk fat globules as a function of time after quenching and as a function of temperature on heating. From a technological point of view, this work will contribute to a better understanding of the rheological properties as well as on the flavor evolutions of goat's milk-based products.

Milk fat and primary fractions obtained by dry fractionation: 1. Chemical composition and crystallisation properties

The chemical composition and crystallisation properties of milk fat and its primary fractions, obtained by dry fractionation at 21 °C, were investigated. The solid fraction (stearin) and the liquid fraction (olein) displayed a different triacylglycerol (TG) composition. Stearin fraction was enriched in long-chain fatty acids, whereas olein fraction was enriched in short-chain and unsaturated fatty acids. Crystallisation properties of milk fat, and both the stearin and olein fractions were studied on cooling at |d T/d t| = 1 °C min −1 by differential scanning calorimetry and time-resolved synchrotron X-ray diffraction (XRD) at small and wide angles. Two main types of crystals corresponding to double chain length structures were characterised in the stearin fraction: α 2 L 1 (47.5 Å) and β′ 2 L 2 (41.7 Å). A triple chain length structure was formed in the olein fraction: α 3 L (72.1 Å). Crystallisaton of milk fat showed the formation of two 2 L (47.3 and 41.6 Å) and one 3 L (72.1 Å) lamellar structures with an hexagonal packing (α form). A schematic representation of the 3 L packing of olein fraction was proposed to explain how a wide diversity of TG can accommodate to form a lamellar structure with a thickness of 72 Å. Furthermore, the sharpness of the small-angle XRD lines associated to the α form was explained by the formation of liquid crystals of smectic type.

Crystallization kinetics of polymorphic forms of a molecular compound constructed by SOS (1,3-distearoyl-2-oleoyl- sn-glycerol) and SSO (1,2-distearoyl-3-oleoyl- rac-glycerol)

Crystallization kinetics of binary mixture phases of SOS (1,3-distearoyl-2-oleoyl- sn-glycerol) and SSO (1,2-distearoyl-3-oleoyl- rac-glycerol) were examined with time-resolved X-ray diffraction (XRD) and DSC methods. Formation of three polymorphic forms, named α c, β′ c and β c, of a molecular compound was observed at a SOS/SSO=50/50 concentration ratio. The XRD data showed that all three polymorphs were arranged in a double chain-length structure, although a triple chain-length structure is formed in α and β′ forms of SSO, and β′ and β forms of SOS. The time-resolved XRD studies showed that α c and β c forms of the molecular compound crystallized more rapidly than the corresponding polymorphic forms of SOS and SSO. The results were discussed in terms of molecular chain–chain interactions of SOS and SSO, by assuming that kinetic processes of nucleation may prefer the formation of the α c and β c forms arranged in the double chain-length structure.

Thermal and structural properties of sn‐1,3‐dipalmitoyl‐2‐oleoylglycerol and sn‐1,3‐dioleoyl‐2‐palmitoylglycerol binary mixtures examined with synchrotron radiation X‐ray diffraction

AbstractThermodynamic and polymorphic behavior of POP (sn‐1,3‐dipalmitoyl‐2‐oleoylglycerol) and OPO (sn‐1,3‐dioleoyl‐2‐palmitoylglycerol) binary mixtures was examined using differential scanning calorimetry and conventional and synchrotron radiation X‐ray diffraction. A molecular compound, βC, was formed at the 1:1 (w/w) concentration ratio of POP and OPO, giving rise to two monotectic phases of POP/compound and compound/OPO in juxtaposition. βC has a long‐spacing value of 4.2 nm with a double chainlength structure and the melting point of 31.9°C. A structural model of the POP‐OPO compound is proposed, involving the separation of palmitoyl and oleoyl chain leaflets in the double chainlength structure. In the polymorphic occurrence of the POP‐OPO mixtures, the POP fraction transformed from α to β′ with no passage through γ, then transformed to β. The presence of OPO in POP promoted the β′‐β transformation of POP during the melt‐mediated crystallization.

FT–IR study on microscopic structures and conformations of POP–PPO and POP–OPO molecular compounds

The microscopic structures and conformations of the molecular compounds of POP–PPO and POP–OPO were examined using microprobe polarized Fourier transform infrared (FT–IR) spectroscopy. The two molecular compounds are formed in a double chain length structure due to specific acyl–acyl interactions through oleic acid moieties, yet peculiar differences were observed between the two molecular compounds. The subcell structure of β form of the POP–PPO compound was T ‖, whose b s-axis was approximately parallel to the unit cell b-axis. The olefinic conformation were neither of skew- cis-skew′ type nor of skew- cis-skew type. On the other hand, two subcell structures were assumed for β form of the POP–OPO compound: (a) the palmitoyl leaflet forms the T ‖ subcell and the oleoyl leaflet O′ ‖, or (b) the two leaflets are both of T ‖ subcell in which the directions of subcell axes of the palmitoyl and oleoyl chains are different. The olefinic conformation of the β form of the POP–OPO compound revealed the skew- cis-skew′ type. It was assumed that steric hindrance between the palmitoyl and oleoyl chains resulted in the formation of the molecular compounds of two types of double chain length structure: (a) complete separation of the palmitoyl and oleoyl chain leaflets as revealed in the POP–OPO compound, and (b) one leaflet of the palmitoyl chains and other leaflet of the palmitoyl-oleoyl mixed acid chains in the POP–PPO compound. The olefinic conformation of the POP–PPO compound was largely deformed compared to that in the PPO–OPO compound.

Thermodynamic and Kinetic Study on Phase Behavior of Binary Mixtures of POP and PPO Forming Molecular Compound Systems

The phase behavior of PPO (sn-1,2-dipalmitoyl-sn-3-oleoylglycerol) and POP (sn-1,3-dipalmitoyl-sn-2-oleoylglycerol) binary mixture was examined using differential scanning calorimetry, conventional and synchrotron radiation X-ray diffraction (XRD), and highly pure samples. A molecular compound, βC, was formed at the 1:1 concentration ratio of PPO and POP, giving rise to two monotectic phases of PPO/compound and compound/POP in a juxtapositional way. βC has a long spacing value of 4.1 nm of double chain length structure. In the PPO/compound region, the DSC melting peak increased with increasing PPO concentration, whereas the DSC melting peak increased with increasing POP concentration in the compound/POP region. The melting point of βC was lowest at 31.2 °C. Time-resolved XRD study unveiled the formation of molecular compounds in metastable forms, αC and β‘C, having the same PPO/POP concentration ratio as βC. αC and β‘C exhibited monotectic phases with corresponding metastable forms of the pure components. In αC, two hexagonal packing XRD short spacing peaks were obtained for αC, corresponding to a differently packed hexagonal subcell of two leaflets. A structure model of the PPO/POP molecular compound is proposed, involving separation of palmitoyl chain leaflet and palmitoyl−oleoyl mixed-acid chain leaflet in the double chain length structure. This model justifies the structure of βC of SOS/SSO proposed by Engstrom (Engstrom, L. J. Fat Sci. Technol. 1992 94, 173−181), in which the saturated and unsaturated chains are packed in the same leaflet of the double chain length. The present work additionally proved that the formation of the molecular compound was present in the αC and β‘C metastable forms, as directly proved by dynamic X-ray diffraction study using synchrotron radiation.

Physical Properties of Equally Mixed Systems of 1, 3-Dioleoyl-2-stearoylglycerol/Cocoa Butter and 1, 3-Dioleoyl-2-stearoylglycerol-added Dark Chocolate

Explanation has been made for thermo-rheological properties of mixture systems of 1, 3-dioleoyl-2-stearoylglycerol (OSO) /cocoa butter, and dark chocolate involving OSO. A OSO/cocoa butter mixture was shown to form a compound at a ratio such that the sum total amount of POP (P : palmitoyl), POS and SOS in cocoa butter was the same as that of OSO equal. X-ray diffraction patterns showed the stable form of the compound to possess a double chain length structure. Stress values required for constant strain were determined with a rheometer at different temperatures. The OSO/cocoa butter compound showed stress values much less than that of the compound formed by OSO/SOS. Lowering stress values in OSO/cocoa butter would thus possibly be due to interactions between compound crystals and the liquid oil fraction in the cocoa butter. Possible molecular mechanisms of the interactions are discussed based on the relationship of double chain length lamellar structures to lamellar slipping movement induced by the incorporation of liquid oil at the lamellar interface.

Polymorphism of POP and SOS. I. occurrence and polymorphic transformation

AbstractThe polymorphic modifications of POP and SOS were identified with X‐ray diffraction (XRD), DSC and Raman spectroscopy by using pure samples (99.9%). In POP, six polymorphs, α,γ, pseudo‐β′2, pseudo‐β′1, β′2 and β′1, were obtained, whereas five polymorphs, α, γ, pseudo‐β′, β2 and β1, were isolated in SOS. Thermodynamic stability increased from α to β1 straightforwardly both in POP and SOS, because the polymorphic transformation went monotropically in the order described above. Additionally, the 99.2% sample of POP crystallized another form, δ, but the 99.9% sample did not, implying subtle influences of the impurity. The four forms, α, γ, β2 and β1, of POP, revealed XRD and DSC patterns identical to the four forms of SOS designated by the same symbols. The chain length structure was double inα and triple in the other three forms in both POP and SOS. Peculiarity of POP was revealed partly in the chain length structure of pseudo‐β′2 and pseudo‐β′1 which were double, whereas pseudo‐β′ of SOS was triple. This apparently showed contrast to the facts that the three forms revealed rather similar XRD short spacing patterns. Another peculiarity of POP was revealed in enthalpy value of the melt crystallization of α: ΔHc (α) = 68.1 kJ/mol which was much larger than that of SOS (47.7 kJ/mol), and also than AOA and BOB. These peculiarities mean that the double chain length structures of POP are more stabilized than the others. Raman bands of CH2 scissoring mode of SOS indicated parallel packing in γ, β2 and β1, and orthorhombic perpendicular packing in pseudo‐β′. The polymorphic transformation mechanisms were discussed based on the proposed polymorphic structure models.

Lipid structures

AbstractMuch of the function of lipids, as in shortenings, confections, biological membranes, etc., depends on the structures they exhibit, i.e., their crystalline and mesomorphic states. Triglycerides may serve as a point of departure for considering the crystal state of lipids. The three principle cross‐sectional arrangements of long chains‐α, β′ and β‐in which there is kinship to hydrocarbons, offer a major basis for classification. Different crystal forms are rheologically different. Complications in the chain length direction lead to stepped configurations in the methyl planes and to double and triple chain length structures. A detailed consideration of the time‐honored melting point alternation of homologs sheds light on the lipid crystal picture. A startling feature is the catalysis of isomerization by the solid state of diglycerides, with a consequent shift in equilibrium far toward 100% 1,3‐diglyceride. Stepwise reversible transformations, as in hydrocarbons, are documented for high molecular weight 1‐monoglycerides. A common feature, the sharp drop with mixture of stable form melting points, to “expose” metastable forms as stable forms for intermediate compositions is strikingly exhibited by fatty alcohols. The two bases for mesomorphism, or liquid crystal formation, among lipids are discussed: (1) dichotomy between molecular moieties in degree of polarity, as in phospholipids; and (2) dichotomy in molecular geometry as in cholesteryl esters. The former type is associated with three main basic arrangements‐smectic or lamellar, middle or hexagonal (normal and reversed), and viscous isotropic or cubic (normal and reversed). The latter is associated with two main arrangements‐cholesteric (essentially limited to cholesteryl compounds) and smectic. The importance of mesomorphism in consideration of membranes and emulsions is emphasized. Developments in understanding the structures of interfacial lipid states (including monomolecular surface films) and the correspondence of interfacial with bulk states is a matter of great continuing interest.