Computer modeling of theoretical structures of monoacid triglyceride α‐forms in various subcell arrangements

The spray-freezing of two food fats, tripalmitin (PPP) and cocoa butter (CB) and mixtures thereof, has been modeled experimentally using a novel single droplet freezing apparatus configured so that temperature profiles or samples for microstructure analysis can be obtained. For 2 mm diameter droplets suspended in a cold air flow at temperatures around 2-15°C, initial cooling rates were on the order of 10 K s -1 and the temperature profiles could be correlated directly to DSC data collected at 20 K min -1 , indicating that minimal supercooling of the materials occurred in the droplet form. Microstructure analysis confirmed that PPP crystallized preferentially in mixtures, and that the surface structure was very sensitive to storage conditions. The bulk structure was much less sensitive, and the internal microstructure of the PPP droplets revealed distinct nucleation sites, which were absent from the CB: These persisted in the mixtures up to 50 wt%. X-ray analysis indicated that the fats crystallized in their more stable forms, namely, β for PPP and Form V/VI in CB.Spray freezing of liquids and emulsions by exposing fine drops to cold, dry gas (usually air) has been advocated as a manufacturing route for new products and product forms by several workers (e.g., 1-3). This route allows thermally labile products to be converted to the form of a free-flowing powder, avoiding structure and activity modification associated with the temperatures required for spray-drying. It offers very high cooling rates, both via large surface area-to-volume ratios and high rates of convective heat and mass transfer (i.e., where volatiles give rise to evaporative cooling) and, when operated correctly, yields a fine particulate product with well-defined size range. For aqueous systems, a subsequent freeze-drying step is required to give a stable product. Commercial processes based on this technology are available (see Ref.2), particularly for spray-chilling for microencapsulation. A number of patents have been filed recently on a variant where the feedstock is contacted with a cryogen spray, typically liquid nitrogen, to give extremely high cooling rates (4-6). Spray-freezing also offers the opportunity to generate new microstructural forms, or direct routes to known forms, driven by the high rate of cooling achievable in drops compared with bulk liquid.Spray-freezing into liquid has received considerable attention in the pharmaceutical and other controlled release sectors as a way of generating novel microstructures and microencapsulates and for stabilizing enzymes. Interest in the food sector is relatively recent, partly because of the cost associated with the cooling media: spray-freezing into cold air avoids the need to separate the product solids from liquid, which in addition would have to be food grade and noncontaminating. Hindmarsh and co-workers have shown that spray-freezing in air or cold nitrogen gas can yield novel structures from sucrose solutions (7) and emulsions (8), and MacLeod et al. (9) have demonstrated that spray-f...

Section: Resultssupporting

confidence: 82%

Microstructures formed by spray freezing of food fats

Gwie

Griffiths

Cooney

et al. 2006

“…The above procedure is known as the atom-atom potential method (26)(27)(28) and has been widely used, e.g. by Hageman and Rothfus for the analysis of fat crystal structures (29)(30)(31).…”

Section: Lattice-energy Calculations--methods Of Calculationmentioning

confidence: 99%

Crystal structures and melting points of saturated triacylglycerols in the β‐3 phase

Soest

Jong

Roijers

1990

A comprehensive packing analysis is presented of the crystal structures of a large number of saturated triacylglycerols in the/3-3 phase. The triacylglycerols p.q.r have been grouped into four classes: r=p, p-t-2, p+4 and p+6. The length of the middle chain, q, dictates whether a ~-2 or/3-3 packing occurs. The latter packing arrangement is adopted when q differs at least 4 from por r.The model-building approach starts from the known molecular conformation of {3-2 10.10.10 and the T// subcell packing mode of the hydrocarbon chains. Purely geometrical model-building allows a preliminary assignment of crystal structures. Crystal lattice energy calculations using the atom-atom potential method support these tentative assignments and clarify some details regarding the stacking of layers. In common with the ~-2 phase crystal structures, the structure of the terrace-like arrangement of the terminal methyl groups plays a crucial role. Triacylglycerols with chains 1 and 3 differing 2 or 4 carbon atoms have a common arrangement of the end-methyl groups (type ~-3A). Those with a difference of 0 (i.e. symmetrical) or 6 carbon atoms have a different, less favorable methyl terrace (type/3-3B). The available experimental evidence (x-ray powder diffraction, unit-cell data and melting points) is entirely compatible with these proposals.The growth of good single crystals of acylglycerols is beset with extremely great difficulties. A complete solution of the crystal structure is hard to obtain, even if single crystals are available. There is therefore a need for alternative approaches toward the solution of acylglycerol crystal structures.For an up-to-date review of crystal structure analyses of lipid systems see (1} and (2); for a more general treatment of the polymorphism of fatty systems see (3) and (4).In a previous study (5) we derived detailed packing modes of a large number of saturated triacylglycerols in the/~-2 phase, extrapolating from one known crystal structure by using a model-building approach. Particularly important features of this approach were the systematic packing of the hydrocarbon chains (T//subceU) and the terrace-like structure of the end methyl groups at the layer boundaries. For each triacylglycerol, two or three distinct solutions could be advanced, only one of which, in general, was compatible with powder diffraction and melting-point data. In the present work, a similar approach is applied to the ~-3 phase of triacylglycerols.Roughly, the ~-3 phase is observed for those triacylglycerols having a middle chain which differs considerably in length from (one of) the outer chains. This

Section: Methodsmentioning

confidence: 99%

“…The bent g'-forms, to which the a-forms transformed, were as described in (2), but due to the unsymmetrical packing found in some /T-form subcell arrangements, only four preferred arrangements (2), were used in this study. Subcell positions for the starting a-forms and ending /3'-forms were as determined previously (1,2). Oscillatory motion about the long hydrocarbon chain axis was not incorporated into a-form structures.Rotation about bonds required to change an a-form into a bent p'-form (2) and differences between X-, Yand Z-axis translation values i.e., amount of whole-mc~ lecule movement on each axis from molecule P (2), for starting a-forms and ending/T-forms were divided into 200 steps.…”

mentioning

confidence: 99%

“…In recent publications (1,2), studies were reported on conformational structures and packing arrangements of theoretical monoacid triglyceride molecules in a-and p'-forms in which computer modeling was used. These studies on the lowest and intermediate melting polymorphs were among the first to use intermolecular energy calculations to examine triglyceride packing arrangements.…”

mentioning

confidence: 99%

See 1 more Smart Citation

Computer modeling ofα‐ toβ′‐ form phase transitions using theoretical triglyceride structures

Hagemann

Rothfus

1988

1493Intermolecular energy calculations were performed on theoretical triarachidin a-form structures as selected bond rotations converted them into /Y-forms with a chain-tilt change in the glycerol region. Interactions across the methyl gaps amounted to only 2-3% of the total energy in initial a-and final /Y-forms, but computer-generated energy profiles during a-to fl'-phase transitions revealed highly repulsive regions due to the close approach of methyl groups. This methyl gap interaction, plus additional repulsive interactions in the lateral packing of molecules during rigid chain rotations, necessitated modification of certain chain movements during phase transition to reduce excessive repulsive energy. These results suggest that phase transitions proceed in a particular sequence of events that either distribute energy to promote further phase excitation or that lead to collapse into the stable polymorphic form. Phase transition energy curves also reveal that secondary a-and fY-forms are possible and are dependent on the starting a-forms, the direction of chain rotation and the subcell arrangement.In recent publications (1,2), studies were reported on conformational structures and packing arrangements of theoretical monoacid triglyceride molecules in a-and p'-forms in which computer modeling was used. These studies on the lowest and intermediate melting polymorphs were among the first to use intermolecular energy calculations to examine triglyceride packing arrangements. Computer modeling had been used previously for intramolecular conformational analyses of phospholipids (3) and short-chain triglycerides (4,5) and, only more recently, for a combination of intra-and intermolecular interactions for phospholipid conformations (6). Rapid conversion upon heating of a-forms into /Y-forms or the highest melting p-form (7) necessitates use of techniques other than conventional x-ray diffraction (8-10) or thermal analysis (9,11,12). Computer modeling is well-suited for examining these fluctuating polymorphic forms and associated transformation processes.Computation of atom-atom interactions during the a-to p'-form transformation is a logical extension from analyses of packing energies for the a-and p'forms (1,2). Intermolecular minimization procedures thus far indicate that, for theoretical a-and bent/Y-form (13) structures, several equally preferred structure-subcell arrangement combinations are possible. For a-forms, subcell arrangement had a greater effect on packing energy than did the configuration of the triglyceride. This was also quite true for bent p'-forms, where several subcell arrangements revealed unsymmetrical packing. Useful understanding of lipid phase behavior requires knowledge of both individual crystalline forms and the atomic-level mechanics that obtain during changes between forms. Therefore, a theoretical pathway for a-*To whom correspondence should be addressed.to p'-form phase transition was modeled with previously determined subcell packings to examine the effect of triglyceride conformation ...