Crystalline Lipids and Polymorphism
Going from a liquid to an ordered state in lipids is more complex than described for water. Depending on the type of lipid and if it is dispersed in water or a solution in an apolar solvent, it can assemble into 1 or 2-dimensional liquid crystals or 3D colloidal fat crystal networks. Liquid crystals have a degree of orientation and alignment, similar to a solid crystal, yet the molecules are free to diffuse and flow like a liquid resulting in interesting physical properties, such as anisotropy (the physical properties of the material depend on the direction or orientation of the molecules). Many different lipid 1D or 2D assemblies are possible, including liposomes, micelles and lamella (e.g., the phospholipid bilayer of cells). While liquid crystals have numerous applications in food, biomedical, nutritional, and agricultural sciences, including displays, optical devices, biosensors, and drug delivery systems, our focus is on traditional fat crystal networks consumed regularly in our diet, such as butter and chocolate.
Butter, margarine, and chocolate have 3D colloidal fat crystal networks where lamella stack leading to the third dimension of crystal growth. Polymorphism describes the molecular arrangement of lipids in the crystal, and because the molecules are asymmetric, they require a unit and subcell to be characterized fully. The unit cell is the long axis describing how the long hydrocarbon chains arrange and is two or three times the fatty acid chain length. The subcell characterizes the short axis and is the cross-section of the unit cell, which is the molecular arrangement of fatty acid CH2 groups. Polymorphism is important because how the molecules pack alters the intermolecular force strength between them, altering the melting temperature of the fat and sensory properties of many foods, especially chocolate. The hexagonal (α) polymorph is the least dense crystal with the lowest melting temperature, followed by the orthorhombic perpendicular (β՝) and the densest triclinic parallel (β), each with increasing melting points.
The Sequence of Fat Crystallization
Lipid crystallization is a complex step-by-step sequence initiated by decreasing the solution temperature below the melting point of the crystalline fat phase, thereby decreasing the solubility and causing supersaturation. After supersaturation, further removal of sensible heat decreases the temperature, and the solution is super- or undercooled. Supercooling is required to overcome the energy barrier for nucleation or the activation energy of nucleation. The creation of nuclei is not thermodynamically favorable, and the activation energy of nucleation is associated with the entropic loss of creating an interface. Supercooling leads to the crystal embryo reaching a critical radius, at which point the entropic loss (a function of surface area) is exceeded by the enthalpic gain of the intermolecular interactions associated with increased molecular order in the crystalline phase (a function of volume). The volume of a spherical crystal embryo is proportional to the third power of the radius, while surface area is only to the second power; the critical radius is reached as the volume parameter exceeds the surface area, after which growth proceeds spontaneously.
After reaching the critical radius, nucleation allows nuclei to persist, allowing subsequent TAGs to diffuse and adhere to the growing crystal. Following nucleation, spontaneous crystal growth immediately precedes. The molecular arrangement or polymorphism of TAGs in nuclei templates subsequent crystal growth. At very low degrees of supercooling, only hexagonal-polymorphic crystals emerge as they have the lowest energy barrier for formation. Greater supercooling, achieved by more rapid cooling, overcomes the activation energy of nucleation for the β՝ polymorph allowing it to crystallize from the melt. Since the α-polymorph has a lower energy barrier to nucleation, it will form in the presence of β՝ and β.
The densest and most stable polymorphic form is β, corresponding to the greatest barrier to nucleation of the three major polymorphic forms; when it is crystallized from the melt, it will do so in the presence of both other polymorphs. In complex food systems where lipids play an essential role in the structure of the food, control of processing parameters is needed to obtain and maintain the desired polymorphic form. Through time fat crystal networks anneal, which simultaneously undergoes Ostwald’s ripening or crystal coarsening (increase in crystal size) and polymorphic transitions from least to most dense. As crystals anneal, the product attains a grainy texture as the fat crystals can be sensed.
Colloidal Fat Crystal Networks
Fat crystal networks’ physical properties and structure are greatly influenced by heat (.e.g., cooling rate) and mass transfer (e.g., viscosity, presence of shear, etc.). Colloidal fat crystal networks contain levels of structure ranging from nano to macroscale, starting with the lamella, the smallest structural unit influenced by heat and mass transfer and the type of fatty acids and their position on triglycerides (TAGs). Lamella stack without any crystal imperfections to domains, and these domains stack to form primary crystals (platelets). The primary crystals arrange into flocs, the characteristic spherulitic crystals observed with light microscopy. Finally, spherulites aggregate into the supramolecular colloidal fat crystal network. Every level of structure is influenced by the heat and mass transfer conditions driving crystallization, and each level determines the final physical properties. Therefore, depending on how the fat is cooled, different polymorphic forms, microstructural element size (i.e., domain size and crystallite size) and shape (i.e., clusters and flocs), as well as supramolecular arrangement of the crystals (i.e., flocs and networks) differ greatly with each parameter intertwined, dictating the final physical properties of the colloidal fat crystal network.
Tempering of Chocolate
Tempering is most commonly done when making chocolate products and is a process of heating and cooling to specific temperatures to obtain and stabilize the desired polymorphic form for cocoa butter crystals. The chocolate is first melted above 50 oC, the highest melting point for cocoa butter lipids, to ensure no crystal structures remain in the melt. After, the fat is cooled to 27oC, which is sufficiently low to induce nucleation of the β-form V but not more stable forms, as supercooling does not overcome their activation energy. The activation energy of nucleation is lower for the α and β՝ polymorphs than β-form V; therefore, they form in the presence of β-form V. The fat is then warmed to above the melting point of the undesirable α and β՝, eliminating their polymorphic forms, and the result is cocoa butter fat crystals with only β-form V.
Form V crystals are small and closely packed, and associated with the smooth mouthfeel, a glossy appearance, and chocolate snap. Tempering chocolate ensures that the cocoa butter crystals crystallize uniformly, resulting in a glossy, firm texture and a smooth, even finish. When crystallized improperly, the cocoa butter crystals contain several polymorphic forms, resulting in a dull surface, uneven color and texture, and a soft, crumbly texture. Tempering is especially important in chocolate decorating, truffles, or products requiring a shiny and stable finish. In addition, tempered chocolate has a longer shelf life and is less prone to melting at room temperature.