A Comparative Study of Shea Stearin Crystallization in Rapeseed Oil and Octyldodecanol: Effects on Crystallization Kinetics, Polymorphism, and Structural Properties

This work examines the crystallization, polymorphism, and mechanical properties of the stearin fraction of shea butter (Butyrospermum parkii) blended with two liquid diluents: rapeseed oil and octyldodecanol. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), texture analysis, and polarized light microscopy were used to assess crystallization kinetics and structural properties. Rapeseed oil promoted faster crystal growth, a shorter induction time (≈ 25%–50% shorter in the studied temperature span of 20°C—23°C), with the transformation to the β polymorph occurring after 2 days in rapeseed oil and after 5–7 days in octyldodecanol (in room temperature). Microscopic analysis showed that the rapeseed oil system produced smaller, densely packed crystal aggregates, while the octyldodecanol system generated fewer, larger crystal aggregates, resulting in a difference in firmness up to 10 times lower than in rapeseed oil. Furthermore, a higher ΔG and lower Δμ for nucleation were observed in the octyldodecanol system. The variation in growth rate and particle size distribution between the two diluents highlights the importance of liquid phase selection in determining the final mechanical properties and bloom stability of fat systems. These findings provide valuable insights for optimizing fat-based formulations for food and cosmetic applications.