64 EMULSIONS
Crystallizing materials in formulation stability
Staffan Norberg, Mahmoud Shallah - AAK ABSTRACT
This study demonstrates how emulsions with high levels of refined shea butter can transition from a monodisperse to a polydisperse droplet size distribution, correlating with changes in viscosity and the onset of crystallization. In contrast, emulsions formulated with shea olein or lower concentrations of refined shea butter remained monodisperse, with stable viscosity and no signs of crystallization. Comparative emulsions were stored at 20°C and 25°C and analysed with respect to droplet size distribution, viscosity, and crystallization behaviour during a three-month period. The results show that even slow crystallization can destabilize emulsions over time by altering droplet size and viscosity. These findings underscore the importance of understanding the role of crystallizing materials in formulation stability and demonstrate how liquid shea oil can mitigate risks
Cosmetic emulsions are known to change in viscosity over time after production. This study hypothesizes that crystallizing materials could significantly induce these changes in viscosity. The aim is to investigate how crystallization correlates with changes in viscosity and droplet size. Crystallization in emulsions poses
several challenges for cosmetic formulators. Uncontrolled crystallization can lead to unexpected changes in droplet size and viscosity, resulting in unstable emulsions.1
This
instability can affect the texture, appearance, and overall performance of the cosmetic product, leading to customer dissatisfaction and product returns. When formulating emulsions with solid and semi-solid ingredients, a crystallization process is initiated within the emulsion droplets and in contact with the emulsifier system. Depending on the crystallization rate and the emulsifier system, crystals may stay within the droplet, penetrate and grow on the surface of the droplet, or even leave the droplet.2
Therefore, understanding
the behaviour of crystallizing materials in emulsion droplets is crucial for ensuring stable emulsions. Shea butter, derived from the kernels of
PERSONAL CARE November 2025
the African shea tree (Vitellaria paradoxa), is primarily composed of triglycerides.3 Approximately 90 % of the triglyceride composition is characterized by eight types, with stearic-oleic-stearic (SOS) and stearic- oleic-oleic (SOO) being the most prevalent.4 Shea butter contains a high level of symmetric triglycerides such as SOS, while asymmetric stearic-stearic-oleic (SSO)-triglycerides are present in very low quantities.5 Beyond triglycerides, shea butter contains
significant amounts of diglycerides and non- glyceride compounds collectively known as unsaponifiables.6
These unsaponifiables primarily consist of
triterpene esters and unsaturated isoprenoidal hydrocarbons with a typical level of around 6%.7
This combination of glycerides and
unsaponifiable matter has made shea butter highly valued in the cosmetic industry. Shea olein is derived from shea butter through solvent fractionation, which separates lower- melting triglycerides from higher melting triglycerides.8
SOS while increasing the level of SOO.9 Crystallization in oil-in-water emulsions
significantly differs from bulk oil. Nucleation can be heterogeneous or homogeneous. A crystalline layer can serve as a template
heterogenous nucleation of triglycerides. Nucleation starts at the surface of emulsion droplets, followed by crystal formation. Crystals can grow within the droplet or reach its surface, depending on the crystallization rate and the emulsifying system’s properties. Crystals growing outside the droplet can lead to flocculation, partial coalescence, or coalescence, affecting droplet size and viscosity and potentially leading to emulsion instability.10 The rate of nucleation and crystallization depend on the relative supercooling. The emulsion droplet size influences the supercooling level required, with smaller droplets stabilizing metastable polymorphic phases.11 Furthermore, crystallization occurs
individually within each droplet, in contrast to bulk oil.12
to complex polydisperse crystallization behaviour.13
This process reduces the level of
These phenomena can lead Emulsification causes a 7-26 °C
supercooling of the alpha form when fats are emulsified, compared to bulk.14 By investigating the crystallization
behaviour of standard refined shea butter and shea olein during isothermal storage, the study aims to elucidate how the crystallization of these fats in emulsions correlates with
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