94 FORMULATING
mobility of reactants, facilitating hydrolysis and oxidation reactions that degrade potency. Furthermore, the surface tension forces
present during the evaporation of liquid water can cause the collapse of delicate structures, such as collagen sponges or liposomes. Sublimation eliminates these surface tension forces, preserving the original 3D structure of the frozen matrix.
Engineering the cycle: nucleation to desorption The operational execution of freeze-drying is a complex, multi-stage process involving precise control of temperature, pressure, and time. The cycle is universally divided into three stages: freezing, primary drying, and secondary drying.1,2,3,4,5
Stage 1: Freezing The freezing step is arguably the most critical determinant of the final product’s morphology and reconstitution rate. The objective is to immobilize the solvent and solutes.
Nucleation and Supercooling As the temperature of the formulation is lowered, it does not freeze immediately at 0°C. Instead, it undergoes supercooling, remaining liquid below its freezing point until a nucleation event triggers crystallization. The degree of supercooling influences crystal
size. High degrees of supercooling (rapid cooling) lead to the formation of many small ice crystals. Conversely, slower cooling with low supercooling promotes the growth of fewer, larger ice crystals.
Crystal habit and pore structure In cosmetic applications, the “habit” or shape of the ice crystals acts as a template for the final porosity. When the ice sublimes, it leaves behind voids or pores. Large ice crystals result in a product with large, open pores. This open structure reduces the resistance to
vapour flow during drying (making the process faster) and, crucially, facilitates rapid rehydration when the consumer adds serum or water to the freeze-dried bead or tablet.
Annealing To optimize this pore structure, a thermal treatment called ‘annealing’ is often employed. This involves freezing the product, then raising the temperature to slightly below the melting point and holding it there. This promotes ‘Ostwald ripening’, where small,
energetically instable crystals melt and redeposit onto larger crystals, effectively increasing the average pore size and improving product homogeneity.
Eutectic vs. glass transition temperatures For crystalline vehicles (like mannitol), the critical parameter is the Eutectic Temperature (Teu), the point where the solute and solvent crystallize simultaneously. However, most cosmetic formulations (containing peptides, sugars, polymers) are amorphous. For these, the critical parameter is the Glass
PERSONAL CARE MAGAZINE March 2026
SEM micrograph of a lyophilized bead
Transition Temperature of the freeze-concentrated solution (T’g). If the product temperature exceeds T’g during primary drying, the viscosity of the interstitial glass drops, leading to “collapse”—a loss of structure where the cake shrinks into a sticky residue.
Stage 2: Primary drying Once the product is fully frozen, the chamber is evacuated to a deep vacuum, typically between 0.05mbar and 0.2mbar. Heat is applied to the shelves to supply the energy for sublimation. The process is a balancing act: heat must be supplied fast enough to drive sublimation efficiently, but not so fast that the product temperature at the ice interface rises above Tc (collapse temperature, usually near T’g) As drying progresses, a distinct interface
moves from the outer surface of the product inward. The water vapour must travel from this interface through the ‘dry layer’ of the product to escape. The resistance of this dry layer is a limiting factor; if the pores are too small (due to improper freezing), the vapour cannot escape, causing pressure to build up inside the cake, which can raise the temperature and cause melt-back. This phase removes approximately 95% of the
water content.
Stage 3: Secondary drying After the ice has cleared, the shelf temperature is ramped up, often to 20°C–40°C, to drive off the bound water. The goal is to reach a residual moisture
level typically between 1% and 5%. This level corresponds to a water activity (aw) generally below 0.6, which inhibits all microbial growth and significantly slows chemical degradation rates.6
The reduced microbiological growth also allows formulations with no additional preservatives. The duration of this phase depends on the
adsorption isotherm of the material. Over-drying can be detrimental for some biologicals (like proteins), which require a monolayer of water molecules to maintain their conformational stability.
Applications in active ingredient stabilization The application of freeze-drying in cosmetics is not merely for shelf-life extension and preservative-free claims, but for enabling the use of ingredients that are otherwise too unstable for commercial liquid formulations.
The stabilization of L-ascorbic acid (vitamin C) An exemplary benefit of lyophilization of active ingredients is vitamin C. L-ascorbic acid is the biologically active form of vitamin C, essential for collagen synthesis and pigmentation control, yet it oxidizes rapidly in aqueous solutions to form dehydroascorbic acid. Lyophilization offers a superior preservation method; a pivotal study comparing a standard oil-water emulsion against a freeze-dried emulsion revealed that under accelerated ageing conditions (48°C), the freeze-dried formulation retained 90.1% of its vitamin C content after 28 days, whereas the liquid emulsion retained only 28.3%.7
This dramatic preservation of antioxidant
potency confirms that eliminating the aqueous solvent effectively arrests oxidative kinetics, a principle that is immediately translatable to the stabilization of all-trans-retinol, a molecule that
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