Heat Phase or Cool-Down? When to Add Each Ingredient

Making a cream or lotion is a bit like cooking pasta. Some things go in the boiling pot, and some things (fresh basil, a grating of parmesan) get added at the end, or you'll ruin them. In formulation, that "end" is called the cool-down phase, and getting the timing right can decide whether your pricey actives actually survive.

How a hot-process emulsion works

In a classic hot process, you heat the water phase and oil phase separately, usually to around 70–80 °C, combine them with mixing to form the emulsion, then stir as it cools. Trade guides are clear that temperature-sensitive ingredients such as fragrances, actives and preservatives go in only once the emulsion has cooled (Cosmetics & Toiletries; COSSMA).

Cool-down usually means below about 40 °C, though some delicate ingredients prefer even cooler (35 °C or below).

Team Heat: these need (or tolerate) the hot phase

• Butters and waxes need to melt fully: Shea Butter, Candelilla Wax, Beeswax.
• Emulsifiers and fatty alcohols must be melted with the oils: Cetearyl Olivate and Sorbitan Olivate, Cetearyl Alcohol, BTMS.
• Ceramides and cholesterol need heat and co-lipids to dissolve: Ceramide NP, Cholesterol.
• Some water-phase actives dissolve better warm, like Caffeine.
• Humectants and gums such as Glycerin and Xanthan Gum are usually happy in the water phase from the start.

Team Cool-Down: add these below ~40 °C

• Most vitamins: Niacinamide, Sodium Ascorbyl Phosphate, Panthenol, Tocopherol.
• Retinoids: a study of retinoids in commercial cosmetics found far greater losses at 40 °C than at 25 °C over six months, and light did even more damage (Temova Rakuša et al., Journal of Cosmetic Dermatology). Add Retinol or Retinal cool, and protect them from light.
• Peptides: for example Palmitoyl Tripeptide-1 and Copper Tripeptide-1.
• Ferments, extracts and hydrosols: Galactomyces, Rose Hydrosol, most botanical extracts.
• Delicate novelties: plant extracellular vesicles want below 35 °C.
• Fragrance and essential oils: they evaporate and change in the heat.
• Most preservatives: Phenoxyethanol and Ethylhexylglycerin and similar go in at cool-down.

The cheat code: our dictionary tells you

Every entry in our Ingredient Dictionary has an "Add at" line. For example, the niacinamide entry says "Water phase, cool-down", shea butter says "Oil phase, melt at 60–70 °C", and retinol says "Oil phase, cool-down, inert atmosphere". Check it for every ingredient in your recipe before you start heating.

Five cool-down habits

1. Use a thermometer. Guessing "it feels warm-ish" doesn't count.
2. Keep stirring as it cools so the emulsion stays smooth.
3. Pre-dissolve powders in a little water or glycol before adding them at cool-down, so they disperse evenly.
4. Weigh water losses. Heating evaporates water, so many formulators weigh the batch and top it back up with distilled water.
5. Check the pH last, once everything is in and the batch is at room temperature.

Bonus: skip the heat entirely

Cold-process emulsifiers and pre-neutralised thickeners let you make lovely gels and lotions without heating at all. Try Sodium Carbomer or browse Emulsifiers and Thickeners & Rheology Modifiers. It's a great way to protect your most delicate actives.

Treat your actives like fresh basil, and your formulas will taste, er, perform much better.