What happens to products when it gets hot

The temperatures around here have been hot and humid for a couple of weeks now and this weather looks set to continue.  Every year at about this time I start getting emails and phone calls about products changing, becoming thinner,  separating, feeling different, growing mould even.   The weather is partly to blame but the other side of the coin is a lack of stability testing.

When we carry out stability testing we place a product into a dry 40C oven, a fridge set at around 6C and an air-conditioned cupboard set at 20-25C.  We monitor the changes in the samples over twelve weeks, measuring, sniffing, looking and feeling.  Just recently we bought a small humidity chamber and started testing in there too – 30C, 70% humidity.  The data we get back is extremely interesting but not everyone opts to do this testing as it costs $1200 plus GST, requires you to be set on your formula (as the results are formulation dependent) and be willing to part with 1Kg of bulk.  It’s a bit of a process but I think it is really worth it!

Heat really affects a product and so does humidity (or lack of it).   Here in my home town yesterday we went from 20C and 73% humidity up to 40C and 11% humidity. This puts a formula under stresses that exceed those of typical stability testing (although you can cycle your testing to replicate night and day if you need to) and can be a product killer.

Before we go on you might be thinking to yourself ‘well that doesn’t apply to me because I have my products in an air-conditioned unit/ shop or warehouse.  To that I’d say ‘well they need transporting don’t they?  Are you sure that everyone buying your goods has air conditioning?  What happens if your product becomes the in-car moisturiser?  The beach bag lippy?  The sports SPF moisturiser?  The jungle trecking foot massage balm or insect repellant?  What indeed……

We measure a thing called ‘relative humidity’.  This is a measure of how much water is in the air vs how much water the air can hold as a maximum at that temperature, the humidity is relative (or related) to the temperature.  As air gets colder it can hold less humidity so 100% saturated air at say 10C will hold less water than 100% saturated air at 30C.  As temperatures get very hot the water evaporates thus drying out the air again so humidity tends to drop.  The relationship between humidity and temperature  at a moment in time also has a name – Dew Point.   The dew point is expressed using the measurement of temperature and is the temperature at which the air becomes saturated with water and dew, fog or mist appears.   The Dew Point is also listed above and the closer the lines get together the heavier the air will feel, if they cross we will experience fog or mist.  How interesting!

All of this weather puts pressure on your formulation especially if, like me you are living in a property that doesn’t have air conditioning.  So what is happening?  While I am not 100% sure as I haven’t carried out specific research my observations, logic and existing data has given me some ideas of what might be happening inside that cream, serum or lotion.  Here is what I think.

• A word on Humidity focusing on Humectants.
  • Humectants such as glycerine, Hyaluronic acid, Beta Glucan, Betaine and the like suck up water like a sponge sucking (or binding) water.  A typical cosmetic formula has a water phase of between 60-90% which means that the humectant is usually saturated with water  from within the formulation UNLESS it is competing for water with other things – thickeners, surfactants,  other actives etc.  One reason for general stability failure can caused by humectant greed where your humectant has literally sucked too much water from the formula leading to its demise.  It happens, I’ve seen this with my own eyes in my own formulations.
  • When humectants are in a low humidity situation (such as air conditioning, hot and dry environments, cold and dry etc) they can start to leak water out to the air if given a chance – glass is permeable as are some plastics – this leads to a formula shrinking, becoming rubbery, hard and crusty even and in extreme cases breaking down (although this is less likely as the lack of water phase leaves the emulsifier present at a higher percentage than before).   This usually leaves the product looking diminished and being hard to rub on or into the skin although in most cases it is still usable.
  • When humectants are in high humidities they can take on water from the atmosphere.  This water makes the product heavier in terms of its actual weight (we pick this up during stability testing),  feel stickier on use and be more susceptible to micro contamination – the extra water being un-preserved.

• A word on emulsion stability.
  • An emulsion is an unstable beast at the best of times as water and oil don’t mix and are only made to mix by creating a number of barriers between them.  These barriers go through various personality changes when the temperature rises and falls.
    • Emulsifiers can be anionic, non-ionic, cationic or a mixture.  They are trying to hold the oil and water phases in a pattern and as the temperature rises the movement between the phases increases and as it falls the movement decreases.  While most emulsifiers can hold up their end of the bargain if formulated in the right way they often can’t do the emulsifying on their own and may break under the load if other things in the formula fail.  In any case a combination of at least two types of emulsifier will give better protection than just relying on one.
    • Thickeners such as gums, clays and particulates are far better able to withstand heat and cold than oils, butters and waxes.  If your product is thickened with shea butter for example Shea melts at 40-45C changing from a solid to a liquid.  This results in a loss of viscosity and a changing of shape in the oil phase.  If it is cold then the butters and waxes will be harder and your cream will feel less greasy and more waxy.  Also keep in mind that the oil phase is usually inside the cream (as the dispersed phase) and you might end up with a catastrophic formula fail if the cream isn’t re-mixed.
• Actives.
  • We are often careful to add our extracts, vitamins and peptides to the formula after it has cooled to 40C or less only to give the formula no further thought when it goes onto the market.  Oxidation happens faster under hot conditions and also when a product is exposed to UV light so do keep this in mind in the summer.  Changes in product colour and odour should be carefully monitored and taken as signs that your vitamins might well have left the building.  A product doesn’t have to reach 40C for this to happen,  a long bout of 30-35C days typical of an Australian summer can be more than enough stress for things like Vitamin A.
• Aroma.
  • Synthetic perfumes are usually formulated to be very robust and should be able to withstand a few hot and humid days but  Essential oils are not always so lucky.  Many essential oils contain volatile components that start to change chemistry and form at between 30-40C thus leaving your formula smelling and in some cases looking different.  Oxidative changes to essential oils in a cream or serum may lead to it becoming darker in colour – a sure sign that chemical changes have taken place.  This is nearly always important in terms of formula efficacy as oxidised aroma chemicals can be irritating and in some case can lead to photosensitisation.  On top of that a flatter smell is just not as appealing.

It is absolutely possible to create cosmetic products that withstand all sorts of weather conditions but only if you plan for it.  In the days where we used synthetic materials weather related changes were less noticeable as materials were simpler and had more clear-cut melting points and chemistries.  Natural materials are much more complex than that which is both good and bad and ignoring that and hoping that the formula will look after its self is just not a good plan.

Above: Coconut Oil that is solid at 20C but liquid by the time it reaches 30C. 

My advice is to think about the conditions that your product is likely to encounter and do some testing ahead of time and if things do go a bit thinner, thicker, waxier or stickier before panicking and thinking that the sky is falling in just stick your head out of the window and double-check.  It might just be 100% natural.

Amanda