Quick and Dirty Guide to Stability Testing

Have you ever had a product change colour, a cream separate into oil and water or the smell of a product change? I have.  The most recent one was with a natural dupe I did of a Lush shampoo bar. I had used Spirulina as the colourant  and  in about a week the colour had faded.   I have had creams separate over time and had them turn pink due to the preservative. Luckily, I have the ability to do stability testing on products now which saves me an awful lot of time.  But previously I have had to wait and see.

So, what is a stability test and why do we need them?

A stability test is  a way to find out over a period of 1 to 3 months whether a product will hold up during its intended lifetime.  These are called accelerated tests that put the product under extreme conditions and are supposed to indicate how long the product would last under real life conditions.  So, if we want to give a shelf life to a product and we also want to get that product to market in 6 months then we will need to check that nothing will change to it over time.  We need to know that it will smell and look the same, it's viscosity is unchanged, it's pH is the same (will indicate chemical changes which we don't want.)   We will also need to know if the packaging is right for the product (it doesn't leak) or that it is generally compatible with the ingredients.  We might also need to know whether it will stand up to ongoing insult from the consumer.  

There is also  preservative efficacy testing (PET) or challenge tests which will determine whether the product is microbiologically sound - but that is a whole different post.

When to do a test.

Normally, we should do stability tests on all new products.  We should also do stability tests if we need to change an ingredient or packaging. We would need to do one if we are making scale up batches or if we have a change of machinery or processes.

So how does this happen?

According to some sources there is little published research into how to conduct a stability test, this is because of the great variation between different products.  It is generally up to the manufacturer to agree their own protocols on determining when it is appropriate to do a stability test and how that test should be conducted, depending on the type of product they are testing.  A salve or balm which has no water in it and is expected to melt at high temperatures and will have different parameters than an emulsion which is inherently unstable.  That said there are some guidelines offered by the pharmaceutical industry which have informed the cosmetic industry and there are some useful guides by the FDA and COLIPA.

What to do Generally speaking, it involves making a batch of product and separating it into a number of different containers and storing it at different temperatures and conditions.  You need to keep one which is your control.  You test and record the control sample for viscosity, pH, scent profile and keep it in the cupboard to test against the ones you put under stress. Colipa advises that the product is put in an oven at 37c, 40c and 45c.  Depending on who you ask, or what protocol you are following, a product at a different temperature over a certain period will indicate a different shelf life, there is actually no prescription (of a few different ones.)  So we can say that one month at 40-45c will indicate 1 year at room temperature - or thereabouts.  As a little side note, in the EU, in order to get a period after opening sign on the product we need to know that the product will be durable for 30 months minimum, so it would need to pass the 3 months in the oven with no significant changes. There are also tests like the freeze thaw cycle which are supposed to be good for testing liquid products.  This is where a product is put in a freezer at -10c for 24 hours and then left to to thaw at room temperature.  This can be done over 3 cycles - that is three times in the freezer and three times at room temperature.  There are also vibration tests which test that the product won't leak out of the intended container and UV tests that will assess how sunlight will affect the product.

Unconventional ways to do stability tests.

It can be quite costly to do stability tests. Most manufacturing companies have their own in-house equipment and protocols to test for stability but if you are just starting out you might not have the budget and will need to get them sent off for testing. This is always advisable, but it can be expensive if you have to keep repeating the tests in the event of failed tests.  So it is good to be able to get formal testing done only after you have done your own preliminary tests.

Stability Testing on a Shoe String

Oven test

There are reptile incubators that cost about £150 which can be great to conduct your own oven tests.  Set it to 40c and keep a sample in there for 3 months. Check it every few weeks against your control (which will be in the cupboard.)

Vibration tests

Put your product in its intended packaging and send them to a friend who lives far away and ask them to send them back to you. If they live in a warm country, the product will be shipped on a cold plane and then might be waiting around in heat at the destination. Baggage handlers will probably throw your product around a bit and there will be a lot of vibration when on the plane and on the van. You can also buy chicken egg incubators which vibrate and turn eggs at a warm temperature. You could probably do that for a few days and see what happens?

Freeze thaw cycle

Most people have a freezer. Put your product in the freezer for three cycles. Simple.

UV Test

Put one sample in a clear glass container and one in the container intended for sale.  Leave them on the window sill for 3 months and check regularly to see any change in colour.

How to test

You don't need any fancy machinery. Okay, so in the industry they have viscometers to check the viscosity and other fancy stuff.  But you don't really need all that to do your own preliminary tests.  You just need your eyes, nose, skin and a pH meter.   Simply, take your product out of the oven or freezer and leave it at room temperature for 24 hours. Then test it against your control.  Does it smell the same?  Does it look the same? How does it feel? Is the pH as expected? Remember - it is your test, you decide the parameters.  You are going to expect some changes to your product.  But you have to decide whether those changes are acceptable.  For instance, when it comes to pH - being +/- 0.1 pH might be ok if your product is supposed to be between pH 4.6-5.2.  It may come out of the oven having started at pH 4.8 but the pH has dropped by 0.2 units making it pH 4.6.  You may have decided that as long as it stays within a certain range it is acceptable.  Similarly, if you notice  that the colour has changed only slightly, this might be acceptable too.

Record your findings at every stage

Create a form with dates, times and places to describe changes, if any, you have seen.  You can even take pictures and attach them to your form!

Real time tests / Samples

Even the big guns keep samples for the expected lifetime.  They will make the initial batch and keep them in a storage cupboard.  After each batch of product made for market they will keep a sample of that batch too. This is good practice. Below is some other more in depth reading on the subject.  Enjoy :-)

COLIPA Guidelines on stability testing

Slideshare - stability tests on drugs

Presentation on stability testing

Update 13/08/2020

This study into the stability of emulsions gives hints on doing a rapid stability test. They state, “The obtained emulsions, both before and after passing through high-pressure homogenizer, were subjected to stability test under environmental stress conditions, that is, temperature cycling at 4 °C/40 °C for 6 cycles and centrifugal test at 3000 rpm for 10 min.” You can buy a centrifuge pretty cheaply, I have seen some going for around £80 from eBay. You can also buy small egg incubators for around £30-40 that can be set with or without different levels of humidity, and you can use a freezer to create extreme temperature fluctuations. These measures may give you an idea of the stability of your formulations.