Sunday, August 3, 2014

A review of rancidity and anti-oxidants

I'm always surprised to see how much worry there is about rancidity when it's something we don't encounter as often as you'd imagine. Won't you join me on an exciting adventure to learn more about rancidity in our products?

What is rancidity? Rancidity is when our oils "go off" and smell horrible after being exposed to light, heat, or beasties. You can smell when an oil is really off, but the process starts the day the oil is pressed or extracted from the fruit. It is a chemical process during which the double bonds in an oil are broken and a chemical reaction happens, such as the insertion of oxygen in place of that broken bond. There are a few different reactions that can happen: I encourage you to read this post on rancidity to learn more!

How does rancidity differ from contamination? Contamination is something that happens in water containing products or products that are exposed to water. It's when bacteria, mold, or yeast grows in our products. A contaminated lotion might not have any rancid oils and a rancid lotion might not have contamination - they are two completely separate things.

To avoid contamination, you want to add a preservative. To retard rancidity, you want to use an anti-oxidant.

How can we avoid rancidity? We can't. Rancidity is a natural process that starts the day the oil is exposed to air when it is made. There are ways to retard rancidity - by adding anti-oxidants, by keeping it away from heat and light, by freezing the oil - but you can't stop it completely.  This is why we add anti-oxidants. which will slow down the process of oxidation

What can we use as an anti-oxidant? Most of our anti-oxidants are oil soluble, which makes sense because we are using them to retard the rancidity of the oils.

Vitamin E, rosemary oleoresin extract (ROE), and rosemary essential oil are oil soluble anti-oxidants that you can add to your products at the suggested usage rate. (Rosemary essential oil will impart a rosemary fragrance to your products.)

Citric acid is a water soluble anti-oxidant, but it's not a great choice as a little can adjust the pH of your product with as little as 0.1% in the cool down phase. And if you don't have a pH meter, you could end up with a very acidic ingredient if your hand slips and you add a little too much!

And Vitamin C is a water soluble anti-oxidant, but you have to get a special kind so it won't react with your product. It really is easier to go with an oil soluble anti-oxidant most of the time. (In fact, I can't think of a time off the top of my head that you would want a water soluble anti-oxidant solely for the purposes of retarding rancidity, but I am hedging my bets as I'm sure there is at least one reason!)

Anti-oxidants are not preservatives. They do not prevent contamination, meaning they do not prevent bacteria, mold, and yeast from growing in your products. Adding them will not prevent the growth of icky things in your product. They will, however, retard rancidity in your oils. 

Related posts:
How can grapeseed oil have a one year shelf life? (Spoiler: It can't)

Join me tomorrow for more interesting stuff about rancidity in our products! 


susana so said...

Hi Susan,
your blog is awesome.I like it very much...
How to prevent eye shadow (in cream) from dry up.

jody said...

good-morning! i have my coffee and have been touring your wonderful blog!
i have been making CP soap for years and never had a problem with DOS til now! and it has only been 2 batches of 8 bars (Eucalyptus) i have heard of it but like i said never had a problem. i have always done everything the same but see now a few things i can change to help prevent this. i was thinking of adding the ROE to my batches and cant seem to find a suggested usage rate? i have 1.8 lbs of oils / 823 grams per batch. can you help me out or point me in the right direction? thank you so much! jody

Susan Barclay-Nichols said...

Hi Jody! I've made exactly three batches of soap ever, so I'll send you over to Modern Soapmaking where Kenna has a great post on this topic.