Thursday, October 15, 2009

Rancidity: A primer

EEEK! Rancidity! The curse of lotion makers everywhere! You know how oils can go rancid now - through the breaking of the double bonds - but how exactly does that happen and how can we prevent it from ruining your lotions and potions!

For a short post on anti-oxidants, please click here. I will be going into more detail about various anti-oxidants and chelating agents in the next few weeks!

The main destructors of our oils are heat, exposure to light, exposure to air, and time. In lotions, water can also be our enemy.

Heat and time are pretty basic concepts. When we heat anything, the chemical process speeds up. Let's say it take 6 months for an oil to go rancid. When we heat it, we reduce that time dramatically. It can take a few hours instead of months!

This is attributed to the rate of reaction. Every chemical reaction has a rate of reaction, which is determined by various factors. When we heat up a chemical reaction, the molecules bounce around more, which increases the number of collisions (this is called collision theory). The more bouncing molecules, the more collisions, the faster the reaction. So when we heat up an oil, even slightly, we increase the bouncing around of the molecules, which means more possibility of colliding into a free radical, which can cause oxidation. Most chemical reactions are rated at SATP - standard atmosphere (1 atm, or sea level) and temperature (25C).

And time...well, it's the enemy of all things, isn't it? Time is what makes our oils go rancid, even if we have stored them in a cool, dark place with a wonderful bottle and seal! (Time is really not accurate as it is really the chemical reaction happening over time, but it is still our enemy!)

If heat can speed up a chemical reaction, then, logically, cold can slow down a chemical reaction, at least with oils. Hence the suggestion to keep your oils in a cool, dark place to avoid sunlight and heat!

The main culprit in rancidity is the free radical. A free radical is an atom, molecule, or ion with unpaired electron. As you might remember, atoms really like to have 8 electrons and will do just about anything - no matter how foolhardy! - to get those 8 electrons. So they are highly reactive, looking for electrons to make up that complete valence shell.

The free radicals that we worry most about are O2 (oxygen, but can be found alone in O form), H2O2 (hydrogen peroxide, more about this later), and OH- (hydroxide). When these free radicals are involved in rancidity, we call it an oxidative process (because of the oxygen atoms). And the oxidative process is pretty much the main cause of rancidity (with the possible exception of microbial contamination).

Join me tomorrow for more on the different types of rancidity!

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