found in this post): How does adding 1% vitamin e or rosemary oleoresin affect the shelf life of that ingredient (which is usually an oil)? And how does refrigeration affect the shelf life of our oils? For example, if I buy rosehip seed oil (a 6 month shelf life oil) now in April, refrigerate it until I open it in June to make a product, to which I add 1% rosemary oleoresin, when is my product good until? I've been assuming it's good for well over 6 months past the date I made the product, because of all that antioxidizing power - though this estimate matches my experience of my creations going rancid, I realize now I'm sort of pulling that shelf life out of thin air.
How do we turn our techniques for preventing oxidation (refrigeration before using, adding antioxidants) into an estimated shelf life? Maybe you're planning to address this already - if so, sorry for my impatience!
An excellent question! Here's a post on the various anti-oxidants we can choose for our products, a post on how anti-oxidants work, a primer on rancidity, and a post on the mechanisms of rancidity. Now that you have your homework for the week, let's try to answer the question "how do anti-oxidants affect the shelf life of our oils and products?".
We can figure out the theoretical shelf life of our oils - our suppliers generally give us something like "6 to 12 months upon opening" - but there are so many variables in rancidity, it is really hard to come up with a hard answer without doing all kinds of exciting and interesting chemistry involving large machines that I don't own yet but one day will! (And that shelf life doesn't include how long the oil has been at your suppliers after being decanted from that giant metal bin!)
Oils can be oxidized by light (photo-oxidation) and by heat (heat speeds up chemical reactions), which is why you always see suggestions to keep your oils in a cool, dark place. So let's say you leave your olive oil on a shelf in a window that gets an unexpected amount of sun on a lovely spring-like April day because you had to run to work (I have never done this!). How much of an impact will 6 hours of direct sunlight have on that oil? Will it speed it up by a day, two days, a week, a month, or create instant rancidity? We have no idea: There are simply too many variable.s.
Refrigeration and freezing are good ideas for oils or butters you aren't using right now. Chemical processes, like rancidity, generally slow down when the temperature is reduced (and speed up when when the temperature is increased). So you can retard rancidity to the point where it's negligible with freezing, and consider the best before date to be the date you take it out of the freezer (if you put it in right after buying it). In the fridge, I don't know. Again, without a lot of special equipment and a fantastic lab filled with beakers and Ehrlenmeyer flasks and test tubes, I couldn't give you an exact number.
Remember to heat the oils so you can make them uncloudy (the cloudiness is called the titer point and you can read more about it here) before using!
Adding an anti-oxidant will ensure you get to that expected shelf life, but I really wouldn't count on it to go further. If you're using a clear bottle, are you guaranteed that the end user won't leave it in a sunny car or on a beside table that gets a lot of sunlight? Are you assured she doesn't have the room temperature at 75˚F or 22˚C, which would increase the general warmth of the lotion? And what about those products that end up in steamy, warm bathrooms with lots of humidity?
There are some lotions that seem to repel rancidity for ages. I have a lotion I made three years ago - it's an experiment - and it still doesn't smell rancid. But that doesn't mean that rancidity hasn't set in. Rancidity is happening every single day. There's a threshold where we can smell it, but it's there long before my nose notices it. (I don't use that lotion because pretty much every ingredient is well past its shelf life date, but I keep it around and observe it for changes.)
There are just too many variables to determine the shelf life of a product, but we can make a guess by sticking to the shelf life of our oils and other ingredients. And by using anti-oxidants, we're assured we can at least get to that date. As for longer than the suggested shelf life, I really wouldn't count on it as we simply can't figure out how much more life we can get out of an oil using anti-oxidants!