Rosemary Oleoresin Extract (also known as ROE) is NOT a preservative (see note below). It is a very effective antioxidant. Preservatives inhibit the growth of mold, bacteria, yeast, fungus etc. in water-based products like lotions. Antioxidants slow down the oxidation of the oils. It’s the oxidation of the oils that causes rancidity - which can cause bad or off smelling products, and the dreaded orange spots or “DOS”.
If you start making a lot of soap, you'll probably start buying larger quantities of oils to get a better price. This leaves them on the shelf for longer - which means they can start to oxidize.
ROE is a thick, brownish green liquid with a strong herbaceous smell. (The scent doesn’t come through in the final product because you use so little of it.) It’s primary useful constituent is carnosic acid, which you will generally see specified at about 7%.
When added to fresh oils, ROE can greatly extend the shelf life of the oils, especially of more fragile oils like hemp, avocado and grapeseed. But you have to add it to the oils when they are fresh - before the oxidation has started.
Because the rosmary oleoresin is so thick, getting it fully dispersed into the oils can be difficult. I recommend blending the amount you are going to use in a small portion of the oil first, then adding this to the rest of the oil, and mixing thoroughly.
You just need a tiny amount in your oil. Generally, the recommended usage is between .02% and .05%. (Follow the instructions of the vendor you get your ROE from - concentrations do vary.) Yes, that’s between 2 and 5 one-hundredths of a percent. So for 1000 grams of oil (a little over 2 lbs.) you’d use between .2 and .5 grams of ROE.
Note: I stated that rosemary oleoresin extract is not a preservative. Technically, it isn’t. But...it does demonstrate some antimicrobial characteristics. Just not enough to preserve a product all by itself. So...to clarify, rosemary oleoresin can function as part of a preservative system, but is not a broad-spectrum antimicrobial preservative.