preservatives! Unpreserved water containing products have a shelf life of about three days in the fridge.
If you don't want to use preservatives, then don't make water containing products. It really is that simple. Enjoy making anhydrous (non-water containing) products and enjoy things like lotion bars, whipped butters, lip balms, and more!
Want to see a video about contamination? Click here, then click on "The Importance of Preservatives". Ick!
Related posts on preservatives:
Preservative section of the blog
Why use a preservative?
Preservatives: What beasties can contaminate our products!
Preservatives: How do they work?
Grapefruit seed extract (GSE) is NOT a preservative
How can I use essential oils as an anti-microbial? (Short answer: You can't.)
As for rancidity, Vitamin E, citric acid, rosemary extract or essential oil, and other anti-oxidants are a great way to retard this inevitable process. Add it at the appropriate amount to the appropriate phase - for instance, I use 0.5% Vitamin E in the cool down phase - and rejoice.
Related posts on rancidity:
Rancidity: A primer
Mechanisms of rancidity
A more in depth look at anti-oxidants (how they work)
Question: How do anti-oxidants affect the shelf life of our products?