Just like us, microbes need water and oxygen to live. Give them the right conditions and they will procreate like silly, showing up as blooms of pink or green or brown. Preservatives tend to live in the water phase of our products to fight them, but we include preservatives in oil only products like scrubs into which water might be introduced, too.
I really encourage you to read this post on water activity and sugar/salt scrubs. I apparently did quite a bit of research on beasties in the comments!
If you doubt the importance of preservatives, I encourage you to watch this video! Ick!
Preservatives: What can get into our creations?
Preservatives: How the heck do they work?
Preservatives: Water activity and sugar/salt scrubs
If you're new to lotion making (heating & holding)Why do we heat & hold?
Why do we heat and hold separately?
Sorry for the short and not at all weekend-y Weekend Wondering but my back is really bad. Look for more later this week and this weekend! Keep your comments coming!