In this post, Nedeia asks: If one has a rebatch to do (e.g. failed to add something in the oil phase , but remembered right after adding Germall Plus), what would happen with the preservative when we reheat the lotion to add the missing ingredient? I know I have to add the preservative again, but I am worried about what would germall plus "do" on our skin. Would such a lotion still be usable, or do I discard the entire batch? I am asking this because I am in this particular situation, but with a 50 grams batch - so not much of a loss if I were to discard it :)
The same question applies to preservatives in general: what happens with the preservative if heated in case you want to add something to the lotion that was forgotten, especially when you cannot afford to throw away the failed batch?
I don't think it's ever okay to re-heat a batch of lotion. There are a number of reasons for this answer, the main one being that your preservative might not be able to handle heat (in the case of liquid Germall Plus, we know it doesn't like heat very much), and you might inactivate it. Or it might not. And if you add more, you might have too much preservative. The problem is that you don't know if the preservative is working or not, so adding more runs the risk of irritating your skin. Not adding it runs the risk of not having enough preservative. Either way, you've got a lotion that could cause problems for the user.
I can't definitively answer what happens to a preservative when it's heated because there are many things that can happen depending upon the type, but the one that is most important here is that the heat could cause the preservative molecule to degrade and not work any more as a preservative.
I hate this idea that if a lotion is "just for me" that it's okay to use inadequate levels of preservatives. Are you unworthy of having a well preserved lotion that won't be contaminated? I see this all the time - I'll reheat it and it'll just be for me - which makes no sense to me!
If you reheat your product, your fragrance and essential oils will dissipate, and you won't have the lovely fragrance you had planned...but adding more means you could irritate someone's skin!
You can't not afford to throw away a failed batch! What are you going to do with a failed lotion that might have an ineffective preservative in it? It's really not fair to give it to someone else and it's silly to keep it for yourself. We will make products that aren't great, and they are learning experiences. It sucks to have to throw out $10 worth of lotion, but it has to be done because we can't use a failed, separated, unpreserved product on our skin. It's not safe. If you aren't in a place in your formulating abilities where you can guarantee you'll have more successful than unsuccessful products, then using really expensive ingredients probably isn't a good idea. Work on getting the base lotion recipe perfect, then start adding or substituting the expensive stuff!
How do I figure out when to add an ingredient?
How to keep a lab notebook!