Whenever you’ve got water in a product, you need a broad spectrum preservative. Here are some common misconceptions about times when a preservative might not be necessary (spoiler alert… the preservative is still necessary!).
Ingredient X and ingredient Y have indefinite shelf lives, so when I combine them that mixture will also have an indefinite shelf life.
Ingredients like clay, honey, alcohol, and water can sit on shelves for ages and be fine, but this by no means translates to anything being made with them also having an indefinite shelf life! Think about flour, salt, water, and yeast—all will last for ages in their pure state when stored properly. Combine them into bread, and you will have a lovely loaf that will sprout mould in a matter of days.
Ingredient X has antibacterial properties, therefore it will preserve my entire final product.
No, it won’t. Once again, food is a great place to look for parallels. Spices like cinnamon, oregano, and cloves all have antibacterial properties, but you’d never make a curry with a cinnamon stick, store it at room temperature, and eat it a week later (I hope!). Ingredients with antibacterial/antiviral/antifungal properties are not broad spectrum preservatives. Please don’t try to use them as such!
Ingredient X is a store bought product that contains a small amount of preservative and since I am including that in part of a larger formula, I don’t need another preservative.
Proper preservation is not, unfortunately, contagious. The concentration of the preservative is very important, so when you dilute it, you weaken it and/or render it completely ineffective. Preservatives in store bought products are also carefully selected and blended based on the precise composition of that exact product, and that blend might not even work with the ingredients you’re working with.
Broad spectrum preservatives will prevent any and all spoilage for all eternity, regardless of ingredients, storage, and manufacturing methods.
Sadly, broad spectrum preservatives have their limits; especially when we’re working at home in less than sterile making environments. The more delicious bacteria food you add to your products (herbal infusions, food, milks, etc.), the faster they will spoil. Less than sterile making environment? Warm storage? Putting your fingers in things? All of those things will shorten the shelf life of your product, and while a broad spectrum preservative will help, it’s not infallible. Food analogy: consider (hypothetically or otherwise, haha!) putting the recommended amount of your broad spectrum preservative of choice in a pot of chili and leaving that on your counter for a few days. There is far too much bacterial temptation in that chili for the broad spectrum preservative to defend it, however valiantly it may want to. Erk!