Tonya Mckay of Naturallycurly writes;
This month I want to address a topic I see pop up on the message boards pretty frequently: use of homemade or diluted products.
I pretty frequently read of products such as hair conditioners or styling products being diluted with water and used as daily leave-in treatments. Many people also enjoy making some of their own products, using natural ingredients such as honey, aloe vera, water, and various oils often found in the kitchen. The results obtained from these can be wonderful. But it is important to exercise a bit of caution regarding spoilage.
Products purchased off the shelf have been very carefully formulated and pretty exhaustively tested in laboratories by microbiologists in order to ensure that the correct amount of preservative is present in order to prevent growth of fungi, mold, and bacteria (sounds yummy, huh?). These antimicrobial compounds also act as antioxidants, protecting the chemical stability of the product. The optimal level of preservative per unit volume is calculated and used in order to be just sufficient for that particular bottle in ordinary use conditions for a specific period of time. It is important to not use too much preservative because preservatives themselves can be harmful to humans or cause allergic reactions. Therefore, there is not typically an excess of preservative in most products.
For this reason, when a product is diluted with water (especially tap water) and placed into a non-sterile container (which is most typical in any household), the preservative level is reduced by as much as 100 times or more. This diluted product is then often stored in the hot and damp conditions of a bathroom. These conditions are perfect for the breeding of all sorts of living entities that just love to live in water and eat organic molecules (such as those found in your conditioner).