Preservatives are a necessary ingredient because it prevents the growth of microorganisms such as bacteria and fungi. Therefore, preservatives are used for the safety of the user as well as to increase the shelf life of the product. Some examples of preservatives are parabens, benzyl alcohol, salicylic acid, formalidehyde and tetra sodium EDTA (ethylenediaminetetra-acetic acid).
Preservatives work by disrupting the cell walls of microbes (lysing the membrane kills the microbe since the intracellular fluid seeps out). However, the way they do this differs between the preservative used. For example, natural preservatives like honey and sugar bind to water molecules. This makes the water inaccessible to microbes, which need it to grow and multiply.
Another way preservatives prevent the growth of microbes is by controlling the pH. Microbes’ ideal pH is between 5-7.5. Therefore, adding salicylic acid to a mixture keeps the pH below acidic making microbes unable to exist.
Parabens, the most common preservative, work at a broad spectrum of pHs and against several microorganisms. The function of these molecules are not well understood but scientists believe that these molecules easily permeate the cell membrane of microbes and are able to disrupt the lipid bilayer and ultimately kills the cells by leaking its contents.
A cosmetic product that is labeled preservative-free or paraben-free may be seen as more attractive since it is debated whether or not these preservative material have a negative health effect. However, you must keep in mind that these products may develop microbes quicker and thus keep an eye out for signs that bacteria is forming. If you see black/green dots in the formula (bacteria and fungi colonies) or if the product smells different than its initial scent, dispose of the product since it is most likely microbe-infected.