Selection of appropriate preservatives

Partition coefficient. Lipophilic preservatives tend to accumulate in the lipid phase of a formulation. This phenomenon is particularly important when planing preservative systems for emulsions, where the partition coefficient, as the ratio of preservative concentrations in the oil and water phases respectively, is a significant index of how they partition in emulsions. Adding alcohol to an aqueous milieu can shift the distribution coefficient in favour of the aqueous phase. In contrast, non-ionic surfactants tend to shift the partition coefficient in favour of the oil phase, resulting in a reduction of preservative efficacy in the water phase (WallhauBer, 1984). 

Temperature tolerance. The stability of some preservatives is also influenced by temperature. Although PHB esters [II, 8.1.11.] are, for example, relatively temperature stable and can be added to a recipe at 80°C, others such as isothiazolinone mixtures [II, 15.3.] and dibromodicyanobutane [II, 17.18.] can only be added to a formulation at temperatures below 40° C. 

Tolerance of other formulation ingredients. Preservatives may well cross-react with other formulation ingredients. This includes all chemical reactions with cationic, anionic or non-ionic compounds, oxidising and reducing agents, and proteins etc. 

Packaging material compatibility (Wallhdufier, 1984). The suitability of a preservative, depends not only on the breadth of its efficacy spectrum and the other criteria already mentioned, but also on primary packaging materials. Efficacy reduction in this case is mainly due to absorption effects, and depends on the type of formulation -how lipophilic the preservatives are and the plastics used. Plastics such as polyethylene absorb quaternary compounds [II, 18.1.], PHB ester [II, 8.1.11.], sorbic acid PI, 8.1.5.] benzoic acid [II, 8.1.9.] etc., from fluid and semi-fluid contents, leading to a reduction in their efficacy, often within weeks. 

On the other hand, packaging plastics should not release any substances into the formulation (e.g. softener) that might inactivate anti-microbial substances. 

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