New Insights into the Structure, Thermal Properties, and Photostability of Industrially Relevant Salts of Sorbic Acid

Potassium, sodium, and calcium salts of sorbic acid (CH3–CH═CH–CH═CH–COOH) are widely used food preservatives. Despite the industrial relevance, their solid-state chemistry associated with photoreactivity and heat-induced transformations of double bonds remains misunderstood, mostly because of the lack of structural information. Herein, we report a comprehensive structural, thermal, and spectroscopic study of the above sorbates along with previously unknown normal and acid ammonium sorbates for comparison. Although all of the crystal structures exhibit a similar layered motif and carboxylate coordination, in potassium sorbate, there is an additional noncovalent interaction between K+ and the Cα═Cβ bond that bestows the salt with unique thermal properties. In turn, only the acid ammonium sorbate has a preorganization for [2 + 2] photocycloaddition, as reflected by its fast amorphization under soft UV irradiation, opposite to the other salts. The experimental results are discussed in the context of related metal carboxylates and the application and analysis of the studied salts in food industry.