Characterization of alpha-cyclodextrin inclusion complexes with trans-cinnamic acid in an acid-based beverage system

In response to a need for a natural antimicrobial to replace sodium benzoate, cinnamic acid was chosen. Due to cinnamic acid's solubility issues, α-cyclodextrin was used as a host molecule to form an inclusion complex with the cinnamic acid molecule. The cinnamic acid: α-cyclodextrin inclusion complex was then characterized using phase solubility analysis, proton nuclear magnetic resonance (H-NMR), and solid inclusion. Phase solubility analysis verified the maximum amount of cinnamic acid that α-cyclodextrin was able to host. H-NMR was used to determine the complex association constant, determine the chemical shifts of available protons, and yield a stoichiometry for the complex. The solid inclusion complex allowed for a physical formation of the complex, yielding further information in support of the complex stoichiometry. Microbiological tests were also performed to quantify the antimicrobial abilities of the complex, the guest, and the host against the yeast Saccharomyces cerevisiae and mold Paecilomyces variotii.

Results indicated that approximately 990.29 ppm in aqueous solution was the maximum amount of cinnamic acid in the complex. The 2:1 stoichiometry yields an association constant of 21.7 M-1. Results also indicated that the cinnamic acid readily conformed to fit within the α-cyclodextrin host molecule, which remained a rigid structure. An 8.9% weight to weight of cinnamic acid was calculated for the solid inclusion again reinforcing a 2:1 stoichiometry. Microbiological studies showed little to no inhibition power by the complex at varying concentrations against S. cerevisiae and P. variotii. Free cinnamic acid showed greater antimicrobial activity compared with free α-cyclodextrin and the complex.