Food Colors as Intrinsic Luminescent Sensors in Edible Products

Abstract

Limited use of luminescence spectroscopy in food and pharmaceutical applications can be related to two main causes: a)the inherent properties of most useful fluorophores (low availability, toxicity, high price and restricted solubility), and b)incomplete photophysical characterization of edible and safe-to-ingest fluorophores.To expand the use of luminescence spectroscopy to monitor quality and safety of edible goods, photophysical properties of five generally-recognized-as-safe(GRAS) food colors that are routinely added to foods or pharmaceuticals were assessed. The sensitivity of the food dyes’ fluorescence emission intensity to the surrounding medium's rigidity and polarity were also determined.Environmental polarity moderately impacted the location of the fluorescence intensity peaks and bathochromic shifts were observed for all dyes as the polarity of the solvents increased. The Stokes shifts,(λem-λexc), were estimated to be 45-90nm depending on the medium and molecular structure of the synthetic color.All these food dyes were practically non emissive in common fluid solvents, which can explain the limited information on their photophysical properties. Excited state tautomerization and/or internal twisting, already reported in synthetic non-edible dyes in low viscosity solutions, can also constitute the predominant non- radiative relaxation pathway of the studied food colors in fluid environments. The medium's rigidity was altered by changing temperature and composition. As the viscosity and consistency index of the surrounding medium increased, the dyes’ fluorescent emission intensity also increased, which suggests the molecular rotor character of these dyes. The maximum fluorescence intensity of each dye vs. viscosity relationship was characterized by a power law model and the sensitivity of each dye to changes in viscosity was evaluated in terms of its parameters.In principle, the large Stokes shift and sensitivity to viscosity supports the potential use of these food colors as probes of microviscosity or molecular crowding in edible goods.