Ponceau 4R azoic red dye Thermal behavior, optical anisotropy and terahertz spectroscopy study Marian Leulescu 1 • Gabriela Iacobescu 1 • Mihaela Bojan 2 • Petre Rotaru 1 Received: 23 February 2019 / Accepted: 9 August 2019 Ó Akade´miai Kiado´, Budapest, Hungary 2019 Abstract The synthetic red azo dye Ponceau 4R is an additive with widely utilization in food processing technology. Thermal behavior of the trisodium (8Z)-7-oxo-8-[(4-sulfonatonaphthalen-1-yl)hydrazinylidene] naphthalene-1,3-disulfonate, appointed Ponceau 4R or E124, was studied. Thermal analysis measurements of Ponceau 4R revealed thermal stability until 300 °C. Ponceau 4R is an anisotropic crystalline dye that has the property of birefringence. The THz spectroscopy allowed the determination of the characteristic absorption frequencies of this colorant and its identification in any food. In the THz spectrum of Ponceau 4R, it can be seen that the strongest absorption is produced for the radiation with frequency of 1.41, 1.74 and 3.81 THz. THz spectroscopy of Ponceau 4R identified spectral ‘‘signatures’’ of azo food dye studied, which were obtained by the processing of the numerical data of THz spectroscopy. Keywords Azoic red food dye Ponceau 4R  Thermal stability  Optical anisotropy  THz spectroscopy Introduction Azoic food dyes are synthetic colorants that can improve the visual appearance of the products in which they are included or on which are applied, coloring them in shades of yellow, red, reddish brown, blue or black. The presence of azo group (–N=N–) in the molecular structure of dyes, typically linking two aromatic groups, determines their classification as azo dyes. The conjugated system so formed, due to the p-electrons from the two aromatic groups and due to the azo groups, can absorb the specific wavelengths from the visible light, obtaining the color of the compounds. If the azo group is damaged, it results in the cleavage of the system into two aromatic amino frag- ments and color loss. By selection of substituents and the number of azo groups, the color of the dyes can be con- trolled [1]. The Ponceau 4R (E124) is a synthetic azo food dye, a heterocyclic aromatic compound. Ponceau 4R is a food coloring substance, used as a food additive (described as the sodium salt) in beverages (non-alcoholic flavored drinks, spirituous beverages, fruit wines, cider and perry, aromatized wines, liquid food supplements) and foodstuffs (confectionery, fine bakery wares, edible ices, desserts including flavored milk products, flavored processed cheese, fish paste and crustaceans paste, smoked fish, savory snack products and savory coated nuts, non-heat- treated meat products, meat and fish analogues based on vegetable proteins, jam, jellies and marmalades and other similar fruit preparations including low-calorie products, candied fruit and vegetables, mostarda di frutta, preserves of red fruits, potato-, cereal-, flour- or starch-based snacks, extruded or expanded savory snack products, chorizo sau- sage, pre-cooked crustaceans, mustard, fish roe, solid food supplements, decorations and coatings, sauces, seasonings, pickles, relishes, chutney and piccalilli, salmon substitutes, surimi, edible cheese rind and edible casings, flavored fermented milk products, dried fruit and vegetables). Acceptable daily intake (ADI) is 0.7 mg/kg bw/day [2, 3]. & Petre Rotaru petrerotaru@yahoo.com; protaru@central.ucv.ro 1 Department of Physics, Faculty of Sciences, University of Craiova, 13 AI Cuza Street, 200585 Craiova, Romania 2 Laser Department, Plasma and Radiation Physics, INFLPR – National Institute for Laser, 409 Bvd. Atomistilor, Ma˘gurele (Ilfov), PO Box MG-16, 077125 Bucharest, Romania 123 Journal of Thermal Analysis and Calorimetry https://doi.org/10.1007/s10973-019-08680-0