Valorization of Red Onion Skin: Thermal Stability and Potential of a Natural Dye for Industrial Applications

The red onion skin (CB), which accumulates more than 550 thousand tons per year, stands out for its high content of bioactive compounds such as phenolics, flavonoids, and anthocyanins, as well as for its potential application in the food industry, combining innovation and sustainability. In this context, the objective of this study was to obtain a natural dye from CB and characterize it in terms of its thermogravimetric properties (TGA). To obtain the natural dye, CB donated by the Zé Miúdos Cooperative, located in Macacos, MG, was used. The CB samples were washed with potable water to remove dirt, sanitized with sodium hypochlorite (200 mg/L), then frozen at −20 °C and freeze-dried at −46 °C for 72 hours. The samples were ground and standardized using 80-mesh sieves, and the resulting natural dye (CNCB) was stored in glass jars protected from light until analysis. For TGA, 20–30 mg of CNCB was placed in an aluminum crucible under a nitrogen atmosphere, with a flow rate of 50 mL/min and a heating rate of 10 °C/min up to 600 °C. The TGA indicated an initial weight loss between 21 °C and 123 °C, corresponding to water evaporation. The range from 123 °C to 170 °C reflected the degradation of volatile compounds and the breakdown of hydroxyl side groups present in the anthocyanin structures. At around 273 °C, the decomposition of remaining compounds, such as phenolics and sugars, began. Above this temperature, the weight loss continued, likely due to ash decomposition. CNCB demonstrated thermal stability up to approximately 300 °C, highlighting its potential as a promising natural colorant for industrial applications. Utilizing red onion skin as a source of this dye presents a sustainable and innovative alternative to reduce agricultural waste, replace synthetic additives, and meet the growing demand for healthier and more environmentally friendly products.