Heat-Induced Phytochemical Changes in Curcuma longa: Effects on Antioxidant Properties and Oxidative Behavior in Illuminated Oil-in-Water Emulsions

Curcuma longa is a medicinal plant valued for its diverse phytochemicals and antioxidant properties. However, the effects of thermal processing on its phytochemical composition and antioxidant function remain insufficiently understood. This study investigated heat-induced changes in curcuminoids, sesquiterpenoids, and their degradation products in C. longa extracts heated at 180 °C for 0–90 min under dry, aqueous, olive oil, and corn oil conditions using UHPLC–MS/MS and GC–MS. Curcuminoids progressively decreased during heating, whereas sesquiterpenoids exhibited greater thermal stability, particularly in oil matrices. Antioxidant properties were also better preserved in oil-treated extracts than in those heated under dry or aqueous conditions. However, in an oil-in-water emulsion system exposed to continuous LED illumination (14,600 lux), extracts obtained after dry heating accelerated lipid oxidation, as evidenced by enhanced oxygen depletion, hydroperoxide formation, and conjugated diene accumulation. This pro-oxidative effect was markedly suppressed by EDTA, suggesting the involvement of metal-mediated oxidation pathways. These findings demonstrate that thermal processing differentially alters the phytochemical profile and antioxidant properties of C. longa depending on the heating matrix, while highlighting that preserved antioxidant capacity does not necessarily translate into improved oxidative stability in illuminated emulsion systems.