Bioengineered gardenia red-Fe²⁺ complex: a heat- and pH-stable dual-function clean-label colourant

Natural pigments are increasingly sought after to meet clean-label demands, yet many suffer from poor stability and limited functionality under processing conditions. In this study, a dual-function pigment was engineered from Gardenia jasminoides for use as a natural clean-label food colourant. Geniposide extraction followed by its enzymatic hydrolysis by β-glucosidase derived from food-grade Lactobacillus plantarum afforded the corresponding aglycone genipin. Genipin was subsequently reacted with various amino acids to form gardenia red (GR). Among the amino acids tested, glycine generated the most intense red pigment. To improve pigment stability and provide additional nutritional functionality, GR was chelated with ferrous sulfate, yielding GR-Fe²⁺ with a chelation efficiency greater than 99%. Coordination followed pseudo-first-order kinetics with a low activation energy of 6.42 kJ/mol, suggesting a diffusion-controlled process. The physicochemical characterisation of GR-Fe²⁺ confirmed pigment formation and iron chelation, and structural analyses further verified successful coordination of Fe²⁺ ions with the pigment matrix. Compared to GR, the GR-Fe²⁺ complex showed greater resistance to heat and pH variation. In conclusion, this bioengineered colour system highlights strong potential as a heat- and pH-stable, clean-label colourant with integrated iron-delivery functionality.