Upcycling of seaweed waste from agar production of red seaweed, Gracilaria tenuistipitata as a source of bioactive and antioxidant potentialities

• Commercial agar extraction from seaweed produces plenty of solid residues. • Residues after agar extraction can be used as a source of phytochemicals and antioxidants. • Methanolic extract of residual powder obtained after hot water agar extraction exhibited higher bioactivity. • A positive correlation was found among TPC, TFC, and different antioxidant assays. • Residues can significantly contribute to the circular economy. A substantial amount of waste biomass is generated during the process of agar extraction from Gracilaria tenuistipitata at the industry level. Considering this issue, this study investigated the possibility of reusing this waste biomass as a source of phytochemicals and antioxidants. Waste biomass of G. tenuistipitata was obtained after agar extraction following three methods: alkali soaking (S 1 ), alkali heating (S 2 ), and hot water extraction (S 3 ). Qualitative and quantitative determination of phytochemicals, Fourier transform infrared (FT‐IR) analysis, and antioxidant activity of different crude extracts of waste biomass and native seaweed (S 0 ) were determined and compared. Results showed phytochemicals presence in waste biomass where the concentration of total phenols (95.27 mg of GA/g) and flavonoids (57.46 mg of quercetin/g) was maximum in the S 3 sample compared to S 0 . S 3 sample showed maximum antioxidant activity measured by DPPH (93.10 ± 1.19 %, IC 50 = 6.69 mg/mL), ABTS (87.68 ± 0.29 %, IC 50 = 6.32 mg/mL), Phosphomolybdenum (28.08 mg of ASE/g) assays followed by S 2 > S 0 > S 1 samples, and reducing power assay (501.22 mg of ASE/g) followed by S 2 > S 1 > S 0 samples. Overall, G. tenuistipitata residues after agar extractions could be upcycled, which could greatly contribute to the circular economy.