Gracilaria tenuistipitata is one of the most important widely available red seaweeds for the production of unique sulfated galactans specially agar. In industries significant volume of solid residues are created during agar extraction processes which are not handled and cause wastage of resources. Considering this issue present study was designed to explore the potential reutilization of the agar extraction residues as a source of phytochemicals and antioxidants. Agar was extracted from Gracilaria tenuistipitata following different methods: alkali soaking (S1), alkali heating (S2) and hot water extraction (S3) worldwide used by the researchers. Phytochemical screening, fourier transform infrared (FT-IR) analysis and antioxidant activity of extracts from agar extraction residues was determined to evaluate the reutilization potentialities with comparison to raw seaweed (S0). Results showed the presence of different phytochemicals in residues where concentration of total phenols (95.27 mg of GA/g) and flavonoids (57.46 mg of quercetin/g) was highest in methanolic extract of S3 sample. FT-IR results confirmed the presence of functional groups of active components. S3 sample also showed the highest antioxidant activity measured by different assays such as Phosphomolybdenum (absorbance 8.71±0.13 at 695 nm wavelength), DPPH (93.10±1.19%), ABTS (87.68±0.29%) followed by S2>S0>S1, and reducing power (absorbance 3.51±0.46 at 700 nm wavelengths) followed by S2>S1>S0. These results indicate that G. tenuistipitata residues after agar extraction can be reutilized as a significant source of natural antioxidants which could be a great contributor in circular economy and support sustainable reutilization of natural resources.
Keywords: Reutilization, Gracilaria tenuistipitata, Agar, Extraction, Antioxidants, Phytochemicals, Potentiality