Agar extraction from Pyropia haitanensis residue after the removal and purification of phycobiliproteins

Pyropia haitanensis is one of the most important and commercial species of edible seaweed in China. However, at its late growth stages it is unsuitable for food processing and is usually treated as waste. The present study investigated the full use of aging Py. haitanensis and attempted to improve its economic value through the purification of R-phycoerythrin combining the extraction of agar from the dry residue after phycobiliprotein extraction. The recovery of R-phycoerythrin from crude extract was 12.12% with a purification ratio of > 4. The agar yield was 5.81% with a gel strength of 796.00 ± 28.10 g cm−2, a gelling temperature of 38.10 ± 0.22 °C, a melting temperature of 89.50 ± 0.55 °C, 3,6-anhydrogalactose content of 34% ± 3.16%, and sulfate content of 3.01% ± 0.24%. There were no significant differences between these parameters and those from the direct extraction of Py. haitanensis. The agar extracted exhibited some physicochemical properties that have industrial applications. It could also be used as a material for DNA electrophoresis and a bacterial culture medium. This new method of comprehensive extraction reduced use of water and alkali, and waste liquid output, which were 36.52 L, 0.40 L, and 39.15 L, respectively, less than that from direct Py. haitanensis extraction. Total nitrogen, suspended solids, phosphorus, ammonia-nitrogen, chemical oxygen demand, and biochemical oxygen demand of the extract after 5 days were 38.37 mg, 4.72 × 103 mg, 62.82 mg, 18.32 mg, 8.46 × 103 mg, and 2.21 × 103 mg, respectively, less than that directly from Py. haitanensis.