Carotenoprotein samples (2.5 g) were mixed with 10 ml of 0.5 M NaOH, followed by incubating at 85 °C for 1 h. The mixture was centrifuged at 8000×g for 10 min. The protein contents of supernatant were measured by the Biuret method ( Robinson & Hogden, 1940) using bovine serum albumin as a standard. To obtain the total protein in shell, ground shells (2.5 g) were dissolved in 10 ml of 0.5 M NaOH for 1 h at 85 °C, followed by centrifugation at 8000×g for 10 min ( Klomklao et al., 2009). Protein recovery was expressed as the percentage of proteins in the extracted carotenoprotein, relative to total proteins of shrimp shells.