Abstract: The stability and antioxidant activity of amaranth pigments extracted from symbiotic fungi isolated from Porphyridium purpureum were investigated.The stability of the pigment was determined by the change of pigments absorbance value using spectrophotometry,and the antioxidant activity was evaluated using the method of scavenging 2,2’-azino-bis(3-ethylbenzo thiazoline-6-sulfonic acid) diammonium salt (ABTS) and hydroxyl free radicals.The results showed that the pigments were stable under these conditions such as heating,UV,indoor natural light,Na2SO3,even in the presence of NaCl,glucose,sucrose,maltose,or metalions (K⁺, Ca²⁺, Mg²⁺ and Zn²⁺).However,the pigments were bleached by H₂O₂ and even unstable under sunlight,the color of the pigments changed with pH and also changed to brown in the presence of Cu²⁺ or Fe³⁺ ions.Besides,the amaranth pigments showed good antioxidant activity,when the concentration of pigments was 12.5 mg/mL,the scavenging percentage of ABTS and hydroxyl free radicals reached 99.69% and 80.95%,respectively.

Key words: Porphyridium purpureum, symbiotic fungi, amaranth pigments, stability, antioxidant