Optimizing ultrasonic extraction of polysaccharides from Spirulina platensis and evaluating their antioxidant and antibacterial activities in acidic environments.

In this study, the response surface method was used to optimize the ultrasonic extraction of polysaccharide from Spirulina platensis. The optimal extraction conditions were ultrasonic time 50 min, solid-liquid ratio 1: 40, ultrasonic power 265 w, and the predicted extraction rate was 6.861 ± 0.116 %. The FT-IR and NMR confirmed that polysaccharide from S. platensis would protonate to form β-glucoside bond in acidic environment, and the original carbonyl group would be converted to carboxyl group and then esterified to form acyl group donor. In the acidic pH range (7.0, 6.0, 5.0 and 4.0), the antioxidant, nitrite scavenging and bacteriostatic activities of the extracted polysaccharide from S. platensis were measured in vitro. The results showed that polysaccharide of S. platensis had better antioxidant activity in acidic environment and better performance in low pH condition. In addition, it has the best inhibitory effect on Salmonella enteritidis (14.3-17.15 mm) in acidic environment, and the inhibitory effect on Staphylococcus aureus (11.95-12.87 mm) is stronger than that in neutral environment (10.48-12.08 mm). To sum up, the structure of polysaccharide from S. platensis will change in acidic environment, and it has good antioxidant and antibacterial properties.