Antibacterial Activity of Red Pigment Extracted from Pitaya Peel and Its Indication Effect on the Freshness of Bighead Carp Meat

In this study, the red pigment of pitaya peel was extracted by ultrasound-assisted ethanol extraction method. Followed by inhibition zone testing method (using drug sensitive testing paper) and DPPH· free radical scavenging method, the antibacterial activity and total antioxidant capacity of the two red pigments were compared. Meanwhile, by detecting the response of the red pigment extracts to the index of both pH change and color difference values, the optimal concentration of the red pigment extract to make an indicator card was selected. Then evaluated the effect of the indicator card on detecting the freshness of bighead carp meat by assessing the changes in pH and TBA values during storage at room temperature. The results showed that the antibacterial activity against E.coli and S.aureus and total antioxidant capacity of red pigment extracted from the red-fleshed pitaya fruit peel was significantly higher than that from the peel of white-fleshed pitaya (P<0.05). The solution of the red pigment extracts of the red-fleshed pitaya also had a sensitive color change reaction to pH2~13. The freshness indicator card was then prepared by using 10 g/L red pigment extracted from the peel of red-fleshed pitaya, and was used to detect the changes of the bighead carp meat. Results showed that during the storage at room temperature (25 ℃) for 12 hours, the pH value of the fish meat first slightly decreased and then increased, while the TBA value continuously increased to 1.96 times that of 0 hours, which indicated a decrease in fish freshness. Meanwhile, the color difference value (ΔE) of the indicator card placed with fish meat also increased from 1.40 (0 h) at the beginning to 18.30 (12 h) at the end, and the color of the peripheral circle of the indicator card changed from light red to rose red to earthy yellow, indicating the deterioration of fish quality more intuitively. This study provides the scientific basis for the comprehensive utilization of the natural red pigment in pitaya fruit peel, also with a preliminary reference for the new food packaging in the aquatic sector of prepackaged food industry.