Effects of ultrasonic-alkali integrated extraction combined with Mannesi reaction on the antimicrobial properties of rice straw lignin and enhancement strategies.

Lignin is one of the most abundant and underused biopolymers in nature with limited antimicrobial activities. Herein, this work aimed to enhance the antimicrobial activity of lignin extracted from waste rice straw by ultrasonic-alkali integrated extraction (USP-AT) and modify the alkali lignin through Mannich reaction to improve its antimicrobial properties. The effects of ultrasonic pretreatment (USP) time on the chemical structure, morphology, antioxidant, and antibacterial activities of lignin were studied. The results demonstrated that the total phenolic content of USP-AT lignin was higher than that of lignin extracted by alkali treatment. Moreover, the antioxidant activity of USP-AT lignin was increased by 49.69 %-69.42 %. The antibacterial activity of USP-AT lignin against Escherichia coli and Staphylococcus aureus increased by more than 40 %. However, the antibacterial capacity of USP-AT lignin is far from meeting the requirements of antibacterial food packaging. The alkali lignin was modified by Mannich reaction in order to improve its antimicrobial properties against common spoilage microorganisms or pathogenic bacteria in food and packaging. The modified USP-AT lignin exhibit remarkable antimicrobial capacities to representative bacteria, molds, and yeasts. The antifungal capacity of modified USP-AT lignin against A. niger and P. citrinum were improved by more than 40 %.