Preparation and characterization of acylated mulberry anthocyanins using biological enzyme method

Mulberry is rich in anthocyanins. The main component is cyanidin-3-O-glucoside. Anthocyanins are hydrophilic and water-soluble polyphenolic plant pigments and metabolites, which are one of the important antioxidants. They possess a wide range of pharmacological properties, such as anti-oxidant, anti-aging, anti-inflammatory, antimicrobial, and anti-cancer. Therefore, anthocyanins have great potential in various fields, such as the pharmaceutical and food industries. However, the relatively low stability of anthocyanin limits its bioavailability and effective applications, particularly on the degradation of anthocyanin. Taking the mature mulberry fruit as research objects, this study aims to prepare and characterize acylated mulberry anthocyanins using biological enzymes. The anthocyanin was extracted with acidified ethanol solvent and then purified by D101 macroporous resin. In order to reduce the degradation of anthocyanin while improving its stability, the mulberry anthocyanin was acylated by biological enzymatic and compared with that of non-acylated anthocyanin. Lipase, reaction solvent, and acyl donor were screened by single factor experiment to explore the anthocyanin acyl conversion rate. Taking the conversion rate as the evaluation index, the optimum conditions for acylation reaction were as follows: Candida Antarctica lipase as an acylating catalytic enzyme, pyridine as a catalytic reaction solvent, methyl benzoate as acyl donor, and the acylation effect was the best. The maximum conversion rate of mulberry anthocyanin was 13.5%. The products were analyzed by Fourier Transform Infrared Spectroscopy (FTIR), Ultraviolet-visible spectrophotometer (UV-Vis), and High-Performance Liquid Chromatography-Mass Spectrometry (HPLC-MS). Acylated anthocyanin was composed of C=O absorption peak at 1 650-1 870 cm-1, -OH bending vibration absorption, and C-O-C stretching vibration absorption of phenolic molecules at 1 000-1 300cm-1. There was also a skeleton vibration peak of the benzene ring (1 420-1 600 cm-1). The non-acylated anthocyanin sample had obvious UV absorption at 280 nm, and the absorption at this wavelength was the most stable. It showed that anthocyanin contained a benzene ring and a phenolic hydroxyl group on the benzene ring. The absorption peak of acylated anthocyanins moved forward from 280 nm to 271 nm, indicating that the structure of anthocyanins had changed. In addition, an attempt was made to explore the effects of acylation on the stability and antioxidant properties of anthocyanin. The acylation could improve the thermal stability, photostability, and acid-base resistance of anthocyanin. At the same temperature, the preservation rate of acylated anthocyanin increased by 5.0%, and it could be better preserved at 40, 50, and 60 ℃. Light had a great influence on the stability of anthocyanin. The preservation rate of non-acylated anthocyanin decreased linearly to 77.3% within 10 days. After 6 days of light exposure, the preservation rate of acylated anthocyanin was still as high as 96.1%. At the pH values of 2, 3 and 8, the stability of acylated anthocyanin was improved. The acylation significantly increased the antioxidant activity of anthocyanin in vitro. DPPH radical scavenging activity was enhanced. The total reducing ability was 30% higher than that of the non-acylated anthocyanin. The chelating ability of metal ions was 90% higher. The inhibition rate of acylated anthocyanin on tumor cell activity was up to 81%, while that of non-acylated anthocyanin was only 50%. Therefore, acylated anthocyanin can effectively inhibit the proliferation of tumor cells. This finding can provide a theoretical basis and technical support for the stable application and performance improvement of anthocyanin in the production fields of functional food, biomedicine, botanical pesticides, and daily cosmetics fields.