Anthocyanins as one class of the polyphenol pigments are widely distributed in fruits and vegetables. An important source of anthocyanins is black rice (Oryza sativa L.), which has been widely cultivated in Southeastern Asian countries for a long time (Loypimai et al., 2015). The total anthocyanin contents of black rice range from 1231 to 5101 mg (Kamiya et al., 2014). More than 600 anthocyanins are found in nature, which are mainly derivated from cyanidin, delphinidin, pelargonidin, peonidin, petunidin, and malvidin (Hou et al., 2013; Tatsuzawa et al., 2008). The main anthocyanins in black rice are cyanidin and paeoniflorin, as well as their glycosidyl structure (Hao et al., 2015).
Anthocyanins are responsible for a variety of bright colors, ranging from red, blue to purple and intermediate colorations of various plant tissues. Anthocyanins not only impart color to food but also will strengthen the antioxidant defense system of body. It has been widely reported that anthocyanins have an effect in reducing risk of oxidative damage and are a kind of the potential drug candidates to treat cancer and cardiovascular diseases (He et al., 2010; Koosha et al., 2019; Prasain et al., 2020; Pahlke et al., 2021).
Despite anthocyanins possess many outstanding bioactive properties, this type of pigments is water-soluble, which make it impossible to use in oil-soluble condition. Furthermore, Anthocyanins are unstable and easily destroyed by different factors, such as pH, light, thermal treatment, enzymes, oxygen, and copigments, which restrict its application severely (Moura et al., 2012; Swer et al., 2019; Zhu et al., 2020). Thus, many researchers have sought to improve the lipophilic property and stability of anthocyanins. A series of studies revealed that the glycosyl in anthocyanins could react with organic acid to form acylated anthocyanins, which exhibited excellent stability through the formation of hydrophobic and “π-π”-interactions (Castro et al., 2014; Fei et al., 2021). The acylation reaction of anthocyanins is mainly located at the C3, or C6, or the unite of C3 and C5 in glycosyl group (Shao et al., 2014). The increasing color stability of acylated anthocyanins is associated with the higher steric-hindrance, which probably protects the anthocyanin from hydration, and water is incapable of attacking the aglycone to prevent the formation of chalcone. Furthermore, acylated anthocyanins exhibited outstanding stability in a wide range of pH, especially in acidic and neutral environment (Zheng et al., 2022).
Recently, many researchers have been concentrated on the acylation of anthocyanins. Lu et al. have reported the stability properties of acylated anthocyanins from black rice, but the antioxidant capacity and degradation kinetics have not been studied yet (Lu et al., 2011). Despite Yan et al. have revealed the degradation kinetic parameters of acylated black rice anthocyanins, the antioxidant capacity and degradation kinetics at different pH have not been yielded (Yan et al., 2016). Liu et al. have explored the color stability and antioxidant activity of acylated blueberry anthocyanins, however, the ability to capture radicals of different polarity for anthocyanins and the stability in different conditions have not been investigated yet (Liu et al. 2020). At present, chemical and enzymatic strategies are the main acylation methods for anthocyanins. Although the stable structure can be formed through chemical strategy, those anthocyanins exbibit low selectivity and weaken antioxidant activity (Howell et al., 2020). Compared with chemical strategy, enzymatic strategy is conducted with high selectivity but poor stability. Consequently, obtaining acylated anthocyanins with high stability has drawn many attentions of scholars. Caffeic acid is a kind of aromatic acid, which is wished to provide both stable C = C bonds in the benzene ring but also phenol structure with antioxidant activity (Chebil et al., 2006).
In consequence, caffeic acid was selected as acyl donor to obtain acylated anthocyanins from black rice with excellent antioxidant activity lipo-solubility, and stability by lipase-catalyzed in this study. Ultraviolet spectrum (UV) and Fourier infrared spectrometer (FTIR) was used to characterize the structure of anthocyanins, Thermogravimetric analysis (TGA) was conducted to investigate the thermal property. The antioxidant capacity of genuine and acylated anthocyanins was evaluated by measuring the 2 - diphenyl − 1 - picrylhydrazyl (DPPH) radical scavenging activity, 2,2′ - azino - bis (3 - ethylbenzothiazoline − 6 - sulfonic acid) diammoniumsalt (ABTS) radical scavenging activity and Power reduction antioxidant capacity (FRAP). Consequently, the degradation kinetics of acylated anthocyanins at different variates (temperature, pH, light) were investigated. This study will provide theoretical basis for the feasibility of anthocyanins in food processing and the improvement of health care value.