**1. Introduction**

Anthocyanins are gorgeous in color and have many human health benefits, including antioxidant, antiobesity, and antidiabetes [1–3]. In recent years, people's awareness of the relationship between diet and health has gradually increased, resulting in an increase in consumer demand for foods containing natural anthocyanins. Therefore, anthocyanins are widely used in the fields of food, natural pigments, cosmetics, and medicine [4]. The distribution of the six most common anthocyanins in nature is cyanidin, delphinidin, pelargonidin, peonidin, petunidin, and malvidin, accounting for 50%, 12%, 12%, 12%, and 7%, 7% of total anthocyanins, respectively [5]. Malvidin has high biological activity, but the overall distribution is less in nature [5]. It is worth noting that the high-yielding *Vitis amurensis Rupr* in the Changbai Mountains of China are rich in anthocyanins (the average content is above 150 mg/100 g·FW, and individual varieties are as high as 400 mg/100 g·FW) [6,7]. Malvidin accounts for 55~65% of total anthocyanins from *Vitis amurensis Rupr*, among which malvidin-3-O-glucoside (Mv3G) is the most abundant monomer [8]. There are

**Citation:** He, Y.; Chen, D.; Liu, Y.; Sun, X.; Guo, W.; An, L.; Shi, Z.; Wen, L.; Wang, Z.; Yu, H. Protective Effect and Mechanism of Soybean Insoluble Dietary Fiber on the Color Stability of Malvidin-3-*O*-glucoside. *Foods* **2022**, *11*, 1474. https://doi.org/10.3390/ foods11101474

Academic Editor: Lavanya Reddivari

Received: 25 April 2022 Accepted: 17 May 2022 Published: 19 May 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

many phenolic hydroxyl groups on the carbon skeleton of anthocyanins, so pH, light, heat, oxygen, metal ions, and other environmental factors can affect the color stability of anthocyanins [6,7]. The instability of anthocyanins greatly affects their application in food. Therefore, enhancing the color stability of anthocyanins is an important problem that needs to be solved urgently.

During food processing, polyphenols can interact with starch, protein, and cellulose to form a complex, which can improve the stability of polyphenols [9–12]. In particular, Quan et al. [13] found that adding soybean protein to purple-fleshed sweet potato anthocyanins could effectively improve the color stability at 100 ◦C. Sun et al. [14] reported that linear dextrin could be used as a food-grade carrier of curcumin. UV–VIS spectrophotometry (UV–VIS) and Fourier transform infrared spectroscopy (FT-IR) analyses showed that linear dextrin and curcumin formed an inclusion complex (a mixture in which the molecules of one component are contained in the crystal lattice of another component) by hydrogen bonding, which improved the stability of curcumin. Li et al. [15] also reported the interaction between soluble dietary fiber and polyphenols through conjugation and hydrogen bonding improved the stability of the polyphenols–soluble dietary fiber complex. Zhang et al. [16] microencapsulated ethylcellulose and polyphenols to diminish the instability of polyphenols as water-soluble compounds, particularly under harsh processing and storage conditions. Thus, the protective effect of macromolecules on polyphenols contributes to their application in more complex food systems.

Okara is the main byproduct during the processing of traditional soybean products such as tofu, soybean milk, and soybean oil. Okara is abundant in nutrients, especially soybean insoluble dietary fiber (SIDF), which accounts for 45~55% of the dry matter of okara [17,18]. SIDF has high stability and is not easy to react with other food ingredients, so it can be used as an inert substrate to build a stable system [19]. SIDF has a loose structure and rough surface, which can promote the interaction with polyphenols by adsorption or embedding [20]. Zhao et al. [21] reported that the hydration of insoluble dietary fiber (IDF) improved as its particle size decreased because of the greater surface area, increased number of polar groups, and exposure to other water-binding sites of IDF to the surrounding water. IDF has a high denaturation temperature, so the thermal characteristics are stable [22]. There is little research on the interaction between IDF and anthocyanins. It is concluded that SIDF has the potential to promote the stability of anthocyanins. In order to further explore the interaction between SIDF and Mv3G, this study explained the stabilization mechanism of SIDF and Mv3G, which will be more conducive to anthocyanins as stable pigments or functional components added to food.

This study aimed to optimize the stable system of SDIF and Mv3G and explain the stabilization mechanism. In this paper, the color stabilities of the stable system of SIDF-Mv3G (hereinafter referred to as SIDF-Mv3G) at different pH values, thermal conditions, or sunlight conditions were investigated. Furthermore, the interaction mechanism of SIDF and Mv3G was investigated through UV–VIS, FT-IR, X-ray diffraction (XRD), and scanning electron microscopy (SEM).
