**1. Introduction**

The functionality of insoluble dietary fiber (IDF) has been gradually accepted by consumers [1–3]. Especially for intestinal health, IDF may have unique physiological activities [4–6]. As a kind of material that can improve intestinal health, IDF had been gradually developed as a kind of functional food material. However, recent studies have shown that some kinds of IDF are not necessarily beneficial to intestinal health [7–9], which may be due to the complex fermentation mode of IDF after entering the colon. It had been proved that roughly ten subtypes of dietary fibers described to date, categorized as soluble or insoluble, with varying chemical structures, and large differences in their fermentation profiles [10]. This means that the changes of gut microbiota and colonic environment are complex. Therefore, the changes of different kinds of IDF in the colon become worthy to study.

**Citation:** Lyu, B.; Wang, Y.; Zhang, X.; Chen, Y.; Fu, H.; Liu, T.; Hao, J.; Li, Y.; Yu, H.; Jiang, L. Changes of High-Purity Insoluble Fiber from Soybean Dregs (Okara) after Being Fermented by Colonic Flora and Its Adsorption Capacity. *Foods* **2021**, *10*, 2485. https://doi.org/10.3390/ foods10102485

Academic Editor: Adriana S. Franca

Received: 10 September 2021 Accepted: 14 October 2021 Published: 17 October 2021

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Okara (Soybean dregs), the main by-product during traditional soybean products processing and the production of soybean protein isolate (SPI), had caused huge environmental pollution and waste of resources [11], which includes a large amount of dietary fiber (DF). Okara-DF had been proved to be functional, such as decreased content of blood glucose [12], protected the intestine [13], used as a food raw material ingredient [14], etc. However, different from soluble dietary fiber (SDF), the application of Okara-IDF had a core problem—IDF was difficult to purify. The traditional processing method of IDF is often accompanied by a large amount of protein and soluble fiber as remains. It is difficult to evaluate the functionality of one component in the mixture, as while as caused a disturbance, In addition, okara was difficult to dry because of its unique structure [15]. For these reasons, the usage or value-increment of Okara-IDF as a material of functional food was restricted.

To solve the problem, we prepared high purity insoluble dietary fiber from okara (Okara-HPIDF) by enzymatic method, which purity could exceed 90% [16], and proved that it had potential bioactivities. Meanwhile, we proved that Okara-HPIDF had certain functions, such as improving lipid metabolism and so on [17,18]. In the process of studying its relationship with the changes of the colonic environment, we found that Okara-HPIDF could significantly change the composition of colonic flora [18]. This means that Okara-HPIDF was fully fermented by microorganisms in the colon, which may lead to changes in structure, composition, functionality, and even bioactivities of Okara-HPIDF. The above changes had not been studied, because we used germ-free simulated digestive fluid (SGF) in the previous study [16].

In this way, this study aimed to explore the changes after Okara-HPIDF enters the colon, while being fermented by colonic microorganisms. In this work, we were committed to treating Okara-HPIDF by simulated intestinal fluid (SIF) with colonic microorganisms and digestive enzymes, to look for the changes of Okara-HPIDF after fermentation. The structure, monosaccharide composition, and physicochemical properties before and after fermentation were studied, and a more accurate model of simulated fermentation in vitro of Okara-HPIDF was established. This study is a supplement to our team's previous research, to explore the changes and properties of HPIDF after entering the colon and be fermented by colonic flora, which will improve our evaluation system for the potential biological activity of HPIDF and provide a theoretical basis for improving the biological activity of HPIDF. This work will also be the basic theory to functional research of Okara-HPIDF, which is also the core data of the relationship between the intake of Okara-HPIDF and colon health.

#### **2. Materials and Methods**

#### *2.1. Preparation of Okara-HPIDF*

Low purity dietary fiber from okara (okara-LPDF) was purchased from Shandong Sinoglory Health Food Co., Ltd., Liaocheng, China. After identification, its main components include: dietary fiber ≥ 60%, protein ≤ 26%, moisture ≤ 10%, ash ≤ 6%. HPIDF from okara was prepared by complex enzymatic method of LPDF [17]. The purity of HPIDF was 91.25% (according to the enzymatic-gravimetric method described in Chinese national standard GB 5009.88-2014).
