**1. Introduction**

Surimi-based products have become an increasingly consumed food with prominent characteristics of convenience, special flavor, and unique texture [1]. In addition, the deep processing technology gave it the qualities of excellent nutritional value and high digestibility [2]. Due to the continuous overexploitation of marine fishing resources, lowvalue or cultured fish has been paid more attention as a potential alternative raw material for surimi production [3]. Silver carp (*Hypophthalmichthys molitrix*), widely cultured in China, is considered a low-commercial value fish due to its muddy flavor and higher by-product content [4]. However, due to its rapid growth, high yield, and low price, silver carp could replace sea fish as raw material for surimi [5]. Through the processes of rinsing, dehydration, and defatting, the myofibrillar protein could be well retained, and more fishy compounds were removed, which improved the flavor characteristics to a certain extent. Raw surimi could be further processed into prepared foods or ready-to-eat foods in order to satisfy different taste demands for surimi-based products, which realize the value addition of silver carp [6].

Throughout the entire process of thermal gel formation, heat treatment is an important step in determining the quality of the gel matrix [7]. Before heating, free myosin molecules and actomyosin complexes were dispersed, while the network structure complex reinforced by actomyosin was completed as the temperature rose [8]. Based on the unique property of fish protein, heating processes can be divided into direct heating and two-step heating,

**Citation:** Jiang, X.; Chen, Q.; Xiao, N.; Du, Y.; Feng, Q.; Shi, W. Changes in Gel Structure and Chemical Interactions of *Hypophthalmichthys molitrix* Surimi Gels: Effect of Setting Process and Different Starch Addition. *Foods* **2022**, *11*, 9. https:// doi.org/10.3390/foods11010009

Academic Editors: Jianhua Xie, Yanjun Zhang and Hansong Yu

Received: 22 November 2021 Accepted: 15 December 2021 Published: 21 December 2021

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**Copyright:** © 2021 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/).

with significant differences in protein networks between these two treatments [9]. Direct heating treatment provided the gel with soft structure but might cause the formation of protein aggregates in different sizes [10]. In case of two-step heating, an extension of setting time at 4–40 ◦C prior to heating at 90 ◦C could strengthen gel properties, which was widely used in making kamaboko [6]. The first setting step, in which more available cross-linking sites become accessible, ensured a gradual sol-gel transition so that an orderly initial protein network formed [11,12]. Subsequent secondary heating at high temperature is used for the final production of thermal gels [13]. Compared with direct heated gels, the two-step heated gels enabled the gel structure to remain more compact even when suffering from physical forces [10]. Hence, the setting process played a vital role in gel processing. However, the addition of some exogenous substances during the actual processing was also necessarily for helping improve gel texture.

Starch, as a popular food additive for improving gels, plays an essential role in reducing the factory processing costs of surimi-based products and meeting consumer taste needs [14,15]. According to the "packing effect" raised by Kong et al. [16], the starch granules swelled and then exerted more pressure on the gel matrix, forming a firm and cohesive gel. Starch swelling in the surimi matrix increased the hardness of the gels, thereby improving overall gel properties. It also found that starch did not crosslink with the surimi protein, whereas it could change the chemical interaction in the surimi matrix [17]. Thus far, studies have mainly emphasized the effects of different starches in improving gel strength [17–19]. Nevertheless, the surimi gel network also affected the filling effect of starch in the mixture, which was rarely reported in research. It was hypothesized that the characteristics of the starch-containing surimi matrix could be affected by the setting processing, showing different change trends in various properties. Accordingly, two heating processes (direct heating process and two-step heating process) were set up to detect the different properties of the mixed matrix and to compare the systematic effect of the setting process on the heated starch–surimi matrix.

Therefore, the research aimed to elucidate water migration, microstructure, chemical interactions, and surimi protein structure in the starch–surimi matrix comprehensively. Physical properties, such as texture profile analysis, whiteness, and water holding capacity, were also detected, providing insight into the combined effect of thermal processing and exogenous additives on texture characteristics.

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

#### *2.1. Materials and Reagents*

Silver carp frozen surimi (AAA grade, cryoprotectants were 6% sucrose and 0.25% polyphosphate) obtained from Jinli Fishery Food Co., Ltd. (Honghu, China) was cut into pieces weighing about 200 g and stored at −20 ◦C after vacuum sealing. The moisture and crude protein contents were 75.20% and 14.37%, respectively. Native potato starch was purchased from Hangzhou Starpro Starch Co., Ltd. (Hangzhou, China). The P0006C Detergent Compatible Bradford Protein Assay Kit was obtained from Shanghai Beyotime Biotechnology Co., Ltd. (Shanghai, China). The other chemicals were analytical grade, with the exception of KBr (spectrography), and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

#### *2.2. Sample Preparation*

Frozen surimi was thawed at 4 ◦C overnight and cut into small pieces. An amount of 200 g of semi-thawed silver carp surimi was blended for 2 min and then mixed with 5 g of NaCl for 4 min. During the blending process, moisture content was adjusted to 80% with iced water that was also used to keep the temperature of mixtures below 10 ◦C. Native potato starch (0 g, 3 g, 6 g, 9 g, or 12 g/100 g surimi) was added to the surimi and mixed in blender (AM-CG108-1, Appliance Co. of America, Zhuhai, China) for 6 min. Then, starch–surimi combinations were filled into the plastic tubes with a diameter of 25 mm and heated in two different processes. In the direct heating process, the tubes were heated in a

water bath at 90 ◦C for 30 min, obtaining cooking gel (CG). In the two-step heating process, a combination of preheating at 40 ◦C for 1 h and cooking at 90 ◦C for 30 min was carried out to obtain setting-cooking gel (SCG) [6]. After heating, all samples were stored at 4 ◦C for 12 h.

#### *2.3. Low Field Nuclear Magnetic Resonance (LF-NMR)*

The relaxation time and moisture distribution were measured by a Niumag Pulsed NMR analyzer (MesoMR23-060H-I, Niumag Electric Co., Shanghai, China). Gel samples were cut into the cylinders with a height of 20 mm, and a CPMG (Carr-Purcell-Meiboom-Gill) pulse sequence was carried out [20,21]. The CPMG parameters are listed in Table 1.

**Table 1.** CPMG parameters.


SF: magnet frequency, MHz; SW: spectral width, kHz; RFD: radio frequency delay, ms; Tw: recycle delay, ms; RG1: regulate analog gain 1, db; DRG1: regulate digital gain 1; PRG: pre-amplified receiver gain; P1: 90◦ pulse lengths, μs; P2: 180◦ pulse lengths, μs; TE: pulse gaps between π and π, ms; NECH: echo number; NS: scanning number.

#### *2.4. Scanning Electron Microscope (SEM)*

The surimi gels were cut into 3 mm × 3 mm × 1.5 mm pieces and fixed with glutaraldehyde (2.5%, *v*/*v*) for 14 h at 4 ◦C. The fixed samples were rinsed with 0.1 M phosphoric acid buffer (pH 7.2–7.4) three times. After that, the samples were dehydrated with a serious of ethanol solution (30%, 50%, 70%, 80%, 90%, and 100%) and then replaced with tert-butanol solution (absolute ethanol: tert-butanol = 3:1, 1:1, 1:3, 0:1). The dehydrated samples were dried by using a freeze dryer (SCIENTZ-10N, Ningbo Scientz Biotechnology Co., Ltd., Ningbo, China) and sputter-coated with gold. The microstructures were analyzed by an SEM instrument (Hitachi SU5000, Hitachi High-Tech Co., Ltd., Shanghai, China) at an acceleration voltage of 5 kV.
