*3.5. Changes in Gel Properties of Starch–Surimi Gels with Non-Setting or Setting Effect* 3.5.1. Texture Profile Analysis

Texture profile analysis simulates chewing twice to obtain the characteristic parameters of the gel matrix (Table 4) [47]. A significant increase in hardness was observed along with the elevated starch content due to reduced moisture content and starch swelling. The starch-absorbed water and swelled during the thermal processing, which increased pressure on the surimi gel [22]. No significant difference in springiness occurred in CG with the addition of starch (*p* > 0.05). However, springiness decreased along with the addition of starch in SCG. It is speculated that if the surimi gel had better gelling properties, then it would play a dominant role in the mixed matrix. Thus, the reduction in springiness in SCG was attributed to decreased surimi concentration. The improvement of cohesiveness and resilience in CG presumably contributed to granule-swelling of starch. The gelation and swelling of starch granules might cause an increase in hydration, which enabled better compatibility of protein networks and starch [14]. SCG showed reduced cohesiveness and resilience with excessive starch, which could be explained by how excessive expansion in starch granules attenuated the stability of the surimi matrix. Thus, potato starch filling could ameliorate the weak gel matrices by increasing texture properties such as hardness and cohesiveness. Nevertheless, the addition of potato starch increased the hardness of the gels but at the same time reduced springiness and cohesiveness. However, compared with a large content of starch, CG still showed lower textural properties, which was connected with the limited filling capacity of natural potato starch and the poor texture of unsetting surimi gel.


**Table 4.** Textural properties of starch–surimi mixtures subjected with different heating processes.

Uppercase letters indicate significant difference (*p* < 0.05) between different heating processes, lowercase letters indicate the difference between gels with different starch content (*p* < 0.05), and values are expressed as mean ± SD.

#### 3.5.2. Whiteness

Whiteness is one of the most essential indications of surimi quality. The variation in whiteness is found to be correlated not only to the conformation of the gel network but also to the color of additives [48,49]. As presented in Table 5, significant increases in whiteness were found with starch contained in both two types of gels (*p* < 0.05). Moreover, two-step heated gels showed higher whiteness in different starch content compared with direct heating gels. The differences in two types of gels might be attributed to a restrictive effect on starch swelling. L\*, a\*, and b\* values decreased as potato starch content increased and presented significant differences between CG and SCG.

**Table 5.** Whiteness of starch–surimi mixtures subjected with different heating processes.


Uppercase letters indicate significant difference (*p* < 0.05) between different heating processes, lowercase letters indicate the difference between gels with different starch content (*p* < 0.05), and values are expressed as mean ± SD. L \*: lightness; a \*: red-green value; b \*: yellow-bule value.

#### 3.5.3. Water Holding Capacity

WHC represents the stability of the mixed matrix suffering external forces, which indirectly reflects the compactness of surimi gels and water absorption capacity of starch [50]. As shown in Figure 6, the gel matrix presented a significant increase in WHC accompanied by adding starch in both heating processes (*p* < 0.05). This was in line with previous research by Mi et al. [17] and Luo et al. [14]. It has been revealed that starch had strong hydrophilicity and showed an effective swelling effect with larger particles, which increases the WHC of the gel matrix. The setting effect promoted the formation of the myofibrillar protein network, which could also reduce water loss caused by an external force, thereby increasing WHC. The matrix treated by direct heating showed better WHC with a continuous increase

in starch content. It was related to the irregular expansion of starch confirmed by the microstructure (Figure 4). In addition, WHC was markedly correlated with the variations in peak area proportion and peak relaxation time [51]. Some possible assumptions could be introduced to describe the changes of WHC: (i) the increase in WHC was associated with the migration of free water by adding starch; (ii) starch showed better swelling states in CG on account of the higher TP2b and lower TP22; and (iii) the setting effect improved the stability of immobile water, whereas it limited starch swelling.

**Figure 6.** Water holding capacity of different water in starch–surimi matrix subjected to direct heating and two-step heating. Caption: see Figure 1.
