# Study of Physicochemical and Gelatin Properties of Fish Gelatin from Different Sources
## 3 Results and Discussion
### Characteristics of Gels Derived from the 4 Fish Gelatins  
Gel strength is one of the three most important properties for characterizing the gelatin source and determining its applications. Previous studies have reported gel strengths of 70-270 g/cm and 100-300 g/cm for fish and mammalian gelatins, respectively. Importantly, low gel strength affects the application of gels [28]. We found that the gel strength of M08 was the highest among self-assembled gels (253 \(\pm\) 1 g/cm), whereas that of M04 was the lowest (35 \(\pm\) 1 g/cm) (Table 4). Moreover, we noticed that the covalent bonds generated by crosslinking with the TG enzyme enhanced the gel strength of fish gelatin, resulting in the enzymatic M08 gel exhibiting the highest gel strength of 800 \(\pm\) 2 g/cm, whereas, even the lowest gel strength of the enzymatic M04 gel was increased to 204 \(\pm\) 6 g/cm (Table 5). Compared with the study of [PERSON] et al. (2001), the gel strength of M08 crosslinked TG enzyme of cold-water cod was increased by 480-500 g/cm, 2.3-2.6 times This indicated that the gel strength of self-assembled and enzymatic gels was determined by physical and chemical factors, such as the molecular weight of fish gelatin itself. Our results show that the gel strength of fish gelatin mainly depends on the physicochemical properties between the source of the fish and the fish itself: Gelatin itself has self-relatability, but because of the different sources of gelatin, the performance of the gel is not the same, also determines the gelatinization temperature and melting temperature of gelatin, limiting the application of gelatin at different temperatures. Therefore, we used the TG enzyme to catalyze cross-linking to improve the properties of fish gelatin, improve the gel strength and improve the properties of fish gelatin. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Gelation** & **Uncrosslinked Gel Strength** & **Gel Strength after** \\  & **(g/cm)** & **Crosslinked (g/cm)** \\ \hline M03 & 67 \(\pm\) 1 c & 351 \(\pm\) 2 c \\ M04 & 35 \(\pm\) 1 d & 204 \(\pm\) 6 d \\ M06 & 156 \(\pm\) 2 b & 476 \(\pm\) 6 b \\ M08 & 253 \(\pm\) 1 a & 800 \(\pm\) 2 a \\ \hline \hline \end{tabular} \(p\)-values \(<\) 0.05 were considered statistically significant. The result of abcd’s significant difference analysis on the data is obtained by SPSS software. As long as there are the same letters, there is no significant difference. The data has significant difference marked from a to d.  
\end{table} 
Table 4: Crosslinked gel strength. Figure 3: Scanning electron microscopy of enzymatic gels and autogels (all magnifications are 10.00 KX) (**A**) M03; (**B**) M04; (**C**) M06; (**D**) M08; (**E**) M03+TG; (**F**) M04+TG; (**G**) M06+TG; (**H**) M08+TG. Water- and oil-holding are important functional properties of gelatin and are closely related to its structure through the interaction of water and oil components [29]. We observed that M04 gelatin had the highest water holding capacity among the four gelatins (245 \(\pm\) 15%), whereas M06 exhibited the lowest capacity (6 \(\pm\) 11%), which was proportional to the subamino acid content of gelatin. We found that the oil holding capacity was highest in M08, reaching a maximum of 253 \(\pm\) 3%, whereas M04 exhibited the lowest capacity at 122 \(\pm\) 4%, which was proportional to the molecular weight content of gelatin. We determined that crosslinking by the TG enzyme improved the water and oil holding properties of gelatin, resulting in the enzymatic M08 gel exhibiting the lowest water holding capacity (327 \(\pm\) 12%). TG enzymes can form cross-linking bonds between protein molecules, which can improve the water and oil retention of gelatin. It can be seen from the table that the water and oil retention of gelatin is both improved after the cross-linking of fish gelatin by TG enzymes. Studies have shown that the water holding capacity of commercial fish gelatin on the market is 5.1 \(\pm\) 1 and the oil holding capacity is 3.45 \(\pm\) 0.06.