# Study of Physicochemical and Gelatin Properties of Fish Gelatin from Different Sources
## 3 Results and Discussion
### Physical and Chemical Properties of Fish Gelatin  
The gelation time, temperature and melting point of gelatin are shown in Figure 2 and Table 3. We observed that the gelation time and melting temperature of cold-water fish gelatin were lower than those of warm-water fish gelatin. Among them, M08 was able to gelate at 4-12 \({}^{\circ}\)C, whereas warm water fish gelatin M04 gelated at 4-23 \({}^{\circ}\)C. Although warm-water fish gelatin exhibited a broader range of gelation temperature than cold-water fish gelatin, M06 and M08 were able to gelate at 4 \({}^{\circ}\)C in 10 min, whereas M03 and M04 required 30 min to gelate at 4 \({}^{\circ}\)C (Figure 2). We specifically found that at the same temperature and time, the gelation of M08 was better than that of M06; therefore, we selected M08 as the optimal gelatin for low-temperature applications. It has been shown that there is a correlation between the content of proline and hydroxyproline in gelatin and the gel melting temperature (Table 3), and this is also pointed out by [PERSON] and [PERSON]'s study [26; 27]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Amino Acid Content** & & & & \\
**(ug/mgCNS)** & **M03** & **M04** & **M06** & **M08** \\ \hline Asp & 54.1922 & 58.8436 & 68.8884 & 73.336 \\ Thr & 26.355 & 24.9026 & 29.016 & 29.9064 \\ Ser & 33.177 & 36.7824 & 66.3272 & 71.7264 \\ Glu & 95.0596 & 103.5844 & 107.4772 & 112.034 \\ Ala & 99.2322 & 103.2566 & 95.828 & 100.0978 \\ Val & 22.0882 & 21.1086 & 20.2046 & 21.1596 \\ Met & 9.5148 & 9.386 & 14.965 & 18.7756 \\ Ile & 12.238 & 11.8736 & 12.9586 & 14.2264 \\ Leu & 26.4908 & 26.1532 & 25.0706 & 27.1428 \\ Tyr & 3.1944 & 3.4752 & 3.9124 & 5.4574 \\ PHe & 17.5284 & 20.2732 & 19.3872 & 19.7182 \\ Lys & 31.5506 & 32.6718 & 34.8594 & 37.0076 \\ His & 5.9378 & 6.2598 & 11.2676 & 12.8436 \\ Arg & 80.0758 & 82.4078 & 84.784 & 91.671 \\ Pro & 145.2082 & 156.3152 & 131.2282 & 140.0648 \\ Total & 661.843 & 697.294 & 726.1744 & 775.1676 \\ Percentage of hydrophobic groups & 35.22\% & 35.15\% & 30.82\% & 31.10\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Amino acid content of four types of fish gelatin. We then characterized the structure of self-assembled and enzymatic gels by scanning electron microscopy (Figure 3). We observed that the surface particles of M03 and M04 gelatins were more tightly arranged in the formed gels, whereas the particles of M06 gelatin were aggregated with fewer adherent particles on the surface of gels. We further detected that among the four fish gelatin sources, the microstructure of M08-derived gels was characterized by a higher number of aggregated particles with nearly non-adherent small particles on the surface of gels compared with those formed by the other three gelatins. This was positively correlated with gel strength. In addition, the enzymatic gel was crosslinked using the TG enzyme, which generated covalent bonds and enhanced the network structure. We observed that the surface of the M08 enzymatic gel was non-porous and flat, and exhibiting the highest gel strength, indicating that the microstructure determined the gel strength. These results are consistent with the previous literature comparing the microstructure of gelatin in cold water fish and mammals. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Gelation** & \multicolumn{2}{c}{**Temperature**\({}^{\circ}\)**C**} & **Melting**\({}^{\circ}\)**C** \\ \hline \multirow{3}{*}{Warm water fish} & M03 & 4–20 & 20–37 \\  & M03+TG & - & >60 \\  & M04 & 4–23 & 25–37 \\  & M04+TG & & >60 \\ \hline \multirow{3}{*}{Cold water fish gelatin} & M06 & 4–10 & <17 \\  & M06+TG & - & >60 \\ \cline{1-1}  & M08 & 4–12 & <17 \\ \cline{1-1}  & M08+TG & - & >60 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Gelation temperature and gel melting point of gelatin. Figure 2: (**A**) 4 \({}^{\circ}\)C 10 min self-gel (**B**) 4 \({}^{\circ}\)C 30 min self-gel.