# Study of Physicochemical and Gelatin Properties of Fish Gelatin from Different Sources
## 3 Results and Discussion  
### Analysis of Protein Content and Composition of Different Fish Gelatins  
According to the national food safety standard (GB6783-2013), the protein content index of food additive gelatin is \(\geq\)82.0%. We found that the total protein content of the M03, M04, M06 and M08 fish gelatins was 90.9 \(\pm\) 0.3c%, 95.4 \(\pm\) 0.5b%, 94.7 \(\pm\) 0.3b% and 99.9 \(\pm\) 0.1a% (Table 1), respectively, as measured using the Kjeldahl nitrogen determination method. These results are consistent with the previous literature that compares the protein content composition of cold-water fish and mammalian gelatin ([PERSON] et al.) [22] reported that the collagen contents in the fish skin waste of Japanese sea bass, chub mackerel and bullhead shark were 51.4%, 49.8% and 50.1% (dry basis), respectively. The protein content of the raw material in this experiment is excellent. Although the protein content of all four gelatins met the national standard, we observed large differences in the protein content between different gelatins, which was attributed to the source of gelatins and processing technology. Gelatin is generally composed of \(\alpha\) chain, \(\beta\) chain and \(\gamma\) chain three kinds of polypeptide chains and between the components of the molecular chain fragments, the \(\beta\) chain is composed of two \(\alpha\) chains connected, \(\gamma\) chain is composed of three \(\alpha\) chains and the distribution of these chains will affect the reliability of gelatin. According to the study of ([PERSON] et al.) [23], it is believed that the gel strength of fish gelatin is related to the distribution of molecular mass, namely the number of \(\alpha\)-chains and \(\beta\)-chains. The more \(\alpha\)-chains, the more hydrogen bonds of components and the more triple helix structure, resulting in a denser microstructure and greater gel strength. We observed some differences in the molecular weight between the four fish gelatin samples (Figure 1). Fish gelatins with high protein content have a wider distribution of proteins with different molecular weights [24]. In addition to the protein content, the physicochemical properties of gelatin are also influenced by protein composition. These results are consistent with the previous literature that compares the protein content composition of cold-water fish and mammalian gelatin. Fish gelatin mainly contains \(\alpha\)1 and \(\alpha\)2 peptide chains with molecular weights of approximately 130 and 110 kDa, \(\beta\) peptide chains with molecular weights higher than 200 kDa and a small number of high and low molecular weight components below 110 kDa. Of note, as \(\beta\) chains promote gel formation, the gel strength of fish gelatin has been correlated with the molecular weight distribution and presence of \(\alpha\) and \(\beta\) chains, with higher \(\alpha\) chain content leading to greater gel strength. We determined that the four fish gelatins had different distributions of \(\alpha\)1 and \(\alpha\)2 peptide chains of 130 and 110 kDa. In particular, we found that the \(\beta\)-chain content of the two gelatins from cold-water fish, M06 and M08, was higher than that of warm-water fish gelatins, M03 and M04. The high total protein content of gelatin and its wide molecular weight distribution (M08) indicated a higher gel strength. The physicochemical properties of gelatin have also been closely associated with its amino acid composition. Interestingly, the proline and hydroxyproline subamino acids \begin{table}
\begin{tabular}{c c} \hline
**[Number]** & **Protein Content (\%Protein)** \\ \hline M03 & 90.9 \(\pm\) 0.3 c \\ M04 & 95.4 \(\pm\) 0.5 b \\ M06 & 94.7 \(\pm\) 0.3 b \\ M08 & 99.9 \(\pm\) 0.1 a \\ \hline \end{tabular}
\end{table} 
Table 1: Gelatin protein content. Figure 1: Molecular weights of four types of fish gelatin. have been closely correlated to fish gelatin gel strength.