## 3 Results and Discussion  
### Yield  
The lipid yields of solvent and enzymatically extracted oils from pre-testing with gutted and beheaded Baltic herring and actual study samples with whole fish and by-products are presented in Table 1. The Bligh and Dyer solvent extraction is assumed to result in the recovery of more than 95% of the total lipids present in the raw materials, and therefore the total lipid contents were estimated to be 9.0% and 9.5% for whole fish and by-products, respectively. The fat content of the fish caught in autumn 2018 was similar to the levels reported in the literature [23]. In contrast, the oil content of the fish caught in autumn 2019 was higher than those reported in most previous studies, especially considering the findings of [PERSON] et al. (2018), who reported a decrease in the oil content of Baltic herring from 5-6% to 1.5% from 1987-2014. However, the oil contents were comparable to the oil content of Baltic herring fillets produced from fish caught in autumn [5]. The fish used was also caught in autumn, when Baltic herring has a higher fat content compared to the spring season [23; 24]. The lipid content is also significantly higher than 4.5 g/100 g fw extracted from a catch from October 2020, which was analyzed by the authors (data not published). There seems to be a high variation in fat content of Baltic herring not only between seasons, but also between years. \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & \multicolumn{2}{c}{**Solvent**} & \multicolumn{2}{c}{**Alcalase**} & \multicolumn{2}{c}{**Neutrase**} & \multicolumn{2}{c}{**Protamex**} \\  & **Extraction** & **35 min** & **70 min** & **35 min** & **70 min** & **35 min** & **70 min** \\  & **g** & **g/\%** & **g/\%** & **g/\%** & **g/\%** & **g/\%** & **g/\%** & **g/\%** \\ \hline Pre-test & & & & & & & \\ (whole fish) & 4.89 \(\pm\) 1.28 & 2.24 \(\pm\) 0.56/45.8 & - & 2.11 \(\pm\) 0.25/43.1 & - & 2.47 \(\pm\) 0.71/50.5 & - \\ \hline Whole fish* & 9.00 \(\pm\) 0.46 & 4.09/45.4 & 3.85/42.8 & 5.39/59.9 & 6.25/69.4 & 4.52/50.2 & 6.08/67.6 \\ By-products* & 9.46 \(\pm\) 0.23 & 4.57/48.3 & 5.13/54.2 & 5.17/54.7 & 5.44/57.5 & 4.57/48.3 & 6.14/64.9 \\ \hline \end{tabular}
* In the actual test, the oil yield and recovery were determined after pooling of the triplicates of each enzymatic treatment to take into consideration the differences among different batches and to limit the total number of analyses.  
\end{table} 
Table 1: Oil yield/recovery from Baltic herring whole fish and by-products from solvent (reference) and enzymatic extractions. The oil yields (g/100 g fw) for pre-tests are presented as mean \(\pm\) standard deviation and oil recovery as percentage of the solvent-extracted oil. Actual test samples are presented as g/100 g fw and percentage of the solvent-extracted oil. The oil recoveries achieved by enzymatic extractions ranged from 3.9 to 6.3 g/100 g and 4.6 to 6.1 g/100 g for whole fish and by-products, respectively. The corresponding recovery percentages ranged from 43-69% for whole fish and 48-65% for by-products. Based on our results (Table 1), a longer extraction time gives higher results with Neutrase and Protamex, but this is not so evident for Alcalase. The best results were achieved with Neutrase using an extraction time of 70 min for whole fish, and Protamex with an extraction time of 70 min for by-products. The oil recoveries were similar to the results from the pre-test, except for Neutrase with 35 min extraction, which gave significantly lower results in the pre-test than in the actual extraction trial. This may have been related to the differences in total lipid content between the two batches of fish used, although this effect was not seen in the extractions using other enzymes. According to previous research, oil recovery from enzymatic hydrolysis has been reported as 72% from salmon by-products using 5% Alcalase [25], 51% from Catfish using 0.91% Alcalase [26], 76% from head, and 76% from whole mackerel using 2% Alcalase [27].