## 3 Results and Discussion
### Concentration of Protein Hydrolysis Products (PHP)  
The process of enzymatic ripening of herring meat leads to the formation of TCA-soluble proteins, peptides and amino acids, the quantitative composition of which allows assessment of the dynamics of the ripening process of marinades [3; 36]. The average NPN content of fillets marinated for 2-14 days was significantly lower by 66-91% than in marinades from whole or headed herring (Figure 7A). NPN content in fillets after 2 days decreased to 156 and 117 mg in the German and Danish methods, respectively. The reason for the decrease in NPN content in the German method was the diffusion of NPN from the meat into the marinating brine [30], while in the Danish method, there was an additional increase in moisture during the salting stage. As a result, fillets marinated for 2 days contained 16-18% less NPN than the raw material. This phenomenon did not occur in whole or headed herring, where diffusion was limited by the skin, causing these marinades to contain 344-356 mg of NPN after 2 weeks, 60-65% more than the raw material (Figure 7A). The higher NPN content was also due to higher proteases activity in marinades form whole and headed herring. Figure 7: Effect of marinating method (G—German, D—Denmark) and degree of primary processing of Baltic herring (whole, headed, fillet) on concentration of (**A**) NPN, (**B**) PHP(R) and (**C**) PHP(A) during marinating. \({}^{\text{a,b,c}}\) Results marked with the same lower case letter do not statistically differ by the influence of marinating time; \({}^{\text{A,B,C}}\) Results marked with the same capital letter do not statistically differ by the influence of the marinating method. The peptide fraction content of G-fillets marinades significantly increased only up to the fourth day, reaching 92-103 mg (Figure 7B), which is characteristic for the German method [32]. On the other hand, during marinating of whole and headed herring, the peptide content increased up to the tenth day, reaching 232-234 mg PHP(R). In the Danish method, the peptide content decreased by a few mg after 2 days due to an increase in the moisture of the salted meat, after which it sharply increased to 7 days in fillets and headed herring or to 14 days in whole herring. After 7-10 days, marinades from fillets and headed herring obtained by the Danish method contained more peptides compared to the German method (Figure 7B). Peptide content was not significantly different after 7-10 days for whole herring marinated using both methods. This shows that the inhibition of cathepsin D by salt, and the lower dynamics of mainly peptide formation in the Danish method than in the German method, were balanced by high digestive protease activity. Additionally, the content of the amino acid fraction changed during marinating, similar to the content of NPN and peptides. After 7 days of marinating, G-fillets marinades contained the least PHP(A) (55.8 mg), followed by D-fillets (73.7 mg), G-whole (81.6 mg), G-headed (90.3 mg), D-whole (96.9 mg), and the most, D-headed (106.1 mg) (Figure 7C). For the amino acid fraction, the increase in content was characteristic up to 7-10 days in fillets, while up to 14 days in marinades from ungutted herrings. Statistical analysis showed that NPN, peptides and amino acid content mainly correlated in G-fillets marinades with cathepsin D (0.637-0.931) and Am-Tp (0.633) activity, in G-headed marinades with the addition of chymotrypsin activity (0.610-0.705) and in G-whole marinades with the addition of Es-Tp and Cp-A activity (0.580-0.946) (Figure 3A,C,E). For the Danish method, PHP content mainly correlated in D-fillets marinades with the addition of cathepsin D activity (0.565-0.746), in D-headed marinades with the addition of cathepsin L (0.704-0.935), and in D-whole marinades with Es-Tp (0.725-0.815) (Figure 3B,D,F). Thus, the lower degree of preliminary processing of herring promoted more significant correlations of digestive proteases with PHP and with higher cathepsin D activity.