Proteolytic Activity in different Rest Raw Material fractions from Herring.

The growing global demand for sustainable protein sources and increasing fish processing waste have intensified interest in the valorisation of rest raw materials (RRMs). This thesis explores the proteolytic activity potential from various herring (Clupea harengus) by-product fractions—backbone, belly flap, skin, viscera, and mixed tissues—as a function of temperature and at different dilution rates. The study aimed to find at what temperature the proteolytic activity was highest and evaluate the biochemical and functional properties of the extracted proteins from different fractions. Protease activity was performed at temperatures ranging from 40°C to 70°C and dilution ratios of 10x and 20x. The Lowry assay was used to quantify protein content. Among all fractions, belly flap demonstrated the highest proteolytic activity measured as cut in % of wet weight (1.06%) and concentration (267.34 mg/g) under optimal conditions (60°C, 20x dilution), followed by backbone (0.75%, 251.30 mg/g). Skin and viscera showed lower activity but improved with higher dilution and moderate temperatures, while the mixed fraction presented consistent, moderate values. Complementary biochemical data from another master thesis revealed variation in protein, fat, ash, and water content across fractions. Belly flap, though lower in protein percentage (12.2%), yielded high amount of water-soluble proteins and high proteolytic activity due to its high muscle content and moderate moisture. Viscera had higher protein content (20.3%) but were less efficient due to enzymatic degradation and structural complexity. Backbone and skin presented challenges due to collagen and bone content but still showed promise with optimized extraction. The results underscore the feasibility of utilizing by-products of herring for the recovery of high-quality protein, supporting a circular economy. Through customized process planning, companies can achieve protein yields in accordance with sustainable resource use and the principles of responsible sourcing in the seafood industry, and apply those yields toward food, feed, or functional ingredients.