Valorization of herring filleting co-products to silage - Control of protein hydrolysis and lipid oxidation during ensilaging and possibilities for separating herring silage into multiple products

Doctoral thesis, 2021

To meet the increased consumer demand for convenience seafood products such as fish fillets, around 70% of all landed fish is industrially processed before final sale. In such processing, around 50% co-products are generated in the form of frames, heads, viscera etc., which essentially have the same nutritional quality as a fish fillet, although a more complex and bony structure. For example, industrial filleting of herring produces around 12,000 tons co-products every year in Sweden, which contain around 12-15% protein and up to 18% omega-3-rich oil, the latter being connected to many documented beneficial health effects. Despite these values, herring co-products have traditionally been considered as low value products that are exported for fish meal and fish oil production in Denmark. The meal and oil process consumes a lot of energy and also requires transport of the co-products to large processing plants.

In this thesis, value-adding of herring co-products was studied using a simpler, milder and less energy demanding process technique called “ensilaging”, where the final product is called “silage”. During ensilaging, fish proteins are cleaved into smaller pieces – i.e. “peptides” or free amino acids – by so called protease enzymes naturally present in the fish. One big advantage of this process technique is that it can be implemented locally on site at the fish processing plant where the co-products are generated, without the need for transport of raw materials or heavy investments. Different from meal/oil-production, the process can be made economically feasible even with small volumes of co-products. Another advantage is that the peptides formed, especially the small ones, may have health-promoting properties in both animals and humans. However, just as herring fillets, herring filleting co-products are highly sensitive to become rancid as the unsaturated lipids quickly oxidize in the presence of blood-derived hemoglobin (Hb). Thus, the raw material must be carefully handled between its generation and the ensilaging and indeed also during subsequent storage/processing of silage prior to its use.

In this thesis, it was studied how parameters important in the ensilaging process, mainly temperature and time, affected cleavage of proteins and unwanted rancidity. The goal was to find optimal process conditions to produce a high-quality silage suitable for both food and feed applications. By monitoring the protein cleavage during the ensilaging, it was possible to produce a silage enriched in small peptides, but with less of free amino acids; the latter which can have adverse effects when the silage is to be used in fish feed. To minimize lipid oxidation during the entire ensilaging process chain, it was studied how this reaction was affected by temperature and time, and also how rosemary-derived antioxidants could be introduced at correct dosage. The produced silage hereby obtained as high quality as has been reported for fish fillets, and thus can be used not only for feed but also as a food ingredient. To further add economic value to the silage, it was also separated into fish oil and a so-called protein hydrolysate (a liquid rich in peptides), both of which can have a high market value.

Overall, the results of this thesis suggest that herring filleting co-products can be valorized on site into silage for both food and feed applications, as well as into high value fish oil and protein hydrolysates. The findings create a basis for design of local “green” herring silage-based biorefineries producing multiple value-added products from a biomass still having a low value.