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

Industrial processing of herring (Clupea harengus) into convenience products such as fillets generates around 60% co-products being rich in both protein and n-3 polyunsaturated fatty acids (PUFAs). A promising cost-efficient strategy to valorize these raw materials into food and/or feed ingredients would be to apply ensilaging, i.e., proteolysis mediated by endogenous proteases under acidic conditions. Although an ancient technique, very little is still known about factors affecting the protein degree of hydrolysis (DH) and lipid oxidation during ensilaging. In this work, the effect of e.g. temperature and time on DH and lipid oxidation during ensilaging of herring co-products was investigated with the aim to maximize the DH while keeping the formation of unwanted free amino acids (FAA) and lipid oxidation to a minimum. Also, the role of hemoglobin (Hb) in ensilaging-induced lipid oxidation was studied, along with different ways of introducing antioxidants to the silage. Finally, possibilities to separate pilot scale-produced herring silage into oil and hydrolysates were investigated.

Ensilaging for 1-7 days between 7-47°C revealed that the highest DH was noticed at 32°C, but the DH increased over time at all studied temperatures, which also applied to FAA. At ambient temperature (i.e. 22°C), being the main focus in this thesis, DH and FAA reached 60% and 14%, respectively, at day 7. Heat-treating the silage for 30 min at 85°C prior to storage (0-6 months; 4°C or 22°C) stopped hydrolysis, and is thus a route to minimize FAA formation. Lipid oxidation proceeded during ensilaging at all temperatures, however, ≥ 22°C, the secondary oxidation product marker malondialdehyde (MDA) underwent hydrolytic cleavage or interacted with proteins/peptides/amino acids, preventing its accumulation. Instead, non-enzymatic browning reaction products e.g. 2-ethylfuran and 2-pentylfuran developed, along with saturated aldehydes as pentanal and hexanal. Upon adjusting the pH from physiological to 3.5 (i.e. ensilaging pH), it was documented that trout oxyHb changed to metHb, facilitating heme group release, suggesting heme-mediated peroxide cleavage was a dominating mechanism behind the ensilaging-induced lipid oxidation. To minimize lipid oxidation, two different strategies were evaluated; (i) pre-incubating the co-products in antioxidant solutions, or (ii), direct addition of 0.25-1.25% antioxidants at the start of ensilaging. Both strategies were effective, and among all the antioxidants studied, the commercial rosemary extract-based antioxidant Duralox MANC-213 provided best protection against lipid oxidation during ensilaging, heat-treatment and storage of silage. The total volatile basic nitrogen (TVB-N) level in silages remained below the acceptable limit of 30 mg TVB-N/100 g fish for human consumption, suggesting that silage can be used in both food and feed. Scaling up the ensilaging to a 1500-liter batch size, ~38% DH was recorded after 2 days at ~22°C, which was similar to the DH found in lab-scale (40% after 2 days; 22°C), suggesting lab-scale data well simulated what happens in pilot scale. Subjecting the silage to centrifugation (3000-8500 x g; 2-20 min) showed that it can be successfully separated into fish oil and protein hydrolysates.

To summarize, these studies provide valuable information on optimal process parameters to use in on site valorization of herring co-products into a high-quality peptide-rich silage, and/or fish oil and hydrolysates, paving the way for silage based-biorefining of herring side streams into multiple products.


fish oil


protein hydrolysis

lipid oxidation



Herring (Clupea harengus)




Lecture hall EE, Hörsalavägen 11, Campus Johanneberg, Chalmers, Gothenburg
Opponent: Professor (emeritus) Ragnar L. Olsen, UiT The Arctic University of Norway, Norway


Mursalin Sajib

Chalmers, Biology and Biological Engineering, Food and Nutrition Science

Sajib, M., Wu, H., Fristedt, R. and Undeland, I. Hemoglobin-mediated lipid oxidation of herring filleting co-products during ensilaging and its inhibition by pre-incubation in antioxidant solutions.

Sajib, M., Langeland, M. and Undeland, I. Effect of antioxidants on lipid oxidation in herring (Clupea harengus) co-product silage during its production, heat-treatment and storage.

Sajib, M., Trigo, J.P., Abdollahi, M. and Undeland, I. Pilot-scale ensilaging of herring filleting co-products and subsequent separation of fish oil and protein hydrolysates.

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.

Engineering Nutritious Seafood by-products to Improve Local Aquaculture Growth and Environment (ENSILAGE)

Formas (2016-20057), 2016-01-01 -- 2020-12-31.

Driving Forces

Sustainable development

Innovation and entrepreneurship


Chalmers Infrastructure for Mass spectrometry

Subject Categories

Food Science

Biological Sciences


Basic sciences

Areas of Advance

Health Engineering

Life Science Engineering (2010-2018)



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5018



Lecture hall EE, Hörsalavägen 11, Campus Johanneberg, Chalmers, Gothenburg

Opponent: Professor (emeritus) Ragnar L. Olsen, UiT The Arctic University of Norway, Norway

More information

Latest update