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

Abstract: 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.