Abstract
The global population is projected to grow by 2 billion by 2050, increasing pressure on food and feed resources. In Norway, the whitefish sector and the deep-sea fleet represent a significant opportunity to enhance the utilization of residual raw materials. In 2022, 70 % of the residuals generated onboard deep-sea vessels were discarded at sea. These raw materials are rich in valuable nutrients, including health-promoting omega-3 fatty acids and proteins, which can be used in food or feed applications. One of the key challenges in utilizing these raw materials is the rapid degradation of viscera and liver, which are highly susceptible to enzymatic and microbial breakdown, as well as lipid oxidation. This highlights the importance of developing optimal strategies for handling, storing, preserving and processing these raw materials onboard. Thermal treatment can be used to produce oil from the whole viscera or from out-sorted livers onboard. However, little is known about how sorting and raw material freshness affect the composition and quality of the oil. Silaging can be used to preserve either the whole viscera or viscera without livers, if livers are sorted out for separate oil production. The technology is cost-effective, and the low pH inhibits the growth of spoilage bacteria while creating favorable conditions for proteolytic enzymes to break down proteins, resulting in a liquid product that can be separated into oil and protein hydrolysate for feed applications. However, silage is often regarded as a low-value product with poor quality, highlighting the need for quality optimization. This study investigates the impact of raw material freshness and sorting on the quality of oil produced by thermal treatment. The findings show that high-quality oil, rich in omega-3 fatty acids, can be produced from both viscera with liver and out-sorted livers. Processing shortly after catch led to a generally higher oil quality, but both viscera and liver could be stored for up to 2 days at 4 °C and still meet quality guidelines for human consumption. It was also demonstrated that significantly higher oil yields could be obtained if the raw materials were stored for 1 day or more. However, after 2 days of storage, an undesired emulsion layer formed during processing of the whole viscera. Additionally, the research examines how raw material sorting, freshness, and the use of antioxidants affect the yield, composition and quality of the oil and protein hydrolysates obtained from silaging. The study demonstrates that preserving the raw material shortly after catch and using antioxidants significantly reduce the formation of lipid oxidation products, thereby improving oil quality. The findings also show that viscera can be stored for 1 day before silaging without compromising protein quality, though immediate processing yields hydrolysates with a higher protein content. Out-sorting the liver reduces the risk of emulsion formation but was not required to produce protein-rich hydrolysates. Overall, this work demonstrates that the viscera and liver of whitefish can be valorized into ingredients that can either directly, or indirectly, enter the food value chain. Valorizing these currently wasted residuals can significantly contribute to sustainability by improving resource efficiency, enhancing human nutrition and reducing pressure on fishery stocks.