Cross-Processing Fish Co-Products with Plant Food Side Streams or Seaweeds Using the pH-Shift Method - a new sustainable route to functional food protein ingredients stable towards lipid oxidation

Sammanfattning: The seafood value chain is highly inefficient as 50-60% of the fish weight end up as co-products in the filleting operation. Despite their abundance in high-quality proteins, fish co-products mainly go to low value products such as fodder. The pH-shift process, i.e. , acid/alkaline solubilization followed by isoelectric precipitation, is an opportunity to instead recover these proteins in a food grade manner while maintaining their functionality. A challenge when subjecting hemoglobin-rich fish raw materials to pH-shift processing is however oxidation of polyunsaturated fatty acids (PUFAs). This thesis investigated, for the first time, cross-processing of fish co-products with antioxidant-containing support materials (''helpers'') to protect the fish protein isolates from lipid oxidation in a clean label and sustainable manner. The helpers, including locally sourced plant food side streams (press cakes from lingonberry (LPC) and apple, barley spent grain, oat fiber residues), shrimp shells, and seaweeds, were also expected to introduce new characteristics to the protein isolates. All helpers, except shrimp shells, reduced lipid oxidation in herring/salmon co-products when added at 30% (dw/dw) at start of the pH-shift process. LPC was the most effective, and even at 2.5% addition it prevented volatile aldehyde formation during production of herring protein isolates while at 10% addition, the isolates were also stable towards oxidation for ≥8 days on ice. When the 10% LPC instead was added during protein precipitation, the oxidation lag phase was extended to 21 days. The oxidative stability of protein isolates correlated with their total phenolic content, and the very high antioxidant ability of LPC's was mainly attributed to anthocyanins, e.g. , ideain and procyanidin A1. LPC also improved the water solubility, emulsifying activity, and gel-forming capabilities of herring protein isolates, expanding their potential applications in food products. The water solubility and emulsifying activity were also boosted by adding shrimp shells and Ulva , while the gel-forming ability was also enhanced by apple press cake. LPC-derived anthocyanins resulted in red isolates under acidic conditions and dark-colored isolates under neutral/alkaline conditions. Ulva resulted in green isolates due to the presence of chlorophyll. The color of protein isolates was also affected by oxidation of fish-derived pigments like Hb and astaxanthin. The addition of helpers also influenced the composition of protein isolates. LPC added at the start of the process reduced lipid content, while shrimp shells and LPC added during precipitation increased it. Seaweeds raised ash content by introducing minerals. Additionally, the organic acids of LPC saved the use of HCl in acid-aided protein solubilization and in isoelectric precipitation of alkali-solubilized proteins. During the latter, adding 30% LPC decreased HCl usage by as much as 61%. Opposite, alkaline protein solubilization in presence of LPC required more NaOH than the control, but this issue was naturally less pronounced at low LPC additions. Another challenge of introducing helpers was that they reduced total protein yield in the pH-shift process. This was however successfully mitigated by optimizing solubilization/precipitation pH, increasing water addition, and employing more powerful high shear homogenization and ultrasound techniques. In summary, this thesis introduced a completely new concept of cross-processing fish co-products with antioxidant-containing food materials, significantly reducing lipid oxidation and enhancing protein isolate techno-functionalities. Herring co-products paired with 10% LPC was particularly promising. Beyond its technical advantages, cross-processing can add economic value to side streams of both fish and other food industries, while stimulating circularity and industrial symbiosis. Altogether, these features reduce food chain losses and promote a more sustainable food system.