Jingnan Zhang, Food & Nutrition Science

Opponent: Brijesh K. Tiwari, Principal Research Officer at Teagasc and adjunct professor at University College Dublin, Ireland

Supervisor: Professor Ingrid Undeland, Chalmers
Examiner: Professor Rikard Landberg, Chalmers

Abstract:

Seafood is very rich in omega-3 fatty acids, high-quality proteins, vitamins, and minerals and is therefore a vital component of our diets and health. Globally, fish consumption is rising, which is a challenge to accomplish given that >35% of the wild fish stocks are overfished and 57.3% are maximally used; facts that also hamper the growth of aquaculture. At the same time, there is an overwhelming focus on only eating fish fillets, i.e., just 40-50% of the fish weight. Frames, heads, skin, fins and guts are hereby dedicated to fodder meal and oil instead of feeding people, despite being very high in all the mentioned nutrients. Taking Sweden as an example, as much as 85% (!) of landed fish is allocated for feed, including both "fodder fish" and fish filleting co-products. This scenario calls for a change.
A promising tool for producing food protein ingredients from fish co-products is the pH-shift method. Here, fish proteins are solubilized in water under acid or alkaline conditions, allowing them to be separated from for example bones and skin. The proteins can then be made “solid” again and recovered from the water by changing the pH. A challenge in the process which needs to be overcome is that the fish lipids easily oxidize, i.e., become rancid. Hereby, unpleasant odor, reduced nutritional value, and impaired dry mouthfeel may develop.
This thesis investigates if cross-processing of herring and salmon co-products with various natural antioxidant-rich raw materials, such as plant-food side streams, shrimp shells or seaweed, can prevent rancidity during the pH-shift process. Results showed that some of the antioxidant raw materials, such as lingonberry juice press cakes (LPC), effectively limited rancidity during both the production and ice storage of fish protein ingredients. In fact, protein isolates made with LPC were stable for up to 21 days. Our research points to that molecules called “anthocyanins” are the secret behind this strong antioxidant effect.
In addition to their antioxidant ability, raw materials such as LPC, shrimp shells, and sea lettuce also made the protein ingredients more water-soluble and easier to emulsify with oil. These properties are crucial in food applications. LPC and apple press cake also helped the protein ingredients to form a “gel” which is important for example in burgers and fish balls. Moreover, the addition of antioxidant-rich material gave new colors to the protein ingredients which could be unusual to the consumer, but also open up new product possibilities. Lingonberry anthocyanins are for example red at low pH but bluish/purple at neutral/alkaline pH. Sea lettuce on the other hand produced green protein ingredients due to chlorophyll.
In conclusion, the cross-processing concept developed in this thesis is highly promising for the production of new food protein ingredients from fish filleting co-products, taking advantage of antioxidant-rich seaweeds and side streams from other food industries. Beyond its technical advantages, this breakthrough can stimulate symbiosis between food companies and a more circular food chain with lower losses and higher sustainability. Thus, the sea can now serve more dishes of food without increasing the fish catches, and at the same time, losses from land-based food production can be minimized.