Contents lists available at ScienceDirect LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt Physicochemical and gel-forming properties of protein isolated from salmon, cod and herring by-products using the pH-shift method Mehdi Abdollahi * , Ingrid Undeland Department of Biology and Biological Engineering–Food and Nutrition Science, Chalmers University of Technology, SE 412 96, Gothenburg, Sweden ARTICLE INFO Keywords: Marine by-products Fish protein isolate pH-shift method Solubilization/isoelectric precipitation ABSTRACT The impacts of variation in fish filleting by-products origin including white muscle (cod), dark muscle (herring) and farmed fish (salmon) on physicochemical and gel-forming properties of protein recovered using the pH-shift method were studied. The effects of different solubilization pHs (acid or alkaline) on protein yield/composition, and its properties were also studied. Alkaline version (pH 11.5–12.5) resulted in maximum protein yield for the three resources which ranked them as: salmon > herring > cod. Increasing solubilization pH from 11.5 to 12.5 increased protein yield in salmon and herring, while maximum protein yield of cod was obtained at pH 12. However, increasing solubilization pH from 11.5 to 12.5 required a two-fold higher amount of alkali compared to the adjustment to 11.5. All recovered proteins had gel-forming capacity; however, cod gels showed higher WHC and breaking force compared with salmon and herring protein gels. Increasing solubilization pH from 11.5 to 12.5 negatively affected the breaking force and color of the salmon and cod protein gels, but improved heme pigment removal, breaking force and whiteness of herring protein gels. The pH-shift process thus showed good potential for recovering high quality protein from the by-products, but protein solubilization pH should be carefully selected based on the target species. 1. Introduction The fish processing industry produces large amounts of side streams which normally form more than 50% of the original fish weight. Globally, these by-products are already mainly used in animal feed production or are even wasted (Aspevik, Totland, Lea, & Oterhals, 2016). Increasing demands for seafood products due to increasing world population and awareness about the health effects of seafood in parallel with wild fish supply decline have caused great interest in using fish processing by-products in food production. Some of these by-pro- ducts, such as head, backbone, tail and trimming, are a good source of high value food-grade products like fish protein, oil and minerals. However, the complex bony nature of these materials and their high heme protein and lipid content have barricaded their successful use in e.g. protein ingredient production (Abdollahi, Marmon, Chaijan, & Undeland, 2016; Aspevik et al., 2016). These challenges have led to continuous research to find methods that can recover proteins from these complex structure, while retaining their functionality. An alternative to classic meat-bone separation using a belt and drum or conventional washing method, the so-called acid-alkaline solubili- zation/isoelectric precipitation or pH-shift method patented by Hultin et al. (2001), has shown high potential for protein isolation from non- conventional complex marine resources. In this process, muscle is so- lubilized in water at low or high pH, < 3.5 or > 11, respectively, which drives the muscle proteins apart from each other. High and low density undissolved material easily be removed, and proteins can be subse- quently de-watered by adjusting the pH to a value where they have minimal solubility (pH∼5.5). Previous studies have shown that when pH-shift process is used for fish processing by-products, the protein yield of the process and functionality of the recovered protein will be dependent on the quality of the initial raw material, process version (acid or alkaline) and solubilization/precipitation pHs (Abdollahi & Undeland, 2018; Chen & Jaczynski, 2007a; Chomnawang & Yongsawatdigul, 2013; Panpipat & Chaijan, 2017). However, there is no comprehensive report that side by side evaluates the effect of by- products variations caused by fish origin, including salmon (Salmo salar), cod (Gadus morhua) and herring (Clupea harengus) representing dark and white muscle fish as well as fatty fish and lean fish on the yield and quality of the protein isolated using the pH-shift method. A handful of studies have also reported that increasing solubiliza- tion pH may increase the protein yield of the pH-shift process. Beyond that, some studies (Chen & Jaczynski, 2007; Taskaya et al., 2009b) have shown that the effect of very high solubilization pHs on the functional properties of the final protein isolate may be species specific and would https://doi.org/10.1016/j.lwt.2018.11.087 Received 2 March 2018; Received in revised form 24 November 2018; Accepted 28 November 2018 * Corresponding author. E-mail address: khozaghi@chalmers.se (M. Abdollahi). LWT - Food Science and Technology 101 (2019) 678–684 Available online 29 November 2018 0023-6438/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/). T