Consumption of fish is rapidly increasing worldwide. It is important to evaluate fish fillet quality because fish undergoes physical and chemical changes during frozen storage. Fish fillets exhibit formaldehyde (FA) accumulation from the decomposition of trimethylamine N-oxide. FA is a powerful protein denaturant; thus, it is important to avoid FA buildup during fish processing to preserve fish quality, especially texture. To determine where FA accumulates, in order to maintain the quality of fish fillets, we performed matrix-assisted laser desorption/ionization mass spectrometry imaging, aiming to identify muscle-derived peptides, which reflect conditions such as denaturation and/or aggregation. We used frozen sections from which lipophilic molecules were washed out and detected various peptide peaks. Furthermore, we tried to identify indices to represent fish fillet softening by protease treatment. We could detect characteristic peaks owing to FA and protease treatment; the findings were consistent with the results of texture profiles showing fish fillet’s real solidity. These molecules might thus serve as effective markers to evaluate fish fillet quality.

Keywords: Alaska pollock, surimi wash-water, protein recovery, pH shift,heat treatment IntroductionSurimi product is a traditional food in Japanese cuisine (Park, 2013). The texture of surimi product is stabilized by myofibrillar protein networks of myosin and actin (Stone and Stanley, 1992), though organic impurities such as sarcoplasmic proteins, lipids, and blood components decrease the strength and whiteness of protein gels. Accordingly, conventional surimi technology involves repeated washing of minced fish muscle to remove undesirable organic compounds. The wastewater thus generated contains a high concentration of organic fish solids (Karayannakidis, et al., 2007), which have a marked impact on the environment due to their high chemical and biochemical oxygen demand (Islam, et al., 2004). Therefore, a demand exists for the development of a recovery method from surimi wash-water (SWW) to increase the yield of fish protein and to reduce the negative environmental impact (Park, et al., 2001).Various SWW protein recovery methods have been developed for industrial applications, such as ultrafiltration (Lin, Park, and Morrissey, 1995;Montero and Gómez-Guillén, 1998), precipitation by flocculation using alginate and chitosan (Wibowo, et al., 2005(Wibowo, et al., , 2007, and ohmic heat treatment of well-denatured protein (Huang, et al., 1997;Kanjanapongkul, et al., 2008). The most versatile method for the industrial application of surimi from fish is pH shift because of its low cost (Okazaki, 1994;Nishioka and Shimizu, 1983;Niki, et al., 1985;Torres, et al., 2007). In particular, the pH shift method has been applied to isoelectric solubilization/ precipitation, which is a high-efficiency protein isolation method that lacks time-consuming washing processes (Gehring, et al., 2011;Martín-Sánchez, et al., 2009;Nolsøe and Undeland, 2008).Briefly, all fish proteins can be easily solubilized at acidic and/or alkaline pH due to the increase in electrostatic repulsion between proteins. In contrast, co-aggregation of proteins occurs at the pH K. IwashIta et al. 744 levels close to their isoelectric point. The driving force of protein aggregation at the isoelectric points of protein complexes is hydrophobic interactions between protein molecules. The pH shift method can be applied on an industrial scale for SWW protein recovery (Nishioka and Shimizu, 1983;Niki, et al., 1985).However, not all of SWW proteins can be recovered by the pH shift method because proteins have different isoelectric points.In this study, we improved the recovery of SWW protein in the pH shift method by combination with heat treatment. Heat treatments induce protein aggregate formation by increasing hydrophobic interactions between thermally unfolded molecules (Hamada, et al., 2009;Shiraki, et al., 2016;Totosaus, et al., 2002).Proteins are prone to form aggregates when subjected to heat treatment around their isoelectric point (Tomita, et al., 2011). The optimized conditions outlined in this paper are useful for the recovery of protein from the wa...