Skipjack tuna (Katsuwonus pelamis) (ST) is the third most harvested species globally, with an annual catch reaching up to 3.2 million tons (Barbosa et al., 2018; Chang et al., 2022; Huang, Wang, et al., 2022). As a kind of high-protein and low-fat food, ST is predominantly traded internationally as canned products, due to its comparatively smaller size compared to other tuna species (Kuroda & Nozawa, 2008). Smoking treatment is one of the most popular traditional food processing techniques, which is intended not only to extend shelf life, but also to enhance the flavor, color and texture of foods (Hitzel et al., 2013; Yin et al., 2021). The conventional manufacturing process for sliced-smoked skipjack tuna comprises five essential stages, including freezing, heating, smoking, steaming, and slicing, with transparent glassy crystals developing during the smoking phase. Such processing affects the application of sliced-smoked skipjack tuna and its by-products in soy sauce or stock (Kosowska et al., 2018). Sliced-smoked skipjack tuna, derived from the slicing process of smoked skipjack tuna meat (SSTM), represents a widely used condiment in the Japanese diet, serving as a flavorful base for takoyaki.

The production of volatile flavor compounds (VFCs) in fish is typically associated with lipid oxidation and hydrolysis, protein oxidation and hydrolysis, as well as the Maillard reaction (MR) (Bassam et al., 2022). The key constituents of smoky flavor in smoked meat are reported to be phenolic compounds such as m-cresol, 2-methoxy-phenol, 4-ethyl-guaiacol, phenol, 4-methyl-phenol, and 2,6-dimethoxy-phenol (Guo et al., 2021; Marušić Radovčić et al., 2021). The phenolic and carbonyl compounds generated through the thermal decomposition of wood during smoking, which are absorbed by the meat, serve as the primary source of smoked flavor formation, followed by the MR between the carbonyl compounds and meat proteins (Albishi et al., 2019; Rieuwpassa et al., 2023). Recent research has shown that the interactions between the proteins and VFCs play a key role in influencing the quality and sensory attributes of smoked products (Merlo et al., 2021; Zou et al., 2024). Carton et al. (2009) found that the carbonyl compounds generated during the smoking process reacted with amino acids, resulting in structural modifications in the myofibrillar proteins of salmon, such as significant shrinkage of the myofibrils. The unfolding of protein structures is the underlying factor for the ionization of carboxyl and amino groups, thereby promoting the establishment of novel intermolecular and intramolecular bonds during the processing of SSTM (Yu et al., 2021). The extent of adsorption and release of smoky flavor from proteins is influenced by changes in protein structure, which also impart a characteristic smoky and crunchy flavor profile to products. However, limited research has explored the relationship between changes in proteins and changes in VFCs during the production of the sliced-smoked skipjack tuna. This study hypothesizes that the protein modification in SSTM is responsible for the development of smoky flavor.

The interaction between protein and VFCs contributes to the conversion of food flavor, especially in low-fat products. This process is influenced by various factors, such as the properties of proteins, conformation of proteins and the chemical properties of VFCs (Chen et al., 2023; Martinez-Arellano et al., 2016; Wang et al., 2017). Structural changes of proteins in meat may alter hydrophobicity, hydrogen bonding, and the formation of non-specific binding forces, which may control the release and adsorption of VFCs (Ma et al., 2019). The exposure or hiding of hydrophobic binding sites in meat may result in a decrease or increase in binding affinity to VFCs (Kühn et al., 2008). Reversible binding and irreversible binding are the two modes of binding between VFCs and proteins. Given that most VFCs are hydrophobic, reversible binding plays a major role in their interactions with proteins (Kühn et al., 2008; Martinez-Arellano et al., 2016).

Herein, the alterations in protein structure and the formation of VFCs in smoked ST throughout the processing sequence were investigated. Additionally, the influences of protein structure changes on the adsorption and release of key VFCs were explored. This study primarily aims to enhance the stability and quality of skipjack tuna products during production, thereby expanding the potential applications of smoked skipjack tuna and its by-products.