Abstract: The purpose of this study was to investigate the effects of different salt concentrations (0.3%, 0.4%, 0.5%, 0.6% and 0.7%) on the formation of micro/nano particles (MNPs) in tuna head soup, prepared under the following conditions: shallow frying 120 ℃ for 40 s and then boiling for 150 min in an eight-fold excess of water. The migration of nutrients from fish head into soup was investigated, and the formation of MNPs during the boiling process was analyzed using a Zetasizer nano ZEN 3600. Furthermore, the morphology of MNPs was observed by optical microscopy, and the interactions among MNPs components were studied by the combined use of laser scanning confocal microscopy and fluorescence labelling. The amount of dissolved nutrients in the soup reached a maximum value at 0.5% salt concentration. The results of Zetasizer nano ZEN 3600 showed that the average particle size, polymer dispersity index (PDI), and zeta-potential value of MNPs were significantly affected by salt concentration (P < 0.05). MNPs had the smallest size at 0.5% salt concentration. These results demonstrated that salt could promote the formation of MNPs in the soup. Moreover, at 0.5% salt concentration, MNPs had an average particle size of (519.1 ± 3.11) nm and formed a stable spherical bilayer in the soup. Cl- was found to permeate into the core of MNPs to bind to triglycerides. Meanwhile, Na+ bound to electronegative polysaccharide molecules dispersed around the surface of MNPs, counteracting the negative charges. At an appropriate salt concentration, the migration of nutrients from fish head into soup reached a maximum value, and so did the stability of MNPs formed by self-assembling of triglycerides, phospholipids, proteins, and glycosylated molecules (glycoproteins and glycolipids) being beneficial to improve the stability of nutrients in the soup.

Key words: tuna head soup; salt concentration; substances migration; micro/nano particles; laser scanning confocal microscope