Koichi Kuwahara

Effect of trehalose on the gel-forming ability, state of water and myofibril denaturation of horse mackerel Trachurus japonicus surimi during frozen storage

The cryoprotective effects of trehalose on fish myofibrillar protein were compared with those of sucrose, glucose and sorbitol. The frozen surimi with trehalose exhibited significantly higher Ca2+-ATPase activity through-out the storage periods, resulting in higher gel-forming ability than that of without trehalose. The amount of unfrozen water was significantly increased in the surimi upon addition of trehalose at any concentrations tested. The findings suggest that trehalose constructed bound water molecules in protein structure, consequently suppressed freeze-induced denaturation of protein and maintained gel-forming ability. An addition of 5.0% to 7.5% concentration of trehalose showed threshold behavior to increase the amount of unfrozen water and to prevent freeze-induced denaturation of protein. The effects of trehalose were almost similar to those of other sugars.

Effect of starvation on lipid metabolism and stability of DHA content of lipids in horse mackerel (Trachurus japonicus) tissues

For the purpose of characterizing the effect of starvation on 22∶6n−3 (DHA) content in marine fish tissues, horse mackerel (Trachurus japonicus) were reared in a tank containing filtered, sterilized seawater under nonfeeding conditions for 107 d (survival rate of the fish was 96.5%). The crude total lipids (TL) of ordinary dorsal muscle, dorsal skin, and viscera of the starved individuals were separated into classes on silicic acid columns, and the constituents of the TL were quantified by gravimetric recovery from column chromatography. The TL, initially>85% TAG in dorsal muscle, and even more in skin lipids, decreased dramatically within the first 44 d of starvation, and then decreased more gradually during the remainder of the test period, whereas the visceral TL decreased more slowly. The percentages of both saturated and monoenoic FA in the muscle TL also decreased somewhat, but those of DHA increased significantly in muscle during the test periods. Decreases in PE and PC initially were much smaller than TAG, but DHA levels remained high in both PE and PC. These findings indicate that all of the FA in the depot lipids of horse mackerel tissues are easily metabolized for energy production during starvation, but DHA in muscle lipids of the starved fish was maintained at a consistently high level, indicating that starvation did not affect DHA stability in phospholipids. The findings suggest that preservation of DHA in cell membrane lipid PE and PC is necessary for self-protection functions in starving fish.

Differences in free amino acid composition between testis and ovary of sea urchin Anthocidaris crassispina during gonadal development

For the purpose of clarifying the differences of taste components between the testis and ovary of the sea urchin Anthocidaris crassispina, variations in free amino acid composition in gonadal extract during gonadal development were investigated using chemical and histopathological methods. Sea urchins were collected at Iki Island, Nagasaki, Japan between 30 April and 5 June 2003, and maturation stages and the free amino acid compositions were determined. The sweet amino acids threonine (Thr), glycine (Gly), and alanine (Ala), the umami amino acid glutamic acid (Glu), and the bitter amino acids lysine (Lys) and arginine (Arg) were the main constituents accounting for more than 80% of total free amino acids. In the testis and ovary, Gly levels were the highest among them and increased with gonadal development. The other free amino acids consisted mainly of bitter amino acids and either showed no variation or decreased with gonadal development. These variations in free amino acid composition were observed more clearly in the ovary than the testis. These findings indicate the possibility of differences in the flavor of sea urchin gonad by sex and gonadal development stages.

Quality aspect of fish sauce prepared from underutilized fatty Japanese anchovy and rabbit fish

To explore the potential utility of underutilized fish in the fish sauce industry, fish sauces were prepared from both raw and heat-induced meat of Japanese anchovy Engraulis japonica and rabbit fish Siganus fuscescens using wheat malt, and their quality aspects and sensory properties were assessed. Proximate composition of the fish meat represents protein as the major component (16.0–17.8%), other than moisture. Analyses of free amino acids, peptides including oligopeptides, and organic acids contents in fish sauces revealed suitability of both raw and heat-induced meat of the selected species in commercial fish sauce production. The variations in taste of raw meat fish sauces were species-specific, but the taste of fish sauces from heat-induced meat were similar.

Docosahexaenoic acid levels in the lipids of spotted mackerel Scomber australasicus

The lipid and FA compositions of various organs and of the stomach contents of Scomber australasicus were analyzed. DHA was characteristically the major FA of all the major lipid classes of all organs except for liver TAG. The mean DHA contents of the various organs accounted for more than 17% of the total FA (TFA), whereas those in the stomach contents, originating from the prey, fluctuated and were generally low. In particular, the DHA levels in the TAG from all organs of S. australasicus accounted for up to 17% of TFA, even though it is a neutral depot lipid. S. australasicus contained markedly high levels of DHA, even though it is a small-sized Scombridae species, and its high levels of DHA were close to those in large-sized highly migratory tuna species. Furthermore, DHA levels in its muscle TAG were consistently high, compared with those in the visceral TAG, which might be directly influenced by the prey lipids. These phenomena suggest that long-distance migration has a close relationship with high accumulation of DHA in fish tissues, since S. australasicus is reported to migrate in offshore water, similar to highly migratory tuna species. Additionally, the physiological selective accumulation of DHA in the muscle during migration is caused by in vivo metabolism of FA in the vascular system, suggesting that DHA is poorly used as a source of migration energy, though it is provided abundantly through the prey lipids.

Year-round high arachidonic acid levels in herbivorous rabbit fish Siganus fuscescens tissues

To identify a stable resource of 20∶4 n−6 (arachidonic acid, AA) in marine fish tissues, the lipid profiles of Siganus fuscescens organs (muscle, liver, and other viscera) and stomach contents were examined throughout the year. Crude total lipid (TL) contents in respective organs showed seasonal variations and were high in winter and low in summer. The main FA in TL were 16∶0, 18∶0, 16∶1n−7, 18∶1n−9, AA, and 22∶6n−3 (DHA). These FA were those generally observed in marine fish lipids, except for comparatively high levels of AA. In TL of muscle and liver, AA showed relatively high values during the period from late May to August (muscle, 4.6–13.1%; liver, 4.5–9.1%), compared with other seasons (muscle, 4.3–9.5%; liver, 3.6–8.4%). The AA levels in TL of other viscera and stomach contents fluctuated (other viscera, 2.0–10.7%; stomach contents, 7.6–26.7%). Regardless of the fishing season, each organ contained a higher level of AA in polar lipids (PL) than in neutral lipids. It was concluded that the fish contain comparatively high levels of AA in their TL throughout the year, and they accumulate AA characteristically in their tissue PL, probably from dietary food sources. Moreover, it was suggested that S. fuscescens has potential utility as a natural marine source of nutritional lipids, because the fish contain comparatively high levels of DHA and AA.

Purification and Characterization of Cathepsin B from the Muscle of Horse Mackerel Trachurus japonicus.

An endogenous protease in fish muscle, cathepsin B, was partially purified and characterized from horse mackerel meat. On SDS-PAGE of the purified enzyme under reducing conditions, main protein bands were detected at 28 and 6 kDa and their respective N-terminal sequences showed high homology to heavy and light chains of cathepsin B from other species. This suggested that horse mackerel cathepsin B formed two-chain forms, similar to mammalian cathepsin Bs. Optimum pH and temperature of the enzyme were 5.0 and 50 °C, respectively. A partial cDNA encoding the amino acid sequence of 215 residues for horse mackerel cathepsin B was obtained by RT-PCR and cloned. The deduced amino acid sequence contains a part of light and heavy chains of cathepsin B. The active sites and an N-glycosylation site were conserved across species. We also confirmed that the modori phenomenon was avoided by CA-074, a specific inhibitor for cathepsin B. Therefore, our results suggest that natural cysteine protease inhibitor(s), such as oryzacystatin derived from rice, can apply to thermal-gel processing of horse mackerel to avoid the modori phenomenon. Meanwhile, this endogenous protease may be used for food processing, such as weaning meal and food for the elderly.

Year‐round gelation characteristic of bassy chub Kyphosus lembus meat

Year-round change in gelation of bassy chub (Kyphosus lembus) meat, collected from October 2001 to September 2002 off the coast of Nagasaki, was investigated. The fish meat exhibited a similar high gel-forming ability for all seasons, including pre- and post-spawning seasons. Freshwater washing of the meat did not improve its gel-forming ability. It was concluded that bassy chub meat could be used for thermal gel production in all seasons.