Suitability of longtail tuna as a raw material for ‘ara‐bushi’

Longtail tuna Thunnus tonggol a type of smallsized tuna, is caught in considerably large quantities in the coastal waters of South-East Asia. The average annual amount of landing between 1987 and 1996 was 127 480 tons. Relatively large-sized fish have commercial value and are marketed. However, smaller fish that are less than 1.5 kg are underutilized and processed only into pet food or fishmeal. Therefore, the present study was undertaken to give small tuna of low commercial value a much higher value by processing. The present study, attempts to process small tuna into arabushi, and evaluates the products by analyzing their chemical components and sensory tests. Five fresh longtail tuna weighing 1.0–1.5 kg were purchased in October at a fish market in Singapore. The fish were frozen and transported to the laboratory of Shizuoka Prefectural Fisheries Experimental Station by air. Each fish was cut into fillets, after being thawed in running water. One fillet of each fish was used for chemical analyses, and the other fillets were processed into ara-bushi by cooking them in boiling water and intermittently smoke-drying them five times using the same methods as those for commercial Katsuo-bushi processing. The yield of ara-bushi was 8% of the raw material by weight. Shavings of the products were subjected to chemical analyses and sensory tests. Skipjack tuna Katsuwonus pelamis, which were caught at the equatorial zone in the Pacific Ocean by purse seine net fishing, were used for comparison. Moisture content was determined by ovendrying at 105∞C for 5 h. Crude protein was determined using an automatic protein analyzer (Mitamura, Tokyo, Japan); crude fat by the Soxhlet method; and crude ash by the ashing method at 550∞C for 12 h. Free amino acid was analyzed by high pressure liquid chromatography (HPLC) according to the method of Bidlingmeyer et al. Inosinic acid was determined using the HPLC method described by Tsuchimoto et al. The Yaizu Smoke-Dried Skipjack Processing Cooperative Association (Yaizu, Japan) kindly conducted the grading tests for color and flavor of the shavings. Table 1 compares the chemical compositions of the raw meats and the product (ara-bushi). With regard to the raw meats, the fat content of longtail tuna was lower than that of skipjack. This suggests that longtail tuna is a suitable raw material for dried products. The total amount of free amino acid (FAA) in longtail tuna was less than that in skipjack. Glutamic acid, taurine, histidine (His), tyrosine, and lysine were the predominant sources of FAA, particularly His, which accounted for 80% of the total FAA in the meat of longtail tuna. The amounts of carnosine and anserine in longtail tuna were considerably lower than in skipjack. As for the product (ara-bushi), both inosinic acid and FAA in the longtail tuna product accounted for 70% of those in the skipjack product, although the pattern of FAA in the skipjack product is almost the same as that obtained for 10 different samples of commercial skipjack products. Therefore, when considering the FAA content; that is, the tasteproducing amino acid content, the longtail tuna product might be somewhat inferior to the skipjack product (Katsuo-bushi). The color of shavings of longtail tuna ara-bushi was whiter than those of skipjack ara-bushi (Fig. 1). A test panel composed of three specialists on Katsuo-bushi processing concluded that, from Short Paper