Title: Physicochemical properties and potential utilization of side raw materials of yellowfin (Thunnus albacares) and albacore (Thunnus alalunga) tuna Efnasamsetning og nýting hliðarafurða yellowfin og albacore túnfisks

Tuna processing companies produce a lot of rest raw materials including skin, viscera, bones, heads, tails, trimmings, belly flaps, and dark meat, which are often irresponsibly disposed of. There is great potential to utilize these tuna raw materials and convert these as valuable products by screening their physicochemical characteristics, bioactivities and potential applications.

This study focuses on the comparison and utilization of tuna heads and skins. Chemical composition such as fatty acids, and minerals were analysed from tuna heads and skins from two tuna species, yellowfin (Thunnus albacares, YF) and albacore (Thunnus alalunga, AL) tuna. The analysed components from the heads were head muscle, eye muscle and eyeball, of which the latter was further separated in aqueous humour, lens, inner casing and outer casing. Gelatin and fish glue were extracted from the skins, and hydroxyapatite was extracted from the scales.

Analysis of fatty acids showed very high docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), arachidonic acid and oleic content in the muscle tissues and aqueous humour. The highest proportion of saturated fatty acids was found in the head muscle, the highest mono-unsaturated fatty acid in the aqueous humour, and the highest poly-unsaturated fatty acid in the head muscle. Palmitic acid, oleic acid and DHA were the dominant fatty acids in both species. Mineral analysis showed high phosphorous and calcium levels in the scales and high sodium and potassium levels in the aqueous humour. High collagen content was found in the eye components, scales and skins, with the lowest value in the YF inner casing and the highest in the YF abdominal skin.

High fatty acid composition in the muscle tissues of the head have shown a potential source of obtaining high omega-3 tuna oil, especially in DHA. High mineral content in the scales and high collagen levels found in the different tissues, show potential in further utilization for e.g. to Ca rich foods and gelatin. The gelatin extracted from the YF tuna and AL tuna dorsal skin had a fairly high and similar yield of 17.5 % and 12.7 % respectively. The hydroxyapatite yield was higher in the YF tuna, but was lower in mineral content. The yield and composition proved the importance of processing these raw materials into valuable end products.