Abstract:
Fish processing offcuts, especially heads, are frequently discarded without realizing its potential
in extracting fish oils. Among many fish oil extracting methods, the wet reduction process (WRP)
is industrially popular as it is environmentally safe. As the processing conditions of the WRP affect
the yield and quality of oil, two heat transfer methods: direct heating and indirect heating at three
pre-treatment temperatures: 55, 75 and 95 °C were evaluated in the present study by using
yellowfin tuna heads. The effect of heat treatment on the quality of the extracted fish oil was
studied by free fatty acids percentage (FFA%), acid value (AV), peroxide value (PV), p-anisidine
value (p-AnV), total oxidation (TOTOX), oil colour, and Attenuated Total Reflectance - Fourier
Transform Infrared (ATR-FTIR) measurements and compared to the oil extracted using Bligh and
Dyer method (BDM). The optimum process conditions were direct heating at 75 ºC for 30 minutes,
followed by pressing whereas the extraction yield and recovery were 4.35 ± 0.55 % and 57.43 ±
7.32%, respectively. Oils extracted by different pre-heat treatments resulted in significant
differences (P < 0.05) of FFA, AV, PV, p-AnV, TOTOX and colour values. Further, the increased
heating temperature increased the amount of extracted yield but reduced the oil stability as all
FFA% had exceeded the maximum allowable limit (3 mgKOH/g) specified by WHO. However, the
PV showed a downtrend with increasing temperature. Comparing the p-AnV of oils extracted, it
can be stated that the WRP had less effect on the secondary oxidation of oil than the BDM. Since
oil from the BDM showed high p-AnV that exceeded the maximum allowable limit (20), thus,
resulted in an elevated TOTOX value. Therefore, the WRP exhibits a favourable effect on the oil
quality than the BDM. All ATR-FTIR spectra confirmed the presence of important functional
groups at wavenumbers of 3012 cm−1, 2922 cm−1 and 2853 cm−1 confirming the existence of PUFA.
Nevertheless, this study identified yellowfin tuna head as a good source of fish oil that can be
extracted by the wet reduction process.
