Viscosity increases with chain length and decreases with the number of double bonds in the chain [33]. Therefore, the lower viscosity found for fish oil biodiesel (4.2 mm2/s) can be explained by its high content in PUFA (26.1 wt.%), comparing to the biodiesel from palm oil (10.0 wt.% PUFA) and waste frying oil (24.3 wt.% PUFA) (Table 1). Moreover, cis double bonds present lower viscosity than trans double bonds, which shows similar viscosity to a correspondent saturated compound [33]. Waste frying oil is usually partially hydrogenated and contains higher amounts of trans fatty acid chains [33]. This explains the fact that frying oil biodiesel, with contents in SFA, MUFA and PUFA similar to fish oil biodiesel, had a higher viscosity (5.3 mm2
/s) (Table 1). The higher viscosity for palm oil biodiesel (5.6 mm2/s) is associated to its high content in SFA (44.3%) (Table 1).