# The Potential of _Phaeodactylum_ as a Natural Source of Antioxidants for Fish Oil Stabilization
## 3 Results and Discussion
### _Phaeodactylum_ Total Lipid Extract-Fish Oil Mixtures (Part II)
#### 3.2.4 Oxidative Stability and Antioxidative Capacity  
The results of both primary (Section 3.2.2) and secondary oxidation (Section 3.2.3) showed a clear separation in oxidative stability between the different systems. 'Mixture 79/21' and 'Mixture 49/51' (containing respectively 78.6% and 49.2% (_w/w_) of _Phaedactylum_ total lipid extract) have been shown to be equally stable as the pure _Phaedactylum_ extract despite the presence of respectively 21.4% and 50.8% (_w/w_) of oxidatively susceptible fish oil. On the other hand, 'Mixture 6/94' and 'Mixture 1/99' (containing respectively 93.6% and 99.3% (_w/w_) of fish oil) have been shown to be equally unstable as the pure fish oil despite the presence of 6.4% and 0.7% (_w/w_), respectively, of _Phaedactylum_ total lipid extract. This clearly showed that apart from the presence of lipophilic antioxidants from the _Phaedactylum_ total lipid extract, of which the role in keeping _Phaedactylum_ lipids stable was already suggested in former research [16; 18; 29; 31] and could again be observed in the oxidative stability of the (n-3 LC-PUFA rich) _Phaedactylum_ total lipid extract of the current research, their amount also plays a role in determining lipid oxidation to be either absent or far advanced. Revising the fucoxanthin/n-3 LC-PUFA ratio of the different systems (Section 3.2.1), it could be stated that systems with a fucoxanthin/n-3 LC-PUFA ratio \(\geq\) 0.101 were shown to be oxidatively stable, whereas systems with a ratio \(\leq\) 0.0078 led to strong lipid oxidation. In this regard, the fact that 2.5% _Phaedactylum_ biomass addition to the fish oil did not show inhibition of oxidation (Part I) is not surprising since this gave rise to a fucoxanthin/n-3 LC-PUFA ratio of only 0.00007 (\(\pm\)0.00002) in the oil phase. These findings, therefore, confirmed the hypothesis of antioxidant concentrations being too low as compared to n-3 LC-PUFA (as the oxidizable substrate) to have an antioxidative effect, although they were present in similar absolute concentrations as in former research where an antioxidative effect had been described [27]. Interesting to remark is that regarding the fucoxanthin migration to the oil phase in Part I, even a full transfer would not have led to a fucoxanthin/n-3 LC-PUFA ratio associated with oxidative stability. Therefore, no effort should be made in trying to improve antioxidant migration by additional treatments in trying to obtain an antioxidative effect of 2.5% _Phaeodactylum_ biomass addition to fish oil. Moreover, significantly increasing the biomass concentration would also not lead to the fucoxanthin/n-3 LC-PUFA ratios of interest. An extreme, tenfold increase to 25% _Phaeodactylum_ biomass, for example, would, whether or not feasible, give rise to a fucoxanthin/n-3 LC-PUFA ratio of 0.0007 (\(\pm\)0.0002) maximally (assuming a similar migration). Unlike other former research adding microalgal biomass as an antioxidant to bulk oil, of which the antioxidative effects of [PERSON] et al. (2014) (_Arthospira_ addition to olive oil) were the most convincing, it is clear that _Phaeodactylum_ biomass addition to stabilizing fish oil thus has no potential. Figure 5: Secondary oxidation of six different _’Phaeodactylum_ total lipid extract–fish oil’ systems during storage at 37 \({}^{\circ}\)C. _’Pune _Phaeodactylum_ total lipid extract’, _’Mixture 79/21’, and _’Mixture 49/51’_ (respectively possessing 100.0%, 78.6% and 49.2% (_w_/_w_) of _Phaeodactylum_ total lipid extract) were stored for 10 weeks (left). _’Mixture 6/94’, _’Mixture 1/99’_, and _’Pure Fish oil’_ (respectively possessing 6.4%, 0.7% and 0.0% (_w_/_w_) of _Phaeodactylum_ total lipid extract) were stored for 4 weeks (right). The content of volatile compounds originating from lipid oxidation [(E,E)-2,4-Heptadien (**a**,**b**), (E)-2-Pentenal (**c**,**d**), (Z)-2-Butenal (**e**,**f**)] in \(\mu\)g/mg (mean \(\pm\) sd, \(n=3\)) is expressed in delta values, representing the relative increase or decrease compared to day zero.