## 3 Results and Discussion
### Characterization of Fried Mackerel Fillets
#### 3.3.3 Nutritional Characterization of Fried Mackerel Filllets
One of the mechanisms proposed for describing oil absorption in food matrices is water replacement, which states that rapid evaporation of water during frying creates holes in the structure, which allows oil to penetrate, thus increasing its uptake [45]. This may explain the results observed in this study, where samples with higher moisture content showed lower fat absorption (Car Pc). Another explanation for oil absorption might be the increase in polar compounds during the frying process as oil degrades. These compounds act as surfactants, promoting contact between the food and the frying oil, thereby enhancing oil uptake [46]. Nevertheless, in this study, this effect might be neglected since the oil was replaced at the end of each frying batch (4 min). Besides the slight differences observed in fat content, the fish samples exhibited markedly distinct fatty acid profiles. The fatty acid profile of the fried fillets results from both the original fish matrix and the sunflower oil absorbed during frying. Also, chemical reactions that take place along the process can provide changes in FA composition in fried samples. The sunflower oil used in this study is predominantly composed by linoleic acid (C18:2 n-6), followed by oleic (C18:1 n-9), palmitic (C16:0), and stearic (C18:0) FA, which accounted for 59.36 \(\pm\) 0.15%, 29.15 \(\pm\) 0.09%, 6.71 \(\pm\) 0.10%, and 3.07 \(\pm\) 0.02% of total FA, respectively [31]. The high content of PUFA, particularly susceptible to oxidation, has a negative impact on the frying performance of sunflower oil. Nevertheless, this oil was selected for this study due to its widespread use in Portuguese cuisine [11]. Atlantic mackerel, classified as a fatty fish, is particularly rich in PUFA, especially the n-3 fatty acids docosahexaenoic acid (DHA, C22:6 n-3) and eicosapentaenoic acid (EPA, C20:5 n-3), with reported values ranging from 19-40% to 5-12% of total fatty acids, respectively. The most abundant saturated fatty acids are palmitic (C16:0) and stearic (C18:0), while oleic (C18:1 n-9) is the predominant monounsaturated fatty acid, typically comprising 7-14% of total FA. Other unsaturated fatty acids present include arachidonic acid (C20:4 n-6), at 3-8%, and linoleic acid (C18:2 n-6) in smaller amounts, ranging from 1 to 2.5% [47; 48]. In the present study, it was observed that the most abundant saturated fatty acids in the post-frying fillets are palmitic (from 15.7 \(\pm\) 1.5 to 18.30 \(\pm\) 0.35%) and stearic (ranging from 5.40 \(\pm\) 0.16 to 6.28 \(\pm\) 0.09%), regardless of the applied coating formulation (Table 3). Statistically significant differences were observed between control and the coated fillets Alg Pc for both myristic (C14:0) and stearic fatty acids. The highest value for total saturated FA was exhibited by the control sample (31.59 \(\pm\) 0.23), conversely to Alg Pc (26.9 \(\pm\) 2.2), which attained a significantly lower value. It is worth mentioning that the presence of algae extract resulted in a decrease in SFA content in both base-coatings. Oleic was the most abundant fatty acid in all the samples, being higher in coated fillets (ranging from 27.14 \(\pm\) 0.66%, in Alg Pc to 44.94 \(\pm\) 0.95% in Car Pc) than in the control (24.56 \(\pm\) 0.10%). Regarding PUFA, a considerable decrease in linoleic acid content in all coated fillets (from 2.39 \(\pm\) 0.16 in Alg to 2.65 \(\pm\) 0.02 in Car Pc) in comparison to the control (15.34 \(\pm\) 0.55%) was observed, except for Alg Pc samples (21.4 \(\pm\) 1.8%). Car Pc stands out for the lowest levels of DHA (10.66 \(\pm\) 0.29) and EPA (4.52 \(\pm\) 0.06) compared to the control, with values of 11.68 \(\pm\) 0.52 and 5.43 \(\pm\) 0.20 for DHA and EPA, respectively. Among the FAs present in the sunflower oil, linoleic (C18:2) acid is the most susceptible to degradation, promoted by either oxidative or thermal reactions.