## 4 Discussion
lipolytica_ showed the strongest antioxidant activity both in DPPH and ABTS assay. To some extent, the higher antioxidant activity of the fresh samples was correlated with the higher degree of hydrolysis and peptide content present in those samples. This aspect has been reported in several publications where protein hydrolysates are assessed for their radical scavenging activity [43; 48; 49; 50]. Surprisingly, _B. subtilis_ B5C, which showed a lower degree of hydrolysis and a lower peptide content, formed hydrolysates with a good radical scavenging activity. Similar values on DPPH assay were reported by [PERSON] et al. [51] using shrimp waste hydrolysates obtained with crude protease from _Bacillus cereus_. After lyophilization, only hydrolysates obtained with YL2 and B5C maintained their stronger activity in DPPH assay, while only YL4 showed good radical scavenging activity in ABTS assay, with respect to the control. Lyophilization reduced the difference of the hydrolysates, since similar amounts of powders were compared. This means that other compounds present in the hydrolysates, other than peptides, may have played a role in the radical scavenging activity. The production of volatile compounds upon microbial fermentation is well documented in the literature. For instance, the fungal fermentation of Fuzhuan brick-tea demonstrated an increase in volatile compounds, especially aldehydes (51%), such as 2-pentenal, 2-hexenal, 1-penten-3-ol and 2,4-heptadien [52]. [PERSON] et al. [53] obtained an increase in 2-methylbutanal and 3-methylbutanal after the fermentation of fish sauce made with freshwater fish by-products. In our work, samples fermented with the two strains of _Y. lipolytica_ showed a higher abundance of aldehydes (more than 50%). In particular, the most represented ones were 2,4-heptadienal (12.1 and 11.6%), benzaldehyde (4.7 and 5.8%), nonanal (3.3-4.1%), 2-pentenal (2.8 and 3.5%), 2-hexenal (1.3 and 5.0%) and 2-nonenal (2.4 and 2.7%) in YL2 and YL4 samples, respectively. However, even pentanal, heptanal, octanal and nonanal were more abundant in samples fermented with yeast. These aldehydes have specific aromatic characteristics: 2,4-heptadienal is described as fatty, green, oily, aldehyde, vegetable, cake, and cinnamon at 1%; benzaldehyde has fruits, almond, nutty and creamy notes; nonanal is described as fatty, citrussy and green; 2-pentenal is characterized by a strawberry, fruity flavor; 2-nonenal has earthy, fishy, cucumber and green notes [54]. Moreover, only the two samples incubated with _Y. lipolytica_ developed 3-methyl-butanal (around 0.2%), which is responsible for the taste and characteristic smell of fermented fish sauces [53]. Aldehydes can be produced during various biochemical reactions starting from free amino acids, and especially free fatty acids [55; 56]. The fact that their abundance was higher in samples containing _Y. lipolytica_ than in the control sample may depend on the stronger lipolytic properties of this yeast on this substrate. The presence of aldehydes in fish waste hydrolysates makes them an interesting source of flavor and aroma compounds that can be reused in food preparation. It is also interesting to mention that aldehydes from oxidized fatty acids, including pentanal, 2-heptenal and 2-nonenal, can react with DPPH [57]. This could explain the stronger antioxidant activity of the fresh hydrolyzed samples compared to the lyophilized ones. In fact, according to [PERSON] et al. [58], freeze-drying can significantly decrease the varieties of total volatile compounds. At the same time, the fact that lyophilized samples of YL2 and YL4 maintained the strongest radical scavenging activity in DPPH and ABTS assay, respectively, suggests that the remaining non-volatile compounds had an impact on their antioxidant power.