## 3 Results and Discussion
### Determination of Volatile Substances
#### 3.2.2 Results of GC-MS Measured Volatile Substances in Hairtail Surimi

Table 1: _Cont._After Monascus fermentation, there were 26 kinds of volatile substances in the surimi gel that were significantly reduced, including octanol, trans-2-octen-1-ol, valeraldehyde, heptaldehyde, etc., and 17 kinds of volatile substances in the fermentation process that first decreased and then increased. Aldehydes may be generated by lipid oxidation, which is connected to the grassy and fatty scent of aquatic items, and aldehydes have a strong odor superposition effect [55; 56; 57]. Heptaldehyde is an aldehyde that has an oil oxidation odor that is similar to a fishy smell [58]. Heptaldehyde was destroyed in the surimi gel during Monascus fermentation. Nonanal has a fishy smell, and its content was eliminated within 3 h of fermentation; at 5 h, the relative content was 42.8% of the control group. Hexanal, which has a fishy odor, a grassy flavor, and other smells, may be generated when n-6 fatty acids are oxidized. According to the research, it is thought to be the primary culprit behind fish's beany flavor [7]. The high content of hexanal is often mixed with volatile compounds of C8 and C9 to contribute to the aroma of fish. The results showed that the relative content of hexanal was reduced, and it was reduced to its lowest at 1 h of fermentation, where it represented 41.0% of the blank group. Therefore, Monascus fermentation effectively reduces the fishy flavor of surimi through the elimination or reduction of fishiness-related aldehydes. 1-octene-3-ol in alcohols has been reported as a fishy component of fish [59]. It can be seen from the Table that when the fermentation is 0-3 h, the relative content of 1-octene-3-ol drops to 0; at 5 h, it represents 75.9% of the blank group. Octanol has an earthy, metallic taste, and trans-2-octene-1-ol is a potential key substance for fishy odor [60; 61]. These two alcohols were eliminated during the Monascus fermentation. Alcohols are often believed to be produced from fatty acids or reduced from carbonyl compounds, and 1-octen-3-ol is a hydroperoxide of linoleic acid breakdown, which results in a mushroom-like odor that is common in volatile fish components. It is believed that 3-methyl-1-butanol concentrations rise as a result of microbial deterioration during fish preservation [62]. Oleic acid was oxidized to produce octanol. Furthermore, alcohols that diminish hairtail surimi after fermentation include cis-2-penten-1-ol and phenylethanol, which are also present in related fish volatile taste compounds. Among the esters, 2-ethylfuran is regarded as a fishy flavor element that is primarily present in the volatile flavor of freshwater fish. It is produced by amino acids and sugars via the Maillard and Strecker degradation reaction and helps give some foods flavors such as burnt, sweet, bitter, cooked meat, and coconut [62]. The relative concentration of 2-ethylfuran was 44.3% and 50.4% of the control group at 1 h and 3 h, respectively, which decreased the fishy smell of the hairtail surimi gel. In general, the fishy substances in the volatile flavor of hairtail surimi gel that was fermented for 1 h and 3 h after Monascus fermentation were significantly eliminated or reduced, which was conducive to improving the flavor of the hairtail surimi. After Monascus fermentation, the volatile flavor of the hairtail surimi gel was enhanced by the presence of 15 new volatile substances, 5 different types of volatile substances (ethanol, 1-penten-3-ol, n-amyl alcohol, benzaldehyde, and naphthalene), and 9 additional volatile substances in the late fermentation stage. Meanwhile, Monascus fermentation increased the variety and content of amines in the hairtail surimi to a certain extent. The newly added volatile substances were mainly methylbenzene (eight kinds), and the inclusion of methylbenzene improved the taste of the hairtail surimi gel. The content of aromatic aldehydes benzaldehyde was 312.5% of the blank group during fermentation for 3 h, which has an almond flavor. Isovaleraldehyde is thought to be the volatile main flavor of cod, pomfret, and other fish.