## 3 Results and Discussion
### Identification of Characteristic Flavor  
The amount of flavor components is insufficient to determine the characteristic flavor of a food product. The key flavor is determined by the concentration of the flavor components and the threshold value, not all of which are active characteristic flavors [21]. The OAV and TAV theories are extensively used to identify active taste and key aroma, respectively [18; 40]. They have similar characteristics, and a value of TAV or OAV exceeding 1 is known to indicate that the presenting substance has a significant influence on the flavor. A greater value corresponds with a greater contribution. In the present study, 15 differential taste substances had TAV \(>\) 1 (Table S2). Each sample contained between 8 and 12 flavor compounds with TAV \(>\) 1, respectively, where Glu, Ala, succinic acid, and betaine had TAV \(>\) 1 in all samples. Gly, Arg, IMP (except the NSD group), and AMP had TAV \(>\) 1 in the dried group. Gly, Arg, and Ala provided the sweet properties of the food. Glu provided the umami. IMP and AMP contributed to the umami alone while synergistically interacting with amino acids to increase the umami intensity. These results indicated that umami and sweet taste attributes were characteristic of fresh oysters and that dried processing further enriched the intensity of this attribute.  
The OAV theory was used to distinguish the volatile-flavor substances of the five groups of samples. There are 18 volatile-flavor substances had OAV values greater than 1(Table S4), including 13 aldehydes (hexanal, octanal, heptanal, (E, E)-2,4-heptadienal, (E)-2-decenal, (E, E)-2,4-decenal, nonanal, pentanal, 2-hexanal, 2-octenal, (E, Z)-2,4-nonadienal, benzaldehyde, and decanal), 2 ketones (1-octen-3-one, 2,3-pentanedione), 1 ester (ethyl heptanoate), 1 alcohol (1-octene-3-ol), and 1 furan (2-pentylfuran). The aldehydes provide the olfactory attributes of dried oysters, such as green, fatty, and fruity aromas. 2,3-Pentanedione has a creamy smell, whereas 1-octene-3-ol and 1-octen-3-one contribute to the mushroom scent, which is also considered a fishy substance in seafood. 2-Pentylfuran has a roasted aroma, a crucial fragrance characteristic of dried oysters Aroma. The drying groups (VFD, VD, NSD, HAD) contained between 10 and 17 substances with an OAV greater than 1. The HAD group had the highest number of substanceswith an OAV greater than 1, with the addition of heptanal, pentanal, 2-hexenal, (E, Z)-2,4-nonadienal, benzaldehyde, 1-octen-3-one, 2,3-pentanedione, 1-octene-3-ol, and ethyl hexanoate compared to CK. This was followed by the VD group, which contained 16 species, and the VFD group, which had the fewest substances with OVA greater than 1, with only 10 species. This result indicates that high-temperature drying increased the richness of the dried oyster flavor characteristics.  
To visually distinguish the characteristic flavor of the five samples, we developed characteristic flavor models for the 11 sensory attributes of umami, bitter, sweet, fatty, caramel, fruity, green, orange, fishy, woody, and mushroom based on substances with TAV > 1 and OAV > 1 (The OAV of substances with the same flavor are added up and finally normalized). As shown in Figure 7, the different drying approaches have a practical influence on the characteristic flavor of the dried oysters. NSD has the highest sweetness,  
Figure 6: Cluster Heat Map of volatile flavor in oysters under different drying conditions. CK: blanched oysters; VFD: vacuum freeze drying; VD: vacuum drying; NSD: natural sun-drying; HAD: hot air drying.  
bitterness, fruit, and caramel properties. HAD has the highest umami and lowest bitterness. The woody of VFD is significantly higher than other drying methods. The fishy and mushroom flavor of the VD is most noticeable in the dry-aged group. Overall, these dried oysters prepared by HAD had better flavor attributes.