## 3 Results and Discussion
### Analysis of Volatile Flavor  
A total of 60 volatile-flavor substances were recognized through HS-SPME/GC-MS from five groups of samples: aldehydes, alcohols, ketones, esters, hydrocarbons, furans, and acids. 28, 23, 37, 41, and 46 volatile compounds were observed in the CK, VFD, VD, NSD, and HAD groups, respectively (Table S3). The functional group classification of volatile flavor compounds was developed to analyze the variations in volatile-flavor substances in oysters after processing by different drying methods (Figure 5). Compared with the CK group, VFD resulted in the loss of some volatile flavor substances, and the hot drying method (VD, NSD, and HAD) increased the amount and concentration of volatile-flavor substances (Figure 5A). This finding indicated that dried oysters had a more intense and complex volatile flavor than fresh oysters. Similar results have been obtained in a previous study on semi-dried fish fillets by [PERSON] et al. [12]. Among the volatile-flavor substances, aldehydes were the main ones in raw and dried oysters, and they were higher in number and concentration than other classes of compounds. The relative content of aldehydes ranged from 54% to 72% in the five groups of samples, suggesting that aldehydes' volatile compounds were the majority aroma components of oysters (Figure 5B). Similar results have been found in _Ostrea edulis_[30] and _Crassostera gigas_[14; 31]. Hexanal, octanal, heptanal, benzaldehyde, (E, E)-2,4-heptadienal, (E)-2-decenal, 4-pentenal, and nonanal were all discovered in dried oyster samples. Aldehydes are produced mainly by the oxidation and degradation of fatty acids [32; 33]; for example, hexanal takes shape oleic, linoleic, and arachidonic acids or through degradation of other unsaturated aldehydes [34], whereas (E, E)-2,4-decadienal is an oxidation product of polyunsaturated fatty acids in the time of heat treatment. Because of the high content and low threshold of aldehydes, they have a strong fatty, orange, and grassy aroma and an effective impact on the flavor of dried-oyster food [12; 35]. Figure 5: The number of species (**A**) and relative content (**B**) of various volatile fractions in dried oysters under different drying conditions. CK: blanched oysters; VFD: vacuum freeze drying; VD: vacuum drying; NSD: natural sun-drying; HAD: hot air drying; Different letters indicate significant differences between groups (\(p<0.05\)). Furthermore, 4, 4, 7, 7, and 8 ketones were detected in the CK, VFD, VD, NSD, and HAD groups, respectively. Ketones are essential aroma components of dried products and are primarily produced through the thermal oxidation of polyunsaturated fatty acids or the degradation of amino acids. They can impart unique aromas and fruity flavors to the products [21; 36]. Furans are produced by Maillard reactions when sugar is exposed to high temperatures and are an important source of flavor for coffee, caramel, and fruity [37; 38]. Only 2-pentylfuran was detected in the CK group, whereas 2-acetylfuran, 2-ethylfuran, trans-2-(2-pentenyl)furan, and 2-5-dimethylfuran were detected in the dried oysters. The concentration of furans was much higher in dried oysters than in fresh ones. Alcohols, which are primarily produced by fat oxidation and reduction of carbonyl compounds, have much higher flavor thresholds than aldehydes and ketones and contribute less to the overall flavor of the oyster. Nevertheless, some unsaturated alcohols have lower thresholds and contribute to the overall flavor of oysters (e.g., 1-octen-3-ol and hepten-3-ol). Notably, 1-octen-3-ol has a mushroom-like flavor and is thought to be the source of the earthy flavor in aquatic products [32; 39]. 1-Octen-3-ol levels are also responsible for the fishy flavor of dried oysters and were detected in VFD, VD, and HAD dried oysters, but the highest levels were found in the VFD group.