## 3 Results and Discussion
### Odor-Active Compounds  
The odor thresholds in water and odor descriptions for volatile compounds are indicated in Table 2. The odor threshold information for propyl hexanoate, pentyl hexanoate, hexol 2-butenoate, butyl octanoate and hexyl octanoate were unknown, so they were not taken into consideration. Moreover, if the OAV was greater than 1 among all the apple varieties, they would be labeled in Table 2, which can be regarded as apple characterized odor-active compounds for each year. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Nn. ** & **Name** & **Rotation** & **Rotation** & **Rotation** & **O-Iail  
In the first year, a total of 6 volatile compounds were identified, including ethyl 2-methylbutyrate, hexanal, 1-hexanol, hexyl 2-methylbutanoate, E-2-nonenal and linalool, which are described as green (grass), fruity and floral (flower) notes. However, there were a total of 19 characterized odor-active compounds in the second year. 14 of these compounds were unique in the second year, consisting of ethyl butyrate, butyl acetate, butyl propionate, pentyl acetate, D-limonene, 2-methyl-1-butanol, E-2-hexenal, hexyl acetate, 6-methyl-5-hepten-2-one, hexyl propionate, 1-hexanol, nonanal, E-2-hexen-1-ol, 1-heptanol and naphthalene, which were regarded as green, fruity and sweet notes. Although the presence of naphthalene was uncertain, it was a characterized odor-active compound in the second year. A total of five compounds were detected in both years, which were ethyl 2-methylbutyrate, hexanal, 1-hexanol, E-2-nonenal and linalool, demonstrating that they were the most critical characterized odor-active compounds in the 35 apple varieties. Ethyl 2-methylbutyrate is frequently used as a food additive to obtain better product flavor. The toxicity values for hexanal, 1-hexanol, E-2-nonenal and linalool in LD50 rat oral were 4890 mg/kg, 4870 mg/kg, 5000 mg/kg and 2790 mg/kg, respectively. Their concentrations in all the apple varieties were much less than their toxicity values, proving all the 5 critical characterized odor-active compounds were safe and harmless for human smell and consumption. Since the odor thresholds were taken into consideration, the results were different to volatile compound concentrations. Hexanal and 1-hexanol not only had high concentrations among all the apple varieties, the odor thresholds were also at high levels, especially for 1-hexanol (500 \(\mathrm{\SIUnitSymbolMicro g}\)/L). They were the most potent odorant in the aldehyde and alcohol group, respectively, which were found in the apple varieties Golden Delicious and Braeburn [4; 32], and also other fruits, such as mulberry [14], peach [48] and cranberry [49]. Although the quantity for butyl acetate, 2-methyl-1-butanol, E-2-hexenal, E-2-hexen-1-ol, hexyl 2-methylbutanoate and 1-heptanol were greater, the thresholds were also high. Thus, the OAV may not be higher than 1 for all apple varieties. For example, the threshold for E-2-hexen-1-ol could achieve 2319 \(\mathrm{\SIUnitSymbolMicro g}\)/L, it was an odor-active compound only in the second year. However, lower concentration compounds contributed more to the characterized odor-active compounds, especially for ethyl 2-methylbutyrate. Because the threshold for ethyl 2-methylbutyrate was also low (0.13 \(\mathrm{\SIUnitSymbolMicro g}\)/L), it was also considered as an odor-active compound in Fuji, Golden Delicious, Starking, Golden Reinders and Braeburn apples [3; 4; 5]. Nevertheless, ethyl 2-methylbutyrate was the main OAV in pineapple [50] and cranberry [49]. In the cases of E-2-nonenal and linalool, they were similar and thresholds were both below one. Based on the literature, E-2-nonenal and linalool were not regarded as odor-active compounds in apple, but they were carried out in other fruit, such as mulberry [14], peach [48], cranberry [49] and strawberry [51]. Figure 4 indicates the total sum of odor active values (OAVs) for each apple variety over two years. The high total OAVs represent a rich aroma in apple.