## 2 Results and Discussion
### Incubation Temperature and Time  
The incubation temperature and time affect the extraction efficiency and selectivity of sweet potato volatiles into the headspace. In this study, the sweet potato samples were heated in the process of incubation to reach the cooked and equilibrium state. A factorial design was performed to evaluate the effect of incubation temperature and incubation time to the sweet potato samples. Three incubation temperatures of 60 \({}^{\circ}\)C, 70 \({}^{\circ}\)C, and 80 \({}^{\circ}\)C, and three incubation times of 5 min, 10 min, and 30 min were investigated. It could be easily seen that with the increase in the incubation temperature and incubation time, both the total peak area and the number of identified volatile compounds gradually increased (Figure 2). The total peak area increased from 5.1 \(\times\) 10\({}^{7}\) (60 \({}^{\circ}\)C, 5 min) to 1.36 \(\times\) 10\({}^{8}\) (80 \({}^{\circ}\)C, 30 min) and the number of identified compounds increased from 42 (60 \({}^{\circ}\)C, 5 min) to 55 (80 \({}^{\circ}\)C, 30 min). Then, the identified compounds were analyzed. The majority of analytes (95%) detected in 5 min at 60 \({}^{\circ}\)C gave a relative standard deviation (RSD) below 20%. When the temperature increased to 70 \({}^{\circ}\)C and incubated for 30 min, RSDs of the relative content of 52.7% analytes were lower than 20%. The volatile compounds released by the samples incubated at 80 \({}^{\circ}\)C for 30 min were more stable, and 88.7% analytes gave a RSD lower than 20% (data not shown).  
The extraction efficiency increased with temperature and time, and was the highest at 80 \({}^{\circ}\)C for 30 min, the upper bound of our incubation parameter. Furthermore, strong sweet and honey notes were released by the samples incubated at this condition, which is similar to the smell of cooked sweet potato. To verify whether 80 \({}^{\circ}\)C for 30 min was the most suitable incubation condition, we extended the incubation time to 50 min and 60 min  
Figure 2: Peak area and number of volatiles detected in different temperature and time of incubation. Means with different lowercase letters indicate significant difference with \(p\) < 0.05.  
at an incubation temperature of 80 \({}^{\circ}\)C. As a consequence, the total peak area captured was 1.30 \(\times\) 10\({}^{8}\) and 1.02 \(\times\) 10\({}^{8}\), and the number of identified compounds were 54 and 53, respectively (data not shown). The extracted compounds and extraction precision were mostly the same as that incubated for 30 min. Incubation temperatures higher than 80 \({}^{\circ}\)C were not tested further since this temperature is enough for the starch of sweet potato to gelatinize. The aldehydes could already been observed as the most abundant volatile compounds in sweet potato samples and the higher temperature could have a strong modification on the pattern of aldehydes [8]. On the other hand, the progressing of analytes absorbed from headspace onto a fiber coating is an exothermic process, which can be hindered by high temperature and the partition of analytes would also be impacted [32]. Hence, incubation at 80 \({}^{\circ}\)C for 30 min was selected for further analysis.