Shaking the tea leaves is the key manipulation to making oolong tea. It contributes to the formation of flavor and fragrance in oolong tea. The dynamic variations of catechins and volatile organic compounds (VOCs) during the shaking process were investigated. The results showed that the contents of epicatechin, epigallocatechin, epicatechin gallate (ECG), and epigallocatechin gallate (EGCG) first decreased after the shaking and then increased to the initial value before the next shaking. Geraniol, linalool and its oxides, and phenylethyl alcohol showed similar variations. The contents of trans-β-ocimene, 1H-indole, and 3-hexenyl hexanoate increased after the second or third shaking (the late fermentation stage). However, the contents of aldehydes showed an opposite trend to other VOCs. The abundance of phenylethyl alcohol was positively related to the content of ECG and EGCG ​during fermentation, whereas the abundance of cis-3-hexenal was negatively related to the content of ECG. The correlations between catechin and VOCs indicated that shaking affected the chemical transformation of the compounds in oolong tea.

Avocados may suffer from short-term waterlogging stress when exposed to high temperatures and heavy rainfall during the summer in Taiwan. We compared the waterlogging responses of own-rooted and grafted seedlings of two Taiwan cultivars, ‘Black-Beauty’ and ‘Hung-Hsin-Yuan’, by stomatal conductance (gs) and chlorophyll fluorescence parameters. Four-day waterlogging and four-day post-waterlogging recovery periods were investigated. Both gs and Fv/Fm of own-rooted seedlings of two cultivars were significant reductions in response to short-term waterlogging. The grafted seedlings on the same cultivar rootstock were evaluated by gs and Fv/Fm during the growth and the growth cessation periods, respectively. The combined responses of gs and Fv/Fm under short-term waterlogging showed that ‘Black-Beauty’ was sensitive to stress because of decreased gs after waterlogging or decreased Fv/Fm after the two-day recovery period. ‘Hung-Hsin-Yuan’ showed more tolerance to waterlogging stress, especially during the growth cessation. This indicates that the vegetative dormancy may affect the evaluation of the stress response of avocados. Our results revealed that gs and Fv/Fm can be effective indicators in the four-day waterlogging of avocado, and the growth status of avocado seedlings should be considered during stress-tolerant variety selection.

The delicate aroma of Bao-chung tea comes from oxidation, followed by fixation in the pan-firing step. Traditionally, the timing of pan-firing has been based on odor perception by tea masters and lacks relevant scientific research. Pan-firing at three different green-note intensities and three stirring sequences was used to explore the relationship between the compositions of volatile organic compounds (VOCs) before pan-firing and in the finished tea. Pan-firing decreased green leaf volatiles and increased the ratio of terpenoid volatiles. The characteristic VOCs of the finished tea were highly related to VOCs before pan-firing (R² = 0.97). Principal component analysis revealed that the traditional judgment of the pan-firing step is based on nonanal, β-linalool, and cis- and trans-linalool oxides. The timing of pan-firing is crucial for VOCs, and VOC composition before pan-firing can be used to predict desired tea aroma.

The operational duration of shaking tea leaves is a critical factor in the manufacture of oolong tea; this duration influences the formation of its flavor and fragrance. The current method to control the duration of fermentation relies on the olfactory sense of tea masters; they monitor the entire process through their olfactory sense, and their experience decides the duration of shaking and setting. Because of this human factor and olfactory fatigue, it is difficult to define an optimum duration of shaking and setting; an inappropriate duration of shaking and setting deteriorates the quality of the tea. In this study, we used metal-oxide-semiconductor gas sensors to establish an electronic nose (E-nose) system and tested its feasibility. This research was divided into two experiments: distinguishing samples at various stages and an on-line experiment. The samples of tea leaves at various stages exhibited large differences in the level of grassy smell. From the experience of practitioners and from previous research, the samples could be categorized into three groups: before the first shaking (BS1), before the shaking group, and after the shaking group. We input the experimental results into a linear discriminant analysis to decrease the dimensions and to classify the samples into various groups. The results show that the smell can also be categorized into three groups. After distinguishing the samples with large differences, we conducted an on-line experiment in a tea factory and tried to monitor the smell variation during the manufacturing process. The results from the E-nose were similar to those of the sense of practitioners, which means that an E-nose has the possibility to replace the sensory function of practitioners in the future.

Despite extensive studies, the relationship between the quality/quantity of tea odorants and oolong tea quality remains unclear. To investigate the key components affecting Bao-chung tea quality, we collected samples of different grades from a tea-tasting competition and determined the content and composition of volatile components and individual catechins using gas chromatography–mass spectrometry and high-performance liquid chromatography. We used an electronic nose (E-nose) to collect odor component signals and established a quality recognition model. The different tea grades did not significantly differ in catechin content, but their specific odor intensity and proportion of odor components varied significantly. Linear discriminant analysis showed that the intensity and proportion of volatile organic compounds could be used for distinguishing the different grades of Bao-chung tea. By combining different quantities of indole, linalool, and butanoic acid and proportions of p-cymene, cis-β-ocimene, nonanal, allo-ocimene, cis-jasmone, and α-farnesene, the ability to distinguish among Bao-chung tea grades was significantly improved. Our results revealed that the quality of Bao-chung tea should be evaluated based on the combined perception of odor component intensity and proportion rather than solely relying on the concentration or composition of specific compounds. Therefore, individuals can judge a Bao-chung tea grade based on the combined perception of odor component intensity and proportion. The E-nose can be used to identify Bao-chung tea grades based on its ability to determine the odorant composition.