## 4 Conclusions  
In this study, the differences in metabolites of vanilla beans were compared at five different curing stages. Hundreds of metabolites were detected based on metabolic analysis of a widely targeted metabolome technique. Blanched vanilla beans (BVB) exhibited the most abundant metabolites, and its total peak area was the highest, whereas the peak areas of sweating vanilla beans (SVB) and drying vanilla beans (DVB) showed a downward trend. Compared with blanched vanilla beans (BVB), sweating vanilla beans (SVB) and drying vanilla beans (DVB), the total peak area of cured vanilla beans (CVB) was on the rise. The score plots of principal component analysis indicated that vanilla beans from the same curing stages were clustered together, revealing that the metabolites were generally similar. On the contrary, vanilla beans from the different curing stages were rather scattered, implying that the metabolites were quite different.  
The contents of vanillin, vanillin acid, \(p\)-hydroxybenzoic acid and \(p\)-hydroxybenzaldehyde were also detected. The highest content (2.69%) of vanillin was observed in cured beans. Vanillic acid content was 10 times lower, whereas \(p\)-hydroxybenzoic acid and \(p\)-hydroxybenzaldehyde contents were 100 times lower than vanillin content in cured vanilla beans. \(p\)-hydroxybenzoic acid content increased, while vanillic acid decreased after the sweating stage. \(p\)-hydroxybenzaldehyde was produced mainly in the period of blanching and curing.  
The precursors of vanillin were also determined by the high-performance liquid chromatography. The hydrolysis of glucovanillin occurred during sweating and drying period. Vanillic acid content was at its maximum in sweating beans, and its conversion into vanillin mainly occurred in the drying stage. The formation of vanillin and its glucoside from ferulic acid and its glucoside may take place in the periods of curing and blanching, respectively. By the detection of \(p\)-coumaric acid, the generation of \(p\)-coumaric may occur in the period of drying. Vanilllyl alcohol may be formed by the hydrolysis of its glucoside during drying and converted into vanillin during curing.  
\(\beta\)-Glucosidase lost all of its activity during blanching and sweating, but reappeared during drying. The activity in cured beans was 25% of that in fresh beans. Compared with \(\beta\)-glucosidase, peroxidase, polyphenol oxidase and cellulase maintained a certain level of activity in the developing vanilla bean. Peroxidase activity decreased during the curing process; activity was lowered by 94% of that in green beans. Polyphenol oxidase activity at the earlier curing stages was lower than that in drying and cured beans. Cellulase activity increased after blanching, with activity in blanching, sweating, and cured beans being 232%, 210% and 105% of that in fresh beans. It is beneficial to improve the quality of vanilla beans by controlling the activity of these enzymes at the appropriate curing stage.  
Software, Y.C. ([PERSON]); formal analysis, Y.C. ([PERSON]), Y.H. and Y.C. ([PERSON]); investigation, Y.C. ([PERSON]); writing--original draft preparation, Y.C. ([PERSON]), Y.H. and K.A.; writing--review and editing, F.G.; supervision, F.G.; project administration, F.G.; funding acquisition, F.G. This research was funded by the National Natural Science Foundation of China, grant number 31671848 and 31801499.  
The authors declare no conflict of interest.