## 1 Introduction  
Vanilla flavor has been the focus of many research groups for many years. Natural vanilla flavor is a complex mixture of more than 200 flavor components and is obtained from cured beans of _Vanilla plantifolia_ and _V. halitensis_[1]. These form the highest-priced flavor ingredient in food (60%), cosmetics (33%) and aromatherapy (7%) [2; 3]. Mature green vanilla beans are flavorless, the characteristic vanilla flavor in beans is formed only by a careful curing process that results in vanillin, vanmillic acid and other flavor compounds [4]. Among these odor-active compounds, vanillin is the greatest contribution to the aroma of beans [5]. Nevertheless, vanillin content is usually ca. 1-2.5% _w/w_ in cured beans. The annual global sales of vanillin have reached more than 15,000,000 kg in 2010, less than 1% of this value was obtained from vanilla beans [6; 7]. Therefore, the synthesis pathway of vanillin has attracted much research interest, and different pathways have been suggested [6; 8; 9; 10; 11]. However, the synthesis pathway of vanillin in the beans has not been conclusive as hitherto demonstrated [11; 12; 13; 14; 15]. Improvement of natural vanillin production remains a major obstacle [16]. Many compounds were involved in the formation of vanillin. Previous research showed that vanillin and vanillic acid were derived from ferulic acid via radiolabelling methods [17]. [PERSON] and [PERSON] suggested a more complex route in which vanillin was formed from caffe acid [8; 9; 10]. With the development in analytical chemistry, increasing compounds from other plants have been identified as precursors of vanillin, such as _p_-coumaric acid [18], glucovanillin [19], _l_-phenylalanine [11] and vanillyl alcohol [20]. [PERSON] et al. reported the results of comprehensive metabolomic fingerprinting of vanilla fruits prepared from the curing process and constructed seven pathways of vanillin biosynthesis, including four new pathways, namely, glucose, cresol, capsaicin and vanillyl alcohol pathways [21]. The influence of enzymes and microorganisms on flavor formation in the vanilla curing process has been revealed recently [22; 23; 24; 19]. \(\beta\)-D-Glucosidase plays the most important role in the curing process. [PERSON] et al. found that colonizing _Bacillus_ isolates produced \(\beta\)-p-glucosidase, which mediated glucovanillin hydrolysis and influences flavor formation [25]. In addition, researchers have suggested that enzymes such as polyphenol oxidases, peroxididases and proteases contribute to flavor development during curing of _V. planifolia_ beans [26]. The main analytical techniques used in metabolomics analysis are nuclear magnetic resonance (NMR) spectroscopy, gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS). These techniques are suitable for the analysis of different concentrations and various physicochemical properties of metabolites [27]. Previous research showed that 22 primary metabolites and 55 secondary metabolites were identified in wines by NMR and GC [28]. [PERSON] et al. made it possible to detect several hundred compounds by GC-MS in wine [29]. However, NMR is the least sensitive of the three techniques. In addition, GC-MS is susceptible to sample derivatization [30]. Therefore, more information can be obtained by LC-MS, which has high sensitivity and is capable of detecting various metabolites [31]. [PERSON]'s research on quantification of compounds based on LC-MS and NMR analysis revealed that vanillin could reach 24% of total vanillin content after eight months of development in vanilla beans [32]. Earlier work indicates that the metabolic changes of vanilla beans and vanilla beans by LC-MS to elucidate the biosynthesis of vanillin. Seven pathways for the biosynthesis of vanillin were constructed [21].