Abstract
This paper compares the differences in metabolites of vanilla beans at five different curing stages. Key vanilla flavors, vanillin precursors and main enzymes during the curing process of Hainan vanilla beans were also analyzed. Hundreds of metabolites were detected based on metabolic analyses of a widely targeted metabolome technique, compared with blanched vanilla beans (BVB), sweating vanilla beans (SVB) and drying vanilla beans (DVB), the total peak intensity of cured vanilla beans (CVB) is on the rise. The score plots of principal component analysis indicated that the metabolites were generally similar at the same curing stages, but for the different curing stages, they varied substantially. During processing, vanillin content increased while glucovanillin content decreased, and vanillic acid was present in sweating beans, but its content was reduced in drying beans. Both p-hydroxybenzaldehyde and p-hydroxybenzoic acid showed the maximum contents in cured beans. Ferulic acid was mainly produced in drying beans and reduced in cured beans. p-coumaric acid increased during the curing process. Vanillyl alcohol in drying beans (0.22%) may be formed by the hydrolysis of glucoside, whose conversion into vanillin may explain its decrease during the curing stage. β-Glucosidase enzymatic activity was not detected in blanched and sweating beans, but was observed after drying. Peroxidase activity decreased during curing by 94% in cured beans. Polyphenol oxidase activity was low in earlier stages, whereas cellulase activity in processed beans was higher than in green beans, except for cured beans. This study contributes to revealing the formation of flavor components and the biosynthesis pathway of vanillin.
Highlights
Vanilla flavor has been the focus of many research groups for many years
The profiles were remarkably different at five different curing stages in vanilla
Ranadive reported that vanillyl alcohol was present in glucosidic form in green beans, and the level of hydrolysis of glucosidic derivatives was affected by the curing and β-glucosidase activity [40]
Summary
Natural vanilla flavor is a complex mixture of more than 200 flavor components and is obtained from cured beans of Vanilla planifolia and V. tahitensis [1]. Molecules 2019, 24, 2781 flavor in beans is formed only by a careful curing process that results in vanillin, vanillic acid and other flavor compounds [4]. Researchers have suggested that enzymes such as polyphenol oxidases, peroxidases 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). High-performance liquid chromatography (HPLC) was used to quantify the main vanilla flavors and vanillin precursors in vanilla beans during curing, and the activities of four enzymes that play an important role in curing were determined. We analyzed and interpreted fluctuations of these compounds to reveal the biosynthesis pathway of vanillin
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