In a paper published recently, Marchal et al. (2015) turned to high resolution mass spectrometry (HRMS) and nuclear magnetic resonance (NMR) imaging to learn more about natural wine sweeteners found in oak wood1.
One of the compelling features sought out by wine lovers is the complex taste sensation with every sip. It is this complexity that gives each wine its own signature and more is being learned about the key volatile and non-volatile constituents responsible. Furthermore, as the wine industry seeks to cater to consumers with new and better products, analytical science is becoming more important to discover and characterize novel ingredients.
Oak is a common material used for wine production. It can be used during the ageing process, where the fermented product matures in barrels for specific periods, and also during fermentation. But wood is more than just a material for making storage vessels; winemakers add oak as staves or chips to the maturing wine itself. During the wine making process, the liquid reacts with the physical properties and phytochemicals found within the wood, thus gaining extra qualities and value.
Marchal et al. turned to liquid chromatography (LC)-HRMS to further elucidate the nature and structural analogs of two naturally occurring quercotriterpenosides (QTT) found in oak wood. Knowledge about these natural sweeteners, they surmised, could enhance wine production knowledge.
The researchers took oak (Quercus petraea) heart wood chips and extracted them over seven days by immersion in ethanol. Following this, the team examined the extracts using an Accela U-HPLC system coupled with an Exactive Orbitrap mass spectrometer (both Thermo Scientific). They also conducted mass spectrometric analysis using a Dionex Ultimate 3000 RSLC system coupled with an LTQ-Orbitrap Elite mass spectrometer (both Thermo Scientific) for direct injection of the compounds purified during the experimental workflow.
First, the team screened the oak wood extracts by LC-HRMS. They constructed extracted ion chromatograms (XIC) acquired as fullscan data by Exactive mass spectrometer and performed a targeted search for QTT I and II isomers and derivatives. Using LC-HRMS to guide purification, the researchers took the fractions thus indicated and subjected them to centrifugal partition chromatography (CPC) followed by high performance liquid chromatographic (HPLC) purification.
Marchal et al. confirmed empirical formulae with further LC-HRMS to determine functional group locations, in addition to mono- and multidimensional NMR spectrometry to elucidate structural details.
Once purified and characterized, the compounds with formulae corresponding to QTT isomers and derivatives underwent a sensory evaluation by six wine experts, who classed the new isolates according to their sweetness. Two of the new isolates ranked at 5/5 for sweetness.
In evaluating all data generated from this investigation, the research team proposes that the workflow enabled the discovery of two novel sweeteners from the oak wood samples. With LC-HRMS purification and structural elucidation, Marchal et al. note that only the two isomers containing either a glucosyl-or a galloyl-group were classed as sweet by the wine experts. They suggest that LC-HRMS-guided purification method described in their paper is a useful tool for novel compound discovery in complex food mixtures.
1. Marchal, A. et al. (2015) “Development of an analytical methodology using Fourier transform mass spectrometry to discover new structural analogs of wine natural sweeteners“, Analytica Chimica Acta 853 (pp.425–34)