“We were part of a European project,” Verpoorte explains. “It was on German wines, trying to identify genes connected with the resistance of the grape and the plants against different fungi.”
For their part of that study, Verpoorte and Choi were sent 100 ml samples of both Riesling and Mueller-Thurgau wines from 2006 and 2007, all of which had been tasted by a panel of oenologists who rated the wines from one to four. The goal: To determine if they could analytically define what makes a good wine “good.”
From each of the samples, they extracted and concentrated the organics with ethyl acetate and then profiled them via NMR, using multivariate data analysis to distinguish the good and bad samples.
In the end, they found most samples contained the same chemicals, only at different levels. “Good” wines contained more longer-chain acids like malic acid and tartaric acid, whereas “bad” wines were enriched in acetic acid and other short organic acids.
Does that mean winemakers now have the chemical secret to fine wine? Not quite, says Verpoorte—as with the rosemary extract, correlation does not equal causation. “The strength of metabolomics is that you can correlate the presence of compounds and good taste. But to really prove it, then you have to start to make all these sort of mixtures and have the same taste panel and see if you take a wine which is high in malic acid and low in acetate, that they think that it's better.”
Still, researchers can use these preliminary data to try and develop better (that is, tastier) wines, say by selecting yeast that produce the longer-chain acids.
Verpoorte and Choi have a track record of studying, shall we say, the metabolites of sin. In 2006, they used proton-NMR to differentiate six different brands of pilsner beer. Two years earlier, it was cultivars of Cannibis sativa—marijuana. But today, Verpoorte and Choi are using their NMR and metabolomic datasets to address a different question: How do plants accumulate water-insoluble secondary metabolites? The answer may lie in a class of fluids called deep eutectic solvents.
From their metabolite profiling studies, Choi and Verpoorte recognized that plants often contain high levels of certain specific small molecules, such as malic acid and choline. Of course, these compounds are expected. But why, they wondered, were they always present at such high concentrations? In 2011, they advanced the theory that perhaps these molecules serve not only as energy storage and metabolic intermediates, but also as a kind of non-volatile ionic liquid, or liquid crystal, which allows plant cells to make and accumulate molecules that are not otherwise soluble in biological systems.
To date, Choi and Verpoorte have identified more than 150 potential natural deep eutectic solvent (NADES) combinations from common cellular metabolites, which could explain the biosynthesis of non-water-soluble products like taxol or cellulose, as well as the storage of high levels of flavonoids in flowers.
Verpoorte has even found something of a “practical” application for this work: While giving a talk in Brazil, Verpoorte explained how a local mixed drink, caipirinha, relies on NADES chemistry. Caipirinha is made by adding lime and ice first, then sugar, and finally cachaça, a sugarcane liquor. Add the ingredients in a different order, and the drink tastes completely different. “Why? Because sugar acts as a solvent. By first mixing the lime with the sugar, you extract very different [terpenoids] than when you mix the lime with the alcohol.”
His Brazilian hosts didn't believe him, so they tested the theory that evening at dinner. “We asked the waiter to make it both ways now and everybody was convinced,” he says. “It's totally different.”WHAT'S IN YOUR STRAWBERRY?
NMR's advantages notwithstanding, many foodomics practitioners still prefer MS for their studies. Valdimir Shulaev, a professor of Biological Sciences at the University of North Texas, recently invested “several million dollars” to establish one such “state-of-the-art” platform at his lab in Denton, Texas.
Located on the first floor of the university's new Life Sciences Complex, Shulaev's lab contains a battery of MS instrumentation, including a high-resolution Thermo Fisher Orbitrap Velos for compound identification, a MALDI-Orbitrap XL for metabolite imaging, a Waters G2 qTOF with ion mobility separation for high throughput metabolite profiling, and more.