Social microbes and Schizosaccharomyces pombe

 

If there’s been a theme to the wine microbiology research of the past few years, it’s been microbial communities. Don’t just study one yeast or bacteria at once; look at an environment’s microbial population. And if there’s been a supporting theme, it’s been non-Saccharomyces yeast. Don’t just look at Saccharomyces cerevisiae; pay attention to at least some of the other, marginalized members of the microbial community, and ask what they can do for you.

Those two themes are obviously related. Studying microbial communities means noticing all of the auxiliary players in the environment. Noticing those players usually leads to asking what they’re doing and then to asking how you can exploit them. In another way, though, those two themes don’t overlap half often enough. Plenty of studies of non-Saccharomyces organisms keep on plodding on in the old microbiology tradition of poking and prodding at one or a few species as though they’ll work alone outside the lab.

Very forgivable in one sense. When we don’t know much about an organism in the first place, sussing out its individual characteristics before querying how it behaves in mixed company doesn’t seem unreasonable. It’s also fair to say that plenty of winemaking involves making an effort to kill all existing microbes before inoculating one selected S. cerevisiae strain that’s supposed to work alone. Then again, single-microbe studies remind me of studies of individual primates held in solitary captivity, which are not only deeply unethical but not very useful. What primate, humans included, is going to behave normally when held in solitary confinement? I’m not claiming that solitary microbe studies are unethical, or that they do harm to the microbes involved, but we have plenty of evidence that microbes are social.* Data from solitary confinement studies is limited.

So a new study on Schizosaccharomyces pombe is heading in an interesting direction, but yields data with some limitations for winemaking.

Is S. pombe a spoilage organism? That’s like asking whether dandelions are weeds: yes, in the lawns of a golf course; no, when you’re growing them for salad greens. S. pombe produces unpleasant quantities of acetic acid. It also efficiently (and even completely) metabolizes malic acid. Scott Labs sells S. pombe “teabags” that can be dropped into overly acidic tanks or barrels and then fished back out again, after malic acid has been degraded but before volatile acidity gets out of hand. New research (open-access article) has considered whether some S. pombe strains, carefully selected for low acetic acid production, might be suitable as primary fermentation organisms to be used instead of S. cerevisiae rather than afterwards. The team was able to find several low acetic-producers, able to ferment a must to dryness (albeit they tested final alcohol concentrations in the 12-12.5% range), and still able to simultaneously metabolize malic acid. Their perfunctory sensory testing, however, pretty much only judged for major faults: acidity, reduction, acceptable aroma. So when the researchers conclude that these strains might be a good option for high-acidity musts instead of malolactic fermentation, they’ve yet to account for whether that solution produces a delicious product or merely an acceptable one. Still, these strains might be incredibly useful in combination, or when a vat of something undrinkably acidic needs to be made inoffensive enough to be blended away into something else. But how do these microbes behave in company, when asked to cooperate on the job of making a drinkable wine?

I hope that this project steps forward in two directions. One: better sensory analysis. Two: what happens when S. pombe and S. cerevisiae (and perhaps some other bugs) are asked to play together.


*The Foster Lab at Oxford is up to interesting research on cooperation between microbes and other species. Here’s another (albeit dated; 2007) excellent resource on microbial sociability, from Annual Reviews in Ecology, Evolution, and Systematics. Unfortunately, it’s also behind an academic publisher’s paywall.

Tracing down tobacco aromas in aged brandies

I’ve become so accustomed to fermented beverages having flavors completely unrelated to their starting ingredients that I all too easily forget that there’s anything strange about it. And then I introduce someone to wine tasting for the first time and find myself having to explain that, yes, the wine tastes like cherries and no, they don’t put cherries in the wine to get it to taste that way. (But also sometimes that, yes, the wine tastes like oak, and yes, they do put oak in the wine, or wine in the oak, to make that happen.)

Wine and aged brandies, like Cognac, sometimes taste like tobacco. No one adds tobacco to them.* What happens instead is an example of what you could liken to chemical convergent evolution. Brandies can indeed contain odiferous molecules also characteristic of tobacco flavors. But those molecules appear to come from the breakdown of compounds present in grapes, not from any exposure to tobacco (and, needless to say, the tabanones in tobacco don’t come from grape-derived compounds).

Megastigmatrienone is more conveniently called tabanone. Or, more properly, megastigmatrienones are are more conveniently called tabanones, because the designations apply to a group of similar molecules. They seem to show up in wine because carotenoids – the highly pigmented class of antioxidant molecules you’ll recognize from it’s best-known carrot-colored member, beta-carotene – can become megastigmatrienones under appropriately acidic conditions. The precise pathway from grape-derived carotenoids to tobacco aromas remains something of a mystery. It seems that brandies, distilled from grape wine, end up with concentrated amounts of tabanones or precursors which then have an extra chance to develop into tabanones during barrel aging.

That mystery makes all the more interesting a recent study to quantify tobacco aroma molecules in Cognac and Armagnac**. Most of the (many) examples they tested contained some tabanones. Concentrations varied widely, but the Armagnacs averaged higher than the Cognacs and – evidently to everyone’s surprise – the aged rums thrown in for comparison ranked even higher than the Armagnacs. Much higher, in fact, even though the molasses from which rum is distilled doesn’t contain significant amounts either of tabanones or of the identified precursor molecules present in grapes.

So, why is this interesting?

  1. Tobacco aromas in aged spirits are evidently not just coming from molecules originally present in grapes.
  2. The oak barrels used for aging rum, as well as various brandies, probably contributes substantially to tobacco aromas in those spirits by leaching precursor molecules out of the wood and into the spirit and providing favorable conditions for their development.
  3. The researchers behind this study seem to think that their new-and-improved means for quantifying particular variants of tabanones could be useful for distinguishing Cognac from Armagnac, presumably to defend against passing one kind of spirit off as another. Looking at the enormous variation in tabanones between one kind of Cognac and another, or one kind of Armagnac and another, this suggestion seems frankly ridiculous. Chemistry backs up tasting experience: individual bottlings vary dramatically in the presence or absence of tobacco aromas as a defining characteristic. Yes, Armagnacs beat out Cognacs on average, but averages don’t help distinguish individuals when individual variation is so high. A more interesting use of this research, I think, is the starting point it provides for thinking about how barrel-aging adds to tobacco aromas, in spirits and possibly in wines. I suspect that the people who produce these spirits already have a lot to say on that point. Joining their knowledge with some well-placed chemical analyses could improve everyone’s understanding of how tobacco aromas happen and how to manipulate them.

 

* Ewwww.

**Camper English has drawn up a helpful comparison between Cognac and Armagnac at Alcademics.com.

Waste not? Capturing every last bit of aroma potential from fermentation

The aroma of baking brownies arouses two categories of responses. Summary of Category One: “Wow, that smells delicious!” Summary of Category Two: “Oh no, just smell all of that chocolate flavor escaping into the air that won’t be in the finished brownies any more.” This new research is for people who empathize with category two.

As Saccharomyces cerevisiae ferment grape sugars into ethanol, they produce massive amounts of carbon dioxide. Most of that CO2 gas escapes from the top of the fermenting liquid (small amounts remain dissolved in the wine) and, with it, escape wine aroma molecules. One of the two defining features of an aroma molecule is that it’s volatile, which is to say that it evaporates readily; you can’t smell a molecule if that molecule can’t make its way into your nose. (The other defining feature of aroma molecules is their ability to activate sensory receptors; even after a molecule makes its way up your nostrils, it still has to trigger a sensorineural response.) Volatile molecules will escape from a stationary liquid to some degree, but they escape a whole lot faster when the liquid is moving (hence our habitual practice of swirl, then sniff), and even faster still when bubbles forming inside the liquid carry those volatile molecules up from the inside out.

In short, the logic of physics and chemistry have it that a lot of aromatic molecules are lost to the air during fermentation. In theory, those molecules could be captured and deposited back into the wine after fermentation to yield a more aromatic final product.

This idea occurred to a bunch of Italian food scientists, who gave it a try with two sangioveses and a syrah and have published the results in the American Journal of Viticulture and Enology. It seems to have worked pretty well. They attached a condenser to the top of their stainless steel fermentation tanks and collected the vapor rising from the tanks as a liquid that they could then dose back into the wine after fermentation was complete. Which is, of course, precisely what they did. A sensory panel was universally able to identify which wines had received more and less condensate and which had received none at all though, to the study’s detriment, only eight test-sniffers* were involved and they weren’t asked to report on what they thought about the wine’s aromas or on how the wines tasted. 

There’s a certain appealing efficiency to the idea of capturing a heretofore lost byproduct of fermentation – all of those lovely aromas of fermentation, plus a healthy dose of alcohol (the liquid condensates contained about 24% ethanol) – and doing something with them. The condenser itself is a simple apparatus consisting, essentially, of a heat exchanger. Adding the condensate back to the wine is an obvious use for the stuff, but it’s certainly not the only one, and so in effect this is an experiment in increasing our efficient, sustainable use of available resources, like turning vine prunings into biochar or using grapeseed flour as a nutritional supplement. Though, the folks with Category One brownie aroma responses might have a different perspective.

 

*Eight test-sniffers who were described only as “panelists,” and who were therefore most likely  happened to be around the lab when testing needed to happen. Since the panelists’ job here was limited to answering the questions, “which of these wines doesn’t smell like the others?” and “which smells most intense?” it’s not frightfully important whether they were trained sensory testing experts or wine professionals or ordinary-Jane wine consumers. If and when this idea is taken into “which of these wines is most appealing to you?” territory, someone will need to decide whether they care about the responses of wine experts, random occasional drinkers, or both.