Episode Summary

Dr. Kate Howell, Associate Professor of Food Chemistry at the University of Melbourne, Australia discusses how microbes impact the flavor and aroma of food and beverages and shares how microbial interactions can be used to enhance nutritional properties of food and beverage sources.
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Ashley's Biggest Takeaways

  • Saccharomyces means sugar-loving fungus.
  • Humans have similar olfactory structures and mechanisms as insects and are similarly attracted to fermenting or rotting fruits produced by Saccharomyces.
  • Research has shown that insects (and humans) prefer yeasts that produce more esters and aromatic compounds.
  • Palm wine is a product that is made from sap collected from palm trees (palm sap) across the tropical band of the world.
  • Fruity flavors appear to be less important to persistence of Saccharomyces strains in an Indonesian palm wine fermentation.
  • This may be because palm wine fermentation is very quick, generally 1-3 days often, with a maximum of 5 days for fermentation to be conducted.
  • Wineries, on the other hand, ferment annually (one fermentation per year/vintage), when the grapes are right. Grape wine fermentations can take 7 days to 2 weeks to complete.
  • So different selections likely take place between the 2 fermentation products.

Featured Quotes:

When we start drawing our lens on how microbes produce food for humans, we're coopting a process that happens quite naturally. Here I'll start off talking about Saccharomyces cerevisiae, the main fermenting yeast in food and beverage production, because it's one of the most studied organisms and was the first eukaryote to be sequenced.

Saccharomyces cerevisiae, as the name implies, loves sugar, and it flourishes when there's a lot of sugar in the environment. Where is sugar found? In fruits, and that's done quite deliberately, because fruits develop sugars and flavors and aromas to attract a birds or insects or anything else that can carry their seeds elsewhere for dispersal.

Now, Saccharomyces lies dormant in the environment in a spore before it encounters a sugar-loving environment. And then it replicates very quickly and tends to dominate fermentation. Humans have coopted that into our kitchens, into our meals, into our lives, and we use that process to produce food.

As Saccharomyces starts to use this sugar, it balances up its metabolism. And as it does this, it produces aromas. These aromas have a lot of important characteristics. Humans love them, but insects also love them too.

I've been interested in the yeasts that are found naturally in sourdough starters. Sourdough is a really interesting system. Because you've got yeast and bacteria interacting with one another.

One of the things we are collaborating on with colleagues in France at Inrae, Dr. Delphine Sicard, is to understand some of the non-Saccharomyces yeasts that are naturally occurring in sourdough starters. So here we're looking at a collection of a yeast called Kazachstania humilis and trying to understand how it has adapted to the sourdough environment, how its sustained over time and how different global populations differ to one another.

And this, of course, is of interest to the baking industry because not only do artisanal bakers have sort of an undiscovered wealth of biodiversity in their starters, baking companies also have an interest in using different sorts of flavors and bread for the commercial markets.

The connection between a chemical profile and a person’s sensory preference isn't something that's complete and direct. So, in every method that we use, there's always caveats, but we try to correlate it. Let's start off with the chemical characterization. We use headspace sampling, analytical chemistry, separation with gas chromatography and identification with mass spectrometry.

And we use different 2-dimensional methods to be able to understand what the very small compounds are, and to be able to identify them. We can semi-quantify these to be able to make comparisons between different fermentations.

We know from wine fermentations and understanding preferences of wine that, in some cases, a particular increase, or an abundance of a particular compound, can be extremely attractive. And that might depend on the style of wine.

What we've discovered through this process is that different people prefer different flavors. Makes sense, doesn't it? We like different things. But some really interesting results from our lab, show that people from different cultural backgrounds have different preferences. And here we're using here in Melbourne, I'm very lucky to work with some very talented postdocs and Ph.D. students from China, who have very different preferences for wine than an Australian does. Of course, Australians are quite heterogeneous in their in their cultural diversity as well. But there's certain flavors that our Chinese colleagues tend to prefer. So we decided to investigate this a little bit more.

So for this study, we recruited wine experts from Australia, actively working in the wine industry, and also wine experts from China, working in the wine industry, and brought them to campus and ask them to rate their preferences on particular aromas and flavor characteristics that they noted in a panel of wines. These were very high-quality wines. We knew with wine experts, we couldn't just give them our loved wines, for example, which can be a little bit patchy quality wise. We asked them to rate their preferences, and then we collected saliva samples.

The saliva samples were really interesting. We looked at 2 different aspects. We looked at the proteins that were present in the saliva samples. And we also looked at the oral microbiome. So the salivary microbiome—the bacteria, in particular—that are present. We found some really interesting things. And this has sparked a big area in our lab.

So while the main enzymatic activities in the different groups of participants were quite similar—so esterase activity, Alpha amylase activity were similar—we found that there was a difference in the abundance of proline rich proteins and other potential flavor carrying compounds. Now, this is quite speculative. We'd like to know why this is the case. And so we're delving a little bit further into this area.

What we do know though is that the abundances, especially if these proline rich proteins, is correlated with how people perceive the stringency. Now stringency is one of those characteristics in wine which is quite difficult to appreciate. It’s a lack of drying characteristic on the tongue and in the mouth and oral cavity. Some people find it quite attractive, others don't.

But we found that the abundance of these polyproline-rich proteins correlates with stringency. This is, in fact, found in other studies because proline-rich proteins interact with polyphenols in the wine, and precipitate, which changes the sensation of astringency in the oral cavity. So here we've got a nice link to protein abundance and how people perceive flavor. But we're talking about microbiology, so maybe I should delve into the microbiological aspects of these studies as well.

In that particular study that I'm referring to, we used wines that were naturally fermented, and that's the other variability that we need to consider when we think about wines from different areas. So, a natural fermentation, in a wine sense, is the grapes are harvested, and whatever microflora is present on the grapes will just ferment, and we often don't know what the main fermenting parties are. But if you contrast that with a majority of commercial wine that's produced, mainly in Australia, but also worldwide, it's inoculated with a selected strain of Saccharomyces or maybe 2 selected strains of Saccharomyces, and that tends to produce a fairly similar flavor profile, regardless of region.

So, you can flatten out geographical characteristics and indications of flavor by inoculating a particular strain of yeast to ferment. That's not true with a natural fermentation, because that's conducted by the yeasts, and also the bacteria which just happened to be in the environment. So, I agree with you there is a lot of regional variation with wine flavor. And we can correlate that with regional diversity of yeast, but only if the wines are naturally fermented not if they're inoculated with a selected strain.

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Dr. Kate Howell