In what ways does yeast absorb or interfere with flavors/aromas?

[piojo]

A couple sources told me that flavors and aromas may be volatile, and can be blown off with the CO₂ if the flavor is added during primary. For instance, I understand oak is usually added to mead after primary. Fruit is often added in secondary, as are dry hops. Supposedly these aromas would be carried away by CO₂.

But some yeast are known for preserving aroma and tastes more than others. This can't be because some yeast will produce more esters which mask flavors, otherwise adding stuff in the secondary (as opposed to primary) wouldn't help. How is it that yeast change or eliminate flavors?

When yeast reduce the honey or fruit essence, is it more because of the CO₂ effect, the yeast doing something, or both? Are there guidelines that would help me understand how yeast is likely to affect flavors and smells?

 

[SoCal-Doug]

I'll keep this really topical and not go full chemistry potato... Some compounds (flavors, aromas, whether desirable or undesirable) are volatile, some are not, some are anywhere in-between and it varies. For example, sulfur (generated by many lager yeasts) is very volatile and by venting a carbonated keg intermittently, can remove the sulfur smell.

Yeasts eat sugars (note that is plural, lots of different types of sugars). They poop many different esters (which may be smelled, tasted or both) and this varies greatly by the strain used. They fart CO2 and pee alcohols/fusels (note that is plural also, and in higher quantities alcohols can be tasted).

When you say "preserving aromas and taste", some yeasts are "cleaner" than others, meaning they generate less esters and compounds that we can smell or taste, and leaving us with only the malt, hops, and added flavors and aromas we brewed and fermented with. A specific yeast doesn't really "preserve" anything, it just doesnt stomp on it with other compounds.

Yeast doesn't really "remove" flavors and aromas. Compounds created by yeast can overpower or mask flavors and aromas. A good example of this might be a really pungent Belgian yeast. If you are using one of those, you will need a bit more fruit or whatever to balance it out, than if you were using a cleaner yeast strain.

When you say "reduce the honey or fruit essence", do you mean the sweetness? Because the yeast doesnt kill the flavor but does remove the sugars while adding new compounds to the mixture. If a flavor is based on a sugar, then yes, the yeast may eat that sugar and reduce or eliminate that flavor. Think of it this way... Sweetened vanilla is yummy. Remove the sugar from it and it tastes super nasty but still smells great.

In general, air contact (or in our case, CO2) leaving the beer, is going to take some volatiles with it. We plan for that in our recipes and quantities and through trial and error, recipes are altered to end up with the desired levels of everything when it comes to actually drinking it.

When wood is used, one of the big reasons (but not the only reason) for adding it in the secondary (or post fermentation aging) is so the pores in the wood dont fill up with gack created during fermentation, and/or a coating is not formed on the surfaces of the wood thus eliminated the flavors to be released. When we keg in wood for long term aging, its for flavor AND a filtering effect. Wood acts as a "porosity filter" for certain harsh compounds, as the liquid is contacting the wood. Kinda like a charcoal filter for water, but wood removes different things than charcoal.

Hope this helps a little

 

[piojo]

Yeast doesn't really "remove" flavors and aromas. Compounds created by yeast can overpower or mask flavors and aromas... When you say "reduce the honey or fruit essence", do you mean the sweetness? Because the yeast doesnt kill the flavor but does remove the sugars while adding new compounds to the mixture.

Thank you for the fantastic clarifications and tips! Yes, I had thought flavor compounds were also removed. So if you ferment apple cider dry so it's tasteless, then add more sugar, it will taste like apples again except for the amount that evaporated with the CO₂?

 

[Northern_Brewer]

When you say "preserving aromas and taste", some yeasts are "cleaner" than others, meaning they generate less esters and compounds that we can smell or taste, and leaving us with only the malt, hops, and added flavors and aromas we brewed and fermented with. A specific yeast doesn't really "preserve" anything, it just doesnt stomp on it with other compounds.

Yeast doesn't really "remove" flavors and aromas.

Yes they can - biotransformation of compounds in the wort is a real thing. The most familiar example is ferulic acid from malt, which certain "Belgian" yeast can convert into 4-vinyl guaiacol (4-VG) which gives that clove aroma typical of some wheat beers. It's not a 100% efficient process as ferulic acid can also be converted into vanillin and a bunch of other compounds, and it's not a process that all yeast can perform.

It's a similar story with hop compounds, but it's more noticeable as often the starting compounds taste/smell of something, so you lose certain aspects of the taste/smell if the process happens. Some yeasts don't touch them - Conan and the typical US yeasts like Chico seem to be examples, so give "bright" if sometimes a bit one-dimensional flavours. But some yeasts like T-58 and 1318 seem to knock the total "volume" of hop flavour by about 20%, but in return those flavours become more complex. I've experienced T-58 turning the grapefruit of Chinook into lime, and Scott Janish saw a Citra/Mosaic/Simcoe beer go from "a huge sticky orange lifesavers aroma" with Conan to "more of an orange sherbert with a slight lemon/lime thing" with 1318.

When I mixed T-58 with M36 and WB-06 the intensity went down >80%, but what flavour there was became a complex mix of citrus flavours.

I'm sure this is the sort of thing that's been quite well studied by breweries, but there's not much in the public domain.