This is a great question.. I think most people using Brett are wondering this, I'll do my best to answer from my perspective of what I've seen during my research and in the beers that have been and released at Crooked Stave.
Flavor is generally the result of two compounds coming together. In the case of an ethyl ester which is what we are most familiar with it's basically an ethyl alcohol compound and an acid. Think of this in general terms as it's more then that. So these two compounds must first have been created on their own as alcohol and an acid. Those flavors are produced by yeast during fermentation and the building blocks for them came from the raw materials used. In the case of beer it is the malt used and hops. In these beers we are generally thinking more about the malt.
The characteristic funky flavors from Brett are polyphenolics like 4-ehtyl guiaicol and 4-ethylphenol. If you imagine there are 1000's of Brett strains (because there are they are just not all sold by yeast companies as it's not used as much as Saccharomyces) the ability for the 4-ethyl derivatives to be produced from the 4-vinyl derivatives is very variable. Not all strains can and will produces them, and some will but not much. So the 4-vinyl derivatives ca be made by normal brewers yeast. think belgian phenolic strains. They will produce the clove and musty, spicy aromas. Well we tend to use these strains before pitching Brett in Secondary. Brett then takes the 4-vinyl and produces 4-ethyl derivatives. These are stronger more flavor active compounds in smaller dosages and the Brett wont work as hards to make them. The 4-vinyl derivatives are produced from p-coumeric acid and ferulic acid which are hydroxycinnimic acids that can be brought out from the cell walls in malt. So higher temps and protein rests will bring the pre-cursers out.
So if the pre-cursers exist then you have the potential for the funkier secondary characteristics to shine through in secondary. So it is based on raw materials, brewing techniques, the compounds made available and the ability for the first strain and the second strain to make the necessary compounds that you in the end have what are considered the characteristic Brettanomyces flavors. So it has many aspects.
Now if you use Brett in primary, the strains in my opinion are more concentrated on fermentation and other metabolic processes that the more delicate esters are produced and the energy is not put into producing 4-vinyl guiaicol. Some strains mind you will not even produce the 4-vinyl or if they do they don't go all the way and produce the 4-ethyl. And in primary even if they do they not have a high affinity to produce the compounds so the other esters and flavor compounds shine through. Were as in secondary the 4-vinyl phenolics already possibly exist and the 4-ethyl are easier to produce because the yeast is less concentrated on fermentation and it's esterification process is lowed so it has reserve to do other various sorts of metabolism. Also with the esterification going on the conditions are right to produce higher amounts of the phenolic derivatives giving a more traditional funky aroma.
If you take a minute and think about an all Brett primary.. There are still the funky type aromas but they are layered or masked to the point where we might skip over them. We are so overwhelmed by these unique fruity aromas and the young character of the beer that we miss the underlying aromas.. Over time depending on the strain they will come out but again could be masked.
In looking at the beers I've done at Crooked Stave. I know exactly the strains I'm using as I cultured and selected them from many sources. I have pitched a few strains at a time into primary and then after harvested from primary and used in a secondary fermentation for our wood-aged Saison. I got almost the exact character in primary in the secondary beer. The wood-aged Saison tasted like the other primary fermented beers having fruity characteristics but not the funky. To me that shows the importance of strain selection. Those yeasts are behaving almost identically in primary as in secondary. Now I have selected my strains to be more fruit forward and exhibit less phenolic funkyness so I guess I should have expected that. I have found other strains which affect aroma and not as much flavor in more experimenting with secondary Saisons. It all depends on the yeast. Each strain I have behaves differently and from experiments that we are participating in at Avery, AC Golden, Epic and at Crooked Stave I'm always seeing that it is all about the strain and they will behave very similar given similar brewing techniques and understand of what compounds are or are not present for the Brett to play with. If you try Epic's Elder Brett , Saison-Brett we did. The oud Beersel and Giridin Gueuze strain I selected are meant to be funkier an the nose has an almost lambic phenolic funk to it. But the flavor was not affected as much by the brett as it was the barrels and the peaches previously in those barrels. Those same strains behave that same way in primary as well giving the same aroma with a little more tartness in the flavor then ester aromas.
I would say in my opinion the difference in flavors is not created by stressing due to sugar levels. Brett is perfectly happy with out sugar. It can just as easily metabolize ethyl alcohol and other nitrogen compounds as energy source so the lack of sugar does not stress Brett. But the compounds which exist in the first place will play the role in what flavors are produced and since active fermentation is not occurring it will be esterification and possibly phenolic derivatives being produced.
So as to the gravity dropping. I'm seeing what looks like full fermentation occuring in 2 weeks, 3 at the most but just one time and was most likely due to my pitching rate.. Again I have also selected strain which primary ferment strong and attenuate well quickly. So this is to say a 1.060 beer is going down to 1.016 or 1.014 over 2 weeks. Vinnie sees the same thing. Allagash sees it in one week sometimes and I've had this happen on occasion. But that is not the full story. I and many other brewers in the industry are seeing further attenuation over an 8-10 week period. I saw activity stop in my first couple of Wild Wild Brett series beers at 1.012 even and crashed the fermenter. Vinnie crashes for 2 weeks I was a bit rushed so mine was less. I have more yeast in my beers and as a result the beers are now over carbonated. This is also due to not filtering which leaves large carbohydrates, and many compounds which cause nucleation sites. My bottles are not as high carbed as Russian River yet their beers are perfect and don't gush... Anyways the point is that attenuation does continue. depending on the strain it could take some time. Avery once saw it take 3 months for a beer to drop below 1.030.. It's all about the strain, healthy pitch rate, etc..
I'm now analyzing how low are beers have gone so I can see where my Bretts are stopping at. My guess is it's somewhere around 1.007 to 1.004. Thats a big difference when you look at what is left. Without the data it's hard to say exactly what is happening but I know the Brett is continuing. I didn't think that at first but after also speaking with other brewers everyone is seeing it. I've also noticed that it seems that Brett will spike in activity once some top pressure is placed on it. Whether that be in the bottle or in the tank. To me this is the most important thing I'm dealing with and trying to figure out when these beers are done. What to carbonate to without getting over carbonation and when to release. Ideally we'd be making 100% Brett beers in 1 month right? Maybe it's not possible but I think there is still away or to let carbonation happen in the bottles. Force carb test have been extremely variable though..
As, far as funky flavors.. I think they could come out more in the tail end and through extended aging. It's hard to say as going back over bottles I'm not seeing but lets see what happens in years time. But again the right brewing techniques to select for those funky aromas and flavors must've happen if the yeast is to use the pre-cursers to make the compounds..
Food for thought and things to watch...
Cheers,
Chad
Sorry for any huge errors in spelling/gramer it was written quickly and not edited.. Back to brewing...
