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Old 03-18-2013, 03:44 PM   #1
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Default Yeast and Bacteria eat ethanol (?)

So. I read somewhere that when yeast runs out of sugar to eat, it can use ethanol as a carbone source and then divide CH3-CH2-OH in CO2 (and H2O).

Well that is not cool from them. After Jesus turn water in wine... yeast turns alcool in water!

Fortunatly, it doesn't happens very much in normal beer brewing, it's good.

But I was wondering about bacterias in lambic. Could they do the same, do they eat alcool to create acids? Sourness and tartness improve with time, even when sugars are all gone...

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Old 03-18-2013, 04:28 PM   #2
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Originally Posted by Tiroux View Post
So. I read somewhere that when yeast runs out of sugar to eat, it can use ethanol as a carbone source and then divide CH3-CH2-OH in CO2 (and H2O).

Fortunatly, it doesn't happens very much in normal beer brewing, it's good.
i'd like to see that book/article/etc, because i'm wondering if you read it wrong. yeast create ethanol as part of their digestive process, i would be suprised to hear that they could re-digest that by-product. certainly something i've never heard of. furthermore alcohol is toxic to yeast, if they could digest it why wouldn't they do so to un-polute their environment?
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Old 03-18-2013, 05:21 PM   #3
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i'd like to see that book/article/etc, because i'm wondering if you read it wrong. yeast create ethanol as part of their digestive process, i would be suprised to hear that they could re-digest that by-product. certainly something i've never heard of. furthermore alcohol is toxic to yeast, if they could digest it why wouldn't they do so to un-polute their environment?
Ethanol is toxic for you too, you know.

If you read french :
http://www.brassageamateur.com/forum/ftopic16326.html

Brettanomyces sp. peuvent, en anaérobiose, croître sur un milieu contenant de l’éthanol comme
seule source de carbone (Rodiguez et al., 2001 ; Silva et al., 2004). Cette aptitude explique la
possibilité qu’ont ces micro-organismes à se développer sur des vins secs exempts de sucres résiduels
(Geros et al., 2000). Mais l’assimilation de l’éthanol n’est pas directe. Elle passe par une première
étape au cours de laquelle l’éthanol est transformé en acétate qui est alors utilisé comme substrat de
croissance (Gilis, 1999).
Medawar et al. (2003) montrent cependant qu’en milieu synthétique simple une teneur en
éthanol supérieure à 13% peut être inhibitrice de croissance chez certaines souches de B. bruxellensis.

Dans le vin, B. bruxellensis présente une sensibilité moins importante à l’alcool queSaccharomyces
cerevisiae, suggérant une capacité plus grande à survivre dans ce milieu (Silva et al,. 2004).
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Old 03-18-2013, 06:00 PM   #4
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ah, so you need to revise your original statement, in the first post: "" I read somewhere that when brett runs out of sugar to eat, it can use ethanol as a carbone source..."

reading that message board, it's clear that "Nezahualcoyotl" seems to be knowledgeable and familiar with lab terminology, but his assertion that Sacc can break down ethanol is something i've never read (including in "Yeast" by White and Zainasheff... you would think it would be somewhere in there). he later quotes the passage that you did, which only mentions brett... so maybe he changed his mind?

(oui, je lis le francais... mais trop paresseux pour utiliser les accents )

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Old 03-18-2013, 06:03 PM   #5
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I forget where, but I've heard the same thing, but only when in the presence of oxygen (so no worries in your brewing).

The explanation I heard was that in the wild (say, on a piece of fruit) the yeasts produce CO2 and alcohol to lower the pH and raise the toxicity of the sugar solution they're eating. The other microbes die off, and then they eat the alcohol. Basically positioning the food supply for everything but them.

Otherwise, they're really inefficient, there's a lot of energy they'd be leaving behind by just breaking a few bonds.

I would love to see something to prove this though.

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Old 03-18-2013, 06:05 PM   #6
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Originally Posted by sweetcell View Post
i'd like to see that book/article/etc, because i'm wondering if you read it wrong. yeast create ethanol as part of their digestive process, i would be suprised to hear that they could re-digest that by-product. certainly something i've never heard of. furthermore alcohol is toxic to yeast, if they could digest it why wouldn't they do so to un-polute their environment?
In an aerobic situation it can occur it appears, only place I have ever seen any mention of yeast changing alcohol.

http://www.brettanomycesproject.com/.../introduction/
"Brettanomyces spp. have been observed to utilize both glucose and ethanol in producing acetic acid under increased levels of oxygen, although Freer (2002) showed not all strains could use both as carbon sources, with high variability seen in the levels of acetic acid produced by different strains. "
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Old 03-18-2013, 06:11 PM   #7
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Ethanol is toxic for you too, you know.
i know! that's why i get rid of it every time someone puts it in front of me

one of the reasons that yeast stop fermenting and go into hibernation is because of excess levels of alcohol. if you put a yeast with a low sugar tolerance into a high sugar environment, it can go dormant before all the sugar is consumed if the alcohol gets to high. it essentially poisons itself with its own waste product, alcohol. seems to me that if yeast could ingest alcohol, they would do so to make the environment more hospitable... but what do i know, i'm not a yeast biologist.
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What hops should I grow? Hop grower's comparison table. Looking for cheap honey?

Drinking: a farmhouse with ECY08 & brett blend
Fermenting: wet-hopped harvest ale x 2, sour cherry mead, imperial chocolate stout and its not-so-small second runnings beer
Aging: oud bruin & a few other sours, acerglyn, a BDSA
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Old 03-18-2013, 06:14 PM   #8
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Brett was an exemple, only a part of the post. At first they were talking about Sacch doing the same.

I know it happens in an aerobic environnment, at least for sacch and brett (and that's why it doesn't occur in beer.

But I was asking about bacterias.

Edit: At first, in the post, the guy mention he did an ANAEROBIC experiment where the alcool content begin to DECREASE after fermentation. It was only sacch. But again, my question is more for bacterias in lambic, I want to understand better the souring process.

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Old 03-19-2013, 01:52 PM   #9
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Sacch. can convert ethanol back into acetaldehyde (not water) in certain conditions. Apparently, there are two important alcohol dehydrogenases - ADH1 that facilitates conversion of acetaldehyde into ethanol and ADH2 that facilitates the reverse reaction. Brewers yeast has the ability to express ADH2 only in certain conditions (low sugar, not sure what else is necessary).

As far as bacteria goes, acetobacter is a very prevalent bacteria in the environment. In aerobic conditions, acetobacter converts ethanol to acetic acid. This reaction won't happen in the absence of oxygen. This is why porous aging vessels such as barrels inevitably lead to the production of at least some acetic acid. If you want to avoid the production of acetic acid in sours that you'll be aging a long time, its best to age in glass, stainless, etc. I can't remember right now how much the other strains (that are intentionally added, e.g. lacto, pedio) contribute to acetic acid production and in what conditions. I know they can produce acetic, but I also know its not one of the major products in a good anaerobic fermentation.

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Old 03-19-2013, 02:16 PM   #10
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Sacch. can convert ethanol back into acetaldehyde (not water) in certain conditions. Apparently, there are two important alcohol dehydrogenases - ADH1 that facilitates conversion of acetaldehyde into ethanol and ADH2 that facilitates the reverse reaction. Brewers yeast has the ability to express ADH2 only in certain conditions (low sugar, not sure what else is necessary).

As far as bacteria goes, acetobacter is a very prevalent bacteria in the environment. In aerobic conditions, acetobacter converts ethanol to acetic acid. This reaction won't happen in the absence of oxygen. This is why porous aging vessels such as barrels inevitably lead to the production of at least some acetic acid. If you want to avoid the production of acetic acid in sours that you'll be aging a long time, its best to age in glass, stainless, etc. I can't remember right now how much the other strains (that are intentionally added, e.g. lacto, pedio) contribute to acetic acid production and in what conditions. I know they can produce acetic, but I also know its not one of the major products in a good anaerobic fermentation.
That's a great reply, thank you! So we can say that in a close non porous vessel such as a carboy, there's no consumtion of alcohol to produce acetic, but only slow and long consumtion of long and complex sugars? That's another story if you age in a barrel.
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