Alcoholic fermentation from grape juice

[Natldnel]

Yeahh... Just like your first post about plasmid DNA was from Molecular Biology and techniques pract... Lol...

Anyway thanks... Although you answered after i handed in my report. Nevermind... Thanks for the explanation anyway...

[MrMistery]

@chemistry_freako
Look at my first post on the topic. Acetobacter pasteurianum was the bacteria i was referring to... But he said that the pH had rissen so it can not be that since that bacteria would make the medium more acidic(because of the ethanal)

[Natldnel]

Ok i never really read her full post... Yeah, it has nothing to do with Acetobacter contamination... That would make it more acidic and drop the pH, not increase it...

My guess that the neglible increase in pH is simply due to the grape having been fermented Probably nothing special about it...

[chemistry_freako]

Yeap - sour taste is a characteristic of acids (eg. malic acid, citric acid etc)
Maybe we should look into the structure of ethanol to find out why it's not acidic, and what pKa of it means? =D

[chemistry_freako]

hmms my mistake - guess it was too late in the night to be typing anything here haha. =D
the changes in the pH should be rather minor, since the glucose having been fermented to ethanol shouldn't cause a change in pH by too much (it's almost like hmm, adding water to dilute a sample? =p)

[eluned]

Before the start of fermentation, which is an anaerobic and involves the production of CO2 (which goes into solution as carbonic acid), the yeast go through a phase of multiplying. The multiplication phase is aerobic (which is why yeast multiply and fermentation resumes after racking), and I believe organic acids may fuel some of the growth. It has been YEARS since I've studied this, so I may be off-base, but I'm pretty sure the correct answer involved the two different metabolic pathways. The other possibility is that the pH resulting from carbonic acid production is higher than the pH due to malic acid (which I think is the primary acid in grape must).

Yeast beasties like more acidic conditions than many of the bacteria that will foul fermentation (and make you sick as a dog). That is why monitoring pH is important - acid conditions promote the yeast growth and inhibit some of the competitors. The increasing alcohol content also helps inhibit growth of some of the bad bugs. As others mentioned, sulfur-reducing bacteria are among the bad bugs. Beer does not keep as well as wine because it is lower in alcohol and less acidic (also less tannin - which is a preservative).

Did you use Campden tablets (sulfite) to stop fermentation?

Can you let us know the correct answers after you get your report back?

[chemistry_freako]

Don't remember stopping the fermentation - all we did was to observe and note down the changes and after one week we disposed of the 'wine'.

Can you let us know the correct answers after you get your report back?

hmms, answers to? =\

[mith]

Does the oxygen in the air oxidate some of the stuff and turn it to acetic acid?

[MrMistery]

No. Only Actobacter can do this. They oxidase ethanol to acetic acid. The reaction needs an enzyme, it can not take place without it.

[mothorc]

Ethanol pH is approximately to 3.00 (acidic properties because it have active hydrogen (not like true acid).
Don't too believe in theoretical because water in practice is acidic (distilled 2 times)
Usually, Acetobacter and Sacharomyces live together.(like symbion but I can't find the right word to express). Sac :glucose ->ethenol. Ace: ethanol->acid acetic. This is nature phenomenon so sometime you can smell ethanol in vinegar(house product).

[xand_3r]

Eluned is right, organic acids are usually more acid than H2CO3. Additionally, carbonic acid is very unstable and it decompose in carbon dyoxide (which is eliminated in air) and water. The pH increase might also be caused by the decreasing quantity of H2CO3 due to its decomposing. As for the H2S, this is toxic for most organisms (including humans) so an increasing quantity of this compound is not welcomed (H2S can cause headaches). Also, some chemoautotrophs bacteria (Thiobacillus) use sulfur compounds (S, H2S) oxidizing them to sulphate to obtain energy. This is also unwanted because sulphur compounds are toxic, have a bad smell and sulphur bacteria are competitors for yeast.
Ethanol has amfoter charachteristics, that's why the pH is neutral (it is weaker than water which also has amfoter characteristics - it's both an acid and a base, it acts like a base in acidic solution and like an acid in basic conditions). Acids have a sour taste and bases a caustic one (for example NaOH). The taste of alcohol isn't related to its acidic nature - as I said before, alcohol is a weaker acid than water (and water isnt' sour).
pKa is the constant of acidity (it measures the hydrolysis degree of the acid - if the acid hydrolyses rapidly and in large quantities, it is a very strong one). For example, acetic acid:
CH3COOH+H2O------>CH3COO- + H3O+

pKa= ([H3COO-][H3O+])/[CH3COOH]

where [] means mollar concentration (Cm=Niu/V moles/liter). The lower the pKa, the stronger is the acid.