Alcoholic fermentation from grape juice

[Natldnel]

During the fermentation of ethanol from a source such as grape juice using the common yeast, a slight increase in pH level is observed as the days go by with the fermentation process going on. The overall pH remains acidic of course.

Why is there a slight increase in pH?

Also, why should we be concerned of the pH level of the grape juice and subsequently the wine?

Also, the production of hydrogen sulfide is also being tracked. Why is there a need to do this?

Thanks...

[MrMistery]

Hmmm... From what i know the pH or alcohol is neutral... But, if there is air in the container when you keep the wine, there could be a possible infectation with a bacteria that, through it's respiration process, produces acetic acid from alcohol:
CH3CH2OH + O2 ----> CH3COOH + H2O
The pH of acetic acid is acidic, so this could be the reason of your fluctuance in pH

[Natldnel]

But if the fluctuation of pH is due to the acetic acid, the pH level should decrease and become more acidic. However, the pH level observed actually increased... By slight amounts of course...

[MrMistery]

Strange... So infectation with that bacteria is out of the question... Maybe the yeast uses something acidic in the wine as food, as a result the pH increases... Just a guess

[Natldnel]

Hmmm.... Well, there should be a definite reason since everyone that did this fermentation process saw a slight increase in pH. Also, since we were told to record the pH, there should be a significant reason...

And also about the hydrogen sulfide... Why do we need to tract its production?

I need a fermentation expert of some sort... Lol...

[mith]

Or maybe ethanol is indeed basic since it's bitter

[MrMistery]

Ethanol can not be basic or acidic since there are special pH indicators(like turnesol paper) that are soluble in ethanol

[mith]

I looked it up at wikipedia, while the pH is neutral, the pKa is 15. I'm not quite sure what that is but it's related to acids/bases.

[opuntia]

Or maybe ethanol is indeed basic since it's bitter

I think bitter taste is a property of acids in solutions[i have read something like that]

[chemistry_freako]

(Are these questions from Industrial Microbiology practical? =p)

Bitter taste's a property of acids in solution?
I thought they're supposed to be sour, while basic substances are the ones that taste bitter?

As for the function of using lead acetate paper (leaving it in the flask, not touching the wine inside), it is for testing for the production of hydrogen sulphide (the lead acetate subsequently gets converted to lead sulphide, which is black in colour).
Hydrogen sulphide can also be a by product of sulphate- reducing bacteria.
Some other anaerobic bacteria liberate hydrogen sulfide when they digest sulfur-containing amino acids, for instance during the decay of organic matter.


Acetobacter belongs to the phylum PROTEOBACTERIA, which also contains bacteria which are predominantly aerobic, and are sulphur reducing (look under deltaproteobacteria). Therefore, it could mean that our conditions were not anaerobic enough, thus allowing contamination, and bacteria growth, which ended up converting our wine to acetic acid (if there is a drop in pH of the wine), or the blackening of the lead acetate paper.

This is what i'm thinking, not too sure if it's on the right track though. =D

[opuntia]

(Are these questions from Industrial Microbiology practical? =p)

Bitter taste's a property of acids in solution?
I thought they're supposed to be sour, while basic substances are the ones that taste bitter?

Well.... it could be wrong coz i'm not sure ...i'll re-check

[mith]

Acid = sour
Think vinegar(acetic acid)