What causes pH to decrease during fermentation?

[Orlando]

I was making some wine recently for my undergraduate biochemistry course and i have to explain why the pH of the wine decreased during fermentation. I am genuinely at a loss; what is responsible for this decrease in pH?

 

[Borek]

What are fermentation products?

 

[Orlando]

The fermentation process produces ethanal and carbon dioxide and the ethanal reacts with H+ to form ethanol. I thought that the production of carbon dioxide could have been responsible because it would disolve to form carbonic acid, but it requires an H+ ion to form so there would be no net gain in H+ concentration. So I am a bit confused as to why the pH does decrease, or at least did so in my experiment and should not have.

 

[Borek]

Ethanal plus H⁺ doesn't give ethanol, at least not directly and there must be other reactants/products (try to write a balanced reaction equation and you will there is a problem with charge balancing).

Carbonic acid doesn't require H⁺ to form, again - try to write reaction equation.

 

[Orlando]

There are other compounds involved, ethanal + H(+) + NADH -> ethanol + NAD(+) where NAD is some really complicated compound. CO2 is produced by the reaction: pyruvate + H(+) -> ethanal + CO2 so the evolution of CO2, and therefore the carbonic acid, requires H(+) ions doesn't it?

Do you have any ideas about what else could cause the decrease in pH? I know it is quite a general question but I was hoping someone might know a lot about fermentation off the top of their heads. I thought perhaps some of the ethanal that did not react to form ethanol might form ethanoic acid, but I added a lot of sulfur dioxide to the wine samples which should bind with any unreacted ethanal to stop this from happening. I can't really think of any other way of explaining the decrease in pH though.

 

[symbolipoint]

Does this need to be so technical? Microbial waste products contain acidic substances.

 

[Orlando]

What type of microbial waste product?