What causes pH to decrease during fermentation?

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Discussion Overview

The discussion centers on the factors contributing to the decrease in pH during the fermentation process, particularly in the context of winemaking. Participants explore various biochemical reactions and products involved in fermentation, as well as the implications of these processes on pH levels.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about the reasons for the decrease in pH during fermentation, expressing confusion over the underlying mechanisms.
  • Another participant mentions that fermentation produces ethanal and carbon dioxide, suggesting that carbon dioxide could lead to carbonic acid formation, but questions the net effect on H+ concentration.
  • A different participant challenges the initial claim about the reaction between ethanal and H+, indicating that there may be issues with charge balancing in the proposed reaction.
  • Further contributions suggest that the reaction involving pyruvate and H+ produces ethanal and CO2, and raises the possibility that unreacted ethanal could form ethanoic acid, although sulfur dioxide was added to prevent this.
  • One participant notes that microbial waste products can contain acidic substances, implying a potential source of pH decrease.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and propose different mechanisms for the pH decrease, indicating that multiple competing views remain without a consensus on the primary cause.

Contextual Notes

Some participants highlight the complexity of the reactions involved and the need for balanced equations, suggesting that assumptions about the reactions may be incomplete or require further clarification.

Who May Find This Useful

This discussion may be of interest to students and professionals in biochemistry, fermentation science, and winemaking, as well as those exploring the biochemical processes involved in pH changes during fermentation.

Orlando
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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?
 
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What are fermentation products?
 
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.
 
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.
 
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.
 
Does this need to be so technical? Microbial waste products contain acidic substances.
 
What type of microbial waste product?
 

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