MadSci Network: Chemistry


Date: Wed Mar 22 10:56:27 2000
Posted By: Carl Custer, Staff, Office Public Health & Science, Scientific Research Oversight Staff , USDA FSIS OPHS
Area of science: Chemistry
ID: 952514300.Ch

Four things limit the pH of fermenting milk:
1) Sugar: The amount of sugar limits the amount of lactic acid produced.  
One molecule of lactose (disaccharide in milk) produces four molecules of 
lactic acid.  Add more sugar, e.g. sucrose or glucose, and you'll have more 
acid production.  
Lactose is  4.8 to 5.2% of milk.  Try adding another 5% sucrose and check 
the pH.  Lactose
This is the main limiter of the pH around pH 5.0. 
Three other effects are:

2) Weak acid: The primary acid produced in milk fermentation is lactic 
acid. It's a weak acid so the pH drop isn't always directly proportional to 
the amount of acid (unlike a strong acid e.g. HCl).  The pKa of lactic acid 
is 3.86 which means that close to pH 3.86, it will take a lot of lactic 
acid to move the pH a little.  Thus, the "buffering" effect of lactic acid 
isn't a main effect at pH 5.0. 

3) Buffering: In addition to lactic acid, the proteins in milk also act as 
weak acids and bases and thus "buffer" the effect of additional lactic 
acid. Actually, not the protein but the reactive groups of the amino acids 
that stick out from the peptide chain. 
For more information on weak acids and "buffering" see: Acid-Base Titrations
Weak Acids

 4) The fermenting cultures tolerance for low pH:  Some bacterial cultures 
e.g. lactobacilli can drop the pH quite low - if they have sugar to 
convert; other cultures stop fermenting around pH 4.0.

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