MadSci Network: Biochemistry

Re: Yeast Metabolism in fructose, lactose, glucose, and sucrose

Date: Fri Jan 21 17:18:09 2000
Posted By: Carl Custer, Staff, Office Public Health & Science, Scientific Research Oversight Staff , USDA FSIS OPHS
Area of science: Biochemistry
ID: 944988586.Bc

Sucrose and lactose are not single sugars (mono-saccharides).  They are 
di-saccharides or double sugars. 
Yeasts, and just about living every living thing has to break complex 
carbohydrates to simple sugars, namely to glucose then to fructose.  

Sucrose is a double sugar made of glucose and fructose linked together.  
The enzyme to break that double sugar into single sugars is fairly 
commonplace among living organisms (and certainly in yeast).  Plus, under 
slightly acidic conditions, sucrose will break into the two single sugars 
automatically.  Thus, sucrose breaks down to glucose plus fructose. 

Lactose is a double sugar made of glucose and galactose linked together. 
That link is both strong and the enzyme is not so common (certainly not in 
common bakers yeast). Even some humans lack the enzyme needed to break 
lactose when they become adults. 

Thus, the answer to your question is: fructose has a head start on becoming 
carbon dioxide over glucose and sucrose.  And because the yeast can't break 
lactose into digestible pieces, there will be no carbon dioxide. 

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