|MadSci Network: Biochemistry|
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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