MadSci Network: Biochemistry

Re: Why do simple sugars produce more alcohol than complex sugars

Date: Mon Mar 19 11:28:27 2001
Posted By: Charlene Wolf-Hall, Faculty, Food Science
Area of science: Biochemistry
ID: 984927406.Bc

Hi Charlie,

The reason relates to the yeast's metabolic processes and to the type of 
carbohydrate available.  Yeast will ferment simple sugars to alcohols 
through the action of enzymes they produce.  These yeast enzymes are only 
able to convert simple sugars like glucose and sucrose to ethanol.  

To give a quick explanation of simple sugars lets discuss the basics of 
carbohydrate chemistry.  Carbohydrates are made up of simple sugars linked 
by glycosidic bonds.  Each simple sugar, i.e. glucose, is called a 
monosachharide.  A disaccharide, i.e. sucrose, is made up of two 
monosachharides, i.e. glucose and fructose connected with a glycosidic 
bond.  The yeast can break apart the sucrose into glucose and fructose and 
utilize these monosachharides resulting in ethanol as a byproduct.  If a 
carbohydrate is made of 3 to 6 monosaccharide units it is an 
oligosachharide.  If it is made of more than 6 monosaccharide units it is a 
polysaccharide.  Starch is a polysaccharide made up of over a thousand 
monosaccharide units linked by glycosidic bonds.

Yeast cells produce enzymes with specific activities to catalyze chemical 
reactions, which give the yeast energy.  Carbohydrates are great sources of 
energy, however, the yeast enzymes only have the ability to extract this 
energy from a few specific mono- and disaccharides.  This is why you get 
more ethanol from the sugars you mention than from starch.

In alcoholic yeast fermentations using cereal grains as a source for 
carbohydrates, there is a special process called malting which allows the 
starch in the grain to be converted to simple sugars for the yeast to 
utilize.  Basically, malting induces the seed's own enzymes to break down 
the starch to simpler sugars.  This is done by putting the seeds under 
conditions which cause them to start sprouting.  The enzyme of importance 
here is amylase which will help convert starch to glucose and maltose (a 
disaccharide of two glucose units) which the yeast can use and which would 
give the seedling energy to grow.

I hope that answers you question.

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