|MadSci Network: Biochemistry|
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.
Try the links in the MadSci Library for more information on Biochemistry.