Glucose is a simple sugar or monosaccharide, a group of sugars that are the simplest forms of carbohydrate. Yeast metabolises glucose and produces carbon dioxide and alcohol as waste in the process known as fermentation. Although the starch in flour doesn't contain glucose, enzymes in the flour and in the yeast themselves are able to break it down into glucose.
Want to find out more? Look at the chapter on ingredients in the book flour and water.
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