The quality of bread is characterised by its flavour, nutritional value, texture and shelf life. In the baking industry there characteristics are improved by addition of so called "improvers" or enzymes (which normally are incorporated in the improvers). Alternatively the addition of sourdough influences all aspects of bread quality and thus meets the consumer demand for a reduced use of these improvers which contain all kind of additives. As sourdough is an intermediate and not an end product, its impact on the bread can only be determined on the basis of the quality of the bread. Biochemical changes during sourdough fermentation occur in protein en carbohydrate components of the flour. The rate and extent of these changes greatly influence the properties of the sourdough and consequently the quality of the bread. The effects are associated with the metabolites produced by the lactic acid bacteria and yeast during fermentation, including organic acids, enzymes and CO2.