Pliny the Elder, a Roman naturalist who lived during the first century BC, said that Gallic and Iberian bread which incorporated beer foam (that is to say the yeast which appeared on the surface of the beer during fermentation) was renowned for its lightness.      Baker’s yeast ...is characterised by the fact that it is composed of living cells. It comes in two forms: fresh yeast, as the European baker generally knows it, and dry active yeast, which is specially dried for transportation over long distances (to Africa, the Middle and Far East, Oceania, etc).      Rising powders Improperly called chemical "yeast" in France, these products are not authorised and could not be used in bread making as the flavour and the texture of the bread result from fermentation. Made up of mineral salts such as bicarbonates, they react chemically in the presence of water and produce carbon dioxide or ammonia during cooking. They are designed for pastry and crackers.      Yeast extracts The enzymatic digestion of cell constituents by yeast enzymes themselves leads to the preparation of a natural biological product which is rich in amino acids coming from yeast proteins and vitamins. Yeast extracts are used in the elaboration of culinary dishes, pharmaceutical preparations and culture mediums. The properties of yeast extracts, rich in ribo-nucleotides, are such that they strengthen the taste, thus making it possible to replace glutamates and hydrolysed vegetable proteins. These particular properties are much appreciated in the food processing industry.      Dietetic yeast The composition of yeast gives it a high nutritional value which can be retained by an appropriate drying process. This thermal treatment deactivates the fermentable power of yeast whilst preserving its dietetic qualities.      Sourdough This is dough in which micro-organisms, which are naturally present in flour, have been cultivated in successive stages. These micro-organisms contain yeast and an acidifying bacteria producing acetic and lactic acids which are characterised in leavened bread. A fraction of this dough is used for seeding each kneading (around 30% of the flour weight) thus producing a new sourdough.      Yeast hybridization The use of pure strains of yeast dates back to Hansen in 1883 who was the first to observe the fusion of two spores. Later on, this form of reproduction, completely different from burgeoning reproduction, was shown to be a phenomenon of sexual reproduction. This crucial discovery showed that the usual generic laws could be applied to yeast. In particular, hybrids could be formed, as is done for more complex plants (ie, corn). This hybridisation makes it feasible for commercialised yeast to be adapted to specific domains by combining the properties of the different strains available.