Baker's yeast spends its winters inside wasps and its days in labs
The single-celled fungus that raises bread and ferments beer cannot fly. To get from one ripe grape to the next it hitches rides, and overwintering wasp queens carry it from autumn to spring. Inside a wasp gut, strains even mate. Meanwhile, biologists use it to decode proteins that also run human cells.
Saccharomyces cerevisiae means roughly sugar fungus of beer, from Greek words for sugar and fungus plus a Latin word for beer. Its cells are round to oval, 5 to 10 micrometres across, and multiply by budding. It likely first came from grape skins, and in the wild it lives mostly on ripe fruit and year-round in oak bark. Because it is not airborne, it needs carriers. Overwintering queens of social wasps such as Vespa crabro shelter it through winter and pass it to their young, and the gut of Polistes dominula encourages spores to form and germinate, which promotes mating between strains.
Early brewers and bakers worked with wild mixtures of yeast and Lactobacillus bacteria, which soured their products. Around the fifteenth century, beer makers found that boiling wort with hops gave a drink without the sourness; boiling killed bacteria and hops held them back. By the nineteenth century bakers bought yeast from brewers, producing sweeter breads such as the Kaisersemmel roll. After Pasteur, pure cultures followed, Britain built dedicated growing vats in 1879, and cake yeast became standard in Western bakeries early in the twentieth century.
During World War II, Fleischmann's made a dry granulated yeast for American troops; it needed no refrigeration and is still the default in American military recipes. Lesaffre introduced instant yeast in the 1970s. Under good conditions a population doubles every 100 minutes, growth peaks at 30 to 35 degrees Celsius, and an average cell buds about 26 times.
Cells come in two mating types, a and alpha, and can live as haploids or diploids; stressed diploids form four spores. Its mating signals pass through a receptor system and three-tier kinase cascade that mirror human versions, which is why many human cell-cycle and signalling proteins were first found through their yeast cousins. A daughter bud emerges at two-thirds its mother's size, an asymmetric split that parallels how stem cells divide.
Source: Saccharomyces cerevisiae
