In food and biology, a culture is a deliberately grown population of microorganisms — bacteria, yeasts, or moulds — maintained for a useful purpose. Starter cultures drive fermentation: Lactobacillus species sour yoghurt and salami, Saccharomyces cerevisiae raises bread and brews beer, Penicillium roqueforti veins blue cheese. As a verb, to culture means to grow these organisms under controlled conditions, feeding them and holding them at the temperatures and pH where they thrive. The same word in its older sense means the customs and way of life of a people — and the two meanings meet in fermented foods, where microbial cultures are among the most durable expressions of a food culture. Modern dairies, bakeries, and breweries keep their cultures as pure, characterised strains, because a contaminated or mutated culture can ruin whole production runs.