Yeast is a microscopic one-celled plant belonging to the fungus order, which ordinarily multiples by a process known as budding and which causes fermentation when placed under suitable conditions.
There are about 3,300 known species of yeast. Only a few of them have been grown for the baking trade. Yeasts have been found in nature wherever there is sugar.
Initially yeast was produced by using the German grain process which yielded both yeast and alcohol. The yeast produced, because of its insolubility, did not meet with the requirements of the bakers and therefore, this process had to be discarded.