<P> When yeast is used for making bread, it is mixed with flour, salt, and warm water or milk . The dough is kneaded until it is smooth, and then left to rise, sometimes until it has doubled in size . The dough is then shaped into loaves . Some bread doughs are knocked back after one rising and left to rise again (this is called dough proofing) and then baked . A longer rising time gives a better flavour, but the yeast can fail to raise the bread in the final stages if it is left for too long initially . </P> <P> Some yeasts can find potential application in the field of bioremediation . One such yeast, Yarrowia lipolytica, is known to degrade palm oil mill effluent, TNT (an explosive material), and other hydrocarbons, such as alkanes, fatty acids, fats and oils . It can also tolerate high concentrations of salt and heavy metals, and is being investigated for its potential as a heavy metal biosorbent . Saccharomyces cerevisiae has potential to bioremediate toxic pollutants like arsenic from industrial effluent . Bronze statues are known to be degraded by certain species of yeast . Different yeasts from Brazilian gold mines bioaccumulate free and complexed silver ions . </P> <P> The ability of yeast to convert sugar into ethanol has been harnessed by the biotechnology industry to produce ethanol fuel . The process starts by milling a feedstock, such as sugar cane, field corn, or other cereal grains, and then adding dilute sulfuric acid, or fungal alpha amylase enzymes, to break down the starches into complex sugars . A glucoamylase is then added to break the complex sugars down into simple sugars . After this, yeasts are added to convert the simple sugars to ethanol, which is then distilled off to obtain ethanol up to 96% in purity . </P> <P> Saccharomyces yeasts have been genetically engineered to ferment xylose, one of the major fermentable sugars present in cellulosic biomasses, such as agriculture residues, paper wastes, and wood chips . Such a development means ethanol can be efficiently produced from more inexpensive feedstocks, making cellulosic ethanol fuel a more competitively priced alternative to gasoline fuels . </P>

How does one identify different strains of yeast