How yeast works

Yeast, like most living organisms, need a good supply of carbon and nitrogen, a source of sulfur, phosphorus, and oxygen, various minerals and micronutrients (e.g. trace elements, and several vitamins) for growth and reproduction. The usual carbon sources are the six-carbon sugars, glucose and fructose. Wine yeast can also use sucrose, which may be added to the juice of underripe grapes (chaptalization) and is used in sparkling winemaking to induce the second fermentation. Grape amino acids and ammonium compounds most often supply the nitrogen, and most fruit juices, including grape juice, provide the other components necessary for growth. Nutrients, based on nitrogen or vitamins, may be specially added to encourage yeast activity at the beginning of fermentation. This is especially important in the case of underripe grapes, rot, grapes from low soil potential vineyards, or grape- and must-processing conditions that deliberately or unwittingly lead to nutrient depletion, such as harvesting and transport under hot conditions that permit extensive microbial growth, or excessive clarification. Grape solids provide a source of lipids, which, in the absence of oxygen, are used to build stronger cell membranes that confer better tolerance to fermentation stresses, such as extremes of alcohol and temperature. Oxygen is an especially important nutrient, which is supplied in large quantity during production of the yeast starter culture. Small amounts of oxygen may also be supplied during the early or middle stage of fermentation to improve yeast survival and fermentation activity later in fermentation, especially in anaerobic, high-sugar, highly clarified musts at low temperatures (see stuck fermentation).