The lag phase usually lasts for 1–2 days, but may be longer if SO2 concentrations are high, there is insufficient yeast present or there are other inhibitory substances such as spray residues in solution. The temperature should be controlled during the exponential growth phase to give the desired rate of fermentation. During the exponential and stationary phase, the bulk of the sugar is converted into alcohol. Towards the end of the stationary phase the population decreases, but significant sugar metabolism still takes place in the absence of growth of the population. The sugar is utilized exclusively as an energy source during this stage, and not as a carbon source in the production of biomass. The conversion to alcohol slows down towards the end of the stationary phase as the yeasts run out of food and die. In the death phase, the total number of cells remains constant but viability decreases. After this, there is little activity and fermentation finishes. Yeast cells will precipitate, and form a dense layer at the bottom of the fermentation vessel. Depending on