The juice is exposed to the microorganisms on the grape skins, grape stems, leaves, equipment surfaces, hands. The high sugar concentration and low pH favor fermentative yeasts. Oxygen which was initially available is quickly consumed. This causes a change in yeast population. Yeast cell population duirng fermentation is 10exp.6 to 5x10exp.7.
Cell growth occurs during the early stage of fermentation.
At this time all oxygen dissolved in the must is quickly metabolized. Yeast fermentation stops when all available sugar has been metabolized. The winemaker may remove some of the dissolved CO 2 by splashing the wine and some oxygen may be reintroduced into the wine. In this new environment certain lactic acid bacteria and some wine spoilage yeast can grow utilizing other sources of energy which were not metabolized by the wine yeast. After alcoholic fermentation Following alcoholic fermentation the winemaker can encourage the growth of other microorganisms.