How is wine made?
Yeast is the main ingredient needed for fermentation yeast is what transforms the grapes into wine. Just as a point of intrest, there are actually wild yeast spores just beign carried all of the place by the wind and to make wine you just need an open container of grape juice patients and time.
The result however, wouldnt be very pleasant to taste.

There are a number of different strains of yeasts, numbering somewhere in 1000's and the types being used to make wine have been
designed for this sole purpose.
So yeast is actually a living organism that feeds off of sugars in the grape juice in a process called fermentation.

When fermenting, yeast spores increase in numbers exponentially, this happens until the sugar content has been consumed.
During this fermentation process, the sugars are converted into alcohol and carbon dioxide.

There are various different flavors the yeast will lend to the finished wine product, the temperature when fermentation takes place,
the particular strain of yeast used s well as a few other things.

When all of the fermentable sugars have been exausted the yeast then falls to the bottom of the container where the fermentation process starts.
The wine is removed, leaving the yeast, and is trasferred to another container to mature to await bottling.