Yeast requires nitrogen for fermentation. If there is not enough, yeast cells are stressed and produce excess H2S—something that gives fermentations an off-odor. If you want to avoid problems, you must be prepared by adding nitrogen to fermentations.
You don’t need to follow these directions to get a ferment to work, but you will be more likely to get a ferment that produces better wine flavors and minimal H2S if your give the yeast a proper start in life.
Grape must and wines are protected from microbial action and oxidation by adequate levels of sulfur dioxide (SO2). The required amount of sulfite solution depends on the pH of your wine.