Pour 4 1/5 cups (1 L) of water into a large pot. Pour in the sugar and stir. Let the solution simmer on the heat 20 min. and take it off of it. Once the liquid cools down to 104°F (40 °C), put the bread slice and yeast (mixed with a little water ahead of time) in it.
Put the lid on the pot and leave it in a warm area. It needs to sit there for 3 days. Then distribute the liquid into 2 glass bottles.
Divide up the grapes among them. Close the bottles with caps and leave them at room temperature for 14 days. Filter the resulting vinegar and use it to season dishes.