Here Miho in Japan explains to thedailyenglishshow how she makes a natural yeast starter using fruit.
They experiment with three fruit based starters Miho prepares for from raisins, pommelo and grapes. The first stage of the process is fermenting the fruit in water until it is fizzy.
Then you strain out the fruit, put the strained liquid back in jars and mix flour into it. The ferment liquid-flour mixture should start to rise. It’s this yeasty dough that you use in your bread making.
The raisins won out in terms of producing a viable bread starter.