I sometimes make sourdough and have found by accident that by adding fresh tomatoes to the dough, it seems to swell substantially more during proving, or more quickly.

There must be some chemical reaction and I just wondered if anyone knows more about it?

  • 3
    have you tried it with other semi-sweet fruits? My guess would be the simple sugars in tomatoes but I'm not sure enough for a Answer. Some more info seems to be here: thefreshloaf.com/node/43028/…
    – Borgh
    Commented Sep 6, 2019 at 8:00
  • 5
    I would be very surprised if it is a chemical reaction. Sourdough expands because of a biological process - apparently whatever bacterial strain you have caught, it loves munching on tomatoes.
    – rumtscho
    Commented Sep 6, 2019 at 9:01
  • It could be as simple as extra hydration from the tomatoes causing more fermentation.
    – user50726
    Commented Sep 7, 2019 at 16:06
  • @Borgh Yes, I’ve tried a mixture of roasted peppers, onions etc and got the same performance but there has always been tomatoes mixed in with it. Will have to experiment.
    – Alan Spark
    Commented Sep 7, 2019 at 16:58
  • 1
    @rumtscho I think you could be right. It is fascinating.
    – Alan Spark
    Commented Sep 7, 2019 at 16:59

1 Answer 1


Apart from the pH and sugar your tomatoes bring (as mentioned in the comments), I think there's a third part to the explanation - you're likely adding more yeast!

In Wild Fermentation by Sandor Ellix Katz, there's a section on how to get a sourdough starter started. They suggest that if your starter never gets going, you put some pieces of fruit - like apples or grapes - in it, and the wild yeast which almost invariably inhabit the peel of the fruit will get the fermentation going. There's no reason why tomatoes wouldn't be likewise inhabited.

I suppose you test this hypothesis by making two doughs, one to which you add fresh tomatoes, and one where you add tomatoes from the same source which have been cooked.

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