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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?

Alan Spark
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    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: http://www.thefreshloaf.com/node/43028/starters-fruit-fermentation-vs-just-flour-starters – Borgh Sep 06 '19 at 08:00
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    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 Sep 06 '19 at 09:01
  • It could be as simple as extra hydration from the tomatoes causing more fermentation. –  Sep 07 '19 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 Sep 07 '19 at 16:58
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    @rumtscho I think you could be right. It is fascinating. – Alan Spark Sep 07 '19 at 16:59
  • @aris Maybe but I always try to account for that by reducing the amount of water to compensate. – Alan Spark Sep 07 '19 at 17:00
  • Tomatoes are a fairly acid fruit. CO2 has a pKa of pH6.5. So you very well may just be forcing more CO2 out of your dough liquid by lowering its pH. That'd make for a faster rise. Since your sourdough mix is fermenting when you put it in, the included liquid is likely already saturated with CO2/carbonate for whatever pH it is at. – Wayfaring Stranger Sep 09 '19 at 23:31

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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.

gustafc
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