2 Changed answer to be more accurate following down vote, I typed acidic instead of metallic.
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It is possible that the tomatoes you are using are under ripe, however could I suggest another possibility, are you using an aluminium pan? Tomatoes are acidic, and they can react with the metal in an aluminium cooking pot to become more acidicchange the flavour in a way that might taste unpleasant or metalic.

http://noshon.it/tips/tomatoes-and-aluminu/

If it is not the pan, then you should concider the variety of tomato you are using, if available try using smaller cherry tomatoes which are sweeter, or if using bigger beef tomatoes try adding a pinch of granulated sugar whilst cooking.

It is possible that the tomatoes you are using are under ripe, however could I suggest another possibility, are you using an aluminium pan? Tomatoes are acidic, and they can react with the metal in an aluminium cooking pot to become more acidic.

http://noshon.it/tips/tomatoes-and-aluminu/

If it is not the pan, then you should concider the variety of tomato you are using, if available try using smaller cherry tomatoes which are sweeter, or if using bigger beef tomatoes try adding a pinch of granulated sugar whilst cooking.

It is possible that the tomatoes you are using are under ripe, however could I suggest another possibility, are you using an aluminium pan? Tomatoes are acidic, and they can react with the metal in an aluminium cooking pot to change the flavour in a way that might taste unpleasant or metalic.

http://noshon.it/tips/tomatoes-and-aluminu/

If it is not the pan, then you should concider the variety of tomato you are using, if available try using smaller cherry tomatoes which are sweeter, or if using bigger beef tomatoes try adding a pinch of granulated sugar whilst cooking.

1
source | link

It is possible that the tomatoes you are using are under ripe, however could I suggest another possibility, are you using an aluminium pan? Tomatoes are acidic, and they can react with the metal in an aluminium cooking pot to become more acidic.

http://noshon.it/tips/tomatoes-and-aluminu/

If it is not the pan, then you should concider the variety of tomato you are using, if available try using smaller cherry tomatoes which are sweeter, or if using bigger beef tomatoes try adding a pinch of granulated sugar whilst cooking.