Disclaimer: I haven't personally used these, hence do not have an experienced input.
Eno is a fruit salt which is composed of basically 60% baking soda and 40% citric acid. So if you have these on hand, then you can use this proportion to substitute for ENO. If citric acid is not available, you can use some other souring agent instead along with Baking soda. Few readers also mention that they have used Baking Soda and Citric acid in equal measures to substitute for the fruit salt to good results. (i.e for 1 tsp eno = 1/2 tsp baking soda + 1/2 tsp citric acid). Some people even suggest using just baking soda (for 1 tsp eno = 3/4 tsp upto 1 tsp baking soda) instead of using it along with citric acid.

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