Fruit salt is marketed under the brand name Eno.
It has a similar effect to baking powder and is used as a leavening agent in several recipes in this book. Dhokla, a lentil cake, and idli batter use fruit salt to make a lighter and fluffier batter.
Fruit salt is also used to soak pulses, helping to soften them and reduce their tendency to creat flatulence.
Whilst fruit salt is not the same as baking soda, in some ecipes you can subsitue baking soda for fruit salt
Baking soda is sodium bicarbonate. When baking soda contacts moisture and an acidic ingredient, the resulting chemical reaction produces carbon dioxide. causing the mixture to expand. This reaction begins immediately after mixing the ingredients.
Sodium bicarbonate only reacts when activated with acids like buttermilk, yogurt, lemon juice or vinegar.
Eno, or generically fruit salt, is a combination of baking soda, citric acid and other carbonates. It has 46% sodium bicarbonate, 44% citric acid and 10% sodium carbonate. Fruit salt does not require the addition of acid ingredients. It is more like baking powder in its chemical composition.
Despite its name, it contains no sodium chloride, or common salt.
The best source of Eno is a pharmacist. Whilst it is available in supermarkets and Indian grocers, they tend to stock flavoured varieties, such as lemon. For cooking purposes, you want to get the unflavoured variety. If you live in the United States be aware that Eno is not approved by the FDA and is unlikely to be seen in a pharmacist, but it is allowed as a specialist food ingredient. You will need to obtain it from an Indian grocer.
Eno can be substituted by baking soda, provided there is sufficient acid content in the mixture you are using.