What are the ingredients which makes fish bone soften without making it sour?

You can't soften fish bones without making them sour. The process of softening bones involves breaking down the collagen and calcium in the bones, which is typically done through a process called acid hydrolysis. This process uses acids to break down the bone structure, and as a result, the bones will inevitably become sour.

Here's why:

* Acids are necessary: Acids like vinegar, lemon juice, or even tomato paste are the primary agents used to break down the tough protein structures in fish bones.

* Chemical reaction: The acid reacts with the calcium and collagen in the bones, breaking them down into smaller molecules. This process releases acids into the surrounding liquid, making it sour.

However, you can minimize the sourness by:

* Using less acid: You can use less acid or a weaker acid like lemon juice instead of vinegar.

* Adding sweeteners: Adding a small amount of sugar or honey can help offset the sourness.

* Shortening the cooking time: The longer the bones are exposed to the acid, the more sour they will become. Cooking for a shorter period can help minimize the sourness.

* Using a pressure cooker: Pressure cooking at high temperatures can soften bones faster, potentially reducing the amount of acid needed and the overall sourness.

While you can't completely eliminate the sourness, these techniques can help make the softened fish bones more palatable.