Dahi is a natural unpasteurised yoghurt eaten as-is and an ingredient in India, aka curd in Indian English (different to curd in Br/Am E. though). Khatta dahi is more soured (khatta means sour).
What's a good commercially-available (outside of India/Asia) substitute? Natural yoghurt for dahi, plus some (how much?) soured cream for khatta dahi perhaps?
I don't think commercial natural yoghurt could be soured at home, since my understanding is that they pasteurise it (to prolong shelf-life) before re-introducing the desirable bacteria (but not the undesirables).
Or perhaps dahi is often only used because it's so common anyway, and actually if we're substituting it needn't be a yoghurt at all; we could just use (soured, for khatta) cream? – which is of course higher in fat, but so is buffalo milk, which is also used (I'm not sure whether it or cow's is more common) for dahi in India anyway.
Best Answer
There is quite a big difference in the process of making dahi vs yoghurt (aside from pasteurisation), as yogurt is made with bacteria (called yogurt cultures) and curd is made by curdling milk with an acidic agent (like lemon juice).
I would personally use full-fat greek yogurt and add a little fresh lemon juice to the yogurt for a quick-fix. You could, however, follow this recipe to make sour yogurt. I see the author also suggests just adding lemon juice to yogurt as a faster solution.
Let me know if this worked for your recipe!