In Jain dharm, the rule is not “potato is bad, dahi is good.” The rule is: which food involves more himsā (violence) and which involves less.

1. Why potato is not eaten

• Potato is a root (underground) vegetable.
• To get a potato, you must uproot the whole plant, pull it out of the soil.
• In Jain shastra, such roots are considered “anantkāy” – they contain innumerable living beings (jīv) in one body (in the root itself + in the surrounding soil).
• When you pull out and wash/cut a potato, countless jīv are destroyed at once.
• Because of this very high level of himsā, Acharyas have clearly said to avoid potato and other root vegetables (onion, garlic, carrot, etc.).

So: Potato = anantkāy, many lives, more violence.

2. Why dahi (curd) is generally allowed

• Dahi is made from milk. Traditional Jain position:

- Milk comes from a one-sensed (ekendriya) being (cow, etc.), and taking milk does not kill the animal. - From that milk, dahi is made by adding culture.

• There are still tiny microorganisms in dahi, so some himsā is there, but:

- It is much less than the himsā caused by uprooting an anantkāy root. - Therefore, in traditional Jain household practice, milk, dahi, ghee, etc. are allowed, within limits.

3. Important nuances about dahi

• Many strict Jains avoid dahi and all fermented food (idli–dosa batter, wine, vinegar, etc.) because fermentation involves growth and killing of many micro‑organisms.
• Dahi is also not taken at night, and not taken during certain vrats (fasts) and paryushans.
• Today, because of modern dairy cruelty, some Jains choose to give up all dairy and go vegan.

But that is a modern ethical choice, not the classical minimum standard.

4. Simple summary

• Potato

- Root, anantkāy, uprooting = kills countless lives at once. - So clearly prohibited in all Jain traditions.

• Dahi

- Made from milk, fewer jīv involved compared to roots. - Traditionally allowed, though: - Not at night - Not during some vows - Some stricter Jains avoid it completely (like other fermented foods).

So the reason is not about taste or nutrition, but purely about ahimsā and number of jīv harmed. Jain dharm always chooses the option with minimum possible violence in day‑to‑day life.

You can read a simple Jain explanation about potatoes here: “Why not to eat potatoes?”