Your logic is very sharp, and this is exactly the type of question Jain dharm tries to make us think about calmly.

Let’s break it simply, in Jain view:

1. Is dahi itself allowed in Jainism?

• Traditionally, for shravaks (householders), fresh dahi made from freshly boiled milk, set and eaten the same day before sunset, is generally allowed in many Shwetambar families. ( jainknowledge.com)
• Monks/nuns and stricter shravaks usually avoid all fermented items (curd, buttermilk, cheese, etc.) because of high microbial life and fermentation. ( jainknowledge.com)

So, dahi is not automatically “paap” in shastra; it is allowed with conditions for laypeople and avoided at higher levels of saiyam.

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2. “If we heat dahi and kill bacteria, is that less or more paap?”

From Jain drishti, we see two main things:

1. How much life is destroyed (quantity and level of jiv)

- Bacteria in dahi are one‑sensed beings (ekendriya). Killing them is himsa, but less serious than killing animals (five‑sensed). - Eating dahi as it is: you are still causing destruction of those jiv in your body (acid, digestion, etc.). - Heating dahi to kill them outside the body also destroys them – the number destroyed may be similar in both cases. - So, from only the “count of dead bacteria” angle, neither is clearly “less paap”; both involve himsa of one‑sensed beings.

1. Bhav (intention) and awareness

- Jain dharm always sees dravyahimsa (physical) and bhavahimsa (mental) together. - If someone knowingly heats dahi exactly to kill and eats with a mind of carelessness towards jiv, the bhav can be worse. - If someone eats small quantity of fresh dahi, within limits, as per traditional niyam, with awareness that “this is a compromise of a grihasthi, I should not get attached to its taste”, the bhav is softer and more aligned with shastra‑permitted level.

So, it is not as simple as: “uncooked = less paap, heated = more paap” or the opposite. Jainism evaluates both: amount of himsa and inner attitude.

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3. “But if bacteria remain alive and go to intestine, is it better because we didn’t kill them?”

Important subtle point:

• Jiv that enter our body through food or air generally cannot survive long in that inner environment; digestion itself is a cause of their destruction.
• So, even if you don’t heat the dahi outside, most bacteria will still die inside due to digestion – so himsa still happens, only at another place.
• Jain dharm doesn’t say: “If killing happens inside your stomach it is fine, only outside is paap.” Himsa is himsa, wherever it occurs; for shravaks it is tolerated to a certain practical extent, but not called punya.

Therefore, “letting them stay alive and go inside” is not really saving them; they are anyway being subjected to conditions where they will perish.

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4. Then what is the Jain way to look at dahi?

Very simply:

1. Ideal (for sadhus / high saiyam)

- Avoid dahi and all fermented items; avoid unnecessary multiplication of jiv.

1. Practical allowance (for shravaks)

- Many traditions allow: - Milk and freshly set curd, made from boiled milk, - Taken in limited quantity, - Same day, - Before sunset, - No over‑fermented, sour, or stored curd. ( jainknowledge.com) - During Paryushan, Chaturmas niyams, or special vows, even this is often stopped.

1. Better vs. worse in everyday choices

- Best: Avoid dahi altogether if you want to walk closer to monk‑like ahimsa. - If you still take dahi: - Take fresh, same‑day, limited quantiy. - Avoid mixing with many other things, avoid over‑fermented products (old curd, cheese, etc.). - Keep bhav of regretful necessity, not attachment.

Whether you heat it only to “kill bacteria” or eat it raw – both involve himsa of one‑sensed beings. The clean Jain answer is:

• For a serious sadhak, less dahi → less fermentation → less himsa → better.
• For everyday grihasthi, follow niyam of your sampradaya / guru, eat simplest, and keep reducing such items with time.

You can read a simple discussion on curd in Jainism here: Curd and Jain principles of ahimsa in food Another related Q&A on Jains eating curd despite bacteria is here: Why Jains do eat curd even if it has bacteria