Jains can eat some breads, but usually avoid yeast-bread. Let’s separate the points.

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1. Why do many Jains avoid bread?

“Bread” usually means soft bakery bread made with yeast (fermented dough).

In Jain aahar-niyam:

• Fermented foods (curd, yeast-bread, idli–dosa batter, vinegar, alcohol, etc.) are generally discouraged, and
• On special days (Aatham, Chaudas, Paryushan, Das-Lakshan) they are usually strictly avoided. ( jainknowledge.com)

Reason:

• Yeast is a living organism.
• In fermentation, countless tiny living beings (yeast cells and other micro‑jiv) are active and then destroyed when we bake/cook.
• Jain dharma tries to avoid himsa even of very small, invisible jiv (sūkshma jiv), so such foods are seen as unnecessary himsa.

Therefore:

• Yeast-bread, pav, buns, pizza base, etc. are avoided by stricter Jains and during vrat/Parv. ( jainknowledge.com)
• Unleavened breads like roti/chapati, phulka, khakhra (made fresh, without yeast/fermentation) are generally acceptable.

You can read a related explanation here

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2. What kind of ekindriya jiv is yeast?

In Jainism, ekindriya jiv = beings with only one sense: touch (sparshendriya).

Traditional categories of ekindriya are:

• Prithvi-kaya – earth-bodied
• Ap-kaya – water-bodied
• Tejas-kaya – fire-bodied
• Vayu-kaya – air-bodied
• Vanaspati-kaya – plant-bodied

Modern science calls yeast a unicellular fungus, but Jain scriptures mainly talk in terms of sensed, subtle life, not modern biology.

From Jain drishti:

• Yeast is treated as sūkshma, ekindriya jiv present in fermenting dough/liquids.
• Many Jain teachers simply explain it as living micro‑organisms involved in fermentation, and therefore to be avoided, rather than arguing over its exact sub‑class.
• Whether one calls it sadharan vanaspati or another subtle category, the practical point is:

- it has one sense (touch) - it is alive - heating/baking etc. kills countless such jiv.

So in simple words:

> Yeast = very tiny, one-sense (ekindriya) living beings (sūkshma jiv) used for fermentation. > Because Jain Dharma avoids harming even such small life, yeast-based foods are avoided, especially by serious sadhaks and on vrata days.