Milk vs water
Milk is water with protein, sugar and fat dissolved in it — so the swap changes browning, softness and flavour without changing whether the bake works.
The short answer
Both supply the liquid phase and either will make the bake work. What differs is everything attached to the water in milk. Lactose is a reducing sugar that ordinary baker’s yeast cannot ferment, so it survives fermentation and is still there to brown — which is why a milk-enriched loaf colours faster and deeper, and why enriched breads are baked cooler than lean ones. Milk protein softens and adds a little structure; milk fat shortens the crumb and slows the perceived firming of staling. The result is that milk gives a browner, softer, richer bake that keeps better, and water gives a leaner, chewier, paler one that stales faster. Neither is an improvement in the abstract: a rustic loaf or a pizza base is lean deliberately, and milk changes its character rather than raising its quality.
Where they differ
| Dimension | First | Second |
|---|---|---|
| Crust colour | Browner, from lactose that survives fermentation. | Paler; the crust browns on whatever sugar the fermentation left. |
| Crumb | Softer and finer, from fat and protein. | Chewier and more open. |
| Keeping | Stays soft longer, because fat slows perceived firming. | Firms faster. |
| Flavour | Mild sweetness and dairy notes. | None — the flour and the fermentation carry it all. |
| Oven temperature | Often baked cooler, because the crust colours before the crumb is done. | Tolerates a hotter bake. |
Differences that do not matter
These are real distinctions that are unlikely to change your decision. They are listed so you can stop thinking about them rather than left out so the comparison looks decisive.
- Liquid contribution. About seven-eighths water. All water.