Why kettle boiling makes a bagel a bagel
Most bread is shaped, proofed, and baked. A bagel has one extra step that changes everything: it goes into boiling water before the oven. That step is not a tradition for tradition's sake. It does specific things to the dough that cannot be replicated by steaming or by simply adjusting the bake temperature.
What the boil actually does
When raw dough hits boiling water, the outer layer of starch gelatinises almost immediately. This sets a thin shell around the bagel before it ever sees the oven. That shell is what gives a kettle-boiled bagel its characteristic chew and its slightly shiny crust. The inside stays relatively soft because the heat does not penetrate far in 60 to 90 seconds.
The boil also limits how much the bagel rises in the oven. Because the outer layer is already set, the oven spring is constrained. This is why a proper bagel is dense rather than airy. The crumb should have small, tight bubbles, not the open, irregular structure you find in a sourdough loaf.
What goes into the water
Plain water works, but most bagel bakers add something to the boiling liquid. The two most common additions are barley malt syrup and baking soda. Barley malt syrup adds a small amount of fermentable sugar to the surface, which accelerates browning in the oven through the Maillard reaction. The result is a deeper, more even colour on the crust.
Baking soda raises the pH of the water, which also speeds up browning and gives the crust a slightly more alkaline, pretzel-like quality. Some bakers use both. At Bold Citadel we use a combination of barley malt syrup and a small amount of honey, which gives a crust that browns evenly without tipping into the heavier flavour of a pretzel.
Why steaming is not the same
Some commercial operations use steam injection in the oven rather than a water bath. Steam delays crust formation in a similar way to the boil, which is useful for baguettes and other breads where you want a thin, crisp crust. But it does not gelatinise the outer starch layer in the same concentrated way that immersion in boiling water does.
The result is a bagel that looks similar but has a softer, less defined crust and a more open crumb. It is not a bad product, but it is a different product. If you have eaten both side by side, the difference is clear within the first bite.
The boil is the step that most shortcuts remove first, because it adds time and equipment. It is also the step that most determines whether what you are eating is actually a bagel.