Lye
In order to cause a Maillard reaction during baking for the characteristic browning effect, a lye roll needs to be coated with a high pH solution. The higher the pH, the stronger the reaction. Lye provides a high pH. However, lye is not the only way to produce this result, it's just the strongest and best for this purpose. A baking soda solution, which is easier to handle and arguably safer to use owing to its lower pH, will provide a similar product but will not power as strong a reaction, so the effect will be less.
The same solution is also used for preparing pretzels; outside of Germany they are often the only baked food being commonly glazed with a lye solution.
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Famous quotes containing the word lye:
“We all cry out that the world is corrupt,—and I fear too justly,—but we never reflect, what we have to thank for it, and that it is our open countenance of vice, which gives the lye to our private censures of it, which is its chief protection and encouragement.”
—Laurence Sterne (1713–1768)