Toast is bread hardened with direct heat that usually carries some degree of brownness. The toasted bread is used in every part of a full-course meal, from appetizers to dessert. It is the size and flavors of the bread being toasted that separates one type of toast from another. Therefore, zwieback toast and Melba toast have several differences, despite both being some version of toasted bread.
The Make Up
Zwieback toast is sweetened white bread that is baked twice to produce a hardened piece of toast. It is made from the same ingredients as regular white bread -- milk, flour, eggs, yeast and salt. The only addition is usually the sweetener, which is sugar. Alternatively, Melba toast is white bread that is sliced thin and toasted until it is crispy. It is not sweetened and usually does not require a special recipe other than the traditional white bread ingredients and cooking specs.
A visual difference between zwieback and Melba toast is the size of the serving. Melba toast is thinly sliced and smaller than the average piece of toast. The thin slices are crisp throughout. Zwieback toast is traditionally the same size as white bread and sliced to the same thickness. It is baked twice to toast and harden the bread. Like Melba toast, zwieback toast does not have a soft center.
These two types of toast cover the savory and the sweet. Zwieback toast is used in dessert preparations as a base for confection, or crumbled and made into a crust. Zwieback also is used as a teething treat for babies. The sweetness makes the toast appealing to a baby's taste buds, while the hard toast can be gnawed without easily breaking. Melba toast is an appetizer staple, making the base of several traditional appetizer dishes. It is a great companion to soups, dips, pates and also works as a snack.
Zwieback is not a great substitute for Melba toast as it is too sweet for most savory dishes in which Melba toast is used. However, Melba toast with a little sweetener added can be a thin substitute for zwieback toast in some preparations. Use this substitution when crumbled zwieback toast is required for a recipe. However, Melba toast is too thin to sub for zwieback toast as a baby's teething treat.