|Anatomy of barley kernel (Palmer, How to Brew)|
The modification process was likely discovered by accident. Barley kernels which are soaked in water and then kept moist for several days will begin to germinate. The figure to the right shows the inside of a barley kernel a few days into the modification process. Endosperm contains globules of starch contained within tough cell walls. As the kernel germinates, the acrospire metabolizes the cell walls of the endosperm as it beings to engulf the volume of the kernel. If the acrospire takes up less than 75% of the volume, it is considered under-modified; if it beings to sprout, it is over-modified.
As the acrospire metabolizes the endosperm, it liberates the starches which will eventually become the sugars that the yeast turn into alcohol. The modification process changes the physical as well as the chemical properties of the kernel. Prior to modification, the kernel is hard as a rock due to the cell wall infrastructure. As the kernel is modified and the cell walls metabolized, the kernel becomes more friable and won't break your tooth during a sensory test.
The modification process requires the kernel to be saturated; up to 50% water by weight. This water content must be reduced to below 5% by slowly increasing the temperature to 100degF. After the malted barley has a low enough water content, it is then kilned to halt enzymatic activity as well as impart a complexity of sweetness. Pale and pilsner malt are kilned and low temperatures for a brief duration, while chocolate and Black Patent malt are essentially broiled for hours. There is a spectrum of malts between the two classes that is kilned at intermediate temperatures and duration. The kilning process was mechanized during the Industrial Revolution by Daniel Wheeler's invention of the drum roaster.
The kilning process provides heats that destroys some of the proteins required to convert starches into sugars. For this reason, you cannot make a beer strictly out of darkly roasted malts; you must have some pale/pilsner malt to provide the catalysts involved in saccrification. The proteins in highly roasted malts are broken down and often react with sugars in a Maillard Reaction
The Maillard Reaction is what gives steak and bread their color and flavor. It occurs between 140 and 160 degrees Fahrenheit, when sugars form compounds with amino acids. This reaction "stores" the sugar as yeast are unable to metabolize it. This reaction is responsible for the sweetness of Samuel Adam's Boston Lager.
The Reinheitsgebot forced brewers to be creative with the four ingredients allowed. This lead to the spectrum of malts as well as the variety of different styles that exist today. The modification and kilning process have become so intricate, that they are typically preformed by maltsters and not your average brewery. While the brewer does not have to prepare their own malt, they are still responsible for converting the starches stored in the malted barley into fermentable sugars in a process known as mashing.