Malting begins by steeping the barley in water, with the barley absorbing up to half of its weight in water. The water is then drained away and the barley is ready to germinate. The barley is spread out in a layer and kept at controlled temperatures to provide the ideal conditions for the barley kernel to begin the process of growing and germinate. The barley is turned with rakes and carefully monitored until it reaches the correct point to halt the process.
Germinating the barley transforms the grain, to begin with the endosperm (the starchy insides of the grain) is broken down by enzymes within the seed. The starches in the endosperm are transformed into simpler carbohydrates and sugars which are used to help the grain grow but are also the exact sugars you need for brewing.
The maltster turns the grain for several days until the endosperm becomes modified, allowing enough time for the starches to break down as much as possible. The barley is now ready for drying and kilning.
Drying & Kilning
A kiln is effectively a large room where the grain is sat usually over multiple levels with the temperature being raised in the room and ventilation in the room being adjusted to first dry the grain at fairly low temperatures and then cure it at a higher temperature
Much of the flavour and the varieties of colours of malted grains comes from the drying and kilning processes. Kilning and then roasting the grains creates reactions called Mailliard reactions. This reaction is what gives food browned during the cooking process its desirable flavour, for example when you caramelise onions they turn brown and become sweet. The exact same reaction occurs when grains are kilned. By controlling the temperature of the kilning process a huge variety of flavours and colours can be produced from the very same raw ingredient.
Before the drying and kilning process starts the grain is known as green malt, the rootlets that formed when the grain was germinated is still attached and these are broken off and removed during the kilning process.
Depending on what type of malt is being produced, the kiln is set to a certain temperature with the grain being dried at this temperature for a determined period. As an example pale malts are dried at a temperature around 40°C with high ventilation for up to 24 hours. Once the moisture content of the green malt falls below 10% the temperature of the kiln is raised to around 95°C for about 5 hours.
Drying the malt slowly at low temperature prevents the enzymes needed for mashing from being destroyed. In the case of a malt like Munich malt which is slightly darker than pale malt the drying phase in the kiln is usually at 50°C and the moisture content is higher at around 20% before being kilned at 105°C for 5 hours. This higher moisture content means some of the enzymes present in the grain are lost, this is why munich malt has less diastatic enzymes, which are needed for mashing than pale malt.
Once malt is kilned there is also then the option to transform the malt completely by roasting or toasting it. To do this a barrel roaster is commonly used which consists of a rotating drum that is heated from beneath with water sprayers to stop the malt from burning too quickly.
The barrel roaster can take a very light pale malt and turn it completely black with any number of possible shades in between by adjusting the time and temperature the malt is held at in the barrel roaster. This process of toasting or roasting the grain destroys the majority or all of the enzymes required for conversion in the mash, the enzymes to convert these malts must come from other malts in the grain bill.