Diary of a Brewer: Malting

With my graduation occurring in the past week and a half I have had little time to keep up with my documentation of brewing techniques and general brewing information. Now that things have calmed down I have found more time to devote to the study of brewing and the pursuit of a brewing career. I know malts must seem like a dull subject to the non-brewer but they are possibly the single most important ingredient in any beer and so should not be overlooked. Here I wish to examine the different types of malt, their flavour characteristics, their creation and wide range of uses.

The malt that we use in brewing is a derivative of cereals (themselves a type of grass). The grains of the cereal are the seeds they produce and come in a variety of forms. These grains consist of several components, most simply the endosperm, germ and bran. The bran is the outermost casing of the grains and contains most of the fibre found in the grain. The germ is the embryo of the grain that will facilitate the reproduction of the plant and as such has a very high protein content. And the endosperm is the soft tissue inside of the grain, rich in carbohydrates and starches, that will provide the nutrients for the germ. It is this starch rich component of the grain that is of most use in the brewing process. 


The process of extracting fermentable sugars from the raw cereal grains begins by harvesting fresh grains and drying them to a moisture content of around 12-15%. These semi-dried grains are then rested for several weeks. This step is undertaken because fresh grains have an intrinsic property known as seed dormancy whereby variations in the characteristics of the grains ensure a natural defence against mass simultaneous germination. Once the grains are held at this dormant stage for approximately 6 weeks they are steeped in water ranging in temperatures from 12-18C depending on the dormancy of the grain. During this steeping process the grain swells to ~1.3 times its original volume at which point the grains need to be agitated by blowing air into the steeping vessel to prevent them from becoming too compacted. This process provides the microbes on the surface of the grain with oxygen and allows them to begin degrading the surface layers of the grain. As this process continues the steep water becomes more and more riddled with microbial metabolites and so is often changed several times during the steeping process. After the steeping is complete the grains are dried by sucking humid air downwards through the grains to eliminate any residual moisture or carbon dioxide. Often at this stage the grains have started to germinate. 


At this stage the grains are transferred to either a germination vessel of a floor malting room where they will continue the process of germination under strictly controlled conditions, often kept at a very precise humidity and turned regularly to prevent the grains from growing together in masses. The indicator of germination is the 'chit'; a sheath that grows from the base of a germinated grain. The grains are held in the germination stage until the acrospire (a protein growth under the husk of the grain) has reached ~75% of the length of the grain. If germination where allowed to progress until the acrospire were longer it would not result in higher extract yields and thus would be inefficient. 

After the germination is deemed complete the malt is ready to be dried, or kilned, to produce the correct colour and flavour profile desired. Kilning consists of layering the grains about a metre deep and blowing warm air through the grains from beneath. Because the grains are still moist the warm air will dry the lowest layers of the grain before cooling and becoming saturated with moisture. Therefore the grains closer towards the surface are still warm and damp and can continue to germinate. The boundary between the dry grains and the damp grains above is called the dry-zone and this dry-zone will move upwards through the grains as time progresses. At a certain point the dry-zone will reach the surface of the grain bed and the break-point occurs. At this point the airflow through the grain is decreased and the temperature is increased. Once the grains have been kilned to ~80-105C the production of pale malt is basically complete.


Some of the pale malt will be diverted and used in the production of speciality malts (although green malts and kilned malts are sometimes used). Malt is loaded into a rotating cylinder that can be heated either directly or indirectly to roast the contents. During this roasting stage the colour of the malt will darken and enzyme destruction will occur. The temperature, type of input grains and duration of the roasting are varied depending on the desired colour and flavour of the final malt. Finally the malt is cleaned, milled and packaged ready for use in mashing.