An Overview of the Malting and Brewing Processes

MALTING

The story of beer starts with ripe barley grain, plump and sound, with a moderate (for a cereal) protein content of 10–12%. The barley kernel is roughly ovoid in shape, surrounded by protective layers of husk, with a small embryo at one end. This embryo is the part that will grow into the new plant, given the chance. The remaining part of the kernel is the endosperm, which is basically just a store of food for the young plant.

Most of the endosperm consists of large dead cells with thick cell walls consisting mainly of β-glucan (a polymer of glucose molecules linked by β-glycosidic bonds) together with some pentosan (an arabinoxylan polymer) and a little protein.

These cells are stuffed with starch granules, which come in two sizes; large (about 15–20 µm diameter) and small (about 2 µm diameter). There are very many more small granules than large granules but they account for less than 5% of the weight of the starch. These starch granules are embedded in a matrix of hordein. This is an insoluble protein which provides a store of peptides and amino acids for the new plant. The whole of the starchy endosperm is surrounded by the aleurone, which is a triple layer of living cells.

The whole aim of the malting process is to get rid of as much as possible of the the β-glucan cell walls and some of the insoluble protein which would otherwise restrict access of enzymes to the starch granules. At the same time enzymes are developed which will, in the brewhouse, convert the starch into soluble sugars.

In the maltings the barley is steeped to raise the water content from 12% to around 45%. This process takes about 48 hours and consists of two or three periods when the grain is totally immersed in water, interspersed with 'air rests' when the water is drained off and fresh humidified air is blown through the grain bed to provide oxygen. The increased water content stimulates respiration in the embryo and hydrates the stores of starch in the endosperm. As the embryo activity increases, gibberellins are produced. These are natural plant hormones that diffuse into the aleurone, where they stimulate the production of hydrolytic enzymes during germination.

The moist grain is then allowed to germinate for a few days. During this time cool humidified air is again blown through the grain bed to keep the temperature down to around 16 °C and to stop the grain drying out. As gibberellins diffuse into the endosperm from the embryo they stimulate the aleurone cells to produce hydrolytic enzymes. These include amylolytic enzymes, which break down starch, proteolytic enzymes, which attack the protein, and cellulytic enzymes, which break down cell walls. Proteolytic enzymes include carboxypeptidases, which release one amino acid at a time starting from the carboxyl end of an amino acid chain, and endopeptidases, which can break peptide bonds in the centre of long amino acid chains. They can therefore very rapidly reduce the size of a protein or polypeptide. Next β-glucanases are produced. These break down the endosperm cell walls, making it easier for the other enzymes to diffuse out into the starchy endosperm. Last, but not by any means least, amylolytic enzymes are produced. The two most important are α-amylase and β-amylase, both of which can break down α-1,4 bonds. A debranching enzyme, which can attack the 1,6 bonds, is also produced, but this enzyme is quite sensitive to heat and so is normally inactivated during malt kilning.

All of these enzymes must diffuse into the starchy endosperm and begin the process of breaking down the cellular structure (the cell walls) and the stores of protein, starch and lipid in order to provide nutrients for the new plant. This process is strictly controlled by the maltster, who...