The Thin Living Layer in a Grain of Barley Is Why Beer Exists

A cereal grain consists of an outer protective coat, a large central mass of starch packed into dead cells, and a small embryo at one end. Between the coat and the starch lies a layer usually one to three cells thick, and unlike the starch store those cells are alive. They contain protein, oil, vitamins and minerals in concentrations far above the rest of the grain, and they are the part of the seed that responds to the signal to germinate.

The sequence is a piece of hormonal signalling with a visible result:

• •Water reaches the embryo and germination begins
• •The embryo releases a hormone that travels to this layer
• •The layer responds by manufacturing enzymes
• •Those enzymes are secreted inwards into the starch store
• •Starch is broken into sugars the embryo can use
• •The starch store is dead tissue and cannot do this itself

Malting is the deliberate exploitation of exactly this mechanism. Barley is steeped in water until germination begins, held while the layer produces its enzymes and converts part of the starch, and then dried with heat at the moment judged best, which stops the process and kills the embryo while leaving the enzymes intact. The result is a grain full of both starch and the enzymes needed to convert it, which a brewer later reactivates by mashing in warm water. Without this layer there would be no enzymes and no straightforward route from barley to fermentable sugar.

Two families do most of the work and knowing which is which explains why malting is a matter of timing. Enzymes that cut starch chains at random points appear first and reduce long chains to shorter fragments, which lowers the thickness of the mixture dramatically. Enzymes that clip sugar units from the ends of those fragments follow, producing the fermentable sugar yeast actually consumes. Enzymes that break down cell walls and proteins act alongside them. Stopping the process early leaves plenty of starch and few enzymes, and stopping late wastes the starch on a seedling.

The nutritional history of refined grain turns on the same layer. Because it lies just inside the outer coat, ordinary milling removes it along with the bran, taking most of the grain's protein, oil, vitamins and minerals with it and leaving almost pure starch. Polished white rice loses it, which is why diets based on polished rice caused widespread deficiency disease in the nineteenth century and why rice is now frequently enriched. Wholemeal flour retains it, which is the main nutritional argument for wholemeal, and also why wholemeal flour goes rancid faster, since the oil it contains oxidises.

A thin layer of living cells between the coat and the starch store of a cereal grain responds to a hormone from the embryo by making enzymes and secreting them inwards, because the starch store is dead and cannot do it. Malting exploits exactly that. Milling removes the layer with the bran, which is why refined grain loses most of its nutrients and wholemeal turns rancid faster.