of barley is essentially the “breathing” of the live barley kernel. Like our own breathing, it involves the uptake of ambient oxygen and the release of carbon dioxide. During dormancy, the seed’s husks and protective coat—called the testa layer and the pericarp layer—serve as barriers to gas exchanges between the internal living cells and the exterior, thus limiting the rate of respiration. But once the grain is awoken during steeping and germination, the grain’s respiration rate depends on several factors, most important on the grain’s protein and moisture content, the amount of oxygen in the environment, and the ambient temperature. As these values rise, so does the grain’s respiratory metabolism. The faster the grain’s breathing, the faster will be its acrospire and rootlet growth. Respiration only slows down again or even ceases when the grain is being dried in a kiln or roaster. Sugars are the principal carbon sources for grain respiration during malting, which is why respiration is also considered a “malting loss.” This is because sugars used up by barley for its respiration will not be available later for fermentation. Controlling respiration during silo storage, steeping, and germinating, however, is a delicate balancing act between, on the one hand, depriving the grain of oxygen—which impedes the malting process and may even kill the kernel—and excessive oxygenation, on the other hand, which may result in reduced brewhouse yields. Phased adjustments in ventilation, temperature, and moisture, therefore, are the maltster’s main tools for maintaining a proper equilibrium between kernel vitality and nutrient preservation for subsequent beer making.