The goal of the lautering process is to thoroughly rinse the grain particles of all the sugars produced during mashing. To achieve this, the particles must be evenly wetted by the sparge water. Extraction efficiency is measured by comparing the amount of sugar extracted from the grain after lautering to the theoretical extraction potential.
In an optimal mash, all available starch is converted to sugar. The optimal extraction varies depending on the malt, but it is typically around 35 points/lb/gal for a two-row barley base malt. This means that 1 lb of malt crushed and mashed in 1 gal of water should yield a specific gravity of 1.035. Most brewers achieve closer to 1.031, which represents an extraction efficiency of 88%. The gap between ideal and actual extraction can be due to factors affecting the mash, such as temperature and pH, and also how effectively the grain bed is rinsed.
A unit volume of the grain bed contains many grain particles and considerable adsorbed sugar. Ideally, particles would be uniform in size with equal spacing, allowing for equal rinsing. In practice, grain particle sizes vary significantly, leading to denser regions within the grain bed. Fluids flow through the path of least resistance, causing preferential flow, or channeling, where some grain bed regions are fully rinsed while others are left unrinsed.
To address channeling, the first step is to supersaturate the grain bed by maintaining an inch of water above it. This keeps the grain bed in a fluid state, preventing compaction from gravity. Each particle can move freely, and the liquid can circulate around it. Loss of fluidity, leading to grain bed settling, is a major cause of stuck sparges.
The next principle for effective lautering is the uniform draining of wort from the grain particles. Fully wetting the grain bed sets the conditions for complete draining, as long as supersaturation is maintained.
The fluid dynamics of draining the grain bed presents the most variables in the lautering process.