Theory and Practice of Lautering
Lautering is the act of separating sweet wort from spent grains. The act of lautering wort is physically very similar to filtration. The flow of wort through a grain bed can be thought of physically as a type of filtration (liquid flowing through a “filter bed” of grain). The basic principles of filtration were established in the 19th century by Henry Darcy.
The important parameters for filtration are described by the “Darcy’s Equation”:
where:
Q = volumetric flow rate of liquid through the filter
K = a constant associated with the specific properties of the fluid and the filter media
A = cross-sectional area of the filter (h1 – h2) = pressure drop across the height L.
L = height or thickness of the filter medium
Darcy’s equation was later modified in a way better suited for process and brewing applications. The modified version of the equation accounts for important brewing process factors such as wort viscosity, grain bed permeability, and particle size distribution of the grain bed. This modified equation is given by:
where:
Q = volumetric flow rate of wort
A = cross-sectional area of grain bed
ΔP = pressure drop across the grain bed
μ = wort viscosity
L = grain bed depth
K = grain bed permeability
Grain bed permeability, K, can further be estimated using:
With:
Y = bed porosity (wort volume/total mash volume)
de = effective particle size diameter of the grain in the bed
It is the interaction of all of the variables in the above equations that determine the total flow rate of wort through the grain bed.
Brewing process factors that affect lautering
Milling
Milling is the process of crushing grain in order to expose the interior of the grain kernel to the wort. The way that milling is performed directly influences the particle size distribution of the grain bed, and also determines if the grain husk is left mostly intact. Grain bed permeability, K, is a function of the effective particle size diameter, de, of the crushed grain. A larger effective diameter means greater permeability. Permeability is also a function of the amount of void space (porosity, Y) within the grain bed, and this is directly related to how “tightly” the grain pieces are packed together in the grain bed. If the grains are milled too finely (small de), then it is likely that the pieces will be packed together rather tightly and the void space between the pieces will be reduced. Also, if the grain husks are not mostly intact, then there will be relatively little void space within the grain bed.
Water to grist ratio
Brewhouse Plant Optimisation by R. Wilkinson states that, in a commercial brewery, an overall (mash + sparge) water/grist ratio of 7.5 liters/kg is used (for roller-milled grain) in order to optimize extract yield. Higher gravity beers require that this ratio be reduced for optimal extract yield. Optimized extract yield is a very important consideration for professional brewers, but the ability to lauter the wort is also impacted by the water/grist ratio. At lower water/grist ratios, the concentration of dissolved solids will be higher in the wort. The viscosity, μ, of the wort is directly affected by the presence of dissolved solids. Higher concentrations of dissolved solids in the wort, cause the viscosity to increase. Viscosity directly impacts the rate of run-off during lautering. Higher viscosity leads to a reduced rate of run-off.
Sparging rate
Wort viscosity changes dramatically during the wort collection/sparging phase of the brewing process. Water has a lower viscosity than wort. As water is added to the grain bed during sparging, the overall concentration of dissolved solids within the wort decreases. Decreased viscosity leads to a faster wort run-off rate. Wort run-off rate is usually at its slowest when sparging is first started and speeds up as sparging continues.
Depth of liquid above grain bed
The depth of liquid above the grain bed is important for two reasons. The height of the liquid directly affects the pressure differential, ΔP, across the grain bed. Higher ΔP means faster wort run-off. Unfortunately, if the liquid level above the grain is too high, the positive effects on ΔP will be offset as the higher pressure begins to compress and compact the grain bed, effectively reducing the permeability of the grain bed and slowing down the rate of wort run-off.
Geometry of the lautering vessel and grain bed depth
Wort run-off rate is directly related to the cross-sectional area, A, and depth, L, of the grain bed. A larger cross-sectional area leads to a faster run-off. Conversely, a deeper grain bed decreases the run-off rate. Ideally, a lauter vessel will have as shallow a grain bed depth and as large a cross-sectional surface area as is practical. Commercial lautering vessels tend to be larger in diameter and shallower in depth than the mash vessel. This allows better brewing performance and allows the commercial brewer to use finer-milled grist. Finer-milled grist results in higher extract rates.
Temperature
TemperatureThe temperature of a liquid directly affects the viscosity of the liquid. The viscosity, μ, of a liquid directly affects the rate of run-off, Q. Increasing temperature causes viscosity to decrease and allows liquids to flow more easily. A graph showing the effect of temperature on viscosity for water is shown here.
Adjuncts in the grain bed
A high percentage of adjuncts in the mash can negatively impact the rate of run-off. Barley contains grain husks that allow the grain bed to act as a relatively stable filter media. The barley husks cause void space within the grain bed and increase the grain bed permeability (K). This allows the liquid to flow more freely through the grain bed. If the grain bed contains a large amount of grain that does not have a husk (e.g. malted wheat), or contains components that tend to form “gummy” starch complexes (e.g. oats), then the ability of the grain bed to act as a relatively permeable, stable filtration media is compromised. Lauter tuns can usually filter recipes that contain up to 50% adjunct. If the grain bed contains more than 50% adjunct, there will likely be insufficient husk material to form an adequate filter bed.
Practical discussion
There are several options available for the homebrewer to extract fermentables and separate the wort from the grain. As homebrewers, we are not as concerned with economic efficiencies as commercial brewers, so we may choose to use no-sparge or batch sparge methods, the parti-gyle method, or even the brew-in-a-bag method. Many homebrewers, however, choose to use the more traditional mashing/sparging approach in which grains are rinsed with a slow, continuous sparge. If done properly, a brewer can optimize both extract efficiency and run-off rate using the traditional method.
To optimize the experience using a traditional lautering approach, be sure that grains are milled properly and that the husks are allowed to remain mostly intact. Ensure that sparge water temperature is appropriate (168–176 °F/~76–80 °C). Allow the sparge water to be introduced to the grain bed slowly and ensure that it completely covers the grain. Adjust the flowrate of the sparge water to ensure that there is no channeling of flow through the grain bed. Slow, even coverage of the grain bed by the sparge water is the goal. Additionally, ensure that the flowrate of the sparge water closely matches the flowrate of the wort from the lauter vessel in order to maintain a fairly constant liquid level within the system. Depending upon your lautering equipment configuration, the liquid level above the grain bed should be maintained at a point where run-off speed is adequate and the grain bed does not become agitated by a too-fast flow, nor does it become compressed from a too-high liquid level. Experiment with your system in order to determine a liquid height that works well for you using your equipment. If you have the ability to specify the geometry of your lautering vessel, you should select a vessel that has dimensions that are as large “horizontally” as is practical in order to achieve a reasonably shallow grain bed depth for the range of amounts of grain that you anticipate using when you brew various styles of beer.
Troubleshooting
During the sparging operation you might need to gently stir the grain bed in order to alter the path of flow of the liquid should channeling be observed. If you must stir, try to stir only the upper part of the grain bed. Leave the grains in the bottom 4–6 inches (10–15 cm) of the bed undisturbed if possible.
If the run-off stops flowing and you experience a “stuck sparge,” stir the whole grain bed in order to briefly re-suspend the solids and re-establish flow. If stirring does not fix the problem and allow flow to begin, then you might need to force compressed air backwards through the outlet drain plumbing in order to clear out any obstruction in the line.
References
1) Leiper, K. & Miedl, M., Handbook of Brewing, chapter 10, p. 398 CRC Press, 2006
2) Wilkinson, R., Brewhouse Plant Optimisation. Part II, Brew. Guard., 130(5):22–28, 2001.
3) Crane Technical Paper No. 410, “Flow of Fluids Through Valves, Fittings and Pipes,” 1985
