I like this chart for an explanation of efficiency. I guess they don't use the term "mash efficiency", but instead use conversion efficiency.
http://www.brewersfriend.com/brewing-efficiency-chart/
What I believe most people use for efficiency is the percentage of potential sugars that make it (out of the mash and) into the boil pot. Brewersfriend calls this "Pre-Boil Efficiency." I think many people refer to this as "Mash Efficiency."
"Conversion Efficiency" is what percentage of the potential sugar actually got created during the mash.
Conversion Efficiency = Actual Sugar in Mash / Potential Sugar
Conversion Efficiency should always be greater than 98%. If Conversion Efficiency is low, it can be due to the following things:
- Mash temp too high. The amylase enzymes get denatured before starch to sugar conversion is complete.
- Mash temp too low. Conversion reactions happen slower at lower temperatures, and not enough time was allowed for conversion to complete. Or, temp too low for significant conversion to even occur.
- Mash time too short. Not enough time allowed for conversion reactions to complete.
- Crush too coarse. Larger grain particles take longer to convert than smaller particles (all else being equal). Mash time and crush size interact very strongly.
- pH too high or too low. Slows down conversion reactions, and may limit completion if far enough off.
- Dough balls. Dry clumps of grain don't participate in the mash, so no sugar gets extracted from them.
For most people having Conversion Efficiency issues, it's the crush too coarse for the mash time that is the root cause. I give an explanation of how to calculate conversion efficiency
here (personally I find Kai's explanation harder to follow, but ymmv.)
"Lauter Efficiency" is the percentage of sugars actually created during the mash that make it into the boil kettle.
Lauter Efficiency = Actual Sugar in Boil / Actual Sugar in Mash
Lauter Efficiency can be impacted by:
- Undrainable volume in the mash tun (dead volume). Some of the sugar always remains in the dead volume.
- Grain absorption. The more wort retained in the grain, the more sugar is retained.
- Insufficient agitation during mash or prior to run off. If the concentration of sugar in the free wort and wort held in the grain particles isn't equal before run off, less sugar is obtained during run off.
"Pre-Boil (or Mash) Efficiency" is the percentage of potential sugars that actually make it into the boil kettle.
Pre-Boil Efficiency = Actual Sugar in Boil / Potential Sugar
Pre-Boil Efficiency = (Actual Sugar in Mash / Potential Sugar) * (Actual Sugar in Boil / Actual Sugar in Mash)
Pre-Boil Efficiency = Conversion Efficiency * Lauter Efficiency
If your Pre-Boil (Mash) Efficiency is low, then you should determine your Conversion Efficiency. If your Conversion Efficiency is low, then you need to address the items above that can reduce Conversion Efficiency. If your Conversion Efficiency is good, then you need to address the items above that can reduce Lauter Efficiency.
Brew on