The original homogenization method! Mortar and pestle homogenizers consist of a hard, blunt object (the pestle) which is used to grind the sample against a solid container (the mortar). Used in life science applications, there are often called tissue grinders. There are also specific styles of mortar and pestle homogenizers which create a tighter fit between the mortar and pestle and thereby increase the effectiveness of homogenization. These include the glass dounce, Potter, Potter-Elvehjem, or Tenbroeck. A mortar and pestle homogenizer can, however, be as simple as a cheap plastic rod which is used in a standard tube.
Mortar & pestle homogenizers may be manual or motorized, although dounce homogenizers are almost always manual. Motorized mortar and pestle homogenizers are usually low-power devices which simply spin the pestle in order to create additional shear forces and thereby assist in homogenization.
Homogenization with a mortar and pestle is a low-throughput method, used most often for relatively small samples which are processed one at a time. They can be used for just about any type of sample, provided that it is not too hard, as pestles typically cannot be used to grind substances harder than themselves.
The primary benefit of the mortar and pestle method is cost and ease of use. A basic plastic pestle costs a few dollars, and even motorized mortar and pestle homogenizers can cost as little as $200. Maintenance is easy - pestles may be washed and re-used or simply discarded after use - and there is very little setup required.
Perhaps the most unique advantage of the mortar and pestle method is its suitability for homogenization under cryogenic conditions. Because both the mortar and the pestle can be made of just about any material, they can be made to tolerate cryogenic temperatures. This is particularly useful in the life sciences, especially when an analyte may be subject to rapid degradation ex-vivo.
Because mortar and pestle homogenization is a somewhat manual process, even if a motorized homogenizer is used, it is more prone to human error and poor reproducibility than other methods.
If you’re homogenizing inside a tube, ensure that your pestle reaches the bottom of the tube. If the bottom of the tube is narrow and the pestle is wide, you may end up with a gap that you cannot reach at the bottom, and homogenization will be poor.
When using a glass dounce, don’t put too much pressure on the pestle, as this may cause it to break. Dounces are designed for a very tight fit, so you do not need to put a lot of force behind it. Repeatedly push the pestle gently into the mortar and rotate the pestle.
If you’re going to be doing a lot of homogenization, be sure to get a motorized homogenizer. Using a manual mortar and pestle can cause wrist and / or hand strain if performed for extended periods of time.
If you are concerned about yield and your application is not cryogenic, consider using a dounce, Potter, Potter-Elvehjem, or Tenbroeck homogenizer for more thorough homogenization.