Recently we did a short blog on the life of burrs! It was well received and generated many more questions from our Baratza community, especially around the topic of the differences between ceramic and metal burrs, and the heat generated in burrs, when grinding. We decided it was time that Kyle Anderson, download some of that knowledge that he's built up over his 20 years in the business of designing and manufacturing coffee machines and grinders, and let you in on some facts on these burrs! By the way, Kyle's the President and Co-owner of Baratza, plus he's the chief designer/engineer/geekhead – he knows a thing or two!
So, here you go.............
by Kyle Anderson
As with the ongoing argument about which is better: a Ford or a Chevy, so goes the banter about steel vs. ceramic burrs in coffee grinders. In the former argument the facts are few and the fur flies readily, in the later the same is true. The few known "facts" about steel and ceramic burrs are mixed in with a large dose of misunderstandings. My goal with this blog is to lay out the facts, dispel the most common misunderstandings, all in the hopes of producing a better educated coffee grinder user/buyer. While we have produced grinders for 11+ years with conical and flat burrs, I will limit this missive to flat burrs only.
First a few facts….
- Ceramic burrs are harder than steel and last (on average) about twice as long as steel burrs.
- Ceramic burrs are more brittle than steel so you could chip the ceramic burr if a very hard rock was hiding in the beans.
- Ceramic burr are less thermally conductive than steel, this means these burrs transfer less heat than steel burrs (See below on heat).
- Ceramic burrs require a custom mold or tool to form them. These tools are not cheap so there are far fewer ceramic burr designs to choose from than steel.
- Unit cost of ceramic burrs are MUCH lower than comparable steel burrs, typically less than half (after the molds to make the burrs are paid for)
- Ceramic burrs are ideal for heavy duty commercial applications (see LaMarzocco Swift grinder for $4500! It is only sold with ceramic burrs)
- The material of the burr has no bearing on the speed it must rotate, this is a function of the design of the teeth.
- The noise generated during grinding is a function of the teeth design not the material of the burr.
- Ceramic and steel burrs both handle all roasts equally
- Steel burrs MAY be more durable than ceramic if a stone is encountered, but the steel burr can dent or chip also.
Heat and grinding coffee beans: the big misunderstanding
Thermal conductivity of the burrs (i.e. The material they are made from) has NO BEARING on the root cause of heat build up in grinders which comes from a combination of internal friction as the coffee bean is crushed, and from minor friction in the actual cutting of the bean by the burrs. The sharper the burrs ,the less heat is created in the cutting of the bean. The majority of heat creation comes from the crushing of the bean. One could actually argue that the higher thermal conductivity of steel burrs can actually decrease the heat of the ground coffee in small batches. This is because the heat is created IN the bean and is then transferred to the burrs. If the burrs are steel, they will do a MUCH better job of transferring this heat away from the coffee and to the housing of the grinding mechanism. The actual surface temperature of the grinding surface will be similar whether the burr is steel or ceramic, except for that heat which is able to transfer through the burr and away from the coffee. In large industrial coffee grinders, the burrs are mounted to a plate that has water cooling running through it. The burrs conduct the heat away from the beans (and the grinding surface) to the water. A secondary source of heat in ground coffee comes from heat that is stored in the burrs and grinder housing (from coffee ground earlier or immediately preceding) and transferred back into the coffee being ground in the moment. And this brings me to another pet peeve:
The speed of rotation of grinding burrs and the effect on heat build up in the the ground coffee.
Dull and/or poorly designed burrs (of any material (steel or ceramic)) will result in inefficient grinding of the coffee which in turn will generate more heat in the beans. The actual speed of rotation of the burrs is not the dominant (or even significant) variable. I have seen designs of conical burrs turning at 3400 rpm and grinding 10+ grams per second and the coffee comes out cool. (Typical conical burr grinders turn at 300-500 rpm) An argument can be made for lessening the contact time that the coffee beans have with the burrs (once the burrs get hot). Remember, the surface of the burrs get hot from the beans that have been ground before, this hot surface can then conduct this heat back into new coffee. As coffee passes through the grind path quickly (this can only happen with well designed, sharp burrs and a clever grind chamber design) the contact time is so minimal that the only heat experienced by the coffee is that from its own crushing and cutting, not from latent heat in the burrs or grinder housing.
I hope this writing has shed some light on common misunderstandings relative to burrs materials and heat issues in the grinding of coffee.