Waiter, there’s a fly in my soup- or was that a stone in my coffee…

Image via CoffeeSnobs (with thanks)

The topic of stones in coffee is somewhat of a taboo amongst coffee roasters. Most are not prepared to comment.

Some smaller coffee roasting operations don’t run a destoner and they just hope that no stones find their way into the final product. They might justify their inaction with statements like “I only buy good grade beans” or “I do a visual check”, but anyone who has run a destoner knows that you find all sorts of objects in all sorts of grades of green coffee. Merely looking and hoping doesn’t always work!

If you are purchasing coffee, it’s not a bad idea to ask your roaster how they handle stones. You may well have some homework.

So what do you do?
We run a small destoner and every roast we do passes through it. I have seen and heard of destoners finding items such as:

  • stones
  • teeth
  • rocks and concrete
  • mud that looks like stones
  • bullets and coins
  • drop earrings
  • engagement rings
  • sticks and twigs
  • nuts, bolts, screws and nails
  • lentils and maize

What do you find?
In most of our roasts, we find nothing. In perhaps 1 in 5 batches, perhaps 1 or 2 small concrete stones. If these were to find their way into a grinder, they might sound a little scary, but will ultimately do no damage. Grinder burrs are much harder. In perhaps 1 in 100 roasts, we might find something nastier. We love seeing coffee flavoured popcorn in the cooling tray as it provides a yummy snack!

So destoning is the panacea then?
Coffee destoning should remove most rocks and hard objects if the destoner is correctly tuned and adjusted for that batch of beans- but they are not infallible. The only way of guaranteeing you won’t find something other than coffee is to do a visual check before you load the hopper of your grinder.

At Talk Coffee, we use a large mesh cooling tray which allows the smallest of the rocks to fall through after roasting. We then destone the output to separate larger stones and other dense objects. Anything with a density lower than a roasted coffee bean can still get through (for example a small twig), but it would be sterile (roasted to over 200C) and the grinder would not notice it. On rare occasions and even with the best of processes, stones can find their way into your bag of beans regardless of where you purchase your coffee. It sometimes happens.

At least, I know we have done our utmost to avoid stones… Some may say that forewarned is forearmed!

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