Can Coffee Beans Be Re-Grinded If the Grind Isn't Fine Enough? The Principles and Effects of Double Grinding Coffee Beans

A friend once shared an interesting technique with FrontStreet Coffee: "Sometimes I forget to adjust the grind setting, and the coffee grounds come out too coarse. But I don't want to waste them, so I adjust the grind to the appropriate setting and grind the coffee grounds again." FrontStreet Coffee laughed and replied: "Which new grinder have you been eyeing recently?"

Is this approach feasible? Some people understand grinders as machines that break large particles into smaller ones. Since coffee beans are relatively large particles, they can be ground into fine powder. If coffee grounds that are coarser than the desired grind size pass through, they should theoretically be ground to a similar fine consistency. This plan seems feasible.

In practice, however, it's not quite that simple. I'm sure everyone has heard of the term "retention" when it comes to grinders. This means that when you grind 20g of coffee beans, you might end up with less than 20g of grounds, with the missing portion getting stuck inside the grinder. This situation can be caused by coffee oils secreted from the beans sticking to the burrs along with coffee grounds, or by static electricity attracting coffee grounds to the burrs.

If you grind already-ground coffee again, the effects of static electricity or oil adhesion become even more pronounced. In severe cases, dense coffee grounds concentrating in the burrs (grinding area) can cause blockages and even machine failure. FrontStreet Coffee's previous grinder broke down and became unusable precisely because the burrs were clogged with excessive fine grounds. So, don't save coffee grounds at the expense of your grinder.

Secondary Grinding

For the safety of everyone's grinders, FrontStreet Coffee doesn't recommend recklessly grinding coffee grounds twice. However, we can learn from the concept and method of secondary grinding used by others.

In the 2017 World Barista Championship (WBC), Japanese competitor Suzuki Itsuki used a secondary grinding method for espresso beans. She first used liquid nitrogen to cool the coffee beans, then ground them at a coarse setting. After that, she used liquid nitrogen again to cool the grounds before grinding them to the fineness required for espresso.

According to her explanation, liquid nitrogen was used to freeze the coffee beans, which helps reduce the loss of aromatic flavors during grinding. Secondary grinding was employed because, at the same grind setting, twice-ground coffee particles have more surface area than once-ground particles. This means there are fewer coarse particles in the grind distribution, resulting in more concentrated and uniform particle sizes.

In the 2019 World Brewers Cup (WBrC), Chinese competitor Du Jianing also used secondary grinding to achieve her desired coffee grind quality. First, she chose a relatively coarse grind setting for the initial grinding. After grinding the coarse coffee particles, she removed the silver skin to improve the coffee's clarity. Then she ground the coarse particles again to the required fineness. Coffee particles obtained through secondary grinding are slightly more uniform than those from single grinding.

The primary purpose of choosing secondary grinding is to pursue ultimate coffee ground uniformity. However, we should pay more attention to the grinding equipment they used. Suzuki Itsuki used an EK grinder, whose large burrs are less prone to clogging, and the grinder was provided by the event organizers. Du Jianing used a C40 hand grinder, which is easy to maintain, though a bit tiring. Then consider your own grinder and wallet before making a decision.

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