The grinding powder is not fine enough for secondary grinding? the principle and function of secondary grinding of coffee beans

A friend told Qianjie an interesting practice: "sometimes I forget to investigate the grinding degree, but the coffee powder is too coarse, but I don't want to waste it, so I set the grinding degree to the right scale and grind the coffee powder again." At that time, Qianjie smiled and replied, "which new bean grinder have you taken a fancy to recently?"

Is this feasible? Some friends' understanding of the bean grinder is to grind large particles into fine particles. Particles as large as coffee beans can also be ground into fine powders, and coffee powders thicker than the grinding quota should naturally be ground into powders of about the same thickness if they are to pass. Such a plan seems to be feasible. In practice, it may not be so simple. I believe you have heard of the word "eating powder" in the bean grinder. That is to say, the original grinding of 20g coffee beans turned out to be less than 20g, and the missing part was stuck in the bean grinder. The cause of this situation may be that the oil secreted by coffee beans sticks to the knife head, together with the coffee powder, or it may be static electricity that absorbs the coffee powder on the knife head.

If the ground coffee powder is then ground, the sticking effect of static electricity or grease is more obvious, if serious, the dense coffee powder is concentrated into the knife head (grinding area), which is prone to blockage and downtime. The last bean grinder in Qianjie store was scrapped because it was blocked by a large amount of fine powder. So, don't save the coffee powder and lose the bean grinder. Secondary grinding for the sake of the safety of everyone's bean grinder, Qianjie does not recommend that everyone rashly grind and re-grind the powder. However, we can learn from other people's ideas and methods of secondary grinding. At the 2017 WBC, Japanese contestant Suzuki used secondary grinding when grinding Italian coffee beans. She first cooled the coffee beans with liquid nitrogen, then took a rough grinding scale to grind the coffee beans into coffee powder, and then used liquid nitrogen to cool the coffee powder and grind it into the thickness required for espresso.

In her introduction, liquid nitrogen is used to freeze coffee beans, which reduces the volatile aroma and flavor of coffee powder, while secondary grinding is used because at the same scale, secondary grinding has more surface area than primary ground coffee particles. In other words, the coarse powder in the powder diameter distribution is less, and the powder diameter is more concentrated and uniform. At the 2019 WBrC, Chinese player du Jianning also used secondary grinding to achieve the coffee grinding quality she needed. For the first time, the first grinding still chooses a relatively coarse grinding scale. After grinding out the coarse coffee particles, remove the silver skin first, which will improve the cleanliness of the coffee. Then the coarse coffee particles are then ground again to the desired grinding thickness. The coffee particles obtained by the second grinding will be slightly more uniform than the first grinding. The choice of secondary grinding, the main function is to pursue a more extreme uniformity of coffee powder. However, we should pay more attention to the grinding equipment they use. Suzuki uses EK, the knife head is not easy to clog, and the bean grinder is owned by the organizer; du Jianning uses C40, hand grinding, easy to maintain, but a little tired. Then look at your bean grinder and wallet before making a decision.