How to tell if coffee is under-extracted or over-extracted? How much coffee liquid is most suitable for espresso extraction?

Friends often share on the Internet some tips based on his years of extraction experience. These tips have different functions. Some can help us determine the node to stop extraction, and some can make coffee exhaust and extract more evenly. All in all, it can help us make a good cup of coffee. But are these techniques really reliable? Yesterday, a friend asked Qianjie while buying beans, saying that he had seen a technique online to judge whether coffee was over-refined by "observing the face and looking at the color" and asked whether Qianjie was really practical. The so-called "observing the color", simply explained, is a way to judge the extraction based on the color of the coffee liquor that seeps out during the extraction process.

To be reasonable, the color of coffee liquor can indeed reflect an extraction state of coffee. Qianjie has also made relevant sharing before. Whether it is espresso or hand-brewed coffee, as long as friends have observed the color changes of coffee liquor during the extraction process, it should be easy to find that the color of coffee liquor will follow the extraction process. Advance and gradually become lighter from the beginning. The reason for this is that the soluble matter in coffee is limited, which only accounts for 30% of the weight of the bean, and a large part of it will be dissolved in the early stage. Therefore, as the extraction process progresses, the coffee The soluble matter is getting less and less, and the extracted coffee liquor will become lighter and lighter due to the lower and lower concentration.

So why can we tell by color whether coffee is over-strong? Here I have to mention another content that is often shared on Front Street, namely, the dissolution rate of soluble substances-soluble substances in coffee will have different dissolution rates depending on the size of the molecules. Sour substances and sweet substances will dissolve faster because their molecules are relatively small; while bitter substances have relatively large molecules, their dissolution rate will be slower. Different dissolution rates will give these substances different dissolution times. Because sour substances and sweet substances dissolve quickly, they will dissolve quickly, while bitter substances dissolve slowly, so it takes longer to dissolve.

This leads to the fact that when the sweet and sour substances are almost dissolved, the bitter substances are still dissolved, which is basically in the middle and late stage of extraction. Because the substances were almost extracted, the concentration of the coffee began to decrease, and the color of the coffee liquor began to fade and turn white (this is a phenomenon that occurs in espresso, please refer to the figure below).

Then, because what is mainly dissolved at this time is bitter substances, and if extracted later, only more bitter substances will be dissolved (there are also sweet and sour, but less than the same period last year), many friends will regard it as a coffee. The point where it is about to be over-extracted. There has been an attempt on Qianjie before, where the object is espresso coffee, and then the extraction is ended when the coffee liquid begins to white. As a result, the final liquid weight obtained was very close to the optimal liquid weight during Qianjie morning conditioning. Both of them were extracted using 20g of coffee powder. The optimal liquid weight for early conditioning was 39ml. The liquid weight after the extraction was completed through color judgment The weight was 40ml, with an error of 1ml.

In terms of taste, because the difference is only 1ml, the difference in taste will not be large. The coffee beans used in Qianjie at that time were a classic blend in Qianjie bean list. Whether it was the best concentration in the early season or the concentrated flavor that stopped extracting based on the color, they were classic Italian flavors such as chocolate, caramel, nuts, and cream. The only difference is that the concentration of the latter will be slightly lower. Hand-brewed coffee can also use the same method to judge the point at which to stop extraction, when the extracted coffee liquid gradually becomes transparent.

But, but oh, this method can only be used for reference or as an emergency situation. When you don't have a measuring tool such as an electronic scale at hand and want to brew coffee, this method can really help the judgment of coffee extraction. If you have measuring tools, or need stable production, then it is best to use an electronic scale and use the data as a reference. The reason is also very simple. There will be many restrictions on using this method. For example, when the flow rate of coffee liquor is fast or the extraction efficiency is not so high, the color of coffee liquor does not change much. If it were not for an experienced friend, it would be very difficult to judge this node. It would be easy to stop the extraction before the coffee had been fully extracted, and then you would get a cup of coffee that was insufficiently extracted. Of course, it may also be the opposite. Because the difference in color is unclear, the coffee is still extracted when it is over-extracted, and finally a cup of bitter over-extracted coffee is obtained. (The picture below shows the car overturned during Qianjie's first attempt)

In addition, the color of coffee liquor is also affected by the degree of roasting of the coffee beans, which is something that Qianjie shared only two days ago. The deeper the roasting degree of the coffee beans, the darker the color of the extracted coffee will be. When the coffee beans we use are lightly roasted, the whitening will appear earlier, but this is not because the extraction is similar. Instead, the extracted coffee liquor is not so dark, so it will whiten earlier. Therefore, although this little trick of "observing people's faces and observing people's faces" can be used, Qianjie does not recommend that everyone use it all the time. It can be used as a rainy day.

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