Turning freshly squeezed lemon juice into sour crystals can be achieved by crystallizing citric acid and sodium citrate in lemon juice. The specific methods are as follows:

1. Filter the freshly squeezed lemon juice to remove impurities from the juice.

2. Pour the filtered lemon juice into a flat-bottomed container, such as a stainless steel pot or a glass container.

3. slowly heat the lemon juice until it boils, and then turn off the fire.

4. Gradually add sodium citrate, and stir evenly with a spoon until the sodium citrate in the lemon juice is completely dissolved.

5. Put the container in the shade, let it cool naturally, and then put it in the refrigerator for refrigeration.

6. After the lemon juice is completely cooled, you can see that white crystals gradually appear in the lemon juice.

7. Filter out the crystals in the lemon juice with a sieve or gauze, leaving pure citric acid crystals.

It should be noted that in the production process, it is necessary to master the amount of sodium citrate and the degree of stirring uniformity, so as not to affect the quality and taste of crystals. At the same time, we should also pay attention to safety during the production process to avoid scalding or eating by mistake.