Prepare equipment ahead of time as you may need to order online. Besides the ume and salt (and red shiso leaves that are required later), you will need a large bowl, a 2-gallon crock, weights (should be double the weight of the ume; the crock comes with 3.85-lb weights, so I use a bag of salt as an addition), a drop lid (John recommended me to get a plastic cutting board on Amazon and cut it slightly smaller than the crock’s opening), a large Japanese bamboo strainer called bonzaru, a mesh hanging dry net.
Day 0 (June 4/Previous Day)
Many recipes mention soaking the ume overnight to remove astringency (“aku” in Japanese). If you are using green ume, you may want to soak for 6 hours. If you ripen the green ume, soak for 2-3 hours. If your ume are yellow and blushed like mine, skip this step as most of them have less astringency, and you are more likely to damage the ume by soaking (discoloration, dent, mold, etc). If your ume got damaged, you can make ume jam after removing the seeds and damaged parts.
Day 1 (June 5)
Rinse the ume under cold running water. If you see blemishes or damages on ume, don’t use it because they can start growing mold from there.
Using a bamboo skewer, remove the woody bits where the fruits are attached to their stems. It’s a tedious work, but please do not skip. Gently dry the ume completely with a clean kitchen towel.
Pour Shochu on a clean kitchen towel and clean the inside of the crock with the alcohol. Make sure your hands are clean.
Sprinkle salt to cover the bottom of the crock. Then add 2 layers of ume.
Sprinkle salt on top followed by 2 layers of ume.
Then continue salt and ume alternatively.
If you use the same size crock, the ume should fill around 2/3 of the way up to the top.
Finally sprinkle the salt. Sterilize the plastic drop lid with shochu where it touches the fruits and place on top of the ume.
Place the weights on top of the drop lid. Cover with the crock’s lid. We use double the weight of the ume. [Optional] If you live in a place with high humidity, you may want to wrap the top tightly with plastic and cover the crock with paper to prevent umeboshi from getting mold. Write down today’s date on the crock. Leave the crock in a dark, cool place.
Day 2-3 (June 6-7)
After a few days, the ume will start releasing moisture and you should see a layer of ume plum vinegar (梅酢, umezu) on top. If the ume plum vinegar does not come up easily, increase the weight so that the ume will sink in the vinegar quickly (this will protect from going bad/growing mold). The picture below shows Day 1 and Day 2.
Day 7 (June 11)
Opening the crock lid for the first time! Use clean hands and equipment to check.
If the ume plum vinegar is 1 inch (2.6 cm) above the plums, decrease the amount of the weight (roughly equal weight as the plums). If the plums are smashed/torn, also decrease the amount of the weight. Store in a cool and dark place for at least 1 month, making sure the ume are soaked in plum vinegar.
Wait Until Red Shiso is Available
Patiently wait until red shiso is in season, usually mid to late June. You can leave your ume in the crock as long as they are soaked in the pickling solution (ume plum vinegar). Get red shiso when you see them in the Japanese grocery store. If your ume haven't reached 7 days at that time, wrap the shiso stems with a damp paper towel and store them in the refrigerator. If you can't get red shiso, you can skip and go straight to the sun drying step.
Day 14 (June 18)
Pick shiso leaves from the stems and put them in a large bowl.
Using plenty of water, rinse them and drain well.
Sprinkle half of the salt and knead the shiso really well. The salt slowly starts to draw the moisture of the leaves as you knead. Please note that red shiso is a powerful dye and your hands will get slightly red for a day or two.
This dark, frothy, purple liquid contains astringency (“aku” in Japanese). Squeeze the shiso tightly and discard the dirty water.
Then add the remaining salt on squeezed shiso and loosen them up. Kneading and rubbing until you get more juice. If you don’t get rid of astringency at this stage, your umeboshi will be darker. So do not skip this step.
You will see more liquid coming out, but this time, it’s slightly prettier color than the first time. Squeeze the shiso and discard the dirty water again.
Open up the pickling crock. Take out 1 cup of clear ume plum vinegar (umezu) from the crock. And add to the squeezed shiso in the bowl.
Loosen up the red shiso, and the ume plum vinegar will soon turn bright purple/red color.
Transfer the red shiso leaves and remaining shiso liquid over the ume.
Evenly distribute the shiso leaves to cover the ume and place the plastic drop lid on top.
Put the equal amount of weights as the ume (6.6 lb or 3 kg). Cover the crock with the top lid and put it back to the dark, cool place. Over the next month, the red color from the shiso leaves will permeate the ume plum vinegar and the ume. When the ume plum vinegar exceeds 1 inch (2.5 cm) above the shiso leaves, you can reduce the weight so the ume will not tear.
A Month After Adding Shiso (July 17-19)
Check the forecast and plan on 3-day ume drying on sunny days. On these 3 days, put the ume under the sunshine for all day. Exposing the ume to the strong summer sun prevents mold growth. The moisture also evaporates, which yields more concentrated flavors and meaty texture.
Open the crock lid and remove the weights and the drop lid. The ume plum vinegar now has a beautiful deep red color.
Scoop away or remove the layer of red shiso so you can see the ume. I just moved them aside so I can pick up the plums. The skin is very fragile so handle it with care.
Pick up all the ume from the crock and drain the liquid over the sieve placed over a large bowl (I use a 4–cup measuring cup here).
The ume closer to the layer of shiso has a darker red color. Transfer the ume to a flat bamboo strainer, keeping them apart from each other. You probably need 2 large strainers.
Take out the shiso leaves and drain over a strainer that is set above a bowl/measuring cup. Squeeze the liquid out.
Place them over a tray lined with foil or parchment paper. Then transfer the shiso to the hanging dry net and place outside for 2-3 days until they dry out completely (in the mid-afternoon, take them inside the house with umeboshi).
Transfer the ume plum vinegar (umezu) from the crock to sterilized mason jars. Keep it in the refrigerator.
Place the ume under the sunshine all day, from morning to mid-afternoon before the air gets cool and the moisture starts to build up.
You may want to put these food cover tents to protect the ume from bugs. When I bring the ume to indoor in the mid-afternoon, I keep them on the strainers. Some people put the ume back in the ume plum vinegar so they can absorb more of the juice and color after being dried all day. It's up to you. The next morning before you take out the strainers again, flip them. I learned that the ume skin does not stick to the strainer after overnight, so it's easier to turn them over in the morning. Repeat this drying process for another 2 days. By the 3rd day, the umeboshi is plump but slightly wrinkled. Some ume may have white salt visible on the surface.
After 3 days of sun drying, they are ready to be packed away. You have 2 ways to do it. 1) putting back to the ume plum vinegar to preserve, 2) keep the umeboshi in the sterlized air-tight mason jar as it is. The first method will yield bright red, juicy, more sour umeboshi. The second method will yield less red, stickier, and less sour umeboshi. I use #2 method.
It’s best to rest umeboshi for at least 3 months so the flavors will mature (I wait for 6-12 months). Even if you can eat the umeboshi immediately, they are just too salty and are not very delicious. If they are stored for 3 months or more, they will mature and mellow out the salty, sour flavors slightly. You can keep the umeboshi in a dark, cool place for 2-3 years.