We say in Japan “One umeboshi a day keep the doctor away.” Pickled plum, umeboshi, which is salty and extremely sour, has many health benefit. Umeboshi is an alkaline food and rich in citric acid. One umeboshi a day maintains our blood pH at neutral. Citric acid in umeboshi dissolves lactic acid and helps our body to regain energy.
My sister, Yoko, is now enjoy making umeboshi, pickled plum in Tokyo. She began this annual ritual several years ago. Here in New York city I watch the photos which she sent to me at this time of the year, and just admire them. Obtaining fresh Japanese plum, ume, to make umeboshi in New York city is very difficult, or close to impossible. Umeharvested in California comes to Mitsuwa Marketplace, New Jersey, in very limited quantity in June in just a week or so window.
Umeboshi has also antiseptic property. In the past when there was no refrigeration system we packed rice with umeboshi in order to prevent the cooked rice from spoilage on the day of making. Onigiri rice ball stuffed with a piece of umeboshi is a good example. 
You can find umeboshi at Japanese food store. Please choose the ones which has no chemical additives, such as MSG, preservatives (the salt reduced type) and food coloring. You can assure the health benefit of umeboshi only from the ones which are made in the traditional way without additives.
If you live in California you can source ume plums much easier locally than us on East Coast. Here is Yoko’s recipe. Please try it next year. You need to start the process around June 20th. Mark the calendar right now!
2 pound ume plum
6 1/2 ounce sea salt for ume plum
7 ounce purple shiso
35g sea salt for shiso
Distilled alcohol
1. Rinse the ume plums. Remove the stems with toothpick.
2. In a clean Ziploc like bag add the plum and sea salt for the plum, and mix them well.
3. In another Ziploc like bag add the cleaned purple shiso and sea salt for shiso. After 30 minutes of resting, squeeze the shiso to remove bitter juice, discarding the juice.
4. Rinse a non-reactive pickling pot with distilled alcohol to sanitize the inside. Transfer the salted plum and shiso to the pot. Cover the pot with a triple layered plastic wrap and top it with 6 pound weight. Keep the pot at cool, breezy and dark place for about 5 days. By the fifth day the plum is completely covered with own liquid. Let it stand as it is.
5. After about one month (choose the sunny days) remove the plums from their own pickling liquid and dry them outside in the sun for about 3 to 4 days.
6. Transfer the dried ume to a clean glass jar. Strain the pickling liquid through a fine sieve and store it separately with the plums. Use the plum vinegar in cooking.