• Before You Start…Please note that this recipe requires 14 days of fermenting time.

Day 1: Layer the Ume and Rock Sugar

• Gather all the ingredients. You will also need a 3–4 L glass jar, which you can buy at a Japanese or Korean grocery store.
  
• Rinse the jar thoroughly with soap and hot water and wipe dry with a clean towel. While the jar is still hot, pour in boiling water and shake to clean and drain. Air dry completely, making sure there‘s no moisture. Dampen a clean towel with liquor (I use shochu) and wipe the inside of the jar.
  
• Wash and dry 2.2 lbs green ume plums thoroughly.
  
• Remove the stem ends from the plums with a bamboo skewer or toothpick. Discard any plums with brown or blemished spots.
  
• In the clean jar, put some of the plums in a single layer. Then, cover the plums with a layer of some of the 2.2 lbs white rock sugar.
  
• Add another single layer of plums again, followed by another layer of rock sugar. Repeat this process until you’re done layering the plums and rock sugar. The last layer of plums should be completely covered with rock sugar.
  
• Seal, write the date on the jar and store in a cool, dark place (not the refrigerator).
  

Day 2 to Day 14: Ferment

• At least twice a day, gently shake and tilt the jar and make sure the plums are coated with syrup. The goal is to extract the plum flavor as soon as possible. If exposed and not coated with sugar/syrup, the plums may grow mold (and ruin the syrup). Once the essence is extracted, the ume will look wrinkled. Tip: If you see white mold growth on a plum, discard it immediately. If you see white foam and smell fermentation, you have to stop this process and move onto the next step (cooking the syrup).
  

Day 15: Cook the Syrup

• Sterilize the jars and tools you will use to store the syrup. Wash the jars, lids, and bands in hot, soapy water. Put the jars on a rack in a pot of water. Boil at least 10 minutes and keep in simmering water until ready to fill. Carefully remove a jar from the water with a jar lifter or tongs, empty out the water and place the jar on a clean kitchen towel. Put the lids and bands in a small saucepan of simmering water (do not boil) until ready to use.
  
• Take out the plums and weigh them (mine weighed 455 g) if you plan to use them for other recipes. I recommend making Ume Plum Compote.
  
• Pour the syrup into a large pot. Simmer for 15 minutes on low heat, skimming the foam that comes up to the surface with a fine-mesh skimmer.
  
• Fill the jar with the hot syrup, leaving ½ inch of headspace. Repeat to fill the remaining jars. Remove the lids and bands from the simmering water with tongs and place them on top of the jars.
  
• Close the lid tightly and turn the jar upside down to seal. Let cool. Check the seal: The lids should not pop in the center (if they do, you can only keep the syrup in the fridge for up to 2 weeks).
  

To Serve and Store

• To enjoy the syrup, dilute it with iced water or carbonated water. Store unopened vacuum-sealed jar in a cool, dark place for 6 months (possibly 12 months, but I never made enough to try keeping for 12 months). Refrigerate after opening and use it in 2 weeks.