Gently wash the wild-foraged mulberries in cold water to remove any forest dust or debris. Drain them completely in a colander.
In your thoroughly sterilized and bone-dry 20L container, begin layering the mulberries and sugar in a 1:1 ratio. Alternate layers of berries and sugar, making sure to save a generous amount of sugar to completely cover and seal the very top layer.
In your thoroughly sterilized and bone-dry 20L container, begin layering the mulberries and sugar in a 1:1 ratio. Alternate layers of berries and sugar, filling the container only up to about 70% to 80% of its capacity to allow proper breathing space. Finally, after adding the last batch of mulberries, pour a thick, generous layer of sugar over the very top to completely cover and bury the berries. This heavy sugar seal is crucial as it blocks oxygen, preventing mold, spoilage, and oxidation.
Close the container lid slightly loosely (or use a gas-release valve) so the natural gases can escape safely. Immediately move the container to a cool, dark place out of direct sunlight, allowing the wild microbes to begin their fermentation in the shade.
During the 100-day aging process, the first month is the most critical. Every 3 days for the first 30 days, insert a completely dry, long-handled ladle deep into the container and stir thoroughly from the very bottom. This ensures the heavy sugar dissolves perfectly and helps the wild mulberries ferment beautifully.
After the first month of active stirring, let the magic happen. Allow the cheong to ferment undisturbed in the dark for the remaining period, reaching a total of 100 days. Once complete, strain out the liquid syrup from the solid berries. Do NOT throw away the leftover berries! Store the pure ruby syrup in a glass bottle for drinks, and save the plump, sugar-soaked berries for baking and toppings.