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Plum, which is often seen in stores in early summer, is a fruit that is effective in relieving fatigue and is perfect for the coming hot season. There are various processed plum products such as pickled plums and vinegars, and among them, Umeshu is popular among all generations and both genders for its sweet and sour taste.
To make Umeshu, plums are soaked and preserved in a solution of white liquor and glacial sugar, during which time the rock sugar slowly melts, and it takes from three to six months until they are ready to drink. But why is rock sugar, which does not dissolve easily, used to make ume liqueur instead of granulated sugar?
The movement of water due to “osmotic pressure”.
➢ What is osmotic pressure?
➢ Osmotic pressure makes pickles tasty.
➢ Why is rock sugar used for Umeshu?
➢ What happens if we use other sugar to make Umeshu?
What is osmotic pressure?
“Osmotic pressure” refers to the pressure that occurs when two liquids of different concentrations are placed next to each other through a semipermeable membrane (a membrane that allows small water molecules to pass through, but not large molecules such as sugar dissolved in water, and allows molecules below a certain size to pass through). This is referred to as pressure.
Imagine that when you add a lump of sugar to coffee, it dissolves. At the moment the sugar is added, a partial difference in concentration is created. However, as time goes by, the sugar dissolves throughout the coffee and the sweetness becomes uniform? This is also a function of trying to keep the concentration difference the same.
Osmosis is not a phenomenon that occurs only in sugar. When salt is sprinkled on cucumbers, water is drawn out from the inside, which is also a function of osmosis to equalize the difference in salt concentration between the cucumber’s surface and inside. Osmosis is effectively used in cooking. Salting cucumbers and sprinkling salt on fish before grilling are also ways to condense umami by removing excess water.
Osmosis pressure makes pickles tasty
Pickles have long been popular in Japan as a way to preserve vegetables. In fact, the more sugar used in the seasoning solution, the more easily the flavor is absorbed. Vegetable cells used for pickles have a semipermeable membrane called a cell membrane, and when vegetables are soaked in a seasoning solution, a large amount of water moves out of the cells (toward the thicker seasoning solution) in an attempt to maintain a constant concentration inside and outside the cells.
Adding caster sugar to the seasoning liquid increases the concentration of the seasoning liquid and thus osmotic pressure is easily generated. When water is removed from the vegetables, the structure of the cell membrane of the vegetables changes, and the semipermeable membrane ceases to function, allowing the seasoning liquid to soak into the vegetables, resulting in delicious pickles.
Why is rock sugar use for Umeshu
Why do we use rock sugar instead of caster sugar or granulated sugar when making Umeshu? The truth is that rock sugar’s hardness to dissolve is utilized.
When Umeshu is freshly made, the extract of the fruit is thicker than the outer liquid, so the water and alcohol move to the fruit.
As time passes, the sugar gradually dissolves from the rock sugar, and the sugar concentration is higher in the outer liquid than in the ume fruit, so the water and alcohol are gradually released from the ume fruit.
The flavor of the plums is transferred to this water and alcohol, resulting in a rich and delicious Umeshu.
What happens if we use other sugar to make Umeshu?
What would happen if other sugar, which dissolves easily, were used instead of rock sugar? Since plums are suddenly soaked in a high-sugar solution, the water in the plums is rapidly drained out to adjust the sugar content due to osmotic pressure, and the plums become hard and shriveled.
In order to prevent this from happening and to fully bring out the sourness and aroma of the plums in the soaking liquid, it is desirable to start with a very light sugar content and gradually increase the sugar content, making slowly dissolving glacial sugar suitable.
Making Umeshu that takes a lot of time and effort has a proper scientific backing. When you understand the principle of osmosis in this way, don’t you want to try your hand at making Umeshu while observing the “osmosis pressure” of the plums?
