Mechanism of Umeshu and Japanese Fruit Wine-1


Umeshu, plum wine and other Japanese fruit wine are easy to make. If you understand how they work, you can make more delicious plum wine without making mistakes.


Wine but NOT Wine

Japanese Umeshu or Kajitsushu is called fruit wine in English. However, its mechanism is quite different from that of normal wine.

Normal wine uses fermentation by yeast. On the other hand, we just pickling fruits into liquor for making Umeshu or Japanese fruit wine.

It is very easy to describe in writing. And when you actually make it, it is very easy. However, something very profound is happening. Let’s take a scientific look at how it works.

Actors are Fruit, Liquor, and Sugar

The process of making Umeshu or fruit wine is very simple. Simply put the fruit, rock sugar, and alcohol in a clean bottle and let it mature, that’s it.

Only three actors play roles in the story of Umeshu: the fruit, the liquor, and the rock sugar. And this umeshu story is in three parts.

Part I: Fruit Meets Liquor

What happens when preparing umeshu or fruit wine? Let’s take a step-by-step approach.

First, fruit and liquor meet. When the fruit meets the liquor, the liquor osmoses into the fruit. Then, the fruit swells up.

In a word, what is happening between the fruit and the liquor is the liquor osmoses the fruit. Since we are here, let’s take a closer look.

The key words here are semipermeable membrane and osmosis.

Cell Membrane is Semi-permeable Membrane

Fruits are made up of many cells. Each cell is surrounded by a skin called a cell membrane. In fact, the skin has many small holes in it.

Smaller particles (molecules and ions) than the holes in the cell membrane can move in and out of the holes. In other words, they can freely move in and out of the cell. On the other hand, particles that are larger than the holes in the cell membrane cannot pass through them.

Thus, a membrane that has holes of a certain size and allows only particles up to a certain size to pass through is called a “semipermeable membrane. In other words, the cell membrane is a semipermeable membrane.

To understand how the semipermeable membrane works, let us first consider the full permeable membrane, which allows anything to pass through.

Diffusion Occurs in a Full Permeable Membrane

What happens when we place different concentrations of solutions across a membrane that allows anything to pass through it?

The red membrane in the center is the total permeable membrane. Since the size of the hole is larger than the △, the △ moves to the right side and the whole area has the same concentration.

In the figure below, there is a lot of △ dissolved on the left side. On the other hand, there are almost no △ on the right side. Since this △ is smaller than the hole in the total permeable membrane, it can move freely between the left and right sides. As a result, the △ on the left side moves to the right side, resulting in the same concentration on both sides.

This movement of the particles in the solution to move freely and make the concentration uniform is called diffusion.

Strictly speaking, both left-to-right (→) and right-to-left (←) movements exist. In this case, however, there is more left-to-right movement. Therefore, in total, we can think that the movement is only from left to right.

Diffusion Does Not Occur in Semi-permeable Membranes

So what happens if we place solutions of different concentrations across a semipermeable membrane and let them touch each other?

The purple membrane is the semipermeable membrane. The hole is smaller than the △, so the △ cannot move to the right side.

A semipermeable membrane is a membrane that allows only particles up to a certain size to pass through. In this case, △ is larger than the hole in the semipermeable membrane. Therefore, the △ cannot move from left to right. In other words, the diffusion seen in the full-permeable membrane does not occur in the case of a semi-permeable membrane.

So nothing happens? Not quite. What is important here is the dissolving side of the solution.

Confirmation of Solute, Solvent, and Solution

A solution is made up of what is dissolved and what is dissolved in it. The dissolved things are sugar, salt, etc., and are called solutes. And what dissolves is water, alcohol, etc., and is called a solvent. Solute + solvent = solution.

So far, we have only focused on the solute, the thing dissolved = △. From now on, we will focus on the solvent, the thing which dissolves =●.

Osmosis Occurs in Semipermeable Membranes

Once again, we are talking about a case where solutions of different concentrations are touching each other across a semipermeable membrane.

The solute △ cannot pass through the semipermeable membrane. However, the solvent ● can pass through the semipermeable membrane. As a result, ● moves from right to left, and the concentrations of the left and right solutions become equal.

We call this movement of the solvent through the semipermeable membrane as osmosis. In other words, osmosis occurs when there is a semipermeable membrane.

Solvent Also Moves by Diffusion

In considering osmosis, we focused on the movement of solvents. In fact, when diffusing through a full permeable membrane, the solvent is also moving to the left or right. However, when viewed as a whole, the left-to-right and right-to-left movements are equal. For this reason, we do not need to consider solvent movement.

Osmosis and Osmotic Pressure are Different

If you search for how plum wine works, you will often see that osmotic pressure is generated. Osmotic pressure means pressure applied to the side of a highly concentrated solution to prevent osmosis. But we don’t apply pressure in making umeshu, do we? The name of the phenomenon occurring in umeshu is osmosis. It is incorrect to say that osmotic pressure occurs.

The difference in the height of the water surface is created by the moving of the dissolved material from the right to the left. If a force is applied to the water surface on the left side, the height of the water surface on the left and right sides will be the same. This force (↓ in the figure) is called osmotic pressure.

What Happened Between the Fruit and the Liquor

Now let’s summarize what happened between the fruit and the liquor.

First, let us compare fruit and liquor. Fruit contains sugar in its cells, whereas liquor does not. In other words, the fruit has a bigger concentration of sugar than the liquor.

The cell membrane of the fruit is semipermeable. The holes in the semipermeable membrane are smaller than the size of the sugar. So, the sugar cannot pass through the holes. This means that the sugar in the fruit cannot pass through the cell membrane and out of the fruit.

However, alcohol and water in liquor are smaller than the holes in the semipermeable membrane. This means that they can pass through the cell membrane and move from the liquor to the fruit.

As a result, the alcohol and water in the liquor permeate into the fruit.

Alcohol and water permeate through the semipermeable membrane into the fruit, causing the fruit to swell up.


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