1. Why Water Comes Out of Cucumber Slices When You Sprinkle Salt on Them
If you sprinkle salt on thin slices of cucumber and wait, water collects on the plate. In an experiment by the Salt Industry Center, hollowing out the center of a cucumber and putting salt inside causes water to fill the hole. Salt has a dehydrating effect that draws moisture out of foods like vegetables [1][2]. Water moves out because plant cells are wrapped in membranes that let water pass but block salt particles. When a strong salt solution forms outside the cell, water moves from the weaker side to the stronger side. This force moving the water is called osmotic pressure [3].
Plant cells also have a strong wall (cell wall) outside the membrane. If the liquid outside is stronger than the liquid inside, water leaves the cell. Only the part wrapped in the membrane shrinks and pulls away from the wall. This is called plasmolysis [4]. Plants rely on the pressure of water inside their cells (turgor pressure) to hold their shape [4]. When water leaves, this pressure drops, making the vegetable limp. Does the same thing happen to bacteria living in food?
Safety Note: Plant cells have a cell wall outside the membrane. The wall does not change shape easily, so only the part inside the membrane shrinks and separates from the wall.
2. How Salt Reduces the Available Water That Bacteria Need
Salt prevents spoilage for two main reasons. First is osmotic pressure. Water leaves bacterial cells, making them weak or unable to multiply. Second, salt reduces the "available water" in the food [5]. The Salt Industry Center explains that salting food reduces the moisture bacteria can use, which stops the bacteria that cause spoilage [1].
Not all water in food is available to bacteria. Water activity (Aw) is a number used as a guide for how well food can be stored, indicating the amount of available water. Pure water is 1, and the number is between 0 and 1 [6]. The US Food and Drug Administration (FDA) defines water activity as the ratio of the vapor pressure of the food itself to the vapor pressure of distilled water under the same conditions [7].
The smaller this number is, the harder it is for bacteria to grow. For common food poisoning bacteria, they can usually grow if the water activity is around 0.900 or higher. At 0.600 or lower, no microorganism can grow [6]. Vibrio parahaemolyticus, a bacterium that causes gastroenteritis, cannot grow if the water activity is 0.94 or lower [8]. Salt and sugar reduce the available water in food, lowering this number. Reducing available water is not just for salt. Dried fish (himono) reduces available water through drying [9]. Frozen water also becomes unusable for bacteria. Salt, sugar, drying, and freezing are all connected by the idea of reducing "water available to bacteria."
Safety Note: The reasons salt prevents spoilage are mainly explained by two factors: osmotic pressure and the reduction of available water.
3. Does Using More Salt Mean Food Never Spoils?
The answer is: not always. If there is not enough salt, if salt-loving bacteria are present, or if other conditions like temperature are right, bacteria can still grow in salted foods.
Vibrio parahaemolyticus is a bacterium found in the sea and river mouths. It is a halophilic (salt-loving) bacterium that needs about 3% salt to grow and can grow in 1–8% salt. It cannot multiply if water activity is 0.94 or lower. Under the best conditions, it divides in less than 10 minutes [8]. If it doubles every 10 minutes, it becomes 64 times larger in 1 hour and over 260,000 times larger in 3 hours. This is a calculated value. In real food, growth changes based on temperature and food type. It does not grow much at 4°C, but grows actively in squid, scallops, shrimp, and crab at 25°C [8].
The Salt Industry Center states that seawater has a salt concentration of about 3% [10]. The US National Oceanic and Atmospheric Administration (NOAA) says the world's average seawater is about 3.5% [19]. A 3% salt concentration means about 3 grams of salt in 100 grams of seawater. The "about 3%" Vibrio parahaemolyticus needs is close to the concentration of seawater.
Staphylococcus aureus is a salt-tolerant bacterium. According to a fact sheet from the Food Safety Commission, it can grow even at salt concentrations of 16–18%. If other conditions are right, it produces toxins (enterotoxins) even at 10%. This toxin is heat-resistant and does not lose its effect with normal cooking [11].
In September 2007, food poisoning caused by low-salt squid shiokara made in Miyagi Prefecture affected 595 people across 12 municipalities. No deaths occurred. A notice from the Ministry of Health, Labour and Welfare in December of that year pointed out that this shiokara had a salt content of about 4%, which is low compared to traditional high-salt aged shiokara. It stated that the effect of preventing food poisoning bacteria from growing could not be expected. The notice reported that poor hygiene management of raw materials and improper low-temperature management at the factory were the main causes [12]. Since Vibrio parahaemolyticus can grow in 1–8% salt, the 4% concentration in this shiokara was not high enough to stop the bacteria. The notice requires low-temperature management at 10°C or below for such low-salt shiokara from production to consumption [12]. How much salt works depends on the combination of salt amount, bacteria type, and temperature.
Safety Note: Staphylococcus aureus can grow at salt concentrations of 16–18% and produce enterotoxins at 10% if conditions are suitable. This is one reason why salting and heating food does not guarantee safety.
4. Why Sugar Also Makes Food Last Longer
Sugar dissolves well in water and makes a concentrated solution. Like salt, it attracts water and reduces the water available to bacteria. Candied fruits, bean paste (anko), and jams are explained as foods where sugar acts as a preservative [13]. Jam is made by boiling fruit with sugar and using pectin and acid to make it jelly-like [14]. The National Center for Home Food Preservation at the University of Georgia explains that sugar in jams and jellies is not just for taste and gel formation, but is also a preservative that prevents microorganisms from growing [15].
For the same weight, salt has many more particles. The size of osmotic pressure is determined by the number of particles dissolved [3]. Salt splits into two particles (sodium ions and chloride ions) in water. Sugar does not split, and one particle is much heavier than salt (the formula weight of salt is about 58, while the molecular weight of sugar is about 342). So, in 100 grams, there are about 10 times more particles in salt than in sugar. This is a calculated value. This calculation alone does not determine preservation, but it is one reason why jams need so much sugar to last.
Safety Note: One reason jams and candied fruits need large amounts of sugar to preserve them is that sugar particles are heavier and fewer per gram than salt particles.
5. How Salt Was Combined with Other Preserving Methods Before Refrigerators
Preserved foods have been made using salting, sugaring, drying, smoking, and fermentation [16]. Salt was often used in combination with other methods. Dried fish (himono) is often made by briefly soaking fish in salt water and then drying it. This combines salt and drying. Drying reduces the water microorganisms can use [9].
In pickles (tsukemono), salt suppresses spoilage bacteria, making it easier for the microorganisms needed for fermentation to work [1]. When lactic acid fermentation makes the food acidic, other microorganisms that cause spoilage or food poisoning have a harder time growing, allowing the food to be stored for a long time [17]. Salt does not just stop all bacteria; it also helps create an environment where helpful bacteria can work.
An example of using a lot of salt is pickled plums (umeboshi). Traditional methods use 25–30% salt, while modern basic products are said to have about 20% or less [18]. In contrast, foods like low-salt shiokara cannot be preserved by salt alone, so they require low-temperature management. You can compare how each product should be stored by reading its label.
6. Where the Salt Used in Japan Comes From
According to the Salt Industry Center, Japan has very little rock salt, seawater has a low salt concentration of about 3%, and it is a country with much rain and high humidity. Because of this, a two-stage method of concentrating seawater and then boiling it developed [10]. It is known that large-scale salt production areas existed around the 8th century [10].
This concentration is close to the 3% Vibrio parahaemolyticus needs. Using seawater alone means the bacterium can grow. To preserve food with salt, it was necessary to use salt stronger than seawater in combination with other methods. Where did the salt in the sea come from? There is more to the story than rivers carrying it. That story is written in the note "Sea salt was not only carried by rivers."
7. How to Test Salt and Cucumber Slices in Your Own Kitchen
You can check this in your kitchen using thin cucumber slices. Ask an adult to use the knife. Sprinkle a pinch of salt on one slice and do nothing to the other. Wait 10 to 30 minutes and compare them. See if water comes out of the salted one and how the cut surface looks different [2]. Eat them right away after checking. Do not leave them sitting out.
When you go to a store, compare the "storage method" labels on bags of pickled plums, salted squid, jam, and dried fish. Some are room temperature, some are refrigerated, and some are labeled "refrigerate after opening." Can you read the differences in salt or sugar amounts and moisture content from the ingredient and nutrition labels?
Safety Note: Prepare two thin cucumber slices (ask an adult to use the knife). Sprinkle a pinch of salt on one and do nothing to the other. Wait 10–30 minutes and compare. The Food Safety Commission's fact sheets on Staphylococcus and Vibrio parahaemolyticus organize the properties of salt-tolerant and salt-loving bacteria with numbers. There is a risk of food poisoning. Do not intentionally rot food to compare it, and do not make your own salt-preserved foods to test.