Why Do Both Sugar and Salt Cake, Yet Need Opposite Fixes?
― One Loses Water, the Other Takes It In
Both are white grains, and both harden in the kitchen. But do to salt what helps sugar, and the salt gets even worse. The end results look alike, but the paths that lead there run in opposite directions.
You open a bag of sugar you haven't touched for a while, and the contents have become one lump. Poke it with a spoon and it makes a hard little clunk.
What about the salt container on the same shelf? It is damp and clammy, and shaking it gets you nothing. Open the lid and press with a finger, and it crumbles with a gritty crunch.
The usual remedies turn out to be opposites, too. For sugar, you tuck in a slice of bread. For salt, you add a desiccant or a few grains of toasted rice. One gets moisture added, and the other has moisture taken away.
There are two main reasons for caking
Each sugar grain is coated with a very thin film of sugar solution. When the air is dry, only the water escapes, and the dissolved sugar turns back into crystals in the gaps where grains touch. This acts as glue.
Salt contains a component that pulls water vapour out of the air. On damp days it gathers water and the surface dissolves. When the air dries again, bridges form that join the grains together.
In both cases, the final result is the same thing: a "crystal bridge". What differs is which way the water moved to build that bridge. That difference in direction is exactly the difference in the fixes.
Why does sugar cake in dry places?
The white sugar (jōhakutō) commonly used at home looks free-flowing, but it holds a little water. That water is packed with dissolved sugar, so each grain is wrapped in a film of thick syrup.
In dry air, only the water evaporates from this film. The leftover sugar has nowhere to go, so it crystallises at the single points where grains touch. Countless thin pillars form, and the contents of the bag become one lump.
That is why the bread trick works. Water vapour from the bread rebuilds the film and dissolves the bridges again. The left side of Figure 1 shows this flow of water leaving.
Why does salt cake on damp days?
Salt itself, sodium chloride, isn't especially eager to absorb water. Its surface is said to start dissolving only once the air's humidity rises above about 75%.
But table salt contains small amounts of other substances that come from nigari (bittern, the mineral-rich liquid left after sea salt forms). The main one, magnesium chloride, is said to start gathering water at around 33% humidity. That is an ordinary level for a kitchen, even outside the rainy season.
The gathered water dissolves a little of the salt's surface. Then a dry day comes, the water leaves, and the dissolved salt sets at the points where grains touch. A desiccant in the salt container is there to stop this very first step.
Refined salt with less bittern resists caking because it has less of the water-gathering component itself. Conversely, sun-dried salt and seaweed salt clump more easily for the same reason. Larger grains have fewer contact points, so coarse salt is easier to break up even after it cakes.
Summary
Sugar and salt cake through the same "crystal bridges". But what starts the bridge-building differs: in sugar, water leaving; in salt, water coming in. That is why opposite treatments, bread in the sugar bag and a desiccant in the salt container, are both correct.
There is more than one reason white grains cake.
The same bridge is left behind whether water escaped or water arrived.
A little water can actually bind grains together more strongly, a story that also appears in the article on wet sand. How sweetness can appear without any sugar is covered in the article on baked sweet potatoes.
- Put one tablespoon each of sugar and salt on separate small dishes, and leave them in the same room for three days to a week. Choosing the rainy season or a run of wet days makes the difference easier to see.
- Each day, gently prod them with the tip of a bamboo skewer and compare the surfaces. The salt should turn damp and shiny, while the sugar should dry out and turn whitish.
- Once they have caked, add a small piece of bread to the sugar only and cover it. After a day, check that the sugar has loosened while the salt is unchanged.
The inside of a fridge is dry, so it is a poor place to keep sugar. Sugar in the fridge turns especially hard because the same thing is happening as in this experiment.
For those who want more ― terms, formulas and links to textbooksEach part is labelled, from middle-school science to university-level courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Chemistry / Chemistry"
- High school+Advanced high-school material, or textbook sidebar content
- UniversityUniversity-level specialist courses (physical chemistry, food engineering) not taught in high school
- ResearchNot taught even at university as settled fact; questions researchers are still investigating
Middle schoolTerms: this phenomenon has names
- Caking (koketsu): powder or grains gradually setting into one mass over time. It is one of the most hated faults in factories that handle powders.
- Deliquescence (chōkai): a substance takes in water vapour from the air and turns itself into a solution. Magnesium chloride and calcium chloride are typical examples.
- Crystal bridge: a thin pillar formed where dissolved material turns back into crystal at the contact point of two grains. This is what caking really is.
Middle schoolHigh schoolCheck with a formula: how much sugar becomes bridge?
The water that dries out of the bag is tiny. Can it still make a lump? Let's work out the amount using solubility.
| In symbols | M ＝ W × （S ÷ 100） |
| In words | Mass of sugar that becomes bridges ＝ mass of water lost × mass of sugar dissolving in 100 g of water ÷ 100 |
| Where it comes from | It is simply the definition of solubility. When only water leaves a saturated solution, the dissolved portion stays behind as crystals, by conservation of mass. |
| Mass of one bag of white sugar | 1000 g |
| Water content of white sugar | About 1% (0.01), it is said |
| Sucrose dissolving in 100 g of water at 20℃ | About 204 g, it is said |
| Meaning of symbols and units | M is the mass of sugar that becomes bridges (in grams), W is the mass of water lost (in grams), S is the solubility (grams per 100 g of water) |
| Mass of water in the bag | 1000 × 0.01 ＝ 10 |
| Sugar that was dissolved in that water | 10 × 2.04 ＝ 20.4 |
| Share of the whole bag | 20.4 ÷ 1000 ＝ 0.0204 |
If all the water in one bag leaves, 20.4 g of sugar returns to crystal. That is only about 2% of the total. But sugar grains are very small, and one bag holds tens of millions of them. The bridges gather only at the contact points, so even 2% is enough to make a lump.
High schoolHigh school+The humidity at which melting begins is fixed
High schoolAbove a saturated solution, water's tendency to evaporate is lower than for pure water. This is called vapour pressure lowering. So a saturated solution never dries out completely, even in dry air, and it takes up water from damp air.
High school+The humidity at which these two balance is called the critical relative humidity. It is about 75% for sodium chloride and about 33% for magnesium chloride hexahydrate. From the moment the room's humidity passes this value, the surface of the grains begins turning into a solution. For sugar, the same idea is handled through a quantity called water activity.
UniversityWhat decides how fast powder cakes
Caking theory is built from the amount of liquid gathered at grain contacts and the crystal growth from it. The Kelvin equation covers how the curved surface of the liquid changes vapour pressure, and liquid-bridge force theory covers how the amount of liquid relates to the attraction between grains. It is also known that the critical relative humidity of a salt mixture is lower than that of each component. In food engineering, these are combined into one chart, the water activity isotherm, and used to design shelf life.
📖 Derivations and further reading: Deliquescence (Japanese Wikipedia) ／ Solubility (Japanese Wikipedia)
ResearchWhat is still not clear
- When a bridge breaks How much force it takes to break up caked powder should depend on the thickness and number of bridges. But there are few ways to measure how contacts are distributed inside a bag, so predictions still rely on experience.
- Effect of temperature swings Caking is known to progress faster on shelves with day–night temperature changes. The cause is thought to be water moving and recrystallising repeatedly, but how much progress each cycle brings is unknown.
- Anti-caking additives How fine particles that help powder flow actually work is not settled. They may coat the surface or they may block bridge growth, and no single explanation has been agreed on.
In other words, this article is also "an explanation of what is known so far". Much of what happens in the kitchen remains an open engineering problem.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: solutions and dissolving | Dissolved material reappearing when water evaporates |
| High school | Chemistry: solubility and vapour pressure lowering | The calculation in the formula check, and the saturated-solution explanation |
| High school+ | Chemistry: critical relative humidity | Salt behaving differently at 75% and 33% |
| University | Physical chemistry, food engineering | The Kelvin equation and water activity isotherms |
| Research | Powder engineering | Bridge strength, and how anti-caking additives work |
| ― | Link to daily life | Opposite remedies: bread for sugar, desiccant for salt |
- Salt Industry Center of Japan (公益財団法人塩事業センター), "Salt Encyclopedia" (塩百科)
- Japan Society for Food Engineering (日本食品工学会) et al., eds., Food Engineering Handbook (食品工学ハンドブック), chapter on water activity and shelf life
- Kagaku Binran (化学便覧), Basic Edition: entries on the solubility and deliquescence of sodium chloride, magnesium chloride and sucrose
- Japan Sugar Refiners' Association (精糖工業会), explanatory material on "Types and Properties of Sugar" (砂糖の種類と性質): sections on the water content of white sugar and on invert sugar
※This article is a general-audience science explainer. The numbers given are rough guides to help you understand the mechanism. For food storage, follow the product label and the manufacturer's instructions first.