Why Do Vegetables Get Sweeter After a Frost?
The Sweetness Is a "Water Holder" That Keeps Cells From Freezing Up
Winter vegetables are sweet. Farmers have known this for a long time, and some even expose their crops to cold air on purpose. But the sweetness isn't there for us. It is a preparation the vegetable makes inside its own body to protect its cells from the cold.
It is a December morning, and the whole field is white with frost. The spinach leaves lie flat against the ground, slightly crinkled and darker in colour.
Boil that spinach and eat it, and it tastes sweet, like a different vegetable from the summer one. People say the same of daikon radish and cabbage: they taste better after a few frosts.
It was the vegetable that went through the cold, so why does it come out sweeter? And why is a frozen leaf still alive after it thaws?
There Are Only Two Reasons It Gets Sweet
The vegetable breaks down the starch it has stored and turns it into sugar. In the cold, it also burns less sugar through respiration, so the sugar keeps building up.
Below freezing, ice forms first outside the cells. That ice tries to draw water out of the cells, and the dissolved sugar holds it back.
In other words, the sweetness is the vegetable's winter coat. We simply get to enjoy the leftovers.
When Vegetables Sense Cold, They Start Making Sugar
Spinach and daikon store the nutrients they make by photosynthesis as starch. Starch is a large grain made of many sugar units joined together. As a grain, it doesn't taste sweet on the tongue.
As the temperature drops, enzymes inside the vegetable chop the starch into small pieces and turn it into sugar. At the same time, the cold slows respiration, so less sugar is burned. The plant makes sugar by photosynthesis in the day and uses little of it at night. The difference piles up, and the vegetable turns sweet.
This change is said to take anywhere from a few days to a few weeks after the cold sets in. One night of frost does not suddenly make a vegetable sweet. Over many chilly days, the vegetable gradually gets itself ready for winter.
Why Can Sugar Protect Cells?
This is the most surprising part of the story. Sugar water is often said to freeze less easily than pure water. But at the concentration found in vegetable sap, the freezing point drops by less than 1°C. On a frosty night at minus 5°C, that alone is not enough.
What really happens is something else. Below freezing, the water in the gaps between cells freezes first. Ice has a tendency to draw nearby water toward itself. So water inside the cells moves out through the cell walls toward the ice outside.
Look at Figure 1. On the left is an ordinary leaf cell. On the right is a cell that was exposed to cold and has stored twice as much sugar. In both, ice forms outside and water flows out in the direction of the arrows. But the amount of water left inside is different.
The more that is dissolved inside, the harder it is for a cell to let go of its water. With twice the sugar, about twice as much water remains in the same cold. A cell that loses too much water shrinks and is damaged, and cannot recover when it thaws. Sugar is what holds the cell back just short of that point.
Sugar is also thought to protect cell membranes from drying out. Together with its small effect on the freezing point, this makes a protection with several layers.
In the Tohoku region of Japan, a growing method became popular in which spinach raised in a plastic greenhouse is deliberately exposed to cold outside air before harvest. The sides of the greenhouse are opened, and the crop spends many chilly days that way. There are reports of sugar levels reaching about twice those of normally grown spinach. In Hokkaido and elsewhere, there are also "overwintered vegetables": cabbages and carrots grown in autumn are left under the snow and dug up in winter.
Potatoes also turn starch into sugar when kept as cold as a fridge. They do get sweeter, but that sugar makes them burn more easily when fried or roasted. Burnt parts are said to contain more harmful substances, so potatoes are best stored in a cool, dark place.
Summary
Vegetables are sweet after a frost because, sensing the cold, they turn starch into sugar and store it. That sugar holds back the loss of water from cells below freezing and protects them from the cold. Farmers know this, and they chill their crops on purpose to make them sweeter.
The sweetness of winter vegetables is preparation against the cold.
We get to enjoy their "winter coat."
The story of spraying water on fields so that freezing releases heat and protects crops is in "Why Do Farmers Spray Water on Their Fields on Nights When Frost Is Expected?", and seasonal differences in nutrition are in "Are Seasonal Vegetables Really More Nutritious?". For how sweet potatoes make sugar as they bake, see "Why Are Baked Sweet Potatoes So Sweet Without Any Added Sugar?"
- Prepare two identical cups. Pour the same amount into each: plain water in one, and water with two tablespoons of sugar dissolved in it in the other.
- Put both side by side in the freezer and check every 30 minutes to compare when ice starts to form on the surface.
- When winter comes, nibble a little of the raw root end of spinach from a field after a frost and compare it with summer spinach.
The sugar water should start freezing a little later. But the difference is small. Remember that what protects the vegetable is the "holding back of water" more than the resistance to freezing. If you eat vegetables raw, wash them well first.
For Those Who Want to Know More: Terms, Formulas and Links to TextbooksEach section is labelled with its level, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school chemistry and biology
- High school+Advanced high school material, or textbook sidebars
- UniversityUniversity specialist courses (plant physiology, cryobiology), not taught in high school
- ResearchTopics researchers are still investigating, not settled even at university level
Junior highTerms: This Phenomenon Has Names
- Cold acclimation (teion junka): the process by which a plant, after experiencing cold for many days, becomes a body that resists freezing. Sugar build-up is part of it.
- Extracellular freezing: freezing of the water in the gaps between cells first, rather than inside them. Cold-hardy plants survive by freezing in this way.
- Freezing point depression: the property that water with something dissolved in it starts to freeze at a lower temperature. It depends on the number of dissolved particles.
- Kanjime: a growing method in which vegetables are deliberately exposed to cold before harvest to raise their sugar and other contents.
Junior highHigh schoolChecking With a Formula: How Much Water Stays in the Cell?
Let's estimate the fraction of water left in a cell on a frosty night at minus 5°C. This is a simple model that treats the cell sap as a water solution with sugar and other things dissolved in it.
| In symbols | ΔT ＝ Kf × m ／ Fraction of water left ＝ m0 ÷ ( ΔT ÷ Kf ) |
| In words | Drop in freezing point = constant × concentration of dissolved material. Fraction of water left = starting concentration ÷ concentration in balance with ice |
| Where it comes from | This is the freezing point depression formula. With ice outside, water inside moves out until the sap reaches "the concentration that balances with ice at that temperature". The amount of dissolved sugar does not change, so only the amount of water goes down. |
| ΔT | Drop in temperature from 0°C (unit: °C) |
| Kf | Molal freezing point depression constant of water: 1.86 (unit: °C·kg/mol) |
| m | Amount of dissolved particles per 1 kg of water (unit: mol/kg) |
| m0 | Starting concentration of the cell sap before freezing (unit: mol/kg) |
| Molal freezing point depression of water | 1.86 °C·kg/mol |
| Concentration of ordinary leaf sap (assumed value) | 0.3 mol/kg |
| Leaf chilled to build up sugar (assumed value) | 0.6 mol/kg |
| Temperature on a frosty night | Minus 5°C (a drop of 5°C) |
| Drop in freezing point for an ordinary leaf | 1.86 × 0.3 ≒ 0.56 °C |
| Drop for a leaf with twice the sugar | 1.86 × 0.6 ≒ 1.12 °C |
| Concentration in balance with ice at minus 5°C | 5 ÷ 1.86 ≒ 2.69 mol/kg |
| Fraction of water left in an ordinary leaf | 0.3 ÷ 2.69 ≒ 0.11 |
| Fraction of water left in a leaf with twice the sugar | 0.6 ÷ 2.69 ≒ 0.22 |
| Ratio of water left | 0.22 ÷ 0.11 ＝ 2 |
Even doubling the sugar lowers the freezing point by only about 1°C. On a minus 5°C night, both kinds of cell will eventually freeze from the outside. The difference lies in how much water is left. An ordinary leaf keeps only about a tenth of its water, while a sugar-rich leaf keeps about two tenths. The numbers in Figure 1 run on the same formula as this calculation.
High schoolHigh school+How Starch Turns Into Sugar, and Osmotic Pressure
High schoolStarch is a substance made of glucose joined in long chains. When enzymes such as amylase cut it, it becomes maltose and glucose. In plants, these are further rearranged into sucrose (the same substance as table sugar) and others, and stored. Breaking one starch grain into sugars increases the number of dissolved particles hundreds of times over. Both freezing point depression and osmotic pressure depend on the number of particles, so breaking up a big grain makes a real difference.
High school+Properties like freezing point depression and osmotic pressure, which depend only on the number of dissolved particles and not on what they are, are called colligative properties. Water keeps moving from the cell to the ice until its "urge to escape" is equal inside the cell and in the ice outside. The formula above is an approximation of that equal condition for dilute solutions.
UniversityExtracellular Freezing and Freezing Dehydration
In plant physiology and cryobiology, most cold-hardy plants are explained as getting through winter by extracellular freezing. The main cause of damage to cells here is thought to be not the ice itself but dehydration caused by freezing, and the resulting deformation of the cell membrane. In cold-acclimated plants, besides sugar, amino acids such as proline and antifreeze proteins, which keep ice crystals from growing large, are also known to increase. The formula above is an ideal-solution approximation. Real cell sap is concentrated, so actual values deviate somewhat.
📖 Derivation of the formula and further reading: Freezing point depression (Wikipedia, in Japanese) ／ Colligative properties (Wikipedia, in Japanese)
ResearchWhat We Still Don't Fully Know
- Where does a plant sense the cold? Changes such as the stiffness of the cell membrane are thought to act as the signal, but what the very first "thermometer" is remains debated.
- Which protection matters, and by how much? Sugar, amino acids, antifreeze proteins, and swaps in membrane components are all layers of defence. The size of each contribution is said to differ between plant species.
- How can sweetness and cold hardiness go together? Research continues into breeding and growing methods that give vegetables that resist frost damage and still taste good.
So the content of this article is also "an explanation of what is understood so far". In particular, Figure 1 and the formula are simple models for seeing how the mechanism works.
Links to Textbooks (by Level)
| Level | Subject / Unit | Where in this article |
|---|---|---|
| Junior high | Science Field 1 "Aqueous solutions" and Field 2 "Structure and function of plants" | Starch and sugar, photosynthesis and respiration |
| High school | Chemistry "Properties of solutions" and Biology "Metabolism" | Freezing point depression, enzymatic breakdown of starch |
| High school+ | Chemistry (advanced) | Colligative properties, and the balance of ice and water |
| University | Plant physiology, cryobiology | Extracellular freezing, freezing dehydration, cold acclimation |
| Research | Plant responses to the environment | How cold is sensed, size of each protective contribution |
| ― | Links to daily life | Sweetness of winter vegetables, kanjime, storing potatoes |
- Wikipedia (Japanese), "Freezing point depression"
- Wikipedia (Japanese), "Osmotic pressure"
- Wikipedia (Japanese), "Antifreeze protein"
- Akira Sakai, 『植物の耐凍性と寒冷適応 ― 冬の生理・生態学』 (Freezing Tolerance and Cold Adaptation of Plants: Winter Physiology and Ecology), Gakkai Shuppan Center
- National Agriculture and Food Research Organization (農研機構) Tohoku Agricultural Research Center, research results and cultivation materials on "kanjime spinach"
- Ministry of Agriculture, Forestry and Fisheries (農林水産省), "Information on acrylamide in food" (on how to store potatoes)
* This article is a popular-science explanation for general readers. The numbers given are rough guides to help you understand the mechanism. The concentration of cell sap is an assumed value, and the actual sugar content and cold tolerance of real vegetables vary greatly with variety and growing conditions.