Is the "Honey" in an Apple Really Sweeter?
― It looks see-through because sap has replaced the air in the gaps
From late October into November, apples with "honey" start to appear in the shops. Cut one open and the area around the core is a see-through amber. In fact, this patch is not especially sweeter than the flesh around it. It looks see-through because the air that filled the gaps between cells has been replaced by sugary sap.
One autumn evening, you cut a gift apple in half. Around the core spreads a glassy, see-through yellow pattern. "A good one," you think, and smile.
You bite into the see-through part. It has a crisp crunch, and it is certainly tasty. But is it several times sweeter than the white flesh around it? Not really, you feel.
So what is this see-through "honey"? And why does it look see-through?
The secret of the honey: just two things
An apple tree sends the sugar made in its leaves to the fruit in a form called sorbitol. In a fully ripe fruit, the cells cannot take it in fast enough. The overflow collects in the gaps between cells.
Ordinary flesh looks white because light bounces many times off the borders of the air gaps between cells. When those gaps fill with sap, the bouncing almost vanishes. Light travels deep inside, and the patch looks see-through.
So the honey is not a lump of sweetness. It is a sign that the apple ripened fully on the tree. Let's go through it step by step.
Honey is sugar that "overflowed" after reaching the fruit
Plants make sugar by photosynthesis in their leaves. Most plants carry this sugar around the body as table sugar. But in fruit trees of the rose family, such as apples, pears and peaches, much of the sugar carried is said to be sorbitol.
Once it reaches the fruit, sorbitol is normally taken into the cells and converted into sugars such as fructose. While the fruit is young, this works well. When the fruit is fully ripe, the cells slow down and cannot keep up with the amount arriving.
The sorbitol with nowhere to go collects in the gaps between cells. Water is drawn toward places where sugar is concentrated, so it flows in from around. In this way, the gaps fill with sugary sap. This is the honey (Figure 1).
White flesh and see-through honey differ only in how light passes
Apple flesh is, at heart, a collection of semi-transparent cells. It looks white because there are tiny air gaps between the cells.
Each time light crosses a border between a cell and air, a little of it is bounced back. One border returns only a small share, but a centimetre of flesh holds about 100 of them. The light scatters in every direction and cannot reach deep. That is why the flesh looks white.
When the gaps fill with sap, the cell and the sap bend light almost equally. The number that describes how strongly a material bends light is called the refractive index. The bouncing at each border then almost disappears. Use the slider in Figure 2 to change what fills the gaps, from air to sap.
So why are apples with honey tasty?
Sorbitol, the main ingredient of the honey sap, is said to be about 60% as sweet as table sugar. To the tongue, it is not an especially sweet sugar. Even if you take out just the honey part and measure it, it is said not to be markedly sweeter than the flesh around it.
Still, there is a reason people say "apples with honey are good". Honey appears in fruit that has ripened for a long time on the tree. Sugar has built up throughout the fruit, and the sourness has eased to a pleasant level. The honey is a "sign" that makes this ripeness visible from the outside.
Honey is said to appear easily in varieties such as "Fuji", and to increase as autumn deepens and the air turns cold. That is why apples with honey are starting to stand out in the shops now.
If you keep an apple with honey for a long time, the see-through part slowly shrinks. This is thought to happen because the sap in the gaps is absorbed by the surrounding cells. Keep it too long and the spot where the honey was may turn brown. If you want to enjoy the honey, it is best to eat the apple soon after buying it.
In Europe and America, the honey is called "watercore", and it is often disliked as a condition that shortens shelf life. The same phenomenon is a treat in Japan and a sign of damage abroad. It is a rare case where the value flips depending on how you look at it.
Summary
Apple honey is sap formed in a fully ripe fruit, when the sorbitol that arrives overflows into the gaps between cells and draws water in. When the air in the gaps is replaced by sap, the bouncing of light almost vanishes and that spot looks see-through. The honey itself is not especially sweet; it is a sign of thorough ripeness.
Honey is not a lump of sweetness, but a sign of full ripeness.
The secret of the whiteness was the air in the gaps.
The way air gaps create whiteness is the same as in why ice in the freezer turns cloudy only in the middle. You can also read about fruit ripening and turning sweet in the black spots on bananas and why dried persimmons get sweet, and about sugar's power to draw water in why honey never goes bad.
- Slice an apple crosswise into rings and look at where the honey shows around the core. The see-through parts often spread near the core and along the veins.
- Hold a thin slice up to a bright window or a lamp. You will see light pass through the honey part, while the white part blocks it.
- Wet just half of a sheet of white tissue paper and hold it up to the light. Only the wet part turns see-through. The air in the gaps between fibres has been replaced by water, the same mechanism as the honey.
Try tasting the honey part and the white part side by side to see how big the sweetness difference is. You may find it is smaller than you expected.
For those who want to know more ― terms, formulas and links to textbooksShows which level each point belongs to, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics, biology and chemistry
- High school+Advanced high-school material, or treated as a textbook sidebar
- UniversityUniversity specialist courses (optics, plant physiology, horticulture), not taught in high school
- ResearchNot yet taught as settled fact, even at university; questions researchers are working on now
Middle schoolTerms: this phenomenon has a name
- Watercore: a state in which the gaps between flesh cells are filled with sugary sap and look see-through. It is seen in apples and pears.
- Sorbitol: a "sugar alcohol" made by adding hydrogen to glucose. In rose-family fruit trees, it is said to be the main sugar carried from leaf to fruit.
- Refractive index: a number showing how much light slows down and bends in a material. It is about 1 for air and about 1.33 for water.
Middle schoolHigh schoolChecking with a formula: how much more easily does light pass when gaps turn to sap?
We calculate the share of light bounced at one cell border for air gaps and for sap gaps, then compare how much light gets through the flesh.
| In symbols | R ＝ { ( n₁ − n₂ ) ÷ ( n₁ + n₂ ) }² 、 T ＝ ( 1 − R )N |
| In words | Share bounced at one border = (difference of refractive indices ÷ sum of refractive indices) squared. Share going straight through = (1 − share bounced), multiplied together once for each border. |
| Where it comes from | From the Fresnel equations, which give the reflection when light strikes the border between two materials of different refractive index head-on. They follow from the condition that light (an electromagnetic wave) joins up smoothly on both sides of the border. |
| R | Share of light bounced at one border (no unit) |
| n₁ | Refractive index of the cell (wall and contents) |
| n₂ | Refractive index of what fills the gaps (air or sap) |
| T | Share of light passing straight through after crossing N borders |
| Refractive index of a cell n₁ (rough guide) | About 1.4 |
| Refractive index of air | 1.0 |
| Refractive index of honey sap (sugar solution) (rough guide) | About 1.35 |
| Number of borders N in 1 cm of flesh (rough guide) | About 100 |
| Air gap: difference in refractive index | 1.4 − 1.0 ＝ 0.4 |
| Air gap: sum of refractive indices | 1.4 + 1.0 ＝ 2.4 |
| Difference ÷ sum | 0.4 ÷ 2.4 ≒ 0.167 |
| Squared, the share bounced R | 0.167 × 0.167 ≒ 0.0279 |
| Sap gap: difference in refractive index | 1.4 − 1.35 ＝ 0.05 |
| Sap gap: sum of refractive indices | 1.4 + 1.35 ＝ 2.75 |
| Difference ÷ sum | 0.05 ÷ 2.75 ≒ 0.0182 |
| Squared, the share bounced R | 0.0182 × 0.0182 ≒ 0.000331 |
| By what factor did the bouncing shrink? | 0.0279 ÷ 0.000331 ≒ 84 |
| With air, share passing through 100 borders T | 0.9721 to the power 100 is about 0.059 |
| With sap, share passing through 100 borders T | 0.99967 to the power 100 is about 0.967 |
| How many times more light goes straight through? | 0.967 ÷ 0.059 ≒ 16 |
The bouncing at each border falls to about one eightieth of its value with air. Through 1 cm of flesh, the light going straight through rises from about 6% to about 97%, around 16 times more. The numbers also confirm why only the honey part turns glassy-clear while the flesh was white. The refractive indices and the number of borders are rough guides, so treat the results as an order-of-magnitude feel.
High schoolHigh school+Refractive index, and water drawn in by osmosis
High schoolAt the border between materials with different refractive indices, part of the light is reflected and the rest is refracted and carries on (physics: "reflection and refraction of light"). The smaller the difference in refractive index, the weaker the reflection. Frosted glass turns clear when wet for the same reason. Also, when sugar collects in the gaps, the outside becomes more concentrated than the inside of the cells, and water is pulled out. This is the idea of osmotic pressure, taught in chemistry and biology.
High school+Sorbitol is a sugar alcohol made by reducing the aldehyde group of glucose. It is about 60% as sweet as table sugar, and because the bacteria behind tooth decay use it poorly, it is also used as a sweetener in chewing gum and similar products.
UniversityThe Fresnel equations, and sorbitol metabolism
The reflectance at a border is given by the Fresnel equations, derived from the boundary conditions of Maxwell's equations; for normal incidence it takes the form of R above. The whiteness of flesh is the result of "multiple scattering" by countless cell-air borders, and can be treated as the same phenomenon that makes paper and snow white. In plant physiology, sorbitol is made in the leaf by sorbitol-6-phosphate dehydrogenase and carried to the fruit through the phloem. Inside the fruit, sorbitol dehydrogenase converts it to fructose. Watercore is explained as arising when this uptake and conversion weaken in ripe fruit, so that sorbitol collects outside the cells (in the apoplast).
📖 Derivations and further reading: Fresnel equations (Wikipedia, in Japanese) ／ Sorbitol (Wikipedia, in Japanese)
ResearchWhat is still not clear
- Why do varieties differ in how easily they develop it? It appears easily in "Fuji", and hardly at all in some varieties. Differences in the transporter proteins that carry sugar into cells, and in the enzymes that convert it, are being studied.
- How does temperature act? Autumn chill and strong sunlight are said to increase honey, but at which stage and by how much is still not well understood.
- Can browning of the honey be spotted in advance? Researchers are studying why the inside of long-stored honey apples turns brown, and how to detect it from outside (for example, techniques that shine light in and measure the interior).
So this article, too, is an explanation of "what is understood so far". Even in the pattern you see every year in an everyday fruit, there are questions still without answers.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science Field 1 "Reflection and refraction of light"; Field 2 "Photosynthesis and movement of nutrients" | Why flesh looks white; sugar made in leaves being carried to the fruit |
| High school | Physics "Reflection and refraction of light"; Biology "Osmosis" | Refractive-index difference and reflection; sugar drawing in water |
| High school+ | Chemistry "Sugars", advanced | Sorbitol as a sugar alcohol |
| University | Optics, plant physiology, horticulture | Fresnel equations, multiple scattering, sorbitol metabolism and watercore |
| Research | Fruit horticulture and postharvest physiology | Varietal differences, how temperature acts, predicting browning |
| ― | Links to daily life | Choosing and storing apples; why wet paper turns see-through |
- Marlow, G. C. and Loescher, W. H. (1984) Watercore. Horticultural Reviews, 6, 189–251.
- NARO Institute of Fruit Tree and Tea Science (農研機構 果樹茶業研究部門), public materials (explanations on apple varieties and fruit quality)
- Ministry of Education, Culture, Sports, Science and Technology (文部科学省), "Standards Tables of Food Composition in Japan (日本食品標準成分表)" (composition of apples)
- Wikipedia, "Sorbitol" (ソルビトール, in Japanese)
- Wikipedia, "Fresnel equations" (フレネルの式, in Japanese)
※This article is a general-audience science explanation. The figures given are rough guides and estimates to help you understand the mechanism. The refractive indices and the number of borders are typical rough values and vary with variety and degree of ripeness.