Do Lemons Start Off Green? The Science of Ripening

Every lemon you have ever seen at the grocery store was, at some point in its life, green. Lemons begin their development covered in chlorophyll, the same pigment that makes leaves green, and only shift to their trademark yellow as the fruit matures and those green pigments break down to reveal yellow ones beneath. But the journey from green to yellow is stranger than it sounds, driven less by the internal clock of the fruit and more by outside temperature, and lemons can even reverse the process and turn green again under the right conditions.

Why Lemons Start Green

When a lemon first forms on the tree, its rind is packed with chlorophyll. This is not unusual. Virtually all citrus fruits start green because the outer layer of the peel, called the flavedo, functions almost like a leaf during early development. The chlorophyll absorbs light and contributes to photosynthesis, helping the young fruit manufacture some of its own energy. At this stage, yellow pigments called carotenoids are present too, but they are masked by the much larger concentration of chlorophyll.

As the fruit matures, the chlorophyll in the rind gradually degrades. Once enough of it breaks down, the carotenoids that were always there become visible, and the peel appears yellow. Research on Fino 49 lemons confirmed this directly: chlorophylls are the predominant pigments in immature green rind, while the yellow color of ripe fruit is mainly due to carotenoids.¹ More specifically, work on Eureka lemons found that the yellow color may be primarily attributed to a carotenoid called phytoene, rather than the more familiar beta-carotene that colors carrots and sweet potatoes.²

Temperature Is the Real Trigger

Here is where lemons get interesting. Unlike bananas or avocados, lemons are what scientists call non-climacteric fruits. Climacteric fruits undergo a burst of respiration and ethylene production that drives their ripening, and they continue to ripen after harvest. Non-climacteric fruits, including all citrus, do not show this burst.³ So what tells a lemon to turn yellow?

The answer, overwhelmingly, is cold. Research in Mediterranean growing regions showed that natural degreening of lemon peel does not begin until minimum temperatures drop below about 10.5 °C for at least two days.⁴ A separate modeling study placed the threshold slightly higher, finding that degreening did not start until temperatures fell below roughly 11.5–16 °C, and continued until temperatures reached about 4.4–7.7 °C.⁵ This is why lemons harvested in winter from cooler climates come off the tree already yellow, while lemons from summer harvests or tropical climates are often still green even though the fruit inside is perfectly ripe.

A controlled storage experiment made the temperature connection even clearer. Lemons kept at 15 °C showed the most pronounced shift from green to yellow over about four to six weeks. Those at 10 °C and 20 °C also degreened, though more slowly. But lemons stored at 25 °C barely changed color at all, holding onto their green rind throughout the storage period.⁶ The moderately cool range around 15 °C appears to be the sweet spot for natural color change.

Ethylene Is Less Important Than You Might Think

If you have read about fruit ripening before, you probably associate the hormone ethylene with color change. Ethylene is the reason a banana in a paper bag ripens faster, and commercial citrus packers routinely use ethylene gas to turn green oranges orange. It would be reasonable to assume the same chemistry drives lemon yellowing. But the evidence suggests otherwise.

In the same storage experiment mentioned above, researchers repeatedly treated lemons with 1-MCP, a chemical that blocks ethylene receptors. They expected this to prevent degreening. It did not. Lemons stored at 5 °C and 15 °C with their ethylene receptors blocked still turned yellow at comparable rates to untreated fruit. Only at 25 °C did the fruit stay green, and it stayed green regardless of whether ethylene was blocked or not.⁶ This was a surprising result. It means that the natural yellowing of lemons on the tree is driven by cool temperatures acting on the chlorophyll-breakdown machinery directly, with ethylene playing at most a minor supporting role.

That said, ethylene still works on lemons if you apply it externally at high enough concentrations. Commercial post-harvest ethylene treatment can force color change in green lemons, and this is standard practice for fruit harvested during warmer months.⁷ The distinction is that natural on-tree degreening and artificial post-harvest degreening appear to work through partially different pathways, even though both produce the same visual result.

Green Lemons Are Not Unripe Lemons

One of the most persistent misconceptions about lemons is that green ones are not ready to eat. In tropical and subtropical growing regions, lemons can be fully mature in terms of juice content, acidity, and flavor while still sporting a green rind, because nighttime temperatures never drop low enough to trigger degreening. A green lemon from a warm climate is the same fruit on the inside as a yellow one from a cool climate.

This disconnect between external color and internal readiness is the reason the citrus industry uses ethylene degreening in the first place. Summer- and autumn-harvested lemons are internally ripe but cosmetically unacceptable to consumers who expect yellow. So packers expose them to ethylene gas after harvest to stimulate the color change shoppers want.⁷ The treatment changes the peel color but does not meaningfully alter the juice quality.

What does change inside a lemon as it develops, independent of peel color, is its acid composition. During the first 50 or so days of growth, quinic acid is the dominant organic acid in the fruit. After that, citric acid takes over and becomes the characteristic acid of the mature lemon.⁸ Vitamin C content, meanwhile, tends to peak earlier in development and decline as the fruit ripens. Across four lemon cultivars, ascorbic acid concentration in the juice dropped from about 31 mg per 100 grams in early stages to about 24 mg per 100 grams by the latest ripening stage, and total antioxidant capacity fell by roughly 38 percent over the same period.⁹ If you are after the most vitamin C per squeeze, a less-ripe lemon may actually deliver more.

Regreening and the Color Reversal Problem

Lemons can also go backward. In climates where winter cold triggers yellowing, the return of warm spring temperatures can cause the rind to re-synthesize chlorophyll and turn green again. This regreening is well documented in other citrus, particularly Valencia oranges, where fruit left on the tree into late spring often develops green patches despite being overripe inside. Research on Valencia oranges showed that the hormone gibberellic acid can promote this reversal by stimulating chlorophyll production in the flavedo.¹⁰ Lemons exhibit similar behavior. A lemon that turned yellow on the tree during a cool spell can green back up if temperatures climb again, which further demonstrates that peel color is a thermometer reading, not a ripeness gauge.

For growers, regreening is a headache. A harvest of already-yellow lemons that sits too long in warm conditions can develop green patches that make them look less fresh, even though the fruit has not become less ripe. This is partly why postharvest storage conditions and wax coatings matter so much. Coated lemons lose less water and maintain better firmness during storage, which helps preserve both quality and appearance.¹¹

Sun Exposure Adds Another Layer

Even on a single tree, not all lemons ripen in exactly the same way. The amount of sunlight hitting each fruit affects the chemistry of its rind. Researchers have categorized lemons into sun-exposed, semi-sun, and shaded based on where they sit within the tree canopy. Sun-facing lemons develop higher concentrations of UV-B-absorbing compounds, a class of protective molecules the fruit produces in response to light exposure.¹² These compounds, many of them flavonoids, are part of the fruit’s sunscreen. They do not directly determine color, but they contribute to differences in rind composition, thickness, and even flavor between lemons growing inches apart on the same branch.

The practical takeaway is that a lemon’s appearance and chemistry reflect a combination of its genetic variety, the temperature it has experienced, its position in the canopy, and how long it has been on the tree. No single factor dominates, though temperature is the strongest predictor of whether the skin is green or yellow at any given moment.

Lemons Versus Limes and the Green Confusion

Part of the reason people associate green citrus with “not ripe” is the existence of limes. We tend to think of limes as the green ones and lemons as the yellow ones, as if they are the same fruit at different stages. They are not. Lemons and limes are genetically distinct, though they share some ancestry. Phylogenetic analysis has shown that the common yellow lemon (Citrus limon) arose from a cross involving sour orange and citron, while the Mexican lime (Citrus aurantifolia) descended from citron crossed with a different wild species called Citrus micrantha.¹³ Other so-called lemons and limes have still different parentage: Rangpur lime and Rough lemon, for instance, are both mandarin-citron hybrids that are genetically closer to each other than either is to a true lemon or a true lime.¹⁴

What this means is that the green-versus-yellow distinction in the produce aisle reflects both real genetic differences in final rind color and the harvesting conventions of different species. Limes are typically harvested while still deep green because that is when their flavor profile is preferred, though most limes will also turn yellow if left on the tree long enough. Key limes, for example, are yellow when fully mature but rarely sold that way. The entire color-ripeness connection in citrus is far messier than the tidy categories shoppers are used to.

What Happens During Commercial Degreening

When a warehouse full of green-skinned, internally ripe lemons needs to look yellow for the market, the standard procedure is ethylene degreening. The fruit goes into sealed chambers where ethylene gas is introduced at controlled concentrations, usually alongside regulated temperature and humidity. The ethylene accelerates chlorophyll breakdown in the rind, mimicking what cool temperatures would do naturally but on a much faster timeline.

Bagging experiments on Eureka lemons showed that ethylene treatment sharply increased concentrations of certain carotenoids, particularly phytoene and capsanthin, while decreasing others like lutein and beta-carotene.² So commercial degreening does not just remove green; it subtly reshuffles which yellow pigments dominate the final color. Whether this difference is visible to the naked eye is debatable, but it means that the precise shade of yellow on a commercially degreened lemon may differ slightly from one that yellowed naturally in cool weather.

Ethylene treatment is generally considered safe for the fruit and for consumers. Ethylene is a natural plant hormone that all citrus produce in small amounts. The external application simply provides a concentrated burst. There is no residue concern, because ethylene is a gas that dissipates once the chamber is opened. The bigger risk is overdoing it: too much ethylene or too long an exposure can cause rind damage, off-flavors, or accelerated decay after the fruit reaches the shelf.

Disease and Abnormal Color Changes

Not all green-to-yellow or yellow-to-green changes on a lemon tree are normal ripening or regreening. Viral infections can dramatically alter the pigment balance in citrus tissue. Citrus yellow vein virus, for example, causes severe chlorophyll loss in infected lemon leaves. Plants grown at 25 °C with the virus showed reductions in total chlorophyll of over 80 percent compared to healthy plants.¹⁵ This kind of chlorophyll loss is not the orderly breakdown of ripening. It reflects damage to the photosynthetic machinery itself and can cause yellowed, mottled, or chlorotic patches that look nothing like the even golden tone of a ripe lemon.

For backyard growers, irregular yellowing on leaves or fruit, particularly with vein-following patterns or mosaic-like patches, is worth investigating rather than chalking up to ripening. Nutrient deficiencies, particularly iron and zinc, can also produce yellowing that mimics some aspects of degreening but occurs in the wrong tissues or at the wrong time of year. Healthy ripening produces a uniform color shift across the entire rind, progressing gradually from the stem end outward. Patchy, asymmetric, or vein-aligned yellowing is usually a sign of something else going on.

Meyer Lemons and Genetic Variation in Color

Not all lemons follow the same color trajectory. Meyer lemons, one of the most popular varieties for home gardens, are not pure lemons at all. Genetic analysis places them as hybrids with contributions from both citron and a mandarin-pomelo cross, making them part orange in their ancestry.¹³ This shows in their color: ripe Meyer lemons are a deeper golden-orange compared to the bright yellow of standard Eureka or Lisbon lemons. Their skin is thinner, their juice is sweeter, and their rind contains a different balance of carotenoids.

Even among true lemons, cultivar differences matter. The Fino 49, widely grown in Spain, and the Eureka, dominant in California, follow the same broad green-to-yellow arc, but the rate and timing of pigment changes differ. Some cultivars hold their chlorophyll longer, others break it down more rapidly under the same temperature regime. The result is that “when does a lemon turn yellow” has no single answer. It depends on the variety, the climate, the position on the tree, and whether the fruit has been treated after harvest. What holds true across all of them is the starting point: green.