Lemons are colored yellow due to the presence of carotenoids, which are natural pigments found in many fruits and vegetables. The main carotenoids that cause the yellow color in lemons are lutein, zeaxanthin, and β-cryptoxanthin.
Carotenoids Give Lemons Their Color
Carotenoids are organic pigments produced by plants and some microorganisms like algae. There are over 600 different types of carotenoids identified so far, but only about 40 are present in a typical human diet. Carotenoids are fat-soluble pigments that provide bright colors of yellow, orange, and red to fruits and vegetables. The carotenoid compounds absorb light in the blue-green and violet region and reflect the longer yellow, orange, and red wavelengths.
In lemons, the predominant carotenoids are:
- Lutein: Gives a yellow color
- Zeaxanthin: Gives a yellow-orange color
- β-cryptoxanthin: Gives an orange color
These carotenoid pigments are synthesized by chloroplasts in the lemon fruit peel and pulp. Lutein and zeaxanthin are found at high concentrations in the flavedo (outer colored layer) of lemon peel. β-cryptoxanthin is abundant in both the peel and juice sacs (endocarp) of lemons.
Carotenoid Biosynthesis Pathway in Lemons
The carotenoid pigments in lemons are synthesized by the following biochemical pathway:
- The starting compound is phytoene, which is colorless.
- Phytoene converts into phytofluene, which is also colorless.
- Phytofluene is converted into ζ-carotene, which has a pale yellow color.
- ζ-carotene is converted into neurosporene, which has a reddish-orange color.
- Neurosporene is converted into lycopene, which has a bright red color.
- Lycopene is cyclized into β-carotene, which has an orange color.
- β-carotene is hydroxylated to form zeaxanthin, which has a yellow-orange color.
- Zeaxanthin is epoxidized to form violaxanthin, which has a yellowish color.
- Violaxanthin converts into neoxanthin, which has a pale yellow color.
- Neoxanthin goes through rearrangements to form lutein, which has a bright yellow color.
The major carotenoids, lutein, zeaxanthin and β-cryptoxanthin accumulate in high concentrations in lemons during ripening. Chlorophyll breakdown also occurs during ripening, unmasking the bright yellow carotenoid pigments.
Factors Influencing Carotenoid Biosynthesis in Lemons
Several factors can affect the types and amounts of carotenoids produced in lemon fruits. These include:
- Light exposure – Exposure to light induces carotenoid biosynthesis in lemon peel and pulp. Lemons grown in full sunlight have higher carotenoid levels.
- Temperature – Cooler temperatures favor carotenoid accumulation. Lemons grown at lower temperatures tend to be more yellow.
- Maturity and ripening – Carotenoid content increases as lemons ripen. Fully ripe lemons have the highest carotenoid levels.
- Post-harvest storage – Continued carotenoid synthesis occurs if harvested lemons are stored at room temperature. Storing lemons at cooler temperatures slows carotenoid accumulation.
- Species and variety – Carotenoid content can vary significantly among different Citrus species and varieties. Some lemon cultivars have dark yellow flesh while others are pale yellow.
- Rootstock – The rootstock used for grafting lemon trees can affect carotenoid accumulation in the scion variety.
- Plant health – Healthy, vigorous lemon trees tend to produce fruits with higher carotenoid levels.
- Soil fertility – Proper soil nutrition supports biosynthesis of carotenoids in lemons.
- Crop load – Heavier crop loads on a lemon tree can result in reduced carotenoid concentrations in the fruits.
Lutein and Zeaxanthin
The predominant carotenoids in lemon peel are lutein and zeaxanthin. These two yellow pigments often occur together in citrus fruits. Lutein and zeaxanthin are structural isomers, meaning they have the same chemical formula but different arrangements of atoms.
Both lutein and zeaxanthin consist of 40 carbon atoms arranged in a symmetrical structure with rings at each end. The main difference is in the placement of a double bond in one of the end rings. In lutein, this double bond is in one beta-ionone ring, while in zeaxanthin it is in both beta-ionone rings.
Lutein and zeaxanthin are hydrocarbon carotenoids, meaning they contain only carbon and hydrogen atoms. Other abundant dietary carotenoids like beta-carotene and lycopene contain oxygen atoms.
In addition to giving lemons their yellow color, lutein and zeaxanthin have important health benefits. They function as antioxidants and blue light filters in the macula of the retina to support eye health.
β-cryptoxanthin is the other major carotenoid pigment found in lemons. It accumulates in both the peel and juice sacs. The highest concentrations are found in ripe lemons that turn from green to yellow.
The chemical structure of β-cryptoxanthin consists of 40 carbon atoms arranged in rings at each end similar to lutein and zeaxanthin. However, it contains one oxygen atom that differentiates it from the hydrocarbon carotenoids. The oxygen atom causes the bright orange color of β-cryptoxanthin.
β-cryptoxanthin provides some of the yellow-orange hue in lemon juice and accounts for about one-third of the total carotenoid content. It has provitamin A activity in the body and can be converted into vitamin A (retinol).
Minor Carotenoids in Lemons
In addition to the major carotenoids described above, lemons contain small amounts of other carotenoid pigments. These include:
- Violaxanthin – Yellow pigment and precursor to lutein
- Neoxanthin – Pale yellow pigment, precursor to violaxanthin
- α-Carotene – Orange pigment with provitamin A activity
- β-Carotene – Orange pigment and main provitamin A carotenoid
- Phytoene – Colorless carotenoid precursor
- Phytofluene – Colorless carotenoid precursor
These minor carotenoids account for less than 10% of the total carotenoid profile in lemons. However, they contribute to the diversity of colored phytochemicals.
Carotenoid Content Compared to Other Citrus Fruits
The carotenoid composition and concentrations can vary between citrus types. Here is a comparison of the major carotenoids in different citrus fruits (per 100g pulp):
|Lemons||0.025 mg||0.038 mg||0.060 mg||0.010 mg|
|Limes||0.033 mg||0.019 mg||0.058 mg||0.005 mg|
|Oranges||0.115 mg||0.099 mg||0.310 mg||0.059 mg|
|Grapefruits||0.148 mg||0.080 mg||0.460 mg||0.038 mg|
|Tangerines||0.085 mg||0.056 mg||0.260 mg||0.034 mg|
This comparison shows that oranges and grapefruits tend to be highest in carotenoids among the common citrus fruits. Lemons have moderate amounts while limes have lower carotenoid levels. Each fruit has a distinctive carotenoid profile that gives it unique health benefits.
Changes in Carotenoid Content During Processing
The carotenoid content of lemons can be affected by processing and preservation methods. Here are some typical changes:
- Juicing lemons causes some loss of carotenoids into the discarded peel.
- Dehydrating or freeze-drying lemons retains carotenoids well if done properly.
- Canning or making preserved lemons results in leaching of carotenoids into the brine.
- Making lemon jam or marmalade reduces carotenoid content due to heat treatment.
- Thermal pasteurization degrades heat-sensitive carotenoids like β-cryptoxanthin.
- Making lemon juice concentrate does not affect carotenoids since they are oil-soluble.
To maximize the carotenoid content, fresh lemons or frozen lemon juice are ideal. Drying methods can also preserve carotenoids effectively when protected from heat, light, and oxygen.
Health Benefits of Lemon Carotenoids
Here are some of the key health benefits associated with the carotenoid antioxidants found in lemons:
- Eye health – Lutein and zeaxanthin protect the retina and lower risks of age-related macular degeneration.
- Immune function – Carotenoids enhance the activity of immune cells and antioxidant defenses.
- Cancer prevention – Carotenoids exhibit anti-cancer activity by inhibiting cell proliferation and tumor growth.
- Skin health – Dietary carotenoids can improve skin texture, protect against sunburn, and reduce photodamage.
- Cardiovascular effects – Carotenoids help prevent LDL oxidation, lower inflammation, and reduce risk of heart disease.
- Brain health – Higher intakes of lutein and β-cryptoxanthin are linked to better cognitive function in older adults.
Lemons are an excellent dietary source of antioxidant carotenoids that can provide many benefits for protecting health and fighting disease. Eating fresh lemons, adding them to recipes, or drinking lemon water are easy ways to increase your carotenoid intake.
In summary, lemons get their bright yellow color primarily from three carotenoid pigments – lutein, zeaxanthin, and β-cryptoxanthin. These fat-soluble phytochemicals are synthesized by lemon trees and accumulate in the peel and pulp, especially as the fruits ripen. Production of carotenoids in lemons is influenced by light, temperature, maturity, variety, plant health, and other factors. Compared to other citrus fruits, lemons have moderate levels of carotenoids with potential health benefits. The carotenoid content of lemons can decrease during juicing, heating, and other processing but is preserved well by freezing or drying. Maximizing consumption of fresh lemons and lemon juice is the best way to obtain these beneficial phytonutrients that give lemons their yellow color.