Lycopene is the natural pigment that gives many fruits and vegetables their characteristic red to pink coloration. This appearance can vary depending on its concentration and form, from a deep red solid in its purest state to the diffused redness seen in a ripe tomato. The molecule’s structure is the key to understanding why it appears red.
The Striking Red Pigment: Chemical Basis
The vibrant red color of lycopene is a direct result of its molecular structure. It is an acyclic carotenoid with a long chain of 11 conjugated double bonds. This extensive conjugation system acts as a chromophore, a part of the molecule that absorbs light.
- Light Absorption: The system of conjugated double bonds allows the lycopene molecule to absorb high-energy light from the visible spectrum, specifically in the blue and green wavelengths.
- Light Reflection: Since the molecule absorbs blue and green light, it reflects the remaining lower-energy light, which our eyes perceive as a vibrant red color.
- Isomerization: The color can change due to exposure to light, heat, or oxygen. This causes the molecule to undergo isomerization, converting its most stable all-trans form into cis-isomers, which can alter its light absorption and result in a slightly different hue.
How Lycopene Colors Foods
In fruits and vegetables, lycopene is not a free-floating pigment but is stored within the tissue. Its concentration increases as the fruit ripens, causing the red color to become more intense. For example, the ripening process in a green tomato involves the degradation of chlorophyll and the accumulation of lycopene, leading to its familiar red color. The pigment is responsible for the red of watermelon, pink grapefruit, and red carrots.
Physical Forms of Isolated Lycopene
In laboratories or for commercial use, lycopene is isolated from its natural sources, revealing a more concentrated physical appearance.
- Crystalline Form: When purified and crystallized from a solvent mixture, lycopene forms long, deep red needles.
- Powder Form: As a fine powder, such as when used as a food additive (E160d), lycopene takes on a dark reddish-brown appearance.
Comparison of Lycopene with Other Carotenoids
Lycopene is part of the larger family of carotenoids, which are pigments responsible for the yellow, orange, and red colors found in nature. Different molecular structures within this family lead to distinct appearances and properties.
| Feature | Lycopene | Beta-Carotene |
|---|---|---|
| Color | Deep red (pure solid) | Yellow-orange |
| Structure | Acyclic (open-chain) | Cyclic (rings at each end) |
| Vitamin A Activity | No provitamin A activity | Provitamin A activity |
| Conjugated Double Bonds | 11 bonds | 11 bonds |
| Solubility | Fat-soluble, insoluble in water | Fat-soluble, insoluble in water |
Other Noteworthy Physical Properties
Beyond its color, lycopene's physical properties influence how it behaves in food preparation and in the human body.
- Fat-Solubility: Lycopene is a fat-soluble compound, meaning it dissolves in oils and fats but not in water. This is why cooking tomatoes with oil, like in pasta sauce, enhances the absorption of lycopene in the body. This property also explains why tomato sauce can stain plastic containers; the pigment dissolves into the porous plastic.
- Oxidative Instability: The molecule is highly sensitive and prone to oxidative degradation upon exposure to light, oxygen, heat, and acidic conditions. This instability can cause a change in color and a loss of its antioxidant effectiveness. Cooking tomatoes with oil can actually increase the concentration of bioavailable lycopene despite heat exposure.
- Stability and Processing: While lycopene itself can be sensitive to heat, the lycopene in processed tomato products like sauce, ketchup, and paste is often more bioavailable than in fresh tomatoes. The processing breaks down cell walls, making the lycopene easier for the body to absorb.
Conclusion
The appearance of lycopene is defined by its intense red hue, a result of its unique molecular structure and light absorption properties. In its pure, crystalline form, it is a deep red solid, while its powdered form is a dark reddish-brown. As the vibrant pigment in foods like tomatoes and watermelon, its concentration and form determine the final coloration. Understanding lycopene’s appearance and its fat-soluble, somewhat unstable nature provides insight into how this powerful antioxidant behaves in food and in our bodies. For further information on its properties and potential benefits, the National Institutes of Health (NIH) is a great resource.
