Fresh greens, bright red, vivid orange, deep purple, golden yellow…Fruits and vegetables often have attractive colours. But sometimes also unattractive discolouration like browning occurs. Knowing the background of what colours these products can help to understand how to control the colour of these fresh products.
For that reason, in this article we will discuss chlorophyll, carotenoids, flavonoids, betalains and melanins. These groups of molecules are the major pigments colouring fresh produce.
What is colour actually?
Colour is visible light human eyes can detect. Visible light is a type of electromagnetic radiation with wavelengths of about 380-750nm. Most electromagnetic radiation has shorter wavelengths (Gamma ray, X-ray, Ultraviolet), other longer wavelengths (Infrared, microwaves, radio waves). These are outside of the ‘visible spectrum’, meaning they are not detectable by human eyes. For example, 700 nm is red, 450 nm is blue and 550 nm is what we perceive as green.
Sunlight (or artificial light) seems white, but in reality is a mixture of different wavelengths. When this white light falls onto a ripe tomato, part of these wavelengths is reflected (the red wavelengths), and we see these reflected waves: a red tomato. Other wavelengths are absorbed by the tomato and do not show.
In order for specific colours to reach our eye, something in the fruit or veggie needs to absorb and reflect the right wavelengths. Special molecules called pigments can do this: their molecular structure can interact with light. These natural plant pigments give colour to the fresh products. Just like artificial pigments or dyes, often you only need a bit of pigment to give something colour.
In fruits and vegetables almost all colours are caused by just 5 groups (sometimes called families) of pigments:
• Chlorophyll (green)
• Carotenoids (yellow, red, orange)
• Flavonoids: anthocyanins + anthoxantins (red, blue, purple)
• Betalains (red, yellow, purple)
• Melanins (brown)
A family of pigments again consists of a range of different molecules. Each molecule in one ‘family’ has a similar basic structure. Let’s zoom in:
Chlorophyll: green energy generator
Grass, leaves and stalks all are green because of chlorophyll, the basic pigment in all green plants. Chlorophyll is crucial for the survival of most plants. It absorbs solar energy, thereby driving the process called photosynthesis. During this process, water and carbon dioxide are converted into glucose, a crucial energy source to sustain life. Since the green wavelengths are not important for photosynthesis, they are reflected and thus make plants look green. This is why modern greenhouses and vertical farms specifically use red/blue LED lighting, not wasting energy into green wavelengths which will be reflected anyway.
There are mainly two types of chlorophyll: a and b. These occur together in most plants, but in different ratios. Since chlorophyll a and b have slightly different colours, the ratio determines the shade of green of products.
Chlorophyll molecules consist of a large ring (the heme ring) and a long phytol chain. While the phytol chain serves as an anchor to keep the molecule in place, the heme ring absorbs light and gives chlorophyll its colour.
After harvest, chlorophyll can break down easily during storage or processing. If the heme ring is damaged, the green colour will be lost or changed. Also the ripening hormone ethylene can accelerate chlorophyll breakdown during storage. To prevent yellowing, ethylene-producing products like apples should be stored separate from for green products such as cucumbers. During cooking, chlorophyll can react to form pheophytin, which is dull green. That is why the bright green colour of broccoli fades when it is overcooked.
Carotenoids: chlorophyll protectants
Carotenoids are a large family of molecules that give F&V red, orange and yellow colours. Carotenoids were first discovered in -as the name indicates- carrots. Carotenoids are very stable molecules, which is why carrots remain orange during cooking. However, it can break down when exposed to oxygen, resulting in some colour loss.
One of the most well-known carotenoids is β-carotene. It can be found in carrots, oranges, sweet potatoes, mangoes, and many other products. Our bodies can convert β-carotene into vitamin A, making it very important for human health.
Some other carotenoids:
• Lycopene - colours tomatoes red
• Lutein - leafy veggies yellow, mostly masked by chlorophyll
• Zeaxanthin - bell peppers, corn
• Capsanthin - peppers
Carotenoids are often present together with chlorophyll: it has a protective effect on it alongside several other vital functions. Most leafy vegetables contain lots of carotenoids, but the green colour of chlorophyll masks its presence. Only when the chlorophyll is broken down, the carotenoid colours become visible. This is the reason broccoli can turn yellow in storage. The same thing happens during the ripening of many fruits; only as chlorophyl is broken down, the carotenoid colours are revealed.
Flavonoids: sunscreen for plants
Flavonoids are phenolic substances isolated from a wide range of vascular plants, with over 8000 individual compounds known. They act in plants as antioxidants, antimicrobials, photoreceptors, visual attractors, feeding repellants, and for light screening (like SPF sunscreen). Flavonoids can be divided in anthocyanins and anthoxantins.
- Anthocyanins have purple, blue, and red colors. Purple carrots, black raspberries, purple cauliflower or potatoes all contain anthocyanins. Most anthocyanins are quite unstable.
- Anthoxantins do not have these bright colours, they are white or yellow and more stable than anthocyanins. You can find them in onions and cauliflower for example.
Betalains: why pee is red after eating beets
Betalains are less common as the first three pigment families. They are found in beets as well as a few other types of produce, and can be red (betalains) or yellow (betaxanthins). They are structurally quite similar to anthocyanins differ but have a nitrogen containing ring structure unlike anthocyanins. Betalains are water soluble and are not digested by humans: that is why your urine turns red after eating beetroots.
Melanins: brown is uncool
Fruits and vegetables may be prone to turn to an unappetizing brown due to bruising, peeling or cutting, but also as result of physiological disorders. This browning is often associated with the actions of polyphenol oxidase (PPO) enzymes, also known as polyphenoloxidases. In healthy cells, these enzymes are stored in compartments and are separated from their substrate. If these compartments are damaged by ageing, force (bruising, peeling, cutting) or disorders, enzymes and substrates can mix and quickly react to form mostly brown pigments called melanins. These are considered unappetizing and can result in high food losses in the value chain. Proper conditioning and protection against mechanical damage is key to prevent this type of unwanted colouring.
In reality, objective measuring or perceiving colour is quite complex, because it depends on a lot of factors (lighting conditions, surface structure and much more). In practice, measuring colour is often done for colour sorting and grading, or to see whether a batch complies with set standards in the chain, e.g. whether a product is still green enough, or if has a sufficient percentage of blush in bicolour apples. In the industry, colour charts are widely used, although sometimes colour is measured instrumentally. WFBR developed a Smart Colour Inspector to objectively measure colour using calibrated imaging technology. If you want to know more about it, please contact our experts below.