Anthocyanin

Anthocyanins are a group of water-soluble pigments found within the vacuoles of plant cells.

These pigments are responsible for a wide spectrum of colors, including red, pink, purple, blue, and black. The specific color displayed by an anthocyanin depends on the pH level, or acidity, of its environment. This property is known as halochromism, where a substance changes color in response to pH shifts. In acidic conditions, these pigments often appear red, while they shift toward blue in more basic environments.

Because of this sensitivity, they are often used as pH indicators.

Chemically, anthocyanins belong to a larger class of molecules called flavonoids. They are synthesized through a biological process known as the phenylpropanoid pathway. To create an anthocyanin, the plant first produces a basic structure called an anthocyanidin. The plant then attaches sugar groups to this structure, turning it into an anthocyanin glycoside.

There are many different types of these molecules in nature. The most common glycosides include those derived from cyanidin, delphinidin, malvidin, pelargonidin, peonidin, and petunidin. Scientists have identified more than 550 different types of anthocyanins.

These pigments serve several vital roles in plant physiology. In flowers, the accumulation of anthocyanins helps attract various animal pollinators. In fruits, the bright colors aid in seed dispersal by attracting herbivorous animals to edible fruit.

Anthocyanins also provide protection against environmental stress. For example, tomato plants use them to counter reactive oxygen species during cold stress, which lowers the rate of cell death in leaves. In some leaves, the red color may also protect against herbivores that are attracted to green colors.

This color can be complementary to the green light absorbed by chlorophyll.

Anthocyanins are found in many plant tissues, including leaves, stems, roots, flowers, and fruits. They are most often located in the outer cell layers, such as the epidermis and peripheral mesophyll cells. While many land plants contain them, some groups like the Caryophyllales do not. In plants like cacti and beets, anthocyanins are replaced by a different pigment called betalains. Interestingly, betalains and anthocyanins are never found in the same plant.

Humans have studied these pigments for nearly two centuries. In 1835, a German pharmacist named Ludwig Clamor Marquart published a treatise titled *Die Farben der Blüthen*, or *The Colors of Flowers*.

In this work, he named a chemical that gives flowers a blue color as "Anthokyan." Since that time, research has expanded to understand the complex chemistry of these colorful molecules.

Many foods are exceptionally rich in anthocyanins. Black crowberry contains approximately 4,180 mg per 100 g, while purple corn husks contain ten times more than the kernels.

Other notable sources include black chokeberry, wild blueberries, and black rice. Even some vegetables, like red cabbage, are high in these pigments.

In the autumn, many tree species produce anthocyanins as the seasons change. This results in the red and purple foliage seen in temperate regions.

In some areas, like New England, up to 70% of tree species may produce them.

Modern agriculture has also utilized these pigments through selective breeding. For instance, the "Indigo Rose" tomato was made available to gardeners in 2012.

High anthocyanin content in tomatoes can double their shelf-life and inhibit the growth of the mold *Botrytis cinerea*. Some scientists have even used genetic transcription factors from snapdragons to produce high levels of anthocyanins in fruit. While the European Union approves anthocyanins as food colorants under the code E163, they are not currently approved as food additives in the United States.