Tomatoes can be purple, and the science behind their color has created two distinct groups of these dark-hued fruits. The purple coloration in tomatoes is due to the presence of a powerful class of plant pigments called anthocyanins, which are also responsible for the deep colors in blueberries and blackberries. These pigments are a type of flavonoid, a naturally occurring compound in plants with known antioxidant properties. The development of a truly purple tomato has involved both traditional breeding methods and advanced genetic engineering to maximize the concentration of this beneficial compound.
The Science Behind the Purple Pigment
The vibrant colors of purple, blue, and black in fruits and vegetables are primarily caused by anthocyanins. These compounds are a subgroup of flavonoids, which are synthesized by plants as a protective mechanism. Anthocyanins are water-soluble pigments, and their exact shade can shift depending on the cellular environment’s acidity, or pH level. In highly acidic conditions, they tend to appear red, while in more neutral or alkaline conditions, they present as blue or purple. In tomatoes, the typical red color comes from a different pigment, lycopene, and the machinery to produce high levels of anthocyanins in the fruit flesh is usually “switched off.” The most nutritionally important development involves activating the pigment production throughout the fruit’s interior.
Genetic Engineering Versus Traditional Breeding
There are two main routes to a purple tomato, resulting in notable differences in the fruit’s final pigment level and distribution.
Traditional breeding has yielded varieties like ‘Black Krim’ and ‘Indigo Rose,’ which are often dark red or brownish-purple. These cultivars accumulate anthocyanins primarily in the skin, and this accumulation is often dependent on exposure to sunlight. The flesh of these traditionally bred varieties remains the standard red or pink color, leading to a lower overall concentration of the purple pigment.
The genetically engineered purple tomato, developed by Norfolk Plant Sciences, represents a different approach to maximizing this beneficial compound. This variety was created by introducing two genes from the snapdragon flower into the tomato plant’s DNA. These snapdragon genes act as a biological “on switch,” activating the tomato’s natural, but dormant, ability to produce high levels of anthocyanins throughout the entire fruit, including the flesh. This genetic modification allows the tomato to accumulate significantly higher concentrations of the purple pigment compared to even the darkest traditionally bred varieties. The U.S. Department of Agriculture (USDA) has found this engineered tomato safe for cultivation.
Nutritional Differences and Antioxidant Power
The purple color in these tomatoes directly corresponds to a boost in their antioxidant power, setting them apart from the typical red varieties. Red tomatoes are rich in lycopene, a carotenoid antioxidant known for its potential benefits for heart health. Purple tomatoes, however, supplement this lycopene with a high concentration of anthocyanins, offering a dual-antioxidant profile. This combination is thought to enhance the overall health benefits of the fruit. Studies on purple tomatoes suggest that their high anthocyanin content may offer anti-inflammatory effects and provide cardiovascular support. This heightened antioxidant capacity is a result of the anthocyanins’ ability to neutralize harmful reactive chemical species within the body. While traditional purple-skinned varieties offer some of this benefit, the engineered purple tomato, with its pigment distributed throughout the flesh, delivers a far greater dose. This nutritional enhancement provides a convenient way for consumers to increase their intake of a compound typically found in less-consumed foods like berries.