Wednesday, 12 January 2011

The Colour of Beetroot

Roasted root vegetables – ideal fodder for this time of year. I cooked roast potatoes, parsnips and beetroot the other night. The beetroot, from an organic vegetable box, had a lovely deep-red colour. Today’s post is an extract from my 2004 book 'Beetroot'. It’s from Chapter 5 and concerns colour pigments.

“Betalains are a group of nitrogen-containing pigments that are yellow, orange, pink, red and purple in colour. Unlike the other three main classes of plant pigment [chlorophylls, carotenoids and flavonoids], betalains have a limited distribution in the plant kingdom. Most red colouration in plants is due to carotenoids and flavonoids. The red colour of most fruit and vegetables, such as strawberries, grapes and red cabbage, is due to anthocyanins, which are in the flavonoid class of pigments. Betalains are restricted to plants in the order Caryophyllales. In addition to Beta vulgaris (family Chenopodiaceae), betalains have been described from Cactaceae fruits (prickly pear), Amaranth seeds (Amaranthaceae), Bougainvillaea bracts (Nyctaginaceae), and flowers or other plant parts within the Aizoaceae, Basellaceae, Didieraceae, Phytolaccaceae and Portulaceae. Nine of the eleven families within the order Caryophyllales have plants containing betalains. The other two families (Caryophyllaceae and Molluginaceae) have anthocyanins (flavonoids) instead, which probably reflects an early taxonomic division within this plant order. Red beetroot and prickly pear (Opuntia ficus-indica) are the only edible sources of betalains.

Betalain pigments were first isolated from the red roots of Beta vulgaris; the betalain class of pigments are named after the plant genus Beta. There are currently over fifty known betalain pigment molecules, which occur in flowers, fruits, shoots and roots. The betalains are subdivided into two structural groups: the red-violet betacyanins and the yellow betaxanthins.

Beetroot contains a complex mixture of betalain pigments. However, the characteristic purple-red-violet colour of beetroot is mainly derived from a particular betacyanin pigment called betanin. Betanin was first discovered in around 1920, while a crystalline form of betanin dye was produced in the 1960s. Up to 200 mg of betanin is typically found in one beetroot.

After betanin, the yellow betaxanthin pigments vulgaxanthin-I and vulgaxanthin-II are the next most significant in beetroot. Mario Piattelli and colleagues, working in Naples, first described these pigments in beetroot in the 1960s. Cultivars with deep purple-red roots have a high ratio of betanin to betaxanthin pigments, while yellow and gold cultivars such as Burpee’s Golden have relatively high levels of betaxanthins and very little or no red betanin pigment. Cultivars with white roots, including Albina Vereduna and Blankoma, have extremely low levels of both betacyanin and betaxanthin pigments.

Distinct light and dark rings are usually visible when beetroot is cut transversely. This is due to different amounts of pigment in the vascular system and storage tissue of the root. The vascular system appears as darker bands due to higher levels of pigment, while the storage tissue appears as lighter bands. In some deep red cultivars like Boltardy and Red Ace this colour difference can be quite subtle. The colour difference is at its most obvious in Chioggia, with its concentric bullseye pattern of rosy red bands (vascular system) and white bands (storage tissue).

Further reading:

1 comment:

  1. Hi Stephen

    Thanks for that information and your online book. It is pareticularly interesting to me as I'm one of those people with an intterest in cultivating ulluco, the basellaceous crop Ullucus tuberosus, with a very wide range of colour forms.