Comparison of carotenoid and anthocyanin profiles of raw and boiled Solanum tuberosum and Solanum phureja tubers

Abstract

Tubers of the closely related species Solanum tuberosum (potatoes) and Solanum phureja belong to the staff of life for humans. The coloration of the often very colorful tubers is caused by carotenoids (especially xanthophylls) and anthocyanins. The new S. phureja cultivars Mayan Gold and Mayan Twilight, the new S. tuberosum cultivar Red Laura and the traditional S. tuberosum variety Shetland Black were analyzed for their pigment content. The dominant anthocyanin in the red-skinned cultivars Red Laura and Mayan Twilight was characterized as pelargonidin-3-coumarylrutinoside-5-glucoside by LC-ESI-MS. The blue pigment of the Shetland Black cultivar was petunidin-3-coumaroylrutinoside-5-glucoside. Red Laura contained 10 mg anthocyanins g⁻¹ dry weight and Shetland Black 31 mg anthocyanins g⁻¹ dry weight. The main carotenoids in raw tubers are 9-cis-violaxanthin, lutein and one unidentified carotenoid in all S. tuberosum and S. phureja cultivars. Minor compounds were identified as neoxanthin and neochrome. Additionally, zeaxanthin and one further unidentified carotenoid could be found in Red Laura and Mayan Gold. No differences in the carotenoid composition between peel and parenchyma could be found. Total carotenoid content ranged from 2.57 ± 0.53 μg g⁻¹ dry weight in Shetland Black to 14.77 ± 2.22 μg g⁻¹ dry weight in Red Laura (calculated as β-carotene-equivalents). In heat processed tubers, high amounts of carotenoids either changed from all-trans to 9-cis and 13-cis-isomeric form or was degraded. The total pigment content was decreased by heat processing in all cultivars. 8.23 ± 2.98 μg g⁻¹ dry weight could be found in Mayan Twilight and 1.51 ± 0.31 μg carotenoids g⁻¹ dry weight in Shetland Black.

Introduction

Because of high contents of carbohydrates, proteins and vitamin C potatoes belong to the staple food in many regions of the world. Whereas in European countries only the cultivation of Solanum tuberosum tubers due to climatic and terristrial conditions was possible, in Latin America the growing and consumption of tubers of Solanum phureja was always usual.

In our days the Scottish Crop Research Institute (SCRI) worked successfully on new varieties of S. phureja tubers, which are able to grow in Europe. The color of tubers depends on anthocyanin and carotenoid content and composition. Present carotenoids of yellow-fleshed tubers are mostly xanthophylls (Iwanzik et al., 1983). Lutein and zeaxanthin are main xanthophylls in the human retina and prevent cells from damage caused by UV-light. For this health benefit carotenoids are of special interest. Previous publications show that tubers could be a regular source for carotenoids. For example Kasim (1967) described β-carotene-5,6-5′,6′-diepoxide, lutein, lutein-5,6-epoxide and 9-cis-violaxanthin as main carotenoids in S. tuberosum varieties. Minor components were neoxanthin and β-carotene. Iwanzik et al. (1983) showed that 9-cis-viola-xanthin, lutein, lutein-5,6-epoxide, and neoxanthin, were substances of content in S. tuberosum tubers. Breithaupt and Bamedi (2002) found 9-cis-violaxanthin, antheraxanthin, lutein and zeaxanthin as main compounds and neoxanthin, β-cryptoxanthin and β,β-carotene as minor compounds. Bonierbale et al. (2009) worked on original S. phureja tubers. They found that 9-cis-violaxanthin, antheraxanthin, lutein, zeaxanthin and β-carotene were present in those tubers. Andre and co-workers analyzed the S. phureja cultivar Chaucha and one other variety (Andre et al., 2007) and found neoxanthin next to other carotenoids in this tuber.

Most staple foods are poor in zeaxanthin. An increasing amount of lutein and zeaxanthin in daily foods by traditional plant breeding or biotechnological methods could prevent sicknesses. So Römer et al. (2002) blocked the enzyme zeaxanthin epoxidase by sense–antisense techniques. The conversion of zeaxanthin to violaxanthin was impossible. Ducreux et al. (2005) chose another way to boost total carotenoid and especially zeaxanthin content. They produced tubers with an Erwinia uredovora crtB gene, encoding phytoene synthase. This and two other enzymes were influenced by the research group of Diretto et al. (2007). Bub et al. already showed that zeaxanthin from modified tubers is bioavailable (2008).

Red and blue color of skin and flesh is based on the composition of anthocyanins. It was found by several researchers that red-fleshed and -skinned potatoes predominantly contain acylated pelargonidin derivatives while acylated derivatives of petunidin, malvidin, and peonidin are well known in blue-fleshed and -skinned tubers (Hillebrand et al., 2009). The anthocyanin content of red- and blue-colored potatoes varies in large ranges. According to the intensity of color many varieties contain high amounts of anthocyanins (>100 mg per 100 g fresh weight) and a good correlation between the total monomeric anthocyanin concentration and the antioxidant level was observed (Hillebrand et al., 2010). It was shown by several papers that anthocyanin-rich breeding cultivars exhibit antioxidant values which are up to 10-fold higher than in yellow- and white-fleshed ones (Gromes and Herrmann, 2008). Due to this fact, a high intake of anthocyanins has been linked to health preventive effects and reduced risks of e.g., certain form of cancer (Zhao et al., 2004, Cooke et al., 2006), ocular disorders (Jang et al., 2005) or vascular failures (Mazza, 2007).

The purpose of this study was to investigate the carotenoid and anthocyanin profiles of the new and commercially available tubers of S. phureja. For comparison one traditional and one new variety of S. tuberosum, Shetland Black and Red Laura, were analyzed as well. Furthermore, the change of carotenoid composition after heat treatment was of interest.