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J. Agric. Food Chem. 2007, 55, 366−378
Andean Potato Cultivars (Solanum tuberosum L.) as a Source of Antioxidant and Mineral Micronutrients CHRISTELLE M. ANDRE,*,†,‡ MARC GHISLAIN,§ PIERRE BERTIN,# MOUHSSIN OUFIR,‡ MARIÄA DEL ROSARIO HERRERA,§ LUCIEN HOFFMANN,‡ JEAN-FRANC¸ OIS HAUSMAN,‡ YVAN LARONDELLE,† AND DANIEÅ LE EVERS‡ Institut des Sciences de la Vie, Universite´ catholique de Louvain, B-1348 Louvain-La-Neuve, Belgium; Department ‘Environment and Agro-biotechnologies’, Centre de Recherche Public-Gabriel Lippmann, Rue du Brill 41, L-4422 Belvaux, Luxembourg; Centro Internacional de la Papa, Applied Biotechnology Laboratory, P.O. Box 1558, Lima 12, Peru; and Unite´ d’Ecologie des Grandes Cultures, De´partement de Biologie applique´e et Productions Agricoles, Universite´ catholique de Louvain, B-1348 Louvain-La-Neuve, Belgium
Potato tubers were evaluated as a source of antioxidants and minerals for the human diet. A genetically diverse sample of Solanum tuberosum L. cultivars native to the Andes of South America was obtained from a collection of nearly 1000 genotypes using microsatellite markers. This size-manageable collection of 74 landraces, representing at best the genetic diversity among potato germplasm, was analyzed for iron, zinc, calcium, total phenolic, total carotenoid, and total vitamin C contents. The hydrophilic antioxidant capacity of each genotype was also measured using the oxygen radical absorbance capacity (ORAC) assay. The iron content ranged from 29.87 to 157.96 µg g-1 of dry weight (DW), the zinc content from 12.6 to 28.83 µg g-1 of DW, and the calcium content from 271.09 to 1092.93 µg g-1 of DW. Total phenolic content varied between 1.12 and 12.37 mg of gallic acid equiv g-1 of DW, total carotenoid content between 2.83 and 36.21 µg g-1 of DW, and total vitamin C content between 217.70 and 689.47 µg g-1 of DW. The range of hydrophilic ORAC values was 28.25-250.67 µmol of Trolox equiv g-1 of DW. The hydrophilic antioxidant capacity and the total phenolic content were highly and positively correlated (r ) 0.91). A strong relationship between iron and calcium contents was also found (r ) 0.67). Principal component analysis on the studied nutritional contents of the core collection revealed that most potato genotypes were balanced in terms of antioxidant and mineral contents, but some of them could be distinguished by their high level in distinct micronutrients. Correlations between the micronutrient contents observed in the sample and the genetic distances assessed by microsatellites were weakly significant. However, this study demonstrated the wide variability of health-promoting micronutrient levels within the native potato germplasm as well as the significant contribution that distinct potato tubers may impart to the intake in dietary antioxidants, zinc, and iron. KEYWORDS: Potato; Andean tuber; Solanum tuberosum; antioxidants; ORAC; carotenoids; phenolics; vitamin C; minerals; iron; calcium; zinc; genetic diversity; microsatellite
INTRODUCTION
Population-based epidemiological studies have stressed the important role of diet and lifestyle in the emergence of many degenerative chronic diseases such as cancers and cardiovascular diseases, in both developed and developing countries. In industrialized countries, chronic diseases constitute the main
cause of premature mortality (1). Over the past decade, the prevalence of those pathologic disorders has surprisingly increased in low-income countries as well and become a significant public health concern (1, 2). In addition, infections and inadequate micronutrient intake remain major causes of death and disability in the developing world. Iron, zinc, and vitamin A deficiencies are the most widespread forms of micronutrient malnutrition (3).
* Address correspondence to this author at the Department ‘Environment and Agro-biotechnologies’, Centre de Recherche Public-Gabriel Lippmann, Rue du Brill 41, L-4422 Belvaux, Luxembourg [telephone (+352) 47 02 61 416; fax (+352) 47 02 64; e-mail
[email protected]]. † Institut des Sciences de la Vie, Universite ´ catholique de Louvain. ‡ Centre de Recherche Public-Gabriel Lippmann. § Centro Internacional de la Papa. # Unite ´ d’Ecologie des Grandes Cultures, Universite´ catholique de Louvain.
Potato is currently the fourth most important crop worldwide after maize, wheat, and rice, with a production in 2005 of >323 million tonnes (4). In many developed countries, potato represents a secondary staple crop, with an average per capita consumption of 75 kg year-1 in 1999-2001. In developing countries, its consumption (20 kg year-1 per capita) is less
10.1021/jf062740i CCC: $37.00 © 2007 American Chemical Society Published on Web 12/22/2006
Andean Potato Cultivars widespread and mainly concentrated in Turkey, Tunisia, South America, and Malawi (5). In the Andes of South America, the potato consumption can reach 250 kg year-1 per capita (6). In these regions, potato constitutes the main staple crop, and most households cultivate 10-12 varieties in order to reduce their vulnerability to environmental conditions (7). The nutritional value of potato is worth considering with regard to the high consumption of potato and the body of evidence showing the relationship between diet and human diseases. Potato is notably recognized as a source of high-quality proteins, carbohydrates, vitamin C, vitamin B6, vitamin B3, and certain minerals such as potassium, phosphorus, and magnesium (8). Beyond these basic nutrients, potatoes have been found to contain significant amounts of antioxidant phytochemicals (911). Nowadays, there is an increasing interest for their potential effect on human health. Several epidemiological studies have shown that a dietary intake of foods rich in natural antioxidants, such as fruits and vegetables, correlates with a reduced risk of cardiovascular diseases and certain cancers (12, 13). Among the different groups of naturally occurring antioxidants in plants, vitamin C, vitamin E, carotenoids, and polyphenols are the best known for their health-promoting effects in human. They may indeed protect proteins, lipids, and DNA against reactive oxygen species (ROS), which are known to be involved in the pathogenesis of aging and many chronic diseases (14). Lachman et al. in 2000 (15), as well as Brown in 2005 (16), reviewed the contents of the main antioxidants present in potato tubers. Hydrophilic antioxidants, that is, polyphenols [1226-4405 mg kg-1 of dry weight (DW)] and vitamin C (170-990 mg kg-1 DW), predominate in potato tubers, whereas lipophilic carotenoids occur to a lesser extent (1-60 mg kg-1 of DW in the literature). Moreover, the health-promoting effects of potato are very promising for humans as a recent study showed that the consumption of unpeeled cooked potatoes improves the lipid metabolism and antioxidant status in cholesterol-fed rat (17). Native to the Andes, potato landraces (Solanum tuberosum L.) are extremely diverse, ranging from diploids to pentaploids. They show a wide variability in tuber shape, flesh and skin color, and flavor and in storage and cooking quality. This wide genetic diversity among native cultivated potatoes may show a considerable variability in nutritional contents. Published data on the extent of variation with regard to antioxidant contents and mineral contents within the native potato germplasm is scarce. To our knowledge, only one study exists regarding the antioxidant compounds in Andean tubers, but it dealt with a limited number of potato genotypes (13, 18). Most of the research was developed on modern potato varieties, which have been shown to have lower genetic diversity as compare to the native Andean potato landraces (19). The objectives of the work reported here were to determine the extent of variation of health-promoting micronutrients in the cultivated potato and to evaluate their potential to contribute to dietary antioxidant and mineral intake. Using microsatellite markers, a size-manageable collection of 74 potato genotypes was first built up as a representative sample of the genetic diversity among potato germplasm and was then analyzed for both potato antioxidants (total phenolics, total carotenoids, and vitamin C) and selected minerals (calcium, iron, and zinc). The efficacy of the major antioxidants present in the potato tubers has also been evaluated using an in vitro antioxidant capacity test, the oxygen radical absorbance capacity (ORAC) assay. MATERIALS AND METHODS Chemicals. Solvents (of analytical or HPLC grade as required) and Folin-Ciocalteu’s phenol reagent were obtained from VWR Interna-
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tional (Leuven, Belgium). Gallic acid, ascorbic acid, isoascorbic acid, metaphosphoric acid, butylated hydroxytoluene (BHT), dithiothreitol (DTT), fluorescein sodium salt, Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), 2,2-azinobis(2-amidinopropane) dihydrochloride (AAPH), and hexadecyltrimethylammonium bromide (cetrimide) were purchased from Sigma-Aldrich (St. Louis, MO). Plant Materials. Molecular fingerprinting data have been produced for approximately 1000 Andean cultivars held in CIP (Centro Internacional de la Papa, Lima, Peru) gene banks (20). On the basis of this dataset for 10 simple sequence repeat (SSR) (or microsatellite) loci, a genetically diverse sample was selected on the basis of maximum SSR allele retention. A total of 79 accessions that bear >60% of the genetic diversity assessed by microsatellite markers have been identified and retained. However, 5 of them were not available due to inappropriate health status. From the 74 remaining healthy genotypes, the SSR-based genetic distances were calculated (using the proportion shared allele coefficient). A dendrogram was then generated on the basis of unweighted pair-group method with arithmetic averaging (UPGMA) cluster analysis using the Phylip software (Felsenstein, University of Washington, Seattle, WA) to illustrate the SSR-based genetic relationships between the genotypes. The core collection comprises samples of the eight taxonomic groups of the S. tuberosum species (21). The Andigenum group accounts for >78% of this group. The high level of polymorphism and the wide morphological and physiological variability of this group have been stressed in several studies (22). The 74 genotypes have been cultivated in 2004 in Huancayo (Peru) at the CIP experimental station, located in the highlands (altitude of 3280 m). They were field-grown using normal agronomic practices with minimal or no use of pesticide. Mature tubers were harvested between April and May 2004 after 6-7 months of growth, depending on the genotypes. The genotype, the cultivar group, and the skin and flesh color characteristics are listed in Table 1 for the 74 potato cultivars of the core collection. All tubers were shipped to the Public Research CentreGabriel Lippmann, where they were washed and placed for storage in incubators at 10 °C for 4 months prior to sampling and analysis. All genotypes were treated in the same way. The moisture content of the tubers is given in Table 2. Sample Preparation. Whole tubers were ground, freeze-dried, and stored at -20 °C under nitrogen prior to extraction and analysis. Tubers were processed with their skin due to the difficulty of uniformly peeling certain potato tubers with irregular shape. For each cultivar, three samples (each made of three tubers from one plant) were used for the antioxidant analyses. Each sample was extracted in duplicate. Four samples (each made of three tubers from one plant) were used for the mineral analyses of each cultivar. Total Carotenoid Analysis. The concentration of total carotenoids was estimated according to the method reported by Morris et al. (11) with a few modifications. Briefly, 150 mg of powdered freeze-dried material was submitted to a double extraction with an acetone solution containing 1% (w/v) BHT. Both supernatants were combined, and the extract was centrifuged at 4 °C for 10 min at 5000g. An aliquot of the carotenoid extract was taken for spectrophotometric determination at 450 nm using a Beckman DU 800 spectrophotometer (Fullerton, CA). The total carotenoid content was calculated as described by Britton et al. (23) and expressed as micrograms of carotenoids per gram of DW. Total Vitamin C Analysis. Approximately 150 mg of powdered freeze-dried material was mixed with 1.5 mL of 5% (w/v) aqueous solution of metaphosphoric acid containing 1% (w/v) DTT to allow reduction of dehydroascorbic to ascorbic acid (24). Isoascorbic acid was used as an internal standard. This mixture was homogenized and shaken for 1 h at 4 °C. After centrifugation at 9000g for 10 min at 4 °C, the supernatant was collected. A second extraction was done on the residue using the same extraction solvent. Both supernatants were pooled, and sample aliquots were filtered through a 0.45 µm syringe filter prior to injection. HPLC analysis was performed using a Dionex Summit system (Sunnyvale, CA) equipped with a P580 gradient pump, a GINA 50 autosampler, a UVD 340S diode array detector (DAD), and an external Bio-Rad column heater. A 15 µL aliquot was injected onto a Zorbax Bonus-RP (250 × 4.6 mm internal diameter; 5 µm particle size) (Agilent Technologies, Palo Alto, CA). The mobile phase was methanol/water (5:95, v/v) containing 50 mM potassium dihydro-
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Table 1. Total Phenolics, Total Carotenoids, Total Ascorbate, and Hydrophilic Antioxidant Capacity by ORAC Assay of the 74 Native Cultivated Potato Genotypes of the CIP Core Collectiona genotype Ajanhuiri group 702683-Laram Ajawiri 702802-Jancko Ajawiri 704229-Jancko Anckanchi Andigenum group 700141-Wiishilla Paqui 700347-SS-2613 700815-Sale Papa 701127-Ojo De Buey 701589-Muro Chaleco 702316-Pulu 702408-Huatay Runi 702477-Yana Puma Maqui 702535-Sipancachi 702568-Pichea Papa 702599-unknown 702829-Alcca Tarma 702867-unknown 703248-Wila Huaka Lajra 703346-Huaycha Pacena 703365-Holandesa 703370-unknown 703426-Wacapa Naguin 703428-Samba 703462-unknown 703499-Higos 703505-Puca Papa 703558-Tuquerrena 703578-Quincha 703721-Jallga Huarmi 703739-Lisan 703750-Carganaca 703759-Chiar Imilla 703905-Huata Colorada 703988-Canchillo 704078-Malcachu 704152-Runa Bola 704201-Curipamba 704338-Violeta 704353-Puma 704354-unknown 704429-Guincho Negra 704437-Chata Colorada 704469-Wuiclush 704497-Huaccoto 704504-Puca Huato 704592-Yurac Putis 704647-unknown 704828-Wila Immilla 704864-Koli 704865-Holendesa 704903-Keny Luky 704916-Coyu 704964-Sutumari 705082-Tocana Rosada 705191-unknown 705214-Morada 705264-Roscalena 705424-Chatilla 705428-San Jose 705445-Uqi Nawi 705556-Wayru 705601-Yana Palta Chaucha group 700145-Chimbina Colorada 702196-Mauna Huaman Uma 703305-Chiar Surimana o phinu Chilotanum group 703610-Papacacho 703671-Negrita Curtilobum group 702455-Yuraq Ocucuri o Wana
total phenolicsc (mg of GAE g-1)
total carotenoids (µg g-1)
total ascorbate (µg g-1)
H-ORACd (µmol of TE g-1)
1.76 ± 0.24 1.53 ± 0.47 2.00 ± 0.11
7.13 ± 0.86 10.73 ± 1.09 4.61 ± 1.23
459.10 ± 57.61 374.64 ± 11.14 459.30 ± 77.54
52.96 ± 3.25 29.92 ± 10.92 56.34 ± 20.57
Yr/Y Y/C Ro/W Py/Cp P/Cp P/Cp R/Y DP/Cp Wp/Y Py/C Rp/C Py/Wp Y/W Ry/C Ry/C Yr/C Y/W Po/Wp Pr/Yp Py/C P/Cp Ry/C Ry/C Rp/Y R/C Y/C Pr/Wp P/C Py/Y P/C Yp/W Py/Y Pr/Y P/C Py/Y Yp/C DP/P Yr/W Bp/C P/Cp Y/C Wp/C P/W Ry/Y P/Cp Y/C Wp/Y Y/Cp Ry/C Wp/Y Y/W Py/C P/W Ry/w R/C Py/C P/Cp P/C
1.70 ± 0.43 2.62 ± 0.11 1.46 ± 0.34 2.24 ± 0.06 2.23 ± 0.62 4.43 ± 0.95 2.23 ± 0.82 5.99 ± 1.22 1.44 ± 0.08 1.16 ± 0.27 1.33 ± 0.61 1.78 ± 0.22 2.05 ± 0.14 1.32 ± 0.39 2.54 ± 0.26 1.92 ± 0.37 1.71 ± 0.17 3.77 ± 1.39 2.68 ± 0.51 1.94 ± 0.25 2.90 ± 0.95 1.12 ± 0.21 2.20 ± 0.50 1.87 ± 0.16 1.53 ± 0.05 2.10 ± 0.57 3.41 ± 0.81 2.72 ± 0.94 3.07 ± 0.67 1.31 ± 0.51 1.69 ± 0.31 2.82 ± 0.09 1.61 ± 0.34 1.38 ± 0.42 2.23 ± 0.47 1.95 ± 0.26 12.37 ± 0.65 1.56 ± 0.14 2.00 ± 0.33 3.17 ± 1.29 1.67 ± 0.28 1.63 ± 0.32 2.77 ± 0.16 2.52 ± 0.38 2.36 ± 0.49 1.53 ± 0.26 1.57 ± 0.34 2.54 ± 0.26 2.03 ± 0.09 2.30 ± 0.48 2.05 ± 0.36 1.84 ± 0.31 1.56 ± 0.84 2.49 ± 0.65 2.64 ± 0.09 2.17 ± 0.84 3.51 ± 0.24 1.94 ± 0.23
17.07 ± 2.75 8.94 ± 1.01 8.24 ± 1.20 10.89 ± 0.48 7.46 ± 0.70 11.69 ± 1.62 20.16 ± 1.05 9.52 ± 1.72 14.93 ± 2.43 4.66 ± 0.85 12.02 ± 2.28 8.42 ± 0.68 6.40 ± 0.67 7.79 ± 1.24 14.74 ± 1.92 8.94 ± 2.80 8.10 ± 2.69 8.47 ± 0.68 16.40 ± 0.33 12.13 ± 2.18 8.92 ± 0.71 4.89 ± 1.01 13.74 ± 1.48 15.39 ± 4.27 6.75 ± 0.34 12.85 ± 3.42 10.06 ± 2.00 8.74 ± 2.44 31.23 ± 4.02 6.81 ± 0.91 9.79 ± 1.13 10.31 ± 2.77 22.15 ± 1.88 9.60 ± 2.74 28.06 ± 3.00 14.16 ± 1.04 9.90 ± 0.60 6.17 ± 2.35 11.12 ± 1.12 9.88 ± 2.31 17.06 ± 2.06 6.73 ± 1.71 6.99 ± 0.71 13.53 ± 2.48 7.58 ± 1.76 9.60 ± 0.58 13.31 ± 1.38 7.92 ± 1.47 9.06 ± 1.87 19.30 ± 1.21 13.82 ± 2.96 7.49 ± 1.09 7.96 ± 1.68 7.65 ± 0.99 11.62 ± 1.14 13.02 ± 1.18 13.02 ± 2.82 10.07 ± 1.75
358.09 ± 18.50 447.71 ± 40.84 540.69 ± 44.17 438.41 ± 14.66 689.47 ± 7.46 531.06 ± 84.29 338.16 ± 48.15 299.42 ± 69.69 494.53 ± 93.18 362.04 ± 11.96 304.20 ± 21.61 349.40 ± 44.14 391.42 ± 54.05 368.27 ± 1.10 367.72 ± 40.00 343.12 ± 56.19 374.80 ± 46.05 449.36 ± 38.72 513.12 ± 49.30 288.26 ± 41.39 380.60 ± 42.22 217.70 ± 13.91 299.01 ± 69.86 313.27 ± 50.21 284.71 ± 33.79 542.89 ± 100.91 340.30 ± 21.27 526.14 ± 75.49 464.68 ± 95.93 515.78 ± 1.29 364.81 ± 42.55 266.70 ± 47.97 513.28 ± 106.14 451.06 ± 20.47 391.90 ± 39.16 291.15 ± 19.48 483.62 ± 40.82 417.68 ± 21.50 396.23 ± 16.07 431.15 ± 14.89 409.19 ± 59.34 308.28 ± 46.14 285.88 ± 19.58 350.55 ± 20.71 505.19 ± 19.10 286.04 ± 34.74 337.93 ± 37.90 266.22 ± 42.75 370.56 ± 14.42 384.35 ± 44.31 543.58 ± 57.89 328.26 ± 37.33 355.27 ± 19.22 268.76 ± 41.93 340.24 ± 29.68 382.84 ± 59.03 429.67 ± 93.94 232.79 ± 15.65
49.69 ± 11.95 81.55 ± 15.29 40.63 ± 11.35 58.64 ± 18.96 76.02 ± 13.35 110.45 ± 27.56 54.88 ± 11.79 142.61 ± 16.16 28.25 ± 3.21 32.09 ± 5.13 35.08 ± 13.97 49.21 ± 14.29 41.73 ± 5.02 32.43 ± 4.46 63.05 ± 15.50 50.96 ± 4.80 42.60 ± 1.76 73.04 ± 16.55 60.93 ± 19.15 40.34 ± 19.03 79.98 ± 10.65 32.15 ± 8.92 53.93 ± 7.10 38.90 ± 8.53 49.84 ± 11.39 57.31 ± 6.71 94.26 ± 19.59 62.83 ± 9.50 71.93 ± 26.19 33.21 ± 6.20 53.26 ± 6.38 63.62 ± 23.10 54.51 ± 15.87 35.70 ± 14.24 51.53 ± 1.24 42.34 ± 4.50 204.41 ± 19.74 50.68 ± 8.46 47.56 ± 8.28 75.54 ± 22.49 50.19 ± 7.68 45.74 ± 3.43 73.52 ± 15.57 56.76 ± 8.63 46.03 ± 9.33 32.34 ± 10.06 42.65 ± 2.91 57.18 ± 13.28 46.28 ± 16.64 53.05 ± 13.42 43.26 ± 6.77 45.44 ± 8.50 34.33 ± 4.60 45.41 ± 2.12 64.13 ± 14.29 58.29 ± 17.41 70.95 ± 21.07 45.87 ± 11.87
Ro/Y Y/C Py/W
1.49 ± 0.09 1.61 ± 0.23 1.48 ± 0.74
21.46 ± 5.21 11.25 ± 1.38 8.53 ± 1.18
493.04 ± 16.77 420.67 ± 18.55 273.82 ± 35.11
44.25 ± 9.68 50.12 ± 7.03 30.08 ± 10.31
P/Wr Pr/Cp
2.05 ± 0.11 2.12 ± 0.34
10.87 ± 3.19 8.87 ± 0.84
255.56 ± 37.47 319.62 ± 32.67
51.18 ± 9.04 45.35 ± 3.04
W/W
1.64 ± 0.54
5.21 ± 1.02
306.69 ± 43.00
40.63 ± 10.10
skin and flesh colorb P/W W/C W/W
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Table 1. (Continued)
genotype Juzepczukii group 702305-Chimi Lucki 703258-Laram Canchali Phureja group 701570-Chaucha 705172-unknown Stenotonum group 702472-Amarilla del Centro 702961-Garhuash Pashon 703288-Yana Poccoya
total phenolicsc (mg of GAE g-1)
total carotenoids (µg g-1)
total ascorbate (µg g-1)
H-ORACd (µmol of TE g-1)
W/W Pw/W
1.50 ± 0.74 1.27 ± 0.01
2.83 ± 0.63 2.99 ± 0.14
456.43 ± 78.14 366.88 ± 67.77
30.86 ± 10.52 33.37 ± 4.11
P/Y Py/Cr
2.82 ± 0.18 1.62 ± 0.27
25.43 ± 2.60 15.14 ± 1.05
322.12 ± 11.50 307.64 ± 8.31
69.32 ± 4.68 49.29 ± 17.59
Y/Y O/Y Pr/W
1.49 ± 0.19 1.94 ± 0.50 1.66 ± 0.14
36.21 ± 1.47 29.59 ± 3.99 7.20 ± 0.46
474.64 ± 57.03 425.03 ± 31.83 330.99 ± 23.68
36.17 ± 5.21 55.45 ± 3.61 38.60 ± 3.62
skin and flesh colorb
a Data are expressed on a dry weight basis. The mean values represent analyses of three samples from three different plants (n ) 3), each assayed in duplicate. Primary (in capital) and secondary skin color/primary (in capital) and secondary flesh color. DP, dark purple; P, purple; R, red; O, orange; Y, yellow; C, cream; W, white. c Data expressed as mg gallic acid equiv (GAE) g-1 of dry weight. d H-ORAC, hydrophilic oxygen radical absorbance capacity. Data expressed as micromoles of Trolox equiv (TE) g-1 of dry weight. b
gen phosphate (pH 4.6) and 5 mM hexadecyltrimethylammonium bromide (cetrimide) as described by Zapata et al. (25). It was filtered through a Millipore Isopore membrane filter (0.2 µm) and degassed under vacuum. Ascorbic acid was eluted isocratically at a flow rate of 1 mL min-1 and a column temperature of 40 °C. Ascorbic acid and isoascorbic acid were identified by their retention time and spectral data as compared to authentic standards. Quantification was accomplished at the maximum absorbance detected in the spectrum of ascorbic acid (265 nm) by comparing integrated chromatographic peak areas from the samples to peak areas of known amounts of ascorbic and isoascorbic acids. Total ascorbate content was expressed in micrograms per gram of DW. Total Phenolic Analysis. Five hundred milligrams of powdered freeze-dried material was mixed with 10 mL of 80% (v/v) methanol in a 15 mL graduated tube. This mixture was homogenized using a vortex for 30 s and allowed to stand with intermittent shaking for 2 h in the dark at room temperature. After centrifugation at 5000g for 15 min at 4 °C, the supernatant was collected and evaporated to dryness in a Speedvac (Heto, Thermo Electron Corp., Waltham, MA). Phenolic compounds were resuspended in 5 mL of water and stored at -20 °C under a nitrogen atmosphere until analysis. Total phenolic concentrations, measured as gallic acid equivalents, were estimated using the Folin-Ciocalteu assay (26). Five hundred microliters of appropriately diluted samples, 1250 µL of a 7.5% sodium carbonate solution, and 250 µL of Folin-Ciocalteu reagent (1 N) were mixed in a test tube and allowed to react at room temperature for 30 min. Absorption at 755 nm was measured using a Beckman DU 800 spectrophotometer (Fullerton, CA). Total phenolic content was expressed as milligrams of gallic acid equivalents (GAE) per gram of DW from a gallic acid standard concentration curve. Hydrophilic Antioxidant Capacity. The hydrophilic antioxidant capacity was determined using the ORAC assay. ORAC analyses were performed on the phenolic extracts in a 96-well microplate fluorometer (Ascent F.L. Fluoroscan, Labsystem, Vantaa, Finland) and were adapted from the procedures described by Ou et al. (27), Huang et al. (28), and Cao and Prior (29). AAPH, a water-soluble azo compound, was used as a peroxyl radical generator. Trolox, a water-soluble tocopherol analogue, was used as standard and fluorescein as fluorescent probe. Fluorescence filters were used for an excitation wavelength of 485 nm and an emission wavelength of 520 nm. Briefly, 25 µL of blank, Trolox standard, or diluted samples were mixed with 250 µL of 55 nM fluorescein and incubated for 10 min at 37 °C before automatic injection of 25 µL of AAPH solution (153 mM). The fluorescence was measured every minute for 50 min. All samples were analyzed in duplicate at three different dilutions. The final ORAC values were calculated using the net area under the decay curves and were expressed as micromoles of Trolox equivalents (TE) per gram of DW. Calcium, Iron, and Zinc Analyses. Samples, 300-500 mg of DW, were subjected to acid digestion. Reagent (4.4 mL) (0.42 g of selenium, 14 g of Li2SO4, 350 mL of H2O2, and 420 mL of H2SO4) was added to the sample in a Kjeldahl flask. Acid digestion was performed by increasing temperature until the digest had cleared. At the end of the procedure, samples were diluted with H2O up to 75 mL and kept at 4
°C prior to analysis. Blank digestions were performed in the same way. Samples were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Varian, Palo Alto, CA). Certified reference material (white cabbage, BCR-679) was used as quality control. Statistical Analyses. Pearson correlation coefficients were determined on log-transformed data to evaluate relationships between variables. Principal component analysis (PCA) was performed on centered and standardized data to compare the micronutrient profile of the different potato genotypes. For that purpose, the computer program Canoco 4.0 for Windows and SigmaPlot 7.101 software were used. The Euclidean distance matrices from data on micronutrient analysis were calculated using the DARwin 4.0 software (CIRAD, Montpellier, France). Mantel tests were performed to determine the correlation between the genetic distance matrix and the Euclidean distance matrices from nutritional data using Genalex6 software (30). RESULTS AND DISCUSSION
Antioxidants. The contents in total carotenoids, total vitamin C, and total phenolics are presented in Table 1 for the 74 potato genotypes of the core collection. All mean values are expressed per gram of dry weight with standard deviations. Total Carotenoid Content. The levels in total carotenoids varied greatly among the 74 potato genotypes of our core collection. The contents ranged from 2.83 to 36.21 µg g-1 of DW, and the mean value was 11.77 µg g-1 of DW. The highest ranking cultivar, 702472-Amarilla del Centro (Stenotonum group, formerly S. goniocalyx), was intensely yellow-fleshed and contained 13 times more carotenoids than the lowest ranking white-fleshed cultivar, 702305-Chimi Lucki (Juzepczukii group). These levels of carotenoids are consistent with the levels reported by Morris et al. (11) in stored potato tubers (1.8-34.3 µg g-1 of DW). A large variation in total carotenoid content was observed within the Andigenum group, which was the most largely represented group in our core collection (58 of 74 cultivars). Among the 74 potato cultivars, 8 genotypes (11%) contained >20 µg of carotenoids g-1 of DW. They were all highly yellow pigmented and included one genotype from the Phureja group and two from the Stenotonum group (formerly S. goniocalyx). On the other hand, five genotypes from the core collection (7%) showed
