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Trans-β-carotene, selected mineral content and potential nutritional contribution of 12 sweetpotato varieties
Journal of Food Composition and Analysis 27 (2012) 151–159
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Original Research Article
Trans-b-carotene, selected mineral content and potential nutritional contribution of 12 sweetpotato varieties S.M. Laurie a,*, P.J. van Jaarsveld b, M. Faber b, M.F. Philpott c, M.T. Labuschagne d a
Agricultural Research Council - Roodeplaat Vegetable and Ornamental Plant Institute, Private Bag X293, Pretoria 0001, South Africa Nutritional Intervention Research Unit, Medical Research Council, PO Box 19070, Tygerberg 7505, South Africa c Agricultural Research Council - Institute for Soil Climate and Water, Private Bag X79, Pretoria 0001, South Africa d Department of Plant Sciences, University of Free State, PO Box 339, Bloemfontein 9300, South Africa b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 January 2012 Received in revised form 24 May 2012 Accepted 27 May 2012
The content of trans-b-carotene and selected minerals was determined in 12 sweetpotato (Ipomoea batatas) varieties produced at 4 agro-geographical production sites in South Africa. All 9 orange-fleshed varieties have the potential to contribute 100% of the recommended dietary allowance of 4–8 year-old children for vitamin A, 27% for magnesium, 15% for zinc and 11% for iron. Orange-fleshed varieties were superior to cream-fleshed ones in calcium and magnesium content. The trans-b-carotene content of the varieties varied over the geographical sites. The mean content in the 9 orange-fleshed varieties was between 5091 and 16,456 mg/100 g fresh weight. The mineral content in fresh roots of the 12 varieties ranged from 34 to 63 mg/100 g for calcium, 15 to 37 mg/100 g for magnesium, 28 to 51 mg/100 g for phosphorus, 191 to 334 mg/100 g for potassium, 0.73 to 1.26 mg/100 g for iron, and 0.51 to 0.69 mg/ 100 g for zinc. Variation within varieties over geographical sites could be ascribed to differences in soil mineral content, soil pH and the interaction of these. The variation in nutritional content of sweetpotato indicated here, needs to be considered in varietal selection for different production sites and in calculating nutrient contribution of sweetpotato toward dietary intake. ß 2012 Elsevier Inc. All rights reserved.
Keywords: Sweet potato Ipomoea batatas Carotenoids Environment Dry matter content Minerals Recommended daily allowance Vitamin A deficiency VAD Biodiversity Varietal differences Genetic variability in nutrient content Horticulture Agricultural practices and nutrition Food security Food analysis Food composition
1. Introduction Sweetpotato (Ipomoea batatas), one of the major staple crops of the world, is an excellent source of energy (438 kJ/100 g edible portion) and, as a crop, has advantages such as versatility, high yield, hardiness and wide ecological adaptability. In developing countries, sweetpotato is important in alleviating major problems such as malnutrition, food insecurity, consequences of droughts and limited agricultural technologies (Bovell-Benjamin, 2007). There is renewed focus on particularly the orange-fleshed sweetpotato, as it offers one of the best sources of naturally bioavailable b-carotene, the major precursor of vitamin A (Burri,
* Corresponding author. Tel.: +27 12 8419611; fax: +27 12 8080844. E-mail address: [email protected] (S.M. Laurie). 0889-1575/$ – see front matter ß 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jfca.2012.05.005
2011; Van Jaarsveld et al., 2005). The efficacy and effectiveness to improve vitamin A status of orange-fleshed sweetpotato have been confirmed (Low et al., 2007; Van Jaarsveld et al., 2005). Carotenoids are important to human health for their ability to boost the immune system, and to maintain eye health and vision (Blomhoff and Blomhoff, 2006; Montrone et al., 2009). Children who are vitamin A deficient have a lower resistance to common childhood infections such as respiratory and diarrheal diseases, measles, and malaria (Rice et al., 2004). Worldwide, 33.3%, or 190 million children younger than 5 years are vitamin A deficient (WHO, 2009). Orange-fleshed sweetpotato is one of the major provitamin A-rich crops promoted in crop-based programs to address vitamin A deficiency in developing countries (Burri, 2011). In sweetpotato there is a very large genetic diversity in bcarotene content, ranging from <100 mg/100 g to 26,600 mg/100 g in raw sweetpotato (Kidmose et al., 2007; K’osambo et al., 1998;
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Takahata et al., 1993; Teow et al., 2007; Wu et al., 2008). Whiteand cream-fleshed types are devoid of, or contain very little, bcarotene. The b-carotene content in food crops is used to calculate the vitamin A value as mg RAE (retinol activity equivalents) (Trumbo et al., 2001) and the potential contribution food crops can make to the vitamin A dietary requirement of humans. Orange-fleshed sweetpotato is one of six staple crops in the HarvestPlus Crop Biofortification Program which aims to breed key micronutrients into staple crops (Nestel et al., 2006; Pfeiffer and McClafferty, 2007). Nutrient content of the crop and the influence of the environment during growth on micronutrient expression are key considerations in biofortification programs (Pfeiffer and McClafferty, 2007). The target level when breeding for orange-fleshed sweetpotato to be used as sole source of pro-vitamin A is 7500 mg b-carotene/100 g, and 3750 mg b-carotene/100 g in mixed diets (Nestel et al., 2006). The b-carotene content in sweetpotato varieties can significantly be influenced by air and soil temperature, radiation, fertilization, location, season and maturity of roots (K’osambo et al., 1998; Liu et al., 2009; Wu et al., 2008). To obtain reliable estimates, b-carotene content in sweetpotato therefore needs to be measured over a range of geographical production sites. The main nutritional focus of the orange-fleshed sweetpotato is its b-carotene content and subsequently its potential to alleviate vitamin A deficiency. However, sweetpotato also contains several minerals. Reported values for mineral content of sweetpotato (per 100 g raw roots) show a large variation for e.g. potassium (129–470 mg), iron (0.16–0.94 mg), zinc (0.27–1.89 mg), calcium (8–101 mg), magnesium (18–35 mg) and phosphorus (41–70 mg) (Leighton, 2007; STA, 2005; USDA, 2009; Wolmarans et al., 2010; Woolfe, 1992). Courtney et al. (2008) reported high heritability estimates for iron (H = 0.74) and zinc (H = 0.82) in sweetpotato, while Gru¨neberg (2006) found a positive correlation between iron and zinc content, and to a lesser degree between zinc and b-carotene content in sweetpotato, indicating promising prospects to breed simultaneously for improved content of these 3 micronutrients. This is of particular importance as vitamin A, iron and zinc deficiencies are among the leading causes of disease burden and mortality globally (Ezzatti et al., 2002). Tumwegamire et al. (2011) reported significant genotypic variance in dry matter, protein, starch,
b-carotene, iron, zinc, calcium and magnesium content of 89 farmers varieties from East Africa. The present study was conducted (1) to determine the content of b-carotene and selected minerals in 12 sweetpotato varieties; (2) to investigate the effect of agro-geographical production sites on the content of b-carotene and selected minerals; and (3) to calculate the potential contribution to human nutrition. Minerals selected for analysis were iron, zinc, potassium, phosphorus, calcium and magnesium. 2. Materials and methods 2.1. Plant material Nine orange-fleshed I. batatas varieties with varying intensities of orange flesh color and 3 local cream-fleshed varieties (Blesbok, Ndou and Monate) were investigated. The orange-fleshed varieties included 3 local varieties (Khano, Serolane and Impilo), 2 promising local advanced breeding lines (1999_1_7 and 2001_5_2) and 4 varieties imported from the USA (Resisto, W119, Excel and Beauregard). Samples of storage roots were collected from plots consisting of 60 plants in multi-location cultivar trials over 4 agro-geographical production sites in South Africa. Geographical, climatic and cultural conditions at the 4 sites are summarized in Table 1. The highest atmospheric temperature was experienced at Giyani and Empangeni, while Empangeni had the highest humidity. Agricultural management was very good at Roodeplaat and Empangeni where trained full-time technicians were based, while management at Giyani and Hazyview was typical of production conditions in communities. The latter two sites experienced water stress at some stages due to unavailability of irrigation water. Analysis of soil samples from the 4 sites indicated loam-clay soils which are generally acceptable for vegetable production, and pH level within acceptable range (Table 2). Soil phosphorus content was low at Empangeni and high at Roodeplaat. Soil potassium content was generally within the optimum range, but lower at Giyani. Soil calcium content was high at Hazyview, while soil magnesium content was high at Roodeplaat and Hazyview. At Roodeplaat the soil Ca:Mg ratio was low. For Giyani the soil sodium content was low. Fertilizer application is indicated in Table 1.
Table 1 Geographical, climatic and cultural conditions at the 4 agro-geographical production sites.
Province Type of site Location Climatic area Altitude Growth period Temperature (8C)a Rainfall (mm)a Humidity (%)a Radiation (MJ/m2/day)a Irrigation Irrigation frequency Fertilization: preplant Fertilization: top dressing Total nutrient application (kg/ha)
Roodeplaat
Giyani
Hazyview
Empangeni
Gauteng Research Institute 25.6048S; 28.3458E Warm temperate 1164 m 5 months 21.6 31.3 51.7 22.2 Sprinkler (400 mm) 1–2 times/week (depending on rainfall) 200 kg/ha 1:0:1 (18.5% N, 0% P, 18.5% K) 150 kg/ha limestone ammonium nitrate (LAN; 28% N) N = 79, P = 0, K = 37
Limpopo Community garden 23.3008S; 30.5338E Dry subtropical 716 m 5 months 24.5 39.6 59.1 18.5 Furrow Infrequent
Mpumalanga Farmer’s plot 25.0498S; 31.1448E Humid subtropical 564 m 6 months (winter) 19.5 27.0 59.4 13.6 Furrow Infrequent
500 kg/ha 2:3:2 (8.5% N, 13% P, 8.5% K) 150 kg/ha LAN (28% N)
150 kg/ha 2:3:4 (6.7% N, 10% P, 13.3% K) 268 kg/ha LAN (28% N), 170 kg/ha potassium chloride (KCl; 50% K) N = 85, P = 15, K = 105
KwaZulu-Natal Agricultural College 28.7258S; 31.8988E Humid subtropical 100 m 4.5 months 24.8 94.1 71.7 19.4 Sprinkler 1–2 times/week (depending on rainfall) 500 kg/ha 2:3:2 (8.5% N, 13% P, 8.5% K) 150 kg/ha LAN (28% N)
N = 84, P = 64, K = 42
Source: Climatic database ARC-Institute for Soil Climate and Water, South Africa. a Monthly mean.
N = 84, P = 64, K = 42
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Table 2 Physical and chemical properties of the topsoil at the 4 agro-geographical production sites. Units
Roodeplaat Soil physical properties Clay (%) % Soil chemical properties mg/kg Phosphorousa Potassiumb mg/kg Calciumb mg/kg b Magnesium mg/kg b Sodium mg/kg b Iron mg/kg Zincb mg/kg pH (water)
Interpretationc
Site
28 92 147 1413 548 62 – – 6.9
Giyani 34 19 116 959 252 16 64.8 1.27 7.0
Hazyview
Empangeni
26
32
24 168 2991 611 42 – – 6.8
9 193 1126 321 74 156 3.89 6.1
Low content
High content
Optimum content
<35% <15 <40 <200 <50 <50 na na <5.5
>60 >250 >3000 >300 >250 na na >7.3
40–90 120–240 400–2500d 100–400d na na
‘‘–’’ = Fe and Zn not analyzed; na = not available (micronutrient). a Bray I extraction method (Non-Affiliated Soil Analysis Work Committee, 1990). b Ammonium acetate method (Non-Affiliated Soil Analysis Work Committee, 1990). c Indication of levels for vegetable production according to Buys (1988). d Less critical.
At each of the 4 sites, the sweetpotatoes were lifted from the soil by garden fork, and 25 medium-sized sweetpotatoes were randomly selected from different parts of the plot, providing a representative random sample for each variety. Each laboratory sample was reduced by randomly selecting 8 of the 25 mediumsized sweetpotatoes (approximately 3 kg). The harvested roots were washed with tap water to remove the soil and placed on a brown paper sheet to allow the roots to air-dry for about 30 min before the intact unprocessed roots were transported to the laboratory where sample preparation for the nutrient analysis was done. 2.2. Analytical procedures At the laboratory of the Medical Research Council (MRC), 5 medium-sized sweetpotatoes per variety for each site were peeled, thoroughly washed with tap water and dried with paper toweling. Two opposite quarters from longitudinally quartered roots were combined and homogenized, and aliquots of 2.0–2.5 g weighed and stored at 20 8C until analysis for b-carotene content. The total b-carotene as well as trans-b-carotene content was determined in duplicate by high-performance liquid chromatography (HPLC) (SpectraSERIES; Thermo Separation Products, Fremont (CA)) using a monomeric C18 column (Waters Spherisorb ODS 2, 3 mm, 4.6 mm 150 mm). This method has been evaluated for sweetpotato (Van Jaarsveld et al., 2006) and was slightly modified as described by Low and Van Jaarsveld (2008). Extraction of bcarotene was done with tetrahydrofuran:methanol (1:1, vol/vol). A b-carotene standard was purified by HPLC as previously described (Van Jaarsveld et al., 2006) and a 5-point standard curve that bracketed the concentration of the unknown samples was constructed in triplicate. An aliquot of the purified standard solution with a known concentration was used as the external standard for quantification of b-carotene in the sample extracts (Van Jaarsveld et al., 2006). Sub-samples (0.5 g) from the same homogenized mixture were used at the Agricultural Research Council (ARC)-Institute for Soil, Climate and Water for triplicate determinations of zinc, iron, potassium, magnesium, phosphorus and calcium through inductively coupled plasma (ICP) optical emission spectrometric determination according to Huang and Schulte (1985). The following wavelengths were used: calcium 422.673 and 317.933 nm, magnesium 383.826 nm, phosphorus 213.618 nm, potassium 769.896 nm, iron 259.940 nm and zinc 213.856 nm. The calibration curves were found to be very good for all 6 elements, with a correlation coefficient of 0.9995 for K and greater than 0.9999 for the other 5 elements.
Aliquots were dried at 105 8C for 16 h for duplicate determination of dry matter content (AOAC, 2007). Care was taken throughout the sample preparation to avoid contamination. 2.3. Statistical analysis The data were analyzed with GenStat12007. In case of the minerals, the triplicate determinations were first screened for normality, and outliers examined and eliminated. The values were rejected when identified as outliers through at least two of the following three criteria: (a) the difference between the maximum or minimum determination and the median determination was larger than an appropriate constant for that element; (b) the ratio of the difference between the maximum and minimum determination to the difference between the two determinations closest to each other was larger than an appropriate constant for that element; and (c) the maximum determination was much larger than the maximum determination for all the other samples or the minimum determination was much smaller than the minimum determination for all the other samples. The number of outliers removed was 10 for iron, 5 for zinc, and 1 each for magnesium, phosphorus and calcium. The mean of duplicate/triplicate determinations per variety was calculated. To assess the variability of nutrient content among sites and varieties, an ANOVA was performed. The Student’s protected t-Least Significant Difference (LSD) test was calculated at the 1% significance level for b-carotene content, and 5% significance level for mineral content to compare varietal means and site means. The mean nutrient content per variety over the 4 sites was used to calculate the potential nutrient contribution to dietary requirements of 4–8 year old children (National Academy of Sciences, 2004; Trumbo et al., 2001). A portion size of 125 g was used, based on consumption data available for South African children (Nel and Steyn, 2002). Calculations were based on retention of nutrients when boiled, using the USDA retention factors of 85% for b -carotene and 95% for minerals (USDA, 2007). 3. Results and discussion 3.1. Dry matter content The commercially available cream-fleshed variety Blesbok had the lowest dry matter content at 18.7% (Table 3). The dry matter
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Table 3 Means for trans-b-carotene content (mg/100 g raw root) for 12 sweetpotato varieties at 4 agro-geographical production sites, mean over the 4 sites, and mean dry matter content (%). Variety
Resisto Khano 2001_5_2 W_119 Beauregard 1999_1_7 Excel Serolane Impilo Ndoub Monateb Blesbokb Mean (site) Overall mean Mean orangefleshed varieties P-Value (variety) P-Value (site) LSD (variety) LSD (site) CV%
Trans-b-carotene content (mg/100 g raw root)
Dry mass content (%)
Hazyview
Giyani
Empangeni Roodeplaat Mean (variety)
18,372 15,565 12,372 10,236 9922 9453 4434 5500 2978 110 16 23
20,525 14,439 12,470 12,978 11,609 11,869 5945 4255 6125 212 21 10
11,987 12,234 11,716 8806 8453 8384 5534 4766 4227 21 62 9
7415 ab
8372 a
14,939 13,906 10,640 9836 8599 8524 4889 5903 7034 177 25 12
6350 b
%Trans Hazyview Giyani Empangeni Roodeplaat Mean configurationa (variety)
16,456 3490 a 96.9 14,036 1293 ab 97.2 11,800 799 bc 92.6 10,464 1649 c 96.7 9646 1362 c 95.8 9558 1495 c 96.9 5106 687 d 96.5 5200 622 d 95.8 5091 1697 d 94.5 134 77 e 99.9 31 0 e 100.0 14 0 e 100.0
7040 b 7294
9706
24.3 19.4 21.3 25.1 17.3 24.1 27.9 25.8 23.1 27.2 24.8 15.5
28.9 24.2 25.7 31.8 21.1 27.9 28.8 34.9 24.8 28.8 23.9 22.6
28.5 24.9 25.5 30.4 21.5 23.7 26.1 30.6 22.9 23.6 27.1 20.1
27.5 21.2 23 27.8 18.8 24.7 23.5 30.5 21.9 28 23.8 16.1
23.0 c
26.9 a
25.4 b
23.9 c
96.2
27.3 2.1 22.4 2.6 23.9 2.1 28.7 2.9 19.8 2.0 25.1 1.9 26.7 2.4 30.5 3.7 23.1 1.0 27.0 2.3 24.9 1.5 18.7 3.4
bc e de ab f cd bc a de bc cde f
24.8
95.8
25.3 <0.001 0.009 2640 1525 18.7
<0.001 <0.001 1.5 2.6 7.2
Means for site in a row followed by the same letter and means for variety in a column followed by the same letter, are not significantly different at P 0.01. a %Trans configuration of total b-carotene. b Cream-fleshed varieties.
content of the 9 orange-fleshed varieties varied from 19.8 to 30.5%, which is slightly lower than that reported for selected countries in the sub-Saharan Africa region, which has a dry matter content ranging from 21 to 34.9% (Kapinga et al., 2007). The dry matter content of varieties is a critical parameter in sweetpotato breeding programs, as African consumers generally prefer sweetpotato varieties with high dry matter content (Tomlins et al., 2004; Masumba et al., 2007; Laurie, 2010). From a consumer perspective, orange-fleshed varieties with high dry matter content such as
Serolane, Resisto and W_119 were the most promising varieties. The standard deviations given in Table 3 indicate a variation in dry matter content within varieties over the 4 geographical sites. It has been reported that the dry matter content in root crops can vary according to differences in climate, day length, soil type and cultivation practices, e.g. water application vs. rainfall (Bradhury and Holloway, 1988). The dry matter content was low at Roodeplaat compared to Giyani, where the rainfall was lower and irrigation more erratic, pointing at lower dry matter content
Table 4 Means of triplicate determinations for calcium and magnesium content (mg/100 g raw roots) for 12 sweetpotato varieties at 4 agro-geographical production sites and mean over the 4 sites. Calcium (Ca)
Magnesium (Mg)
Variety
Hazy
Giy
Emp
Rood
Mean (variety)
Variety
Hazy
Giy
Emp
Rood
Mean (variety)
W_119 Resisto Khano Serolane 2001_5_2 Excel Impilo Beauregard 1999_1_7 Blesboka Ndoua Monatea
60 79 60 64 53 56 56 50 53 43 47 46
55 68 43 48 45 49 51 47 55 46 37 30
65 43 52 58 55 47 49 48 23 16 15 16
72 61 65 46 56 56 47 49 61 51 47 45
63 7 63 1 55 10 54 8 53 5 52 5 51 4 49 2 48 17 39 16 37 15 34 14
Khano Serolane W_119 Resisto 1999_1_7 Excel Impilo 2001_5_2 Beauregard Monatea Ndoua Blesboka
39 35 24 33 28 30 30 24 17 20 20 13
31 31 29 31 36 30 25 25 23 19 20 17
35 37 34 27 23 27 28 31 23 18 18 13
38 30 37 28 30 29 25 24 22 22 20 16
37 3 33 3 31 6 30 3 29 6 29 1 27 3 26 3 21 3 20 2 20 1 15 2
Mean (site) Overall mean P-Value (var) P-Value (site) LSD (var) LSD (site) CV%
56 a
48 bc
40 c
55 ab
Mean (site) Overall mean P-Value (var) P-Value (site) LSD (var) LSD (site) CV%
26
27
26
27
a a ab ab ab abc abc bcd bcd cde de e
50 <0.001 <0.001 12.9 7.5 18.1
a ab abc bc bc bc c c d de de e
26 <0.001 0.932 5.0 ns 13.1
Means for site in a row followed by the same letter and means for variety in a column followed by the same letter, are not significantly different at P 0.05. ns = not significant; var = variety; Hazy = Hazyview; Giy = Giyani; Emp = Empangeni; Rood = Roodeplaat. a Cream-fleshed varieties.
S.M. Laurie et al. / Journal of Food Composition and Analysis 27 (2012) 151–159
with higher soil moisture levels. Rautenbach et al. (2010) reported a lower dry matter content in 3 varieties with an increase in water application.
155
double the other sites, ascribed possibly to sunlight, soil type and other environmental factors. 3.3. Mineral content in raw sweetpotato roots
3.2. Trans-b-carotene content in raw sweetpotato roots The trans-b-carotene content in raw roots, determined over 4 sites, varied between 5091 and 16,456 mg/100 g for the 9 orange-fleshed varieties (Table 3), with a mean trans-b-carotene content of 9706 mg/100 g. The cream-fleshed varieties, Ndou, Monate and Blesbok, had negligible content of trans-b-carotene. A very high mean percentage (95.8%, varying between 92.6 and 97.2%) of total b-carotene was in the trans configuration in the orange-fleshed varieties. The predominance of the trans isomer in orange-fleshed sweetpotato agrees with previous reports (Van Jaarsveld et al., 2006; Bengtsson et al., 2008; Takahata et al., 1993) and is an additional beneficial characteristic of orange-fleshed sweetpotato for addressing vitamin A deficiency as the trans isomer has double the provitamin A activity than the cis isomers. Orange-fleshed sweetpotato varieties with high trans-bcarotene content were Resisto, Khano and 2001_5_2 (11,800– 16,456 mg/100 g), with 2001_5_2 showing the least variation, and Resisto the most variation over the 4 sites. Six varieties exceeded the breeding target of 7500 mg/100 g for trans-b-carotene as set for the HarvestPlus Program (Nestel et al., 2006). The trans-b-carotene content of sweetpotato varied across the agro-geographical sites. The highest mean trans-b-carotene content was detected in sweetpotatoes grown at Giyani, significantly higher than those grown at Roodeplaat and Empangeni. The site at Giyani was typical of community-based production and experienced periods of water stress due to unavailability of irrigation water. The b-carotene content of sweetpotato tended to increase with low water application (Rautenbach et al., 2010) and to decrease under high irrigation levels (Constantin et al., 1974). The shortage of irrigation water could thus have contributed toward the higher trans-b-carotene content in sweetpotatoes grown at Giyani. Wu et al. (2008) found a large effect of growing site on the content of b-carotene in the same variety, as much as
3.3.1. Calcium The overall mean calcium content of the 12 sweetpotato varieties was 50 mg/100 g, and ranged from 34 to 63 mg/100 g (Table 4). Significant differences were found in calcium content between varieties. The general trend observed was that calcium content was higher in orange-fleshed varieties than in the creamfleshed varieties, with Resisto and W_119 having the highest calcium content. A variation in calcium content across varieties within geographical sites was observed. The largest variation was observed for sweetpotato varieties from Empangeni (15–65 mg/100 g). On average, sweetpotatoes from Hazyview had the highest calcium content (56 mg/100 g). Hazyview also had the highest soil calcium content (Table 2), suggesting a relationship between soil value and root content. 3.3.2. Magnesium The magnesium content in raw roots of the 12 sweetpotato varieties, determined over 4 sites, ranged from 15 to 37 mg/100 g (overall mean 26 mg/100 g). Magnesium content varied among the varieties (Table 4). Khano, Serolane and W_119 had the highest mean magnesium content, while cream-fleshed varieties Monate, Ndou and Blesbok had the lowest. Higher variability in magnesium content was seen for W_119 and 1999_1_7. Although soil magnesium content (Table 2) varied considerably among sites, the magnesium content in sweetpotato did not differ significantly over the sites. Availability of magnesium in soil is complex and is influenced by the concentration of exchangeable elements such as potassium, sodium and calcium (Mayland and Wilkinson, 1989). 3.3.3. Phosphorus The overall mean content of phosphorus in the 12 sweetpotato varieties was 40 mg/100 g (varying from 28 to 51 mg/100 g;
Table 5 Means of triplicate determinations for phosphorus and potassium content (mg/100 g raw roots) for 12 sweetpotato varieties at 4 agro-geographical production sites and mean over the 4 sites. Phosphorus (P)
Potassium (K)
Variety
Hazy
Giy
Emp
Rood
Mean (variety)
Variety
Hazy
Giy
Emp
Rood
Mean (variety)
Serolane Khano Impilo Resisto 1999_1_7 Excel W_119 Ndoua Monatea 2001_5_2 Beauregard Blesboka
54 49 47 50 46 44 37 43 41 35 31 30
60 47 47 48 48 46 54 49 44 33 33 30
37 33 42 34 32 35 34 31 26 31 26 24
52 54 45 47 46 46 45 45 39 32 31 28
51 10 46 9 45 3 45 7 43 7 43 5 42 9 42 8 38 8 cd 33 1 30 3 28 3e
Serolane Khano Ndoua 1999_1_7 Excel Monatea Impilo Resisto W_119 Beauregard 2001_5_2 Blesboka
309 295 312 255 259 279 252 257 246 235 247 193
414 334 359 335 301 313 324 324 333 283 195 200
268 237 239 277 253 274 261 218 215 224 235 181
345 391 328 320 350 297 307 286 259 253 259 190
334 62 314 65 309 51 297 37 291 45 291 18 286 35 272 45 263 50 249 26 234 28 191 8 g
Mean (site) Overall mean P-Value (var) P-Value (site) LSD (var) LSD (site) CV%
42 a
45 a
32 b
42 a
Mean (site) Overall mean P-Value (var) P-Value (site) LSD (var) LSD (site) CV%
262 b
310 a
240 b
299 a
a ab b b b bc bc bc de e
40 <0.001 <0.001 5.3 3.1 9.1
a ab abc abcd bcd bcd bcde cdef def ef f
278 <0.001 <0.001 41.36 23.88 10.4
Means for site in a row followed by the same letter and means for variety in a column followed by the same letter, are not significantly different at P 0.05. ns = not significant; var = variety; Hazy = Hazyview; Giy = Giyani; Emp = Empangeni; Rood = Roodeplaat. a Cream-fleshed varieties.
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156
Table 5). Significant variation was found in the phosphorus content of varieties, with Serolane having significantly higher mean phosphorus content than all other varieties, except Khano. The cream-fleshed variety Blesbok had the lowest mean phosphorus content. When considering the standard deviations, more variability in phosphorus content was observed over sites for Serolane, Khano and W_119. Although similar trends were seen for varieties over sites, mean phosphorus content of sweetpotato at Empangeni was significantly lower (varying between 24 and 42 mg/100 g) than for the other 3 sites. Soil phosphorus content was the lowest for Empangeni and furthermore poorly accessible to the plants as the soil pH was low (pH = 6.1) (Table 2). 3.3.4. Potassium The overall mean potassium content of the 12 varieties was 278 mg/100 g, varying between 191 and 334 mg/100 g (Table 5). Potassium content of the sweetpotato varieties differed significantly, with Serolane, Khano, Ndou and 1999_1_7 having the highest, and Blesbok having the lowest potassium content. The cream-fleshed varieties exhibited potassium content more comparable to orange-fleshed varieties, than in the case of magnesium and calcium. Significant differences were found in the mean potassium content of the raw roots between the sites. Sweetpotatoes from Empangeni (240 mg/100 g) and Hazyview (262 mg/100 g) had significantly lower potassium content than those from Giyani (310 mg/100 g) and Roodeplaat (299 mg/100 g). Soil potassium content was the highest in Empangeni, yet the sweetpotato roots produced in the area had the lowest potassium content (Table 2). This somewhat contradictory observation was probably caused by interference of sodium in uptake of potassium, as the soil level of sodium was considerably higher than at all other sites. 3.3.5. Iron The overall mean iron content of the 12 sweetpotato varieties over 4 sites was 0.93 mg/100 g (varying between 0.73 and 1.26 mg/ 100 g) (Table 6). No statistically significant differences were found between the mean iron content of varieties. The mean iron content
of sweetpotatoes from Empangeni (1.34 mg/100 g) was significantly higher than that of the sweetpotatoes from the other 3 sites. The soil iron content at Empangeni was 2.4-fold higher than that at Giyani. Soil iron content seems to be negatively associated with soil pH (Table 2). 3.3.6. Zinc As for iron, no statistically significant differences were found between the mean zinc content of varieties (Table 6). The overall mean for the zinc content of the 12 varieties was 0.60 mg/100 g. Zinc content differed significantly between the sites. Sweetpotatoes at Hazyview (0.73 mg/100 g) had the highest mean zinc content, while sweetpotatoes at Roodeplaat had significantly lower mean zinc content (0.42 mg/100 g) than all other sites. 3.4. Potential nutritional contribution to dietary requirements An average portion (125 g) of orange-fleshed sweetpotato provides more than 100% of the RDA for 4–8 year old children (Table 7), even those with the lower b-carotene content (i.e. Impilo, Serolane and Excel). Dark orange-fleshed varieties such as Resisto, provide more than four times the RDA for 4–8 year old children; a quarter of the portion would therefore still be adequate. The potential contribution of sweetpotato toward the dietary requirements of minerals is fairly low (Table 7). The highest contribution is for magnesium. Most of the orange-fleshed varieties contain at least 20% of the RDA for 4–8 year old children for magnesium, while Khano and Serolane contain 30% of the RDA for this age group. Deficiencies of iron and zinc are, together with vitamin A and iodine deficiency, globally the leading causes of malnutrition (Ezzatti et al., 2002). An average portion size of any of the varieties will provide at least 12% of the RDA for zinc and 9% of the RDA for iron for children of age 4–8 years. For phosphorus, potassium and calcium, an average portion size of sweetpotato will provide on average 10% or less of the RDA for children aged 4–8 years. For all minerals included here, the orange-fleshed sweetpotato varieties contributed more to mineral requirements than cream-fleshed sweetpotato.
Table 6 Means of triplicate determinations for iron and zinc content (mg/100 g raw roots) for 12 sweetpotato varieties at 4 agro-geographical production sites and mean over the 4 sites. Iron (Fe)
Zinc (Zn)
Variety
Hazy
Giy
Emp
Rood
Mean (var)
Variety
Hazy
Giy
Emp
Rood
Mean (var)
W_119 Impilo 1999_1_7 Serolane Resisto Monatea Khano Excel 2001_5_2 Blesboka Beauregard Ndoua
1.60 1.57 0.96 1.00 0.36 0.71 0.42 0.85 0.80 0.72 0.60 0.32
1.12 0.75 1.04 1.18 0.96 0.70 1.17 0.62 0.95 0.60 0.84 1.11
1.77 1.54 1.67 1.38 0.92 1.39 1.28 1.12 1.13 1.33 1.34 1.14
0.54 0.82 0.92 0.41 0.88 0.84 0.51 0.66 0.37 0.53 0.24 0.36
1.26 0.55 1.17 0.45 1.15 0.35 0.99 0.42 0.91 0.33 0.84 0.44 0.81 0.23 0.81 0.33 0.80 0.37 0.78 0.28 0.76 0.46 0.73 0.45
Beauregard 2001_5_2 Excel Khano Serolane Resisto W_119 Ndoua Impilo 1999_1_7 Monatea Blesboka
0.90 0.69 0.85 0.80 0.85 0.71 0.64 0.65 0.70 0.74 0.58 0.63
0.68 0.57 0.71 0.63 0.68 0.70 0.77 0.73 0.60 0.51 0.47 0.50
0.82 0.85 0.61 0.63 0.64 0.55 0.62 0.65 0.67 0.57 0.57 0.49
0.38 0.59 0.39 0.51 0.32 0.45 0.37 0.32 0.37 0.40 0.51 0.37
0.69 0.23 0.67 0.13 0.64 0.19 0.64 0.12 0.62 0.22 0.60 0.13 0.60 0.17 0.60 0.18 0.60 0.15 0.56 0.14 0.53 0.05 0.51 0.10
Mean (site) Overall mean P-Value (var) P-Value (site) LSD (var) LSD (site) CV%
0.83 bc
0.98 b
1.34 a
0.59 c
Mean (site) Overall mean P-Value (var) P-Value (site) LSD (var) LSD (site) CV%
0.73 a
0.63 b
0.64 b
0.42 c
0.93 0.191 <0.001 ns 0.24 31.1
0.60 <0.149 <0.001 ns 0.0755 15.1
Means for site in a row followed by the same letter and means for variety in a column followed by the same letter, are not significantly different at P 0.05. ns = not significant; var = variety; Hazy = Hazyview; Giy = Giyani; Emp = Empangeni; Rood = Roodeplaat. a Cream-fleshed varieties.
Table 7 Nutrients supplied by 125 g fresh roots of orange-fleshed sweetpotato varieties when boiled (calculated by using the USDA retention factors), daily nutrient requirement for 4–8 year old children and potential contribution (%) to nutrient requirements of this life stage. Nutrient
RDA/AI (mg/day)
Orange-fleshed varieties
Cream-fleshed varieties
Resisto
Khano
2001_5_2
W_119
Beauregard
1999_1_7
Excel
Serolane
Impilo
Mean
Ndou
1463 1243 311
1229 1044 261
1090 927 232
1005 854 214
996 847 212
533 453 113
514 460 115
530 451 113
1011 859 222
14 12 3
Monate 4 3 1
Blesbok 1 1 0
Mean 6 5 1
400
%RDA 4–8 years oldsc
1714 1457 428
1000
mg/125 g fresh root mg when boiledb %RDA 4–8 years oldsa
79 75 7
68 65 6
66 62 6
79 75 7
61 58 6
60 57 6
65 61 6
68 64 6
63 60 6
68 64 6
46 43 4
43 41 4
49 46 5
46 43 4
130
mg/125 g fresh root mg when boiledb %RDA 4–8 years oldsc
37 35 27
45 42 33
33 31 24
39 37 28
27 25 19
37 35 27
36 34 26
41 39 30
34 32 25
36 35 27
24 23 18
25 23 18
18 18 13
22 21 16
500
mg/125 g fresh root mg when boiledb %RDA 4–8 years oldsc
56 53 11
57 54 11
41 39 8
53 50 10
38 36 7
54 51 10
53 51 10
63 60 12
56 54 11
52 50 10
52 50 10
47 44 9
35 33 7
45 42 8
3800
mg/125 g fresh root mg when boiledb %RDA 4–8 years oldsc
339 322 8.5
393 373 9.8
293 278 7.3
329 312 8.2
311 295 7.8
371 352 9.3
364 345 9.1
418 397 10.4
358 340 8.9
353 335 8.8
387 367 9.7
364 345 9.1
239 227 6.0
330 313 8.2
10
mg/125 g fresh root mg when boiledb %RDA 4–8 years oldsc
1.1 1.1 10.8
1.0 1.0 9.7
1.0 0.9 9.4
1.6 1.5 14.9
0.9 0.9 9.0
1.4 1.4 13.6
1.0 1.0 9.6
1.2 1.2 11.8
1.5 1.4 13.9
1.2 1.1 11.4
0.9 0.9 8.7
1.1 1.0 10.0
1.0 0.9 9.3
1.0 0.9 9.3
5
mg/125 g fresh root mg when boiledb %RDA 4–8 years oldsc
0.8 0.7 14.3
0.8 0.8 15.2
0.8 0.8 16.0
0.7 0.7 14.2
0.9 0.8 16.5
0.7 0.7 13.2
0.8 0.8 15.2
0.8 0.7 14.8
0.7 0.7 13.9
0.8 0.7 14.8
0.7 0.7 13.9
0.7 0.6 12.6
0.6 0.6 11.8
0.7 0.6 12.8
Calcium
Magnesium
Phosphorus
Potassium
Iron
Zinc
RAE = retinol activity equivalents: 12 mg all-trans-b-carotene = 1 mg retinol = 1 mg RAE (Trumbo et al., 2001). b Nutrient content in boiled roots calculated from the fresh content obtained in the present study using USDA retention factors (USDA, 2007): 95% for minerals and 85% for b-carotene. c RDA = recommended dietary allowance (daily dietary intake that is considered sufficient to meet the nutrient requirement of nearly all (97–98%) healthy individuals in a particular life stage and gender group) (Trumbo et al., 2001). d AI = adequate intake (recommended intake assumed to be adequate); no RDA available for calcium and potassium nutrient requirements (mg/day) based on Trumbo et al. (2001), National Academy of Sciences (2004), and Ross et al. (2011).
S.M. Laurie et al. / Journal of Food Composition and Analysis 27 (2012) 151–159
mg RAE/125 g fresha mg when boiledb
Vitamin A
a
Unit
157
158
S.M. Laurie et al. / Journal of Food Composition and Analysis 27 (2012) 151–159
4. Conclusions In this study, a random composite sample representative of each variety per site was analyzed. Although environmental variability at each site was minimized by harvesting roots from different parts of the plot to constitute the composite laboratory sample, preferably more than one random sample per variety per site should have been collected and analyzed. Despite this limitation, the values obtained were within the reported range of both trans-b-carotene content and mineral content. The results of the present study showed that orange-fleshed sweetpotato varieties contain considerable amounts of trans-b-carotene and confirmed that nearly all of the total b-carotene in orange-fleshed sweetpotato was in the trans configuration. The large genetic diversity in terms of trans-b-carotene content between sweetpotato varieties was apparent. Six orange-fleshed varieties were identified with b-carotene content above the breeding target level for use in crop-based programs to alleviate vitamin A deficiency. As a precursor of vitamin A, the b-carotene in a 125 g boiled portion of all 9 of the orange-fleshed sweetpotato varieties, even those with the lower b-carotene content, will provide, on average over the 4 sites, more than 100% of the RDA for 4–8 year old children. Six of these varieties at any of the 4 agro-geographical sites will provide more than 100% of the RDA for 4–8 year old children. Blesbok, the commercially available cream-fleshed variety, had the lowest dry matter content (18.7%). Five of the 9 orange-fleshed varieties had a high dry matter content of >25%, which is of importance in taste acceptance and consequently in consumer adoption of this crop. Iron and zinc content did not differ significantly between varieties. For the other minerals, mineral content in orange-fleshed varieties was generally higher than in cream-fleshed varieties. Mineral content varied over the geographical sites, which could be ascribed to differences in soil pH, soil mineral content and the interaction of these factors. The varieties with superior mineral content, Serolane, Resisto and W-119, also had high dry matter content. The mean calculated contribution to the recommended dietary allowance of minerals for 4–8 year old children when 125 g of these sweetpotato varieties are boiled was 24% for magnesium, 14% for zinc, 11% for iron and 10% or less for phosphorus, potassium and calcium. The orange-fleshed varieties were superior to creamfleshed varieties in terms of calcium and magnesium content. This study showed variation in b-carotene and mineral content in sweetpotato between varieties, as well within varieties over agro-geographical sites. This variation needs to be considered in varietal selection for different agro-geographical production sites and in calculating nutrient contribution of sweetpotato toward dietary intake. Acknowledgements This research was funded by the ARC and the South African Sugar Association (Project 202). The authors thank Ms Liesl Morey and Ms Marie Smith of the Biometry Unit of the ARC for assistance with statistical procedures, the ARC-Institute for Soil, Climate and Water for conducting the mineral analyses, Ms Beula Pretorius ARC-Animal Nutrition and Products Institute for dry matter determination and Mr Eldrich Harmse and Ms Johanna van Wyk of the Nutritional Intervention Research Unit of the Medical Research Council for laboratory assistance. References AOAC, 2007. Official Methods of Analysis of AOAC International, 18th edition revision 2. Association of Official Analytical Chemists International, Maryland, USA.
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Journal of Food Composition and Analysis journal homepage: www.elsevier.com/locate/jfca
Original Research Article
Trans-b-carotene, selected mineral content and potential nutritional contribution of 12 sweetpotato varieties S.M. Laurie a,*, P.J. van Jaarsveld b, M. Faber b, M.F. Philpott c, M.T. Labuschagne d a
Agricultural Research Council - Roodeplaat Vegetable and Ornamental Plant Institute, Private Bag X293, Pretoria 0001, South Africa Nutritional Intervention Research Unit, Medical Research Council, PO Box 19070, Tygerberg 7505, South Africa c Agricultural Research Council - Institute for Soil Climate and Water, Private Bag X79, Pretoria 0001, South Africa d Department of Plant Sciences, University of Free State, PO Box 339, Bloemfontein 9300, South Africa b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 January 2012 Received in revised form 24 May 2012 Accepted 27 May 2012
The content of trans-b-carotene and selected minerals was determined in 12 sweetpotato (Ipomoea batatas) varieties produced at 4 agro-geographical production sites in South Africa. All 9 orange-fleshed varieties have the potential to contribute 100% of the recommended dietary allowance of 4–8 year-old children for vitamin A, 27% for magnesium, 15% for zinc and 11% for iron. Orange-fleshed varieties were superior to cream-fleshed ones in calcium and magnesium content. The trans-b-carotene content of the varieties varied over the geographical sites. The mean content in the 9 orange-fleshed varieties was between 5091 and 16,456 mg/100 g fresh weight. The mineral content in fresh roots of the 12 varieties ranged from 34 to 63 mg/100 g for calcium, 15 to 37 mg/100 g for magnesium, 28 to 51 mg/100 g for phosphorus, 191 to 334 mg/100 g for potassium, 0.73 to 1.26 mg/100 g for iron, and 0.51 to 0.69 mg/ 100 g for zinc. Variation within varieties over geographical sites could be ascribed to differences in soil mineral content, soil pH and the interaction of these. The variation in nutritional content of sweetpotato indicated here, needs to be considered in varietal selection for different production sites and in calculating nutrient contribution of sweetpotato toward dietary intake. ß 2012 Elsevier Inc. All rights reserved.
Keywords: Sweet potato Ipomoea batatas Carotenoids Environment Dry matter content Minerals Recommended daily allowance Vitamin A deficiency VAD Biodiversity Varietal differences Genetic variability in nutrient content Horticulture Agricultural practices and nutrition Food security Food analysis Food composition
1. Introduction Sweetpotato (Ipomoea batatas), one of the major staple crops of the world, is an excellent source of energy (438 kJ/100 g edible portion) and, as a crop, has advantages such as versatility, high yield, hardiness and wide ecological adaptability. In developing countries, sweetpotato is important in alleviating major problems such as malnutrition, food insecurity, consequences of droughts and limited agricultural technologies (Bovell-Benjamin, 2007). There is renewed focus on particularly the orange-fleshed sweetpotato, as it offers one of the best sources of naturally bioavailable b-carotene, the major precursor of vitamin A (Burri,
* Corresponding author. Tel.: +27 12 8419611; fax: +27 12 8080844. E-mail address: [email protected] (S.M. Laurie). 0889-1575/$ – see front matter ß 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jfca.2012.05.005
2011; Van Jaarsveld et al., 2005). The efficacy and effectiveness to improve vitamin A status of orange-fleshed sweetpotato have been confirmed (Low et al., 2007; Van Jaarsveld et al., 2005). Carotenoids are important to human health for their ability to boost the immune system, and to maintain eye health and vision (Blomhoff and Blomhoff, 2006; Montrone et al., 2009). Children who are vitamin A deficient have a lower resistance to common childhood infections such as respiratory and diarrheal diseases, measles, and malaria (Rice et al., 2004). Worldwide, 33.3%, or 190 million children younger than 5 years are vitamin A deficient (WHO, 2009). Orange-fleshed sweetpotato is one of the major provitamin A-rich crops promoted in crop-based programs to address vitamin A deficiency in developing countries (Burri, 2011). In sweetpotato there is a very large genetic diversity in bcarotene content, ranging from <100 mg/100 g to 26,600 mg/100 g in raw sweetpotato (Kidmose et al., 2007; K’osambo et al., 1998;
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Takahata et al., 1993; Teow et al., 2007; Wu et al., 2008). Whiteand cream-fleshed types are devoid of, or contain very little, bcarotene. The b-carotene content in food crops is used to calculate the vitamin A value as mg RAE (retinol activity equivalents) (Trumbo et al., 2001) and the potential contribution food crops can make to the vitamin A dietary requirement of humans. Orange-fleshed sweetpotato is one of six staple crops in the HarvestPlus Crop Biofortification Program which aims to breed key micronutrients into staple crops (Nestel et al., 2006; Pfeiffer and McClafferty, 2007). Nutrient content of the crop and the influence of the environment during growth on micronutrient expression are key considerations in biofortification programs (Pfeiffer and McClafferty, 2007). The target level when breeding for orange-fleshed sweetpotato to be used as sole source of pro-vitamin A is 7500 mg b-carotene/100 g, and 3750 mg b-carotene/100 g in mixed diets (Nestel et al., 2006). The b-carotene content in sweetpotato varieties can significantly be influenced by air and soil temperature, radiation, fertilization, location, season and maturity of roots (K’osambo et al., 1998; Liu et al., 2009; Wu et al., 2008). To obtain reliable estimates, b-carotene content in sweetpotato therefore needs to be measured over a range of geographical production sites. The main nutritional focus of the orange-fleshed sweetpotato is its b-carotene content and subsequently its potential to alleviate vitamin A deficiency. However, sweetpotato also contains several minerals. Reported values for mineral content of sweetpotato (per 100 g raw roots) show a large variation for e.g. potassium (129–470 mg), iron (0.16–0.94 mg), zinc (0.27–1.89 mg), calcium (8–101 mg), magnesium (18–35 mg) and phosphorus (41–70 mg) (Leighton, 2007; STA, 2005; USDA, 2009; Wolmarans et al., 2010; Woolfe, 1992). Courtney et al. (2008) reported high heritability estimates for iron (H = 0.74) and zinc (H = 0.82) in sweetpotato, while Gru¨neberg (2006) found a positive correlation between iron and zinc content, and to a lesser degree between zinc and b-carotene content in sweetpotato, indicating promising prospects to breed simultaneously for improved content of these 3 micronutrients. This is of particular importance as vitamin A, iron and zinc deficiencies are among the leading causes of disease burden and mortality globally (Ezzatti et al., 2002). Tumwegamire et al. (2011) reported significant genotypic variance in dry matter, protein, starch,
b-carotene, iron, zinc, calcium and magnesium content of 89 farmers varieties from East Africa. The present study was conducted (1) to determine the content of b-carotene and selected minerals in 12 sweetpotato varieties; (2) to investigate the effect of agro-geographical production sites on the content of b-carotene and selected minerals; and (3) to calculate the potential contribution to human nutrition. Minerals selected for analysis were iron, zinc, potassium, phosphorus, calcium and magnesium. 2. Materials and methods 2.1. Plant material Nine orange-fleshed I. batatas varieties with varying intensities of orange flesh color and 3 local cream-fleshed varieties (Blesbok, Ndou and Monate) were investigated. The orange-fleshed varieties included 3 local varieties (Khano, Serolane and Impilo), 2 promising local advanced breeding lines (1999_1_7 and 2001_5_2) and 4 varieties imported from the USA (Resisto, W119, Excel and Beauregard). Samples of storage roots were collected from plots consisting of 60 plants in multi-location cultivar trials over 4 agro-geographical production sites in South Africa. Geographical, climatic and cultural conditions at the 4 sites are summarized in Table 1. The highest atmospheric temperature was experienced at Giyani and Empangeni, while Empangeni had the highest humidity. Agricultural management was very good at Roodeplaat and Empangeni where trained full-time technicians were based, while management at Giyani and Hazyview was typical of production conditions in communities. The latter two sites experienced water stress at some stages due to unavailability of irrigation water. Analysis of soil samples from the 4 sites indicated loam-clay soils which are generally acceptable for vegetable production, and pH level within acceptable range (Table 2). Soil phosphorus content was low at Empangeni and high at Roodeplaat. Soil potassium content was generally within the optimum range, but lower at Giyani. Soil calcium content was high at Hazyview, while soil magnesium content was high at Roodeplaat and Hazyview. At Roodeplaat the soil Ca:Mg ratio was low. For Giyani the soil sodium content was low. Fertilizer application is indicated in Table 1.
Table 1 Geographical, climatic and cultural conditions at the 4 agro-geographical production sites.
Province Type of site Location Climatic area Altitude Growth period Temperature (8C)a Rainfall (mm)a Humidity (%)a Radiation (MJ/m2/day)a Irrigation Irrigation frequency Fertilization: preplant Fertilization: top dressing Total nutrient application (kg/ha)
Roodeplaat
Giyani
Hazyview
Empangeni
Gauteng Research Institute 25.6048S; 28.3458E Warm temperate 1164 m 5 months 21.6 31.3 51.7 22.2 Sprinkler (400 mm) 1–2 times/week (depending on rainfall) 200 kg/ha 1:0:1 (18.5% N, 0% P, 18.5% K) 150 kg/ha limestone ammonium nitrate (LAN; 28% N) N = 79, P = 0, K = 37
Limpopo Community garden 23.3008S; 30.5338E Dry subtropical 716 m 5 months 24.5 39.6 59.1 18.5 Furrow Infrequent
Mpumalanga Farmer’s plot 25.0498S; 31.1448E Humid subtropical 564 m 6 months (winter) 19.5 27.0 59.4 13.6 Furrow Infrequent
500 kg/ha 2:3:2 (8.5% N, 13% P, 8.5% K) 150 kg/ha LAN (28% N)
150 kg/ha 2:3:4 (6.7% N, 10% P, 13.3% K) 268 kg/ha LAN (28% N), 170 kg/ha potassium chloride (KCl; 50% K) N = 85, P = 15, K = 105
KwaZulu-Natal Agricultural College 28.7258S; 31.8988E Humid subtropical 100 m 4.5 months 24.8 94.1 71.7 19.4 Sprinkler 1–2 times/week (depending on rainfall) 500 kg/ha 2:3:2 (8.5% N, 13% P, 8.5% K) 150 kg/ha LAN (28% N)
N = 84, P = 64, K = 42
Source: Climatic database ARC-Institute for Soil Climate and Water, South Africa. a Monthly mean.
N = 84, P = 64, K = 42
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Table 2 Physical and chemical properties of the topsoil at the 4 agro-geographical production sites. Units
Roodeplaat Soil physical properties Clay (%) % Soil chemical properties mg/kg Phosphorousa Potassiumb mg/kg Calciumb mg/kg b Magnesium mg/kg b Sodium mg/kg b Iron mg/kg Zincb mg/kg pH (water)
Interpretationc
Site
28 92 147 1413 548 62 – – 6.9
Giyani 34 19 116 959 252 16 64.8 1.27 7.0
Hazyview
Empangeni
26
32
24 168 2991 611 42 – – 6.8
9 193 1126 321 74 156 3.89 6.1
Low content
High content
Optimum content
<35% <15 <40 <200 <50 <50 na na <5.5
>60 >250 >3000 >300 >250 na na >7.3
40–90 120–240 400–2500d 100–400d na na
‘‘–’’ = Fe and Zn not analyzed; na = not available (micronutrient). a Bray I extraction method (Non-Affiliated Soil Analysis Work Committee, 1990). b Ammonium acetate method (Non-Affiliated Soil Analysis Work Committee, 1990). c Indication of levels for vegetable production according to Buys (1988). d Less critical.
At each of the 4 sites, the sweetpotatoes were lifted from the soil by garden fork, and 25 medium-sized sweetpotatoes were randomly selected from different parts of the plot, providing a representative random sample for each variety. Each laboratory sample was reduced by randomly selecting 8 of the 25 mediumsized sweetpotatoes (approximately 3 kg). The harvested roots were washed with tap water to remove the soil and placed on a brown paper sheet to allow the roots to air-dry for about 30 min before the intact unprocessed roots were transported to the laboratory where sample preparation for the nutrient analysis was done. 2.2. Analytical procedures At the laboratory of the Medical Research Council (MRC), 5 medium-sized sweetpotatoes per variety for each site were peeled, thoroughly washed with tap water and dried with paper toweling. Two opposite quarters from longitudinally quartered roots were combined and homogenized, and aliquots of 2.0–2.5 g weighed and stored at 20 8C until analysis for b-carotene content. The total b-carotene as well as trans-b-carotene content was determined in duplicate by high-performance liquid chromatography (HPLC) (SpectraSERIES; Thermo Separation Products, Fremont (CA)) using a monomeric C18 column (Waters Spherisorb ODS 2, 3 mm, 4.6 mm 150 mm). This method has been evaluated for sweetpotato (Van Jaarsveld et al., 2006) and was slightly modified as described by Low and Van Jaarsveld (2008). Extraction of bcarotene was done with tetrahydrofuran:methanol (1:1, vol/vol). A b-carotene standard was purified by HPLC as previously described (Van Jaarsveld et al., 2006) and a 5-point standard curve that bracketed the concentration of the unknown samples was constructed in triplicate. An aliquot of the purified standard solution with a known concentration was used as the external standard for quantification of b-carotene in the sample extracts (Van Jaarsveld et al., 2006). Sub-samples (0.5 g) from the same homogenized mixture were used at the Agricultural Research Council (ARC)-Institute for Soil, Climate and Water for triplicate determinations of zinc, iron, potassium, magnesium, phosphorus and calcium through inductively coupled plasma (ICP) optical emission spectrometric determination according to Huang and Schulte (1985). The following wavelengths were used: calcium 422.673 and 317.933 nm, magnesium 383.826 nm, phosphorus 213.618 nm, potassium 769.896 nm, iron 259.940 nm and zinc 213.856 nm. The calibration curves were found to be very good for all 6 elements, with a correlation coefficient of 0.9995 for K and greater than 0.9999 for the other 5 elements.
Aliquots were dried at 105 8C for 16 h for duplicate determination of dry matter content (AOAC, 2007). Care was taken throughout the sample preparation to avoid contamination. 2.3. Statistical analysis The data were analyzed with GenStat12007. In case of the minerals, the triplicate determinations were first screened for normality, and outliers examined and eliminated. The values were rejected when identified as outliers through at least two of the following three criteria: (a) the difference between the maximum or minimum determination and the median determination was larger than an appropriate constant for that element; (b) the ratio of the difference between the maximum and minimum determination to the difference between the two determinations closest to each other was larger than an appropriate constant for that element; and (c) the maximum determination was much larger than the maximum determination for all the other samples or the minimum determination was much smaller than the minimum determination for all the other samples. The number of outliers removed was 10 for iron, 5 for zinc, and 1 each for magnesium, phosphorus and calcium. The mean of duplicate/triplicate determinations per variety was calculated. To assess the variability of nutrient content among sites and varieties, an ANOVA was performed. The Student’s protected t-Least Significant Difference (LSD) test was calculated at the 1% significance level for b-carotene content, and 5% significance level for mineral content to compare varietal means and site means. The mean nutrient content per variety over the 4 sites was used to calculate the potential nutrient contribution to dietary requirements of 4–8 year old children (National Academy of Sciences, 2004; Trumbo et al., 2001). A portion size of 125 g was used, based on consumption data available for South African children (Nel and Steyn, 2002). Calculations were based on retention of nutrients when boiled, using the USDA retention factors of 85% for b -carotene and 95% for minerals (USDA, 2007). 3. Results and discussion 3.1. Dry matter content The commercially available cream-fleshed variety Blesbok had the lowest dry matter content at 18.7% (Table 3). The dry matter
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Table 3 Means for trans-b-carotene content (mg/100 g raw root) for 12 sweetpotato varieties at 4 agro-geographical production sites, mean over the 4 sites, and mean dry matter content (%). Variety
Resisto Khano 2001_5_2 W_119 Beauregard 1999_1_7 Excel Serolane Impilo Ndoub Monateb Blesbokb Mean (site) Overall mean Mean orangefleshed varieties P-Value (variety) P-Value (site) LSD (variety) LSD (site) CV%
Trans-b-carotene content (mg/100 g raw root)
Dry mass content (%)
Hazyview
Giyani
Empangeni Roodeplaat Mean (variety)
18,372 15,565 12,372 10,236 9922 9453 4434 5500 2978 110 16 23
20,525 14,439 12,470 12,978 11,609 11,869 5945 4255 6125 212 21 10
11,987 12,234 11,716 8806 8453 8384 5534 4766 4227 21 62 9
7415 ab
8372 a
14,939 13,906 10,640 9836 8599 8524 4889 5903 7034 177 25 12
6350 b
%Trans Hazyview Giyani Empangeni Roodeplaat Mean configurationa (variety)
16,456 3490 a 96.9 14,036 1293 ab 97.2 11,800 799 bc 92.6 10,464 1649 c 96.7 9646 1362 c 95.8 9558 1495 c 96.9 5106 687 d 96.5 5200 622 d 95.8 5091 1697 d 94.5 134 77 e 99.9 31 0 e 100.0 14 0 e 100.0
7040 b 7294
9706
24.3 19.4 21.3 25.1 17.3 24.1 27.9 25.8 23.1 27.2 24.8 15.5
28.9 24.2 25.7 31.8 21.1 27.9 28.8 34.9 24.8 28.8 23.9 22.6
28.5 24.9 25.5 30.4 21.5 23.7 26.1 30.6 22.9 23.6 27.1 20.1
27.5 21.2 23 27.8 18.8 24.7 23.5 30.5 21.9 28 23.8 16.1
23.0 c
26.9 a
25.4 b
23.9 c
96.2
27.3 2.1 22.4 2.6 23.9 2.1 28.7 2.9 19.8 2.0 25.1 1.9 26.7 2.4 30.5 3.7 23.1 1.0 27.0 2.3 24.9 1.5 18.7 3.4
bc e de ab f cd bc a de bc cde f
24.8
95.8
25.3 <0.001 0.009 2640 1525 18.7
<0.001 <0.001 1.5 2.6 7.2
Means for site in a row followed by the same letter and means for variety in a column followed by the same letter, are not significantly different at P 0.01. a %Trans configuration of total b-carotene. b Cream-fleshed varieties.
content of the 9 orange-fleshed varieties varied from 19.8 to 30.5%, which is slightly lower than that reported for selected countries in the sub-Saharan Africa region, which has a dry matter content ranging from 21 to 34.9% (Kapinga et al., 2007). The dry matter content of varieties is a critical parameter in sweetpotato breeding programs, as African consumers generally prefer sweetpotato varieties with high dry matter content (Tomlins et al., 2004; Masumba et al., 2007; Laurie, 2010). From a consumer perspective, orange-fleshed varieties with high dry matter content such as
Serolane, Resisto and W_119 were the most promising varieties. The standard deviations given in Table 3 indicate a variation in dry matter content within varieties over the 4 geographical sites. It has been reported that the dry matter content in root crops can vary according to differences in climate, day length, soil type and cultivation practices, e.g. water application vs. rainfall (Bradhury and Holloway, 1988). The dry matter content was low at Roodeplaat compared to Giyani, where the rainfall was lower and irrigation more erratic, pointing at lower dry matter content
Table 4 Means of triplicate determinations for calcium and magnesium content (mg/100 g raw roots) for 12 sweetpotato varieties at 4 agro-geographical production sites and mean over the 4 sites. Calcium (Ca)
Magnesium (Mg)
Variety
Hazy
Giy
Emp
Rood
Mean (variety)
Variety
Hazy
Giy
Emp
Rood
Mean (variety)
W_119 Resisto Khano Serolane 2001_5_2 Excel Impilo Beauregard 1999_1_7 Blesboka Ndoua Monatea
60 79 60 64 53 56 56 50 53 43 47 46
55 68 43 48 45 49 51 47 55 46 37 30
65 43 52 58 55 47 49 48 23 16 15 16
72 61 65 46 56 56 47 49 61 51 47 45
63 7 63 1 55 10 54 8 53 5 52 5 51 4 49 2 48 17 39 16 37 15 34 14
Khano Serolane W_119 Resisto 1999_1_7 Excel Impilo 2001_5_2 Beauregard Monatea Ndoua Blesboka
39 35 24 33 28 30 30 24 17 20 20 13
31 31 29 31 36 30 25 25 23 19 20 17
35 37 34 27 23 27 28 31 23 18 18 13
38 30 37 28 30 29 25 24 22 22 20 16
37 3 33 3 31 6 30 3 29 6 29 1 27 3 26 3 21 3 20 2 20 1 15 2
Mean (site) Overall mean P-Value (var) P-Value (site) LSD (var) LSD (site) CV%
56 a
48 bc
40 c
55 ab
Mean (site) Overall mean P-Value (var) P-Value (site) LSD (var) LSD (site) CV%
26
27
26
27
a a ab ab ab abc abc bcd bcd cde de e
50 <0.001 <0.001 12.9 7.5 18.1
a ab abc bc bc bc c c d de de e
26 <0.001 0.932 5.0 ns 13.1
Means for site in a row followed by the same letter and means for variety in a column followed by the same letter, are not significantly different at P 0.05. ns = not significant; var = variety; Hazy = Hazyview; Giy = Giyani; Emp = Empangeni; Rood = Roodeplaat. a Cream-fleshed varieties.
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with higher soil moisture levels. Rautenbach et al. (2010) reported a lower dry matter content in 3 varieties with an increase in water application.
155
double the other sites, ascribed possibly to sunlight, soil type and other environmental factors. 3.3. Mineral content in raw sweetpotato roots
3.2. Trans-b-carotene content in raw sweetpotato roots The trans-b-carotene content in raw roots, determined over 4 sites, varied between 5091 and 16,456 mg/100 g for the 9 orange-fleshed varieties (Table 3), with a mean trans-b-carotene content of 9706 mg/100 g. The cream-fleshed varieties, Ndou, Monate and Blesbok, had negligible content of trans-b-carotene. A very high mean percentage (95.8%, varying between 92.6 and 97.2%) of total b-carotene was in the trans configuration in the orange-fleshed varieties. The predominance of the trans isomer in orange-fleshed sweetpotato agrees with previous reports (Van Jaarsveld et al., 2006; Bengtsson et al., 2008; Takahata et al., 1993) and is an additional beneficial characteristic of orange-fleshed sweetpotato for addressing vitamin A deficiency as the trans isomer has double the provitamin A activity than the cis isomers. Orange-fleshed sweetpotato varieties with high trans-bcarotene content were Resisto, Khano and 2001_5_2 (11,800– 16,456 mg/100 g), with 2001_5_2 showing the least variation, and Resisto the most variation over the 4 sites. Six varieties exceeded the breeding target of 7500 mg/100 g for trans-b-carotene as set for the HarvestPlus Program (Nestel et al., 2006). The trans-b-carotene content of sweetpotato varied across the agro-geographical sites. The highest mean trans-b-carotene content was detected in sweetpotatoes grown at Giyani, significantly higher than those grown at Roodeplaat and Empangeni. The site at Giyani was typical of community-based production and experienced periods of water stress due to unavailability of irrigation water. The b-carotene content of sweetpotato tended to increase with low water application (Rautenbach et al., 2010) and to decrease under high irrigation levels (Constantin et al., 1974). The shortage of irrigation water could thus have contributed toward the higher trans-b-carotene content in sweetpotatoes grown at Giyani. Wu et al. (2008) found a large effect of growing site on the content of b-carotene in the same variety, as much as
3.3.1. Calcium The overall mean calcium content of the 12 sweetpotato varieties was 50 mg/100 g, and ranged from 34 to 63 mg/100 g (Table 4). Significant differences were found in calcium content between varieties. The general trend observed was that calcium content was higher in orange-fleshed varieties than in the creamfleshed varieties, with Resisto and W_119 having the highest calcium content. A variation in calcium content across varieties within geographical sites was observed. The largest variation was observed for sweetpotato varieties from Empangeni (15–65 mg/100 g). On average, sweetpotatoes from Hazyview had the highest calcium content (56 mg/100 g). Hazyview also had the highest soil calcium content (Table 2), suggesting a relationship between soil value and root content. 3.3.2. Magnesium The magnesium content in raw roots of the 12 sweetpotato varieties, determined over 4 sites, ranged from 15 to 37 mg/100 g (overall mean 26 mg/100 g). Magnesium content varied among the varieties (Table 4). Khano, Serolane and W_119 had the highest mean magnesium content, while cream-fleshed varieties Monate, Ndou and Blesbok had the lowest. Higher variability in magnesium content was seen for W_119 and 1999_1_7. Although soil magnesium content (Table 2) varied considerably among sites, the magnesium content in sweetpotato did not differ significantly over the sites. Availability of magnesium in soil is complex and is influenced by the concentration of exchangeable elements such as potassium, sodium and calcium (Mayland and Wilkinson, 1989). 3.3.3. Phosphorus The overall mean content of phosphorus in the 12 sweetpotato varieties was 40 mg/100 g (varying from 28 to 51 mg/100 g;
Table 5 Means of triplicate determinations for phosphorus and potassium content (mg/100 g raw roots) for 12 sweetpotato varieties at 4 agro-geographical production sites and mean over the 4 sites. Phosphorus (P)
Potassium (K)
Variety
Hazy
Giy
Emp
Rood
Mean (variety)
Variety
Hazy
Giy
Emp
Rood
Mean (variety)
Serolane Khano Impilo Resisto 1999_1_7 Excel W_119 Ndoua Monatea 2001_5_2 Beauregard Blesboka
54 49 47 50 46 44 37 43 41 35 31 30
60 47 47 48 48 46 54 49 44 33 33 30
37 33 42 34 32 35 34 31 26 31 26 24
52 54 45 47 46 46 45 45 39 32 31 28
51 10 46 9 45 3 45 7 43 7 43 5 42 9 42 8 38 8 cd 33 1 30 3 28 3e
Serolane Khano Ndoua 1999_1_7 Excel Monatea Impilo Resisto W_119 Beauregard 2001_5_2 Blesboka
309 295 312 255 259 279 252 257 246 235 247 193
414 334 359 335 301 313 324 324 333 283 195 200
268 237 239 277 253 274 261 218 215 224 235 181
345 391 328 320 350 297 307 286 259 253 259 190
334 62 314 65 309 51 297 37 291 45 291 18 286 35 272 45 263 50 249 26 234 28 191 8 g
Mean (site) Overall mean P-Value (var) P-Value (site) LSD (var) LSD (site) CV%
42 a
45 a
32 b
42 a
Mean (site) Overall mean P-Value (var) P-Value (site) LSD (var) LSD (site) CV%
262 b
310 a
240 b
299 a
a ab b b b bc bc bc de e
40 <0.001 <0.001 5.3 3.1 9.1
a ab abc abcd bcd bcd bcde cdef def ef f
278 <0.001 <0.001 41.36 23.88 10.4
Means for site in a row followed by the same letter and means for variety in a column followed by the same letter, are not significantly different at P 0.05. ns = not significant; var = variety; Hazy = Hazyview; Giy = Giyani; Emp = Empangeni; Rood = Roodeplaat. a Cream-fleshed varieties.
S.M. Laurie et al. / Journal of Food Composition and Analysis 27 (2012) 151–159
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Table 5). Significant variation was found in the phosphorus content of varieties, with Serolane having significantly higher mean phosphorus content than all other varieties, except Khano. The cream-fleshed variety Blesbok had the lowest mean phosphorus content. When considering the standard deviations, more variability in phosphorus content was observed over sites for Serolane, Khano and W_119. Although similar trends were seen for varieties over sites, mean phosphorus content of sweetpotato at Empangeni was significantly lower (varying between 24 and 42 mg/100 g) than for the other 3 sites. Soil phosphorus content was the lowest for Empangeni and furthermore poorly accessible to the plants as the soil pH was low (pH = 6.1) (Table 2). 3.3.4. Potassium The overall mean potassium content of the 12 varieties was 278 mg/100 g, varying between 191 and 334 mg/100 g (Table 5). Potassium content of the sweetpotato varieties differed significantly, with Serolane, Khano, Ndou and 1999_1_7 having the highest, and Blesbok having the lowest potassium content. The cream-fleshed varieties exhibited potassium content more comparable to orange-fleshed varieties, than in the case of magnesium and calcium. Significant differences were found in the mean potassium content of the raw roots between the sites. Sweetpotatoes from Empangeni (240 mg/100 g) and Hazyview (262 mg/100 g) had significantly lower potassium content than those from Giyani (310 mg/100 g) and Roodeplaat (299 mg/100 g). Soil potassium content was the highest in Empangeni, yet the sweetpotato roots produced in the area had the lowest potassium content (Table 2). This somewhat contradictory observation was probably caused by interference of sodium in uptake of potassium, as the soil level of sodium was considerably higher than at all other sites. 3.3.5. Iron The overall mean iron content of the 12 sweetpotato varieties over 4 sites was 0.93 mg/100 g (varying between 0.73 and 1.26 mg/ 100 g) (Table 6). No statistically significant differences were found between the mean iron content of varieties. The mean iron content
of sweetpotatoes from Empangeni (1.34 mg/100 g) was significantly higher than that of the sweetpotatoes from the other 3 sites. The soil iron content at Empangeni was 2.4-fold higher than that at Giyani. Soil iron content seems to be negatively associated with soil pH (Table 2). 3.3.6. Zinc As for iron, no statistically significant differences were found between the mean zinc content of varieties (Table 6). The overall mean for the zinc content of the 12 varieties was 0.60 mg/100 g. Zinc content differed significantly between the sites. Sweetpotatoes at Hazyview (0.73 mg/100 g) had the highest mean zinc content, while sweetpotatoes at Roodeplaat had significantly lower mean zinc content (0.42 mg/100 g) than all other sites. 3.4. Potential nutritional contribution to dietary requirements An average portion (125 g) of orange-fleshed sweetpotato provides more than 100% of the RDA for 4–8 year old children (Table 7), even those with the lower b-carotene content (i.e. Impilo, Serolane and Excel). Dark orange-fleshed varieties such as Resisto, provide more than four times the RDA for 4–8 year old children; a quarter of the portion would therefore still be adequate. The potential contribution of sweetpotato toward the dietary requirements of minerals is fairly low (Table 7). The highest contribution is for magnesium. Most of the orange-fleshed varieties contain at least 20% of the RDA for 4–8 year old children for magnesium, while Khano and Serolane contain 30% of the RDA for this age group. Deficiencies of iron and zinc are, together with vitamin A and iodine deficiency, globally the leading causes of malnutrition (Ezzatti et al., 2002). An average portion size of any of the varieties will provide at least 12% of the RDA for zinc and 9% of the RDA for iron for children of age 4–8 years. For phosphorus, potassium and calcium, an average portion size of sweetpotato will provide on average 10% or less of the RDA for children aged 4–8 years. For all minerals included here, the orange-fleshed sweetpotato varieties contributed more to mineral requirements than cream-fleshed sweetpotato.
Table 6 Means of triplicate determinations for iron and zinc content (mg/100 g raw roots) for 12 sweetpotato varieties at 4 agro-geographical production sites and mean over the 4 sites. Iron (Fe)
Zinc (Zn)
Variety
Hazy
Giy
Emp
Rood
Mean (var)
Variety
Hazy
Giy
Emp
Rood
Mean (var)
W_119 Impilo 1999_1_7 Serolane Resisto Monatea Khano Excel 2001_5_2 Blesboka Beauregard Ndoua
1.60 1.57 0.96 1.00 0.36 0.71 0.42 0.85 0.80 0.72 0.60 0.32
1.12 0.75 1.04 1.18 0.96 0.70 1.17 0.62 0.95 0.60 0.84 1.11
1.77 1.54 1.67 1.38 0.92 1.39 1.28 1.12 1.13 1.33 1.34 1.14
0.54 0.82 0.92 0.41 0.88 0.84 0.51 0.66 0.37 0.53 0.24 0.36
1.26 0.55 1.17 0.45 1.15 0.35 0.99 0.42 0.91 0.33 0.84 0.44 0.81 0.23 0.81 0.33 0.80 0.37 0.78 0.28 0.76 0.46 0.73 0.45
Beauregard 2001_5_2 Excel Khano Serolane Resisto W_119 Ndoua Impilo 1999_1_7 Monatea Blesboka
0.90 0.69 0.85 0.80 0.85 0.71 0.64 0.65 0.70 0.74 0.58 0.63
0.68 0.57 0.71 0.63 0.68 0.70 0.77 0.73 0.60 0.51 0.47 0.50
0.82 0.85 0.61 0.63 0.64 0.55 0.62 0.65 0.67 0.57 0.57 0.49
0.38 0.59 0.39 0.51 0.32 0.45 0.37 0.32 0.37 0.40 0.51 0.37
0.69 0.23 0.67 0.13 0.64 0.19 0.64 0.12 0.62 0.22 0.60 0.13 0.60 0.17 0.60 0.18 0.60 0.15 0.56 0.14 0.53 0.05 0.51 0.10
Mean (site) Overall mean P-Value (var) P-Value (site) LSD (var) LSD (site) CV%
0.83 bc
0.98 b
1.34 a
0.59 c
Mean (site) Overall mean P-Value (var) P-Value (site) LSD (var) LSD (site) CV%
0.73 a
0.63 b
0.64 b
0.42 c
0.93 0.191 <0.001 ns 0.24 31.1
0.60 <0.149 <0.001 ns 0.0755 15.1
Means for site in a row followed by the same letter and means for variety in a column followed by the same letter, are not significantly different at P 0.05. ns = not significant; var = variety; Hazy = Hazyview; Giy = Giyani; Emp = Empangeni; Rood = Roodeplaat. a Cream-fleshed varieties.
Table 7 Nutrients supplied by 125 g fresh roots of orange-fleshed sweetpotato varieties when boiled (calculated by using the USDA retention factors), daily nutrient requirement for 4–8 year old children and potential contribution (%) to nutrient requirements of this life stage. Nutrient
RDA/AI (mg/day)
Orange-fleshed varieties
Cream-fleshed varieties
Resisto
Khano
2001_5_2
W_119
Beauregard
1999_1_7
Excel
Serolane
Impilo
Mean
Ndou
1463 1243 311
1229 1044 261
1090 927 232
1005 854 214
996 847 212
533 453 113
514 460 115
530 451 113
1011 859 222
14 12 3
Monate 4 3 1
Blesbok 1 1 0
Mean 6 5 1
400
%RDA 4–8 years oldsc
1714 1457 428
1000
mg/125 g fresh root mg when boiledb %RDA 4–8 years oldsa
79 75 7
68 65 6
66 62 6
79 75 7
61 58 6
60 57 6
65 61 6
68 64 6
63 60 6
68 64 6
46 43 4
43 41 4
49 46 5
46 43 4
130
mg/125 g fresh root mg when boiledb %RDA 4–8 years oldsc
37 35 27
45 42 33
33 31 24
39 37 28
27 25 19
37 35 27
36 34 26
41 39 30
34 32 25
36 35 27
24 23 18
25 23 18
18 18 13
22 21 16
500
mg/125 g fresh root mg when boiledb %RDA 4–8 years oldsc
56 53 11
57 54 11
41 39 8
53 50 10
38 36 7
54 51 10
53 51 10
63 60 12
56 54 11
52 50 10
52 50 10
47 44 9
35 33 7
45 42 8
3800
mg/125 g fresh root mg when boiledb %RDA 4–8 years oldsc
339 322 8.5
393 373 9.8
293 278 7.3
329 312 8.2
311 295 7.8
371 352 9.3
364 345 9.1
418 397 10.4
358 340 8.9
353 335 8.8
387 367 9.7
364 345 9.1
239 227 6.0
330 313 8.2
10
mg/125 g fresh root mg when boiledb %RDA 4–8 years oldsc
1.1 1.1 10.8
1.0 1.0 9.7
1.0 0.9 9.4
1.6 1.5 14.9
0.9 0.9 9.0
1.4 1.4 13.6
1.0 1.0 9.6
1.2 1.2 11.8
1.5 1.4 13.9
1.2 1.1 11.4
0.9 0.9 8.7
1.1 1.0 10.0
1.0 0.9 9.3
1.0 0.9 9.3
5
mg/125 g fresh root mg when boiledb %RDA 4–8 years oldsc
0.8 0.7 14.3
0.8 0.8 15.2
0.8 0.8 16.0
0.7 0.7 14.2
0.9 0.8 16.5
0.7 0.7 13.2
0.8 0.8 15.2
0.8 0.7 14.8
0.7 0.7 13.9
0.8 0.7 14.8
0.7 0.7 13.9
0.7 0.6 12.6
0.6 0.6 11.8
0.7 0.6 12.8
Calcium
Magnesium
Phosphorus
Potassium
Iron
Zinc
RAE = retinol activity equivalents: 12 mg all-trans-b-carotene = 1 mg retinol = 1 mg RAE (Trumbo et al., 2001). b Nutrient content in boiled roots calculated from the fresh content obtained in the present study using USDA retention factors (USDA, 2007): 95% for minerals and 85% for b-carotene. c RDA = recommended dietary allowance (daily dietary intake that is considered sufficient to meet the nutrient requirement of nearly all (97–98%) healthy individuals in a particular life stage and gender group) (Trumbo et al., 2001). d AI = adequate intake (recommended intake assumed to be adequate); no RDA available for calcium and potassium nutrient requirements (mg/day) based on Trumbo et al. (2001), National Academy of Sciences (2004), and Ross et al. (2011).
S.M. Laurie et al. / Journal of Food Composition and Analysis 27 (2012) 151–159
mg RAE/125 g fresha mg when boiledb
Vitamin A
a
Unit
157
158
S.M. Laurie et al. / Journal of Food Composition and Analysis 27 (2012) 151–159
4. Conclusions In this study, a random composite sample representative of each variety per site was analyzed. Although environmental variability at each site was minimized by harvesting roots from different parts of the plot to constitute the composite laboratory sample, preferably more than one random sample per variety per site should have been collected and analyzed. Despite this limitation, the values obtained were within the reported range of both trans-b-carotene content and mineral content. The results of the present study showed that orange-fleshed sweetpotato varieties contain considerable amounts of trans-b-carotene and confirmed that nearly all of the total b-carotene in orange-fleshed sweetpotato was in the trans configuration. The large genetic diversity in terms of trans-b-carotene content between sweetpotato varieties was apparent. Six orange-fleshed varieties were identified with b-carotene content above the breeding target level for use in crop-based programs to alleviate vitamin A deficiency. As a precursor of vitamin A, the b-carotene in a 125 g boiled portion of all 9 of the orange-fleshed sweetpotato varieties, even those with the lower b-carotene content, will provide, on average over the 4 sites, more than 100% of the RDA for 4–8 year old children. Six of these varieties at any of the 4 agro-geographical sites will provide more than 100% of the RDA for 4–8 year old children. Blesbok, the commercially available cream-fleshed variety, had the lowest dry matter content (18.7%). Five of the 9 orange-fleshed varieties had a high dry matter content of >25%, which is of importance in taste acceptance and consequently in consumer adoption of this crop. Iron and zinc content did not differ significantly between varieties. For the other minerals, mineral content in orange-fleshed varieties was generally higher than in cream-fleshed varieties. Mineral content varied over the geographical sites, which could be ascribed to differences in soil pH, soil mineral content and the interaction of these factors. The varieties with superior mineral content, Serolane, Resisto and W-119, also had high dry matter content. The mean calculated contribution to the recommended dietary allowance of minerals for 4–8 year old children when 125 g of these sweetpotato varieties are boiled was 24% for magnesium, 14% for zinc, 11% for iron and 10% or less for phosphorus, potassium and calcium. The orange-fleshed varieties were superior to creamfleshed varieties in terms of calcium and magnesium content. This study showed variation in b-carotene and mineral content in sweetpotato between varieties, as well within varieties over agro-geographical sites. This variation needs to be considered in varietal selection for different agro-geographical production sites and in calculating nutrient contribution of sweetpotato toward dietary intake. Acknowledgements This research was funded by the ARC and the South African Sugar Association (Project 202). The authors thank Ms Liesl Morey and Ms Marie Smith of the Biometry Unit of the ARC for assistance with statistical procedures, the ARC-Institute for Soil, Climate and Water for conducting the mineral analyses, Ms Beula Pretorius ARC-Animal Nutrition and Products Institute for dry matter determination and Mr Eldrich Harmse and Ms Johanna van Wyk of the Nutritional Intervention Research Unit of the Medical Research Council for laboratory assistance. References AOAC, 2007. Official Methods of Analysis of AOAC International, 18th edition revision 2. Association of Official Analytical Chemists International, Maryland, USA.
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