African Crop Science Journal, Vol. 9, No. 1, March 2001, pp. 309-316
J. M. Ssebuliba, E. N. B. Nsubuga1 and J. H. Muyonga2
Code Number: CS01062
The potential of orange and yellow fleshed sweetpotato cultivars as a dietary source of Vitamin A in Mpigi and Luwero Districts of central Uganda was evaluated. On-farm agronomic performance, acceptability and β-carotene content of two orange (SPK004 and 316) and two yellow fleshed (Tanzania and 52) sweetpotato cultivars were determined and compared to the farmer's best local cultivars. There were significant differences in yield performance between cultivars with 52 yielding highest. Yields for TZ and SPK004 were comparable to those of best yielding local checks while 316 yielded lowest. Dry matter content of all the cultivars was above 30%. Overall, 52 was the most acceptable to farmers but children prefered SPK 004. β-carotene content of orange fleshed cultivars was higher than that of yellow and white fleshed cultivars. It was concluded that orange fleshed sweetpotato have highest potential for improving Vitamin A nutrition in the study area.
Key Words: β-carotene, Ipomoea batatus, sensory acceptability, Uganda, vitamin A malnutrition
Le potentiel des cultivars de patate douce ayant une chair jaune et orange comme source de vitamine A dans les districts de Mpigi et Luwero au centre de l'Uganda a été évalué. Des performances agronomiques en milieu réel, l'acceptabilité et le contenu en β-carotène de deux cultivars à chair orange (SPK004 et 316) et deux à chair jaune (Tanzanie et 52) ont été étudiés et comparés aux meilleurs cultivars des agriculteurs. Des differences significatives pour le rendement ont été obtenues entre les cultivars et le cultivar 52 était le meilleur. Les rendements de Tanzanie et SPK004 étaient semblables à ceux des mielleurs cultivars témoins locaux alors que le cultivar 316 était le plus pauvre. Le contenu en matière sèche de tous les cultivars était supérieur à 30%. En général, le cultivar 52 était le plus accepté par les agriculteurs mais les enfants ont préféré le cultivar SPK004 . Le contenu en β-carotène des cultivars à chair orange était le plus élevé plus que les cultivars à chair jaune et blanche. Il a été conclu que les patates douces à chair orange ont la plus grande potentialité pour améliorer la nutrition en Vitamine A dans la région.
Mots Clés: β-carotène, Ipomoea batatus, acceptabilité sensorielle, Uganda, malnutrition de la vitamine A
Vitamin A deficiency is a problem of public health significance in over 70 countries (Chakravarty, 2000). It has been associated with diets predominantly consisting of Vitamin A poor stables, such as tubers and grains, little diversity and low consumption of good sources such as meats, yellow and green vegetables and fruits (Underwood, 2000). Sweetpotatoes (Ipomoea batatus) which are a major staple in Mpigi and Luwero districts of Central Uganda vary widely in their content of the Vitamin A precursors. The orange and yellow fleshed sweetpotatoes may contain up to 4,000 µg/100g (fresh weight basis) of β-carotene while the white fleshed sweetpotatoes β-carotene content may be as low as 70 µg/100g (Woolfe, 1992). The low βcarotene, white fleshed sweetpotatoes predominate in Uganda. This could be a contributing factor to the widespread deficiency of Vitamin A. In Kamuli, another district which heavily depends on white fleshed sweetpotatoes as a staple, very high prevelance of clinical and subclinical Vitamin A deficiency have been reported (Kawuma & Sserunjogi, 1992). About 5.4% of children under six years of age in Kamuli district were reported to have signs of Vitamin A deficiency. The prevelence of corneal ulceration and scarring among children was found to be 20 times more than the WHO minimum criteria defining Vitamin A deficiency as a public health problem. Food based strategies that promote the consumption of orange fleshed sweetpotatoes can be an effective way to improve Vitamin A status of young children and their families around the world (Hagenimana et al.,1999). Orange fleshed sweetpotatoes have been identified as the least expensive year round sources of dietary Vitamin A in Kenya (Low et al., 1997). The potential of orange and yellow fleshed sweetpotatoes in alleviation of Vitamin A deficiency may not be realised unless the sweetpotato cultivars are good performers and are acceptable to farmers. Our objective was to evaluate the agronomic performance, acceptability and β-carotene content of two orange (SPK 004 and 316) and two yellow (Tanzania and 52) fleshed sweetpotato cultivars and compare them to widely consumed white fleshed cultivars in Mpigi and Luwero districts of Uganda.
MATERIALS AND METHODS
Location of experiments. The study was conducted in Mpigi and Luwero, two districts in central Uganda that border the capital city Kampala. Mpigi district is wetter with annual rainfall ranging from 1250-1625 mm, while Luwero is drier with 850-1250 mm of rainfall per annum. Small land holdings, low crop productivity and erratic rainfall characterise these two districts.
Selection of women/farmers groups. Women groups were selected from the two districts to grow and evaluate the sweetpotato cultivars. Women were chosen because in this area, women are normally responsible for farming and feeding families. A total of 20 groups were selected 10 from each of the districts. The selected groups were distrubuted by sub-county as follows; Luwero district - Katikamu -1, Kalagala -6 and Zirobwe -3; Mpigi district - Kiira - 4, Nangabo -5, and Busukuma -1. The criteria for selection of groups included being well organised, easily accessible, willingness and ability to grow new potato cultivars.
On-farm sweetpotato production. Each of the selected groups planted two orange fleshed sweetpotato clones (SPK 004 and 316), two yellow-fleshed clones (Tanzania and 52) and one local check cultivar provided by the group during the 1999 short rainy season (March - May) and long rainy season (September - November). The local check cultivars (most popular cultivar in the area) varied among the groups and included Dimbuka, Kawogo and Sekanyolya. Planting materials for the yellow and orange fleshed sweetpotatoes were obtained from Namulonge Agricultural Research Institute.
The members of the women groups together with the researchers were involved in the management of the trials from planting to harvest. Trials were planted on mounds that were approximately 1 meter apart with 3 cuttings per mound. The number of mounds per clone varied among women's groups depending on the available plot size. However, data at final harvest was collected on 6 mounds per clone. Crop management practices for each trial included weeding, earthing up, and physical destruction of the sweetpotato caterpillars. Trials were harvested six months after planting. Tubers were weighed and the tuber yield in tons per hectare was determined. Weevil damage to tubers was rated per plot using a score of 0 - 5 based on external damage (0 - no damage, 1 = mild damage on a few tubers, 2 =moderate damage on many tubers, 3 = severe damage on many tubers, 4 = very severe damage on most tubers, and 5= very severe damage on all the tubers). Analysis of variance was carried out on the sweetpotato traits using the General Linear Model (GLM) procedure of the statistical Analysis system (SAS Institute, Inc., Cary, NC, 1994). Mean separations were done using Tukey's studentised range test. Results from all the women groups in a district were analysed as a separate experiment with women groups treated as replicates.
Acceptability analysis. Following the harvest of the 1999 short and long rainy trials, the members of the women groups were asked to cook the tubers of each clone separately and the group members were asked to separately evaluate them for ease of cooking, absence of fiber, appearance, taste, and acceptability to children in the ranges 7-12 month, 1-5 years and 5-12 years. These are the most important traits that were mentioned by the members of the women groups in a preliminary survey. Acceptability analysis was carried out using a questionnare that was administered to the members of the women groups. Since most of the group members are semi or illiterate, they were assisted in recording their answers. The scoring was on a scale of 1-5, with 1 corresponding to very poor, 2 - poor, 3 - fair, 4 - good and 5 -very good. The farmers were also requested to identify desirable and undesirable attributes for each of the sweetpotato cultivars.
Tuber dry matter and β-carotene content determination. β-carotene content of the sweetpotato cultivars was determined by spectrophotometry (Ameny and Wilson, 1997). Stock solutions of β-carotene were prepared by weighing 25 mg into a 100 ml brown low actinic volumetric flask and making to volume using cyclo-hexane. Four working solutions were prepared by taking 1, 2, 3 and 4 ml of the stock solution and making up to 100 ml. Absorbency by the working solutions were determined at 450 nm. A standard curve prepared by plotting the concentration of β-carotene in working solutions against the absorbency was used to determine the concentration of β-carotene in sample extracts.
Sweetpotato tubers were peeled and grated using a cheese grater. The peeled samples were then separated into two lots. One lot was placed in pre-weighed moisture cups. The cups and samples were weighed and then dried in the oven at 70 °C, to a constant dry weight. They were then cooled and re-weighed. The dry matter content (DM) was then derived as a percentage. The other lot was put into a blender, with an opaque jar. To each 10 g of grated potatoes, 20 g of anhydrous sodium sulphite, 1g of magnesium carbonate and 100 ml of cyclo-hexane were added. The samples were blended at slow speed for 20 minutes and then vacuum filtered through a Buchner funnel fitted with Whatman # 1 filter paper. The absorbency of the extract at 450 nm was measured and the β-carotene content determined from the standard curve.
RESULTS AND DISCUSSION
On-farm perfomance. The highest yield recorded for the two seasons for both Luwero and Mpigi districts was for cultivar 52 while the lowest was for cultivar 316 (Table 1). The yields for Tanzania and SPK 004 were not significantly different from those for the local checks for Luwero district short rainy season and Mpigi district long rainy season harvests. Yields were however lower for the long and short rainy seasons, respectively, for Luwero and Mpigi district. Yield differences between the two districts can be attributed to differences in environmental factors during these periods. For example, rainfall was better distributed in the long rainy season in Luwero district than in Mpigi district.
Sweetpotatoes produced in Mpigi district had a higher number of tubers harvested per plant compared to those from Luwero in the first rainy season of 1999 (Table 1). However, this trend was reversed in the long rainy season. Sweetpotatoes produced from Luwero showed a higher number of root tubers per plant in the long rainy season while those from Mpigi had a higher number of tubers per plant in the short rainy season. There was no significant difference in number of tubers per plant between yellow and orange cultivars produced during both seasons in the two districts.
Weevil damage was recorded in the first rainy season while in the long rainy season it was negligible. This may be because in the short rainy season sweetpotato harvesting started at 6.5 months after planting as compared to the long rainy season when sweetpotato was harvested after exactly 6 months. Sweetpotato weevil damage increases with age of the crop and delay in harvesting (Ociti P'Obwoya and Namakula, 1997). Significant differences in weevil damage were only observed between cultivars 316 and the rest of the cultivars with 316 showing less weevil damage. This can be attributed to the smaller tuber sizes of cultivar 316. Smaller tubers are less prone to weevil attack because they do not produce cracks in the soil.
Dry-matter and β-carotene content of the sweetpotato cultivars. All the cultivars generally had dry matter content above 30 % (Table 2). This concurs with the eating preferences of the people in Luwero district who prefer cultivars with high dry matter content (Ociti P'Obwoya and Namakula, 1997).
Cultivars SPK004 and 316 had the highest β-carotene content, suggesting that the β-carotene content of orange fleshed cultivars was higher than that of yellow fleshed cultivars. The local checks that were evaluated contained amounts of β-carotene comparable to that of yellow fleshed cultivars (Table 2). This is in agreement with earlier reports that some white-fleshed sweetpotato cultivars may also contain a good amount of β-carotene (Martin, 1983; Ameny and Wilson, 1997). However, flesh colour intensity of sweetpotato correlates highly with pro-Vitamin A value (Takahata et al., 1993), with deeper yellow and orange-fleshed tubers being likely to have higher β-carotene content.
Acceptability of orange and yellow fleshed sweetpotato cultivars. Acceptability
of the sweetpotato cultivars by the farmers was in the order 52> SPK 004
> Tanzania > 316 > Kawogo > Sekanyolya> Dimbuka (Table 3). The
acceptability in terms of flesh colour was in the order yellow > orange >
local (white) while acceptability to children was in the order SPK004 > 52
> Tanzania > 316> Kawogo > Sakanyolya > Dimbuka. Overall, 43.5%
of all the farmers ranked the new orange and yellow fleshed sweetpotatoes as
superior to local checks, 26% ranked them inferior and 30.5% ranked them as
Attributes of the sweetpotato cultivars identified as important by farmers. Attributes identified as desirable and undesirable for the different sweetpotato cultivars are listed in Table 4 and those identified by at least 5% of the respondents are given in Table 5. These attributes may serve as guidelines for future cultivar selection and breeding.
Other attributes considered important by farmers are: big tubers (mentioned by 4.5% of respondents); liked by children (3.5%); have tubers all along the vines (2.5%); good taste (1.5%); have vines all year (1.5%); cook quickly (1.5%); long tubers (1.0%); resistant to weevils (0.5%); cooked appearance resembles that of bananas (0.5%); absence of fiber (0.5%); not soft (0.5%); remains good when kept over night (0.5%); good vine cover (0.5%); good tuber shape (0.5%); high yield on all soils (0.5%).
There are a few contradictions in results for the desirable and undesirable sweepotato attributes identified by farmers. For example, some farmers mentioned ability to keep vines during drought as a desirable attribute for cultivars while others mentioned disappearance of vines during drought as an undesirable attribute for the same cultivar. This difference could be due to variation in other environmental conditions.
For the local checks, some farmers mentioned resistance to weevils as a desirable attribute while others mentioned high weevil damage as an undesirable attribute. This too could be due to variations in environmental conditions or variation in the checks since farmers did not grow the same local check.
The smell of Tanzania was considered desirable by some farmers and undesirable by others. Such differences in acceptability are expected since preferances differ.
Orange fleshed sweetpotatoes offer great potential as an inexpensive way for improvement of Vitamin A nutrition in the two districts studied and possibly in other areas where sweetpotato is an important food crop. Of the cultivars investigated, 52, TZ and SPK004 performed either better or as good as local checks and were acceptable to farmers. Adoption of these cultivars is therefore likely to be high if they are promoted. In order to ensure fast adoption, there is need for nutritional education alongside provision of planting materials.
The authors thank Thrasher Research Fund for financial support, the extension staff in the two districts for mobilising farmers, Mr. Gard Turyamureeba of Namulonge Agricultural Research Institute for providing the planting materials and women group leaders for the co-operation accorded.
TABLE 1. Agronomic performance of different sweetpotato cultivars in Luwero and Mpigi Districts during the 1999 short and long rainy season
Means followed by the same letter within a column are not significantly different according to Tukey's studentised range test
*0=No damage 5=Very severe damage on all tubers
TABLE 2. Dry matter and β-carotene content of different sweetpotato cultivars grown in Luwero and Mpigi Districts during the 1999 short and long rainy seasons
Means followed by the same letter within a column are not significantly different according to Tukey's studentized range test
#dry matter and β-carotene content of peeled tubers
TABLE 3. Acceptability of different sweetpotato cultivars by farmers in Luwero and Mpigi Districts
Means followed by the same letter within a column are not significantly different according to Tukey's studentised range test
*1 = Very Poor, 2 = Poor, 3 = Fair, 4 = Good, 5 = Very Good
#Acceptability by children provided by mothers
TABLE 4. Desirable and undesirable attributes identified by farmers in Mpigi and Luwero Districts for different potato cultivars
TABLE 5. Ranking of the most desirable potato characteristics identified by farmers in Luwero and Mpigi districts