Sweet potato ( Ipomoea batatas ) is one of the most important, versatile and unexploited crops in Kenya. The crop is well adapted to smallholder farming systems, inexpensive to produce, relatively drought tolerant and gives high yields even with minimum inputs. Although widely produced in Kenya, sweet potato remains primarily a subsistence crop. Lack of organized marketing, limited consumer interest and low value addition activities are some of the factors that have contributed to low commercialization of the crop. The overall objective of the current study was to investigate the suitability of incorporating sweet potato leaves into the roots to produce nutritious sweet potato flakes with high vitamin A and protein content. Sweet potato roots were cured to increase the endogenous amylase enzyme and then washed and pre-cooked to enable starch hydrolysis to increase sweetness of the flakes and then heated to boiling to enable mashing. Dried sweet potato leaves powder was then added to the mash sweet potato roots at varying percentages and then dried using single drum drier. Addition of sweet potato leaves was found to significantly (p < 0.05) increase protein content from 6.6% protein to 15.40% when the leaves constituted 50% of the flakes. Beta carotene content of the flakes decreased from 7986 μg/100g when no leaves were added to 3979 μg/100g when the leaves constituted 50% of the flakes. The addition of the leaves reduced the overall acceptability. Colour was the most adversely affected while texture was the least affected by the addition of sweet potato leaves. However, all the flakes with up to 30% leaves were acceptable to the panelists with respect to colour, taste, texture and overall acceptability. Incorporating sweet potato leaves into the roots can therefore improve sweet potato protein and hence improve nutrition and value addition of root-based products such as flakes and flour.
Sweet potato (Ipomoea batatas) is a hardy and nutritious staple food crop, which is grown throughout the humid tropical and subtropical regions of the world. It is a perennial plant of the family Convolvulaceae and is among the most important food crops in the world and is ranked seventh based on total production and the fifth most important crop in developing countries [
In Kenya, sweet potato growing is mainly concentrated in eastern Kenya including Kakamega, Bungoma, Busia, Homa Bay, and Kisiicounties. It is also grown to a small extent in the coastal and central regions with production being concentrated in mid-altitudes i.e. 1000 - 1600 m above the sea level [
Orange fleshed sweet potato (OFSP) is one of the new crops that have been introduced due to its high beta- carotene content. It is an excellent source of energy and important nutrients such as vitamin A that can contribute towards improvement of the nutritional status of the community [
Studies have demonstrated that orange fleshed sweet potato improves vitamin A status in children given that the vitamin in the OFSP is highly bioavailable [
Sweet potatoes, just like cereal and potato snacks tend to be low in protein and have poor biological value due to their limited essential amino acid content. Crude protein content of sweet potato has been reported to vary between 1% and 10% (on a dry weight basis), but this includes 10% - 15% non-protein nitrogenous components [
In addition to high protein content, the sweet potato leaves have high antioxidative polyphenolics such as anthocyanins and phenolic acids such as caffeic, monocafeoylquinic (chlorogenic), dicaffeoylquinic, and tricaffeoylquinic acids [
Ready-to-eat sweet potato breakfast cereals have been developed [
Raw orange fleshed sweet potatoes used in the current study were mature roots of variety SPK 004 and its tender sweet potato leaves from Kenya Agricultural Research Centre (KARI) Muguga. They were grown following standard cultural practices and harvested at maturity from the KARI Centre and transported to the University of Nairobi, Department of Food Science, Nutrition and Technology for analysis.
The morphological pictures of the OFSP roots and leaves used for the study are presented in
Sweet potato roots were cured for 5 days at a temperature of 32.2˚C and a relative humidity of 85% as described by Ikemiya and Deobald (1966) [
Sweet potato roots were precooked at a temperature of 75˚C for two hours to enhance starch hydrolysis to enable production of sweet flakes. These parameters were decided based on the idea that alpha-amylase enzymes are heat stable up to a temperature of 70˚C - 75˚C as reported by Ikemiya and Deobald (1966) [
The sweet potato mashed roots and the ground sweet potato leaves were blended as shown in
Individual constituents | Percent blending ratios | |||||
---|---|---|---|---|---|---|
T1 | T2 | T3 | T4 | T5 | T6 | |
Ground sweet potato leaves (%) | 0 | 10 | 20 | 30 | 40 | 50 |
Mashed sweet potato roots (%) | 100 | 90 | 80 | 70 | 60 | 50 |
intervals of 10% until the composite contained the maximum acceptable amount of leaves.
Orange fleshed sweet potato root and leaves were obtained from KARI, Muguga center and were analyzed before and after processing. Before processing, the roots and leaves were washed and then macerated to a smooth puree in a conventional kitchen type electric blender (model NO. BL 60603G, China) and then samples drawn for analysis.
Standard methods of AOAC [
Duplicate samples of 100 g of orange fleshed sweet potato roots were measured accurately and diced and then homogenized with 100 ml of water in a kitchen blender (model NO. BL 60603G, China) for 5minutes. The aqueous fraction containing suspended starch was collected by screening through fine nylon. The residue was recycled twice with the same amount of water and screened through a nylon cloth after each recycling in the blender. All the aqueous fraction was collected together and balanced out in two (4) centrifuge bottles and centrifuged for 10 minutes at about 3000 rpm. The supernatant was gotten rid of and the scum on top of the starch cake removed and the surface rinsed with a little water and suspended in a little water and centrifuged again and the cleaning steps repeated. Finally the starch was purified in a Buchner funnel lined with Whatman paper No. 1 by percolating water, 95% ethanol and acetone through the starch and the starch content determined on a dry matter basis
Total carotenoids from the sweet potato were extracted as described by Khachik (1992) [
Sensory evaluation was based on 7-point hedonic scale [
Samples were analyzed in duplicates and the data generated analyzed using analysis of variance (ANOVA) and comparison of means carried out using least significant difference test (LSD) calculated at 5% using GenStat 15th edition.
The dry matter (DM) content of Orange Fleshed Sweet Potato (OFSP) roots was 28.1%. The DM of roots were significantly (p < 0.05) higher than the dry matter content of the sweet potato leaves which was relatively low at 18.5%. The DM of the sweet potato leaves reported in the current study was comparable to the value of 17.7% reported by Antia (2006) [
The dry mater content of the roots found in this study was higher than 27.16% which was reported in the US Department of Agriculture, Nutrient database of 2001 [
The starch content for the roots was 86.87% on DM basis. The starch content of the roots was significantly (p < 0.05) higher than the starch content of the leaves which was 50.96% on DM basis. There is therefore an adequate quantity of starch for hydrolysis to sugars to enhance the sweetness of the flakes. Omodamiro (2013) [
The protein content of the OFSP roots used in the current study was found to be 6.64% on DM basis. This value is close to the figure of 6.22% on DM basis which was reported by US Department of Agriculture, Nutrient data base of 2001 [
The protein content of the sweet potato leaves was 24.16% which was significantly (p < 0.05) higher compared to the protein content of the roots. The protein content found in the current study was comparable to the result reported by Antia (2006) [
The ash content of the OFSP roots was found to be 3.18% on DM basis. This was a little lower compared to the results reported in the US Department of Agriculture, Nutrient database of 2001 [
Dry matter (%) | Starch content (%) | Protein content (%) | Total sugars (%) | Total ash (%) | β-carotene (mg/100g) | |
---|---|---|---|---|---|---|
Roots | 28.1 ± 0.14a | 86.87 ± 0.07a | 6.64 ± 0.04a | 15.21± 0.01 | 3.18 ± 0.04a | 8.65 ± 0.10a |
Leaves | 18.5 ± 0.14b | 50.96 ± 0.08b | 24.16 ± 0.14b | ND | 11.43± 0.04b | 0.53 ± 0.03b |
Values are means of duplicate determinations plus or minus standard deviation; ND = not determined. Values with similar letters in the same column are not significantly different at 5%.
dried sweet potato leaves that was reported to be 11.10% by Antia (2006) [
The β-carotene content in OFSP root was found to be 8.65 mg/100g. The OFSP roots can provide much higher β-carotene than the RDA for Vitamin A which is measured in retinol equivalents (RE) as a means of making standard comparisons among foods [
The level of carotenoids could be used as a means of choosing the variety for processing as this plays an important role in the overall acceptability and enhances the nutritional image of the flakes [
Adding sweet potato leaves to the roots significantly (p < 0.05) increased the protein content of the flakes (
Increasing the sweet potato leaves reduced the beta carotene content significantly (p < 0.05) as indicated in
The sweet potato flakes presented to the panel varied in appearance as shown in
Addition of sweet potato leaves to the tuber significantly (p < 0.05) affected the colour of the flakes. The flakes containing 100% roots was the most accepted with an average score of 6.62. This decreased as more sweet potato leaves were added with the flakes containing 40% leaves scoring a mean score of less than 4 showing they were not acceptable to the panelists. Addition of leaves may only be done up to 30% if the product has to be acceptable using visual color, beyond which acceptability is greatly reduced.
Addition of sweet potato leaves to the tuber significantly (p < 0.05) affected the taste of the flakes with 100% roots getting a mean score of 6.75. The addition of the leaves reduced the acceptability with one of the treatment containing 50% leaves being rejected since it had a mean score of less than 4. The current results show that it is possible to add leaves up to 40% percent and the taste will still be acceptable.
Texture was the most highly scored parameter which means that it was the most accepted quality attribute with a mean score for all the treatments being 5.92. Despite this the addition of the leaves to the roots significantly (p < 0.05) affected the score for flakes. All the treatments for the flakes were acceptable in terms of texture with the least mean score for the treatment being 4.12, no mastication problems irrespective of the amount of leaves added.
Scores for overall acceptability revealed that all the treatments were accepted by the panelists with the least mean score being 4.12 which is above 4 while the average mean score for all the treatments was 5.52. It is, however, noted that the addition of the leaves to the roots significantly (p < 0.05) lowered the overall acceptability of the flakes.
Addition of sweet potato leaves to the roots increases the nutritional content of the flakes mainly in terms of pro- tein while still maintaining the high beta carotene to levels that can meet the recommended daily allowance.
Attribute | T1 | T2 | T3 | T4 | T5 | T6 |
---|---|---|---|---|---|---|
Colour | 6.62a | 5.88b | 5.75c | 4.12d | 3.38d | 2.75e |
Taste | 6.75a | 6.50b | 6.00c | 4.88d | 4.25e | 3.13f |
Texture | 6.75a | 6.75a | 6.25b | 5.75c | 5.25d | 5.13d |
Overall acceptability | 6.63a | 6.50ab | 6.13abc | 5.00bcd | 4.75cd | 4.12d |
Values are mean of ten panels. Same letters along the row indicate there is no significant difference between the treatments (p > 0.05).
Addition of the leaves to the root significantly (p < 0.05) adversely affects the taste, colour, texture and the overall acceptability of the flakes. Despite this, incorporation of sweet potato leaves up to the level of 30% of the composite results in flakes that are nutritious and acceptable.
Department of Food Science, Nutrition and Technology, University of Nairobi is highly acknowledged for their invaluable financial and technical support.
Nicanor Obiero Odongo,George Ooko Abong’,Michael Wandayi Okoth,Edward G. Karuri, (2015) Development of High Protein and Vitamin A Flakes from Sweet Potato Roots and Leaves. Open Access Library Journal,02,1-10. doi: 10.4236/oalib.1101573