Sweet potato (Ipomea batatas) is an extremely important crop in many
parts of the world and survey reports placed Nigeria as the number one producer
of sweet potato in Africa with annual production output of 3.56 million metric
tons, making it the second world producer with China as the number one FAO
In comparison with other tubers, sweet potato contains an average amount of
proteins and carbohydrates mainly starch. They also contain some free sugars
which gives the tuber its sweet taste. Vitamins A and B are also present in
significant amounts and the tubers are rich in Vitamin C (Rose
and Vasanthakaalam, 2011). Sweet potatoes have white, yellow or orange flesh
and their skin may either be white, yellow, orange, red or purple. White colour
is due to the presence of lycopene while yellow contain higher amounts of β-carotene
than white types and the roots of red sweet potatoes contain anthocyanin pigment
(Salunkhe and Kadam, 1998). Orange fleshed sweet potatoes
are high in carotenoids and β-carotene (Takahata et
al., 1993; Simon, 1997). Infect, consumption
of orange fleshed sweet potato roots can provide sustainable vitamin A which
plays a role in preventing night blindness.
Starch has a wide application possibility in many industries such as food and
sugar industry. Many factors affect the starch content of sweet potato such
as variety difference, long storage, etc. (Li and Liao,
The nutritional value of sweet potato (especially high levels of Vitamin A)
offers an added benefit to processed products. Its processing, marketing and
nutritional, could significantly contribute to alleviation of Vitamin A deficiency
in parts of Africa where sweet potato is grown (Low et
al., 1997). Sweet potato has an abundance of uses ranging from consumption
of fresh roots or leaves, to processing into flour, starch, animal feed, candy
and alcohol (Woolfe, 1992).
However, the bulkiness and perishability of harvested sweet potato storage
is a major barrier to the wider utilization of the crop. A promising avenue
for expanding demand is to diversify sweet potato uses. In addition, the nutritional
content and microbial loads of sweet potato varies widely but depends on gene,
variety, location, etc. (Woolfe, 1992). Five new sweet
potato varieties planted at the field trial of National Root Crops Research
Institute, Umudike, Umuahia, Abia State, Nigeria were harvested and analyzed
with an aim to study the varietal effect on the physico-chemical, carotenoid
and microbial loads of the fresh and flours samples of the stored sweet potato
MATERIALS AND METHODS
Five new sweet potato varieties TIS 2532.OP.1.13 (White), TIS 8164 (Cream), TIS 87/0087 (White), CIP 1999024.2 (Light orange) and CIP 440293 (Pale orange) were harvested at maturity from field trial of National Root Crops Research Institute (NRCRI), Umudike, Umuahia, Abia State, Nigeria in November, 2009.
Production of fufu flour: Sweet potato fufu flours were produced from
each of the sweet potato varieties using the processing methods described by
Etudaiye et al. (2009).
Microbial analysis: The bacterial and fungal counts of the samples were
determined using the pour plate techniques (Ezeama, 2007).
Proximate composition: The method of AOAC (1990)
was used to determine the Moisture, ash, crude fiber and starch contents of
the flours while the method of Rodrigueze-Amaya and Kimura
(2004) was used to determine the total carotenoid contents of both the fresh
and flour samples of the sweet potato varieties.
Sensory evaluation: The sensory evaluation of the sweet potato fufu
flour samples was carried out using the method of Dewi et
Statistical analysis: Data was analyzed using the Mean±Standard Deviations of triplicate experiments and results were considered significant at p<0.05. Data was further subjected to Anova using SPSS 15.0 windows version.
RESULTS AND DISCUSSION
The microbial loads of the flours of the sweet potato varieties stored at ambient temperature (28±2°C) for six months as shown in Table 1 and 2 revealed that the microbial load increased with increase in storage time. The bacterial counts ranged from 1.5x101 to 7.0x105 cfu g-1 while the fungi counts ranged from 1.5x101 to 6.5x105 cfu g-1. However, TIS 8164 was more susceptible to bacterial growth than any of the varieties studied at the 6th month of storage while CIP1999024.2 and TIS 87/0087 were the least susceptible to bacterial growth.
In terms of fungal growth, TIS 8164 was more susceptible than any of the varieties studied at the 6th month of storage while CIP 1999024.2 was the least susceptible to fungal growth than any of the varieties evaluated. The gradual microbial increase during storage could be attributed to contamination during packaging and storage condition of the products.
The values obtained for the bacterial and fungal counts are within the limits
of 106 cfu g-1, being the standard set by the international
commission on microbiological specification of food (ICMSF,
1978) as aerobic counts for foods.
|| The bacteria average plate count (cfu g-1) of
stored fufu flour at ambient temperature (28±2°C)
|Values are means of triplicate experiments and having different
letters are significant at p<0.05
|| The fungi average plate count (cfu g-1) of stored
fufu flour at ambient temperature (28±2°C)
|Values in each row with the same superscript are not significantly
different from each other and are significant at p<0.05
Some micro-organisms produce chemicals that can color, flavor and stabilize
foods thereby increasing their storage lives (Eleazu et
al., 2011). Such types of foods are important because of their improved
aroma and flavor characteristics. Some microflora isolated during the period
of storage include Rhizopus stolonifer and Aspergillus sp. while
the bacteria that were identified include Lactobacillus planterum Streptococcus
sp., Bacillus cereus and Staphylococcus aureus.
The presence of Lactobacillus sp. in this study is thought to have prolonged
the shelf life of the resulting fufu flour. Lactic acid bacteria are a group
of gram positive bacteria that are of economic importance because they are applied
extensively in both the production and p reservation of a wide variety of food
products (Bron et al., 2004). Theyre thought
to produce bacteriocin which helps to prolong the shelf lives of stored flours.
This lends credence to earlier reports by Gobbetti et
al. (2005) and De Vuyst and Vancanneyt (2007).
The isolation of Lactobacillus sp. from the fufu flour from the first
to the sixth month may be attributed to the fact that the fermenting microbes
continued their activities throughout the storage period. The result reveals
that fufu flour stored at ambient temperature (28±2°C) in sealed
polyethylene is stable.
The total carotenoid contents of the 5 varieties of sweet potato flours as
presented in Fig. 1 showed that they ranged from 0.54x103
to 6.23x103 μg/100 g for the fresh samples while the processed
flours ranged from 0.02x103μg/100 g to 3.99x103 μg/100
g. TIS87/0087 showed the least carotenoid contents for both fresh and flour
samples (0.54x103 μg/100 g fresh and 0.02x103 μg/100
g flour) while CIP440293 had the highest carotenoid content for both fresh and
flour samples (6.23x103 μg/100 g fresh and 3.99x103
μg/100 g flour). The higher carotenoid contents of both the fresh and flours
of 2 varieties of orange fleshed sweet potato (CIP440293 and CIP1999024.2) would
be expected since orange fleshed sweet potato are known to contain higher carotenoid
contents (Fresco and Boudion, 2002).
||Total carotenoids of fresh and flour samples of 5 sweet potato
The reduction in the carotenoid contents of the processed flours is very significant
in this study as it was slightly higher than the results given by Hagenimana
and Low (2000) who reported that processing of sweet potatoes results in
25-30% losses in β-carotene content. The reduction in carotenoid content
of the processed flours to the extent that was observed in this study shows
that the processed flours cannot meet the recommended daily intake of β-carotene
(5000 to 25000 I.U/100 g) (Fresco and Boudion, 2002)
while the fresh samples can. Its important to note that β-carotene serves
a dual role: One as a precursor of Vitamin A where its cleaved by a di-oxygenase
using molecular oxygen to form retinal in the intestine (The retinal so formed
is hydrolysed to retinol which is absorbed by the intestinal mucosal cells and
hence, Vitamin A deficiency could lead to night blindness): Secondly as a chain
breaking antioxidant in scavenging free radical species in the body which are
generated from normal oxidation of food stuffs due to leaks in the electron
transport chain, normal metabolism of the body, cigarette smoking, etc. Antioxidants
also help to increase the shelf life of products in food industries. There is
therefore need to determine method of processing that will retain the total
carotenoid content of the sweet potato flour for proper utilization of its β-carotene
The proximate composition of the sweet potato samples as shown in Table 3 indicated that the moisture content of the flours ranged from 7.90-9.74% with TIS5325OP.1.13 having the lowest moisture content (7.9±0.01%) which is a good attribute for storage quality while TIS8164 had the highest moisture content (9.74±0.03%).
Ash content refers to the mineral content of flour. CIP440293 had the highest
quantity of minerals (0.52±0.02%) while CIP1999024.2 had the least mineral
content (0.22±0.08%). This agrees with the literature value of 0.35-0.40%
(Lakra and Sehgal, 2009).
Crude fibre represents that portion of food not used up by the body but mainly
made up of cellulose together with a little lignin and is known to increase
bulk stool. The values obtained for the fibre content in all the varieties of
sweet potato studied as represented in Table 3 were within
the range obtained by Lakra and Sehgal (2009). CIP440293
had the highest crude percentage fibre content (0.36±0.05%) while CIP1999024.2
had the least (0.21±0.02%).
Because of the limited supply of fossil fuels and other possible difficulties
like political, technical and secure problems, all nations in the world have
been trying to explore possible energy resources.
|| Proximate composition of 5 sweet potato varieties
|Values are Means±SD of triplicate experiments
|| Sensory evaluation on fufu samples made from stored sweet
|Scoring system: 1 = Very poor ; 2 = Poor; 3 = Fair; 4 = Average;
5 = Good; 6 = Very good; 7 = Excellent. Values with the same super script
in each column are not significantly different from each other at p<0.05.
NS: Not significant SD: Significant difference
Alcohol, one of the many found substitute energy resources is especially attractive
because starch or sugary biomass can easily produce it. Starch on its own also
has a wide application in both food and sugar industries. The high content of
starch obtained from both the fresh and processed flours of the 5varieties of
sweet potato evaluated as observed in Table 3 is another significant
finding in this study as it shows the wider utility of these sweet potato varieties
in alcohol production on the one hand and their application in both food and
sugar industries on the other hand. In addition, processing was found to increase
the starch content of the sweet potato varieties studied.
Descriptive analysis is one of the most useful tests for sensory profiling
and uses trained panelists to detect and rate the intensities of sensory attributes
in a product (Dewi et al., 2011; Grosso
et al., 2008). The determination of the culinary quality and other
attributes for processing sweet potato storage roots is very important as it
gives an estimate of the quantity of nutrients and anti-nutrients contained
in the storage roots and to identify preferences within a collection. The quality
of a flour and storage condition after milling is very important in the shelf
life of the flour (Akpe et al., 2010). The sensory
evaluation of the flours as presented in Table 4 showed that
there was no significant difference (p>0.05) in the means of the colors of
the flours of all sweet potato varieties evaluated in the first month of storage
when compared with the control (p<0.05). However, there was significant difference
in the means of the colors of CIP1999024.2, TIS87/0087, TIS8164 and CIP440293
when compared with the control (p<0.05) by the second month of storage.
In terms of mouldability of the flours, the means of CIP1999024.2 and TIS87/0087
were found to be significantly different when compared with the control in the
first month of storage (p<0.05).
||Flow chart for processing of sweet potato roots into fufu
However, there was no significant difference in the mouldability of the flours
by the second month of storage when compared with the control indicating that
the flours of all sweet potato varieties studied could be easily moulded by
the second month of storage.
The flours of all sweet potato varieties evaluated were not found to be significantly different from the control in terms of acceptability in the first and second months of storage when compared with the control (p<0.05) and this is another significant finding as it indicates the general acceptability of all the flours evaluated.
The processing of sweet potato roots in fufu flour is shown in Fig. 2.
In conclusion, that variety affects the physico-chemical properties, microbial loads and carotenoid contents of both fresh and stored sweet potato flours has been demonstrated in this present study. The flours of the sweet potato varieties studied were found to have good microbial loads by the 6th month of storage indicating that their shelf lives could be extended if packaged well and stored. Both fresh and processed flours of all sweet potato varieties evaluated were found to be widely accepted and had high starch content indicating the possibility of their utilization in both alcoholic, food and sugar industries. Processing also increased the starch contents of the sweet potato varieties in addition.
In terms of carotenoid contents, all fresh samples of sweet potato varieties elucidated, were found to contain significant quantities of carotenoids indicating their antioxidant potentials. However, about 35% of this was lost during processing. Thus the development of a better processing procedure that can retain the carotenoid contents of these sweet potato varieties is recommended.
The authors are grateful to Mrs Omodamiro of Postharvest Technology Programme, National Root Crops Research Institute, Umudike, Umuahia, Abia State, Nigeria and Mrs Awa of Biochemistry Department, National Root Crops Research institute, Umudike, Umuahia, Abia State, Nigeria for their contributions to this study.