Studies on the Characteristic Properties of Fermented, Sun-Dried Orange-Fleshed Sweet Potato Flour

Studies on the Characteristic Properties of Fermented, Sun-Dried Orange-Fleshed Sweet Potato Flour

Nigerian Food Journal Official Journal of Nigerian Institute of Food Science and Technology www.nifst.org NIFOJ Vol. 32 No. 1, pages 45 –...

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Nigerian Food Journal



Official Journal of Nigerian Institute of Food Science and Technology



www.nifst.org

NIFOJ Vol. 32 No. 1, pages 45 – 53, 2014

Studies on the Characteristic Properties of Fermented, Sun-Dried OrangeFleshed Sweet Potato Flour Amajor, J.U.*, Oti, E., Ekeledo, N., Omodamiro R., Amajor, E.E., Aniedu, C.

ABSTRACT Microbiological, functional, chemical and sensory properties of flours of two (2) cultivars of orange-fleshed sweet potato (centinnial and CIP 440293) being multiplied for distribution in South East and South-South Nigeria were investigated using standard techniques. The flours were observed to have good functional properties and high dry matter contents which were respectively 88.09 ± 0.00% and 86.0 ± 0.00%. The respective values for fat content (0.85 ± 0.00% and 1.75 ± 0.00 %), ash content (2.04 ± 0.00% and 2.40 ± 0.42%) and crude fibre content (2.90 ± 0.28% and 2.67 ± 0.00%) were low but the respective levels of carotenoids values (3.74 ± 0.00 and 5.28 ± 0.00 µg/g) were high indicating a promising source of pro-vitamin A. The sensory evaluation had no statistical difference among the parameters. The flours had high nutritive values with good functional and sensory properties. The microbial loads were within the acceptable limit of < 105 recommended by ICMSF, making the flour samples microbiologically safe and with appropriate packaging, the shelf life could be extended under good storage conditions.

Keywords: Orange-fleshed sweet potato, flour, sun-drying, physicochemical properties, functional properties. Introduction Sweet potato (Ipomoea batatas (L.) Lam) belongs to the convolvulaceae family and is a root crop cultivated in many countries. It is grown extensively in the tropical and subtropical zones (Islam et al., 2002). Sweet potato has become an important global crop and is grown in diverse ecologies (Firon et al., 2009) with a long history as a life saver. It has been recognized as having an important role to play in improving household and national food security, health and livelihoods of poor families in subsaharan Africa (CIP, 2009). This is due to its wide range of agronomic and nutritional advantages capable of combating the food shortages, malnutrition that may arise as a result of increase in

National Root Crops Research lnstitute, Umudike, Umuahia, Abia State corresponding author: [email protected]

population and pressure on land utilization (Woolfe, 1992). Increased production and consumption of the oranged-fleshed sweet potato roots rich in β-carotene would improve the nutritional status of the Nigeria population. With recent introduction of orange-fleshed sweet potato OFSP varieties having high beta-carotene from high land areas of East Africa, there has been an upsurge of interest to include the roots and leaves as part of the food based strategy to compensate vitamin A deficiency in Nigeria. The clamour for the OFSP is due mainly to the health advantages over the white fleshed cultivars. The better health engendered by consumption of orange-fleshed sweet potato will ultimately translate to reduced expenditure on hospital bills, provision of sound body to engage in productive activities leading to more income generation in rural communities (Echendu et al.,

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2011). There is need to promote the consumption of OFSP involving households with young children in rural areas in Nigeria in regular consumption of OFSP which is rich in β-carotene, significantly alleviates the vitamin A deficiency in children (Low et al., 2007). It is also important to create awareness on marketing and consumption of OFSP in Nigeria where new ways of consuming OFSP such as confectionery products; its use in bread making will produce nutritional benefits. Prolonged storage is achieved by drying (usually sun-drying) slices. During the period of drying, microorganisms settle on the exposed surfaces of the slices and the high moisture of the sweet potato permits the growth of these microorganisms during sun-drying. Sweetpotato flours are refined products from sun-drying which are produced by a milling machine. Orangefleshed sweet potato flour produced by grinding dried sweet potato slices could either be used as whole flour or as a composite. Orange-fleshed sweet potato flour being a food product with high nutritional content can harbour a variety of bacteria including pathogenic and non-pathogenic forms (Berghofer et al., 2003). Micro-organisms can cause diseases and food spoilage whenever they exceed their threshold as a result of increase in certain parameters such as moisture (Delong, 2001). Fresh roots can not be stored beyond three months under ambient temperature (Tomlins et al., 2007). The processing of the roots into chips and flours could extend the shelf life of the sweet potato for more than three months. There is scanty information on the microbial, functional, chemical and sensory properties of fermented and sundried flours of orange-fleshed sweet potato which can be utilized as composite or whole flour. It is therefore the objective of this study to evaluate the microbiological, chemical, functional and sensory properties of the fermented, sun-dried orange fleshed sweet potato flour. Materials and Methods Materials The study was carried out in the post harvest technology programme of National Root Crops

Research Institute, Umudike, Umuahia, Abia State, Nigeria. Two orange-fleshed sweet potato cultivars: Centinnial and CIP 440293 were harvested at maturity from the field trial of the institute. Processing and storage of orange-fleshed sweet potato flour Fresh undamaged oranged-fleshed sweet potato roots were washed with tap water peeled and chipped into 2 mm-thickness slices with the use of a manual chipping machine. Fermentation for a period of 24 h was carried out (Achi and Akubor, 2000). After steeping, the chips of each variety were laid out on clean polythene sheets for sundrying. Each variety was sun-dried for 6 days with an average temperature of 32.9oC during the dry season. After sun-drying, the fermented chips were pulverized using a milling machine into flour with particle size of about 200 µm and the resulting flour was sieved with muslin cloth to obtain a fine flour. The flours were properly packaged in cellophane bags (1 kg) per bag, sealed and stored at ambient temperature (28 ± 2oC) for 12 weeks (3 months). From which samples were taken for microbial, chemical, functional and sensory evaluations at monthly intervals. Microbiological analysis Total viable counts of microorganisms in the orange-fleshed sweet potato flours were done by pour plate techniques (Morton, 2001). Five grams of the flours were dissolved in 45 ml sterilized water to obtain a dilution of 1:10, from which subsequent dilutions were made and appropriate aliquot used to determine the total viable counts (TVC) on Trypton soya agar plates incubated at 37oC for 48 h while potato dextrose agar plates used for the fungal counts were incubated at 25oC for 72 h. At the end of incubation period, colony forming units (cfu/g) were estimated using colony counter. Colonies were purified by sub-culturing on fresh Trypton soya Agar and gram stained for morphological examination and biochemical tests (Catalase, coagulase, oxidase, citrate, glucose, sucrose, mannitol, lactose, maltose, and inositol) were done for characterization and identification of the isolates,

Studies on the Characteristic Properties of Fermented, Sun-Dried Orange-Fleshed Sweet Potato Flour ... Amajor et al.

as described by Buchanan and Gibbons (1975). The colonies of the fungi emerging within 2-5 days of inoculation were identified under the light microscope (× 40) and recorded using the scheme of Barnett and Hunter (1992). Each experiment was performed in triplicates. Chemical analyses of orange-fleshed sweet potato flours The flour products were stored at ambient temperature for 12 weeks before samples were taken from the stored products for chemical, functional properties and sensory evaluation. The ash, moisture/dry matter, fat and crude fibre contents were determined by the AOAC (2000) methods. Total carotenoids content analysis The Harvest Plus procedure for β-carotene analysis was used to analyze the total carotenoids content of the fresh orange-fleshed sweet potato roots and the orange-fleshed sweet potato flour. Five grams of each fresh root sample was ground with the aid of hyflosupercel in 50 ml of cold acetone and filtered with suction through a Buchner funnel with filter paper; the filtrate was extracted with 40 ml of petroleum ether (P.E) using separating funnel. Saturated sodium chloride solution was used to prevent emulsion formation. The lower phase being water was discarded while the upper phase was collected into a 50 ml volumetric flask, making the solution pass through a small funnel containing anhydrous sodium sulfate to remove residual water. Then, the separating funnel was washed with P.E and the standard flask made up to 50 ml mark. The absorbance of the solution at 450 mµ was taken using spectrophotometer and the total carotenoids content was calculated as follows: Total carotenoid (µg/g) = ( A × V × 104)/( A1cm1% × W (g)) Where A = absorbance, V = total volume of extract (50 ml), W = Sample weight (g) A1cm 1% = absorption coefficient of β-carotene in P.E. (2590)

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Analysis of the functional properties of the flour samples Gelatinization Temperature (GT) The method of Onwuka (2005) was used for gelatinization temperature determination. A 10% flour suspension of each sweet potato cultivar was prepared in a test tube. The suspensions were heated in a boiling water bath with continuous stirring. The temperature at which gelatinization was observed was recorded as the gelatinization temperature (GT) after 30 sec. Oil Absorption Capacity (OAC) The oil absorption capacity of the flour samples were determined using the method of Beuchat (1977) as modified by Adepeju et al. (2011). One gram of each flour sample was mixed with 10 ml of oil (pure canola) for 60 sec. The mixture was allowed to stand for 10 min at ambient temperature, centrifuged at 4000 x g for 30 min and the oil that separated was carefully decanted. The tubes were allowed to drain at an angle of 45oC for 10 min and then weighed. Oil absorption capacity was expressed as percentage increase of the sample weight. Water Absorption Capacity (WAC) One gram of each flour sample was dispersed in 10 ml of distilled water in a graduated centrifuge tube. The samples were thoroughly mixed for 30 sec and allowed to stand at ambient temperature for 30 min. The volume of the supernatant was measured directly from the graduated centrifuge tube. The amount of the water absorbed was multiplied by the density of water (1 g/ml) and results were expressed as g/100 g (Onwuka, 2005). Bulk Density (BD) The method of Okezie and Bello (1988) was used to determine the bulk density. A graduated cylinder of (10 ml) previously tarred was gently filled with the sample. The bottom of the cylinder was gently tapped on a laboratory bench several times until there was no further diminution of the sample levels after filling to the 10 ml mark. The bulk

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density of the samples was calculated as weight of the sample per unit volume of sample (g/ml). Swelling Index (SI) Ten grams of each flour sample were weighed into a calibrated measuring cylinder. Then, 80 ml of water was added and the cylinder was gently tapped to remove air void after which the initial volume was recorded. The cylinder containing the sample in water was allowed to stand for 1 h undisturbed at room temperature (25oC). At the end of 1 h, the swelling capacity of the sample was expressed as an index, that is, the ratio of the final volume (Narayana and Narasinga, 1982). Measurement were made in duplicates and average values recorded. Sensory evaluation The sensory evaluation of the oranged-fleshed sweet potato flour samples were determined using the method of Larmond (1977). A 20-member panel comprising semi-trained panelists were used for sensory evaluation. The scores were based on the degree of preference of organoleptic qualities of colour, odour, texture, moudibility and overall Table 1: Total viable count (cfu/g) of flours produced from orange-fleshed sweet potato cultivars

Total viable counts (cfu/g)

Months of Week 0 storage

Week 4

Week 8

Week 12

Samples Centinnial

4.3 × 105 6.6 × 105 7.3 × 105 9.3 × 105

CIP 440293 3.3 × 105 4.0 × 105 6.3 × 105 7.6 × 105

Values are means of triplicate determinations

The total viable counts of the centennial and CIP 440293 flour samples after processing were respectively 4.3 × 105 and 3.3 × 105 (Table 1) and increased during storage ranging from 4.3 × 105 to 9.3 × 105 for centinnial cultivar and 3.3 × 105 to 7.6 × 105 for CIP 440293. The total fungal counts after processing were respectively 4.0 ×

acceptability using a 9-point hedonic scale, 9 for like extremely, 5 for neither like nor dislike and 1 for dislike extremely. The flour samples were reconstituted using 250 ml of boiled clean water (100oC) to 100 g of flour samples; which was stirred continuously until the consistency was thick and brought to doneness; the dough (fufu) was rolled, wrapped, labelled and put in a food warmer for the panelists. Statistical analysis Data was subjected to analysis of variable using the “Statistical Package for Social Sciences” (SPSS) version 15.0. Statistically significant differences (p < 0.05) among meals were separated using the Duncan multiple range text. Results and Discussion The results of Tables 1 and 2 show the total viable counts for bacteria and fungi observed in the fermented and sun-dried flours of orange-fleshed sweet potato cultivars, after processing (0-week) and during storage at ambient temperature (28 ± 2oC ) for 12 weeks. Table 2: Total fungal count (cfu/g) of flours produced from Orange fleshed sweet potato cultivars

Total fungal counts (cfu/g)

Months of storage

Week 0

Week 4 Week 8 Week 12

samples Centinnial

4.0 × 103 4.3 × 103 5.6 × 103 7.3 × 103

CIP 440293 2.3 × 103 3.0 × 103 6.3 × 103 8.0 × 103

Values are means of triplicate determinations

103 and 2.3 × 103 (Table 2) which also increased gradually to 7.3 × 103 for centinnial and increased to 8.0 × 103 for CIP 440293. The centinnial cultivar was more susceptible to bacterial growth while CIP 440293 was susceptible to fungal growth at week 12. The study revealed a gradual microbial increase during storage, which could be attributed

Studies on the Characteristic Properties of Fermented, Sun-Dried Orange-Fleshed Sweet Potato Flour ... Amajor et al.

to contamination due to packaging and storage environment. However, the moisture content for centennial and CIP 440293 flours were respectively 11.1% and 12.0% and was not high enough to produce any harmful effect. The values of the TVC and total fungal count (TFC) obtained were within the recommended range of 30 – 250 cfu/g (ICMSF, 1978). This makes the flour product safe and acceptable for formulation of foods. The flour products obtained from the sweet potato cultivars had total plate count and total fungal count within the limit recommended by ICSMF (< 105 for total

plate count and 103 for fungal count). ICMSF recommended limit of microbial contaminants for food of less than 105 cfu/g. Table 3 shows the dominant microorganisms isolated from the samples. The organisms were Staphylococcus aureus, Lactobacillus bervis, Bacillus cereus, Aspergillus niger, and Rhizopus stolonifer. The presence of Staphylococcus aureus in the samples could be due to contamination from handlers during processing. And its presence has been associated with fermented foods of plants origin (Adams and Moss, 1995).

Table 3: Morphological and biochemical characteristics of bacterial and fungal isolates Cultural and morphological Gram Mortility Catalase Coagulase Oxidase Citrate Glucose Sucrose mannitol Lactose Maltose lnositol characteristic reaction

Wet, shiny surface, regular smooth raised clonies 1-2um

+ve

_

+

+

_

_

A +

A

_

A

+

Staphylococcus aureus

Gray round, wavey edge flat and irregular rods 2-5um on TSA

+ve rods

_

+

_

+

+

A A

A

A

A

_

Bacillus cereus

White to cream round rough-edged colonies, slightly rocished with dull appearance

+ve

_

_

_

_

_

+ +

+

+

_

_

Lactobacillus bervis

FOR FUNGI Cultural ldentification Staining Black, irregular roughened and powdery



+ ve with hyphae



Gray-white, + ve with hyphae brown-Black patches. Hyphae rapidly cover the madia



KEY: += positive, - = negative, A = acid production.

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Identified Isolates



Aspergillus niger

Rhizopus stolonifer

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Bacillus cereus is a frequent inhabitant of the soil and leaf surfaces. Its presence in the flours might be due to sun-drying on the ground. The isolation of fungi like Aspergillus niger and Rhizopus stolonifer as well as Bacillus cereus and Staphylococcus aureus is also in agreement with the finding of Akinrele et al. (1970) who worked on a traditional African starch cake, Eko.

processors/handlers; it determines the shelf life of foods and brings about organoleptic changes in stored foods (Berghofere et al., 2003). The result of the chemical composition of the stored orange-fleshed sweet potato flours is presented in Table 4. Moisture is an important parameter when considering flour quality because it significantly affect the shelf life and growth of the microbes (ICMSF, 1998). The moisture level (11.1 – 12.0%) attained in this study is in agreement with Codex Alimentarius Standards (1995). This indicates that moisture is important when considering flour quality. The low moisture content of the flour is a good attribute for storage quality. This agrees with the report of Destrosier (2004), that low moisture content prolonged storage.

The low microbial counts recorded from the samples is likely to preclude them from causing health hazard and the high temperature commonly involved in preparation of confectionaries (cooking, frying) is sufficient to eliminate them. The total viable count is used as an indicator of bacterial population on a sample; the level of hygiene practised by food

Table 4: Chemical composition of stored orange-fleshed flours Sample

Moisture %

Fat %

Centinnial

11.1 ± 0.00

0.85 ± 0.00

CIP 440293

12.0 ± 0.00b

1.75 ± 0.07b

a

Ash % a

Crude fibre %

Carotenoids ( µg/g)

2.04 ± 0.00

2.90 ± 0.28

3.74 ± 0.00a

2.40 ± 0.42a

2.67 ± 0.00a

5.28 ± 0.00b

a

a

Values with different superscripts down each column are significantly different (p < 0.05) Values are means of duplicate determinations.

The fat contents were 0.85 and 1.75% for centinnial and CIP 440293, respectively. There was significant difference (p < 0.05) in the fat content between the cultivars’. Some microorganisms may have utilized some level of fat to thrive during steeping (fermenting) (Talaro and Talaro, 2002). Ash content refers to the mineral content of the flours. There was no significant difference in the ash content between the two cultivars. The ash contents were low and this may be attributed to leaching during steeping. Sun-drying at high temperature for 6 days may also have effect on the ash content since heat strongly affect some minerals such as phosphorus, and calcium (Ihekeronye et al., 1985). Crude fibre content of the flours were low with values 2.90% and 2.67% for centinnial and CIP 440293, respectively. There was no significant (p >

0.05) difference between the cultivars. The crude fibre is important in the diet, since it increases the faecal output, lowers faecal pH and the daily excretion (Cummings et al., 1996) and prevents gastrointestinal diseases in man. The total carotenoids content of the orangefleshed sweet potato flours as shown in Table 4 indicates some good level of carotenoid content retention during storage. The carotenoid content of the fresh orange-fleshed sweet potato were 13.68 µg/g for centinnial and 12.45 µg/g for CIP 440293, respectively. The carotenoid values after storage were 3.74 and 5.28 µg/g for centinnial and CIP 440293, respectively. The percentage retention of carotenoid during storage. This is about 27.3% for centinnial and 42.4% for CIP 440293. There was significant (p < 0.05) difference between the cultivars, CIP 440293 having a higher retention of

Studies on the Characteristic Properties of Fermented, Sun-Dried Orange-Fleshed Sweet Potato Flour ... Amajor et al.

carotenoid content during storage. The carotenoid values obtained in this work were better when compared with yellow roots cassava fufu flours that gave 1.20 – 2.00 µg/g (Omodamiro et al., 2012). The level of sun-drying may have contributed to the reduction of carotenoid content in agreement with the report of Bechoff et al., (2009) that carotenoid losses from sweet potato chips during sun-drying are usually low and that losses of carotenoid are more critical during storage. However, losses could have been sustained during processing as the carotenoids are sensitive to heat treatments (Rodriguez-Amaya and Kimura, 2004). Table 5 shows the functional properties of the flours. Functional properties help to determine the behaviour of nutrients in food during processing, storage and preparation, since they affect the overall quality of foods, as well as their acceptability. Gelation is one of the important factors that determine starch behaviour in various food and industrial applications. It affects the quality of

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starch-based food products such as stability, texture and digestibility (Tsakama et al., 2010). Gelatinization is the temperature at which starch molecules in a food substance lose their structure and leach out from the granules as swollen amylase. Gelatinization affects the time required for the cooking of food. The gelation temperature of the flour samples were 89.00C and 87.50C. With this gelation temperature the flour samples can be incorporated in food formulation that requires such gelation temperatures such as in bread making. The oil absorption capacity of the flours contained considerable levels of oil absorption capacity. The values were 1.45% and 1.90% for centinnial and CIP 440293 cultivars, respectively. The water absorption capacity values were 2.20% and 2.50%, respectively. The water absorption capacity helps to determine the ability of the flours to absorb water and swell for improved consistence in food. Flours are used as food ingredient because of their interaction with water (Nide et al., 2001). Bulk

Table 5: Functional properties of stored orange-fleshed sweet potato flours Sample Gelatinization Bulk Density (g/ml) Swelling Index (%) Temperature (0C)

Water Absorption Capacity (%)

Oil Absorption Capacity (%)

Centinnial

89.0

0.59

175

2.20

1.45

CIP 440293

87.5

0.52

136

2.50

1.90

Values are means of duplicate data.

diversity of the flours are 0.59 g/ml and 0.52 g/ ml for the cultivars, respectively. This indicates the flours heaviness and its suitability to be used for production of confectioneries. Increase in bulk density offers a greater advantage for packaging. Hence greater quantity may be packed within a constant volume (Fagbemi, 1999). The swelling indices were 1.75% and 1.36%, respectively. This is related to associative binding within starch granules, strength and character of the micelle network as related to the amylose content. High amylose content produces low swelling index (Adebowale, et al., 2005).

Table 6 shows the sensory evaluation of the orange-fleshed sweet potato flours. There were no significant (p > 0.05) differences in the colours, odour, mouldibility, texture and overall acceptability of the flours of the two cultivars. The result revealed that the flours had no much difference in all the parameters and were acceptable. The quality of flours and storage condition after milling is vital in shelf life and sensory properties of the flours (Akpe et al., 2010).

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Table 6 : Sensory evaluation of stored orange-fleshed sweet potato flours Sample

Colour

Odour

Moldibility

Texture

Overall acceptability

Centennial

3.5 ± 0.70a

4.5 ± 0.70a

4.5 ± 0.20a

5.0 ± 1.40a

4.5 ± 0.70a

CIP 440293

4.0 ± 1.40a

5.5 ± 1.40a

5.0 ± 0.00a

5.5 ± 0.70a

5.5 ± 0.70a

Means with different superscripts along each column are significantly different (p < 0.05). Values are means of the duplicate experiments.

Conclusion The microbial load, chemical composition, functional properties and sensory evaluation of processed and stored orange-fleshed sweet potato flours were investigated. The flours of the orange-fleshed sweet potato cultivars studied were microbiologically (bacteria and fungi) safe for human consumption, since the total plate counts were within the acceptable limit of <105 cfu recommended by international commission on microbiological specification for food (ICMSF). The study also indicated that the shelf life of the flours could be prolonged if packaged well before storage. The carotenoid content of the flours were drastically reduced by processing. However, Amajor et al., (2011) found that about 35% of total carotenoids were lost during processing and storage. A good processing procedure to retain more of the carotenoid content is required, to sustain the promising source of pro-vitamin A from the cultivars. Acknowledgements The authors wish to appreciate the sweet potato programme and the post harvest technology programme of National Root Crops Research Institute, Umudike, Umuahia, Abia State, Nigeria for their contributions to this study. References

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