Physico - chemical properties of five cultivars of sweet potato (Ipomea batatas (L) Lam) roots grown in Sri Lanka

0041-3216/2013/020096-10 Trop. Agric. (Trinidad) Vol. 90 No. 2 April 2013 96 © 2013 Trop. Agric. (Trinidad)

Physico - chemical properties of five cultivars of sweet

potato (Ipomea batatas Lam) roots grown

in Sri Lanka

Suraji Senanayake1, Anil Gunaratne

2, K.K.D.S Ranaweera

1,

and Arthur Bamunuarachchi3

1Department of Food Science and Technology, University of Sri Jayewardenepura, Sri Lanka

2Faculty of Agricultural Sciences, Sabaragamuwa University of Sri Lanka, Belihuloya, Sri Lanka

3”ON – SITE” Consultancy, Training and Trade Systems, 128/22, Poorwarama Rd, Kirulapone,

Colombo 5, Sri Lanka

The flours and starches obtained from matured roots of five different cultivars of sweet potato (Ipomea batatas

.Lam) roots commonly grown in Sri Lanka (SWP1 – Wariyapola red, SWP3 – Wariyapola white, SWP4 –

Pallepola variety, SWP5 - Malaysian variety and SWP7 – CARI 273) showed significant differences in the

chemical composition and slight variation in functionality. Starch levels in studied cultivars on dry weight basis

were significantly different at P<0.05 and SWP7 contained the highest (64.1±0.1%) while SWP1 contained the

lowest (33.7±1.7%). The protein levels ranged from 1.2 ± 0.1 to 3.0 ± 0.1% and the total fat levels in tubers

varied ranging from 1.1 ± 0.1 to 1.7 ± 0.1%. The crude fibre contents in tubers were found to range from 2.1 ±

0.2 to 13.6 ± 0.3% and SWP7 contained a significantly higher level of fibre (P<0.05) than the other cultivars. The

ash levels ranged from 1.9 ±0.2 to 2.8 ±0.1. The sweet potato contained higher levels of calcium and iron and

high level of magnesium was observed in SWP5. Lowest swelling was observed in the SWP7 cultivar and there

was no significant difference in swelling of the starches obtained from the other cultivars. Scanning electron

microscopic studies showed different shapes such as oval, pentagonal and hexoganal in the native starches of

studied cultivars. Based on the thermal properties, SWP7 starch showed a high energy requirement and thus

gelatinized at a higher temperature. High peak viscosity, high breakdown and high final viscosity were observed

in the SWP4 cultivar and lower values for those parameters were observed in SWP7. The high level of enthalpy

and the low level of peak viscosity in SWP7 were associated with its low level of swelling. The DSC curves of the

melting properties of retrograded amylopectin were similar in all tested cultivars because melting temperature

reflects the quality and the perfection of the crystallites. Results revealed a high nutritional significance and

possible application of sweet potato flour and starch as jelling and/ or thickening agents in food industry.

Keywords: Chemical composition, Ipomea batatas, Swelling power, Mineral elements, Thermal properties

The main sources of starch supply of the

world today are corn, cassava, potato, wheat

and rice (Rosa M. Alves et al. 2002, 476-481).

Sweet potatoes (Ipomea batatas Lam) are an

important staple food of large sections of the

world population in the tropics where both the

tubers and tender shoots are eaten as a vital

source of nutrients. The bulk of production is

now concentrated in Asia, which accounts for

the major production of this crop in

developing countries.

The sweet potato is a tuberous – rooted

perennial plant belonging to the

Convolvulaceae or morning glory family.

This family includes about 45 genera and

1000 species, but only Ipomea batatas is of

economic importance as food (Onwueme

1978, 167-175). In Sri Lanka, fresh tubers of

sweet potatoes are commonly eaten boiled or

as a curry with other food ingredients and in

rural areas leaves are also consumed as a leafy

vegetable. Flour or starch may also be able to

be applied as an important ingredient in

various food systems as a thickener, binder

and a gelling agent depending on the physico

– chemical nature. Corn, potato and cassava

are the most common sources of starch for

such industries (Tester and Karkalas 2002,

381-438). The sweet potato plant consists of

herbaceous prostrate or climbing stems,

branches, petioles with a groove on the upper

surface, leaves spirally arranged on stems and

root (Hahn and Hozyo 1984, 551-567). It

produces one to several tuberous roots at

maturity, generally 4 to 7 (Chandra et al.

1985, 153-156).

Sweet potato has a remarkable ability to

convert solar and soil energies into

pc

Inserted Text

(L)

pc

Inserted Text

(L)

Physico-chemical properties of sweet potato; Senanayake et al.

97 Trop. Agric. (Trinidad) Vol. 90 No. 2 April 2013

carbohydrates giving impressive yields under

marginal and stress situations and it requires a

minimum amount of agricultural inputs and

little attention. Adaptability to extreme

environmental conditions, flexibility in crop

management and non-seasonality make sweet

potato a viable crop in many agro ecological

zones excluding high elevations. The high

nutritive quality, the richness of starch and the

physico –chemical properties in sweet potato

flour and starch can be used to increase its

utilization as a food crop and in food

industrial applications in Sri Lanka. The

objectives of this study were to analyze the

physicochemical characterization of the flour

and the starch of locally cultivated sweet

potatoes in order to popularize its utilization

and to evaluate their potential use in various

food industrial applications.

Materials and methods

Raw material

Mature tubers of sweet potatoes namely,

SWP1 (Wariyapola red), SWP 3 (Wariyapola

white), SWP 4 (Pallepola variety), SWP 5

(Malaysian variety) and SWP 7 (CARI 273)

were randomly collected from Dhambulla,

Horana and Gokarella areas in Sri Lanka and

prepared for analysis two to three days after

harvesting.

Separation of different fractions

Flour extraction

The tubers were washed, hand peeled and

trimmed to remove defective parts. Then the

tubers were grated into thin chips (~ 5 mm)

and dried in an air convection oven at 40 °C

for 30 hours up to 14% moisture. The dried

chips were powdered using a laboratory scale

grinder and sifted through a 300 µm sieve.

The flour samples were sealed and packed in

air tight containers for further analysis.

Starch separation

Starch separation was carried out according to

the method described by Takeda et al., 1988

with slight modifications. Fresh tubers were

washed, peeled and diced. Dipped in ice water

containing 100 ppm sodium metabisulphite to

minimize browning wet milled at low speed in

a laboratory scale blender with 1:2 w/v of tap

water for 2 minutes and filtered through a

gauze cloth. Residue was repeatedly wet

milled and filtered thrice and the suspension

was kept overnight to settle starch. The

supernatant was decanted and the settled

residue was further purified with repeated

suspension in tap water (1:2 v/v) followed by

the settling for 3 hours. The purified starch

was dried at 35 °C, sifted through 300 µm

sieve, sealed and packed for analysis.

Physico – chemical characterization

Chemical composition analysis of flour

Moisture, lipid, protein (N x 6.25), ash and

fibre were determined according to AOAC

(1980) methods. Starch content was estimated

by the complete acid hydrolysis method (Kent

Johns and A. J Amore, 1960). Flour sample of

2.5 g was suspended in a mixture of 200 ml of

water and 20 ml of HCl acid. (Sp. Gravity

1.125) The mixture was heated in a flask

provided with a reflux condenser for 2.5

hours. Contents were cooled, and neutralized

with NaOH (5 N). Volume was made to 250

ml and the sugar formed was determined as

dextrose by Lane and Eynon reducing sugar

estimation method. The dextrose multiplied

by 0.9 was taken as starch.

Mineral elements (Ca, Mg, Fe, K and Zn)

were determined by dry the ashing method,

(AOAC 1980). The ash was dissolved in conc.

HCl, filtered and diluted to 50 ml with

distilled water. Prepared solutions were

analysed with standards for elemental analysis

by Atomic Absorption Spectrophotometer

(GBC Avanta Ver 1.33).


Trop. Agric. (Trinidad) Vol. 90 No. 2 April 2013 98

Swelling power (SP)

Swelling power (SP) of the starch was

determined according to the method of

Gunaratne et al. (2010). Flour (100 mg, db)

was weighed directly into a screw – cap test

tube, and 10 ml distilled water was added.

The capped tube were placed on a vortex

mixer for 10 seconds and incubated at 85 °C

water bath for 30 min with frequent mixing.

The tubes were cooled to room temperature in

an iced water bath and centrifuged at 2000 X

g for 30 min and the supernatant removed and

remaining sediment in the tube weighed (Ws).

The supernatant was dried to constant weight

(W1) in a drying oven at 100 °C. The water

swelling power was calculated as follows:

SP = Ws/ [0.1 X (100% - WSI)] (g/g)

Where WSI = W1/ 0.1 X 100%

Morphology of starch

Morphology of the native starch granules was

evaluated by the scanning electron

microscope (HITACHI SU 6600, Japan) by

the procedures suggested by Lares, Perez and

Gonzalez (1997). Starch samples were

completely dried overnight (50 °C) and then

suspended in ethanol to obtain 1% suspension.

One drop of the starch – ethanol suspension

was applied on an aluminium stub and stub

was placed in dryer for 2 hrs. After drying,

stubs were coated with gold – palladium

(60:40), placed in microscope with an

accelerating potential of 15 kV.

Differential scanning calorimetry

Thermograms were obtained using a TA 2920

Modulated DSC Thermal Analyzer

differential scanning calorimeter (DSC)

equipped with a thermal analysis data station

(TA Instruments, Newcastle, DE). Starch (3

mg) was weighed onto the aluminium DSC

pan and distilled water (9 µl) was added with

a micro syringe. Pans were sealed and

allowed to stand for 1 hour at room

temperature. The scanning temperature range

and heating rate were 30 – 140 °C and 10

°C/min, respectively, using an empty pan as

reference.

Retrogradation

After gelatinization in the DSC, pans were

stored at 4°C for 48 hours to initiate

nucleation. Samples were kept at 40°C for 5

days before rescanning in the DSC with the

same heating rate and the temperature range.

Pasting properties

Pasting properties of starches were

determined in duplicate replications using a

Rapid Visco – Analyzer (RVA) model 3D

(Newport Scientific, Warriiewood, Australia).

Flour (3.5 g, 14% moisture basis) was mixed

with distilled water (25 g) in the canister and

loaded using STD2 heating and cooling

profile.

Statistical analysis

MINITAB software package (version 14 for

Windows) was used for data analysis.

Analysis of variance (ANOVA) with Tukey’s

HSD test (p< 0.05) was performed for

samples in triplicate.

Results and discussion

Chemical composition

The results indicate that the SWP7 is

generally richer in starch than the other

studied sweet potato cultivars. (Table1). From

the five cultivars, SWP5 contained the lowest

level of starch on a dry weight basis. Moisture

contents of the fresh tubers were in the range

of 65.0±3.4 -74.6%. Protein contents in

studied cultivars were significantly different

at P<0.05 and SWP7 contained a significantly

higher level of protein than the other sweet

potatoes cultivars. There was no significant

difference (P<0.05) in the crude fat contents

of sweet potatoes (Table 1) but fat was

slightly lower in SWP3 cultivar.

Crude fibre levels were comparatively

high in most of the studied varieties of sweet



potato cultivars. Literature reveals that the

crude fibre content of sweet potato tuber has

been reported to range from 2.5 to 5% dry

basis and crude fibre in roots had been

regarded as a defect factor of sweet potato

quality (Jones et al. 1980, 797-802). Crude

fibre level of SWP7 was significantly higher

(P<0.05) than the rest of the cultivars and

SWP5 contained a comparatively lower level.

Calcium level ranged between 2.1±0.1 to

5.9±0.1 mg/100 g db and swp5 contained a

significantly high level (P<0.05) than the

other 3 cultivars. Significantly, higher levels

of iron (P<0.05) were observed in swp3 and

swp7 than the other two varieties. A higher

level of Magnesium was in swp5 compared to

other varieties. Considerable levels of

Potassium and zinc were observed in swp1

and the lowest level of potassium was found

in swp5 (Table 2).

Swelling power

The swelling power of the tested starch

samples varied from 5.9 g (SWP7) to 8.8 g

(SWP3) per 1 g of dry starch (Table 3).

Swelling power of SWP7 was comparatively

lower (p< 0.05%) than other sweet potato

cultivars. Swelling power of starch depends

on the starch granular size and the amylose

content in the starch; and the degree of

swelling and solubility rely on the extent of

chemical cross bonding within the granules

(Schoch 1964, 106-109). There was no great

variation in swelling of the studied cultivars

but more variations have been reported in

swelling and solubility of sweet potato starch

of different genotypes (Rasper et al. 1969,

642-646).

Table 1: Chemical composition of flours obtained from sweet potato cultivars

Cultivar Moisture Crude protein* Total fat Crude fibre Total starch

Content (%) (g/100 g db) (g/100 g db) (g/100 g db) (g/100 g db)

SWP1 70.1±0.4b 1.2±0.1e 1.7±0.1a 8.5±0.4b 33.7±1.7e

SWP3 65.8±2.5c 3.0±0.1b 1.1±0.1b 7.3±0.2c 58.6±0.5b

SWP4 74.6±1.9a 2.3±0.1c 1.3±0.2a,b 6.5±0.4d 49.0±0.3c

SWP5 71.0±0.5b 1.7±0.2d 1.5±0.3a,b 2.1±0.2e 43.0±0.6d

SWP7 65.0±3.4c 3.3±0.1a 1.7±0.2a 13.6±0.3a 64.1±1.9a

N* X 6.25, Data represent the mean of three replicates. Values followed by the different superscript in each column are

significantly different at (P<0.05) Table 2: Composition of mineral elements in sweet potato roots (mg/ 100 g dry weight)

Cultivar Calcium Iron Magnesium Potassium Zinc

Swp3 2.1±0.1b 6.2±0.9a 11.0±0.8b 8 05.6±5.7a 2.5±0.3a

Swp4 3.4±0.8b 4.5±0.9b 12.1±0.1b 7 08.5±2.7b 1.9±0.1b

Swp5 5.9±0.1a 4.2±0.1b 15.3±0.1a 526.8±2.3c 1.6±0.1b

Swp7 2.2±0.1b 6.3±0.2a 11.0±0.3b 691.5±5.9b 2.6±0.1a

Data represent the mean of three replicates. Values followed by the different superscript in each column are

significantly different at (P<0.05)



Morphological characteristics

The scanning electron micrograph (SEM) of

native sweet potato starches of different

cultivars is shown in the figure 1. The starch

granules consisted of a mixed population of

large, medium and small sizes and various

shapes such as oval, pentagonal and

hexagonal were observed. The average

diameters ranged between 16 and 24 µm in

five types of cultivars

.

SWP1 SWP3 SWP4

SWP 5 SWP 7

Figure 1: Scanning electron micrograph (SEM) of native sweet potato starch of different

cultivars

Table 3. Gelatinization parametersa and swelling power of sweet potato roots

Cultivar To ( °C) Tp ( °C) Tc( °C) ∆H (J/g) Swelling power

at 85 °C (g/g)

SWP1 78.3±1.2 82.0±0.7 94.0±1.1 15.7±0.4 8.0±0.9

SWP3 77.6±0.4 81.9±0.5 94.5±1.4 16.4±0.7 8.8±0.9

SWP4 77.0±0.3 81.8±0.3 94.3±0.9 14.3±0.3 8.7±0.4

SWP5 77.3±0.6 80.5±0.2 92.8±1.5 15.5±0.6 8.1±0.7

SWP7 78.6±0.4 83.6±0.2 95.7±1.8 20.1±0.5 5.9±0.1

aTo= onset temperature, Tp=peak temperature, Tc=conclusion temperature, ∆H=gelatinization enthalpy, values are mean

of triplicate determination ± SD

Gelatinization and retrogradation

properties

The To and Tp values of starches of the sweet

potato samples were in the range of 77.0 °C to

78.6 °C and 80.5 °C to 83.6 °C respectively (

Table 3). The lowest To ( 77.0 °C) and Tp (80.5

°C) values were observed in the starches

obtained from SWP4 and SWP5 respectively.

Highest values for To ,Tp and enthalpy

was observed in SWP7 cultivar.

Gelatinization is a swelling driven process

and swelling power of SWP7 starch was

comparatively lower compare to other

cultivars (Table 3). High gelatinization



enthalpy, To andTp is related to the low

swelling capacity of the SWP7 starch (Fig.1).

Literature reveals the requirement of higher

energy for gelatinization for sweet potato than

starches like cassava and Zhang and Oates

(1999) observed that the Tp and To of

different sweet potatoes were 83 - 78 °C and

81 – 75 °C respectively. The enthalpy for

starch gelatinization of studied cultivars was

in the 15.5 – 20.1 J/g.

Figure 2: Gelatinization DSC curves of sweet potato starch (starch water ratio 1:3): curves from

top to bottom are SWP1, SWP3, SWP4, SWP5 and SWP7.

Table 4: Melting properties of retrograded amylopectin of sweet potato starches.

Cultivar To ( °C) Tp ( °C) Tc( °C) Tc - To ( °C) ∆H (J/g)

SWP1 66.5 72.1 80.2 13.7 3.9

SWP3 66.4 72.1 80.1 13.7 5.3

SWP4 66.4 72.2 80.3 13.9 5.4

SWP5 65.9 71.8 79.9 14.0 3.5

SWP7 66.2 71.9 80.0 13.8 5.8

To= onset temperature, Tp=peak temperature, Tc=conclusion temperature,

∆H= melting enthalpy of retrograded Amylopectin

Temperature (°C)

Hea

t F

low

(g/W

)



Figure 3: DSC curves obtained from amylopectin retrogradation of sweet potato starches: curves

from top to bottom are SWP1, SWP3, SWP4, SWP5 and SWP7

Table 5: RVA data of different varieties of sweet potato grown in Sri Lanka

Cultivar PV HPV BD CPV SB

Swp1 222± 4.5 131±5.9 91±5.3 180±6.1 49±2.4

Swp3 225±2.1 145±6.1 79±3.6 208±2.8 62±1.8

Swp4 257±4.2 162±3.2 95±4.2 251±3.2 89±5.2

Swp5 248±3.2 129±4.2 118±2.9 178±2.6 48±4.1

Swp7 214±4.1 141±3.1 73±6.3 212±3.2 71±3.2

PV= peak viscosity, HPV=hot paste viscosity, BD=break down, CPV=cold paste viscosity, SB= setback

The melting properties of retrograded

amylopectin showed that quality and the

perfection of the crystallites are similar in all

tested cultivars because melting temperature

reflects the quality and the perfection of the

crystallites. Differences in ΔH suggest that

different cultivars are retrograded to different

extent. ΔH primarily reflects the loss of

double helical order and also is a measure of

the overall crystallinity of the amylopectin.

Takeda et al. (1986, 132-135) reported that

sweet potato amylose retrogrades at the same

rate as the cassava but more slowly than that

of potato amylose. Del Rosario and

Pontiveros (1983, 86-92) found that sweet

potato starch retrogrades more slowly than

cassava, wheat and corn starch.

-3.2

-3.0

-2.8

-2.6

-2.4

He

at

Flo

w (

W/g

)

40 50 60 70 80 90 100

Temperature (°C)Exo Up Universal V2.5H TA Instruments

Temperature (°C)

Hea

t F

low

(g

/W)



Pasting properties by RVA

The pasting properties, peak viscosity,

breakdown viscosity, setback and pasting

temperature are shown in the Table 5. Highest

peak viscosity was shown in SWP4 and the

lowest was observed in SWP7 cultivar (Figure

3). The increase in viscosity during the

heating cycle is influenced by the extent of

amylose leaching, granular swelling and the

extent of friction between swollen granules

(L. Jayakody et al. 2007; 148-163). The

higher peak viscosity exhibited by SWP4

could be attributed to its high level of

swelling compare to SWP7 (Table 3). Pasting

properties are dependent on the rigidity of

starch granules, which in turn affect the

granule swelling potential (Sandhya Rani and

Bhattacharya 1989; 127-137) and the amount

of amylose leaching out in the solution

(Morris 1990; 2-6). The higher peak viscosity

exhibited by SWP4 could be attributed to its

high level of swelling compare to SWP7

(Table 3). The highly swollen granules in

SWP4 are more liable to disintegrate at high

temperature under shear, resulting more

granular breakdown. During the cooling

phase, regaining viscosity (cool paste

viscosity) is primarily due to realignment of

amylose chain into a certain order

(retrogradation).

Figure 4: RVA curves of sweet potato starches (S1= SWP1, S3= SWP3, S4= SWP4, S5= SWP5

and S7= Swp7)

0 0

80

160

240

320

20 20

40

60

80

100 100

0 0 5 10 15 20 25 25

Time (Mins)

Vis

cosi

ty (

RV

U) T

emp

eratu

re (°C)

S4

S5

S7

S3

S1



Conclusion

Flours obtained from sweet potato cultivars

contained a considerable level of starch as

the main constituent on dry matter basis.

Results revealed the presence of significant

quantities of fibre, protein, considerable

amount of crude fat and ash in the studied

cultivars. Significant levels of calcium and

iron in the studied varieties show the

nutritional importance of sweet potato as an

important starch source. Moreover, the

cultivars which show high swelling, low

gelatinization peak temperatures and high

peak viscosity can be used as thickeners,

binders and gelling agents in various food

industrial applications. Further studies on

apparent amylose content, gel textural

strength and flow behavioral nature of

starch suspensions are required to use these

starch sources as food and industrial

additives in Sri Lanka.

Acknowledgement

The authors thank the Horticultural Crop

Research Institute, Gannoruwa, Sri Lanka for

the support extended and the University of Sri

Jayewardenepura, Sri Lanka for providing the

financial assistance for this study.

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Documents

Physico - chemical properties of five cultivars of sweet potato (Ipomea batatas (L) Lam) roots grown in Sri Lanka