Effects of various traditional processing methods on the all-trans-b-carotene content of orange-fleshed sweet potato

ARTICLE IN PRESS
JOURNAL OF
FOOD COMPOSITION
AND ANALYSIS
Journal of Food Composition and Analysis 21 (2008) 134–143
www.elsevier.com/locate/jfca
Original Article
Effects of various traditional processing methods on the
all-trans-b-carotene content of orange-?eshed sweet potato
A. Bengtssona,Ã, A. Namutebib, M. Larsson Almingera, U. Svanberga
aDepartment of Chemical and Biological Engineering, Food Science, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden
bDepartment of Food Science and Technology, Makerere University, P.O. Box 7062, Kampala, Uganda
Received 30 March 2007; received in revised form 20 September 2007; accepted 21 September 2007
Abstract
The effects of traditional preparation methods and drying procedures on the provitamin A carotenoid content of orange-?eshed sweet
potato (OFSP) roots was determined by a high-performance liquid chromatography (HPLC) method. All-trans-b-carotene was the major
provitamin A carotenoid and the mean content of seven improved OFSP cultivars ranged from 108 to 315 mg/g dry matter. The retention
of all-trans-b-carotene was 78% when OFSP were boiled in water for 20 min. When OFSP were steamed for 30 min the retention was
77%, whereas deep-frying OFSP roots for 10 min resulted in retention levels of 78%. Drying slices of OFSP roots at 57 1C in a forced-air
oven for 10 h reduced the all-trans-b-carotene content by 12%. Solar drying and open-air sun drying OFSP slices to a moisture content of
p10% resulted in all-trans-b-carotene losses of 9% and 16%, respectively. The cis-isomer 13-cis-b-carotene was found in noticeable
amounts in all processed samples, but not in any raw samples. The formation of 13-cis-b-carotene correlated with the original amount of
all-trans-b-carotene found in the raw OFSP root. The high content of all-trans-b-carotene in the investigated improved OFSP varieties
and the moderately low losses due to degradation and isomerization renders OFSP a suitable food source of provitamin A.
r 2007 Elsevier Inc. All rights reserved.
Keywords: Orange-?eshed sweet potato; Ipomoea batatas; b-Carotene; Food processing; Drying; Retention; Vitamin A de?ciency (VAD); Bio-forti?ed
staple foods
1. Introduction
use of bio-forti?ed staple foods (Welch, 2002). However,
an inadequate health infrastructure and limited ?nancial
The prevalence of vitamin A de?ciency (VAD) is high in
resources of many poor rural households calls for an
sub-Saharan Africa and particularly so in Uganda (Aguayo
alternative strategy to supplementation and forti?cation
and Baker, 2005). In 2001, a Ugandan Demographic and
(Hagenimana and Low, 2000). The use of bio-forti?ed
Health Survey (UDHS, 2001) recorded the prevalence of
staple foods (i.e. varieties bred for increased mineral and
VAD to 28% for 0–59-month-old children and to 52% for
vitamin content) has been justi?ed as a sustainable food-
15–50-year-old women. VAD is thus one of the major
based approach to reach a large section of the rural
micronutrient de?ciencies in the country. A number of
population who may not be reached by other intervention
different intervention strategies to address VAD are being
strategies (FAO/ILSI, 1997).
promoted in developing countries. These include forti?ca-
Orange-?eshed sweet potato (OFSP) is among the bio-
tion (Dary and Mora, 2002), supplementation (Beaton et
forti?ed staples bred for high provitamin A carotenoid
al., 1993), diet diversi?cation (Gibson and Hotz, 2001) and
content (CIP, 2006). Sweet potato (Ipomoea batatas) is a
major food crop in developing countries (Woolfe, 1992),
and it is mainly consumed as boiled roots. Sweet potato is
ÃCorresponding author. Tel.: +46 31 772 38 22; fax: +46 31 772 38 30.
also commonly processed into dried slices and ?our to
E-mail addresses: anton.bengtsson@chalmers.se (A. Bengtsson),
preserve the roots for household use during off-season.
asnamutebi@agric.mak.ac.ug (A. Namutebi),
OFSP may have the potential to prevent and combat VAD,
marie.alminger@chalmers.se (M.L. Alminger),
ulf.svanberg@chalmers.se (U. Svanberg).
as was indicated by a South African ef?cacy study of
0889-1575/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jfca.2007.09.006

---

ARTICLE IN PRESS
A. Bengtsson et al. / Journal of Food Composition and Analysis 21 (2008) 134–143
135
school-aged children that consumption of boiled and
2.2. Chemicals and standards
mashed OFSP improved their vitamin A status (van
Jaarsveld et al., 2005). In order for communities to bene?t
All chemicals were obtained from Sigma–Aldrich (Stock-
from the high provitamin A carotenoid content of OFSP,
holm, Sweden) or Fischer Scienti?c GTF (Go¨teborg,
preparation and processing methods should be optimized
Sweden). The water used for extraction and high-perfor-
since these steps in?uence the retention levels of provitamin
mance liquid chromatography (HPLC) analysis was
A carotenoids (Rodriguez-Amaya, 1997). Promotion of
generated by Millipore Milli-Q plus ultra-pure water
OFSP to farmers in Uganda has started and therefore
system (Millipore, Solna, Sweden). All-trans-b-carotene
knowledge about the retention levels of b-carotene in the
standard (synthetic, crystalline, Type II, product C-4582)
processed sweet potato products is of great concern.
was purchased from Sigma–Aldrich (Stockholm, Sweden).
Provitamin A carotenoids are degraded by heat treat-
ment and exposure to light during processing and during
2.3. Preparation of samples
prolonged storage, but there is a lack of information
regarding the retention of provitamin A carotenoids from
Four roots from each OFSP cultivar were randomly
traditional processing methods. Also, there is concern over
selected for each preparation method. Each root was
con?icting values for carotenoid retention due to the
washed, peeled and quartered longitudinally (from the stem
intricate nature of the carotenoid analysis (Rodriguez-
end to the root end). Two opposite quarters were combined
Amaya, 1997). The following study was undertaken to
and kept frozen as a reference sample, while the remaining
acquire knowledge about the retention of provitamin A
two quarters were prepared either by boiling, steaming or
carotenoids in boiled, steamed, deep-fried, and dried OFSP
deep-frying. The weight was recorded before and after
roots. In addition, the aim was to assess the natural
processing. Sweet potato samples were prepared as ready-
variability of b-carotene content in improved OFSP
to-eat products. Therefore, the processing times varied for
cultivars and to measure the formation of cis-isomers
the different methods. Sweet potatoes used for boiling were
during processing and drying. An additional objective was
immersed in tap water contained in an open aluminum pot
to evaluate color measurements of OFSP ?our as a rapid
and boiled for 20 min. Sweet potatoes used for steaming
screening method for the b-carotene content of fresh OFSP
were wrapped in banana leaves and placed on top of cut
roots.
banana leaf stem ends to serve as a separator from the
added tap water. The sweet potatoes were then steamed at
93 1C for 30 min in an aluminum pot with the lid on. Sweet
2. Materials and methods
potatoes used for deep-frying were immersed in locally
produced sun?ower oil with a temperature between 160
2.1. Plant material
and 170 1C and deep-fried for 10 min.
OFSP (I. batatas) root samples, which are improved
2.3.1. Drying
sweet potato cultivars under ?eld tests, were harvested 4.5
Sweet potato roots (variety Ejumula) used for drying
months after planting from Namulonge Agricultural and
experiments were sliced to 1–2 mm thickness using a food
Animal Production Research Institute (NAARI), Uganda.
processor (Braun Combi Max K600). A portion of
The variety Ejumula was harvested 4 months after planting
approximately 50 g, taken as a reference sample, was
from a farmer site in the vicinity of NAARI. Table 1 shows
placed in an amber polystyrene bottle, screw capped and
the OFSP cultivars (one commercially released variety
stored in a freezer (–20 1C) prior to subsequent carotenoid
Ejumula and six ?eld test varieties) used for the present
analysis. The remaining slices ($450 g for each method)
study.
were used for drying. The procedure was repeated for four
consecutive days. Sliced sweet potato for oven drying was
spread out on aluminum foil lined trays, which were placed
Table 1
in an HT 4 forced-air cabinet oven dryer (Innotech,
Orange-?eshed sweet potato cultivars procured from Namulonge Agri-
Altdorf, Germany) and dried at 57 1C for 10 h to a brittle
cultural and Animal Production Research Institute, Uganda
texture. Sliced sweet potato for solar drying was spread out
Sweet potato variety
on 0.97 m  0.77 m-sized plastic-meshed trays and placed in
a tunnel solar dryer. The base of the solar dryer was lined
Ejumula
with a black high-gauge polyethylene sheet. Drying
SPK004
temperatures varied between 45 and 63 1C in the solar
SPK004/6/6a
dryer. Sliced sweet potato for open-air sun drying was
SPK004/6a
SPK004/1/1a
spread out on a transparent low-gauge polyethylene sheet
SPK004/1a
placed on a papyrus mat. Sliced sweet potato was dried
Sowola 6/94/9b
under direct sunlight and occasionally turned to improve
a
the drying process. Drying temperatures varied between 30
Advanced yield trial.
bIntermediate yield trial.
and 52 1C, measured directly above the sweet potato slices.

---

ARTICLE IN PRESS
136
A. Bengtsson et al. / Journal of Food Composition and Analysis 21 (2008) 134–143
Solar and open-air sun-dried slices were dried to a moisture
was used in the comparison with the all-trans-b-carotene
content of p10%. The time of drying varied between 6 and
HPLC measurements of the fresh OFSP samples.
10 h depending on weather conditions.
2.5. Preparation of all-trans-b-carotene standard
2.3.2. Treatment after processing
Boiled, steamed and deep-fried samples were drained on
A stock solution of all-trans-b-carotene in n-hexane
absorbent paper, cooled for 15 min at room temperature,
(100 mg/mL) was prepared and stored at À20 1C. The purity
packaged in thick polyethylene bags, and frozen at –20 1C.
of the standard was veri?ed by HPLC and photodiode
All dried slices were packaged and sealed in similar
array detection. Concentration of the standard solution
polyethylene bags and immediately frozen at –20 1C. All
was calculated using the absorbance at 450 nm against n-
samples were transported to Sweden under frozen condi-
hexane as blank by spectrophotometric measurement in a
tions in cool boxes of expanded polystyrene. Both
Cary 50 Series UV–visible spectrophotometer (Varian
processed and reference samples were freeze-dried in a
Australia Pty Ltd., Mulgrave, Australia) and with the
Heto DW6-55 Dry winner (Ninolab, Upplands Va¨sby,
molar absorption coef?cient of 2592 for all-trans-b-
Sweden) before determination of carotenoid content. To
carotene. A standard curve was constructed with eight
ensure that the freeze-drying of the OFSP samples did not
different concentrations for all-trans-b-carotene in dupli-
affect the b-carotene content this pre-treatment step was
cate. The detector’s response showed good linearity and
evaluated prior to the study. No signi?cant difference was
reproducibility. One-point calibration on each day of
found in all-trans-b-carotene content of seven OFSP
analysis was performed, verifying any change in the
samples analyzed before and after freeze-drying. The mean
detector’s response.
difference was less than 2.7% between non-freeze-dried
and freeze-dried samples. The freeze-dried samples were
2.6. Carotenoid extraction
overlaid with nitrogen prior to storage at –20 1C until
carotenoid analysis in order to avoid degradation of b-
Finely ground ?our samples ($0.2 g) in duplicate were
carotene due to presence of oxygen. All analyses were
reconstituted with 1 mL of ultra pure water in a test tube
performed within 7 months on receipt of fresh sweet potato
for 20 min followed by addition of 2 mL acetone containing
roots. Prior to the carotenoid measurements all freeze-
0.1% (w/v) butylated hydroxytoluene. The test tube was
dried samples were homogenized into a ?nely ground ?our
vortexed and then centrifuged at 4750 g for 3 min. The
(particle size $0.2 mm) with a coffee grinder (Braun
resulting supernatant was saved in a new test tube. The
Cafe´Select KMM 30).
residue was re-extracted with 2 mL acetone and centrifuged
again. This was repeated up to four times until the residue
2.3.3. Dry matter determination
was colorless. To the resulting acetone extract 3 mL
Dry matter (DM) content was determined in the fresh
petroleum ether was added together with 5 mL ultra pure
samples by measuring sample weights before and after
water to aid in the separation of two phases. The organic
freeze-drying. Solids content of the boiling water was also
and water phases were separated by centrifugation at 4750g
measured by drying in an oven at 105 1C until constant
for 4 min and the organic phase was transferred to a new
weight.
tube. This step was repeated once. The pooled organic
phases were evaporated in a heating block at 35 1C under a
2.3.4. Extraction of fat
stream of nitrogen. The residue was dissolved in 5 mL
Determination of oil content in the deep-fried samples
(processed samples) or 10 mL (raw samples) mobile phase
was done according to the method of Lee et al. (1996) with
(methanol:methyl tert-butyl ether (1:1, v/v)). Precautionary
chloroform and methanol (1:1, v/v) as the extraction
measures such as exclusion of oxygen, protection from
solvent. The mean fat content of 12 samples was 5.5% with
light, avoiding temperatures above 35 1C, avoiding contact
a standard deviation of 0.9%.
with acids and use of high-purity solvents were taken to
prevent carotenoid losses during analysis (Schiedt and
2.4. Sweet potato ?our L*a*b* color value determination
Liaaen-Jensen, 1995).
Color values of the fresh samples were determined using
2.7. HPLC analysis of carotenoids
a Konica Minolta CR-400 chroma meter (Konica Minolta
Sensing Inc., Osaka, Japan). The equipment was calibrated
Carotenoids were analyzed by reversed phase HPLC
before each measurement using a calibration plate. Color
using a Waters 600 system equipped with auto sampler
measurements were performed on sweet potato ?our and
injector, degasser, pump and a Waters 996 UV–visible
the values were averaged from four consecutive readings.
photodiode array detector operating at 450 nm. The data
The color was described based on the values of L*, a* and
were stored and processed by means of Millennium 4.00
b*, where L* is a measure of lightness, a* de?nes
Software (Waters, Stockholm, Sweden). Absorption spec-
components on the red–green axis, and b* de?nes
tra were recorded between 250 and 500 nm. Separations
components on the yellow-blue axis. The a* parameter
were carried out on a C30 carotenoid column (5 mm,

---

ARTICLE IN PRESS
A. Bengtsson et al. / Journal of Food Composition and Analysis 21 (2008) 134–143
137
250 mm  4.6 mm i.d.) (YMC Europe GMBH, Scherm-
% Retention
beck, Germany). The mobile phase used for isocratic
carotenoid content per g of cooked food ðdry basisÞ
elution consisted of methanol:methyl tert-butyl ether:water
¼
 100.
carotenoid content per g of fresh food ðdry basisÞ
(55:41:4, v/v/v). The ?ow rate was 1 mL/min and the
injection volume 20 mL.
As each OFSP root was quartered, the retention value
calculated for each preparation method was obtained by
2.8. Carotenoid identi?cation and quanti?cation
comparing the processed value with its own reference
sample. Normally, the retention is overestimated when it is
Carotenoids were identi?ed using retention times and
calculated on a dry weight basis (Rodriguez-Amaya, 1997).
UV/vis absorption spectra and the absorption spectra were
However, in the present study changes in DM due to loss of
compared with literature data. Quanti?cation of all-trans-
soluble solids, loss or uptake of water, and uptake of fat in
b-carotene was achieved using a calibration curve with
the deep-fried samples were accounted for. As a result the
eight concentrations and with the correlation coef?cient of
calculated retention values can be regarded as close to true
0.995. Quanti?cation of cis-isomers was achieved using
retention.
relative response factors. Response factors used were 0.806
for 13-cis-b-carotene and 0.702 for 15-cis-b-carotene
2.11. Statistical analysis
(Mulokozi and Svanberg, 2003). The analysis of carote-
noids was carried out on duplicate samples from each root,
The statistical comparison of data was performed by
and the coef?cient of variation (CV) for the analysis was
ANOVA using SPSS software to reveal signi?cant differ-
below 3%. The concentration of each carotenoid was
ences among the cultivars and processing methods studied.
expressed as micrograms per milliliter, given as the mean of
The correlation coef?cients and their probability levels
duplicate extractions.
were obtained from linear regression analysis. Determina-
tion of signi?cance of differences among processing
2.9. Method validation
methods was obtained by Tukey’s HSD multiple rank test.
P values ofo0.05 were considered signi?cant.
Our analytical method was validated in an interlabora-
tory pro?ciency study that included analysis of a lyophi-
3. Results
lized sweet potato test material that was distributed to 15
laboratories. Both total b-carotene and all-trans-b-carotene
The DM content of the storage roots was high with
were analyzed in three replicate samples and our results
mean values ranging from 30.3% to 35.0% for the different
(with a CV of 3%) did not differ signi?cantly from the
varieties. Cultivars SPK004, SPK004/1, Sowola 6/94/9 and
values reported by the reference laboratory that used the
Ejumula had the highest DM content while cultivars
HPLC method described for sweet potato in HarvestPlus
SPK004/6 and SPK004/6/6 had the lowest (Table 2).
handbook for carotenoid analysis (Rodriguez-Amaya and
The mean all-trans-b-carotene content ranged from
Kimura, 2004).
108.1 to 314.5 mg/g DM. Cultivar SPK004/6 had the
highest content whereas the lowest content was found in
2.10. Calculation of carotenoid retention
SPK004 and SPK004/1 (Table 2). The natural variability in
the all-trans-b-carotene content was considerable among
Retention of all-trans-b-carotene was calculated based
the 12 root samples from each cultivar as can be seen in the
on the following equation:
large range in Table 2. All-trans-b-carotene comprised the
Table 2
The contents of dry matter, all-trans-b-carotene, a* color value, and vitamin A activity in seven different improved OFSP cultivars
Cultivara
Dry matter contentb
All-trans-b-carotenec,d
a* color valueb
Vitamin A activitye
(%)
(mg/g DM)
(mg RAE/100 g FW)
SPK004/1
34.672.6
108.1 a (44.3–192.7)
6.4172.4
311
SPK004/6
30.772.3
314.5 d (206.3–460.3)
13.7472.0
804
SPK004
35.072.4
116.3 a (48.2–191.7)
6.4172.1
339
Sowola 6/94/9
35.070.8
212.0 b, c (150.6–251.1)
12.3571.3
619
SPK004/1/1
33.571.8
143.6 a, b (85.6–219.3)
8.3971.8
400
SPK004/6/6
30.372.8
246.2 c (97.9–363.8)
12.1472.6
622
Ejumula
34.671.6
261.9 c, d (185.6–324.8)
12.3171.0
755
aDuplicate analysis of each sweet potato sample (n ¼ 12) for each cultivar except cultivar Sowola 6/94/9 (n ¼ 4).
bMean7standard deviation.
cMean (lowest and highest value shown in parentheses).
dValues in the same column not sharing the same letters (a–d) are signi?cantly different (Po0.05) using ANOVA and Tukey’s HSD multiple ranks test.
eConversion factor 1 mg RAE ¼ 12 mg all-trans-b-carotene.

---

ARTICLE IN PRESS
138
A. Bengtsson et al. / Journal of Food Composition and Analysis 21 (2008) 134–143
500
450
400
350
y = 25.4 x - 55.6
R = 0.96
300
?
g/g DM)
250
200
150
?
-carotene (
100
50
0
0
2
4
6
8
10
12
14
16
18
a? color value
Fig. 1. Correlation between all-trans-b-carotene content and a* color value of individual samples from seven OFSP varieties. Each point represents one
sample (n ¼ 76).
major provitamin A carotenoid found in the OFSP roots.
amount of all-trans-b-carotene in the fresh sweet potato
Neither a-carotene or b-cryptoxanthin were found in
root. The 15-cis-b-carotene was identi?ed in a few
detectable amounts in any of the samples.
processed samples but the amounts were small. The
The ?esh color of the varieties varied from yellow or
maximum amount of 15-cis-b-carotene was 4.0 mg/g DM
light orange to deep orange. The a* color value was
in steamed and deep-fried roots of cultivar SPK004/6. The
measured on sweet potato ?our and ranged from 6.41 for
fraction of 13-cis-b-carotene to total b-carotene varied
cultivars SPK004 and SPK004/1 to 13.74 for cultivar
from 5.7% to 8.8% in the boiled samples, 4.0–10.3% in the
SPK004/6 (Table 2). This is consistent with the all-trans-b-
steamed roots, and 3.0–8.5% in the deep-fried samples,
carotene content for each individual sample of seven
respectively.
cultivars measured by HPLC as shown in Fig. 1 by a
Drying slices of Ejumula variety signi?cantly (Po0.05)
signi?cant relationship (r ¼ 0.96, Po0.001).
reduced the concentration of all-trans-b-carotene (Table 4).
The retention of the all-trans-b-carotene in processed
The reduction of the all-trans-b-carotene content was 12%
samples of OFSP is affected by losses due to degradation,
in oven-dried samples and 9% in solar-dried samples.
trans-cis-isomerization and potential leakage. All three
Open-air sun drying reduced the all-trans-b-carotene
processing methods caused a signi?cant reduction in the
content with 16%, however not signi?cantly different from
all-trans-b-carotene content of the sweet potato roots
the two other methods. Only small amounts of 13-cis-b-
(Po0.001). However, there were no signi?cant differences
carotene were found in the dried samples (Table 4). The
between the effects of boiling, steaming and deep-frying.
moisture content was 10.8%, 6.8% and 10.0% for oven-
The retention of all-trans-b-carotene varied between 70%
dried, solar-dried and open-air sun-dried sweet potato
and 81% for the boiled samples, between 69% and 81% for
slices, respectively.
the steamed roots, and between 76% and 80% for the deep-
fried samples (Table 3). Of the boiled samples, the retention
4. Discussion
in SPK004/6/6 was signi?cantly higher than in SPK004
(Po0.05), and of the steamed samples the retention in
The high DM content observed for all cultivars was in
SPK004/6 was close to signi?cantly higher (Po0.07) than
accordance with the expected DM content above 30%. The
in SPK004/1 and SPK004. There was no correlation
values found in the present study were higher than
between percentage retention and all-trans-b-carotene con-
previously reported values for six orange-?eshed varieties
tent of the fresh roots of OFSP (data not shown).
(Hagenimana et al., 1999). Additional breeding targets to
The isomer 13-cis-b-carotene was found in all processed
high provitamin A carotenoid content are good resistance
samples. The amount of 13-cis-b-carotene ranged from 4.7
against sweet potato virus and high DM content as desired
to 18.1 mg/g DM in the boiled sweet potato samples, the
in sub-Saharan Africa. Although cultivars SPK004/6 and
steamed roots contained between 4.5 and 15.8 mg/g DM 13-
SPK004/6/6 had the lowest DM content (Table 2), on
cis-b-carotene, while the 13-cis-b-carotene content in the
average 30%, this still complies with the breeding target of
deep-fried samples ranged from 3.6 to 23.5 mg/g DM (Table
more than 30%.
3). Fig. 2 shows the correlation between all-trans-b-
Freeze-drying has been shown to cause no degradation
carotene in the fresh and 13-cis-b-carotene in the processed
or isomerization of b-carotene in carrots (Regier et al.,
samples (r ¼ 0.88, Po0.001). The amount of 13-cis-b-
2005; Sweeney and Marsh, 1971). This is as expected since
carotene formed during processing is clearly related to the
oxygen is excluded during the process (Desobry et al.,

---

ARTICLE IN PRESS
A. Bengtsson et al. / Journal of Food Composition and Analysis 21 (2008) 134–143
139
Table 3
The contents of all-trans-b-carotene and 13-cis-b-carotene, and the retention of all-trans-b-carotene in seven different OFSP cultivars after boiling for
20 min, steaming for 30 min, and deep-frying for 10 min
Cultivara/preparation method
All-trans-b-caroteneb (mg/g DM)
13-cis-b-carotene
Retention of
all-trans-b-carotene (%)d
(mg/g DM)c
(% of total b-carotene)
SPK004/1
Fresh
108.1 (44.3–192.7)
Boiled
73.0 (41.5–83.9)
4.770.5
6.1
78.1
Steamed
73.7 (27.7–119.7)
4.572.7
5.8
72.0
Deep-fried
103.1 (59.4–161.0)
9.072.9
8.0
80.4
SPK004/6
Fresh
314.5 (206.3–460.3)
Boiled
252.6 (223.0–275.4)
18.171.8
6.8
79.5
Steamed
249.9 (161.0–404.4)
15.876.5
5.9
84.4
Deep-fried
253.1 (194.9–320.3)
23.572.9
8.5
76.7
SPK004
Fresh
116.3 (48.2–191.7)
Boiled
90.2 (71.8–109.6)
8.771.8
8.8
70.1e
Steamed
59.3 (33.3–89.4)
6.872.1
10.3
68.6
Deep-fried
106.2 (60.1–158.7)
6.572.1
5.8
79.4
Sowola 6/94/9
Fresh
212.0 (150.6–251.1)
Boiled
160.6 (114.3–199.9)
12.172.5
7.0
75.8
SPK004/1/1
Fresh
143.6 (85.6–219.3)
Boiled
107.1 (72.7–129.1)
6.472.0
5.7
79.5
Steamed
116.3 (99.7–132.9)
6.671.3
5.4
79.8
Deep-fried
114.3 (70.8–191.8)
3.670.9
3.0
76.1
SPK004/6/6
Fresh
246.2 (97.9–363.8)
Boiled
201.3 (152.2–224.3)
15.972.6
7.3
81.2e
Steamed
215.7 (126.4–266.1)
15.776.9
6.8
77.4
Deep-fried
168.9 (80.8–258.2)
10.674.1
5.9
79.7
Ejumula
Fresh
261.9 (185.6–324.8)
Boiled
199.4 (159.2–229.4)
12.472.4
5.8
77.9
Steamed
213.5 (201.7–222.9)
8.971.5
4.0
80.5
Deep-fried
210.3 (185.6–244.7)
11.571.8
5.2
79.5
Mean valuef
Boiled
11.2
6.7
77.6
Steamed
9.7
5.9
77.0
Deep-fried
10.8
6.3
78.3
aDuplicate analysis of each sweet potato sample (n ¼ 4) for each cultivar and preparation method.
bMean (lowest and highest value shown in parentheses).
cMean7standard deviation.
dRetention values calculated for each preparation method were obtained by comparing the processed value with its own reference value.
eRetention values are signi?cantly different (Po0.05) using ANOVA and Tukey’s HSD multiple ranks test.
fMean values calculated based on 28 samples for boiling and 24 samples for steaming and deep-frying.
1998). In a subset of samples we found no degradation of
4.1. Variability in all-trans-b-carotene content of OFSP
all-trans-b-carotene due to freeze-drying. During storage at
–20 1C in the presence of oxygen degradation of b-carotene
The mean total all-trans-b-carotene content was found to
may occur. Therefore, the samples in the present study
vary between 108.1 and 314.5 mg/g DM in the cultivars
were stored under nitrogen. No losses of all-trans-b-
analyzed in the present study. These values correspond well
carotene were found in the freeze-dried OFSP samples in
with previously reported values using HPLC ranging from
the present study during storage at –20 1C. After repeated
less than 1 mg/g fresh weight (FW) in white-?eshed varieties
analysis of a set of samples after 3 months the difference
to 265 mg/g FW in orange-colored varieties (Huang et al.,
was less than 1.3%.
1999; K’osambo et al., 1998; Kidmose et al., 2006; Kimura

---

ARTICLE IN PRESS
140
A. Bengtsson et al. / Journal of Food Composition and Analysis 21 (2008) 134–143
500
450
y = 14.0 x + 50.5
400
R = 0.88
350
300
?

g/g DM)
250
200
150
?
-carotene (
100
50
0
0
5
10
15
20
25
30
13-cis-?-carotene ( ?g/g DM)
Fig. 2. Correlation between all-trans-b-carotene in fresh and 13-cis-b-carotene in processed OFSP samples (n ¼ 76).
Table 4
The contents of all-trans-b-carotene and 13-cis-b-carotene, and the retention of all-trans-b-carotene in Ejumula OFSP variety after three different drying
proceduresa
OFSP sample
Drying temperature
All-trans-b-caroteneb
13-cis-b-carotene
Retention of all-trans-
(1C)
(mg/g DM)
b-carotene (%)
(mg/g DM)b
(% of total b-
carotene)
Fresh
310.8716.3
Oven-dried
57
274.3722.8
3.570.3
1.3
88.2
Fresh
290.574.3
Solar-dried
45/63c
264.677.6
2.071.3
0.7
91.1
Fresh
301.7718.3
Open-air sun-dried
30/52c
252.7715.0
0.970.2
0.4
83.8
aDuplicate analysis of each sweet potato sample; each drying method was repeated four times.
bMean7standard deviation.
cLowest/highest temperature during drying.
et al., 2007; Takahata et al., 1993; van Jaarsveld et al.,
WHO, 1988). Thus, the vitamin A value calculated in RAE
2006). There were large differences in the all-trans-b-
of the OFSP cultivars in the present study ranged from 311
carotene content of sweet potato roots from the same
to 804 mg RAE/100 g FW (Table 2). With an average
cultivar. Similar ?ndings have been reported by van
retention of about 78% after cooking, these OFSP cultivars
Jaarsveld et al. (2006). Analyzing the OFSP variety
will provide about 243–627 mg RAE/100 g ready-to-eat
Resisto, they observed a variation in the b-carotene content
portion. Hence, a medium-sized sweet potato root in the
between 132 and 194 mg/g FW for medium-sized sweet
present study (mean weight 177 g) could more than well
potatoes from the same harvest batch. The fact that sweet
cover the safe daily recommended vitamin A intake level of
potatoes of not exactly the same size were analyzed in the
400 mg RE for pre-school children (FAO/WHO, 1988),
present study may explain some of the large variation seen
even after processing. Moreover, in a recent intervention
within the same cultivar.
study in South Africa (van Jaarsveld et al., 2005), the
vitamin A status in school children was signi?cantly
4.2. Provitamin A activity of OFSP
improved when the children were served a daily meal of
cooked OFSP that provided $830 RAE/100 g cooked root.
In 2001, the US Institute of Medicine (IOM, 2001)
derived new conversion factors for estimating the amount
4.3. Correlation between ?our color and all-trans-b-carotene
of retinol activity equivalents (RAE) obtained from
content of OFSP
provitamin A carotenoids in foods, stating that 12 mg of
all-trans-b-carotene is equivalent to 1 mg RAE. This value is
The ?our color of the sweet potato cultivars was
twice as high as the previously recommended factor (FAO/
measured with a chroma meter and the mean a* color

---

ARTICLE IN PRESS
A. Bengtsson et al. / Journal of Food Composition and Analysis 21 (2008) 134–143
141
values ranged from 6.41 to 13.74 based on 12 root samples
reported to contain the 13-cis isomer. This was explained
for each cultivar. These values were found to highly
by a 9 months long storage period employed for the sweet
correlate (r ¼ 0.96, Po0.001) with the amount all-trans-b-
potatoes prior to the initiation of the study. Kidmose et al.
carotene found in the fresh sweet potato roots. This
(2006) reported similar ?ndings with signi?cantly higher
supports earlier ?ndings that color measurements of sweet
content of cis-isomers when frying 3 min compared with
potato ?ours could be used as a screening tool to estimate
1 min, and besides formation of 13-cis-b-carotene, 15-cis-b-
the total b-carotene content in fresh OFSP roots. Takahata
carotene and 9-cis b-carotene were also detected. In the
et al. (1993) reported a correlation coef?cient of 0.90
present study 13-cis-b-carotene was formed during proces-
between color value a* and the b-carotene content in
sing in amounts ranging from 3.6 to 23.5 mg/g DM. The
orange-?eshed varieties, which is in accordance with the
fraction of 13-cis-b-carotene to the total b-carotene content
results in this study. In white-?eshed varieties the b* color
varied between 3.0% and 10.3%. Boiling generally resulted
value has been reported as the best measure of correlation
in the largest fraction of 13-cis-b-carotene to the total
between color variable and b-carotene content as shown by
b-carotene content. The amount of 13-cis-b-carotene was
a correlation coef?cient of 0.74 (Ameny and Wilson, 1997).
positively correlated to the amount of all-trans-b-carotene
Thus, it is less reliable to estimate the b-carotene content
found in the fresh roots (r ¼ 0.88, Po0.001). Furthermore,
from color measurements of pale cultivars.
15-cis-b-carotene was found in small quantities in a few of
the processed samples.
4.4. Effect of degradation and isomerization on the all-trans-
b-carotene content of OFSP
4.5. Retention of all-trans-b-carotene in processed OFSP
Factors such as variety, growing conditions, stage of
It is very important to analyze paired samples (i.e.
maturity, harvesting and post-harvest handling, processing
comparable raw and cooked samples) in retention studies
and storage of OFSP may have a large in?uence on the b-
to be able to preclude differences due to non-uniform
carotene content (Rodriguez-Amaya, 2000). In addition,
distribution of b-carotene in the sweet potato root
the analysis of fresh and processed samples may introduce
(Rodriguez-Amaya, 1997). Consequently, by using paired
further variability of the results due to dif?culties in
samples sampling errors could be avoided. Retention is
achieving a complete extraction of carotenoids. While
de?ned as the proportion of carotenoids remaining in the
processing softens the cell wall and facilitates carotenoid
processed sweet potato root in relation to the amount of
extraction, incorporation of oil and formation of degrada-
carotenoids originally present in the sweet potato. Loss of
tion products may have the opposite effect (de Sa´ and
solids during processing may lead to an overestimation of
Rodriguez-Amaya, 2004). However, the extraction of
the retention values during processing if not accounted for
carotenoids from both fresh and processed freeze-dried
(Ogunlesi and Lee, 1979; Rodriguez-Amaya, 1997). In the
samples in the present study was repeated until no color
present study, the retention of all-trans-b-carotene in
was noticed in the acetone extract. Retention values could
processed and dried OFSP was calculated based on dry
be overestimated due to the fact that the b-carotene content
weights of the sweet potato samples, and corrected for
in fresh samples is decreased as a result of enzymatic
changes in DM due to loss of soluble solids, loss or uptake
oxidation. However, van Jaarsveld et al. (2006) observed
of water, and uptake of fat in the deep-fried samples.
no loss of b-carotene in chopped raw OFSP that were
Hence, these calculations are comparable to the expression
allowed to stand for 4 h, indicating that enzymatic
of true retention as suggested by Murphy et al. (1975).
oxidation is not a major issue in sweet potato. Although
Moreover, carotenoid measurements on a dry weight basis
processing reduces the carotenoid content, heat processing
allow for easier comparisons among different laboratories.
has the potential to enhance the bioavailability of
The mean retention of all-trans-b-carotene was found to be
carotenoids in cooked vegetables (van het Hof et al.,
77.6% in boiled, 77.0% in steamed, and 78.3% in deep-
1998), which may compensate for the loss.
fried OFSP. This is in contrast to the common idea that
Structural alterations of carotenoids during processing
deep-frying affects the b-carotene content to a larger extent
are attributed to geometrical isomerization and enzymatic
than steaming and boiling (Rodriguez-Amaya, 1997). One
or non-enzymatic oxidation (Rodriguez-Amaya, 2002).
explanation for the similar results obtained for the
These changes result in reduced provitamin A activity of
retention values could be that the OFSP roots were
the sweet potato. Chandler and Schwartz (1988) investi-
prepared as ready-to-eat products, hence different pre-
gated isomerization and degradation of all-trans-b-caro-
paration times were applied (20 min for boiling, 30 min for
tene in sweet potato during different processing treatments.
steaming and 10 min for deep-frying). Using the same
Heat processing induced the formation of 13-cis-b-caro-
preparation time might lead to a higher retention for the
tene, and the quantity formed was related to the severity
steamed samples compared to the samples prepared by the
and length of the heat treatment. The amount of 13-cis-b-
other processing methods. Van Jaarsveld et al. (2006)
carotene found in the processed samples ranged from 25.2
reported a true retention of b-carotene ranging from 83%
to 101 mg/g DM, corresponding to 5.2–28.9% of the total
to 92% in boiled OFSP under different conditions. The
b-carotene content. However, the raw samples were also
highest retention was noticed when boiling for 20 min in a

---

ARTICLE IN PRESS
142
A. Bengtsson et al. / Journal of Food Composition and Analysis 21 (2008) 134–143
pot with the lid on, whereas longer boiling times resulted in
these improved cultivars, particularly SPK004/6, SPK004/
a true retention ranging from 70% to 80%. Kidmose et al.
6/6 and Ejumula, which contained signi?cantly higher
(2006) did not ?nd any signi?cant difference in true
amounts of all-trans-b-carotene among the cultivars
retention of all-trans-b-carotene between varying frying
investigated.
times. However, the frying times used varied only from 1 to
3 min for sweet potato, and they proposed that longer
Acknowledgments
frying times may result in lower retentions. K’osambo et al.
(1998) reported that boiling for 30 min reduced the total
We thank Dr. Robert Mwanga at NAARI, Uganda for
carotenoid content by 14–59% in four different cultivars,
providing the sweet potato roots, Emmauel Okalanyi for
but no consideration was made regarding weight changes
assistance during collection and processing of the sweet
due to loss or gain of water and soluble solids.
potatoes, and The Swedish Agency for Research Coopera-
tion in Developing Countries (SAREC) (Grant SWE-2004-
4.6. Retention of all-trans-b-carotene in dried OFSP
005) for funding.
Solar drying of vegetables and fruits is an appropriate
References
technique to prolong shelf-life and improve quality of plant
foods for consumption during off-season periods (Ruel and
Aguayo, V.M., Baker, S.K., 2005. Vitamin A de?ciency and child survival
Levin, 2000). With solar drying, higher air temperatures
in sub-Saharan Africa: A reappraisal of challenges and opportunities.
and lower humidity can be achieved compared with open-
Food and Nutrition Bulletin 26, 348–355.
Ameny, M.A., Wilson, P.W., 1997. Relationship between Hunter color
air sun drying, thereby shortening the drying time. This
values and b-carotene contents in white-?eshed African sweetpotatoes
may result in larger b-carotene retention in solar-dried
(Ipomoea batatas Lam.). Journal of the Science of Food and
compared with open-air sun-dried vegetables. The three
Agriculture 73, 301–306.
different drying methods evaluated did not bring about any
Beaton, G.H., Martorell, R., Aronson, K.J., Edmonston, B., McCabe, G.,
substantial decrease in the all-trans-b-carotene content.
Ross, A.C., Harvey, B., 1993. Effectiveness of vitamin A supplementa-
tion in the control of young child morbidity and mortality in
Sweet potato slices, which were dried in a solar tunnel drier
developing countries. ACC/SCN State-of-the-art Series Policy Discus-
or forced-air oven retained more all-trans-b-carotene than
sion Paper No. 13. World Health Organization, Geneva, Switzerland.
open-air sun-dried sweet potato slices, but the difference
Chandler, L.A., Schwartz, S.J., 1988. Isomerization and losses of trans-b-
was not signi?cant. The slight difference observed could be
carotene in sweet potatoes as affected by processing treatments.
attributed to the direct exposure to sunlight in the open-air
Journal of Agricultural and Food Chemistry 36, 129–133.
CIP, 2006. International Potato Center (CIP) Report. McKnight
sun-dried samples as the drying time on average was the
Foundation CCRP East African Regional Sweetpotato Project Annual
same as for the solar-dried samples. Still, the retention of
Report (April 2005–March 2006). International Potato Center,
all-trans-b-carotene was high with a mean value of 84% for
Nairobi, Kenya.
the open-air sun-dried samples. Isomerization was not
Dary, O., Mora, J.O., 2002. Food forti?cation to reduce vitamin A
substantial in any of the dried samples and lower compared
de?ciency: International vitamin A consultative group recommenda-
tions. Journal of Nutrition 132, 2927S–2933S.
with the processed samples. The time of drying seems to be
de Sa´, M.C., Rodriguez-Amaya, D.B., 2004. Optimization of HPLC
an important factor for the degradation of b-carotene, and
quanti?cation of carotenoids in cooked green vegetables. Comparison
indeed, Mdziniso et al. (2006) showed that a higher
of analytical and calculated data. Journal of Food Composition and
dehydration rate affected the b-carotene content in solar-
Analysis 17, 37–51.
dried OFSP to a lesser extent, resulting in higher retention
Desobry, S.A., Netto, F.M., Labuza, T.P., 1998. Preservation of b-
carotene from carrots. Critical Reviews in Food Science and Nutrition
values. They reported b-carotene losses of 4.0–5.8% in
38, 381–396.
solar-dried sweet potatoes when drying slices to moisture
FAO/ILSI, 1997. Preventing micronutrient malnutrition: A guide to food-
content of 8–10%.
based approaches. Food and Agriculture Organization of the United
Nations/International Life Sciences Institute, Washington, DC.
5. Conclusions
FAO/WHO, 1988. Requirements of vitamin A, iron, folate and vitamin
B12. Report of a Joint FAO/WHO Expert Consultation. FAO Food
and Nutrition Series No. 23. Food and Agriculture Organization of the
To conclude, this paper showed that the retention of all-
United Nations, Rome, Italy.
trans-b-carotene in OFSP after boiling, steaming and deep-
Gibson, R.S., Hotz, C., 2001. Dietary diversi?cation/modi?cation
frying is similar in prepared, ready-to-eat products. Both
strategies to enhance micronutrient content and bioavailability of
solar and oven drying can be considered as appropriate
diets in developing countries. British Journal of Nutrition 85,
S159–S166.
drying methods, resulting in high retention values. The
Hagenimana, V., Low, J., 2000. Potential of orange-?eshed sweet potatoes
retention of all-trans-b-carotene in OFSP after open-air
for raising vitamin A intake in Africa. Food and Nutrition Bulletin 21,
sun drying was somewhat lower due to the exposure to
414–418.
direct sunlight. The retention was substantial, both after
Hagenimana, V., Carey, E.E., Gichuki, S.T., Oyunga, M.A., Imungi, J.K.,
processing and drying, which means that OFSP with high
1999. Carotenoid contents in fresh, dried and processed sweetpotato
products. Ecology of Food and Nutrition 37, 455–473.
b-carotene content can be considered as an excellent source
Huang, A.S., Tanudjaja, L., Lum, D., 1999. Content of alpha-, beta-
of provitamin A. Of interest for further study is to
carotene, and dietary ?ber in 18 sweetpotato varieties grown in
determine the bioavailability of all-trans-b-carotene in
Hawaii. Journal of Food Composition and Analysis 12, 147–151.

---

ARTICLE IN PRESS
A. Bengtsson et al. / Journal of Food Composition and Analysis 21 (2008) 134–143
143
IOM, 2001. Dietary reference intakes for vitamin A, vitamin K, arsenic,
Rodriguez-Amaya, D.B., 2002. Effects of processing and storage on food
boron, chromium, copper, iodine, iron, manganese, molybdenum,
carotenoids. Sight and Life Newsletter 3, 25–35.
nickel, silicon, vanadium, and zinc. US Institute of Medicine, Food and
Rodriguez-Amaya, D.B., Kimura, M., 2004. HarvestPlus Handbook for
Nutrition Board, Standing Committee on the Scienti?c Evaluation of
Carotenoid Analysis. International Food Policy Research Institute and
Dietary Reference Intakes. National Academy Press, Washington, DC.
International Center for Tropical Agriculture, Washington, DC.
K’osambo, L.M., Carey, E.E., Misra, A.K., Wilkes, J., Hagenimana, V.,
Ruel, M.T., Levin, C.E., 2000. Assessing the potential for food-based
1998. In?uence of age, farming site, and boiling on pro-vitamin A
strategies to reduce vitamin A and iron de?ciencies: A review of recent
content in sweet potato (Ipomoea batatas (L.) Lam.) storage roots.
evidence. FCND Discussion Paper No. 92. Food Consumption and
Journal of Food Composition and Analysis 11, 305–321.
Nutrition Division, International Food Policy Research Institute,
Kidmose, U., Yang, R.-Y., Thilsted, S.H., Christensen, L.P., Brandt, K.,
Washington, DC.
2006. Content of carotenoids in commonly consumed Asian vegetables
Schiedt, K., Liaaen-Jensen, S., 1995. Isolation and analysis. In: Britton,
and stability and extractability during frying. Journal of Food
G., Liaanen-Jensen, S.H., Pfander, E. (Eds.), Isolation and Analysis.
Composition and Analysis 19, 562–571.
Birkha¨user, Basel, Switzerland, pp. 81–108.
Kimura, M., Kobori, C.N., Rodriguez-Amaya, D.B., Nestel, P., 2007.
Sweeney, J.P., Marsh, A.C., 1971. Effect of processing on provitamin A in
Screening and HPLC methods for carotenoids in sweetpotato, cassava
vegetables. Journal of the American Dietetic Association 59, 238–245.
and maize for plant breeding trials. Food Chemistry 100, 1734–1746.
Takahata, Y., Noda, T., Nagata, T., 1993. HPLC determination of b-
Lee, C.M., Trevino, B., Chaiyawat, M., 1996. A simple and rapid solvent
carotene content of sweet potato cultivars and its relationship with
extraction method for determining total lipids in ?sh tissue. Journal of
color values. Japanese Journal of Breeding 43, 421–427.
AOAC International 79, 487–492.
UDHS, 2001. Uganda Demographic and Health Survey (UDHS) Report.
Mdziniso, P., Hinds, M.J., Bellmer, D.D., Brown, B., Payton, M.E., 2006.
Uganda Bureau of Statistics and Macro International, Inc., Calverton,
Physical quality and carotene content of solar-dried green leafy and
MD.
yellow succulent vegetables. Plant Foods for Human Nutrition 61, 13–21.
van het Hof, K.H., Ga¨rtner, C., West, C.E., Tijburg, L.B.M., 1998.
Mulokozi, G., Svanberg, U., 2003. Effect of traditional open sun-drying
Potential of vegetable processing to increase the delivery of carotenoids
and solar cabinet drying on carotene content and vitamin A activity of
to man. International Journal for Vitamin and Nutrition Research 68,
green leafy vegetables. Plant Foods for Human Nutrition 58, 1–15.
366–370.
Murphy, E.W., Criner, P.E., Gray, B.C., 1975. Comparisons of methods
van Jaarsveld, P.J., Faber, M., Tanumihardjo, S.A., Nestel, P., Lombard,
for calculating retention of nutrients in cooked foods. Journal of
C.J., Benade, A.J.S., 2005. b-Carotene-rich orange-?eshed sweet
Agricultural and Food Chemistry 23, 1153–1157.
potato improves the vitamin A status of primary school children
Ogunlesi, A.T., Lee, C.Y., 1979. Effect of thermal processing on the
assessed with the modi?ed-relative-dose–response test. American
stereoisomerisation of major carotenoids and vitamin A value of
Journal of Clinical Nutrition 81, 1080–1087.
carrots. Food Chemistry 4, 311–318.
van Jaarsveld, P.J., Marais, D.W., Harmse, E., Nestel, P., Rodriguez-
Regier, M., Mayer-Miebach, E., Behsnilian, D., Neff, E., Schuchmann,
Amaya, D.B., 2006. Retention of b-carotene in boiled, mashed orange-
H.P., 2005. In?uences of drying and storage of lycopene-rich carrots
?eshed sweet potato. Journal of Food Composition and Analysis 19,
on the carotenoid content. Drying Technology 23, 989–998.
321–329.
Rodriguez-Amaya, D.B., 1997. Carotenoids and Food Preparation: The
Welch, R.M., 2002. Breeding strategies for bioforti?ed staple plant foods
Retention of Provitamin A Carotenoids in Prepared, Processed, and
to reduce micronutrient malnutrition globally. Journal of Nutrition
Stored Foods. John Snow, Inc./OMNI Project, Arlington, VA.
132, 495S–499S.
Rodriguez-Amaya, D.B., 2000. Some considerations in generating
Woolfe, J.A., 1992. Sweet potato—past and present. In: Sweet Potato: An
carotenoid data for food composition tables. Journal of Food
Untapped Food Resource. Cambridge University Press, Cambridge,
Composition and Analysis 13, 641–647.
UK, pp. 15–40.

---

Document Outline

• Effects of various traditional processing methods on the all-trans-beta-carotene content of orange-fleshed sweet potato
  • Introduction
  • Materials and methods
    • Plant material
    • Chemicals and standards
    • Preparation of samples
      • Drying
      • Treatment after processing
      • Dry matter determination
      • Extraction of fat
    • Sweet potato flour L*a*b* color value determination
    • Preparation of all-trans-beta-carotene standard
    • Carotenoid extraction
    • HPLC analysis of carotenoids
    • Carotenoid identification and quantification
    • Method validation
    • Calculation of carotenoid retention
    • Statistical analysis
  • Results
  • Discussion
    • Variability in all-trans-beta-carotene content of OFSP
    • Provitamin A activity of OFSP
    • Correlation between flour color and all-trans-beta-carotene content of OFSP
    • Effect of degradation and isomerization on the all-trans-beta-carotene content of OFSP
    • Retention of all-trans-beta-carotene in processed OFSP
    • Retention of all-trans-beta-carotene in dried OFSP
  • Conclusions
  • Acknowledgments
  • References

---