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Journal of Food Composition and Analysis 19 (2006) 562–571

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Original Article

Content of carotenoids in commonly consumed Asian vegetables andstability and extractability during frying

U. Kidmosea,�, R.-Y. Yangb, S.H. Thilstedc, L.P. Christensena, K. Brandtd

aDepartment of Food Science, Danish Institute of Agricultural Sciences, Research Centre Aarslev, Kirstinebjergvej 10, 5792 Aarslev, DenmarkbAsian Vegetable Research and Development Centre (AVRDC), P.O. Box 42, Shanhua, Tainan 741, Taiwan

cDepartment of Human Nutrition, Royal Veterinary and Agricultural University, Rolighedsvej 30, 1958 Frederiksberg C, DenmarkdSchool of Agriculture, Food and Rural Development, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK

Received 26 October 2004; received in revised form 21 December 2005; accepted 5 January 2006

Abstract

In order to investigate the variation in b-carotene and vitamin A in commonly consumed vegetables in Asia, different leafy vegetables

were analyzed. The mean b-carotene content varied between 16 and 6630 mg/100 g fresh weight (FW) with the highest content in

drumstick leaves and the lowest content in common cabbage and Garland chrysanthemum leaves. In six tuber and fruit vegetables, the

mean b-carotene content varied between 311 and 15,400mg/100 g FW with the highest content in a chili pepper variety. Vitamin A

activity varied significantly between the investigated vegetables (1–1280mg retinol activity equivalents (RAE)/100 g FW). The retention of

b-carotene and formation of cis-isomers were investigated in selected vegetables during stir-frying. Retentions of all-trans-b-carotenevaried between 73% and 98% in sweet bell pepper, sweet potato and tomato that were fried for 1

223 min. In sweet potato, 13-cis-b-

carotene was the major cis-isomer of b-carotene, while only minor amounts of 15-cis- and 9-cis-b-carotene were formed. The total

amount of cis-isomers of b-carotene formed during frying depended on the frying time and the size with the highest amount in cubes, that

were fried for 3min (1070mg 13-cis-b-carotene/100 g FW). In leafy vegetables, only 13-cis-b-carotene was detected during frying.

Extraction of b-carotene into the frying oil was only observed in low amounts after 3min frying of sweet potato shreds.

r 2006 Elsevier Inc. All rights reserved.

Keywords: b-Carotene; Vitamin A activity; Frying; Retention; Vegetables

1. Introduction

Many vegetables and fruits contain significant amountsof b-carotene and other provitamin A carotenoids that canbe absorbed and converted to vitamin A in the humanbody. In many developing countries, the largest contribu-tion of vitamin A intake comes from the provitamin Acarotenoids in plant foods, which may contribute up to82% of the total vitamin A intake, whereas the contribu-tion from fish and meat is of minor importance, becausethese foods are expensive and/or are not accessible (vanden Berg et al., 2000).

At least 254 million pre-school-aged children globallysuffer from clinical and subclinical vitamin A deficiency

e front matter r 2006 Elsevier Inc. All rights reserved.

a.2006.01.011

ing author. Tel.: +4588 99 19 00; fax: +4589 99 34 95.

ess: ulla.kidmose@agrsci.dk (U. Kidmose).

(VAD) (WHO/UNICEF, 1995; WHO, 2000). Alleviationof VAD is thus a major objective, in particular amongvulnerable groups of the population such as children andpregnant and lactating women. Food-based strategies areone of the means that are used for combating VAD indeveloping countries (Gibson and Hotz, 2001; Ruel, 2001).With focus on vegetable foods, some of the approaches infood-based strategies include identification of vitamin Adense plants among the commonly consumed foods andimproved vegetable food processing and preparation thatallow optimal retention and bioavailability of vitamin A(Ruel, 2001; Tontisrin et al., 2002).A heat treatment is often carried out prior to consump-

tion of vegetable food. One of the commonly used heatingmethods in developing countries is frying in oil, whichincludes stir-frying, deep frying, or frying with subsequentaddition of water (stewing or sauteing) (Renqvist et al.,

ARTICLE IN PRESSU. Kidmose et al. / Journal of Food Composition and Analysis 19 (2006) 562–571 563

1978; Speek et al., 1988). During frying, degradation due tooxidation and/or trans–cis-isomerization of the provitaminA carotenoids may lower vitamin A activity of thevegetables (Bauernfeind, 1972; Rodriguez-Amaya, 1997).

Little attention has been paid to the extraction ofcarotenoids into the oil during frying, as only a smallproportion of the original carotenoid content was recov-ered in the oil (Renqvist et al., 1978). However, recenthuman intervention studies have shown that extraction ofthe carotenoids into the oil may potentially improve theirbioefficacy, since the bioefficacy of b-carotene was higherwhen dissolved in oil compared with the bioefficacy indifferent vegetables (van Lieshout et al., 2001). Based onthese studies, new differentiated conversion factors for 1retinol activity equivalent (RAE) has been recommendedfor provitamin A carotenoids dissolved in oil and invegetables: 1 RAE is equivalent in vitamin A activity to1 mg retinol, 2 mg b-carotene dissolved in oil, 12 mg b-carotene or 24 mg of other provitamin A carotenoids inmixed food; (IOM/Food and Nutrition Board, 2000).These new conversion factors highlight the importance ofextraction of carotenoids into the oil during frying ofvegetables for the vitamin A activity of vegetable food. Theextraction may be affected by the frying conditions andprobably also by the location and the bindings of thecarotenoids, which influence the bioavailability of carote-noids (Kidmose et al., 2005; Bryant et al., 1992; Zhou et al.,1996).

Table 1

The contents of dry matter, b-carotene, lutein, other xanthophylls and vitamin

deviation, n ¼ 6–12)

Type of vegetable Latin name Dry matter

content (%)

Leafy vegetables

Amaranth, green Amaranthus mangostanus 7.1

Common cabbage Brassica oleracea 6.9

Garland chrysanthemum Chrysanthemum coronarium 4.4

Drumstick leaves Moringa oleifera . 19.8

Radish leaves Raphanus sativa 7.5

Bottle gourd leaves Langenaria vulgaris 7.8

Malabar spinach Baselba alba 5.9

Edible rape turnip leaves Brassica rapa L. (B. campestris) 7.4

Water convolvulus leaves Ipomoea aquatica 8.3

Fruit vegetables and tubers

Chili (red) Capsicum annuum var —

Drumsticks Moringa oleifera 16.5

Sweet potato Ipomoea batatas 24.3

Sweet gourd Cucurbita moschata 21.7

Sweet bell pepper Capsicum annuum var 8.3

Tomato, ordinary Solanum esculentum 6.2

Tomato, high b-carotene Solanum esculentum 7.9

aFresh weight (FW) in raw, cleaned vegetables.bSum of (all-trans)-neoxanthin, (90-cis)-neoxanthin, violaxanthin, neoluteincmg retinol activity equivalent (RAE)/100 g FW ¼ 0.083mg b-carotene / 100

2000).dViolaxanthin and zeaxanthin.eLycopene.

The main objective of the present study was toinvestigate vitamin A values and carotenoid content anddistribution in commonly consumed vegetables in devel-oping countries in Asia. The objective was also toinvestigate effects of vegetable type and frying conditionsduring stir-frying on the retention, the trans–cis-isomerisa-tion, and the degree of extraction of carotenoids into the oiland thus the vitamin A activity in prepared vegetables.

2. Materials and methods

2.1. Plant material

The plant material used in the present trials was collectedin Taiwan. Nine leafy vegetables, 5 fruit vegetables and 1tuber were analyzed as raw material (Table 1). Most of thevegetables were purchased from two local vegetablemarkets in the village Shanhua, close to the AsianVegetable Research and Development Center (AVRDC)in January 2003. One variety of drumstick fruit anddrumstick leaves (Moringa oleifera, TOT 4951, Origin:Thailand, Tree age: 3 years), a common tomato and a highb-carotene tomato variety (FMTT 1098) (Solanum escu-

lentum) and three genotypes of red chili (Capsicum annuum

var; PBC 385, origin: Malaysian, VC16a, origin: India/France and PBC 370-2-2-1-1-1, origin: Thailand) wereharvested from field trials at AVRDC in 2003. One to twohours after purchase or harvest, samples were either frozen

A values in raw commonly consumed vegetables in Asia (mean7standard

b-Carotene(mg/100gfresh FWa)

Lutein

(mg/100gfresh FWa)

Other xanthophyllsb)

(mg/100g fresh FWa)

Vitamin A activity

(mg RAE/100 g

fresh FWa)c

17807591 23107577 25307653 151749

1678 30720 — 1.370.8

6827153 11407279 11307287 57713

66307775 69407528 62807433 551793

157072 190078 1750745 13070.2

16207312 30907321 26907316 135726

17007276 34907202 27807267 141723

32607189 57907885 49007761 271716

1890758 4780768 34207144 15875

6530–15,400 542–1280

311792 697777 493760 2678

783071013 50718 610784

85407289 6097 94 709725

10207221 668727 1505754d 84718

778738 195907255e 6173

23507395 245071140e 224735

A and neolutein B.

g FW + 0.042mg a-carotene/100 g FW (IOM/Food and Nutrition Board,

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raw or used for further frying trials (amaranth and waterconvolvulus leaves, tomato, sweet bell pepper and sweetpotato).

2.2. Sampling and household preparation of vegetables

2.2.1. Preparation and frying

Amaranth leaves (Amaranthus mangostanus): Samples ofamaranth leaves (1.5 kg) were prepared by removal ofdamaged leaves. After cutting off the leaf stalks, the leafblades were washed in cold tap water and shaken toremove the water. The leaf blades were carefully mixed andrandomly divided into 4� 3 replicates: Three replicates ofthe raw material were packed in thick polyethylene (PE)bags and were frozen immediately at �18 1C. Two samplesof each three replicates of 50 g were each fried in 50 g ofsoybean oil for 1 or 2min at 160 1C. In addition, threereplicates of 50 g were each fried in 100 g of soybean oil for1min at 160 1C.

Water convolvulus leaves (Ipomoea aquatica): Samples ofwater spinach leaves (1 kg) were prepared by removal ofdamaged leaves. After cutting off the leaf stalks, the leafblades were washed in cold tap water and shaken toremove the water. The leaf blades were carefully mixed andrandomly divided into 2� 2 replicates: Two replicates ofthe raw material were packed in thick PE bags and werefrozen immediately at �18 1C and two replicates of 50 g ofusable leaves were each fried as whole leaves in 50 gsoybean oil for 1min at 160 1C.

Sweet potato (Ipomoea batatas): Sweet potato (2 kg) werewashed, peeled and cut into 10� 10mm cubes or 6mmshreds. The cubes were carefully mixed and randomlydivided into 4� 3 replicates: Three replicates of the rawmaterial were packed in thick PE bags and were frozenimmediately at �18 1C and two samples of each threereplicates of 75 g of 10� 10mm cubes were each fried in75 g of soybean oil for 1 or 3min at 160 1C. In addition,three replicates of 75 g of 6mm shreds were each fried in75 g of soybean oil for 1min at 160 1C. The weight afterfrying was registered and varied between 53 and 67 g forthe different treatments.

Sweet bell pepper (Capsicum annuum var): Samples oforange sweet bell pepper (2 kg) were washed and theplacenta was removed. The sweet bell peppers were cut incubes of 10� 10mm or 25� 25mm. The 25� 25mm cubeswere carefully mixed and randomly divided into 4� 3replicates: Three replicates of the raw material were packedin thick PE bags and were frozen immediately at �18 1Cand three samples, each of three replicates of 50 g were eachfried in 50 g of soybean oil for 1

2, 1 or 2min at 160 1C. In

addition, 50 g of 10� 10mm cubes were fried in 50 g ofsoybean oil for 1min at 160 1C. The weight after frying wasregistered and varied between 47 and 50 g.

Tomato (Solanum esculentum): The two different types oftomatoes, ordinary (2 kg) and high-b-carotene (2 kg)tomatoes, were washed. The high-b-carotene tomatoeswere cut into halves or quarters, whereas the ordinary

tomatoes were cut only in halves. One of the halves orquarters from each tomato were used for the raw samples,whereas the opposite halves or quarters were used forfrying. For high-b-carotene tomatoes, three replicates of50 g of the raw material were packed in thick PE bags andwere frozen immediately at �18 1C. Three replicates of100 g of two samples of halved tomatoes were each fried in100 g soybean oil for 1 or 2min at 160 1C. Three replicatesof 100 g of the quarters were each fried for 1min at 160 1C.For halved ordinary tomatoes, three replicates of 100 g ofthe raw material were packed in thick PE bags and werefrozen immediately at �18 1C, whereas three replicates of100 g of the opposite halves were fried each in 100 gsoybean oil for 2min at 160 1C. The weight after frying wasregistered and varied between 79 and 98 g.

2.2.2. Treatment after processing

After frying and registration of the weight, the sampleswere cooled for 15min, packed in thick PE bags and frozenat �18 1C. The remaining oil was collected for carotenoidanalysis. The frozen samples were transported in boxes ofexpanded polystyrene with cooling elements by flight toDenmark. The samples were kept at �24 1C for 5 monthsuntil the carotenoid analysis was conducted.

2.3. Analysis of pigments in leafy vegetables

Carotenoids (b-carotene and the xanthophylls all-trans-lutein, all-trans-violaxanthin, all-trans-neoxanthin, 90-cis-neoxanthin, neolutein A and neolutein B) in leafyvegetables were extracted with 100% acetone in red lightat room temperature as described by Edelenbos et al.(2001). After homogenization in ultra-pure water, sub-samples of approx. 8 g were extracted with 10mL of cold100% acetone, homogenized for 30 s by ultrasonic agita-tion and centrifuged as described by Edelenbos et al.(2001). The samples were re-extracted twice until they werecolourless. The resulting extracts were pooled and analyzedby reversed-phase (RP) high-performance liquid chroma-tography (HPLC) on a Shimadzu HPLC system equippedwith a SPD 10AV UV-VIS detector operating at 440 nm.Carotenoids were separated on a LiChrospher 100 RP-18column (5 mm; 244� 4mm i.d., Merck Kebo Lab, Albert-slund, Denmark) by gradient elution with solvent A (80%CH3OH–20% H2O) and solvent B (100% ethyl acetate)according to the method described by Edelenbos et al.(2001). Identification and quantification of carotenoidswere carried out as described by Edelenbos et al. (2001).The content of carotenoids in the raw vegetables wereexpressed as mg carotenoid/100 g FW, which means that thecontent is expressed per 100 g of the raw, cleanedvegetables.

2.4. Dry matter

Dry matter (DM) content was determined in rawsamples. DM content was determined twice on an aliquot

ARTICLE IN PRESSU. Kidmose et al. / Journal of Food Composition and Analysis 19 (2006) 562–571 565

sample of 25 g by drying for 20 h at 80 1C in a heatingcabinet (Lytzen, CBM-spec, Herlev, Denmark) with forcedair.

2.5. Analysis of all-trans- and cis-isomers of provitamin A

carotenoids

The carotenoids were extracted, separated and quanti-fied as described by Kidmose et al. (2004). The methodused was based on a method described by Lessin et al.(1997). Ten grams of subsamples were homogenized withwater/methanol, centrifuged and then extracted withacetone–n-hexane. The samples were separated in aseparation funnel. The hexane/carotenoid epilayer waswashed, diluted and filtered. The samples were analyzed byanalytical HPLC on a Merck Hitachi L7000 system(Merck, Albertslund, Denmark) using a photodiode array(PDA) detector operating at 450 nm. Separations wereperformed at 25 1C on a C-30 column (Carotenoid S-5 mm,4.6� 250mm, YMC/Waters, Microlab, Aarhus, Denmark)by gradient elution with solvent A (CH3OH–THF (v/v),95:5) and solvent B (H2O–CH3CH2OH–THF (v/v/v),90:5:5). The gradient was changed linearly from 0 to60min with solvent B: 0-100% and solvent A: 100-0%.The flow rate was 1.5mL/min and the injection volume20 mL. Identification was carried out by spiking ofauthentic standards and comparing the spectral data withreported values (Britton 1995). a- and b-Carotene, 9-cis-,13-cis- and 15-cis-b-carotene and lutein were quantifiedusing the individual carotenoids as external standards(Sigma-Aldrich Denmark A/S, Vallensbaek Strand, Den-mark; CaroNature, Bern, Switzerland).

2.6. Analysis of b-carotene and lycopene in tomato samples

Lycopene and b-carotene were quantified in tomatosamples by a modified method of Sadler et al. (1990).Samples of 25–50 g were homogenized for 2min. Sub-samples of 2 g were extracted with 50mL hexane–aceto-ne–ethanol (v/v/v), 50:25:25, at ambient temperature byshaking for 15min. Ten milliliters of ultra-pure water wasadded and the samples were extracted for another 5minwith shaking. After 5min without shaking, the sampleswere filtered through a 0.45 mm Minisart SRP 25 filter (Bieand Berntsen, Rødovre, Denmark). The filtrate wastransferred directly into 4mL brown HPLC vials. Thesamples were analyzed by analytical HPLC on a MerckHitachi L7000 system (Merck, Albertslund, Denmark)using a PDA detector operating at 450 or 475 nm.Separations were performed at 25 1C on a LiChrospher100 RP-18 column (Microlab, Aarhus, Denmark) byisocratic elution with a solvent of H2O–CH3OH–THF:67:27:6 (v/v/v). The flow rate was 2mL/min and theinjection volume 20 mL. Identification was carried out byspiking of authentic standards and comparing the spectraldata with reported values (Britton, 1995). b-Carotene andlycopene were quantified using the individual carotenoids

as external standards (Sigma-Aldrich Denmark A/S,Vallensbaek Strand, Denmark).

2.7. Quality control of the analytical methods

The validity of the HPLC methods used for determina-tion of the carotenoids was checked with regard toaccuracy, linearity and precision. The accuracy was verifiedby addition of specific amounts of carotenoid standards toa standard vegetable extract. To verify the repeatability,the same extract was analyzed three to five times. Thecoefficient of variance (CV) was less than 5% for all threemethods. To determine the reproducibility of the extrac-tion, a sample was extracted two to five times for the threemethods. The CVs between the extractions were below 6%for all three methods. The analysis of the carotenoids wascarried out as two extractions, and the CV for determina-tion of b-carotene between the two extractions was below10% for all samples analyzed.

2.8. Calculation of true retention of individual carotenoids

and vitamin A activity

The true retention (TR) defines the ratio of the contentof carotenoids retained in the fried portion to the contentof carotenoids in the raw portion and is therefore related tothe loss. It is thus adjusted for the weight loss or gainduring processing in the calculations (Murphy et al., 1975).The true retention was calculated as

%TR ¼ ½ðnutrient content per g of cooked food

� g of food after cookingÞ=ðnutrient content per g

of raw food� g of food before cooking� � 100.

Vitamin A values in the raw and fried samples werecalculated, using the conversion ratio of 2 mg b-carotenedissolved in oil, 12 mg b-carotene or 24 mg of otherprovitamin A carotenoids in foods to 1 RAE (IOM/Foodand Nutrition Board, 2000). Vitamin A activity of cis-isomers of b-carotene were estimated relative to the activityof all-trans-b-carotene (13-cis-b-carotene 53%, 9-cis-b-carotene 38%) (Bauernfeind, 1972). The value for vitaminA activity in the total fried portion of vegetables wasadjusted to 100 g starting material (original material)taking into account the weight loss during frying, but notthe possible effect of oil absorbed into the vegetablematerial. The values for the fried samples per 100 g startingmaterial were directly comparable to the raw samples.

2.9. Statistical analysis

For analysis of variances, the general linear model(GLM) procedure of the Statistical Analysis System (SASInstitute, Cary, NC) was used. The data were checked foroutliers. The sources of variances were vegetable andtreatment during processing. Least Significant Difference

ARTICLE IN PRESSU. Kidmose et al. / Journal of Food Composition and Analysis 19 (2006) 562–571566

(LSD) at Pp0:05 was used to assess the location of thesignificant differences.

3. Results and discussion

3.1. Provitamin A and non-provitamin A carotenoids in raw

Asian vegetables

3.1.1. Leafy vegetables

b-Carotene is the primary provitamin A carotenoid andoccurs together with the three main xanthophylls lutein,violaxanthin and neoxanthin, which are called the ‘‘chlor-oplast carotenoids,’’ and the chlorophylls a and b as well assmaller amounts of their cis-forms in the chloroplast ofleafy vegetables (Gross, 1991; Lessin et al., 1997). Therange in the mean b-carotene content was between 16 and6630 mg/100 g FW, where drumstick leaves clearly had thehighest content, whereas the lowest content of b-carotenewas found in common cabbage and Garland chrysanthe-mum leaves (Table 1). A similar trend was observed for thecontent of lutein as well as for the total content ofviolaxanthin, neoxanthins, neolutein A and neolutein B(Table 1). The distribution of b-carotene, lutein andviolaxanthin in the analyzed vegetables varied between13–20%, 19–26% and 8–15% of the total carotenoidcontent, respectively.

Both fresh and dried drumstick leaves are commonlyconsumed in certain parts of Asia, for example in ruralIndia. Drumstick leaves are easy to cultivate and have ahigh nutritional value due to high contents of b-carotene,vitamin C, iron and calcium. Thus, drumstick leaves arepotentially a good vitamin A source (Nambiar andSeshadri, 2001). Tee and Lim (1991) reported a b-carotenecontent of 7500 mg/100 g FW in raw Malaysian drumstickleaves, in accordance with the present data (Table 1),whereas the b-carotene content in different Indian drum-stick leaves has been reported to be 17.4 and 19.2mg/100 gFW, respectively (Subadra et al., 1997, Nambiar andSeshadri, 1998).

A wide range of amaranth types is consumed in manydeveloping countries. b-Carotene levels between 8000 and8600 mg/100 g FW have been determined in differentamaranth types from Bangladesh and India (Rahman etal., 1990; Reddy et al., 1995), whereas the mean b-carotenecontent was 1530 mg/100 g FW in leaves from Bangladesh(Hels et al., 2004), in accordance with the value found inthe present trial (Table 1). In Garland chrysanthemumleaves that are commonly consumed in China, the mean b-carotene content was two-fold higher (1280 mg/100 g FW),whereas the content of b-carotene in bottle gourd leavesfrom Bangladesh (1530 mg/100 g FW) was slightly lowerthan the values found in the present study (Chen et al.,1993; Hels et al., 2004). In Taiwanese water convolvulusleaves, the b-carotene content was 1893 mg/100 g FW,which was similar with the content in the present study(Chen et al., 1991). The highest amount of lutein was foundin leaves of Malabar spinach, edible rape turnip and bottle

gourd and the amount of violaxanthin was highest indrumstick leaves (Table 1).

3.1.2. Fruit vegetables and tubers

The mean b-carotene content varied between 311 and15,400 mg/100 g FW (Table 1). As expected, the luteincontent was generally low (Table 1), since lutein is one ofthe ‘chloroplast carotenoids’ (Gross, 1991).In sweet potato, b-carotene was the major carotenoid

(Table 1), which may constitute up to 80% of the totalcarotenoid content (Gross, 1991). The b-carotene contentvaries greatly with the colour of different varieties of sweetgourd/pumpkin and sweet potato, which ranges from whiteto light yellow, deep yellow and dark orange. Takahata etal. (1993) reported a b-carotene content of 18.7mg/100 gFW in dark orange sweet potato variety similar to the oneused in the present study, whereas Chen et al. (1993)reported a lower content of b-carotene. According to Gross(1991), the total carotenoid contents in yellow and orangepumpkins were 2340 and 3240 mg/g dry weight, respec-tively, in accordance with the level found in the presentstudy (Table 1). a-Carotene constituted only a minor partof the total carotenoid content in sweet gourd/pumpkin(Gross 1991).b-Carotene contents in the three red chili pepper

genotypes varied very much (6530, 13,700 and 15,400 mg/100 g FW in the Malaysian, Indian/French and Thaivariety, respectively) and were much higher than thecontent in sweet bell pepper, which contained considerableamounts of violaxanthin and zeaxanthin (Table 1).Ordinary tomato had a very high content of lycopene

and a low content of b-carotene, whereas the high b-carotene tomato was associated with a lower content oflycopene (Table 1). The carotenoid distribution in the twodifferent tomato types in the present study was alsoobserved by Fraser et al. (2001), who reported a two-foldhigher b-carotene content but a two-fold lower totalcarotenoid content in high b-carotene mutant comparedwith the wild-type. An increase in the b-carotene contentwas also associated with a decrease in total carotenoidcontent in transgenic tomato lines. However, the content oflycopene was not measured in this investigation (Fraser etal., 2001).Factors such as variety (genetic variation), growing site,

climate, growing conditions and stage of maturity can leadto different values for the b-carotene content (Gross, 1991;Rodriguez-Amaya, 1997). In addition, analytical methodsused earlier for determination of b-carotene tended tooverestimate the content in some types of plant materialand thus caused different levels in the b-carotene contentbetween different investigations (Hulshof et al., 1997).Different levels in the b-carotene content in differentinvestigations is therefore common, and re-investigationwith modern analytical methods is often required, inparticular for vegetables that are used only in certain areas.Vitamin A activity varied between 1 and 1280 mg RAE/

100 g FW in the raw vegetables (Table 1). The highest

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vitamin A activity was found in one of the chili genotypes,and among the leafy vegetables, in drumstick leaves (Table1).

The values of b-carotene content in commonly con-sumed vegetables are necessary in order to upgrade foodcomposition tables, which contain data for mean nutrientcontent of foods. Food composition data are essential forestimating intakes of specific nutrients, which can berelated to the nutrient recommendations for differentpopulation groups (FAO/UN, 1997). However, to estimatethe intake of specific nutrients, the content of the nutrientin the food consumed should be related to the dietaryintake of the food. Even when a high content of b-caroteneis found in a chili variety, the contribution may beinsignificant, as it also depends on the dietary intake ofchili. It is assumed that the intake is low compared withother vegetables; however, specific data on dietary intakeof chili in Asia is limited.

3.2. Changes in carotenoids during frying

The effects of frying on the carotenoid content can beconsidered in terms of the retention of the all-trans-carotenoids during frying due to losses caused bydegradation and other chemical changes, such as trans-cis-isomerisation. Both loss and trans–cis-isomerizationdecrease the vitamin A activity. The retention can beexpressed as true retention (TR), which was defined inSection 2.8.

3.2.1. Retention of major provitamin A and non-provitamin

A carotenoids during frying

TR of all-trans-b-carotene, which is the major provita-min A carotenoid originally present in the investigated rawvegetables, is shown in Table 2. The data used forcalculation of TR are shown in Tables 3 and 4.

TR of b-carotene was not affected significantly by theduration of the heat treatment for sweet potato, tomatoand sweet bell pepper (Table 2). The size of the vegetableseemed not to affect TR, however, non-significantly lowerTR of b-carotene in high-b-carotene tomato were obtainedafter frying of quarters for 1min compared with frying ofhalved tomatoes (Table 2). The very similar TRs for b-carotene obtained by various frying times may be due tothe very similar frying times used in the present study. Thefrying times used differed only between 1

2and 2min for

sweet bell pepper and between 1 and 2min for sweetpotato. In many developing countries, the frying time ofvegetable foods differs, as this depends on frying methodand cultural practices. The time regime for frying usedduring household preparation of vegetables may be up to10min or more, and thus studies conducted with such longduration of frying may result in a much lower retentioncompared with stir-frying for a very short time.

In many studies about retention of b-carotene duringfrying, different frying conditions were used for differentvegetables. Stir-frying of three leafy vegetables for 3.5,

3 and 2min resulted in retention of b-carotene between58% and 72% with the lowest retention in water mimosathat were fried for 2min (Wasantwisut et al., 1995). Speeket al. (1988) reported retentions of b-carotene, calculatedon fresh weight basis, between 19% and 84% after fryingleafy vegetables or cabbage for 3–5min, whereas frying ofsugar pea resulted in a retention of 92%. Retention of thecarotenoids seemed to depend on the type of vegetable(Table 2). However, since different frying conditions wereused for the different vegetables, the data are not directlycomparable.

3.2.2. Trans–cis-isomerization during frying

The content of b-carotene and other carotenoids inthe investigated raw and fried vegetables is presented inTables 3 and 4. The contents of cis-isomers of b-carotenewere determined in amaranth leaves, water convolvulusleaves and sweet potato and are presented in Table 3.The proportion of cis-isomers formed in relation to the

total b-carotene content was slightly higher in the raw leafyvegetables than in raw sweet potato (Table 3). This is inaccordance with earlier studies, which reported no or onlyvery small amounts of cis-isomers in yellow vegetables, forexample carrot, but a higher amounts in green leafyvegetables. The higher proportion of cis-isomers of thetotal carotenoid content in green vegetables may be due tothe ability of chlorophylls to act as sensitizers of b-carotenephoto-isomerization (Lessin et al, 1997).In fried sweet potato, 13-cis-b-carotene was the cis-

isomer of b-carotene formed in the highest amount,whereas 15-cis- and 9-cis-b-carotene were formed in minoramounts. In contrast, 13-cis-b-carotene was the only cis-isomer of b-carotene that was detected in the leafy vege-tables, amaranth and water convolvulus leaves (Table 3).The trans–cis-isomerization during frying depended on thefrying time used for each vegetable and varied betweendifferent vegetables (Table 3). In sweet potato, frying ofcubes for 3min resulted in a significantly higher content ofcis-isomers than frying of shreds and cubes for 1min. 9-Cis-b-carotene was only detected after 3min of frying ofsweet potato (151 mg/100 g FW). In both leafy vegetables, asignificant increase of 13-cis-b-carotene was observed withincreasing heating time. However, the proportion of cis-isomers formed in percentage of the total b-caroteneincreased only slightly with increasing frying time for theseleafy vegetables due to the simultaneously enhancedextractability of all-trans-b-carotene (Table 3). 13-Cis-b-carotene is mainly formed during thermal treatment,whereas 9-cis-b-carotene dominates, when all-trans-b-carotene is exposed to light (Chen et al., 1995, 1996).However, formation of 9-cis-b-carotene has also beenreported in canned vegetables including sweet potato,whereas no formation of 9-cis-b-carotene was seen afterblanching, steaming, dehydration and baking of sweetpotato (Chandler and Schwartz, 1988). The present studyshowed that the formation of 9-cis-b-carotene depended on

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Table 2

True retention (TR) of all-trans-b-carotene in sweet potato, tomato and sweet bell pepper after frying

Type of vegetables Size

(mm)

Frying time

(min)

Amount of

oil (g)

True retention of

all-trans-b-carotene (%)

Sweet potato (Ipomoea batatas)

Cubes 10� 10 1 75 81 a

10� 10 3 75 84 a

Shreds 6 1 75 95 a

Tomato (Solanum esculentum)

High -b-carotene Halves 1 100 84 a

High -b-carotene Halves 2 100 95 a

High -b-carotene Quarters 1 100 73 a

Sweet bell pepper (Capsicum annuum var)

Cubes 25� 25 0.5 50 98 a

25� 25 1 50 97 a

25� 25 2 50 83 a

10� 10 1 50 83 a

Means followed by different letters, within a column , are significantly different (Pp0:05).

Table 3

The contents of all-trans-b-carotene, cis-isomers and the estimated vitamin A values in sweet potato and leaves of amaranth and water convolvulus before

and after frying

Vegetables Frying

time

(min)

Amount of

oil (g)

Trans-b-carotene

(mg/100 g)a

15-Cis-b-

carotene

(mg/100 g)a

13-Cis-b-

carotene

(mg/100 g)a

% Cis-isomers

(% of total

b-carotene )

Vitamin A activity

(mg RAE/100 g

starting material)b

Amaranth leaves (Amaranthus mangostanus)

Raw 0 0 1780 d 48 b 2.6 151 c

Fried 1 50 2620 c 85 ab 3.2 189 b

2 50 3760 a 128 a 3.4 203 b

1 100 3140 b 101 ab 3.1 254 a

Water convolvulus leaves (Ipomoea aquatica)

Raw 0 0 1890 b 32 b 1.7 160‘

Fried 1 50 4980 a 171 a 3.3 —

Sweet potato (Ipomoea batatas)

Raw 0 0 7830 a 37 b 97 c 1.7 656 a

Cubes 1 75 7460 a 48 b 131 c 2.4 556 a

(10� 10) 3 75 8990 a 128 a 1070 a 15 566 a

Shreds (6) 1 75 8540 a 37 b 332 b 4.3 630 a

—not determined.

Means followed by different letters, within a column, are significantly different (Pp0:05).a100 g fresh weight (FW) as eaten of raw or fried, cleaned vegetables.bmg retinol activity equivalents (RAE)/100 g starting material ¼ 0.083mg b-carotene+0.042 mg a-carotene in mixed foods in the fried portion after

adjusted the values to 100 g starting material (original portion), vitamin A activity of cis-isomers was estimated relative to the all-trans-b-carotene.

U. Kidmose et al. / Journal of Food Composition and Analysis 19 (2006) 562–571568

the heating conditions, as it was only formed and in verylow amounts after 3min frying of sweet potato.

The amount of 13-cis-b-carotene formed during frying ofsweet potato also depended on the size as frying of shredsfor 1min resulted in a higher amount than frying of cubesfor the same time (Table 3). The proportion of cis-isomersformed in percentage of the total b-carotene increasedwith increasing frying time during frying of sweet potato(Table 3).

3.2.3. Extraction of the carotenoids into the oil

Taking into account the higher bioefficacy of pro-vitamin A carotenoids when dissolved in oil, the extractionof carotenoids into the oil during frying was investi-gated. In the present study, only traces of carotenoidswere measured in the oil after frying of tomato, sweetpepper, amaranth leaves and water convolvulus leavesfor all the treatments. In sweet potato, a small, detectableamount of b-carotene, present as cis-isomers, was

ARTICLE IN PRESS

Table 4

The contents of b-carotene, lycopene, zeaxanthin, violaxanthin and vitamin A values in tomato and sweet bell pepper before and after frying

Vegetables Size

(mm)

Frying

time

(min)

Amount of

oil (g)

Lycopene

(mg/100 g)ab-Carotene(mg/100 g)a

Zeaxanthin

(mg/100 g)aViolaxanthin

(mg/100 g)aVitamin A activity

(mg RAE/100 g

starting material)b

Tomato (Solanum esculentum)

Common Raw 0 0 19,860 a 778 b 61 b

Halves 2 100 21,400 a 853 b 60 b

High b-carotene Raw 0 0 2450 b 2700 a 224 a

Halves 1 100 2250 b 2350 a 192 a

Halves 2 100 2370 b 2960 a 227 a

Quarters 1 100 1290 b 2500 a 166 a

Sweet bell pepper (Capsicum annuum var)

Raw 0 0 — 1020 a 714 a 791 a 85 a

25� 25 12

50 — 1040 a 755 a 777 a 88 a

25� 25 1 50 — 1120 a 747 a 1130 a 83 a

25� 25 2 50 — 1120 a 903 a 879 a 74 a

10� 10 1 50 — 1010 a 915 a 1050 a 71 a

Means followed by different letters, within a column, are significantly different (Pp0:05).a100 g fresh weight (FW) as eaten of raw or fried, cleaned vegetables.bmg retinol activity equivalents (RAE)/100 g starting material ¼ 0.083mg b-carotene + 0.042mg a-carotene in mixed foods in the fried portion after

adjusted the values to 100 g starting material (original portion), vitamin A activity of cis-isomers was estimated relative to the all-trans-b-carotene.

U. Kidmose et al. / Journal of Food Composition and Analysis 19 (2006) 562–571 569

measured in the oil (73 mg/100 g oil) after frying of shredsfor 1min.

Thus, the frying time and the sizes used in the presenttrials did not affect the amounts of carotenoids in the oil inthe investigated vegetables, except for sweet potato.

Since carotenoids are fat soluble, a certain extent oftransfer of the carotenoids from the vegetable matrix intothe oil phase may be expected during frying. Van het Hof etal. (1998) reported that boiling tomato paste with oilresulted in a transfer of lycopene from tomato paste intothe oil and that the degree of transfer depended on theamount of oil, whereas Renqvist et al. (1978) reported thatapproximately 5% of the original carotenoid content wasrecovered in the oil after frying of papaya and waterleaf indifferent amounts of fat (100–150mL fat). The degree ofextraction of b-carotene from carrot slices into the oildepended on the frying time, as the highest content of b-carotene was seen after 2min frying after which itdecreased with increasing frying time (Kidmose et al.,2005). Differences in extractability during frying betweenvegetables may also depend on the location and thephysical/chemical structure of carotenoids in the vegetable,as the structure and location of the carotenoids differbetween different parts of the plant. In photosynthesizingtissues, the carotenoids are located in light-collectingchlorophyll/carotenoid-binding protein complexes in thechloroplasts (Demmig-Adams et al., 1996). In yellow/orange plant parts, for example fruits and some roots, thecarotenoids exist in different structures, such as crystallineforms or in lipid droplets, and are located in thechromoplasts (Bryant et al., 1992; Erdman et al., 1993).These different structures of carotenoids in the chromo-and chloroplasts probably affect the extractability of thecarotenoids into the oil and may be a part of the

explanation for the different results for different vegeta-bles. Further studies with frying of vegetables that differ inlocation and physical/chemical structures of carotenoidsare needed to elucidate the relationship between extrac-tability, the retention and the physical/chemical structureof the carotenoids in the vegetables.In summary, the type of vegetable, the degree of

maceration (cubes or shreds, tomato paste or halvedtomato), the concentration of b-carotene in the vegetable(amaranth, sweet potato or carrot) and/or the frying timeseem to play a role for the degree of extraction ofcarotenoids into the oil during frying. However, this couldnot be demonstrated in the present study, since only verysmall amounts of carotenoids were extracted into the oilfor all the treatments.

3.2.4. Vitamin A activity in the fried samples

Estimations of vitamin A activity in the raw and friedvegetables investigated in the present study were calculatedas described in Section 2.8 and shown in Tables 3 and 4. Inorder to estimate vitamin A activity obtained whenconsuming the raw portion of the vegetables comparedwith consuming the corresponding portion after frying,vitamin A activity was calculated on the basis of 100 g ofthe starting material. Thus, the weight loss of the portionduring frying is taken into consideration. This is aminimum estimate, since the extraction into the oil withinthe vegetables, which was not measured in the presentstudy, may increase the values further. The data showedthat the extraction of carotenoids into the oil is notimportant in the present study and contribution of vitaminA from the oil were not included in the calculations,whereas vitamin A activity of the cis-isomers formedduring frying was included, if measured. Similar vitamin A

ARTICLE IN PRESSU. Kidmose et al. / Journal of Food Composition and Analysis 19 (2006) 562–571570

activities were obtained before and after frying of 100 gstarting material of sweet potato shreds and cubes (Table3). Raw and fried portions of sweet bell pepper, commonand high b-carotene tomato also had similar vitamin Aactivities. However, a large difference in vitamin A activitywas observed between the raw samples of high b-carotenetomato and the ordinary tomato (Table 4). The higherconcentration of b-carotene in 100 g FW of the friedsamples of amaranth leaves compared with the rawsamples resulted in a significantly higher vitamin A contentin the fried samples of amaranth leaves (Table 3).

4. Conclusions

A large variation in the b-carotene content in Asianvegetables was found in the present study. As vegetablesare important as a vitamin A source in developingcountries, this variation, together with the amounts ofspecific types of vegetables consumed as well as thedifferences in vitamin A activity are important considera-tions.

The retentions of all-trans-b-carotene in different vege-tables were not significantly affected by the processingconditions. Trans–cis-isomerization leads to a decreasedvitamin A activity and frying causes different degree oftrans–cis-isomerization, depending on the vegetable, thefrying time and the way of cutting. The choice of vegetableand the way of frying can reduce the trans–cis-isomeriza-tion and thus increase vitamin A activity. The highestproportion of cis-b-carotenes of the total b-carotene was15% after 3min frying of sweet potato cubes, whichresulted in a reduction of vitamin A activity by approxi-mately 7%. The degree of extraction of carotenoids intothe oil during frying was minimal. The present studyshowed that consumption of the frying oil after stir-fryingof the investigated vegetables is not important in order toincrease vitamin A content in meals in developingcountries.

Acknowledgements

This work is part of the framework programme ‘Contentand bioavailability of vitamin A, iron, and zinc incommonly consumed foods in developing countries’, whichis funded by The Council for Development Research,Danish International Development Assistance (DANIDA),Ministry of Foreign Affairs, Denmark. We gratefullyacknowledge the excellent technical assistance of Mrs.Lisbeth Stage, Mrs. Birgitte Foged, Mr. Kim Vitten andMrs. Elisabeth Kjemtrup.

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