Profile of phenolics,carotenoids and antioxidative capacities of thermal processed white,yellow,orange and purple sweet potatoes grown in Guilin,China

Yayuan Tang,Weixi Cai,Baojun Xu

Food Science and Technology Program,Beijing Normal University-Hong Kong Baptist University United International College,Zhuhai,Guangdong 519085,China

Abstract The objectives of this study were to systematically compare phenolic profiles carotenoids profile and antioxidant activities of raw and cooked sweet potatoes of fie varieties(white,yellow,orange,light purple and deep purple).Total phenolic content(TPC),monomeric anthocyanin content(MAC), total carotenoid content (TCC), 2-diohenyl-1-picryhydrazyl (DPPH) free radical scavenging capacities and ferric reducing antioxidant powder(FRAP)were determined by colorimetric methods.Higher anthocyanin contents and antioxidant capacities were detected in purple sweet potato species,while higher carotenoid contents were detected in yellow and orange sweet potato.All cooked sweet potato exhibited significantl(p ＜0.05)lower TPC,MAC,TCC,DPPH and FRAP values as compared to the respective raw samples.Under the same cooking time,steaming was good for the retention of TPC,roasting was good for keeping anthocyanins,and boiling was beneficia to preserve carotenoids.

Keywords: Sweet potato;Processing;Phenolics;Carotenoids;Antioxidant capacities

1.Introduction

Sweet potato is a crop with rich nutritional values including carbohydrates, dietary fibers vitamins, and minerals [1].Currently, it is the sixth most popular and abundant staple food worldwide.It plays an important role in solving the issues of food, energy, natural resources and environment.Four commonly available colored sweet potato species in China are white,yellow, orange, and purple, which have completely different chemical compositions.

The major bioactive substances in purple sweet potato are phenolics and anthocyanins.Phenolics are the antioxidant molecules with at least one aromatic ring and one or more hydroxyl groups[2].Anthocyanins,are a group of water-soluble fl vonoids.As the predominant pigments and functional phenolics in purple sweet potato, anthocyanins are the naturally strong free-radical scavengers, which provide many pharmaceutical values including anti-oxidation, anti-tumor capacities,and prevention and treatment of cardiovascular diseases.In yellow or orange sweet potato species,carotenoids(such asβcarotene) act as the primary pigment molecule [3] as well as the source of provitamin A,which shows vitamin A activity[4].Carotenoids have strong antioxidant capacity to scavenge free radicals because of their conjugated double bonds[5].

Generally,sweet potato is cooked,either by boiling,steaming or roasting, before consumption.Such thermal processing can cause impairment of the functional compounds of sweet potato.There have been reports of negative correlation between heat treatments(steaming and baking)and some bioactive substances, such as anthocyanins.Carvalho et al.[6] reported a dramatic decrease in both total carotenoid andβ-carotene contents of sweet cassava after cooking.

Although the benefit of sweet potato are widely established through numerous studies, there is limited information about how their functional components (e.g., phenolic substances,carotenoids), and antioxidant capacities are affected by different home-cooking ways.In the present study, we investigated the changes in total phenolic content(TPC),monomeric anthocyanin content (MAC) and total carotenoid content (TCC), as well as antioxidant capacities(DPPH and FRAP)of fie species of sweet potato after three types of ordinary thermal processing,such as boiling, steaming and roasting with a view to understand detail changes in the functional compositions of different chemical constituents.

2.Materials and methods

2.1.Chemicals and reagents

Folin–Ciocalteu reagent, 2-diohenyl-1-picryhydrazyl(DPPH), and 2,4,6-tri(2-pyridyl)-s-triazine (TPTZ) were purchased from Shanghai Yuanye Biological Technology Co.,Ltd(Shanghai, China).The 6-hydroxy-2,5,7,8-tetramethlchroman-2-carboxylic acid (Trolox) was obtained from Sigma–Aldrich Co.(Shanghai, China).Absolute ethanol was obtained from Tianjin Fuyu Fine Chemical Co.,Ltd.Other chemical reagents were supplied by Tianjin Damao Chemical Reagent Co., Ltd.(Tianjin, China).All chemicals were analytical grade unless specially mentioned.

2.2.Sweet potato samples

Five species of sweet pot atoes were sampled,including light purple,yellow,white,orange,and deep purple(shown in Fig.1 and Table 1).All of them were cultivated in Guilin Agricultural Research Institute in Guilin of Guangxi Province (China) in 2013.

2.3.Cooking approaches and cooking time

Three thermal processes were performed for sweet potatoes with fie species(light purple,yellow,white,orange and deep purple).All sweet potato samples were not peeled before and during heat treatment.After cooking,they were peeled.Boiling,steaming and roasting processes imitated cooking methods at home as far as possible.

For the boiling treatment, about 130 g of sweet potato was added to 650 mL tap water(sample/water–1:5,w/v).The water was heated to its boiling point before being added to the different kinds of sweet potatoes, and then cooked in the electric hot plate cooker for about 30 min.For the steaming process,approximately 130 g sweet potato was placed in a steam cooker,in which 1 L tap water was filled Steaming was conducted for about 30 min after the water generated steam.For the roasting process,an electric oven(Galanz,China)was applied to preheat to 230°C.After that,about 130 g of sweet potato was placed in the oven and roasted for 30 min at 230°C.

All samples (including non-cooked and cooked) were lyophilized by freeze-dryer (Labconco Corporation, Kansas City,MO,U.S.A.),and then sweet potato samples were ground by a grinder(Beijing Zhongxing Weiye Instrument Co.,LTD).Ultimately,sweet potato powder were passed through 80#mesh and stored at 4°C in a refrigerator(Dukers)for further studies.

2.4.Color measurement

The color attributes of sweet potato were measured by Khroma Meter Difference with Colorimeter CR-410 (Konica Minolta,Japan)according to the method of Wang et al.[7]with slight modifications The color was expressed inL*a*b*,where theL*represents lightness (L*=0 yields black andL*=100 denotes white), thea*expresses red (+) or green (?), and theb*indicates yellow (+) or blue (?).L*,a*andb*parameters were measured against a white calibration plate and were directly obtained from the apparatus.ΔEis directly displayed and calculated in this colorimeter by the following formula:

2.5.Extraction of samples

Accurately, 0.5 g of each ground dry sample was weighed,and extracted with 5 mL of acidic 70% acetone (acetone/water/acetic acid – 70:29.5:0.5, v/v/v) in a set of capped centrifuge tubes by shaking on an orbital shaker at ambient temperature for 3 h and setting in the dark for 12 h.Then,the extracts were centrifuged at 5000 r per minute for 10 min,and the supernatants were collected.Residues were extracted with 5 mL of extraction solvent for two more times.Three extracts were combined and stored at 4°C in the dark.The extraction of every sample was conducted in triplicate.The fina volume of each extract was recorded.

2.6.Determination of total phenolic content(TPC)

The total phenolic content was evaluated according to the method of Xu and Chang [8] with no modifications Briefl ,the absorbance was measured by UV-Visible spectrophotometer(TI-1901,Beijing Purkinje General Instrument Co.,Ltd,China)at 765 nm against blank-distilled water.The total phenolic content(TPC)was expressed as milligrams of gallic acid equivalents(mg of GAE/g sweet potato).

2.7.Determination of monomeric anthocyanin content(MAC)

Monomeric anthocyanin content (MAC) was based on a pH differential method described previously by Lee et al.[9]with no modifications The MAC was calculated in the form of w/w%of total anthocyanin in the samples using the molecular weight for cyanidin-3-glucoside (449.2 g/mol) and its extinction coefficien (26 900 L cm?1mol?1).MAC was expressed as cyanidin-3-glucoside equivalents because of its historical usage for similar assays and its wide commercial availability[9].

Fig.1.Sweet potatoes–f ve varieties(1:Gui 04-53;2:Gui 09-75;3:Guishu#2;4:Guineng 05-6;5:Guijingshu 09-7).

2.8.Determination of total carotenoid content(TCC)

The method of extraction was based on that of Sanusi and Adebiyi [4] with slight modifications Briefl , a 0.5 g sweet potato sample in triplicates was extracted with 5 mL of ethanolic butylated hydroxyl toluene (ethanol/BHT – 100:1, v/w)for isolation and the release of carotenoids.Then,it was mixed completely,and placed in a water bath at 85°C for 5 min.After that, 0.5 mL of 80% KOH was added for saponificatio and properly vortexed before putting it back to 85°C water bath for 10 min.The mixture was cooled down in an ice-water bath and was added to 3 mL of cold deionized water.n-Hexane (3 mL)was mixed with the mixture before centrifugation at 7500 r per minute for 5 min for the separation of two layers.The upper layer with yellow was transferred and collected.This procedure was repeated four times until the upper layers became colorless[6].Therefore, a total of 12 mL of hexane was put into each centrifuge tube and the fina volume of each tube was recorded.The samples were read at the wavelengths of both 450 nm and 503 nm against the hexane as the blank[10].The concentration of total carotenoid in μg/mL was expressed by the following equation:total-carotene=4.642×A450?3.091×A503[10].

2.9.Determination of DPPH free radical scavenging activity(DPPH)

DPPH free radical scavenging capacity was determined by a colorimetric method with Trolox as an external standard[8].The absorbance was measured by the UV-Visible spectrophotometer at 517 nm against a negative control.DPPH scavenging rate(%)asYvalue of the standard curve was calculated according to the equation=[(Ablank?Asample)/Ablank]×100%.The results were shown as the equivalent concentration of Trolox(μmole of TE/g sweet potato), through the calibration curve of Trolox with a linearity range from 20 μmol/L to 1.0 mmol/L(r＞0.99).

2.10.Ferric reducing antioxidant power assay(FRAP)

According to the method of Xu and Chang [8] with slight changes, the FRAP assay was applied to determine the antioxidative competence of sweet potato.The absorbance was measured by the UV-Visible spectrophotometer at 593 nm against blank.A calibration standard curve of FeSO4?7H2O solution was used to calculate the FRAP value with millimolesof Fe2+equivalents(FE)per gram of sweet potato(i.e.mmol of FE/g sweet potato).

Table 1 Physical characteristics and origin information of fie varieties of sweet potatoes.

Fig.2.Color of raw and processed sweet potato powder(1:Guishu#2,white;2:Gui 09-75,yellow color;3:Gui 04-53,light purple;4:Guineng 05-6,orange;5:Guijingshu 09-7,deep purple).

2.11.Statistical analysis

All samples from different processes were performed on the basis of duplicate or triplicate.The data were evaluated as mean±standard deviation.The statistically significan differences (p＜0.05) were conducted by analysis of variance(ANOVA)applying SPSS package.

3.Results

3.1.Color values of raw and processed sweet potatoes

The color values of the non-processed and processed sweet potatoes from fie varieties were presented in Fig.2 and Table 2.CIEL*a*b*color system (CIELAB) is adopted by this colorimeter,and it is based on opponent-colors theory.TheL*a*b*model is a three-dimensional model(shown in Fig.3).The color was expressed inL*a*b*, where theL*represents lightness(L*=0 yields black andL*=100 denotes white), thea*indicates the position between red (+) and green (?), and theb*indicates the position between yellow(+)and blue(?).The last column in Table 2, deltaE(ΔE) represents color difference,which can be define as Euclidean distance.

Fig.3.The L*a*b* model from CIELAB color space (Source: HunterLab,Reston,VA).

Obviously,raw sweet potato samples with light purple,yellow,white,orange and deep purple differed in their color values.In addition, the fie sweet potato samples treated with three home-cooking methods(boiling,steaming and roasting)demonstrated distinctly(p＜0.05)diverse color values.

3.2.Chemical compositions of sweet potatoes

3.2.1.Total phenolic content(TPC)

Total phenolic content(TPC)of sweet potato before and after home-cooking processes were exhibited in Table 3.The extracts

from different types of raw sweet potato samples(light purple,yellow, white, orange, and deep purple) exhibited significan differences(p＜0.05)in their TPC values,except fresh white and orange samples.In Table 3,the highest TPC was shown in fresh deep purple sweet potato (Guijingshu 09-7) at 16.8±0.27 mg GAE/g,followed by raw Gui 04-53(light purple,6.31±0.11 mg GAE/g),Guineng 05-6(orange color,6.17±0.12 mg GAE/g),Guishu#2(white color,4.70±0.10 mg GAE/g)and Gui 09-75(yellow,4.15±0.03 mg GAE/g).

Table 2 Effects of different thermal treatments on color values of sweet potatoes.

Table 3 Effects of different thermal treatments on phenolic profile of sweet potatoes.

Compared to their fresh sweet potatoes, the TPC of all the processed samples that underwent the thermal treatments(boiling, steaming and roasting) declined distinctly.Among three thermal processed samples,the highest TPC values were retained in steamed sweet potato samples for all fie varieties.In contrast, the least phenolic content was reserved in boiled samples.For instance, about Guijingshu 09-7, the TPC of 15.8±0.44, 12.9±0.25 and 11.5±0.29 mg GAE/g were illustrated in steamed, roasted and boiled deep purple sweet potato samples, respectively.Another example was presented in Table 3, for yellow species, the TPC of boiled one was 2.28±0.12 mg GAE/g, the roasted one was 3.01±0.18, and the steamed one was 3.91±0.17 mg GAE/g.

Additionally, the TPC in cooking water for boiling sweet potato samples were determined.The TPC values in cooking water for the light purple one was 11.4±0.28 mg GAE/g, it was 11.1±0.12 mg GAE/g for the yellow one,11.7±0.29 mg GAE/g for the white one,13.4±0.23 mg GAE/g for the orange one,and 24.9±0.42 mg GAE/g for the deep purple one.

3.2.2.Monomeric anthocyanin content(MAC)

Monomeric anthocyanin contents(MAC)of the extracts from non-processed and processed sweet potatoes were presented in Table 3.The sweet potato extracts from different thermal processing (boiling, steaming and roasting) differed signifi cantly(p＜0.05)in their MAC.Meanwhile,the sweet potatoes with fie different colors exhibited various MAC values.Among fie varieties of raw sweet potatoes, the dark purple sample(Guijingshu 09-7)indicated to have the maximum anthocyanin content(15.7±1.30 μg CyE/g),and the second highest one was the light purple sample(Gui 04-53)with 2.47±0.36 μg CyE/g.On the contrary,the white sweet potato(Guishu#2)without any heat processing had the lowest MAC,at 1.23±0.07 μg CyE/g.

There was a decreasing trend of anthocyanin content in sweet potato samples after different thermal treatments.For example,forthe deep purplespecies,compared with the raw one,the MAC values of the roasted,steamed and boiled ones were 12.1±0.80,10.4±0.99 and 9.24±0.26 μg CyE/g.Another example was for the light sweet purple sweet potato, the MACs declined to 1.80±0.21 μg CyE/g in the roasted one,0.50±0.06 μg CyE/g in the steamed one and 0.50±0.02 μg CyE/g in the boiled one(shown in Table 2).In addition,except for two purple species,MACs were not detectable in the rest of the three species of sweet potato samples (yellow, white and orange) processed by steaming and boiling.However, the MACs were shown in roasted sweet potato samples (Gui 09-75 in the yellow sample)at 0.95±0.07 μg CyE/g;Guishu#2 in the white sample at 0.52±0.06 μg CyE/g;and Guineng 05-6 in the orange sample at 1.72±0.15 μg CyE/g).

In Table 3, MACs were also detected in the boiling water among all fie species.For the two purple sweet potatoes,the MAC of boiling water for the deep-purple one was 13.8±0.31 μg CyE/g, and MAC values of boiling water for the light-purple one was 0.57±0.04 μg CyE/g.The MAC with 0.20±0.03 μg CyE/g was found in boiled orange sweet potato Guineng 05-6, which had 2.19±0.19 μg CyE/g in the fresh sample acting as the third maximum MAC.

3.2.3.Total carotenoid content(TCC)

The total carotenoid contents (TCC) of fresh and cooked sweet potatoes from fie varieties were shown in Fig.4.The extracts from different varieties of raw sweet potato samples and processed samples by different cooking approaches differed significantl (p＜0.05) in their total carotenoid contents.In Fig.4, it can be observed clearly that the TCC of both raw Gui 09-75 (the yellow sample) and raw Guineng 05-6(the orange sample) were much more than those of the nonprocessed Gui 04-53(the light-purple sample),Guishu#2(the white sample) and Guijingshu 09-7 (the deep-purple sample).Among fie kinds of raw sweet potato samples, the raw orange one (Guineng 05-6) had the highest carotenoid content(157.9±1.75 μg/g).The TCC value of boiled orange sweet potato was 137.9±1.28 μg/g,the TCC value of steamed orange sweet potato was 127.0±3.01 μg/g, and the TCC value of roasted sweet potato sample was 95.4±1.88 μg/g.Just like the orange samples,the TCC values of yellow sweet potato samples(Gui 09-75)had a downward tendency upon thermal processing.The TCC of the raw yellow one were 75.4±2.95 μg/g,while the TCC of the processed yellow sweet potatoes were 67.4±3.18 in boiled one,61.9±1.94 in steamed one,and 60.6±0.66 μg/g in roasted one,respectively.

However, among other three kinds of sweet potato samples (the light and dark purple, and the white samples), there were nearly no significan (p＜0.05) differences.The light purple sweet potato (Gui 04-53) had 5.19±0.04 μg/g in the fresh one,4.50±0.07 μg/g in the boiled one,3.90±0.03 μg/g in the steamed one, and 3.62±0.05 μg/g in the roasted one.In the white sweet potato (Guishu #2), the total carotenoid contents was 4.46±0.19 μg/g raw sample, 4.28±0.09 μg/g boiled one,4.13±0.06 μg/g steamed one and 3.84±0.04 μg/g roasted one.As for the deep purple sweet potatoes(Guijingshu 09-7), they had the lowest carotenoid content (the raw sample with 2.85±0.19 μg/g,the boiled one with 1.99±0.14 μg/g,the steamed one with 1.81±0.13 μg/g, and the roasted one with 1.74±0.12 μg/g).

3.3.Antioxidant activities of sweet potatoes

3.3.1.DPPH free radical scavenging activity

Fig.4.Effect of different thermal treatments on total carotenoid content of sweet potatoes(B:boiled sweet potato;S:steamed sweet potato;R:roasted sweet potato.1:Gui 04-53,light purple color;2:Gui 09-75,yellow color;3:Guishu#2,white color;4:Guineng 05-6,orange color;5:Guijingshu 09-7,deep purple).

Table 4 Effects of different thermal treatments on antioxidant activities of sweet potatoes.

DPPH free radical scavenging capacities (DPPH) of the non-processed and processed sweet potatoes were presented in Table 4.Obviously, the raw samples (light purple, yellow,white, orange and deep purple) differed in their DPPH values.In addition, the different kinds of sweet potato processed with three home-cooking methods(boiling,steaming and roasting)preformed distinctly(p＜0.05)with diverse DPPH values.Without thermal processing, for various sweet potato species,the dark-purple one (Guijingshu 09-7) had the highest DPPH value with 27.8±0.34 μmol TE/g, yet the lowest two DPPH values were demonstrated in Gui 09-75 (the yellow specie) at 23.4±0.69 μmol TE/g and Guishu #2 (the white specie) at 23.3±0.52 μmol TE/g.

All thermal treatments would decrease the antioxidant capacity of each sweet potato sample(light and deep purple,yellow,white and orange).For the two varieties of purple sweet potato,steaming had the least influenc on the DPPH values, but boiling gave the most impact on those values, such as, for the light purple one, 24.4±0.92 μmol TE/g in the steamed sample, 24.2±0.42 μmol TE/g in the roasted sample and 23.4±0.26 μmol TE/g in the boiled sample.For the white Guishu #2, steaming (22.7±0.27 μmol TE/g) also had minimal impact on the DPPH value,and boiling(16.7±0.48 μmol TE/g) ranked as the second cooking method for the reservation of the antioxidant capacity.Unlike the purple sweet potato,for the yellow and orange species, boiling had the least effect on the antioxidant capacity, yet the most influenc was shown in the roasted sweet potato.For example, the orange sweet potato, DPPH value was 23.6±0.96 μmol TE/g boiled one,15.2±0.08 μmol TE/g steamed sample,and 10.9±0.46 μmol TE/g roasted sweet potato.

3.3.2.Ferric reducing antioxidant power(FRAP)

The FRAP values of the antioxidant extracts from sweet potatoes were demonstrated in Table 4.These extracts from fie different varieties of sweet potatoes processed with different cooking methods showed significan (p＜0.05) differences in their FRAP values.Meanwhile, the FRAP value of the boiling water of each sweet potato sample was also determined,which differed significantl (p＜0.05).The highest antioxidant capacity(274.0±6.56 mmol FE/g)was determined in raw Guijingshu 09-7 (the deep purple sample), whereas the lowest FRAP value was shown in non-processed yellow Gui 09-75 with 52.9±0.10 mmol FE/g.The light purple sweet potato exhibited the second highest FRAP value (75.2±1.77 mmol FE/g),followed by 70.2±0.47 mmol FE/g orange sweet potato.

In Table 4,thermal processing(boiling,steaming and roasting) had different influence on each sweet potato sample;however, it can be seen clearly that the FRAP of all sweet potato species declined after thermal treatments.For instance,as compared to the raw deep-purple sweet potato,the FRAP in the boiled (153.7±5.96), steamed (224.4±6.47) and roasted ones (257.6±7.27 mmol FE/g) presented decline trends.For white sweet potato – Guishu #2, the antioxidant capacity in terms of FRAP was 61.9±1.26 mmol FE/g, yet after thermal processing,the steamed one reserved FRAPat51.8±0.93 mmol FE/g, followed by the boiled sample with 37.1±3.12 mmol FE/g, and then the roasted sample with 17.5±0.41 mmol FE/g.For the orange sweet potato samples, the FRAP values of boiled, steamed and roasted were 55.1±0.96 mmol FE/g,37.9±0.77 mmol FE/g and 28.6±0.65 mmol FE/g, respectively.

4.Discussion

4.1.Color value of sweet potatoes

Different kinds of sweet potatoes exhibited various color values, expressingL*a*b*, as well as the visual differences.The reason may be that in sweet potato,there are several types of pigments,such as anthocyanidins which present red or purple color,β-carotene which appears dark green, yellow or orange color,and fl vonoids that display yellow.In other words,purple sweet potato species mainly contain anthocyanins; however primarily yellow and orange sweet potato species possess carotenoids(such asβ-carotene).Additionally,in Table 2,the color values of sweet potato samples were significantl affected by thermal processing,expressingL*a*b*,it can be seen directly that under three thermal treatments,the color of sweet potato from fie varieties had been changed dissimilarly.Under the same cooking time(30 min)but not the same temperature,the degree of degradation of each pigment was different, which can lead to different color values.Meanwhile, the physical and chemical properties of these pigments are completely different.For example,anthocyanins belong to the group of water-soluble pigments,yet carotenoids,which are isoprenoid compounds,cannot dissolve in water but can dissolve in organic solvents.Therefore, thermal treatments would change the color value of each species,and based on processing conditions,these changes were not similar.

4.2.Effects of thermal processing on chemical compositions of sweet potatoes

There were two kinds of substances (phenolics and carotenoids) investigated in this study.Phenolic substances(organic compounds with the presence of at least one aromatic ring hydroxyl-substituted)are secondary metabolites of plants,with various functions,such as protection against pathogens and predators,mechanical support,attraction of pollinating animals,and prevention of ultraviolet radiation [11].The total phenolic substances in the potatoes in current study were determined colorimetrically using Folin–Ciocalteu phenol reagent[8].The amount of anthocyanin present in a material was determined by measuring the color change in absorbance at two different pH values, based on a reversible structural transformation of anthocyanins as a function of pH [9].Carotenoids are frequently based on a C40tetraterpenoid structure with a centrally located,extended conjugated double-bond system[5].Just like anthocyanins,this type of bioactive substance is a large group of natural pigments occurring in plants,algae and many microbes[5].

After analysis of TPC and MAC, it can be observed distinctly that the contents of phenolic substances were rich in purple sweet potato.In other words, white, yellow and orange species had the lower contents of this kind of functional composition as compared to the purple one.When it comes to the determination of total carotenoid content,it can be seen clearly that the carotenoid contents of yellow and orange sweet potato samples were higher as compared to the purple and white ones(shown in Fig.4).All these results reflec an identical trend with the results from the color measurement;meanwhile,it can explain the reasons why different species present different colors.Although three thermal processing (boiling, steaming and roasting)exhibited a negative effect on phenolic and carotenoid contents, the degree of influenc was different.First of all,talking about total phenolic content, in Table 3, among three thermal treatments, steaming is the best way for sweet potato to keep phenolic contents,while boiling lost the most of TPC.Then, about monomeric anthocyanin content, roasted sweet potato can reserve the most of anthocyanin content(MAC),followed by the steamed sweet potato.Therefore, boiling as heat approach for sweet potato retained least MAC.For example,for fresh deep-purple sweet potato species which contained the highest MAC with 15.7±1.30 μg CyE/g; its MAC decreased to 12.1±0.80 μg CyE/g after roasting; its MAC declined to 9.24±0.26 μg CyE/g(shown in Table 3)after boiling.Kim et al.[12]have reported that the Korean fresh purple sweet potato contained total anthocyanin at 1342±65.1 mg/100 g dried weight),baked sweet potato kept 1086±42.2 mg/100 g DW, yet the steamed one only preserved 751±22.8 mg/100 g DW.Therefore, it was found that anthocyanins presented in more stable forms in roasted sweet potato.What is more, for the determination of total carotenoid content, the boiled sweet potato had greater TCC than the sweet potato that underwent the other two thermal processes, the roasted one had the lowest carotenoid contents.

Different thermal processing had totally different influence on sweet potatoes with different colors.The differences may be from diverse sweet potato species with their own special physical and chemical features.Phenolic substances are easily dissolved in water, but carotenoids are not.At the same time,phenolic substances had thermal instability.Therefore,boiling had a larger impact on TPC and MAC.Additionally,the temperature for roasting was 230°C,and the temperature of steaming and boiling was around 100°C.Therefore,based on conditions,roasting had a larger influenc on TCC.

4.3.Effects of thermal processing on antioxidant activities of sweet potatoes

According to previous literatures, sweet potato has plenty of phenolic substances and carotenoid contents–both of them contribute to radical scavenging performance.Therefore, it is crucial to determine the radical scavenging effect of antioxidants in sweet potato.The anti-oxidative efficien y of components is indicated by the degree of discoloration of the violet solution containing DPPH which is a free radical and is stable at room temperature [8].The specificit and sensitivity of one method does not provide the complete finding of all antioxidants in the extracts [13].Hence, a combination of two tests could supply a more reliable assessment of the antioxidant properties of fie varieties of sweet potato.The principle of FRAP assay states that, with reductant (antioxidants) at low pH, ferric tripyridyltriazine (Fe (III)-TPTZ) is reduced to the ferrous tripyridyltriazine (Fe (II)-TPTZ) that has an intensive blue color and can be detected at the wavelength of 593 nm[8,14].

After heat treatment, antioxidant capacities in all species(light purple,yellow,white,orange and deep purple)declined,and there were the same tendency of the results in DPPH and FRAP.For the two varieties of purple sweet potato,steaming had the least influenc on the DPPH values,but boiling exhibited the most impact on those values, such as, for the dark purple one, 26.0±1.90 μmol TE/g in the steamed sample, 24.0±1.31 μmol TE/g in the roasted sample and 22.1±0.53 μmol TE/g in the boiled sample.For Guishu#2(the white sample),steaming(22.7±0.27 μmol TE/g)also had minimal impact on the DPPH value,and boiling(16.7±0.48 μmol TE/g)ranked as the second treatment for the reservation of the antioxidant capacity.For the yellow and orange species,boiling as the treating approach had the least effect on the antioxidant capacity, while the most influenc was shown in the roasted sweet potato.For instance,in the raw yellow sweet potato,the DPPH scavenging capacity were 20.7±0.24,15.7±0.29,and 9.80±0.10 μmol TE/g in boiled, steamed and roasted ones,respectively.The FRAP values also showed a similar trend to the DPPH values for sweet potato samples(shown in Table 4).

All these results can conform to the consequences from total phenolic content,monomeric anthocyanin content and total carotenoid content.Thermal processing exhibited a moderate negative effect on DPPH scavenging capacity and FRAP of sweet potato samples.

5.Conclusions

The dominating pigments were carotenoids in yellow and orange sweet potato species, and anthocyanins can act as the principal pigment for purple sweet potato.Thermal treatment could reduce nutritional values of sweet potatoes by chemical degradation.Heat treatments can decrease bioactive compounds(phenolics and carotenoids) significantl (p＜0.05).Boiling as the heat method may retain much more carotenoids; steaming may preserve greater phenolic substances(except the group of anthocyanin contents), and roasting may keep higher anthocyanins.Therefore,to maintain diverse bioavailability of active components,no cooking method can simultaneously provide a higher retention of both carotenoids and phenolics in different species of sweet potatoes.At present,there is no cooking method that can protect and retain all chemical compositions and antioxidant capacities.It is difficul to digest raw sweet potatoes and absorb their nutrients due to the existence of abundant starch.Based on current dietary habit and cooking practice,consumers need to cook sweet potato before eating.Therefore,cooking has to damage the content of bioactive substances and their antioxidant capacities.Consumers may obtain bioactive substances and health promotion effects from sweet potatoes by choosing different means of cooking.

Conflict of interest

None.

Compliance with ethics requirements

This article does not contain any studies with human or animalsubjects.

Acknowledgements

This research was jointly supported by two research grants(UICRG 201327 and UICRG 201402) from Beijing Normal University-Hong Kong Baptist University United International College,China.The authors would like to thank Guilin Agricultural Research Institute of Guangxi Province for providing the sweet potato samples.