2016 Volume 22 Issue 2 Pages 219-226
Ten pigmented potato cultivars native to China were investigated for an evaluation of differences in total anthocyanin content (TAC), composition of individual anthocyanidins, total phenolic content (TPC) and antioxidant activity (AA) between peel and flesh of the tubers. On average, TAC, TPC and AA in the peel were 15.34 times, 7.28 times and 5.75 times higher than in the flesh, respectively. High positive correlations between TAC, TPC and AA were observed. Types and contents of anthocyanidins in the peel were more than in the corresponding flesh, but they had the same dominant anthocyanidins. Results showed peel of pigmented potato tuber can be used as an important source material in production of natural pigments and natural antioxidants.
Potato (Solanum tuberosum L.) is one of the most important food crops for human consumption with world annual production of 472 million tones on 2013 according to the Food and Agriculture Organization. Potato becomes a cornerstone in human nutrition in which nutritional quality was well established, documented and considered as a source for many nutrients such as carbohydrate, protein, vitamins, mineral elements and so on (Al-Weshahy and Rao, 2012).
While fresh potato consumption is decreasing in many countries, more potatoes are processed into value-added products to meet the demand especially from the fast food and convenience food industries (Schieber and Saldana, 2009). Therefore, potato peels, as by-products from potato processing, represent a serious disposal problem to industry as well as environment and sustainability (Schieber and Saldana, 2009). However, potato peels contain lots of nutritional and functional compounds such as dietary fibers (Lazarov and Werman, 1996), antioxidant (Singh and Rajini, 2004), and polyphenols (Albishi et al., 2013), which could be used in foods and non-food applications.
Recently, people become more interested in anthocyanins since they have shown antioxidant activity and have potential health beneficial effects on various disorders like cancer, aging, neurological diseases and so on (Burgos et al., 2013), and pay much attention to pigmented potatoes. There have been a great number of studies on pigmented potato flesh. Red-fleshed potatoes contain acylated glucosides of pelargonidin and the purple-flesh potatoes contain acylated glucosides of malvidin, petunidin, peonidin, and delphinine (Brown, 2005; Lachman et al., 2012).
Though previous studies have revealed that the peel of pigmented potato tubers contained considerably higher levels of anthocyanins than the flesh (Jansen and Flamme, 2006; Lewis, 1996; Lewis et al., 1999), there have been a few studies about the anthocyanin compositions of potato peels. And also, related researches on pigmented potato native to China are very few. The objective of present experiment was to investigate and estimate the differences in total anthocyanin content (TAC), total phenolic content (TPC), antioxidant activity (AA) and anthocyanidin composition between the potato peel and its corresponding flesh. These results will help to develop the economic value of pigmented potato, especially to improve the utilization of potato peel.
Plant materials Ten pigmented potato cultivars (Table 1) native to China were chosen and grown at Yunnan Academy of Agricultural Sciences (Kunming, Yunnan Province, China) in 2013. Potatoes were harvested at full maturity and immediately transported to the laboratory. Tuber samples (five tubers each cultivars) were washed clean with tap water and dried by natural air. The peel (0.5–1 mm thick) and flesh were separated with a hand peeler, and a laboratory sample (0.5 g) consisted of randomly selected quarters of potato tubers (Ieri et al., 2011; Lachman et al., 2012).
Preparation of extraction The extraction was carried out following the procedure described by (Leite-Legatti et al., 2012), with minor modifications. Peel or flesh (0.5 g) of each cultivar was put into a test tube with 15 mL of extract solution (95% ethanol: 1.5 mol/L hydrochloric acid = 85:15, v/v), then mashed and crushed by ultrasonic treatment (45°C, 100 W) for 30 min, and then centrifuged at 5000×g for 5 min. The supernatant was collected and the precipitate was extracted again. Two supernatants were put together and stored at −20°C in the refrigerator for analysis.
Determination of TAC TAC was determined by the pH differential method described by (Albishi et al., 2013) with minor modifications. One milliliter of extraction was diluted with 9 mL of potassium chloride buffer, pH 1.0 and 9 mL of sodium acetate buffer, pH 4.5, separately. The diluted solutions were then left at room temperature for 1 h, and the absorbance was measured at 520 nm and 700 nm, respectively, against a blank cell filled with distilled water. Tests were conducted in triplicates. The results were expressed as mg of cyanidin-3-glucoside equivalents (CGE) per 100 g of fresh potato peel/flesh.
High performance liquid chromatography (HPLC) analysis of anthocyanidins Anthocyanins were hydrolyzed to study the composition of anthocyanidins. Two hundred microliter of concentrated hydrochloric acid and 600 µL of extracts were added in a screw-cap test tube, flushed with nitrogen and capped, then hydrolyzed at 90°C for 40 min, then cooled in an ice bath (Rodriguez-Saona et al., 1998). The hydrolysate was filtered through a 0.45 µm membrane filter for HPLC analysis.
HPLC analysis was carried out using an Agilent Technology 1200 Series liquid chromatography equipped with a VWD detector (Agilent-Technologies, Palo Alto, USA). The column was an Agilent Zorbax Eclipes XDB-C18 (4.6 mm×150 mm, 5 um). The mobile phase was 0.5% formic acid (A) and acetonitrile (B). The following multi-step linear gradient was applied: from 12% to 15% of B in 3 min, and 20 min to reach 25% B. Total time for the analysis was 23 min. Wavelength was set at 520 nm. The flow rate was 0.8 mL/min and oven temperature was 30°C. Six anthocyanidin standards (delphinidin, cyanidin, petunidin, pelargonidin, peonidin and malvidin) were purchased from PhytoLab (Germany).
Determination of TPC TPC was measured by Folin-Ciocalteu method described by Burgos et al. (2013) with minor modifications. The extract (0.1 mL), Folin-Ciocalteu reagent (2 N) (0.2 mL) and 0.7 mL of distilled water were added to a screw-cap test tube, and then mixed fully. Six minutes later, 2 mL of 15% Na2CO3 was added and then the mixture was allowed to stand for 60 min at 30°C. The absorbance was read at 765 nm. All samples were tested in triplicates. The results were expressed as mg of gallic acid equivalents per 100 g of fresh potato peel/flesh.
AA evaluation AA evaluation was determined by DPPH assay following the procedure described by Teow et al. (2007) and Li et al. (2012), with minor modifications. The extractwas evaporated to get dry powder on a rotary evaporator (40°C), then dissolved in an equal volume of ethanol since the results of DPPH assay would be affected by HCl and H2O (Dawidowicz et al., 2012). The re-dissolved solution (0.1 mL) was added to 3 mL of DPPH· (25 µg/mL) to initiate the reaction under dark conditions for 60 minutes to get a reaction equilibrium. The absorbance was determined at 517 nm. Ethanol was used as a blank. Analysis was done in triplicates for each sample. The antioxidant capacity was reported as mg of ascorbic acid equivalents per 100 gram of fresh potato peel/flesh (ascorbic acid equivalent antioxidant capacity, AEAC) (Lim et al., 2007).
Statistical analysis Variance was analyzed and differences between means were compared with Tukey's multiple comparison tests using SPSS 19.0 for windows. Pearson correlations were analyzed between various parameters. Column charts and scatter diagrams were drawn by Origin Pro 9.0. All data were expressed as mean ± SD from three determinations.
TAC analysis TAC of the peel, flesh and whole tuber of selected pigmented potato cultivars were shown in Table 2. TAC of the peel ranged from 59.67 to 293.57 mg/100 g, TAC of the flesh ranged from 0 to 56.11 mg/100 g, and that of the whole tuber was in the range of 1.34 to 63.32 mg/100 g. Average TAC of the peel (150.83 mg/100 g) was significantly higher (P < 0.01) than that of the flesh (9.80 mg/100 g). The former was about 15.34 times of the latter. TAC was significantly different (P < 0.01) among different cultivars. Cultivar Purple Cloud No.1 showed the highest TAC in the peel as well as in the flesh. Cultivar S05-603 had the lowest TAC in the peel. TAC in the flesh of Yunnan Potato 303, S03-2677, S03-2685 and S05-603 were too low to be detected by using the pH differential method. Pigmented potato peels are richer of anthocyanins than the flesh. Similar results were obtained by previous studies. Jansen and Flamme (2006) found the average anthocyanin contents in the peel were about 3.0 times higher than in the flesh and Albishi et al. (2013) found 10.69 times higher anthocyanins in the peel than in the flesh of purple potato. There were differences among the times obtained by different researches may be resulted from multiple factors, including variety, number of cultivar investigated, thickness of peel peeled and methods of anthocyanin extraction.
Note: “−” indicates the corresponding TAC value was below limit of quantitation. Means with same superscripts in the same column are not significantly different at the P ≤ 0.05 level.
Anthocyanidin compositions analysis Anthocyanin is glycosylated flavonoid and its aglycone form is called anthocyanidin. In present study, anthocyanidin compositions of the potato peel and flesh were investigated by HPLC method (Table 3, Table 4 and Fig. 1). Four cultivars including Purple Cloud No.1, Yunnan Potato 303, S03-2677 and S03-2685 contained six kinds of anthocyanidins in the peel, with petunidin being the highest composition (66.05%, 65.99%, 66.72% and 65.78%, respectively). Their dominant anthocyanidins were petunidin, peonidin and malvidin, collectively contributing 95.33%, 97.29%, 97.48% and 95.58%, respectively. Five anthocyanidins were observed in the peel of S03-2796, which was lack of pelargonidin. Its main anthocyanidin was petunidin and the dominant anthocyanidins were petunidin, peonidin and malvidin (96.97%). S05-603 and S06-1693 also had petunidin being the highest anthocyanidin, but their dominant anthocyanidins were petunidin and malvidin (97.48% and 97.83%, respectively). However, no cyanidin or pelargonidin was detected in S05-603. Three cultivars, Red Cloud No.1, Yunnan Potato 603 and S06-277, had their highest anthocyanidin of the peel being pelargonidin (88.10%, 92.30% and 91.51%, respectively), which was quite different from the other seven cultivars. And their dominant anthocyanidins were pelargonidin and peonidin (97.55%, 98.20% and 97.66%, respectively). The three cultivars contained no malvidin, evidently distinguished from the other seven cultivars. It was shown that the total anthocyanidin contents varied significantly (P < 0.01) in the peel of tubers among the ten pigmented potato cultivars. Delphinidin and peonidin were both detected in the tuber peels of the ten pigmented potato cultivars, which implicated these two anthocyanidins were widely distributed in the pigmented tuber peels.
Note: “−” means no anthocyanidin was detected. Numbers in brackets represent the proportion of individual anthocyanidin in total anthocyanidins content in each cultivar. Means with same superscripts in the same column are not significantly different at the P ≤ 0.05 level.
Note: “–” means no anthocyanidin was detected. Numbers in brackets represent the proportion of individual anthocyanidin in total anthocyanidins content in each cultivar. Means with same superscripts in the same column are not significantly different at the P ≤ 0.05 level.
Chromatograms of six anthocyanidins standards (a) and the anthocyanidins in the peel and flesh of Purple Cloud No.1 and Red Cloud No.1 at 520 nm. (b): the peel of Purple Cloud No.1, (c): the flesh of Purple Cloud No.1, (d): the peel of Red Cloud No.1, (e): the flesh of Red Cloud No.1
Compared with the peel, the compositions of anthocyanidins in the flesh appeared much fewer. Only the flesh Purple Cloud No.1 showed similar compositions of anthocyanidins to the peel. Flesh of Yunnan Potato 303, S03-2677, S03-2796 and S05-603 had only one anthocyanidin, i.e., petunidin. Also only one anthocyanidin, i.e., pelargonidin, was observed in the flesh of Yunnan Potato 603 and S06-277. Flesh of S03-2685 contained two anthocyanidins (petunidin, 77.46%; peonidin, 22.54%) and that of S06-1693 contained three anthocyanidins, with petunidin (89.50%) being the main anthocyanidin. Flesh of Red Cloud No.1 had four anthocyanidins and pelargonidin accounted for 82.00%.
Results showed that the concertration of anthocyanidins was higher, and the number of compositions was bigger in the peel than those in the flesh, which was completely different from previous studies. Rodriguez-Saona et al. (1998) found there were no qualitative difference in anthocyanin compositions between the peel and the flesh. Since the number of pigmented potato varieties they observed was only two and the anthocyanin compositions in the peel were not shown in the paper, their results appeared controversial. Petunidin was the predominant anthocyanidin of purple-colored potatoes and pelargonidin was the dominant anthocyanidin of the red-colored potatoes. Both red-skinned and red-fleshed potatoes contained no malvidin and little petunidin. And anthocyanidin compositions of the peel and flesh can also be proposed as fingerprints of pigmented potato cultivars.
TPC analysis TPC of the peel ranged from 364.79 to 864.79 mg/100 g (Table 1), TPC of the flesh ranged from 38.74 to 189.44 mg/100 g, and that of the whole tuber was in the range of 51.98 to 206.97 mg/100 g. Average TPC of the peel (648.65 mg/100 g) were significantly higher (P < 0.01) than in the flesh (89.12 mg/100 g), and the former was about 7.28 times of the latter. Similar studies were observed before Albishi et al. (2013), Lewis et al. (1999), and Rodriguez-Saona et al. (1998). Phenolic compounds usually accumulate in the peel of potato tubers (Mendel, 1997). Nearly 50% of phenolics were found in the peel and adjoining tissues of the tubers, and their concentration decreased towards the centre of the tuber (Al-Weshahy and Rao, 2009; Mendel, 1997).TPC varied significantly (P < 0.01) among different pigmented potato cultivars. The highest TPC was observed in the peel of Yunnan Potato 303 while the lowest TPC was found in the peel and flesh of S05-603. Purple Cloud No.1 had the highest TPC of the whole tuber.
AA analysis Antioxidant activity was expressed as ascorbic acid equivalent antioxidant capacity. AEAC of the tested potato varieties were shown in Table 2. It can be observed that AEAC in the peel ranged from 274.04 to 643.29 mg/100 g, while that of the flesh ranged from 50.42 to 138.75 mg/100 g. AEAC of peel was significantly higher (P < 0.01) than its corresponding flesh. The former was 5.75 times higher than the latter, averagely. In the peel, Yunnan Potato 303 showed the strongest antioxidant activity and S05-603 showed the weakest. In the flesh, Purple Cloud No.1 showed the strongest while S03-2677 showed the weakest. AEAC of the whole tuber ranged from 65.09 to 149.76 mg/100 g, varied significantly (P < 0.01) among the different cultivars. These results were in consistence with previous studies. Hale (2003) observed that peel of purple potatoes had higher antioxidant activity, possibly due to the presence of anthocyanins that serve as major contributors to antioxidant activity.
Correlation among TAC, TPC and AA Pearson's coefficients of correlation among TAC, TPC and AA were shown in Fig. 2. High positive correlation were observed between TAC and TPC (r = 0.885, P < 0.01), probably because anthocyanins belonged to polyphenolics. Strong positive correlation was also present between TAC and AEAC (r = 0.838, P < 0.01) and between TPC and AEAC (r = 0.989, P < 0.01). It has been observed that high total anthocyanin content and total phenolic content are closely associated with strong antioxidant activity (Reyes et al., 2005). The results demonstrated that anthocyanins and phenolics were mainly responsible for the antioxidant capacity in the pigmented potato tuber (Hamouz et al., 2011; Lachman et al., 2008, 2009; Reddivari et al., 2007; Takahata et al. 2011).
Correlation analysis between total anthocyanin content (TAC), total phenolic content (TPC) and antioxidant activity (AEAC) (Dates come from the peel, flesh and whole tuber, n = 30)
The results in the present study revealed that a big difference was present in total anthocyanin content, total phenolic content, composition of individual anthocyanidins and antioxidant activity, especially the composition of anthocyanidins, between the peel and flesh in the tubers of the 10 pigmented potato cultivars. Anthocyanin content, phenolic content and antioxidant activity were all significantly higher in the peel than in the flesh. As a by-product of pigmented potato processing industry, the peel can be used for production of natural pigments and natural antioxidants. It may increase the economic values of pigmented potatoes. The contribution rates of peel to the whole tuber in anthocyanin contents were higher than the flesh in the pigmented potatoes. This was consistent with previous researches. The contribution rates of peel in antioxidant activity were first reported in the present study. Number of compositions of anthocyanidin was obviously bigger, and the contents of individual anthocyanidins were higher in the peel than those in the flesh. These results are helpful for further studies on the mechanisms of synthesis and accumulation of anthocyanins in pigmented potatoes.
Acknowledgements This study was supported by the fund for China Agriculture Research System (CARS-10-ES17).