Polyphenolic Content and Color of Seedless and Seeded Shade Dried Chinese Raisins

Dongyan Shao; Lijuan Zhang; Shuangkui Du; Wallace Yokoyama; Junling Shi; Nan Li; Jixun Wang

doi:10.3136/fstr.22.359

Abstract

29 grape cultivars, of seeded and seedless raisins from black, reddish-brown and yellowish-green grape varieties were examined to determine the relationship between grape color, seeded status and raisin phenolic content. The raisins were analyzed for phenolic profile, total phenolic (TP) and flavonoid contents (TF), and antioxidant activities (DPPH, ABTS and FRAP). We found that despite the perception that dark colored raisins, contain higher levels of polyphenolic antioxidants, only seedless raisins from black grapes were higher in TP. Black raisins exhibited significant higher values of TP and FRAP activity than reddish-brown and yellowish-green raisins. The values of TP, TF, DPPH, ABTS, and FRAP capacities for seeded raisins were higher than those of seedless samples. Raisins could be classified into three groups and some raisins differing in seed-bearing status or color were grouped together, indicating the antioxidant properties of raisins were not only determined by raisin color, but also by their seed-bearing status.

Introduction

China is the world's third largest producer of raisins after the U.S. and Turkey. Most grapes are grown in Xinjiang province, China's largest province, spanning an area the combined size of France, Spain, Italy, and Germany or the six western states of the USA. The large geographical area encompasses diverse climatic zones requiring grapes varieties that are adapted to these zones. Xinjiang Province is the largest producer of green raisins (Zhang et al. 2011). Green raisins are highly preferred by Chinese consumers over imports even though they command a higher price because of their flavor and appearance.

Raisins rank the highest in concentration of total phenolic compounds and antioxidant activities among solid fruit products (Williamson and Carughi 2010; Karakaya 2001). Thompson seedless is the variety most widely grown in the world including China for both dark and green raisin production, however, other grapes such as Muscat or black grapes such as black Corinth are also grown for specialty markets. Darker colored grapes and raisins are often perceived to have higher levels of “antioxidants” and perceived to be healthier. The interest in health benefits related to polyphenolic compounds have resulted in studies of the polyphenolic content of grape varieties. Breksa et al. (2010) analyzed the polyphenol content of white grapes from six commercial U.S. varieties and ten potential selections for raisin production (Breksa III et al. 2010). The composition of the fresh grapes, but not raisins were analyzed, cafteric acid also called monocaffeyltartaric acid, a phenolic acid and not a polyphenol compound, predominates in fresh grapes but is typically absent in raisins. Baiano and Terracone (2011) analyzed the polyphenol content of peels, seeds and juice of fresh grapes: four white and three red/black varieties. In seeded varieties the content of total polyphenols was 3 – 5 times higher in seeds than in skins. The red/black grapes tended to have higher total polyphenol content in their skins than the white varieties. The polyphenol content of some commercial raisin grape varieties grown in Xinjiang province has been reported (Meng et al. 2011). The varieties analyzed included two green grape and seven reddish-brown or black varieties including two seeded varieties. In contrast to the Baiano study, the seeded varieties did not contain higher amounts of total polyphenol content (TP) than the seedless varieties. The range of TP of raisins from the two white grapes ranged from 3.20 – 4.70 g gallic acid equivalents kg⁻¹ and 1.90 – 6.80 g gallic acid equivalents kg⁻¹ for the seven red varieties. They also reported that the drying process resulted in almost total loss of cafteric acid and the predominant phenolic acid was dihydroxybenzoic acid. This study shows the enormous differences in phenolic acid and polyphenolic content due to raisin variety and processing. They also showed a significant positive correlation between TP and the antioxidant assays: DPPH, CUPRAC and FRAP.

Previous studies have looked at the polyphenolic content and antioxidant activity of a small number of predominantly green grape varities and some using fresh grape samples rather than raisins. The present paper aims at determining the profiles and contents of phenolic compounds, and antioxidant capacities of raisin samples from 29 grape varieties differing in color and seed-bearing status from the major grape growing region of China. Correlations between color appearance, seed-bearing status and phenolic compound contents and antioxidant capacities were evaluated to provide a more comprehensive assessment of potential biological activities of this popular dried fruit food.

Materials and Metods

Reagents and raisins The chemicals 2,2′-azinobis (3-ethylbenzenothiazoline-6-sulfonic acid) (ABTS), 6-hydroxy-2,5,7,8-teramethylchroman-2-carboxylic acid (Trolox), Folin-Ciocalteu reagent, and 2,2-diphenyl-1-picrylhydrazyl (DPPH) were obtained from Sigma Chemical Co. (St. Louis, MO, USA). Ethanol, sodium carbonate, NaOH, and ascorbic acid were all analytical grade. Deionized water was used throughout the study.

Standards of gallic acid, protocatechuic acid, (+)-catechin, chlorogenic acid, vanillic acid, caffeic acid, epicatechin, ferulic acid, rutin and resveratrol were purchased from Shanghai Winherb Medical Science Co., Ltd (Shanghai, China). The purity of all standards was higher than 98%. Methanol and phosphoric acid (HPLC grade) were obtained from Sigma Chemical Co. (St. Louis, MO, USA).

Raisin samples were prepared from 29 varieties of grapes collected from August to September 2011, the peak season of raisin-making in Xinjiang province. Raisins were prepared from fresh grapes using air drying in a special drying house. The house is naturally ventilated and completely kept out of sunshine. The air temperature was not manually controlled during the whole drying season. Normally, it varied from 11°C in night to 40°C at noon in a day. The drying period lasted 30 to 45 days depending on the grape varieties and was stopped when the raisin weight kept stable for two days. All obtained raisin samples were stored at ambient temperature (∼ 25°C) until the analyses were performed about 6 mo later. Other detailed information about the color and seeded status on the raisins is presented in Table 1.

Table 1. Color, seeded characteristics and content of different polyphenolic compounds (µg/g) determined by HPLC in 29 raisin varieties.

No.	Name	Berry color	Seeded status	GA	PA	CAT	CHA	VA	CAA	EPI	FA	RUT	RES
1	Joanna Charnice	Black	Seedless	0.27	nd	3.47	0.31	7.39	nd	30.01	0.14	34.12	2.38
2	Jingkejing	Black	Seedless	0.39	0.98	2.26	0.03	8.31	nd	2.66	nd	427.16	nd
3	Nijiala	Black	Seeded	2.49	nd	40.15	nd	6.57	nd	69.82	0.14	4.45	0.15
4	Otilia	Black	Seeded	1.51	nd	9.51	nd	16.13	nd	72.76	0.37	9.69	0.86
5	Black Monukka	Black	Seedless	0.46	nd	1.32	nd	9.96	15.03	11.30	nd	23.97	0.13
6	Jingzijing	Black	Seeded	0.73	1.62	16.42	nd	10.79	nd	12.75	0.02	107.99	nd
7	SP528	Black	Seeded	0.69	2.01	2.33	nd	13.69	nd	6.64	nd	122.54	nd
8	Beauty Seedless	Black	Seedless	0.28	0.58	3.53	0.06	20.37	nd	45.69	nd	19.12	1.95
9	Zixiang Seedless	Black	Seedless	0.45	1.53	1.36	nd	15.47	nd	68.96	nd	127.25	nd
10	Crimson Seedless	Reddish-brown	Seedless	0.26	nd	1.24	nd	3.17	nd	nd	nd	33.52	nd
11	Flame Seedless	Reddish-brown	Seedless	0.36	0.96	3.73	nd	3.04	nd	1.12	nd	41.90	nd
12	Blush Seedless	Reddish-brown	Seedless	0.53	0.60	5.69	nd	2.51	nd	1.77	0.03	80.76	nd
13	SP5145	Reddish-brown	Seeded	0.71	2.19	1.54	0.11	11.27	nd	11.77	nd	240.65	nd
14	Yatomi Rosa	Reddish-brown	Seeded	8.39	0.47	1.87	nd	4.35	19.74	5.42	0.01	29.23	nd
15	98-6-164	Reddish-brown	Seeded	0.86	2.62	nd	nd	7.36	nd	53.54	nd	50.18	nd
16	SP577	Reddish-brown	Seeded	0.84	1.98	2.95	0.12	nd	6.41	1.15	nd	365.23	nd
17	SP9645	Reddish-brown	Seedless	3.70	1.16	3.73	nd	11.04	11.04	1.17	0.06	163.49	nd
18	Melissa Seedless	Yellowish-green	Seedless	0.25	0.57	5.04	0.05	1.07	nd	0.45	0.13	95.29	nd
19	Jingzaojing	Yellowish-green	Seedless	0.24	0.43	1.06	0.05	1.12	nd	0.33	nd	33.53	nd
20	Delight	Yellowish-green	Seeded	0.28	0.57	2.01	0.18	1.37	nd	nd	nd	203.60	nd
21	Victoria	Yellowish-green	Seeded	0.53	0.51	31.59	0.07	1.03	nd	nd	nd	57.63	nd
22	Aimaina	Yellowish-green	Seeded	0.29	0.54	1.38	0.41	1.78	nd	nd	nd	34.04	0.33
23	Centenial Seedless	Yellowish-green	Seedless	0.14	0.10	0.94	0.06	0.92	nd	nd	nd	39.94	nd
24	KuulMuul Xuulpay	Yellowish-green	Seedless	0.18	nd	125.21	0.07	1.37	nd	5.58	nd	50.54	0.16
25	Dali Thompson Seedless	Yellowish-green	Seedless	0.09	nd	0.72	0.04	1.03	nd	nd	nd	32.40	nd
26	Thompson Seedless	Yellowish-green	Seedless	0.09	0.22	1.96	nd	0.72	nd	nd	nd	76.00	nd
27	Superior Seedless	Yellowish-green	Seedless	0.14	0.65	3.56	nd	1.04	nd	0.17	nd	19.32	nd
28	Jiahuangputao	Yellowish-green	Seeded	1.33	nd	49.99	nd	0.67	nd	28.98	0.05	48.92	nd
29	Perlette	Yellowish-green	Seedless	0.22	0.52	4.15	0.17	1.74	nd	nd	nd	16.94	0.32

GA, gallic acid; PA, protocatechuic acid; CAT, (+)-catechin; CHA, chlorogenic acid;

VA, vanillic acid; CAA, caffeic acid; EPI, epicatechin; FA, ferulic acid; RUT, rutin;

RES, resveratrol; nd, not detectable.

SP528, SP5145, 98-6-164, SP577, SP9645 were grapes of new varieties.

Determination of color parameters The surface color of the raisin samples was measured using a colorimeter (CR-200, Minolta Camera Co., Ltd., Osaka, Japan) and expressed in terms of L*(lightness and darkness), a*(redness and greenness), and b*(yellowness and blueness) according to the method reported by Li et al. (2014) and Arias et al. (2000) with minor modifications. The samples were illuminated with diffused illumination and a 0° viewing angle, and was calibrated against a white reference plate (L* = 97.7; a* = −0.5; b* = 2.3). Color measurements were performed 5 times at 5 different locations of raisin samples with about 1.5 cm² of surface area, and each reported color parameter was the average of the five measurements. Total difference in color (ΔE*) was calculated by using the following formula:

where, ΔE* is the overall difference in color, a₀, b₀, and L₀ are all zero.

Alcoholic extraction of raisins The phenolic components were extracted from raisins according to a modified method reported by Zhao and Hall III (2007). Raisins, 2.0 g, were ground in 100 mL of 80% (v/v) ethanol and ultrasonicated at 20°C for 30 min (Unique-1400A ultrasonic bath, Shumei, Kunshan, China) at a power level of 250 W. The suspension was filtered through a 0.22 µm filter and the filtrate stored at 4°C prior to further analysis. All analytic assays were completed within 48 h and performed in triplicate.

HPLC analysis of the polyphenolic composition The alcoholic extracts of raisins were analyzed for ten polyphenols (gallic acid, protocatechuic acid, (+)-catechin, chlorogenic acid, vanillic acid, caffeic acid, epicatechin, ferulic acid, rutin and resveratrol) by HPLC according to the method reported by Zhao and Hall III (2007) and Suárez Valles et al. (1994) with slight modifications. The alcoholic extracts were concentrated in a rotary evaporator (Shanghai Shensheng Science and Technology Co. Ltd. Shanghai, China) at 38°C, and the residue was extracted with ethyl acetate. The ethyl acetate phases were collected and evaporated to dryness. The dried extract was redissolved in 50% (v/v) ethanol and filtered through a 0.45 µm filter before injection into a Waters 600E HPLC system (Waters, Milford, USA) equipped with a Waters 2487 UV detector and a Waters Symmetry C18 reversed-phase column (250 × 4.6 mm, 5 µm). The mobile phase consisted of solvent A (methanol) and solvent B [0.2 mol/L phosphoric acid (pH 2.6)] and eluted with the following gradient program: 0 – 15 min, 15 – 25% solvent A; 15 – 25 min, 25% solvent A; 25 – 65 min, 25 – 75% solvent A; 65 – 70 min, 75 – 15% solvent A. All the standards were dissolved in 50% methanol. The flow rate was 1.0 mL/min for a total run time of 70 min. The column temperature was set at 30°C, and the injection volume was 5 µL. The UV detection wavelength was monitored at 280 nm. The identification of individual phenolic compound in the sample was confirmed by comparison of their UV-vis spectra and retention times with standards.

Total phenolic (TP) and total flavonoids (TF) content Total polyphenol content (TP) was determined by the Folin-Ciocalteu method adapted from Yi et al. (1997). Results were reported as µmol gallic acid equivalent per gram of raisin weight (µmol GAE/g).

Total flavonoid content (TF) was determined by absorption at 510 nm according to a previous report Tai et al. (2011). Results were expressed as µmol rutin equivalent per gram of raisin weight (µmol RE/g).

Antioxidant capacity assays: DPPH, ABTS, FRAP DPPH (2,2-diphenyl-1-picrylhydrazyl free radical) assay. The free radical scavenging activity of the extracts was determined by measuring the bleaching of a purple DPPH^• free radical in ethanol solution, according to the method described by Enayat and Banerjee (2009). Results were expressed as µmol Trolox equivalent per gram of raisin weight (µmol TE/g RW).

ABTS (2,2′-azinobis (3-ethylbenzenothiazoline-6-sulfonic acid)) assay. The ABTS assay was carried out using a method described by Chun et al. (2005). Results were expressed as µmol Trolox equivalent per gram of raisin weight (µmol TE/g RW).

FRAP (Ferric reducing power) assay. This assay was performed according to the method reported by Babbar et al. (2011). Results were expressed as µmol ascorbic acid equivalent per g raisin weight (µmol AAE/g RW).

Statistical analysis All reported results were on a dry basis and each reported result represents the average of triplicate experiments except for the determination of the color parameters. SPSS software (Ver.17.0, SPSS Inc., Chicago, IL, USA) was used to determine the significant differences between the values and to calculate the correlation coefficients. Significant differences were defined as P < 0.05. Furthest neighbor and Euclidean distance analysis in Minitab 15 (State College, PA, USA) was used for hierarchical cluster analysis (HCA) for what kind of samples.

Resutls and Discussion

Visual Color Grapes classified visually as either black, reddish-brown, or yellowish-green also produced raisins of similar color because of the shaded drying method used in Xinjiang province (Fig. 1). In the U.S., raisins are dark brown or black regardless of the color of the grape because they are sun dried. The CIELAB values of the raisin samples are shown in Fig. 1 and Table S1. Most of the raisin samples were quite dark and achromatic, without strong color saturation (low L^*, a^* and b^* values). As expected, the lightness (L^*) values of yellowish-green group (35.44 ± 1.75 to 43.26 ± 4.55) was significant higher than those of reddish-brown group (32.24 ± 1.28 to 37.12 ± 1.04) and then black samples (29.67 ± 1.94 to 33.42 ± 2.04) (P<0.05). Most raisins had similar and low but positive a^* values, indicating reddish color. Negative b^* values, indicating a slight bluish-purple color, were found in grape varieties such as Black Monukka and Zixiang Seedless. Among the black and reddish-brown raisins, a^* and b^* values of the Crimson Seedless were significantly higher than the others (P<0.05). Much higher b^* values were found in the yellowish-green raisins (7.98 ± 2.87 to 14.27 ± 0.92) than in other types (1.01 ± 0.87 to 4.87 ± 1.05 and −1.15 ± 0.80 to −0.61 ± 0.39 for reddish-brown and black groups, respectively) (P<0.05). The ΔE* value ranged from 36.80 ± 2.08 to 45.51 ± 1.27 in the yellowish-green group, significantly higher than reddish-brown (32.30 ± 1.32 to 37.73 ± 1.10) and black (29.68 ± 1.94 to 33.45 ± 2.06) raisin groups, respectively (P<0.05). In addition, it was noted that seeded status had almost no effect on the color of the raisin (P>0.05).

Table S1. Color parameters (CIELAB) of 29 raisin samples.

Code		Grape variety	Seeded status	Color parameters
Code		Grape variety	Seeded status	L*	a*	b*	ΔE*
	Black grape variety
1		Joanna Charnice	Seedless	31.76 ± 0.74	−0.10 ± 0.16	0.07 ± 0.17	31.76 ± 0.74
2		Jingkejing	Seedless	30.70 ± 0.90	1.24 ± 0.43	0.61 ± 0.39	30.74 ± 0.91
5		Black Monukka	Seedless	30.35 ± 0.62	0.20 ± 0.14	−0.09 ± 0.22	30.36 ± 0.61
8		Beauty Seedless	Seedless	31.03 ± 0.88	0.54 ± 0.59	0.14 ± 0.29	31.04 ± 0.87
9		Zixiang Seedless	Seedless	33.42 ± 2.04	−0.17 ± 0.26	−1.15 ± 0.80	33.45 ± 2.06
	Mean ± SD	Seedless		31.46 ± 1.22	0.34 ± 0.58	0.08 ± 0.65	31.47 ± 1.22
3		Nijiala	Seeded	29.67 ± 1.94	0.63 ± 0.43	0.15 ± 0.20	29.68 ± 1.94
4		Otilia	Seeded	30.77 ± 2.55	−0.12 ± 0.11	0.22 ± 0.40	30.77 ± 2.55
6		Jingzijing	Seeded	30.90 ± 2.00	1.32 ± 0.83	0.54 ± 0.55	30.94 ± 2.00
7		SP528	Seeded	32.99 ± 3.59	−0.13 ± 0.19	0.46 ± 0.77	33.00 ± 3.58
	Mean ± SD	Seeded		31.08 ± 1.38	0.43 ± 0.69	0.34 ± 0.18	31.10 ± 1.39
	Mean ± SD	Seedless and Seeded		31.29 ± 1.22^a	0.38 ± 0.59^a	0.11 ± 0.52^a	31.30 ± 1.23^a
	Reddish-brown grape variety
10		Crimson Seedless	Seedless	37.12 ± 1.04	4.52 ± 1.21	4.87 ± 1.05	37.73 ± 1.10
11		Flame Seedless	Seedless	36.16 ± 2.30	3.22 ± 0.71	2.75 ± 1.73	36.45 ± 2.29
12		Blush Seedless	Seedless	32.38 ± 1.16	2.41 ± 0.66	1.81 ± 0.98	32.53 ± 1.16
17		SP9645	Seedless	32.46 ± 1.79	3.66 ± 1.41	1.32 ± 0.82	32.72 ± 1.89
	Mean ± SD	Seedless		34.53 ± 2.47	3.45 ± 0.88	2.69 ± 1.57	34.86 ± 2.63
13		SP5145	Seeded	32.96 ± 2.49	1.50 ± 1.44	1.25 ± 0.61	33.05 ± 2.53
14		Yatomi Rosa	Seeded	32.68 ± 1.49	2.16 ± 1.13	1.74 ± 1.17	32.83 ± 1.59
15		98-6-164	Seeded	32.24 ± 1.28	1.41 ± 0.44	1.01 ± 0.87	32.30 ± 1.32
16		SP577	Seeded	32.69 ± 0.55	2.87 ± 0.53	1.55 ± 0.15	32.86 ± 0.51
	Mean ± SD	Seeded		32.64 ± 0.30	1.98 ± 0.68	1.39 ± 0.32	32.76 ± 0.32
	Mean ± SD	Seedless and Seeded		33.59 ± 1.91^a	2.72 ± 1.07^a	2.04 ± 1.26^a	33.81 ± 2.07^a
	Yellowish-green grape variety
18		Melissa Seedless	Seedless	36.07 ± 3.77	3.61 ± 0.50	8.33 ± 2.88	37.26 ± 4.13
19		Jingzaojing	Seedless	41.15 ± 3.54	3.00 ± 2.04	13.59 ± 2.02	43.50 ± 3.75
23		Centenial Seedless	Seedless	38.79 ± 3.72	1.51 ± 1.07	9.56 ± 2.89	40.03 ± 4.27
24		KuulMuul Xuulpay	Seedless	37.17 ± 2.77	2.06 ± 2.12	8.79 ± 2.90	38.35 ± 3.41
25		Dali Thompson Seedless	Seedless	43.20 ± 1.28	−0.59 ± 0.35	14.27 ± 0.92	45.51 ± 1.27
26		Thompson Seedless	Seedless	41.79 ± 3.69	1.25 ± 1.47	11.43 ± 2.51	43.40 ± 4.08
27		Superior Seedless	Seedless	42.97 ± 1.47	2.11 ± 0.97	12.94 ± 2.16	44.96 ± 1.85
29		Perlette	Seedless	39.49 ± 3.31	−0.72 ± 1.19	9.48 ± 1.74	40.64 ± 3.65
	Mean ± SD	Seedless		40.08 ± 2.64	1.53 ± 1.55	11.05 ± 2.32	41.71 ± 3.07
20		Delight	Seeded	39.48 ± 3.49	2.75 ± 1.60	11.24 ± 4.42	41.30 ± 4.25
21		Victoria	Seeded	43.26 ± 4.55	0.85 ± 1.37	13.65 ± 2.25	45.40 ± 4.91
22		Aimaina	Seeded	35.44 ± 1.75	3.47 ± 1.16	9.08 ± 2.02	36.80 ± 2.08
28		Jiahuangputao	Seeded	38.59 ± 4.33	3.55 ± 0.77	7.98 ± 2.87	39.62 ± 4.75
	Mean ± SD	Seeded		39.19 ± 3.22	2.66 ± 1.26	10.49 ± 2.50	40.78 ± 3.60
	Mean ± SD	Seedless and Seeded		39.78 ± 2.73^a	1.91 ± 1.50^a	10.86 ± 2.28^a	41.40 ± 3.12^a

Values are means ± standard deviation (SD), n = 5.

For mean values of different color parameters (L*, a*, b*, and ΔE*), respectively, different superscripts in a, b and c indicate significant difference at P < 0.05, respectively.

Fig. 1.

The L*, a*, b*, and ΔE* of raisins produced from grapes visually classified as black, reddish-brown or yellowish-green with or without seed.

Phenolic and polyphenolic compounds Phenolic compounds, such as gallic acids, rutin, vanillic acid, (+)-catechin and epicatechin, were considered to be major contributors to the antioxidant activities of fruits (Fu et al. 2011). Ten phenolic compounds were selected and quantitated. The representative chromatograms of the standard mixture solution are depicted in Fig.2 and the phenolic contents are listed in Table 1.

Fig. 2.

HPLC trace of individual polyphenolic constituents (1, gallic acid; 2, protocatechuic acid; 3, (+)-catechin; 4, chlorogenic acid; 5,vanillic acid; 6, caffeic acid; 7, epicatechin; 8, ferulic acid; 9, rutin; 10, resveratrol) of the standard mixture solution (a) and a chromatogram of Yatomi Rosa raisins (b).

Rutin was the most predominant phenolic compound in a majority of the raisin samples, except of Nijiala, Otilia, and Beauty Seedless which were mainly rich in epicatechin. Dietary rutin has been shown to have a protective effect against spatial memory impairment accompanying hippocampal pyramidal neuron loss (Ksouri et al. 2009). KuulMuul Xuulpay had the highest (+)-catechin content (125.21 µg/g). (+)-Catechin has been reported to have a positive correlation with DPPH and ABTS scavenging capacity and reducing power (Tsai et al. 2007). Yatomi Rosa had the highest amounts of gallic acid (8.39 µg/g), while sample 98-6-164 had the highest amounts of protocatechuic acid (2.62 µg/g). Beauty Seedless had the highest amounts of vanillic acid (20.37 µg/g).

Overall, gallic acid and rutin were detected in all raisins in the study, whereas, the (+)-catechin was not detected in sample 98-6-164, and vanillic acid was not detected in sample SP577. Caffeic acid was detected only in Black Monukka, Yatomi Rosa, SP577 and SP9645, chlorogenic acid, ferulic acid and resveratrol were not detected in most tested raisin samples. The results of this research were different from Meng et al. who found that the individual polyphenolic compounds of Thompson Seedless from different regions were significantly different (Meng et al. 2011). Processing treatments also affect composition, for example Thompson golden raisins (which were treated with sulfur dioxide (SO₂)) had the highest amount of caftaric and coutaric acids while sun-dried and dipped raisins, contained oxidized cinnamics (Karadeniz et al. 2000). These reports indicate that the polyphenol composition was also related to the cultivation region and processing procedures as well as analytical methodology such as extraction solvents and extract concentration (Zhao and Hall III 2008).

Polyphenolic content Total polyphenolic content (TP), total flavonoids content (TF), and antioxidant assays (DPPH, ABTS, FRAP) are often used to characterize the antioxidant potential of foods. The perception that dark color grapes contain a higher polyphenolic content is not completely true as shown in Fig. 3 and Table S2. There was a potential wide range of TP in all color classes of grapes. A black seeded raisin, Otilia, and a yellowish-green seeded raisin, Jiahuangputao, had two of the highest TP contents. The TP content of seeded and seedless black raisins were similar, however, only seedless black raisins were significantly higher in TP than those of seedless reddish-brown or yellowish-green raisins (P<0.05). Unlike the black raisins, the TP content of seeded reddish-brown and yellowish-green raisins were generally higher than the seedless. These results showed that the higher TP content of black raisins compared to the reddish-brown and yellowish-green was due to TP in the skin.

Fig. 3.

The total polyphenolic (TP) and flavonoids (TF) content of seeded and seedless raisins visually classified as black, reddish-brown or yellowish green.

Table S2. Characteristics of the 29 raisin samples tested in the study.

Code.		Grape variety	Seeded status	TP [µmol GAE/g]	TF [µmol RE/g]	DPPH[µmol TE/g]	ABTS[µmol TE/g]	FRAP[µmol AAE/g]
	Black grape variety
1		Joanna Charnice	Seedless	14.03 ± 0.60	2.02 ± 0.11	7.20 ± 0.22	20.50 ± 0.47	23.80 ± 0.60
2		Jingkejing	Seedless	14.25 ± 2.66	2.86 ± 0.67	7.19 ± 1.08	19.65 ± 3.75	24.71 ± 3.31
5		Black Monukka	Seedless	13.18 ± 0.47	2.27 ± 0.15	6.30 ± 0.43	19.74 ± 0.69	29.68 ± 0.91
8		Beauty Seedless	Seedless	15.73 ± 0.59	2.84 ± 0.17	6.45 ± 0.31	20.07 ± 0.83	19.00 ± 0.08
9		Zixiang Seedless	Seedless	20.63 ± 1.19	3.82 ± 0.19	7.43 ± 0.13	24.04 ± 0.80	23.70 ± 1.25
	Mean ± SD	Seedless		15.56 ± 2.97	2.76 ± 0.69	6.91 ± 0.50	20.80 ± 1.84	24.18 ± 3.80
3		Nijiala	Seeded	18.71 ± 0.49	4.21 ± 0.08	8.96 ± 0.13	28.04 ± 0.38	36.98 ± 1.06
4		Otilia	Seeded	26.58 ± 2.71	5.64 ± 0.61	10.20 ± 0.11	30.87 ± 0.03	39.07 ± 2.89
6		Jingzijing	Seeded	15.75 ± 0.49	2.49 ± 0.20	5.47 ± 0.23	19.62 ± 0.59	26.64 ± 0.74
7		SP528	Seeded	16.91 ± 0.94	2.79 ± 0.28	7.27 ± 0.17	21.51 ± 1.06	27.23 ± 1.65
	Mean ± SD	Seeded		19.49 ± 4.88	3.78 ± 1.45	7.98 ± 2.06	25.01 ± 5.32	32.48 ± 6.47*
	Mean ± SD	Seedless and Seeded		17.31 ± 4.20^a	3.22 ± 1.15^a	7.39 ± 1.42^a	22.67 ± 4.15^a	27.87 ± 6.48^a
	Reddish-brown variety
10		Crimson Seedless	Seedless	6.79 ± 0.66	1.20 ± 0.14	4.24 ± 0.33	12.22 ± 0.61	16.16 ± 1.18
11		Flame Seedless	Seedless	7.62 ± 0.48	1.29 ± 0.07	4.07 ± 0.15	9.76 ± 0.42	16.46 ± 0.54
12		Blush Seedless	Seedless	11.32 ± 1.24	2.35 ± 0.34	6.01 ± 0.49	24.00 ± 1.71	27.86 ± 2.65
17		SP9645	Seedless	16.26 ± 0.92	4.52 ± 0.42	8.87 ± 0.17	23.60 ± 2.07	22.67 ± 0.83
	Mean ± SD		Seedless	10.50 ± 4.32	2.34 ± 1.55	5.80 ± 2.23	17.40 ± 7.46	20.79 ± 5.59
13		SP5145	Seeded	10.89 ± 0.31	1.99 ± 0.04	6.34 ± 0.20	13.37 ± 0.48	13.22 ± 1.00
14		Yatomi Rosa	Seeded	16.35 ± 1.39	3.48 ± 0.40	6.46 ± 0.26	22.38 ± 1.67	24.01 ± 1.41
15		98-6-164	Seeded	16.71 ± 2.18	4.66 ± 0.83	9.25 ± 0.64	27.28 ± 2.45	22.93 ± 1.15
16		SP577	Seeded	9.19 ± 0.94	1.81 ± 0.17	6.35 ± 0.70	11.87 ± 1.55	10.89 ± 1.16
	Mean ± SD	Seeded		13.28 ± 3.81	2.99 ± 1.34	7.10 ± 1.43	18.73 ± 7.35	17.76 ± 6.67
	Mean ± SD	Seedless and Seeded		11.89 ± 4.05^b	2.66 ± 1.38^a	6.45 ± 1.87^a	18.06 ± 6.90^a	19.28 ± 5.92^b
	Yellowish-green variety
18		Melissa Seedless	Seedless	9.54 ± 0.71	3.08 ± 0.14	6.49 ± 0.70	18.79 ± 0.21	17.38 ± 0.80
19		Jingzaojing	Seedless	13.25 ± 1.87	3.21 ± 0.40	7.65 ± 0.87	22.68 ± 2.82	20.30 ± 2.22
23		Centenial Seedless	Seedless	7.15 ± 0.79	1.62 ± 0.14	4.33 ± 0.36	11.43 ± 1.50	13.06 ± 1.23
24		KuulMuul Xuulpay	Seedless	9.79 ± 0.94	2.01 ± 0.29	7.01 ± 0.65	14.52 ± 1.48	15.21 ± 0.64
25		Dali Thompson Seedless	Seedless	7.89 ± 0.59	1.64 ± 0.11	4.08 ± 0.31	12.70 ± 1.06	17.01 ± 1.78
26		Thompson Seedless	Seedless	9.24 ± 0.60	2.25 ± 0.11	5.91 ± 0.12	13.18 ± 0.82	15.78 ± 0.49
27		Superior Seedless	Seedless	9.55 ± 0.47	2.56 ± 0.16	4.79 ± 0.25	15.23 ± 0.70	15.41 ± 0.65
29		Perlette	Seedless	13.76 ± 1.09	3.56 ± 0.30	7.36 ± 0.57	20.17 ± 0.86	26.02 ± 0.67
	Mean ± SD	Seedless		10.02 ± 2.34	2.49 ± 0.73	5.95 ± 1.40	16.09 ± 4.00	17.52 ± 4.02
20		Delight	Seeded	12.97 ± 0.23	2.60 ± 0.15	6.47 ± 0.29	18.56 ± 0.24	20.83 ± 0.65
21		Victoria	Seeded	14.59 ± 1.78	4.08 ± 0.55	8.99 ± 0.72	23.86 ± 2.90	19.80 ± 2.03
22		Aimaina	Seeded	10.67 ± 0.37	2.39 ± 0.26	6.17 ± 0.30	17.65 ± 0.85	19.55 ± 0.14
28		Jiahuangputao	Seeded	25.23 ± 1.34	6.90 ± 0.68	10.16 ± 0.04	30.67 ± 0.03	29.82 ± 4.53
	Mean ± SD		Seeded	15.86 ± 6.45*	3.99 ± 2.08	7.95 ± 1.95	22.68 ± 5.99*	22.50 ± 4.91
	Mean ± SD	Seedless and Seeded		11.97 ± 4.81^b	2.99 ± 1.44^a	6.62 ± 1.80^a	18.29 ± 5.53^a	19.18 ± 4.78^b

Values are mean ± standard deviation, n = 3.

For samples with the same color but different seeded status, * indicates significant difference at P < 0.05; For all seeded and seedless samples in different color, different superscripts in a and b in each column indicate significant difference at P < 0.05, respectively.

TP, total phenolic content; TF, total flavonoid content;

DPPH, DPPH radical scavenging activity; ABTS, ABTS radical cation scavenging activity;

FRAP, reducing power; GAE, gallic acid equivalent; RE, rutin equivalent; TE, trolox equivalent;

AAE, ascorbic acid equivalent.

There was a six-fold range of TF content, 1.20 ± 0.14 to 6.90 ± 0.68 µmol RE/g among all the test raisin samples (Fig. 2 and Table S2). Jiahuangputao had the highest amount of TF (6.90 ± 0.68 µmol RE/g) and Otilia had the second (5.64 ± 0.61 µmol RE/g), whereas Crimson Seedless (reddish-brown and seedless) had the lowest amount of TF (1.20 ± 0.14 µmol RE/g) among all the test samples. Relatively, the difference in TF contents of raisins from different color or different seeded status was not significant as those of TP values.

Antioxidant activity assays For a more comprehensive and accurate evaluation of the antioxidant capacity of raisins, 3 antioxidant activity assays (DPPH, ABTS, and FRAP) were selected for their different functions. DPPH^• and ABTS^• are stable free radicals, which are widely used to study the free radical-scavenging activities of natural antioxidants (Brand-Williams et al. 1995). The model system can also be used to determine the scavenging potential of compounds (Krings and Berger 2001). In general, the linear correlation coefficients between DPPH and ABTS have been found to be significant (Chun et al. 2005). Ferric ion is also often used as an indicator of electron-donating activity. The ferric-reducing antioxidant power assay treats the antioxidants in the samples as reductants in a redox-linked colorimetric reaction. The value reflects the reducing power of the antioxidants. Many reports have demonstrated that the reducing power of natural plant extracts correlated with their antioxidant activities (Xu et al. 2010; Chun et al. 2005). In the current study, as shown in Fig. 4 and Table S2, Otilia (black and seeded) had the strongest antioxidant activity in DPPH assay (10.20 ± 0.1 l µmol TE/g), ABTS assay (30.87 ± 0.03 µmol TE/g), and FRAP assay (39.07 ± 2.89 µmol AAE/g). Jiahuangputao (yellowish-green and seeded) had the second highest values of DPPH (10.16 ± 0.04 µmol TE/g) and ABTS (30.67 ± 0.03 µmol TE/g). Nijiala (black and seeded) had the second highest value of FRAP (36.98 ± 1.06 µmol AAE/g). The FRAP value of Jiahuangputao was 29.82 ± 4.53 µmol AAE/g, almost equal to that of Black Monukka (black and seedless), Blush Seedless (reddish-brown and seedless), SP528 (black and seeded), Jingzijing (black and seeded), and Perlette (yellowish-green and seedless). Seedless grape cultivars Crimson Seedless (reddish-brown), Flame Seedless (reddish-brown), Centennial Seedless (yellowish-green), Dali Thompson Seedless (yellowish-green), and Superior Seedless (yellowish-green) had weak antioxidant activity in both DPPH and ABTS assays. SP577 (reddish-brown and seeded) had the lowest FRAP value (10.89 ± 1.16 µmol AAE/g) followed by Centenial Seedless (13.06 ± 1.23 µmol AAE/g).

Fig. 4.

The DPPH, ABTS, and FRAP of seeded and seedless raisins visually classified as black, reddish-brown or yellowish green.

Correlation between antioxidant assays The linear correlation coefficients of TP, TF, antioxidant activity parameters, and color parameters for the raisin varieties are shown in Table 2. The linear correlation coefficients among TP, TF, DPPH, ABTS, and FRAP were all significant at the 0.01 level (P<0.01), indicating that TP and TF are the components contributing to the antioxidant activity of raisins and also that the three antioxidant assays are comparable and interchangeable. These results were consistent with other reports in the literature (Fu et al. 2011; Xu et al. 2010). Strong positive correlations were found between TP and DPPH (0.83^**), TP and FRAP (0.80^**), and DPPH and FRAP (0.62^**), values that were substantially higher than those reported by Meng et al. (0.79^**, 0.76^**, 0.31, respectively) which studied in raisins from other different grape cultivars (Meng et al. 2011).

Table 2. Linear correlation coefficients among TP, TF, antioxidant activity, and color parameters of 29 raisin

	TP	TF	DPPH	ABTS	FRAP	L*	a*	b*	ΔE*
TP	1
TF	0.88**	1
DPPH	0.83**	0.89**	1
ABTS	0.90**	0.88**	0.87**	1
FRAP	0.80**	0.63**	0.62**	0.83**	1
L*	−0.37*	−0.08	−0.24	−0.34	−0.48**	1
a*	−0.29	−0.08	−0.15	−0.18	−0.33	0.03	1
b*	−0.39*	−0.07	−0.2	−0.3	−0.44*	0.96**	0.09	1
ΔE*	−0.37*	−0.08	−0.23	−0.33	−0.47**	1.00**	0.03	0.97**	1

L*, a*, b*, and ΔE*: color parameters;

TP, total phenolic content;

TF, total flavonoid content;

DPPH, DPPH radical scavenging activity;

ABTS, ABTS radical cation scavenging activity;

FRAP, reducing power;

** Significant at the 0.01 level;

* Significant at the 0.05 level.

Correlation between color with TP, TF, and antioxidant capacity Differences in TP, TF, and antioxidant capacity among raisin color categories are shown in Table 2, Table 3, Fig. 3 and Fig. 4. TP exhibited a significant negative correlation with color parameters L^*, b^* and ΔE (P<0.05) suggesting that color could significantly affect the TP content of the raisin. No significant correlation was found between TF and color parameters. Only FRAP had a correlation with the color parameters. TP content and FRAP activity of raisins with black, reddish-brown and yellowish-green color were 17.31 ± 4.20, 11.89 ± 4.05, and 11.97 ± 4.81 µmol GAE/g and 27.87 ± 6.48, 19.28 ± 5.92, and 19.18 ± 4.78 µmol AAE/g, respectively. Black raisins exhibited significantly higher values of TP (31.31% and 30.85%), FRAP (30.82% and 31.18%) than reddish-brown and yellowish-green raisins (P<0.05), respectively. Xu et al. (2010) also reported that black varieties of grapes had higher TP content than the red varieties. It was noted that no significant differences were found in the values of TP, TF, and antioxidant capacity between the tested reddish-brown and yellowish-green raisins (P>0.05) (Table 3, Fig. 3 and Fig. 4). Although it has been reported that commercial red raisins generally had higher content of total phenolic compounds than the white raisins (Sério et al. 2014), however, the findings in this research demonstrated that the phenolic compounds and antioxidant activity of raisins were not exclusively related with the color category.

Table 3. Antioxidant activity characteristics of raisins by visual color and seeded/seedless category.

Category	Type	Seeded status	TP (µmol GAE/g)	TF (µmol RE/g)	DPPH (µmol TE/g)	ABTS (µmol TE/g)	FRAP (µmol AAE/g)
	Black		17.31 ± 4.20^a	3.22 ± 1.15^a	7.39 ± 1.42^a	22.67 ± 4.15^a	27.87 ± 6.48^a
		Seeded	19.49 ± 4.88	3.78 ± 1.45	7.98 ± 2.06	25.01 ± 5.32	32.48 ± 6.47*
		Seedless	15.56 ± 2.97	2.76 ± 0.69	6.91 ± 0.50	20.80 ± 1.84	24.18 ± 3.80
	Reddish-brown		11.89 ± 4.05^b	2.66 ± 1.38^a	6.45 ± 1.87^a	18.06 ± 6.90^a	19.28 ± 5.92^b
Color		Seeded	13.28 ± 3.81	2.99 ± 1.34	7.10 ± 1.43	18.73 ± 7.35	17.76 ± 6.67
		Seedless	10.50 ± 4.32	2.34 ± 1.55	5.80 ± 2.23	17.40 ± 7.46	20.79 ± 5.59
	Yellowish-green		11.97 ± 4.81^b	2.99 ± 1.44^a	6.62 ± 1.80^a	18.29 ± 5.53^a	19.18 ± 4.78^b
		Seeded	15.86 ± 6.45*	3.99 ± 2.08	7.95 ± 1.95	22.68 ± 5.99*	22.50 ± 4.91
		Seedless	10.02 ± 2.34	2.49 ± 0.73	5.95 ± 1.40	16.09 ± 4.00	17.52 ± 4.02
	Seeded		16.21 ± 5.37^a	3.59 ± 1.56^a	7.67 ± 1.71^a	22.14 ± 6.29^a	24.25 ± 8.45^a
Seeded status	Seedless		11.76 ± 3.81^b	2.54 ± 0.91^b	6.20 ± 1.44^b	17.78 ± 4.76^b	20.25 ± 5.01^a

Data are presented as means ± standard deviation (For black grape varieties, n=9 (4 for seeded samples and 5 for seedless samples); reddish-brown grape varieties, n=8 (4 for seeded samples and 4 for seedless samples); yellowish-green grape varieties, n=12 (4 for seeded samples and 8 for seedless samples); seeded grape varieties, n=12; seedless grape varieties, n=17).

For difference in color or seeded status, respectively, different superscripts in a and b in each column indicate significant difference at P < 0.05, respectively. For samples with the same color but different seeded status, * indicates significant difference at P < 0.05.

TP, total phenolic content; TF, total flavonoid content; DPPH, DPPH radical scavenging activity;

ABTS, ABTS radical cation scavenging activity; FRAP, reducing power; GAE, gallic acid equivalent;

RE, rutin equivalent; TE, trolox equivalent; AAE, ascorbic acid equivalent.

Correlation between seed-bearing status with TP, TF, and antioxidant capacity The effects of seeds in raisins on TP, TF, and antioxidant capacity are shown in Table 2, Fig. 3 and Fig. 4 and summarized in Table 3. Higher levels in all parameters were found on the seeded raisins than the seedless ones, although the difference in the FRAP value was not statistically significant (P>0.05). The respective values of TP and TF contents, and DPPH, ABTS and FRAP capacities for seeded raisins were 16.21 ± 5.37 µmol GAE/g and 3.59 ± 1.56 µmol RE/g, and 7.67 ± 1.71 µmol TE/g, 22.14 ± 6.29 µmol TE/g and 24.25 ± 8.45 µmol AAE/g. The values of DPPH, ABTS and FRAP capacities for seedless were lower by 19.17%, 19.69% and 16.49% than the seeded samples, respectively. Similarly, for the same color variety, TP, TF, and antioxidant capacities were higher in seeded raisins than the seedless ones, although the difference was not statistically significant for most of samples. In grapes, Costa et al. analyzed 24 different varieties cultivated in two Portuguese wine regions and concluded that seeds contained the highest antioxidant capacity followed by the skins and pulp (Costa et al. 2014). Seed-bearing status seemed to have a greater influence on TP, TF, and antioxidant capacity than color of raisins.

Hierarchical cluster analysis (HCA) HCA method was used to group all raisin samples according to the values of TP, TF, and antioxidant activities. As shown in Fig. 5, the raisin samples were grouped into three clusters on the basis of their nearness and similarity. Raisin cultivars of Nijiala (#3, black and seeded), Otilia (#4, black and seeded), and Jiahuangputao (#28, yellowish-green and seeded) were grouped in the first cluster (A) with similar characteristics of high values of TP, TF, and antioxidant activities (Table 1). The second cluster (B) included the black and seedless varieities Joanna Charnice (#1), Jingkejing (#2), Beauty Seedless (#8), Zixiang Seedless (#9), and the black and seeded varieities Black Monukka (#5), Jingzijing (#6), SP528 (#7), Blush Seedless (#12, Reddish-brown and seedless), and the reddish-brown and seeded varieties Yatomi Rosa (#14), 98-6-164 (#15), and SP9645 (#17). These samples possessed similar characteristics of moderate values of TP, TF, and antioxidant activities. The third cluster (C) contained the lowest amount of TP, TF, and antioxidant activities.

Fig. 5.

Dendrogram resulting from hierarchical cluster analysis of 29 raisin samples. (Furthest neighbor and Euclidean distance were used in the data analysis. Varieties are listed in Table 1. Cluster A: higher antioxidant activities; Cluster B: moderate antioxidant activities; Cluster C: lower antioxidant activities.)

This HCA classification differed from the perception that black or seeded raisins had the highest values of TP, TF, and antioxidant activities. Each HCA group contained samples of different color and seeded-bearing status. The results demonstrated that the phenolic compounds and antioxidant activity of raisin samples depended on both color and seeded status.

It is also worthy to be noted that in clusters A and B, most of the samples were with black and reddish-brown color, and for cluster C, samples were mainly with yellowish-green color. However, all the three groups contained samples with or without seed. It indicated that color appearance had greater influence on antioxidant activity characteristics than the seeded status.

Conclusions

Seedless and seeded raisin samples including 9 black, 8 reddish-brown, and 12 yellowish-green raisins were evaluated for phenolic composition, polyphenol and flavonoid contents, antioxidant activity, and color appearance. Large variations were found in phenolic compounds and their contents among different raisins. Significant correlations between TP, TF content, DPPH, ABTS, and FRAP were observed (P<0.01), indicating that TP and TF could be the main factors contributing to raisin antioxidant activity. Color parameters of L*, b*, and ΔE* were significantly negatively correlated with TP and FRAP which indicated that color could significantly affect the TP contents and FRAP activity of the raisin. Higher values of TP and FRAP were found for black raisins than reddish-brown and yellowish-green ones. However, only seedless black raisins were higher in TP than those of reddish-brown or yellowish-green raisins. No significant differences in antioxidant assays were found between the reddish-brown and yellowish-green raisins. The values of TP and TF contents, and DPPH and ABTS capacities for seeded raisins were higher than those of seedless samples. Based on the similarity of values of TP, TF, and antioxidant capacities, the raisin samples were classified into three groups using HCA. Some raisin samples differing in seed-bearing status or color were grouped together, indicating that the antioxidant properties of raisins were not only determined by color, but also by their seed-bearing status. This study shows that high levels of polyphenolics can be found in all color categories of raisins and that they are good sources of healthful polyphenolics and phenolic acids.

Acknowledgments The authors acknowledge financial support from the Ministry of Agriculture of the People's Republic of China through the funds of Agro-scientific Research in the Public Interest (No. 201003021) and Modern Agricultural Industry Technology System (No. CAR-30) and support from the project of Shaanxi Provincial Natural Science Foundation (No.2015JQ3083), Fundamental Research Funds for the Central Universities (No. 3102014JCQ15011) and the China Postdoctoral Science Foundation (No. 2014M562451).

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