A comparative study on anthocyanin, saponin, and oil profiles of black and red seed coat peanut (Arachis hypogacea) grown in China

ABSTRACT

The objective of the current study was to investigate the chemical profiles among red and black peanut cultivars grown in China. Boiled peanut seeds lost colour values due to the loss of water-soluble pigments into the boiling water. The oil contents of eight out of ten cultivars are over 49 g/100g on dry weight basis. Peanut seed coat had much higher saponin compared with peanut cotyledon and the boiled peanut. Zhonghua No. 16 peanut had the highest saponin content in both seed coat (381.5 mg/g) and cotyledon (10.6 mg/g). Anthocyanin was comparatively higher in the black peanuts. It was also observed that the anthocyanin content had a close relationship with the colour of the peanuts. Cyanidin-3-O-sambubioside was one of the main anthocyanins present in black peanut cultivars. Seed coat was found to be the most nutritive part in the entire peanut kernel. It could also be considered as a potential source of nutraceuticals.

KEYWORDS:

• Anthocyanin
• Black seed coat
• Colour values
• HPLC
• Peanut

Introduction

Peanut (Arachis hypogaea L.) is one of the most abundant crops and its seeds are widely consumed food products in the world. As one of the most common food legumes in the world, it is produced on a huge scale across the globe. The world production of peanut was estimated to be 41.1 million tons set in 2013/14.^[1] Of this, China is the leading producer, accounting for about 42% of the entire production, followed by India and USA. Although the production of peanut in China is widely spread, the main producing areas are distributed across Shandong, Henan, Hebei, and Anhui.^[2] Generally, the colour of seed coat ranges from whitish to dark purple, with mahogany red, rose, and salmon being the predominate colours. People recently started to pay attention towards peanut for its nutritional values as a functional food.^[3] Peanut is well known for its oil content, so it is most commonly used as a source of vegetable oil and an excellent source of calories. Monounsaturated and polyunsaturated fats and vitamin E in peanut are reported to decrease the LDL-cholesterol level, which can further reduce the risks of heart diseases.^[4] Apart from oils, phytochemicals (such as phenolic and anthocyanins) are also found to be rich in peanut, especially in peanut skin.^[5]

Peanuts with red seed coat have been used as medicinal materials for decades. It is well known for homeostasis function by increasing thrombocytes in the human body. Apart from its pharmaceutical functions, red seed coat peanut is also considered to have antioxidants and a high level of phytochemicals that have beneficial effects.^[6] Previous studies have indicated that total phenolic content, total flavonoids content, and total proanthocyanidins content presented positive correlation with seed coat colour.^[7] Black seed coat peanut has a deeper colour compared to red seed coat or yellow seed coat peanut cultivars. The black colour of seed coat results from the polyphenols pigment. Generally speaking, deeper-colour cultivars will contain a higher amount of phenolic compounds, which contribute to the higher health benefits in prevention against cardiovascular diseases, cancers, and other metabolic diseases. Besides nutritional compositions, saponins and cyanidin can also be found in peanut plants.^[8] Nuts and food legumes are recommended as dietary food in the US to reduce the risks of CVD due to the abundant level of phytochemicals.

In the food industry and home cooking practice, the seed coats of peanuts are generally removed to get cotyledon before processing it into peanut oil, peanut butter, or other peanut foods. Recent research proves that seed coat of peanut is a good source of phytochemicals and antioxidants,^[9] yet the seed coat is usually treated as a by-product during food processing.^[10] Our latest research indicated that the seed coat of different peanuts was found to be rich in phenolic compounds.^[11]As a result of our continued study of the health-promoting effects of peanuts, we further investigated the chemical compositions of red and black seed coat peanut cultivars, and compared the chemical differences in 10 cultivars originally from six provinces in China.

Materials and methods

Materials

Black seed coat, red seed coat, and common seed coat (light yellow) peanut cultivars (shown in Fig. 1 and Table 1) were collected from different parts of China. Peanut cultivars (#1, #2, and #4) were purchased from seed companies in 2013. Three peanut cultivars (#3, #5, and #6) were donated by Crop Research Institute in Jiangsu Academy of Agricultural Sciences (Nanjing, Jiangsu, China), all cropped in 2013. Peanut cultivars (#7, #8, and #9) were donated by Peanut Research Institute in Guangdong Academy of Agricultural Sciences (Zhanjiang, Guangdong, China) in February 2014. One peanut sample (#10) was bought from a local market in February 2014.

Table 1. Information of peanut cultivars.

Sample code	Sample name	Colour	Place of origin	Year of cropping	Source
1	Heiba No. 1	Dark brown	Tai’ an, Shandong	2013	Taian Lvde Agricultural Co., Ltd.
2	Heifeng No. 1	Dark brown	Sichuan	2013	Sichuan Nongweixian Agricultural Products
3	Xuzhou	Black	Nanjing, Jiangsu	2013	Crop Research Institute, Jiangsu Academy of Agricultural Sciences
4	Tangshan	Light brown	Tangshan, Hebei	2013	Tangshan Liyuan Co., Ltd
5	Zhonghua No. 16	Pink	Nanjing, Jiangsu	2013	Crop Research Institute, Jiangsu Academy of Agricultural Sciences
6	Taihua No. 5	Light yellow	Nanjing, Jiangsu	2013	Crop Research Institute, Jiangsu Academy of Agricultural Sciences
7	Zhenzhuhei	Black	Zhanjiang, Guangdong	2014	Peanut Institute, Guangdong Academy of Agricultural Sciences
8	Zhenzhuhong No. 1	Light red	Zhanjiang, Guangdong	2014	Peanut Institute, Guangdong Academy of Agricultural Sciences
9	Yueyong No. 13	Light yellow	Zhanjiang, Guangdong	2014	Peanut Institute, Guangdong Academy of Agricultural Sciences
10	Commercial product	Dark red	Hubei	2014	Guangzhou public market

Figure 1. Morphology and appearance of 10 peanut cultivars.

Chemicals and reagents

Acetic acid was purchased from Damao Co., Ltd. (Tianjin, China). Methanol was purchased from Baishi Chemical Co., Ltd. (Tianjin, China). Butylated hydroxytoluene (BHT) was purchased from Yuanye Co., Ltd. (Shanghai, China). Trifluroacetic (TFA), acetonitrile (HPLC grade), and methanol (HPLC grade) were purchased from Sigma Chemical Co., Ltd. (St. Louis, MO, USA). Cyanidin-3-O-sambubioside, cyanidin-3-O-glucoside, and peonidin-3-O-glucoside were purchased from ChemFaces Biochemical Co., Ltd. (Wuhan, China).

Sample preparation

Peanut hulls were first shelled from the kernel manually. The broken, damaged, and immature seeds were manually excluded from peanut samples. Then, the seed coat was manually separated from the peanut kernel with the help of a knife and collected for further use. The boiled peanut samples were obtained by boiling peanuts in water in the ratio of 1:4 at 100°C for 20 min. Boiled peanuts were then freeze-dried before grinding. All peanut samples were ground by a high-speed pulveriser for 40 s (FW-400A, Zhongxin Co. Ld., Beijing, China) and finally stored at −20°C in dark until further use.

Colour determination

A colorimeter (DP-400 & CP-400, Konica Minolta Sensing Inc., Japan) was used to determine the colour values of peanut samples based on the CIE system. Ground peanut was evenly spread on the bottom of the colorimeter cup and read three times. The raw whole peanut and boiled peanut were also read for seed coat colour before grinding.

Determination of oil content

Percentages of oil in peanuts were determined with the Soxhlet extraction method. Soxhlet extractors and round-bottomed flasks were dried in oven before each usage. Raw whole peanut samples were dried in an oven at 102°C for 2 h. Moisture content was determined by the Satorius electronic moisture analyser (MA150, Germany) before Soxhlet extraction. Next, 200 mL petroleum ether was used to extract oil from 3 g peanut sample at 52°C water bath for 12 h. Oil content was calculated according to the Soxhlet method.

Determination of total saponin content

Total saponin content was determined colourimetrically based on the method of Sim et al.^[12] using vanillin and sulphuric acid. Briefly, 0.5 g sample was defatted with 10 mL petroleum ether by shaking for 4 h. The defatted dry peanut residue was extracted twice with 5 mL 80% methanol for 4 h. The final extracts were stored at −4°C until further use.

For quantification of total saponin, 100 μL extract was mixed with 400 μL 80% methanol, 500 μL freshly prepared 8% vanillin in ethanol, and 5 mL 72% sulphuric acid. The mixture was warmed at 60°C for 10 min, and then cooled in an ice water bath prior to colourimetic analysis at 544 nm by a UV-visible spectrophotometer (TU-1901, Puxi Co. Ltd, Beijing, China). The resulting values were calculated with a best-fit calibration standard curve of soyasaponin Ba.

HPLC analysis of anthocyanin

Acidic extraction solvent (methanol/water/acetic acid/BHT; 85:15.0.5:0.2; v/v/v/w) was used to extract anthocyanin from peanuts. Briefly, 0.5 g peanut sample (or 0.1 g seed coat) was extracted twice with 5 mL extraction solvent by shaking in the dark for 4 h. Both extracts were combined after being centrifuged at 6000 rpm for 10 min, and then were concentrated at 35°C in a vacuum dryer to remove the solvent. The dry residue was reconstituted with 2.5 mL 25% methanol. An aliquot of sample solution was filtrated into the HPLC sample vial through 0.2 μm polytetrafluoroethylene (PTFE). The filtrated sample solution was stored at 4°C in dark before analysis.

Determination of anthocyanins was carried out by an RP-HPLC system (Agilent 120, Wheaton, USA) with a UV-DAD detector (Agilent G1315D) at 520 nm. A Zorbax Eclipse XDB-C₁₈ column (Analytical, 4.6 × 250 mm, 5 μm) was used for the separation of anthocyanins. Mobile phase for the solvent gradient program was 0.1% TFA in water (solvent A) and 0.1% TFA in acetonitrile and water (50:50, v/v) (solvent B). The program was set as follows: 85% A and 15% B at 0–60 min, 60% A, and 40% B at 60–65 min. The flow rate of the mobile phase was set at 0.5 mL/min. The inject volume was 20 μL for each injection. Cyanidin-3-O-sambubioside, cyanidin-3-O-glucoside, and peonidin-3-O-glucoside were dissolved in methanol as stock standard solution to give a concentration of 1.0 mg/mL. Then the stock solution was diluted into different gradient concentrations: 100 μg/mL, 50 μg/mL, 25 μg/mL, 10 μg/mL, 5 μg/mL, 2.5 μg/mL, 1 μg/mL, 0.5 μg/mL, and 0.25 μg/mL.

Statistical analysis

All samples were analysed at least in duplicates. The results and regression analyses were evaluated by ANOVA in Microsoft Excel 2007. SPSS packages (version 19.0, IBM software) were used to evaluate significant difference among sample results in different assays.

Results

Colour values of peanuts

The colour values of peanut samples determined with a colorimeter are shown in Table 2. In the CIE system, L*, a*, and b* represent different colour intensities. L* shows lightness, a* shows redness, and b* shows yellowness. The colour of the peanut seed coat varies from black to light red-yellow and red; their differences can be revealed by the CIE values. Theoretically speaking, the lower L* values represent a deeper colour, L* of black peanuts (#1, #2, #3, #4, and #7) were in the range of 22–29; pink and yellow peanuts (#5, #6, and #9) were in the range of 64–67; and red peanuts (#8 and #10) were around 41 with no significant differences. Relatively, dark red colour peanuts contributed to the highest a* (35–37); light red-yellow peanuts had a* around 19–24; black peanuts had lower a* between 3 and 15. Negative values were of b* were observed for black peanuts, indicating a hint of blue colour. Both light red-yellow seed coat peanut and dark red colour peanut showed positive b*, indicating the presence of yellow colour. ΔE (a comprehensive metric calculated using L*, a*, and b* values) was used for direct comparison. In this study, the ΔE values of black peanuts lied in the range of 67–72, while dark red colour peanuts had slightly lower values around 66. Pink and yellow peanuts had the lowest ΔE values in the range of 39–44. Each parameter exhibited significant differences. There were no significant differences in the ΔE values for four black peanut varieties (Heiba No. 1, Heifeng No. 1, Xuzhou, and Tangshan). Zhenzhuhei cultivated in Guangdong had the highest ΔE, indicating the deepest colour amongst all the 10 peanut cultivars. On the other hand, boiled peanut of Heiba No. 1, Heifeng No. 1, Xuzhou, and Tangshan did show significant difference, unlike their raw kernel (p < 0.05). However, Zhenzhuhong No. 1 and commercial product showed no significant differences as their raw kernel.

Table 2. Color (CIE L*, a*, b*) and ΔE of 10 peanut cultivars.

Sample code	Sample name	Portions	L*^p	a*^p	b*^p	ΔE^p
1	Heiba No. 1	Raw peanut	26.967 ± 0.31^n	10.697 ± 0.47^ijk	−1.037 ± 0.24	68.370 ± 0.30^b
		Boiled peanut	56.430 ± 0.54^h	10.117 ± 1.23^ijk	6.400 ± 0.28^g	39.413 ± 0.43^i
2	Heifeng No. 1	Raw peanut	28.307 ± 0.20^lm	13.237 ± 1.60^gh	−1.177 ± 0.11^j	67.547 ± 0.33^b
		Boiled peanut	63.380 ± 0.78^f	10.290 ± 0.99^ijk	6.240 ± 0.43^g	32.847 ± 1.10^k
3	Xuzhou	Raw peanut	27.400 ± 0.70^mn	5.077 ± 0.49^m	−1.953 ± 0.43^jk	67.973 ± 0.76^b
		Boiled peanut	40.983 ± 0.70^k	10.633 ± 0.03^ijk	4.920 ± 0.23^h	54.447 ± 0.70^d
4	Tangshan	Raw peanut	28.717 ± 0.36^l	15.477 ± 0.367^f	−1.260 ± 0.36^j	67.647 ± 0.30^b
		Boiled peanut	55.307 ± 0.59^i	8.973 ± 0.23^k	5.040 ± 0.21^h	40.100 ± 0.61^hi
5	Zhonghua No. 16	Raw peanut	67.523 ± 0.73^c	24.773 ± 0.45^b	27.727 ± 0.99^a	44.083 ± 0.72^f
		Boiled peanut	74.263 ± 1.09^b	11.997 ± 0.11^hi	14.410 ± 0.56^d	26.000 ± 1.10^l
6	Taihua No. 5	Raw peanut	66.160 ± 0.61^d	21.267 ± 1.41^c	23.613 ± 0.28^b	40.870 ± 0.95^h
		Boiled peanut	75.023 ± 0.57^b	10.857 ± 0.57^ij	13.130 ± 0.69^e	24.343 ± 0.87^m
7	Zhenzhuhei	Raw peanut	22.993 ± 0.22°	6.640 ± 0.45^m	−2.283 ± 0.23^k	71.853 ± 0.20^a
		Boiled peanut	42.837 ± 0.67^j	11.510 ± 0.79^hi	2.517 ± 0.76^i	52.830 ± 0.83^e
8	Zhenzhuhong No. 1	Raw peanut	40.823 ± 0.38^k	35.263 ± 3.39^a	12.837 ± 0.51^e	66.290 ± 0.71^c
		Boiled peanut	59.953 ± 0.35^g	14.863 ± 0.26^fg	8.557 ± 0.11^f	37.957 ± 0.39^j
9	Yueyou No. 13	Raw peanut	64.640 ± 0.47^e	19.367 ± 0.83^d	19.440 ± 0.93^c	39.097 ± 0.44^i
		Boiled peanut	77.470 ± 0.66^a	8.857 ± 0.18^jk	12.870 ± 1.40^e	21.413 ± 0.91^n
10	Commercial Product	Raw peanut	41.550 ± 0.39^k	36.933 ± 0.43^a	13.150 ± 0.43^e	65.420 ± 0.25^c
		Boiled peanut	59.867 ± 0.46^g	13.200 ± 0.45^gh	8.050 ± 0.21^f	37.323 ± 0.27^j

^pValues are mean ± standard deviation of triplicates.

^a-oMeans with the same letter in a column are not significantly (p < 0.05) different.

Oil content of peanut

Oil contents of almost all peanut samples were found to be in the range of 49.5%–53.0%, except for Yueyou No. 13 (light yellow-red) peanut cultivated in Guangdong, Yueyou No. 13 had only 45.5% oil on dry weight basis. Peanut sample #10 was bought from the public market, which was originally cultivar from a private farm in Hubei. However, there were no significant differences in the oil content among most peanut cultivars. Eight of them had a similar oil content. The oil content of eight peanut cultivars out of a total often had high oil content (more than 49%).

Total saponin content of peanut

Raw whole peanuts had a higher saponin content compared to peanut cotyledon and the boiled whole peanuts. Peanut seed coat contributed to the highest saponin content as predicted. As shown in Table 3, Tangshan peanut seed coat contained the lowest amount of saponin. Hence, raw whole peanut and dehulled peanut saponin content, unlike other cultivars, did not have much differences between whole and dehulled peanut. Zhonghua No. 16 peanut cultivar seed coat contributed the highest saponin content, which had a relatively high saponin content in its raw peanut. All 10 peanut cultivars had higher saponin (p < 0.05) in raw whole kernel compared with their peanut cotyledon and the boiled peanut. Zhenzhuhong No. 1 was found to have no differences between its peanut cotyledon and the boiled peanut.

Table 3. Total saponin content (mg/g) of peanuts *

Sample name	Seed coat	Raw whole	Cotyledon	Boiled whole
Heiba No. 1	239.23 ± 3.45^c	4.76 ± 0.16^ef	2.71 ± 0.10^i	2.71 ± 0.15^ijk
Heifeng No. 1	199.27 ± 2.65^d	4.68 ± 0.37^efg	2.54 ± 0.20^i	1.87 ± 0.16^jk
Xuzhou	171.04 ± 13.32^f	10.66 ± 0.50^b	2.61 ± 0.37^h	2.27 ± 0.10^ijk
Tangshan	108.08 ± 4.28^g	5.10 ± 0.16^e	4.58 ± 0.39^fg	1.86 ± 0.09^k
Zhonghua No. 16	381.45 ± 13.57^a	10.60 ± 0.46^a	4.44 ± 0.38^fg	3.31 ± 0.16^h
Taihua No. 5	277.84 ± 2.84^b	4.77 ± 0.35^ef	3.58 ± 0.04^h	2.64 ± 0.25^i
Zhenzhuhei	175.59 ± 6.17^ef	6.43 ± 0.16^cd	4.50 ± 0.24^fg	4.25 ± 0.23^g
Zhenzhuhong No. 1	194.32 ± 4.77^de	6.99 ± 0.68^c	3.56 ± 0.31^h	3.56 ± 0.20^h
Yueyou No. 13	276.99 ± 26.48^b	7.14 ± 0.21^b	3.56 ± 0.29^h	2.54 ± 0.08^ij
Commercial Product	200.63 ± 10.25^d	6.64 ± 0.20^d	2.68 ± 0.16^i	3.12 ± 0.17^h

*Values are mean ± standard deviation of duplicates. Means with the same letter in a column are not significantly (p< 0.05) different.

Anthocyanin content

Three kinds of anthocyanins, including cyanidin-3-O-sambubioside, cyanidin-3-O-glucoside, and peonidin-3-O-glucoside, were mixed in the standard solution. The peaks of each sample were identified by comparing with a retention time of compounds in a standard solution; the retention time of cyanidin-3-O-sambubioside, cyanidin-3-O-glucoside, and peonidin-3-O-glucoside were at 37.10 min, 38.53 min, and 47.87 min, respectively (Supplemental Figure S1). However, the HPLC chromatogram of the peanut samples showed that there were mainly two peaks at 33 min and 37 min, respectively. From the spectrum of each peak, cyanidin-3-O-sambubioside was identified to be one of the main anthocyanins in some types of peanuts. Cyanidin-3-O-glucoside and peonidin-3-O-glucoside were not found in the chromatograms of samples. Besides cyanidin-3-O-sambubioside, another anthocyanin existed in peanut. The spectrum of the unknown anthocyanin is nearly the same as that of cyanidin-3-O-sambubioside.

The distribution of cyanidin-3-O-sambubioside in peanut is shown in Table 4. The results indicated that cyanidin-3-O-sambubioside is mainly distributed in the seed coat of peanut. As for the cotyledon, nearly no cyanidin-3-O-sambubioside was detected. The cyanidin-3-O-sambubioside content in the seed coat of #7 peanut (Zhenzhuhei) was the highest (20.53 mg/g) among all samples, followed by #3 (Xuzhou) (8.76 mg/g) and #1 peanut (Heiba No. 1) (6.70 mg/g), respectively. However, cyanidin-3-O-sambubioside was nearly not detected in light-coloured peanuts, such as #5, #6, #8, #9, and #10 peanuts, which is quite different from other dark-coloured peanuts, especially black peanut (#7 peanut). When comparing the whole kernel, the cyanidin-3-O-sambubioside content of #7 peanut was 0.38 mg/g, which was still the highest content among all the samples. The amount of cyanidin-3-O-sambubioside in the seed coat of #4 peanut (Tangshan) was around 100 times higher than that in the whole kernel, followed by #3 peanut (Xuzhou) and #7 peanut (Zhenzhuhei),which were around 60 times and 50 times higher than their whole kernels, respectively.

Table 4. Distribution of cyanidin-3-O-sambubioside in peanuts.

Sample code	Cyanidin-3-O-sambubioside content (mg/g)
	Whole kernel	Seed coat	Cotyledon
1	0.20 ± 0.01^a*	6.70 ± 0.04^c	ND
2	0.08 ± 0.00^a	4.65 ± 0.10^b	ND
3	0.17 ± 0.01^a	8.76 ± 0.67^d	ND
4	0.04 ± 0.00^a	4.35 ± 0.29^b	ND
5	ND**	0.11 ± 0.00^a	ND
6	ND	0.02 ± 0.49^a	ND
7	0.38 ± 0.01^a	20.53 ± 0.01^e	ND
8	0.01 ± 0.00^a	0.23 ± 0.01^a	ND
9	ND	0.11 ± 0.01^a	ND
10	ND	0.03 ± 0.00^a	ND

*Values marked with the same letters have no significant difference (p < 0.05).

**ND = Not detected.

Discussion

As indicated in our previous study, peanut seed coat colour represents its antioxidant abilities and is relevant to its polyphenol content. In other words, the higher colour value (ΔE) represents a deeper colour, which relatively contributed higher polyphenol content and stronger antioxidant ability. The total flavonoid content and total polyphenol content were found to have higher values in deeper-coloured peanut cultivars.^[7] The boiled peanut samples showed lower ΔE value; plant pigments like anthocyanins were lost from peanut to the boiling water during 20 min of boiling. Regardless of the destruction of pigment structures at high temperatures, the loss of water-soluble pigments and polyphenols into the boiling water resulted in a significant decrease of the antioxidative and anti-ageing abilities.^[13] Loss of colour from black peanut might be more serious than in red or light-coloured peanuts. The loss of pigment from black peanuts (Heiba No. 1, Heifeng No. 1, Xuzhou, and Tangshan) resulted in different ΔE values in its boiled counterparts. The compositions of pigments might be different among these four peanuts. Unlike black peanuts, red peanuts (Zhenzhuhong No. 1 and commercial product) showed no significant difference in both raw and boiled peanuts. This might be attributed to the fact that red peanut has lesser water-soluble pigments compared to black peanuts. Because of the lower ΔE values in pink and light yellow peanuts, the loss of colour seemed to be lesser than black or red seed coat peanut.

According to the agriculture research centre, hybridisation can play an important role in obtaining coloured peanuts, which can provide more nutrients. Unlike genetically modified products, traditional peanut cultivars hybridised without safety argument resulted in high-quality peanut products, which might be necessary for agriculture-predominant countries. With a long agriculture history and various regional planting areas, hybridisation was commonly applied to increase yield in old days in China. Researches findings in the past decades pointed out that the higher quality of agricultural products can be achieved by hybridisation, which is vital to Chinese agriculture development.^[14] For example, Zhonghua No. 16 cultivated in Jiangsu provides a higher colour value compared to the common light red peanut.

The crude fat contents of 10 peanut cultivars are shown in Table 5. Seven peanut cultivars (Heiba No. 1, Heifeng No. 1, Xuzhou, Tangshan, Zhonghua No. 16, and Taihua No. 5 and one commercial product) were found to have a generally higher oil content compared to three peanut cultivars from Guangdong (Zhenzhuhei, Zhenzhuhong No. 1, and Yueyou No. 13). This could be mainly because of the differences in the cultural environment and regional planting areas. Proximately, peanut kernel has 47% oil with 5.80% moisture content. Most of the peanuts in this study contribute 49% oil content, generally higher than the literature finding.^[15]

Table 5. Oil content in peanuts.

Sample name	Moisture content (g/100 g dry solids)	Oil content (g/100g) (wet weight basis)*	Oil content (g/100g) (dry weight basis) *
Heiba No. 1	7.34	45.8 ± 0.03^c	49.5 ± 0.03^abc
Heifeng No. 1	3.48	49.4 ± 0.28^abc	51.2 ± 0.29^ab
Xuzhou	9.80	45.4 ± 0.02^c	50.3 ± 0.02^ab
Tangshan	1.90	50.4 ± 0.08^ab	51.4 ± 0.09^ab
Zhonghua No. 16	3.76	49.0 ± 0.21^abc	50.9 ± 0.22^ab
Taihua No. 5	3.47	50.9 ± 0.33^ab	52.7 ± 0.34^ab
Zhenzhuhei	3.96	43.7 ± 0.10^c	45.5 ± 0.10^c
Zhenzhuhong No. 1	5.61	47.0 ± 0.02^bc	49.8 ± 0.02^abc
Yueyou No. 13	1.96	44.6 ± 0.16^c	45.5 ± 0.17^c
Commercial product	1.76	52.0 ± 0.18^ab	52.9 ± 0.18^a

*Values are mean ± standard deviation of triplicates. Values marked with the same letter in the same column are not significantly (p < 0.05) different.

The highest amount of saponins in Zhonghua No. 16 was originally from Jiangsu Academy of Agriculture Sciences. It was stated that controlled sunshine duration and intensity can result in a deeper seed coat. Together with hybridisation, the deep brown and black colours were obtained. These varieties contained higher amounts of nutrients (trace minerals, amino acids) compared with their traditional counterparts. Peanut products also revealed regional differences. Peanut cultivars from Guangdong Academy of Agriculture Sciences had a low oil yield but a middle level of saponin content in their seed coats and cotyledons. Environmental factors, cropping year, light, temperature, and moisture affect oil composition as well.^[16] Both the duration of peanut soaking and cooking influence saponins reservation. The longer soaking duration and cooking time resulted in reduced levels of saponins in kernel. According to Table 3, the raw seed coat had the highest amount of saponins, followed by raw whole peanut kernel. Boiled whole peanut exhibited a lower amount of saponins without much reduction as compared to its peanut cotyledon, such as Zhenzhuhong No. 1. The current results verified the fact that peanut seed coat contains a vital amount of saponin, which gets destroyed upon boiling.^[17] All 10 raw whole peanuts contributed a significant amount of saponins due to the existence of seed coat and cotyledon.

Anthocyanins, one category of phenolic compounds, are responsible for the dark purple, red, and blue colours of many fruits and vegetables. Fruits rich in anthocyanin include blueberries (Vaccinium L. species), black berries (Rubus L. hybrids), black currants (Ribes nigrum L.), and strawberries.^[18] Anthocyanin content was found to have a high correlation with the colour of black soybean (Glycine max) and coloured common bean (Phaseolus vulgaris).^[19] In the current study, anthocyanin in the peanut also mainly concentrated in the seed coat of the kernel, whereas there was no anthocyanin detected in the cotyledon. As one of the major categories of flavonoids, anthocyanins also contribute to the antioxidant properties in plant food. Among peanuts in the current study, cyanidin-3-O-glucoside and peonidin-3-O-glucoside were not detected, whereas cyanidin-3-O-sambubioside was one of two anthocyanins found among black peanuts.

A previous study also found two major peaks in the HPLC chromatograms in the dark coloured peanuts at around 520 nm.^[8] One of the major peaks was also identified as cyanidin-3-O-sambubioside by mass spectrum and NMR spectrometry.^[8] From the UV spectrum of the unknown anthocyanin in the current samples, it can be found that the spectrum of the unknown anthocyanin is nearly the same as that of cyanidin-3-O-sambubioside. Therefore, it could be inferred that, excluding cyanidin-3-O-sambubioside, there is still one more kind of cyanidin that exists in the seed coat of peanut and contributes to the total anthocyanin content. This unknown anthocyanin may also have very similar physiochemical properties as cyanidin-3-O-sambubioside with higher polarity. Based on a previous study, cyanidin-3-β-xylosyl was a predominant anthocyanin in purple-black seed coat of peanut.^[20] Therefore, the unknown anthocyanin in the current research could be cyanidin-3-β-xylosyl.

As far as regular consumption and food processing of peanut is concerned, seed coats are usually shedding out from kernel. Therefore, although peanut has been found to have an antioxidant effect, consumers may only gain parts of antioxidants from peanuts if the peanut skin is not consumed. As mentioned before, the world production of peanut was estimated to be 41.1 million in 2013/2014 ^[1]. At the same time, the seed coats of peanut were majorly discarded as by-products. If the seed coat of peanut can be utilised, it will be a good source of anthocyanin when compared with berries because the biomass of seed coat is huge in comparison. As one of the natural pigments, anthocyanin is not only safe, but also regarded as a natural colourant, antioxidant, and bioactive ingredient.^[8] Nowadays, synthetic antioxidants in the food industry are gradually being restricted, and thus more attention is now being paid to natural antioxidants. In the future, anthocyanins can play a very important role in the fields of medicine, cosmetics, supplements, and health food. Therefore, investigation about anthocyanin in the seed coat of black peanut is a potential economic issue in the future.

Among all peanuts, the anthocyanin content was found to be high in dark-coloured peanuts (#1, #2, #3, #4, and #7 peanuts), whereas it is hardly found in light-coloured peanuts (#5, #6, #8, #9, and #10 peanuts). The current research also found higher anthocyanin content in #7 peanut, which is the darkest peanut sample among all the samples studied. Recent research has investigated the relationship between skin colour and polyphenolic content of different peanut cultivars, and found that black peanut has a higher content of monomeric anthocyanin.^[11,21] Yvonne’s research did not find high correlation between proanthocyanin and colour, but the hug angle of peanut skin was found to be a possible biomarker for the total phenolic content rather than the total flavonoid content.^[7] The peanut cultivar studied in Chukwumah’s research are mostly red-coloured peanuts, and hence the correlation between the total flavonoid content and colour is not very obvious. Therefore, anthocyanin content as a biomarker may be used among black peanuts rather than red peanuts. Moreover, the colour of cotyledon is commonly white or light yellow. Therefore, it could be explained that anthocyanin content is rich in the seed coat rather than cotyledon.

Conclusions

Comparative study on black and red seed coat peanuts products in China was performed to understand the differences of chemical composition between different colours. As a widely consumed crop, peanut is vital in China for oil and commercially processed products. Higher oil contribution and more nutritious chemical compositions in seed coat extend the usage level of peanut. According to this investigation, colours of seed coat varied across regions. Oil and saponin content did not exhibit significant difference between the red seed coat and black seed coat. The current research found that anthocyanins were mainly concentrated in the seed coat of the peanut. Moreover, cyanidin 3-sambubioside was the main anthocyanin existing in peanut, especially in the black seed coat of the peanut. Anthocyanin is also found to be rich in deep-coloured peanuts and the content was found to have a correlation with colour. Therefore, the seed coat of peanut is a potential source of antioxidant for not only health food products, but also cosmetic and medical products.

Funding

This research was jointly supported by one grant (UICRG201624) from Beijing Normal University-Hong Kong Baptist University United International College, China, and a research grant (R1053) from Zhuhai Key Laboratory of Agricultural Product Quality and Food Safety.

Additional information

Funding

This research was jointly supported by one grant (UICRG201624) from Beijing Normal University-Hong Kong Baptist University United International College, China, and a research grant (R1053) from Zhuhai Key Laboratory of Agricultural Product Quality and Food Safety.