Chemical composition and antioxidant capacity of black pepper pericarp

Piper nigrum L. is a widely used spice because of its flavour and health effects. It is prepared as black and white pepper, according to the harvest time and inclusion of the outer skin. Pepper pericarp is usually considered waste when making white pepper. In this study, bioactive and flavour compounds and minerals in the pericarp of black pepper were determined to identify its applications. The pericarp contained total phenol, total flavonoid and piperine contents of 1421.95 ± 22.35 mg GAE/100 g, 983.82 ± 8.19 mg CE/100 g and 2352.19 ± 68.88 mg/100 g, respectively. There were higher levels of total phenols and total flavonoids in the pericarp compared with black pepper and white pepper. Piperine content was lower in the pericarp than in black pepper. The principal monoterpene compounds in the pericarp were α-pinene (9.2%), 2-β-pinene (14.3%), δ-3-carene (21.5%) and dl-limonene (18.8%), and the primary sesquiterpenes were α-copaene (5.1%) and caryophyllene (17.2%). The higher percentages of flavour compounds found in the pericarp would impart a more potent odour, and the pericarp exhibited higher minor and tiny differences based on electronic nose analysis. It had more minerals than black pepper and peeled black pepper.


Introduction
Piper nigrum L. is an early and widely used spice in human history and is highly valued for its characteristic pungency. It is prepared as black and white pepper, according to the harvest time and processing method [1,2]. Black pepper is the dried immature but fully developed fruit of pepper plants, whereas white pepper consists of the mature fruit devoid of the outer skin (pericarp) [3,4]. Black pepper has been used as a spice and in herbal medicines. It also has applications as a preservative and biocontrol agent [5]. The essential oil or extract of black pepper displays antioxidant, anti-fungal, anti-amoebic, anti-asthmatic, anti-diabetic and immunomodulatory activities [3,6,7]. White pepper has many different applications too, such as a preservative, insecticide, anti-bacterial, anti-fungal and anti-protozoal herb [5,8,9]. Especially, black and white peppers contain a bioactive compound named piperine, also found in other members of the pepper family (Piperaceae), including long pepper (Piper longum). Piperine is the most abundant and active alkaloid in pepper. It is used as a therapeutic agent because of its many health benefits associated with its antioxidant, anti-inflammatory and drug activity-enhancing activities, respiratory effects and inhibitory action on lung metastasis [10]. Spatial memory and neurodegeneration of Alzheimer can be improved by piperine in animal models [11].
Although the chemical composition of black and white peppers has been investigated, so far, no study has determined the chemical components in the pericarp of pepper. When the pericarp is removed from pepper to make white pepper, it is normally discarded. If the pericarp can be used to manufacture other pepper products, it would be economically beneficial. In this study, we identified the chemical components and antioxidant activity of pepper pericarp, and the advantages of using this part of the pepper fruit for food.

Samples
Pepper of 5 kg was prepared from mature peppercorns harvested in March 2017, by Ottogi Sesame Mills Co., Ltd (Eumseong, Choungcheonbuk-do, Korea). The peppercorns were separated from the stems after harvesting them, and then they were naturally sun-dried for 7-10 days to obtain a black colour. Finally, pericarp was mechanically peeled off from the pepper using a decorticating machine (Sinco Mechanical JSC, Vietnam). It was ground and sieved by 60-mesh and then it was stored at room temperature in darkness.

Sample preparation
The ground pericarp sample of 5 g was homogenised with 80% ethyl alcohol of 100 mL using KWG-150 of Sunway Electric Manufacture (Heshan, China). The mixture was incubated for a day in shaking incubator (a DH.WIS 02011, Daihan Scientific Co., Ltd., Daegu, Gyeongbuk, Korea), followed by centrifugation at 15,000 rpm for 10 min (Mega21R, Hanil Scientific, Inc., Gimpo, Gyenggi-do, Korea). The upper layer was taken and filtered by a Minisart filter (0.45 μm pore size, regenerated cellulose) and then concentrated to 20 mL on a rotary evaporator (N-1000, Eyela, Tokyo, Japan). The extract was stored at − 80 °C until analyzing chemical components and antioxidant activities.

Bioactive compounds and antioxidant activities of pepper pericarp Total flavonoids
The total flavonoids were determined by a colorimetric assay [19,20]. A 1 mL of sample extract and distilled water (4 mL) were placed in a 15-mL tube, then 0.3 mL of 5% NaNO 2 was added to the tube, and the mixture was left to react at room temperature for 5 min. After adding 0.3 mL of 10% AlCl 3 , the mixture was left to react further at room temperature for 6 min before 2 mL of 1 N NaOH was added. Distilled water was added to adjust the total volume to 10 mL. The absorbance was recorded by a spectrophotometer (Optizen POP, Mecasys, Daejeon, Korea) at 510 nm. Catechin was used as the standard to establish the calibration curve. Total flavonoids were expressed as milligrams of catechin equivalents (CE)/100 g fresh weight (FW).

Total phenols
The total phenols were analysed by the Folin-Ciocalteu colorimetric method [19,20]. Folin-Ciocalteu reagent (0.2 mL) was added to a 15 mL tube containing 0.2 mL of the sample extract and 2.6 mL of distilled water. The mixture was left at room temperature for 6 min, and then 0.2 mL of 7% Na 2 CO 3 was added, followed by incubation at room temperature for 90 min in the dark. Spectrophotometric absorbance was determined at 750 nm. The total phenol contents were shown as milligrams of gallic acid equivalents (GAE)/100 g FW by evaluating a gallic acid calibration curve.

Piperine
Piperine was determined, as described by Santosh et al. with some modifications [21]. A 50 mL conical tube containing 0.1 g of the sample extract and 50 mL methanol was extracted by ultrasonication at 50 °C for 20 min. The extracted sample was cooled down at room temperature and filtered through a Minisart syringe filter (0.45 μm, regenerated cellulose). Piperine was detected using a high-performance liquid chromatography (HPLC) apparatus (Agilent, Santa Clara, CA, USA) equipped with a diode array detector (340 nm) and Eclipse C18 Plus column (4.6 × 150 mm, 5 μm; Agilent) maintained at 25 °C. Acetonitrile-1% citric acid (45:55, v/v) was used as the mobile phase in isocratic mode at the flow rate of 1 mL/min for 20 min. The injection volume was 10 μL. Peak of piperine and its spectra in the HPLC chromatogram was identified (Fig. 1).

DPPH radical scavenging activity
DPPH radical scavenging activity was analyzed by the method reported by Brand-Williams et al. with some modifications [22]. DPPH solution (100 μM) was diluted with 80% methanol to have an absorbance of 0.65 ± 0.02 at a wavelength of 517 nm. The extracted sample (50 μL) was mixed with 2950 μL of DPPH diluted from 0.1 mM of DPPH solution and reacted for 30 min. The absorbance at 517 nm (Optizen POP spectrophotometer, Mecasys) was shown as milligrams of vitamin C equivalents (VCE)/100 g FW.

ABTS radical scavenging activity
The ABTS radical scavenging activity was measured with an analytical method described by Floegel et al. [23]. ABTS (137.175 mg) and AAPH (27.117 mg) were prepared in 100 mL of phosphate-buffer, followed by reaction in a water bath at 70 °C for 40 min to produce ABTS radicals, and then cooled down at room temperature. The working solutions were diluted with the ABTS radical solution by phosphate-buffered saline to obtain an optical density at 734 nm of between 0.63 and 0.67. Finally, the sample (20 μL) and working solution (980 μL) were mixed and reacted at 37 °C for 10 min. The absorbance of the solution at 734 nm was determined by spectrophotometer (Optizen POP, Mecasys). The antioxidant activity of the solution was expressed as milligrams of VCE/100 g FW.

Flavour compounds by gas chromatography/mass spectrometry (GC/MS)
Two grams of sample was added into an amber vial of 20 mL with PTFE/silicone septa, and it was equilibrated at 80 °C for 30 min with the headspace sampler under agitation at 300 rpm. GC/MS analysis was performed using a 6890 GC/MS system (Agilent Co., Santa Clara, CA, USA) installed with an HP-INNOWax column (60 m × 0.25 mm × 0.25 μm; Agilent Co.) [24]. The sample (1 mL) was injected by a syringe in split injection mode (20:1) at 230 °C, with helium (> 99.9% purity) as the carrier gas at a flow rate of 1 mL/min. The oven was initially maintained at 40 °C for 2 min, then heated at a rate of 5 °C/min to 230 °C and maintained at this temperature for 10 min.

Flavour compounds by electronic nose (e-nose)
A GC-type e-nose (Heracles II, Alpha MOS, Toulouse, France) with dual column (non-polar MXT-5 and slightly polar MXT-1701, 10 m length × 180 μm diameter; Restek, Lisses, France) and dual flame ionisation detector (FID) was used for flavour component profile analysis. The oven temperature was increased from 50 °C (maintained for 2 s) to 260 °C at 1 °C/s. The sample (0.8 g) was transferred to a 20-mL headspace vial and incubated at 50 °C for 5 min, with agitation at 500 rpm. Then, 1000 μL was injected at a flow rate of 1 mL/min and measured five times. For peak identification and principal component analysis (PCA), AroChemBase (NIST retention index database) and AlphaSoft (version 17.0; Alpha MOS) were used.

Minerals
Ca, Cu, Fe, K, Mg, Mn, Na and Zn were identified by the procedure developed by Jung et al. [24]. Aliquot of the sample of 0.6 g was prepared in a Teflon digestion vessel with concentrated nitric acid of 7 mL, and it was digested for 40 min under incrementally increasing pressure heating at 1000 W. The digested sample was diluted with water in flasks of 50 mL for analysis of the minerals using an inductively coupled plasma atomic emission spectrometer (ACTIVA-M, HORIBA Jobin-Yvon, Longjumeau, France) with a charge-coupled device. Argon gas plasma was used (flow rate of 13 L/min, sheath flow of 1.5 L/min). The absorption wavelengths were 317.933, 324.754, 238.207, 766.490, 285.213, 275.610, 589.592 and 213.857 nm for Ca, Cu, Fe, K, Mg, Mn, Na and Zn, respectively. A multielement standard solution was diluted for calibration. The determined concentrations were shown as milligrams/100 g FW.

Statistical analysis
All experiments were conducted triplicately. Data were presented as mean ± standard deviation using Microsoft Office Excel 2010 (Microsoft Corporation, Redmond, WA, USA). Analysis of variance was performed using SAS version 9.4 (SAS Institute, Inc., Cary, NC, USA) for the statistical analysis of each experiment. Significance was determined by Duncan's multiple range test with p < 0.05.

Bioactive compounds and antioxidant activity of pepper pericarp
The total phenols, total flavonoids and piperine are important antioxidants in plants, including black pepper [11,25,26]. Pericarp contained total phenol, total flavonoid and piperine contents of 1421.95 ± 22.35 mg GAE/100 g, 983.82 ± 8.19 mg CE/100 g and 2352.19 ± 68.88 mg/100 g, respectively (Table 1). These data were compared with the corresponding

Flavor compounds by an electronic nose
Black pepper, pericarp and inner kernel after removing the skin were analysed by an e-nose. Some of the discriminative volatile compounds were identified by the AroChemBase database (Table 3). Figure 3 shows the GC/FID of discriminative volatile compounds of black pepper, pericarp and inner kernel. 1R-(+)-α-pinene, o-chlorotoluene, cyclopentane, psi-cumene, heptyl mercaptan, dipentene, terpene, limonene, 3-methyldecane, 1-octanethiol, heptyl benzene and methyl undecanoate were identified as the main volatile compounds by an e-nose with two different columns. Although  Figure 4 shows an odour map based on PCA of all volatile compounds of black pepper, and the three-dimensional odour map based on PCA of the discriminative volatile compounds of black pepper, pericarp and inner kernel. PC1 and PC2 accounted for major differences (99.379%) and minor differences (0.5935%), respectively.  The flavour compounds in the various parts of black pepper had different flavour profiles, with the pericarp exhibiting higher minor and tiny differences. It shows that pericarp has a fruitful flavour than the other parts, as determined by GC/MS analysis.

Minerals
Based on the mineral composition of pericarp, K was the most abundant, followed by Ca, Mg, Fe, Mn and Na (Table 4). Comparatively lower concentrations of K, Ca, Mg, Fe, Mn, Zn and Cu have been reported in black pepper (Table 4) [13,30]. Ca and Mg are important minerals in forming bone, strengthening heart functions, relaxing muscle, conducting memory and metaboliting glucose [31][32][33]. Black pepper pericarp would have stronger health effects than black pepper because of its higher concentrations of minerals. Moreover, it contained a much higher level of K relative to white bean (463.1 mg/100 g), one of the best sources of K. Black pepper contained higher concentrations of Ca compared with yoghurt (181.6 mg/100 g) and milk (120 mg/100 g), which are known to contain high levels of Ca. The Mg level in black pepper pericarp was more than that mentioned in spinach (87 mg/100 g), tuna (64 mg/100 g) and brown rice (44 mg/100 g), and only slightly lower than that in almonds (270 mg/100 g) [34][35][36].