Chemical composition and antioxidant activity of the coriander cake obtained by extrusion

2.1 Chemicals

All solvents used in these experiments (diethyl ether, chloroform, acetonitrile and methanol) were purchased from Merck (Darmstadt, Germany). Sulphuric acid (H₂SO₄), acetic acid, sodium hydroxide (NaOH), hydrochloric acid (HCl), sodium carbonate (Na₂CO₃), sodium nitrite (NaNO₂), butylated hydroxytoluene (BHT), 1,1-diphenyl-2-picrylhydrazyl (DPPH), polyvinyl polypyrrolidone Folin-Ciocalteu reagent and aluminium chloride (AlCl₃) were purchased from Sigma–Aldrich (Steinheim, Germany).

2.2 Plant material

All trials were carried out using a single batch of coriander fruits obtained from Korba area (Northwestern Tunisia; latitude 36°34″38.22″(N); longitude 10°51″29.63″(E); altitude 637 m). In this study, the fruits from coriander were extracted with single screw press extruder, the cake samples were immediately collected for further analysis.

2.3 Extrusion

Extrusion was done by a Single-screw (Model OMEGA 20, France) with a motor (0.75 kW, 230 V of maximal tension, 5.1 A of maximal intensity), a screw length of 18 cm, a pitch screw of 1.8 cm, with an internal diameter of 1.4 cm, a channel depth of 0.5 cm and a sleeve of 2.5 cm of internal diameter equipped with a filter-pierced outlet for liquid at the end of the screw and at the surface of the nozzles. The filter section was of 2 mm in diameter to separate extracted oil. The feed rate and the screw rotation speed were maintained constant to 15 g/min (0.9 kg/h) and 40 rpm, respectively. Five nozzles of different diameters (5, 6, 7, 8 and 9 mm) were used in the pressing of the coriander seed. The nozzle/screw distance was 3 cm. The screw press was first run for 15 min without seed material but with heating via an electrical resistance-heating ring attached around the press barrel, to raise the screw press barrel temperature to the desired value. Running temperature was adjusted with a thermocouple (Sriti et al., 2011b).

2.4 Essential oil extraction

The cake (300 g of dry matter) recuperated under extrusion was hydrodistilled for 6 h using a Clevenger apparatus and the yield of volatile oil was calculated for meal sample. The obtained essential oil was dried over anhydrous sodium sulphate, then stored at 4 °C until it is tested and analysed.

2.5 Gas chromatography–flame ionisation detector (GC–FID) analysis

Essential oils were analysed by gas chromatography (GC) using a Hewlett–Packard 6890 apparatus (Agilent Technologies, Palo Alto, CA, USA) equipped with a flame ionisation detector (FID) and an electronic pressure control (EPC) injector. A HP-Innowax capillary column (polyethylene glycol: 30 m × 0.25 mm i.d ×0.25 mm film thickness; Agilent Technologies, Hewlett–Packard, CA, USA) was used; the flow of the carrier gas (N₂, U) was 1.6 ml/min. Analyses were performed using the following temperature programme: oven isotherm at 35 °C during 10 min, from 35 to 205 °C at the rate of 3 °C/min and isotherm at 205 °C during 10 min. Injector and detector temperatures were held, respectively, at 250 and 300 °C. Diluted samples of 2.0 μl were injected in the split/splitless mode (60:1 split) mode.

2.6 Gas chromatography–mass spectrometry (GC–MS)

The GC–MS analyses were performed on a gas chromatograph HP 5890 (II) interfaced with an HP 5973 mass spectrometer (Agilent Technologies, Palo Alto, California, USA) with electron impact ionisation (70 eV). A HP-5MS capillary column (60 m × 0.25 mm, 0.25 μm film thickness) was used. The column temperature was programmed to rise from 40 to 280 °C at a rate of 5 °C/min. The carrier gas was Helium with a flow rate of 1.2 ml/min. Scan time and mass range were 1 s and 50–550 m/z., respectively. The injected volume was 1 μl and the total run time was approximately 63 min.

Identification of the oil constituents was based on the comparison of their retention indexes relative to C8–C22 n-alkanes with those in the literature or with those of authentic compounds available in our laboratory. Further identification was made by matching their recorded spectra with those stored in the Wiley/NBS mass spectral library of the GC–MS data system and other published mass spectra (Adams, 2001).

2.7 Preparation of extracts

The air-dried coriander cake was finely grounded with a blade-carbide gringing (IKA-WERK Type: A: 10). Triplicate sub-samples of 2.5 g of each ground sample were separately extracted by stirring with 10 ml of pure methanol for 30 min. The extracts were then kept for 24 h at 4 °C, filtered through a Whatman No. 4 filter paper, evaporated under vacuum to dryness and stored at 4 °C until their analysis (Mau et al., 2001).

2.8 Total phenolic content

Total phenolic contents were assayed using the Folin–Ciocalteu reagent, following Singleton’s method slightly modified by Dewanto et al. (2002). An aliquot (0.125 ml) of a suitable diluted methanolic cake extract was added to 0.5 ml of deionised water and 0.125 ml of the Folin–Ciocalteu reagent. The mixture was shaken and allowed to stand for 6 min, before adding 1.25 ml of 7% Na₂CO₃ solution.

The solution was then adjusted with deionised water to a final volume of 3 ml and mixed thoroughly. After incubation for 90 min at 23 °C, the absorbance versus prepared blank was read at 760 nm. Total phenolic contents of coriander cakes (three replicates per treatment) were expressed as mg gallic acid equivalents per gram (mg GAE/g) through the calibration curve with gallic acid. The calibration curve range was 50–400 mg/ml (R² = 0.99). All samples were performed in triplicates.

2.9 Total flavonoid content

Total flavonoid contents were measured according to the method of Dewanto et al. (2002). A total of 250 μl of methanolic extract was mixed with 75 μl NaNO₂ (5%). After 6 min, 150 μl of 10% AlCl₃ and 500 μl of NaOH (1 M) were added to the mixture. Finally, the mixture was adjusted to 2.5 ml with distilled water. The absorbance versus prepared blank was read at 510 nm. Total flavonoid contents of cakes (three replicates per treatment) were expressed as mg catechin equivalents per gram (mg CE/g) through the calibration curve with catechin. The calibration curve range was 50–500 mg/ml. Triplicate measurements were taken for all samples.

2.10 Condensed tannin content

The total tannin content was measured using the modified vanillin assay described by Sun et al. (1998). A total of 3 ml of 4% methanol vanillin solution and 1.5 ml of concentrated H₂SO₄ were added to 50 μl of suitably diluted sample. The mixture was kept for 15 min, and the absorbance was measured at 500 nm against methanol as a blank. The amount of total condensed tannins was expressed as milligrams of (+)-catechin equivalent per gram of dry weight (mg of CE/g of DW) through the calibration curve with catechin. Triplicate measurements were taken for all samples.

2.11 DPPH assay

The donation capacity of the obtained extracts was measured by bleaching of the purple-coloured solution of the DPPH radical according to the method of Hanato et al. (1998). A total of 1 mL of different concentrations of extracts prepared in 80% acetone was added to 0.5 ml of a 0.2 mmol/l DPPH methanolic solution. The mixture was shaken vigorously and kept at room temperature for 30 min. The absorbance of the resulting solution was then measured at 517 nm after 30 min. The antiradical activity was expressed as IC₅₀ (μg/ml), the concentration required to cause 50% DPPH inhibition. The ability to scavenge the DPPH radical was calculated using the following equation: $$\text{DPPH scavenging effect}(\%)=[(A_0-A_1)/A_0]\times 100$$ where A₀ is the absorbance of the control at 30 min, and A₁ is the absorbance of the sample at 30 min. BHT was used as a positive control. Tests were carried out in triplicate.

2.12 β-Carotene bleaching test

A modified method described by Koleva et al. (2002) was employed. β-Carotene (2 mg) was dissolved in 20 ml of chloroform. Then, 4 ml of this solution was added to linoleic acid (40 mg) and Tween 40 (400 mg). Chloroform was evaporated under vacuum at 40 °C, and 100 ml of oxygenated ultrapure water was added. Then, the emulsion was vigorously shaken. Reference compounds [BHT and butylated hydroxyanisole (BHA)] and sample extracts were prepared in methanol. The emulsion (3 ml) was added to a tube containing 0.2 ml of different concentrations of essential oils and extract. The absorbance was immediately measured at 470 nm, and the test emulsion was incubated in a water bath at 50 °C for 120 min, when the absorbance was measured again. BHT and BHA were used as a positive control. In the negative control, the essential oil or the extract was substituted with an equal volume of methanol. The antioxidant activity (%) of the acetone extracts and essential oils was evaluated in terms of the bleaching of the β-carotene using the following formula: $$\text{percent inhibition}(\%)=[(A_t-C_t)/(C_0-C_t)]\times 100$$ where A_t and C_t are the absorbance values measured for the test sample and control, respectively, after incubation for 120 min, and C₀ is the absorbance values for the control measured at zero time during the incubation. The results are expressed as IC₅₀ values (μg/ml), the concentration required to cause a 50% β-carotene bleaching inhibition. Tests were carried out in triplicate.

2.13 Reducing power

The method of Oyaizu (1986) was used to assess the reducing power of different cake extracts. A total of 1 ml of different concentrations of extracts in 80% acetone was mixed with 2.5 ml of a 0.2 M sodium phosphate buffer (pH 6.6) and 2.5 ml of 1% potassium ferricyanide [K₃Fe(CN)₆] and incubated in a water bath at 50 °C for 20 min. Then, 2.5 ml of 10% trichloroacetic acid was added to the mixture that was centrifuged at 650g for 10 min. The supernatant (2.5 ml) was then mixed with 2.5 ml of distilled water and 0.5 ml of 0.1% ferric chloride solution. The intensity of the blue–green colour was measured at 700 nm. The EC₅₀ value (mg/ml) is the extract concentration at which the absorbance was 0.5 for the reducing power and was calculated from the graph of absorbance at 700 nm against extract concentration. Ascorbic acid was used as a positive control. Tests were carried out in triplicate.

2.14 Statistical analysis

All extractions and determinations were conducted in triplicate. Data are expressed as mean (standard deviation (SD). The means were compared by using the one way followed by Duncan’s multiple range tests. Individual means were deemed to be significant at p ⩽ 0.05. A Cluster Analysis (CA) was performed in order to discriminate between different nozzle diameters on the basis of their essential oil composition. All analyses were performed by the ‘Statistica v 5.1’ software (Stasoft, 1998).

3.1 Effect of operating conditions on essential oil yield

The variation of coriander cake essential oil yield as affected by nozzle diameter is illustrated in Fig. 1. Results showed that the essential oil yield of cake decreases significantly by about 0.11% and 0.04% for 5 mm and 9 mm of nozzle diameter, respectively. It can be explained that the elevation of nozzle diameter contributes to the decrease of pressure on the matter. On the other hand, the decrease in the essential oil yield can be attributed to the fact that the temperature increased similarly to the extracting pressure.

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Figure 1 Effects of nozzle diameter on the essential oil yield (%) of Coriandrum sativum cake. Oil yield values with different subscript (a–c) were significantly different at p < 0.05 (Duncan test).

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According to Buggle et al. (1999), increasing temperature resulted in decreasing in essential oil content of Cymbopogon citratus (DC) Stapf. Similar results have been also reported by Braga et al. (2005) in Piper hispidinervium C.DC and by Khangholi and Rezaeinodehi (2008) in Artemisia annua plants as affected by drying method.

The essential oil yield in coriander fruit before extrusion was 0.37%. Vegetable oil extraction processes therefore reduce the essential oil yield, which is not surprising given the volatile nature of most components of essential oils.

3.2 Chemical composition of coriander cake

Effects of extrusion conditions on the essential oil composition of coriander cake are shown in Table 1.

The GC and GC–MS analysis of these oils resulted in the identification of 39 constituents, representing 70–95% of the oils.

Significant changes (p < 0.05) were observed in the chemical profile of the essential oils. Nevertheless, monoterpene alcohols remained the major class (54–79%) followed by monoterpene esters (4–10%), hydrocarbons (1–14%), monoterpene Ketones (1–3%), monoterpene aldehydes (0.31–0.52%), sesquiterpenes (0.25–0.81%), monoterpene ethers (0.14–0.30%) and phenols (0.05–0.34%). In contrast, the relative percentages of the individual volatiles varied significantly (p < 0.05) under different nozzle sizes. Linalool was the major compound of essential oils from all the trials and it reached a maximum at the nozzle diameter of 6 mm (76.11%).

The second main compound, geranyl acetate, increased as the nozzle diameter increased where the highest percentage was registered at 9 mm with 7.45% and its lowest percentage was detected in the trial of 7 mm of nozzle diameter (3.43%). The γ-terpinene reached a maximum percentage at nozzle diameter of 7 mm with 7.63%. As for linalyl acetate, it decreased significantly in the bigger nozzle diameter (9 mm) and its highest percentage was obtained in the trial of small nozzle (5 mm) where it reached 4.04% of total volatiles. The rates of camphor decreased in the trial of nozzle diameter at 6 mm and 9 mm whereas, the percentages of α-pinene increased significantly in the trial of nozzle diameter at 8 mm (4.63%) and decreased using the small diameter (5 and 6 mm).

The variation of individual compounds under the effect of nozzle diameter affects the composition of the different chemical classes. The percentages of monoterpene alcohols and hydrocarbons decreased significantly (p < 0.05) and reached its lowest value (54.57% and 1.87%, respectively) in the trial of 9 mm in diameter. However, the rates of monoterpene ester increased under nozzle diameter of 9 mm (10.15%) whereas they decreased under the other nozzle diameter. The highest level of sesquiterpenes was detected in the diameter of nozzle 9 mm. As for monoterpene ketones, this class decreased only under nozzle diameter 9 mm (1.92%). Finally, phenols decreased under nozzle diameter 7 and 8 mm (0.05%) except for diameter 5 mm where their rates increased significantly and reached 0.34%.

To the best of our knowledge, the effect of nozzle diameter on essential oil composition is investigated for the first time in coriander cakes. For this reason, it is difficult to compare results of this study with other works. However, some works present the hydrodistillation extracting the highest number of compounds (Msaada et al., 2007; Telci et al., 2009) while others attribute a better performance to the conventional techniques (Mhemdi et al., 2011).

These authors showed that only sixteen compounds were identified in dried seeds before supercritical extraction. This could be explained by the evaporation of some of the volatile compounds, such as camphene, α-thujene, 1,8-cineole, menthol, carvone, during drying.

Essential oil composition depends on many factors affecting the plant such as genetic structures and ecological conditions (Telci et al., 2006). For example, maturation stages constitute an important factor influencing essential oil composition in some plants (Telci et al., 2009).

In order to diagnose and characterise the correlation among the diameter nozzle of cake oil, the resulting dendrogram (Fig. 2), was useful for obtaining pre-selected profiles of high similarity. The data set was divided into two groups. The first cluster included trials of 8 and 9 mm of nozzle diameter. The second cluster can be subdivided into two subgroups: trials obtained with the nozzle of 5 and 7 mm in diameter; trials included the diameter 6 mm.

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Figure 2 Two-dimensional dendrogram obtained from the cluster analysis of the essential oils of the different trial of C. sativum cake based on the data (Table 1): horizontal, samples analysed; vertical, differentiation level between samples. N: nozzle diameter (mm).

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3.3 Antioxidant activity

In the present study, the antioxidant activities of coriander cakes obtained by the different trials as compared with BHT as reference antioxidant compound were determined, and the results are summarised in Table 2.

Free radical scavenging properties of methanolic extract from different trials are presented in Table 2. Lower IC₅₀ value indicated higher antioxidant activity. All methanolic extracts of different coriander cakes (IC₅₀ = 55 μg/ml for nozzle 5 mm, IC₅₀ = 73 μg/ml for nozzle 7 mm, IC₅₀ = 88 μg/ml for nozzle 9 mm) showed lower scavenging ability on DPPH radicals when compared to the synthetic antioxidant BHT (IC₅₀ = 25 μg/ml).

The extract demonstrated a concentration dependent scavenging activity by quenching DPPH radicals; DPPH radical scavenging activities of coriander cake extract increased with increased the nozzle diameter. In this context, Shih et al. (2009) reported that the extrusion process significantly increased the DPPH radical scavenging activity in the different sweet potato extrudates. In the same way, Sharma et al. (2012) were also found the antioxidant activity increased significantly upon extrusion of barley. It has been proved that antioxidant activity of plant extracts is mainly ascribed to the concentration of the phenolic compounds present in the plants (Heim et al., 2002).

In this study, we evaluated the antioxidant activity of extracts of different trials of coriander cakes by the β-carotene-linoleate bleaching method because β-carotene shows strong biological activity and constitutes a physiologically important compound (Kumazawa et al., 2002; Sakanaka et al., 2005). This method is based on the loss of the yellow colour of β-carotene due to its reaction with radicals formed after linoleic acid oxidation in emulsion. The rate of β-carotene bleaching can be slowed down in the presence of antioxidants (Kulisic et al., 2004).

All extracts had lower antioxidant activities than BHA and BHT with IC₅₀ ranging from 543 μg/ml of small nozzle (5 mm) and 610 μg/ml of bigger diameter (9 mm).

The reducing power of a bioactive compound may also serve as a significant indicator of its potential antioxidant activity (Roginsky and Lissi, 2005). Table 2 showed that the Fe³⁺ reducing power of cake trials differs greatly depending on nozzle diameter. The coriander cakes showed the lower reducing capacity (EC₅₀ = 700 μg/ml) compared to that of the ascorbic acid (EC₅₀ = 40 μg/ml).

3.4 Total phenolic, total flavonoid and total tannin contents of coriander cake

Phenolic compounds serve as important antioxidants because of their ability to donate a hydrogen atom or an electron in order to form stable radical intermediates. So, they prevent the oxidation of various biological molecules (Cuvelier et al., 1992). In addition, several oilseeds and their byproducts have been investigated for phenolic compounds in search for safe sources of natural antioxidants (Wettasinghe et al., 2002). In the case of cereal grains it has been shown that their outer layers such as husk and aleurone cells contain the highest concentration of total phenolics (Kähkönen et al., 1999).

Fig. 3 summarises the results from the quantitative determination of the tannins, flavonoids and total phenol contents of the different methanolic cake extracts. The total phenolic and flavonoid contents varied between different nozzle diameters; the highest values unregistered of small diameters (5 and 6 mm). The total phenol contents decreased significantly (p < 0.05) when increased the nozzle diameter and reached its lowest value (9.11 mg GAE/g) of 9 mm in diameter. In the present study, total phenolic compound of coriander cake extract was found to be higher than sesame cake extract (1.94 mg GAE/g DW) (Mohdaly et al., 2011), potato peels (2.91 mg GAE/g DW) (Mohdaly et al., 2010), banana (2.32 mg GAE/g DW) (Nagendran et al., 2006), carrot (1.52 mg GAE/g DW) (Kequan and Liangli, 2006) and wheat bran (1.0 mg GAE/g DW) (Kähkönen et al., 1999). However, total phenol content in our study was found to be lower than black cumin seed cake (27.8 mg GAE/g). It has been reported that the reduction in total phenolic content may be attributed either to the decomposition of phenolic compounds due to the high extrusion temperature or alteration in molecular structure of phenolic compounds that may lead to reduction in the chemical reactivity of phenolic compounds or decrease their extractability due to certain degree of polymerisation (Altan et al., 2009).

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Figure 3 Total phenol (PH), flavonoid (FL) and tannin (TA) contents of different nozzle diameter from coriander cakes. ^∗ Total polyphenol and tannin contents were expressed by mg GAE/g DW and total flavonoid contents were expressed by mg CE/g DW. Values are represented as mean ± standard deviation of triplicates. The data marked with the different lower case letters in the histograms of each phenolic category share significant differences at p < 0.05 (ANOVA test).

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A significant decrease in the total flavonoid content was observed under parameter of extrusion (Fig. 2). The highest values were observed with small nozzle diameter (5 and 6 mm) and reached 2 mg CE/g DW in coriander cake extracts. The lowest total flavonoid contents were signalled of nozzle diameter to 9 mm in cake extract with 1.18 mg CE/g DW. However, Mohdaly et al. (2011) found that the total flavonoid content in sesame cake extract was 0.88 ± 0.02 (mg QE/g DW). The decrease in total flavonoid content may be attributed to the thermal destruction of flavonoids, as this later are reported to be heat sensitive (Sharma and Gujral, 2011; Xu and Chang, 2008), therefore the total flavonoid content decreased upon extrusion cooking. In the same way, these results are in agreement with those previously reported by Im et al. (2003) and Huang et al. (2006) for buckwheat and sweet potato, respectively, upon thermal processing.

Significant differences were also found in total tannin contents among different nozzle diameters, the highest values were observed with nozzle diameter of 7 mm and reached 3.00 mg CE/g DW. Contrary to total phenol and flavonoid contents, total tannin contents increased under the increased nozzle diameter to 9 mm and reached 2.3 mg CE/g DW.