2. Materials and methods
2.1 Chemicals and reagents
Commercial CO2 (purity, 99.99%) was supplied by Air liquide Foshan Co., Ltd. (Foshan, China). Tartaric acid, silver chloride were supplied by Aladdin-Reagent Co, Ltd. (Shanghai, China). Isopropyl alcohol, ether, n-hexane, phenolphthalein, sodium hydroxide, acetic acid, chloroform, potassium iodide, sodium thiosulfate, anhydrous sodium sulfate, soluble starch, potassium chloride, potassium hydroxide, hydrochloric acid and ethanol were purchased from Guangzhou Chemical Regent Co, Ltd. (Guangzhou, China). Ultrapure water was produced by Milli-Q Reference (MERCK Millipore, Germany).
2.2 Camellia seed oil sample and physicochemical characteristic analysis
Five commercial pressed Camellia seed oil samples were obtained and denoted from S1 to S5 (Table 2). Camellia seeds were provided by Guangdong Fanlong Agricultural Technology Development Co., Ltd (Jieyang, China) and were grinded to a proper size by an experimental mill. Oil sample (No. S6) extracted by n-hexane was carried out by soxhlet extraction method (GBT14488.1-2008). Supercritical CO2 extractions of Camellia seed were conducted in a HB120-50-05 device (Jiangsu Hongbo Machinery Manufacturing Co., Ltd). 1 kg of Camellia seeds were loaded in the extraction cell, and then CO2 were pumped in to remove air. After reaching the set temperature, pressure was gradually increased by a CO2 high-pressure pump. Extraction started in a cyclic manner for a period of time after reaching set pressure. Oil samples were collected in the pressure reduction container I per 0.5 h, and then merged and classified in the time period of 0-2h, 3h and 4-6h for the subsequent testing. Oil yield was classified as:
Yield% = moil / Mseed * 100%
Single factor analysis with temperature ranging from 313.15 K to 333.15 K and pressure ranging from 20 MPa to 30 MPa were investigated in this study. Samples achieved by SCCE were listed as S7 to S21 according to the condition and sampling time (Table 3). All the oils were sealed and refrigerated before further tests.
Physicochemical characteristic of the oil samples were analyzed according to the PRC National Standard. The moisture and volatile matter (GB 5009.236-2016), peroxide value (GB 5009.227-2016) and acid value (GB 5009.229-2016) were analyzed following the standard methods. Peroxide value and acid value were shown as mean ± standard deviation (SD) in Table 3.
2.3 E-tongue analysis
2.3.1 Sample pretreatment
12 g of oil sample were added to 85 g ultrapure water (333.15 K), stirred by emulsifying mixer for 3 min. After cooling to room temperature, 50 g standard solution (2.24 g KCl and 0.045 g tartaric acid in 1000 mL ultrapure water) was added and then centrifuged for 10 min at 3000 rpm. Aqueous phase were taken for further determination.
2.3.2 Sensors
A commercial Taste-Sensing System SA 402B (Insent Company, Japan) was applied to test the simulated taste of Camellia oil samples. There are two reference sensors and five detecting sensors (Table 4) representing sourness (CA0), bitterness and its aftertaste (C00), astringency and its aftertaste (AE1), saltiness (CT0), umami and richness (AAE). All the sensors were activated for 24 h before of measurement.
2.3.3 Sample analysis
Sensors consists of artificial lipid membranes which are sensitive to different chemical substances and the potential difference between taste sensor and reference sensor is measured and recorded. Each experiment was repeated four times and the later three data profile were selected averaged for subsequent analysis, shown as mean ± standard deviation (SD) in Table 5.
2.4 Statistical analysis
To compare the taste characteristics of Camellia seed oil, correlation analysis (Pearson correlation), principal component analysis (PCA) and hierarchical cluster analysis (HCA) were performed using SPSS 19.0 software (SPSS Inc., Chicago, IL) at test significant differences (p<0.05) (Lin et al., 2020). Estimation of the quality parameters, acid value and peroxide value were also carried out by SPSS 19.0 software using multivariate linear regression technique.