Characterisation of optimised supercritical carbon dioxide chia seed oil using tocopherol quantification, fatty acid profile, oxidation kinetic and digestion study

Particle size affects the yield of seed oil extraction other than supercritical carbon dioxide (SC-CO2) parameters. Chia seed oil (CSO) extracted by supercritical fluid extraction (SFE) is vulnerable to oxidation which contains more than 80% polyunsaturated fatty acids (PUFA). However, limited study has been conducted on examining the effect of different particle sizes of chia seed in extracting a high yield of oil using SC-CO2. Therefore, the objectives of this study are: i) To determine the optimum SC-CO2 extraction parameters and particle size of the ground sample to extract the highest yield of CSO. ii) To compare the optimised CSO extracted by SCCO2 (SC-CO2-CSO) characteristics with Soxhlet (SOX-CSO) in terms of oxidation levels, tocopherols, PUFA, and oxidative stability. iii) To evaluate the oxidation stability of SC-CO2-CSO during storage with SOX-CSO. iv) To identify the bioaccessibility of tocopherols and volatile oxidation compounds of SC-CO2-CSO (fresh and stored) using in vitro stomach and small intestine. The optimisation parameters include different particle sizes of chia seeds based on grinding times (10-30 s) and SFE at different temperatures (40-80 °C) and pressure (220-340 bar) to maximise the CSO yield. The optimum yield (30.7%) of CSO based on Central Composite Design is similar to the predicted value (31.1%) at the pressure (335 bar), temperature (40 °C), and grinding time of chia seed (20 s). The oxidative stability shows both CSO (SC-CO2-CSO: 0.88 h and SC-CO2-SOX: 1.49 h) are less protected against oxidation due to the high amount of PUFA (85%) in CSO. For objective 3, the oxidation level, degradation of tocopherols, and antioxidant activity of SC-CO2-CSO and SOX-CSO stored at different temperatures (25-60 °C) were evaluated kinetically based on the reaction rate constant (k) and activation energy (Ea). The k for oxidation level (PV:0.047-0.468 mEq O2/kg oil day-1), degradation of antioxidant activity (0.000-0.552 % day-1) and α- (0.031-0.233 mg/kg oil day-1) and γ-tocopherols (0.003- 0.03 mg/kg oil day-1) of SC-CO2-CSO and SOX-CSO increased significantly (p<0.05) as storage temperatures increased from 25-60 °C. Lower Ea for the degradation rates of tocopherols (0.012-0.032) than antioxidant activity (147.429-149.26) and oxidation levels (52.779-54.756) in SC-CO2-CSO and SOX-CSO were obtained due to the tocopherol acted as a hydrogen donor to prevent oxidation during storage. As storage temperature increases, the shelf life of SC-CO2-CSO and SOX-CSO is decreased significantly (p<0.05) from 2.35 to 0.23 months. Finally, fresh (PV: 0.6 mEq O2/kg oil) and stored (PV: 26.7 mEq O2/kg oil) SC-CO2-CSO were submitted to the in vitro stomach and small intestine digestion models to determine the bioaccessibility of tocopherols and fatty acids, antioxidant activity and volatile oxidation compounds. Stored SC-CO2-CSO has significantly (p<0.05) higher concentration of aldehydes (2.58-12.29 Bp x 106), lower tocopherols (37.87-73.54%), α-linolenic acid (36.53- 89.97%) and antioxidant activity (6.6-12.6%) are prone to oxidation than fresh SCCO2- CSO using in vitro stomach and small intestine models due to occurrence of oxidation process. Therefore, fresh SC-CO2-CSO has higher bioaccessibility of PUFAs and tocopherols, antioxidant activity, and lower oxidation than stored SCCO2- CSO, which can be diversified as vegetable-based oil supplement and functional food ingredient.

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