Abstract
Seed oils have been used for centuries as a source of food, energy, medicine and for cosmetic applications. In recent years, demand for seed oils has greatly increased. Carrot, flax, hemp and canola seed oils are composed mainly of unsaturated fatty acids, which pose a positive effect on human health. Linoleic and linolenic acids found in flax, hemp and canola seed oils are essential nutrients that may provide favourable effects against cardiovascular disease, diabetes, cancer, and age-related functional decline. However, carrot, flax, hemp and canola seed oils may be susceptible to oxidation due to the high unsaturated fatty acids content. It is hence of importance to know their physicochemical characteristics.
The major fatty acid C18:1 contributed to 81.45 ± 1.67% of total fatty acids, while polyunsaturated fatty acids (PUFA) and saturated fatty acids (SFA) contributed to13.80 ± 0.23% and 4.75 ± 0.18% respectively in carrot seed oil which was high in yellow colour parameters. It was oxidatively stable and had good quality based on its moisture and volatile content, peroxide value (PV), p-anisidine value (p-AnV), total oxidation value (TOTOX), free fatty acid (FFA), acid value (AV), conjugated diene and triene values, and polar compounds content.
Four major triglyceride (TAG) peaks in carrot seed oil were collected via high performance liquid chromatography (HPLC) and identified by electrospray ionization- mass spectroscopy (ESI-MS). The most abundant TAG was dioleoyl-linoleoyl-glycerol (38.24%), followed by trioleoyl-glycerol (23.98%). Carrot oil was found to contain a higher amount of trioleoyl-glycerol compared to flax, hemp and canola seed oils. The fatty acid composition analysis on TAG fractions confirmed the results from gas chromatography (GC).
Positional distribution of fatty acids in carrot seed oil was determined via pancreatic lipase treatment. The SFA prefer sn-1,3 position (5.38 ± 0.92%) against sn-2 position (2.64 ± 0.34%) while PUFA were predominantly found at sn-2 position (22.02 ± 0.47%) against sn-1,3 position (9.82 ± 1.63%). C18:1 in carrot seed oil was found to be more dominant at sn-1,3 position (84.80 ± 1.89% compared to 75.34 ± 0.13% at sn-2 position), which may be influenced by the other two fatty acids in the same TAG molecule (Kallio et al., 2001). Positional distribution analyses in flax, hemp and canola seed oils were studied by using Novozym 435. SFA was exclusively incorporated into the sn-1,3 position and PUFA were higher at sn-2 position as expected. This treatment showed a similar effect as pancreatic lipase treatment except the two enzymes gave a different ratio. A method using HPLC would be an improvement over TLC for positional distribution analysis and is less time consuming.
Melting and crystallization characteristation of carrot seed oil was carried out using Differential Scanning Calorimetry (DSC) from -60 °C to 40 °C under a nitrogen atmosphere. The melting and crystallization of carrot seed oil occurred over a temperature range of -42.70 °C to 21.31 °C and -18.27 to -46.81 °C, respectively. In comparison of carrot, flax, hemp and canola seed oils, melting profiles were also affected by TAG composition and crystal structure and not only by the fatty acid’s chain length and degree of unsaturation in oil.
Oxidative stability of carrot, flax, hemp and canola seed oils were studied using Thermogravimetric Analysis (TGA) at 70 °C, 80 °C and 90 °C under air atmosphere. Results showed that carrot seed oil was the most stable at all experimental heating temperatures. Prediction of shelf life was calculated based on the onset time for oxidation (to) of isothermal heating using TGA at the different temperature settings. The predicted shelf lives of carrot, flax, hemp and canola seed oils are 2.26, 0.13, 0.22 and 0.58 years at 4 °C, respectively. The data on the effects of temperature on oxidative stability of carrot, flax, hemp and canola seed oils follow the Q10 law for the relation between rate of chemical reaction and temperatures. TGA can be used to predict shelf life of seed oils and could replace conventional shelf life prediction methods due to its simplicity and time-saving nature.
Analysis of thermal stability of carrot, flax, hemp and canola seed oils were carried out using TGA. The thermal decomposition of the four seed oils occurred in two major stages during non-isothermal heating, which were the decomposition of fatty acids (PUFA, MUFA and SFA) and their decomposition products, respectively. Carrot seed oil contained less total phenolic acids and flavonoids compared to the other three oils because of the different oil extraction methods. The effect of natural antioxidants on initial temperature of thermal decomposition was masked because of the significant differences in their fatty acid compositions. The maximum decomposition of seed oils that contained more antioxidants occurred at higher temperatures compared with the oil contained less antioxidants.