Introduction: Niger seed with scientific name of Guizotia abyssinica Cass from Asteraceae family is a dicotyledonous and one year plant that is one of the main and most important oil sources in Ethiopia and India. In different researches the amount of extracted oil from the seed of this plant has been mentioned about 37 to 50 percent. The composition of fatty acid in this plant is similar to safflower and sunflower seed oil (of course with high percentage of Linoleic acid that may reach more than 85 percent). Generally, the mechanical press is one of the most popular methods for extracting oil from vegetable oil seeds all over the world. Increase in temperature will lead to more and faster extraction of oil from seeds and will increase the extraction efficiency. Microwave, is the non-ionized electromagnetic wave with frequency between 300 MHz to 300 GHz and are placed between the radio and infrared waves in the electromagnetic spectrum, consumers’ increasing demand for higher quality products and with minimized waste of nutrients in comparison with conventional thermal methods, caused an increase in use of non-thermal methods such as pulsed electric field methods. In current research it is tried to study the influence of pulsed electric fields and microwave pre-treatments on some characteristics of oil and meals obtained from Niger seed.
Materials and methods: To perform this research, the Niger seeds (contain 40% oil) was prepared from local market of Fars Province (Iran). Then the external materials such as weed seeds, sand and stones were separated and removed manually and the seeds were treated with microwave pre-treatment with power of 900W and different procedure times (100 and 200 seconds), and pulsed electric field with two levels of electric field intensities of (250 and 500 kV/cm with 30 pulses). After performing these treatments, the seeds oil was extracted with screw press at the speed of 34 rpm then various experiments, such as the efficiency amount of oil extraction, refractive index, total phenolic compounds, oxidative stability, protein and ash of meals, were performed on the oil in a complete random model with three repetitions.
Results and discussions: The comparing group mean test with Duncan’s method indicated that applying pulsed electric fields and microwave pre-treatments in lower times and intensities caused an increase in extraction efficiency. But with increase in the microwave time and also increase in the intensity of pulsed electric field, the efficiency of oil extraction was decreased in such a manner that when a pulsed electric field with intensity of 500 kV/m was used, the oil extraction had the lowest extraction efficiency. The increase of oil extraction efficiency using microwave can be related to more fracture or disintegration of cells which contain oil during treating with microwave. The reason for increase in the efficiency of oil extraction using the pulsed electric fields can be attributed to the electrical decomposition of cells and more permeability of them. A decrease in the oil extraction efficiency with excessive increase in the microwave time and intensity of electric field is probably attributed to more degradation of seeds internal structure and closure of oil outlet duct. Analysis of data obtained from oils refractive index indicated that the pre-treatment type didn’t have any significant influence on the oils refractive index (P>0.05) and the amount of refractive index was 1.478 for all of the measured samples (figure 2). Applying various pre-treatments, caused an increase in the amount of total phenol and oxidative stability of oils in the manner that the maximum amount of total phenol and oxidative stability was obtained from pulsed electric field pre-treatment with the intensity of 500 kV/m. The amount of tocopherol and antioxidant compounds can be the reason for this observation because of applying these pre-treatments. Variance analysis of data obtained from performing tests and experiments indicated that the influence of pre-treatment type on the amount of protein and meals ash was completely significant (P<0.01). Comparison of the means in data obtained from experiments showed that the maximum and minimum amount of protein and meals ash in the treated sample was obtained with 100 seconds microwave and the pulsed electric fields with 500 kV/m intensity, respectively, in other words, applying various treatments (in low times and electric field intensities of microwave and pulsed electric fields) in oil extraction led to an increase in the amount of protein and ash in comparison with non-treated sample and the reason of this fact can be attributed to the more extraction of oil from seeds and consequently an increase in the % protein and ash remained in the meals. Finally, the results of this research indicated that applying microwave (for 100 seconds) and pulsed electric field (with electric field intensity of 250 kV/m) can be very useful in extracting oil from Niger seed because of increase in the extraction efficiency and also increase of useful compounds in oil.