## 3 Results and Discussion  
### Comparison of SFE-CO\({}_{2}\) Extraction and Solvent Extraction  
Before analysing the ultrasound effect on the yield of extraction, the yield max and the composition of hemp seed oil obtained with supercritical CO\({}_{2}\) are compared. It was found that 35.33 \(\pm\) 2.00 g hemp oil/100 g of hemp seed and 14.77 \(\pm\) 0.82 g hemp oil/100 g of hemp seed feed of oil was extracted by sokhlet and supercritical CO\({}_{2}\) method, respectively.  
Figure 1 shows the yield obtained for the samples processed at 300 bar and 40 \({}^{\circ}\)C to evaluate the effect of pretreatment with ultrasounds.  
From the obtained trend, it is possible to point out that the penetration of CO\({}_{2}\) inside the tissue is favoured by ultrasound pretreatment because it facilitates the oil extraction, as evidenced by the higher slope of the three pretreated samples compared with the untreated one. In particular, the slope increases increase the sonication time even if the amount of the extracted hemp oil does not increase in a relevant way.  
Specifically speaking, a longer ultrasound pretreatment from 10 to 40 min causes extraction yields of 19.85 \(\pm\) 0.02, 20.04 \(\pm\) 0.03 and 20.92 \(\pm\) 0.02 (% \(w\)/\(w\)) compared with the yield obtained without pretreatment, but it is possible to observe a slight greenish colour due to the probable presence of chlorophyll [10].  
The increase in the penetration rate of supercritical CO\({}_{2}\) solvent into the tissue is evidenced by the differences in the slopes of the curve with and without ultrasound pretreatment. Moreover, it is possible to observe that the penetration velocity of the supercritical fluid increases with increasing the ultrasound time from 10 to 20 min because of the higher yield at the same extraction time. According to [PERSON] and [PERSON] [35], ultrasound pretreatment decreases the extraction time and solvent consumption, also giving a higher penetration of chosen solvent into the cellular material and an enhanced release  
Figure 1: Yield (% \(w\)/\(w\)) of oil extracted by supercritical CO\({}_{2}\) from untreated and ultrasound pretreated seeds.  
of cell content, even if it is possible to highlight that the quantity of oil extracted as the pretreatment time increases remains unchanged contrary to [PERSON] and coworkers [8].  
The quality of the extracted oil is fundamentally related to the extraction technique employed. Hemp seed oil is generally extracted by cold pressing, avoiding the use of organic solvents if intended for human consumption. Solvent extraction takes a long time, and the presence of toxic residues in the final product is a disadvantage, but it is in general used to know the maximum yield of extraction [10]. A green alternative with relatively low extraction times could be optimal because the oil colour is yellow without sonication pretreatment, contrary to cold pressing, and light green using the sonication. Moreover, SFE is considered a clean technology (green process/eco-friendly) that allows the use of low-environmental-impact solvents such as carbon dioxide that allow you to work at low temperatures and extraction pressures [10].