There are various methods to extract biological compounds and oils from plants. However, they are not all created equally, and each has its strengths and weaknesses. Two popular methods are steam distillation and Soxhlet extraction. Both are widely used, effective techniques, but Soxhlet extraction is a more efficient and productive method, as was proven in a recent study published in the Asian Journal of Biological Sciences. 
Researchers chose two plants, Euphorbia milii (crown of thorns) and Cassia occidentalis (antbush), that produce natural insecticidal oils and subjected them to both methods of extraction. The tests occurred over a 24-month period at the University of Calabar and the Federal University of Technology Akure, both in Nigeria. Steam distillation was performed using the method outlined by De Castro and Ayuso.  The Soxhlet extraction was performed using n-hexane as the solvent (researchers recommend a non-polar organic solvent such as hexane). Once the extracts were obtained, researchers analyzed them using gas chromatography and mass spectrometry.
The results showed Soxhlet extraction to be superior to steam distillation in a few areas, yield being one of them. Soxhlet extraction produced E. milli and C. occidentalis yields of 8% and 10% of sample weight, respectively, while steam distillation yielded only 0.25% and 1.12% of sample weight. 
The oils obtained through Soxhlet extraction had a higher chemical content than those produced with steam distillation. Steam-distilled oils contained three chemical compounds in those extracted from E. milii (e.g., D-limonene) and six chemical compounds in those extracted from C occidentalis (e.g., phthalic acid, isobutyl octyl ester). Soxhlet extraction, on the other hand, produced 24 chemical compounds from the E. milii samples and 18 from the C. occidentalis. These included “phenols, acyclic
olefins, esters, ketones, carboxylic acids and alcohols.”
Soxhlet extraction also yielded higher quantities of monoterpenes, including α-pinene and α-phellandrene. Interestingly, the heat-sensitive compound methyl stearate was found in the steam-distilled C. occidentalis but not the Soxhlet-extracted plant, suggesting that the higher heat used for the Soxhlet extraction may damage thermally labile compounds. This was also true for D-limonene in E. milii. 
Overall, researchers concluded that the use of the organic solvent, rather than simply steam, is what gave Soxhlet extraction its higher oil and chemical yields.
1. Okonkwo C, et al. Comparative study of steam distillation and Soxhlet for the extraction of botanical oils. Asian Journal of Biological Sciences. 2020;(13):62-69.
2. De Castro MDL, Ayuso LEG. Environmental Applications|Soxhlet Extraction. In: Wilson ID, ed. Encyclopedia of Separation Science. Academic Press; 2000:2701-2709. [Times Cited: 3 (Semantic Scholar)]