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144 PROCEEDINGS OF INTERNATIONAL SCIENTIFIC CONFERENCE ON APPLIED BIOTECHNOLOGYwater volume. When heating the volume of water and material mixture, the water vapor created permeates the plant epidermis containing essential oil gland, breaking down them, and attracting the oil [7]. At the first hour of heating when the large steam volume is created, the more oil escaped from material glands was extracted. During the distillation, oil and water vapor are competitively evaporated. However, if the low water level used will generate less steam not enough to dissolve colloids. Especially, in the last hour of distillation the water volume remaining in the boiler significantly reduced or exhausted decreases oil recovery efficiency and even burning the material at the bottom of the pot. When using more and enough water level to distill, the oil diffusion into water vapor is better that positively contributes to higher oil yield recovery. Though if too much water added will dissolve oil or emulsify it, leading to reduction of oil yield as well as economically ineffective because of requirement of more energy and time for boiling water volume. In this study, the optimal material to water ratio for black pepper powder applied in a 10 L hydro-distillation reactor system was 1/10 (100g/L or 300g/3L) for a maximum yield of essential oil production within 240 min. It is different from the study of P N Dinh et al. (2020) determining 1/15 as an optimal m/V ratio for a 1 L Clevenger apparatus [6]. The optimal m/V ratio of water-based distillation process may be different among reactor systems with different types and capacity. 4.2. Biochemical analysis of black pepper essential oil Based on the analysis result of chemical composition, the essential oil from black pepper in Dak Lak province of Vietnam have typical biochemical compositions for pepper oil, especially the substance of caryophyllene contains the highest percentage of 26.47% in this study. As shown in Table 2, caryophyllene was