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SECTION I: MOLECULAR GENETIC ENGINEERING AND BIOCHEMICAL TECHNOLOGY 143raw materials to get a high oil yield but remaining chemical compositions of essential oil. This study showed that the maximum oil yield obtained within 240 min of the hydro-distilling extraction process. It was in agreement with the result by Shahin Aziz et al. (2012), the optimal time for black pepper essential oil by water-based distillation was also 240 min [2]. It is unnecessary to prolong the distillation process to save the energy and time. Regarding to the effect of material sizes, it was determined that the material with the black pepper ground into the size of 0.5 mm resulted in the highest oil yield. The result is attributed to the mass transfer between the black pepper powder and water as a solvent. Principally, the material with larger particle sizes has more areas filled with essential oil [25], thus it is hard to evaporate essential oil from large-size plant tissues. Additionally, when the raw material particles were ground finer, the material layer has certain porosity so that the steam can pass through this layer evenly and easily to attract the vapors of essential oils to fly upwards. But if the black pepper grinding is too fine, in this case the size of particles about 0.2 mm in a hydro-distillation system without stirrer, the amount of black pepper essential oil obtained may sometimes be reduced because the material layers are collapsed and reducing porosity that prevents volatile essential oils from escaping. Therefore, it is recommended that the black pepper material should be crushed into powder with a medium fine size of about 0.5 mm to achieve the maximal recovery of the essential oil, that this result was in consistent with other studies [2, 7, 20]. Therefore, this material size will be chosen for the later experiments.In the water-based distillation system for essential oil production, oil recovery yield principally depends on vapor and