Rosemary, Rosmarinus officinalis L., is an herb widely used throughout the world. It is, without doubt, one of the most popular plants in Morocco. For the purpose of examining the factors affecting the extraction of the essential oil of this plant by hydrodistillation, a screening study by Hadamard matrix type Plackett and Burman was conducted. After an appropriate choice of seven variables, sixteen experiments lead to a mathematical model of first degree connecting the response function (yield) to factors. Later than the realization of the experiments and data analysis, we concluded that six factors have a significant effect on the hydrodistillation process, namely: the extracting time, the individuality effect, the harvest period, the mass plant/water ratio, the drying of plant material and the temperature of heating. As for the cutting of plants, it presents a statistically negligible effect. Our study proved the great efficiency to applying the experiments design methodology for characterizing the operational parameters that affect the hydrodistillation. It is about a relevant and economical way which allows to obtain the maximum of information in a short time and, especially, with a minimum of experiments.

Thyme, Thymus vulgaris L., is an herb widely used throughout the world. It is, without doubt, one of the most popular plants in Morocco. For the purpose of examining the factors affecting the extraction of the essential oil of this plant by hydrodistillation, a screening study by Hadamard matrix type Placket and Burman was conducted. After an appropriate choice of six factors, sixteen experiments lead to a mathematical model of first degree connecting the response function (yield) to factors. Later than the realization of the experiments and data analysis, we concluded that five factors have a significant effect on the hydrodistillation process, namely: the extracting time, the harvest period, the individuality effect, the mass plant/water ratio and the temperature of heating. As for the drying of plant material, it presents a statistically negligible effect.